Chemicals of life
Overview
All cells are made up of a variety of substances some of which organic while others are inorganic.
Water forms the largest component and is also a medium for all reactions in the cell. The other
substance includes acids, bases, salts, vitamins, carbohydrates, lipids and proteins. There are
enzymes and nucleic acids which perform a variety of functions.
General objective
By the end of the topic, the learner should be able to describe the composition, structure,
properties and importance of inorganic and organic substances to the life of organism.
Acids, bases and salts
Specific objectives
The learner should be able to
a. Describe properties of acids and bases
b. Explain the role of acids, bases and salts in maintaining a stable internal environment for
physiological processes.
Definition
An acid is a molecule or ion capable of donating a hydrogen (proton or hydrogen ion
H+) in solution. They neutralize alkalis, dissolves some metals with liberation of
hydrogen, and turns litmus red; typically, a corrosive or sour-tasting liquid of this kind:
A base is a substance that can accept a hydrogen ion (H+) from another substance
PH
In chemistry, pH is a numeric scale used to specify the acidity or basicity (alkalinity) of an
aqueous solution. It is roughly the negative of the logarithm to base 10 of the
concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is
the negative of the logarithm to base 10 of the activity of the hydrogen ion.
1
Here are some of the places that acids and bases can be found in the human body:
1.
DNA is a complex NUCLEIC ACID found in cells that contains four unique nitrogen-based
BASES: adenine, cytosine, guanine and thymine. Differing combinations of these four bases
determine the genetic characteristics of all human life.
2.
AMINO ACIDS are the building blocks of proteins! The majority of amino acids consist of
both a carboxylic acid (-COOH) and an amino (-NH2) functional group attached to the same
tetrahedral carbon
atom.
3.
LACTIC ACID is an acid produced when sugars are processed by the body. The
production of too much lactic acid can cause serious health issues and even death.
4.
Vitamin C has the chemical name ASCORBIC ACID and Aspirin and other pain relief
remedies are also organic acids. Here is the chemical structure of Aspirin:
5.
BLOOD is naturally buffered to a pH of 7.35 – 7.45 in healthy humans. While our bodies
naturally work to keep a healthy acid-base balance, problems can arise when acidic or basic
compounds are too concentrated or not present in high enough concentrations. The blood's acidbase balance is precisely controlled, because even a minor deviation from the normal range can
severely affect many organs. The body uses different mechanisms to control the blood's acid-base
balance such as the release of CO2 from the lungs, adjusted kidney function for excretion of
substances and controlled buffering of blood via concentrations of the bicarbonate ion.
6.
Acidosis and alkalosis are the two abnormalities of acid-base balance. In acidosis, the
blood has too much acid (or too little base), resulting in a decrease in blood pH. In alkalosis, the
blood has too much base (or too little acid), resulting in an increase in blood pH. Acidosis and
alkalosis are not diseases, but rather are conditions that result for a variety of reasons.
Effects of unbalanced pH in the body
A person with mild metabolic acidosis may have no symptoms but usually experiences
nausea, vomiting, and fatigue. Breathing becomes deeper and slightly faster (as the body
2
tries to correct the acidosis by expelling more carbon dioxide). As the acidosis worsens,
the person begins to feel extremely weak and drowsy and may feel confused and
increasingly nauseated. Eventually, blood pressure can fall, leading to shock, coma, and
death.
Systems responsible for maintenance of the acid-base balance
Several systems maintain constant pH. The list below is made according to order when
they act:
1) Chemical buffering systems
Buffers react immediately – acute regulation. They minimize pH changes that would otherwise
occur in their absence. They do not correct pH deviations, but only serve to reduce the extent of
the change that would otherwise occur. These buffers include the bicarbonate buffer system, the
phosphate buffer system, and the protein buffer system. Capacity of buffers is not indefinite
that is why chemical buffers act only in the short-term. Chemical buffering systems deal
with pH deviations in common metabolism.
The bicarbonate buffer system is an acid-base homeostatic mechanism involving the
balance of carbonic acid (H2CO3), bicarbonate ion (HCO−3), and carbon dioxide (CO2) in
order to maintain pH in the blood and duodenum, among other tissues, to support proper
metabolic function. Catalyzed by carbonic anhydrase, carbon dioxide (CO2) reacts with
water (H2O) to form carbonic acid (H2CO3), which in turn rapidly dissociates to form a
bicarbonate ion (HCO− 3 ) and a hydrogen ion (H+) as shown in the following reaction:
CO2 + H2O ↔
H2CO3 ↔
H+ + HCO3-
As with any buffer system, the pH is balanced by the presence of both a weak acid (for
example, H2CO3) and its conjugate base (for example, HCO−3) so that any excess acid or
base introduced to the system is neutralized
2) Respiratory system
Respiration reacts in 1-3 minutes. Respiratory system regulates carbon dioxide.
Respiration is able to change pCO2 by its elimination or retention. Respiratory centre is in
brainstem.
3) Kidneys
Kidneys react in hours-days. In the kidney carbon dioxide react with water to form
hydrogen ions (H+) and hydrogen carbonate (HCO3-)
3
CO2 + H2O ↔
H2CO3 ↔
H+ + HCO3-
When the pH is low the collecting duct and distal convoluted tubules secrete hydrogen
ions and retain hydrogen carbonate ions. Therefore, acidic, or that is to say urine with low
pH, will be excreted in that case.
Conversely, if blood too alkaline, then the collecting duct can secrete bicarbonate into
urine and retain H+ lowering the pH of blood; therefore cause the urine to become more
alkaline.
4) Liver
Liver is pivotal organ of the energetic metabolism it also has important influence on the
acidbase balance. Liver is the most important tissue where ammonium is detoxified in both
(1) urea cycle, and (2) glutamine synthesis. Which one of these fates of ammonium is
favored closely depends on status of the acid-base balance:
a) NH4+ → urea + 2 H+ → acidification of the body
CO2 + 2 NH4+ → CO(NH2)2 + 2 H+ + H2O
H+ + HCO3- → H2O + CO2 (consumption of bicarbonate-)
b) NH4+ → glutamine synthesis → H+ is not produced, glutamine is taken up by the
kidneys. In the kidney is H+ excreted as NH4+
Water
Specific objectives
The learner should be able to
a.
b.
c.
d.
e.
f.
Describe the molecular structure of water
State the function of water
Explain the importance of water as solvent
Relate the water properties to its role in the life of organism
To test for water
Determine water content in tissue by using dry weight method.
Water
This is the most abundant compound, typically making up of 60-95% fresh mass of an
organism. Structure of water
4
Water is made of hydrogen and oxygen.
2H2(g) + O2 (g) → 2H2O(l)
The H2O molecule is electrically neutral, but polar because the positive and negative charges
are not distributed uniformly. This lead to apartial positive charges on hydrogen atoms and a
partial negative charge on
oxygen atom
In liquid and solids these partial charge attract to form hydrogen bonds
The hydrogens are responsible high melting and boiling point and strong surface tension
of water.
Uses of water
(i)
It makes up structures of organism
(ii)
It is a solvent
(iii)
It is a reagent in hydrolysis
(iv)
Provide support for aquatic organism
(v)
Is a medium of fertilization through which gametes swim.
(vi)
Medium for removal of waste products
(vii)
Temperature control
(viii)
Hearing and balance as endolymph
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Uses of water to the plants
(i) Aid seed dispersal
(ii) Provide support to herbaceous plants
(iii) Breaks up the testa of a seed during germination
(iv) Reagent in photosynthesis
(v) Loss of water through the leaves cools the plant. (vi) Medium of fertilization
Uses of water to the animals
(i) A medium of transport
(ii) Evaporation cools the animal
(iii) Lubricates joints, eyes, lungs
(iv) Constituent of protecting fluids such as tears, mucus.
Adaptation of Water to its function
•
Water is the universal solvent:
Polar and ionic substances have an electrostatic charge, so they are attracted to the charges
on water molecules and dissolve readily. Non-polar substances, such as oil, do not
dissolve in water, as they do not have charged molecules. When a salt dissolves in water,
the ions separate and a layer of water molecules form around the ions. These layers
prevent ions or polar molecules from clumping together, keeping the particles in solution.
•
Water has a high surface tension:
At an interface between air and water, a water molecule on the surface forms hydrogen
bonds with other molecules around and below it, but not with air molecules above it. The
unequal distribution of bonds produces a force called surface tension; this causes the water
surface to contract and form a surprisingly tough film or ‘skin’ enabling small animals like
insects to walk over. It also protects blood capillaries of gill filaments from bursting.
•
Ice floats on water:
Water is at its most dense at 4oC. When water freezes the hydrogen bonds between the
molecules forms a rigid lattice, that holds the molecules further apart then in liquid water.
Ice, having expanded when freezing, is less dense than its liquid counterpart and so floats
on water. This protests water from further freezing because ice insulates the surface of
water.
•
Water is adhesive and cohesive:
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Water is ‘wet’ because it sticks to things. This is because its molecules can form hydrogen
bonds with other polar substances. This is called adhesion. The attraction between
molecules of similar substances is called cohesion. In this way water molecules stick
together which allows water to enter and move along very narrow spaces, in a process
called capillarity. This enables water ascend in the xylem of a tall tree.
•
Important thermal properties:
Water has a high specific heat capacity meaning that it needs to gain a lot of energy to
raise its temperature. Conversely it also needs to lose a lot of energy to lower its
temperature. Water’s specific heat capacity is 4.2 kJ/g/oC. Thus, this minimizes increase in
temperature on hot days.
Water has a high latent heat of vaporization which means a lot of energy is required to
evaporate it. When it evaporates, water draws thermal energy out of the surface it’s on,
which can be observed in sweating. Thus, sweating cools, the body.
Water also has a high latent heat of fusion meaning that at 0oC water must lose a lot of
thermal energy before it freezes, thus liquid water can reach temperatures of down to 10oC before it forms ice. This implies too much heat loss is required to freeze water body.
• Other physical properties of water:
It is transparent to sunlight. This allows animals to seen in water also photosynthesis in
aquatic plants.
It has a relatively high density compared to air, this support aquatic animals while
swimming.
It is difficult to compress. Thus, support aquatic animals while swimming.
It conducts electricity (when it contains dissolved ions) enable conduction of heat, keeping
water hot.
Inorganic compounds/mineral salts
(a) Inorganic salts include those needed in large amounts such as salts of Na+, Mg2+,
Cl-, K+ and those needed in trace amounts such as manganese, iron, cobalt, copper,
zinc, boron, aluminium, silicon, vanadium, molybdenum and iodine
Functions of inorganic salts
(i) They are components of proteins, e.g. nitrogen, phosphorus and Sulphur.
(ii) They are components of tissues; e.g. calcium and phosphorus are components of
bones (iii)They are constituents of enzymes; e.g. copper and iron.
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(iv)
(v)
(vi)
(vii)
They are metabolic activators for example, magnesium activates enzymes in
phosphate metabolism.
They are constituent of pigments for example, iron in hemoglobin and
magnesium in chlorophyll.
They are determinants of anion-cationic balance in the cell example Na+, Cl-,
and K+.
They are determinants of osmotic pressure, e.g. Na+, Cl-, and K+.
Carbohydrates
Specific objectives
The learner should be able to:
a.
b.
c.
d.
e.
f.
Describe the structure and components of various carbohydrates
Explain the properties of carbohydrates.
Explain the functions of carbohydrates in organisms.
Describe the condensation of carbohydrates
Describe the hydrolysis of carbohydrates
Identify the various categories of carbohydrates.
Carbohydrates are food substances with a general formula (CH2O)n where n is natural number.
Classification
(i)
Monosaccharides e.g. glucose, galactose and fructose
8
Ring structures of common monosaccharides.
Sources: honey, fruits
Properties
- They are sweet
- Are soluble in water
- They reduce blue copper II ions to red precipitates in alkaline medium.
Testing for reducing sugars
When boiled with Benedict’s or Fehling’s solutions the color changes from
blue to green to yellow to oranges ppt.
(ii)
Disaccharides
They are made of two simple sugars by condensation
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Illustration
-glucose
6CH2OH
-glucose
6CH2OH
maltose
6CH2OH
6CH2OH
- Maltose, lactose and sucrose are sweet and are referred to as sugars. Maltose and
lactose are reducing sugars whereas sucrose in not and referred to as non-reducing
sugar.
Disaccharides
Composition
Source
Maltose
glucose + glucose
malt
Lactose
Glucose + galactose
milk
Sucrose
Glucose + fructose
Sugar cane
Sugar beets
Cellubiose
Glucose + glucose
wood
Testing for non-reducing sugars
1. When boiled with Benedict’s solution or Fehling’s’ solution, the color remains blue.
2. When boiled with HCl, the solution cooled, neutralized by NaOH, boiled Benedict’s
or Fehling’s solution, the color changes from green, to yellow to orange.
HCl hydrolyses non-reducing sugars
NaOH neutralizes the excess acid because Benedict’s or Fehling’s solution does not
work in acidic medium.
Polysaccharide (CH2O)n
10
These are made of very many mono saccharides per unit molecule e.g. starch and cellulose
Testing for starch
It changes the color of iodine black or blue.
Functions of carbohydrates
1.
2.
3.
4.
5.
6.
Glucose, galactose and fructose are oxidized to release energy in he body
Glyceraldehyde is an intermediate molecule in photosynthesis.
Ribose is component nucleotides.
Sucrose is a form in which carbohydrates are transported in plants
Lactose is a source of energy in milk
Storage of energy (starch in plants, glycogen in animals, inulin is some plants like
Dahlia)
7. Formation of cellular structures (cellulose in plant cell walls, chitin is
Lipids
Specific objectives
The learner should be able to:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Describe the structure and components of lipid molecules.
State the properties of lipids.
Explain the functions of lipids in organism.
Describe the structure of steroids
Explain the effects of lipids and steroids to organisms
Describe the condensation of fatty acids and glycerol to form lipids
Describe the hydrolysis of lipids to fatty acids and glycerol
Compare waxes and lipids
State importance of cholesterol in organism
To carry out tests to identify lipids on food and extracts.
(a) Lipids
These include natural fats and oil. Fats are solids at room temperature while oils are
liquids
Structure of lipids
Lipids are ester of glycerol and fatty acids
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Table of nature and occurrence of some fatty acid
Name of fatty acid
formula
Saturated/unsaturated
occurrence
Butyric acid
C3H7COOH
Saturated
Butter fat
Linoleic
C17CH31COOH
unsaturated
Linseed oil
Oleic
C17H33COOH
unsaturated
All fats
Palmitic
C15H31COOH
Saturated
Animal and vegetable
fats
Stearic
C17H35COOH
Saturated
Animal and vegetable
fats
arachidic
C19H39COOH
Saturated
Peanut
Ceritic
C25H51COOH
Saturated
Wool oil
Uses of lipids
Structural functions
- Make up cell membrane
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- Protection: lipids are constituents of the waxy cuticle of plants and insects - Lipids are
water repellant thus prevent water loss from or entry into an animal skin - Their spongy
nature protects delicate organs as shock absorbers.
- Being bad conductors, they reduce water loss from the body when deposited beneath
the skin for insulation
- Storage ; they are better storage compounds than carbohydrates due high calories value,
due to high hydrogen content, they are light, insoluble in water, compact to fit in a small
volume and are easily used when required.
Physiological functions
- Source of metabolic water
- Store fat soluble vitamins (ADEK)
- Source of metabolic water
- Raw materials for hormones
Testing for lipids
a. They form a translucent mark on paper that does not disappear when the paper is dried
on a flame.
b. Emulsion test
When 2cm3 of fats or oil are dissolved in 2cm3 of absolute ethanol followed by water, a
white cloudy suspension is formed.
c. Sudan III
When a few drops of Sudan III are added to a mixture of 2cm3 of water and 2 cm3 of oil
and shaken, a red stained oil layer separates out.
Phospholipids.
These are lipids in which of the fatty acid is replaced by phosphoric acid. The phosphoric
acid attracts water (hydrophilic) whereas the rest of the group repel water
(hydrophobic). This property enables them to form a cell membrane.
Waxes
Waxes are formed by combination with an alcohol other than glycerol. This alcohol is
much larger than glycerol, and therefore waxes have a more complex chemical structure.
The main role of waxes is waterproofing plants and animals although, they form storage
compounds in a few organisms, e.g., castor oil and in fish.
Advantage of storing fats over carbohydrate
(i) Has high energy content than carbohydrates
(ii) It is lighter
(iii) It is compact and requires less space
(iv) It is a raw material for hormones
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(v) Insoluble in water that they have low osmotic value
Proteins
Specific objectives
The learner should be able to:
b.
c.
d.
e.
f.
g.
h.
i.
Describe the structure and composition of proteins.
Describe the properties of proteins
Explain the importance of proteins
Explain the functions of proteins in organisms
Describe the condensation of amino acids to form proteins.
Describe the hydrolysis of proteins to amino acids
The effect of heat/temperature changes on proteins
To carry out test and identify proteins on food /extracts.
Proteins
These are classified into two groups
(i)
Structural proteins: insoluble proteins that make up body structures like
bones and muscles. Fibrinogen is a soluble structure protein used in blood
clotting.
(ii)
Globular proteins are soluble proteins such as enzymes, antibodies,
hormones and so on.
Composition of proteins
The basic unit of proteins are amino acids
R
O
H
N
C
H
C
H
R =alkyl group
H
There are about 22 different amino acids in the body of which isoleucine, leucine,
methionine, phenylamine, proline, threonine and valine cannot be synthesized in
human body and they are referred to as essential amino acids.
Amino acids unite to form proteins through formation of peptide bonds by a
condensation reaction in which a water molecule is eliminated.
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R1
R2
O
H2O
C
N
H
C
H
O
N
C
C
OH H
H
H
H
H2O
R1
O
R2
O
H
H2O
N
C
C
N
C
H
H
Peptide bond
C
H
H
Uses of proteins
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
Make up structures, e.g., collagen make up connective tissues.
Make up enzymes such as catalyze and amylase.
Are constituent of hormone such as insulin
Are constituents of antibodies that protect the body from foreign particles.
Make up muscles such as myosin and actin
They are storage food e.g. egg white
Constitute toxins such as snake venom for protection.
Testing for proteins
a. They coagulate on heating
b. They coagulate on addition of Melon’s reagent and on heating they form a pink
precipitate.
c. They form a purple color when mixed with equal amount of NaOH followed by 3
drops of copper sulphate solution.
Vitamins
Specific objects
The learner should be able to:
15
a.
b.
c.
d.
e.
State the type of vitamins i.e., water soluble and fat soluble; essential and nonessential.
State the importance of vitamins in organism
Test for vitamin C.
Demonstrate effect of over boiling vegetables
Demonstrate the effects of storage on the quality of fresh food.
Vitamins are complex organic compounds present in very small quantities in natural food which
are essential for good healthy body and maintains its normal metabolic activities. Some vitamins
are fat soluble (ADEK) while others are not.
Vitamin C / ascorbic acid
Sources: citrus fruits, green vegetables, potatoes, tomatoes, etc.
Function of vitamin C
Concerned with the metabolism of connective tissues and the production of strong skin.
Deficient disease : anemia and scurvy: the germs bleed, wounds fail to heal.
Testing for Vitamin C
It decolorizes DCPIP.
Other vitamins and their deficient diseases
Vitamin
A
Night blindness
K
Delayed clotting
E
Reduced fertility in rats
Functions of vitamins
- protect the body against diseases
- formation of coenzyme that facilitate enzyme reactions
- blood clotting
- components of visual pigment
Revision questions
1. Lack of iodine in the diet causes cretinism because iodine
A. Controls metabolism
B. Is essential in the formation of metabolic enzymes
C. Influence growth of bones
D. Is required for synthesis of thyroxine
2. In the body, proteins combine with acids or bases depending on the
16
A.
B.
C.
D.
Temperature of the medium
Hydrogen ion concentration in the medium
Number of solvent molecules present in the medium
Number of amino acid molecules in the protein
3. Starch, glycogen and cellulose are all composed of
A.
B.
C.
D.
α-glucose
β-glucose
monosaccharides
polysaccharides
4. Some amino acids are known as essential because they are
A.
B.
C.
D.
more important in the body metabolism than other
not made by the body
contained in first class proteins
required in larger amounts than others.
5. Which of the following sugars is not reducing?
A.
B.
C.
D.
Maltose
Fructose
Galactose
Sucrose
6. Among the following compounds, one cannot be hydrolyzed is
A.
B.
C.
D.
Glycogen
Galactose
Lactose
Maltose
7. Which one of the following is the correct formula of a polysaccharide?
A. (C6H10O5)n B. (CH2O)n
C. (C6H12O6)n
D. C12H22O11)n
8. Which one of the following statements is true of essential fatty acids? They
A.
B.
C.
D.
They are the most required lipids in the body
Are required in the body in large quantities
Cannot be synthesized in the body
Are most abundant in animal tissues
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9. Which one of the following properties of water facilitates its efficient transportation of
glucose?
A.
B.
C.
D.
Forms hydrogen bonds with other molecules
Has high surface tension
Has low freezing points
Has high boiling point
10. Which of the following vitamins is water soluble?
A.
B.
C.
D.
A
B
D
C
11. Lack of iodine in diet causes cretinism because iodine
A.
B.
C.
D.
Controls metabolism
Is essential for formation of metabolic enzymes
Influence growth of bones
Is required for synthesis of thyroxine
12. A property of water that makes it suitable component of a hydrostatic skeleton is it
A.
B.
C.
D.
High density
High surface tension
Low viscosity
Incompressibility
13. Aquatic organism survives under solidified water body because
A. Water solidifies from bottom to top of lakes
B. Ice is less dense than water at 40C
C. Cold water is more dense than hot water and falls to the bottom
D. Warm water floats on top of cold water
14. In the body, proteins combine with acids or bases depending on the
A. Temperature of the medium
B. Hydrogen ion concentration in the medium
C. Number of solvent molecules present in the medium
D. Number of amino acid molecules in the protein
15. Which one of the following is not a fibrous protein
A. Keratin
B. Globulin
18
C. Elastin
D. Collagen
16. Sucrose is a non-reducing sugar because it
A. It is not fully digested
B. It lacks reducing groups
C. Is a disaccharide molecule
D. Is a ketose sugar
17. Which one of the following is a fibrous soluble protein?
A. Myosin
B. Collagen
C. Myoglobin
D. Fibrinogen
18. A green plant develops yellow leaves as a result of being deficient in
A.
B.
C.
D.
Magnesium
Manganese
Nitrogen
Calcium
19. Which of the following elements is not required by plants?
A.
B.
C.
D.
copper
iodine
iron
zinc
20. In the blood plasma, proteins can act as bases or acids depending on the
A.
B.
C.
D.
Temperature of the medium
Hydrogen ion concentration of the medium
Nature of the protein
Concentration of the solute in the plasma
21. Which of the following is the function of manganese in the human body?
A.
B.
C.
D.
Essential for formation of erythrocytes
Activate enzymes
Acts as growth factor in bone development
Utilized as a component of bone and teeth
22. Which of the following substances consists of globular proteins
19
A.
B.
C.
D.
Enzymes
Keratin
Elastin
collagen
23. Which one of the following symptoms is likely to be caused by magnesium deficiency in
plants?
A.
B.
C.
D.
Yellow leaves and stunted growth
Poor root growth
Weak stems
Yell spotted leaves
24. Which one of the following is not a function of globular proteins in the body?
A. Acts as buffers in blood plasma
B. Form structural proteins
C. Are vital constituents of plasma membrane
D. Form enzymes
25. The following structural formula is for an amino acid in solution
H
-O
C
C
O
H
NH3
A substance was added to this solution and the structure of the amino acid molecule changed to
H
HO
C
C
O
H
NH3
What substance was added and what effect would this have had on the final pH of the solution?
A. Salt added, pH unchanged
B. Acid added, pH lowered
C. Acid added, pH unchanged
D. Base added, pH higher
26. We need to eat iodized salt in order to
20
A. Prevent obesity
B. Get a balanced diet
C. Improve vision D. Avoid goiter
27. Water has comparatively high surface tension and boiling point in relation to other
substances of similar sized molecules because its molecules are
A. doubly bonded
B. polar
C. ionic
D. covalent
28. Evaporation of water from the body surface causes cooling because water has a high
A. Latent heat of vaporization
B. Latent heat of fusion
C. Boiling point
D. Specific heat capacity
29. The complexity and variety of organic molecules is due to the ability of the carbon atom to
A. form covalent and ionic bond
B. form covalent bonds in three dimensions
C. form strong chemical bonds
D. bonds with very many other elements
30. When a lipid is combined with a phosphate group, it becomes
A.
B.
C.
D.
saturated
a complex molecule
water soluble
amphoteric
31. A property of water which facilitates its efficient transportation of glucose
A.
B.
C.
D.
ability to form hydrogen bonds with other molecules
high surface tension
low freezing point
high boiling point
32. When a lipid is combined with a phosphate group, it becomes A. saturated.
B. a complex molecule.
C. water soluble.
21
D. amphoteric.
33. Water has a comparatively high surface tension and boiling point in relation to other
substances of similar sized molecules because its molecules are
A. doubly bonded.
B. polar.
C. ionic.
D. covalent.
34. Starch and glycogen are suitable storage molecules because they
A. are large in size which makes them less soluble in water
B. are chemistry reactive in the cell
C. can easily be hydrolyzed
D. exert an osmotic pressure in the cell
35. The high heat capacity of water has biological importance of
A. minimizing temperature changes in animal fluids
B. cooling animals
C. Preventing freezing of cell contents
D. controlling heat loss in animals
Paper 1 section 1
36. Fat and glycogen are energy storage compounds in animals
(a) Compare the suitability of the two substances as storage compounds (4marks)
(b) State advantage of storing fat over glycogen (3marks)
(c) Why is glycogen more suitable energy compound in muscle than fat? (3marks)
37. Using the structural formula
H
H
C
OH
H
C
OH
H
C
OH
H
For glycerol, and molecular formula CH3(CH2)nCOOH for a fatty acid show the formation
of
triglyceride from fatty acids and glycerol.
(2marks)
22
(b) What properties do lipids possess as storage food substances?
(2marks)
(c) Outline the structural and physiological functions of lipids in living organisms.
(i) Structural
(3marks)
(ii) Physiological
(2marks)
38. (2000/1/43) (a) state three ways in which water has similar functions in both plants and
animals. (3marks)
(b) Give two ways, in each case, in which flowering plants minimize water loss
through
(i)
behavioral means (4marks)
(ii)
physiological means (4marks)
39. Explain how the structure of proteins enable them to form body tissues and structures
(4marks)
40. (a) Describe the significance of physical properties of water to organism. (12marks)
(b) Explain why lipids are better storage material in animals than carbohydrates (8marks)
41. (a) Giving examples, describe the use of nitrogen to plant and animal bodies (b) how is the
concentration of nitrogen maintained at constant level in nature?
42. Distinguish between the following
(a) Monosaccharide and polysaccharide
(05marks)
(b) Starch and cellulose
(04marks)
(c) Saturated and unsaturated fats
(05marks)
(d) Globular and fibrous proteins
(06marks)
43. Relate the properties of water to its biological importance.
(20 marks0 Answers to
objective questions
1
D
6
B
11
D
16
D
21
B/C
26
D
31
A
2
B
7
A
12
D
17
D
22
A
27
B
32
C
3
C
8
C
13
B
18
A
23
D
28
A
33
B
4
B
9
A
14
B
19
B
24
B
29
D
34
C
5
D
10
D
15
B
20
B
25
C
30
C
35
23
36. Solution
Similarities
- They are both compactly arranged to take up little space.
- they are both less soluble in water and little or none can be lost in solution
Differences
Fat
Glycogen
-Has a high calorific valve.
-Has a lower calorific valve.
-Has a higher hydrogen- oxygen content and -Has a lower hydrogen- oxygen content and
can yield more metabolic water.
yield less metabolic water
Others
- Has less weight and keeps body weight to a -Is heavier and can lead to overweight.
minimum which allows buoyancy
Others
Fat is completely insoluble in water and more can be lost in solution. Also. It prevents
desiccation.
Fat forms an insulating layer under the skin that helps in temperature regulation.
Fat has a low density hence provide buoyancy in aquatic animals.
Muscles have a high content of glycolytic enzymes which readily breakdown glycogen to
utilizable glucose.
24
The glycolytic enzymes, glycogen phosphorylase, has an allosteric site for binding AMP.
When content is low in muscle, AMP content rises and activates this glycolytic enzyme
which readily break down glycogen to glucose that can be used by the muscle.
Break down of fat to free fatty acids which can be utilized by the muscle is a slow process
because it is hormone- mediated.
41. (a) use of nitrogen
(i)
protein synthesis
(ii)
production of enzymes
(iii)
production of muscles and tissues
(iv)
production of nucleic acid
(b) nitrogen cycle
37 (a)
25
(c) Properties of fats as storage compounds
i. Has high energy content than
carbohydrates ii. It is lighter
iii. It is compact and requires less space
iv. It is a raw material for hormones
v. Insoluble in water that they have low osmotic value
(d) (i) Structural functions
- Make up cell membrane
- Protection: lipids are constituents of the waxy cuticle of plants and insects - Lipids
are water repellant thus prevent water loss from or entry into an animal skin - Their
spongy nature protects delicate organs as shock absorbers.
- Being bad conductors, they reduce water loss from the body when deposited
beneath the skin for insulation
- Storage ; they are better storage compounds than carbohydrates due high calories
value, due to high hydrogen content, they are light, insoluble in water, compact to
fit in a small volume and are easily used when required.
(ii)
Physiological functions
26
- Source of metabolic water
- Store fat soluble vitamins (ADEK)
- Source of metabolic water
- Raw materials for hormones
38 (a) similar functions of water in plants and animals
(v)
It is a solvent and medium for transport
(vi)
It is a medium of fertilization
(vii)
Evaporation cools the body
(viii)
Provides support to aquatic organism
(ix)
Component of the cell
(x)
A reagent in hydrolytic reaction
(xi)
A medium in which biological reaction occur.
(b) (i)Minimizing water by behavioral means
- Folding or rolling of leaves on a hot day
- Reduction of number of stomata
- Storage of water in leaves
- Sunken stomata
(ii) minimizing water loss by physiological means
- Shedding leaves in hot season
- Reversal of normal stomata rhythm
- Thickening of waxy cuticle
39. How structure of proteins enable them to form body tissues and structures
(a) Globular - These tend to form ball-like structures where hydrophobic parts are
towards the centre and hydrophilic are towards the edges, which makes them
water soluble. They usually have metabolic roles, for example: enzymes in
all organisms, plasma proteins and antibodies in mammals.
Fibrous - They proteins form long fibres and mostly consist of repeated sequences of
amino acids which are insoluble in water. They usually have structural roles,
such as:
Collagen in bone and cartilage, Keratin in fingernails and hair.
42. (a) Monosaccharide and polysaccharide
(05marks)
Monosaccharide
Polysaccharide
Made of 3 to 6 carbon atoms
Many carbon atoms
Composed of one sugar unit
Composed of many sugar units
Low molecular mass
High molecular mass
Soluble
Insoluble
Used for respiration
Used for storage
27
Low energy content
(a) Starch and cellulose
High energy content
(04marks)
Starch
cellulose
Polymer of alpha glucose
Polymer of beta glucose
(b) Saturated and unsaturated fats
(05marks)
Saturated fatty acids lack double bonds between the individual carbon atoms,
while in unsaturated fatty acids there is at least one double bond in the fatty
acid chain.
Saturated fats tend to be solid at room temperature and from animal
sources, while unsaturated fats are usually liquid and from plant sources.
(c) Globular and fibrous proteins
(06marks)
Fibrous proteins are insoluble in water, weak acids and weak bases but soluble in
strong acids and alkalis whereas globular proteins are soluble in water, acids and
bases.
Fibrous proteins are highly resistant to digestion by enzymes and are extremely tensile
NATURE OF THE GENE AND PROTEIN SYNTHESIS
Specific objectives
(i) Describe the composition of chromosomes and structure of nucleotides
(ii) Describe the structure of DNA and RNA
(iii) Distinguish between DNA and RNA
(iv) Explain the Watson Crick hypothesis of the nature of DNA (v) Explain DNA replication.
(vi) Describe the nature of the gene
(vii) Describe the structure of the genetic code
(viii) Describe formation of RNA
(ix) State the role of DNA and RNA in protein synthesis. Describe protein synthesis.
Gene
This is a segment of DNA that tell the body how to produce specific proteins – contain the
genetic information that is passed from parents to their offspring. These genes are found in
structures called chromosomes, which are passed to the embryo during conception. Because
the DNA passed from one human to another helps determine gender and physical
characteristics, this nucleic acid is necessary for the survival of the species.
28
Evidences that DNA is the hereditary material
DNA is a stable molecule
The amount of DNA in a given species is constant for all cells.
Mutation or changes in the composition of DNA alter the organisms‟ characteristics.
Purified DNA rather than proteins from dead virulent bacterium Pneumococcus strain which
causes pneumonia was shown to transform the non-virulent form into a virulent form. The
ability of DNA to transform nonvirulent form was stopped by addition of an enzyme that
breaks DNA in the dead virulent form before DNA was purified.
5. DNA rather than proteins of bacteriophage (T2 phage) that infect E. coli was shown to be the
hereditary material because it enabled the E. coli to synthesize the new T2 phages viruses.
1.
2.
3.
4.
Nucleic acids
This is a form of genetic material in all living organisms including the simplest viruses. Nucleic
acids are polymers made of subunits called nucleotides.
Types
1. Deoxyribonucleic acid (DNA) is found in the nucleus
2. Ribonucleic acid (RNA) is found in both nucleus and cytoplasm.
A nucleotide is made up of three molecules:
(i) Phosphate group
(ii) Pentose sugar – either Deoxyribose (in DNA) or Ribose (in RNA)
(iii)
Nitrogen base – any purine (Adenine, Guanine) or pyrimidine (Cytosine and either
Thymine in DNA or Uracil in RNA)
Structure of nucleotide
It is made of phosphoric acid, sugar and an organic base.
(a) Phosphoric acid
OH
29
O
P
OH
OH
(b) Sugar: the pentose sugar in RNA is ribose while that of DNA is deoxyribose sugar.
Deoxyribose sugar lacks an oxygen atom on the second carbon atom
5CH2OH
5CH2OH
H
O
4C
C1
H
H
C
3
C2
H
4C
OH
OH
H
O
C1
H
H
C
3
C2
H
OH
OH
OH
Ribose sugar
H
Deoxyribose sugar
(c) Organic Bases: DNA contains four different organic bases; adenine (A), guanine (G)
cytosine (C) and thymine (T). RNA also contains adenine (A), guanine (G), cytosine (C)
and Uracil (U). all these bases are ring compounds, made of carbon and nitrogen.
(i)
Pyrimidines (cytosine, uracil and thymine; CUT) have a six-membered ring.
C
H
N
C
N
C
C
NH2
O
O
C
O
C
C
C
N
C
C
N
H
H
N
C
H
O
C
C
N
CH3
H
N
C
H
H
O
C
C
H
N
H
30
H
H
Cytosine
(ii)
Thymine
Uracil
Purine (Guanine and Adenine (GA) have a two membered ring)
NH2
C
N
C
H
O
C
C
H
C
N
H
Adenine
C
N
C
N
N
N
H
C
H
C
N
C
N
NH2
H
Guanine
Nucleoside forms when a pentose sugar joins an organic base by condensation reaction (a
water molecule is lost).
Nucleotide forms when a nucleoside (pentose sugar + organic base) joins a phosphate by loss of
second water molecule.
31
The sugar-phosphate-sugar backbone is formed when the 3‟ carbon on one sugar joins to the
5‟ carbon on the next sugar by phosphodiester bonds repeatedly to form a polynucleotide
(long chain of nucleotides) with organic bases protruding sideways from sugars.
Nitrogen base
DNA structure according to Watson and Crick
1.
2.
DNA consist of two polynucleotide strands.
The polynucleotide stands are antiparallel (face in opposite directions) i.e. one runs from 3‟
to 5‟ direction while another one runs from 5‟ to 3‟ direction.
3.
A DNA nucleotide is made up of three molecules:
(i) Phosphate group
(ii) Deoxyribose sugar
(iii) Nitrogen base - either adenine (A), guanine (G), thymine (T) or cytosine (C).
Untwisted DNA is ladder-like, in which the sugar-phosphate backbones represent the
handrails while the nitrogen base pairs represent the rungs.
5. Twisted DNA forms a double helix of major and minor grooves.
6. The sugar-phosphate-sugar backbone is held by covalent phosphodiester bonds, while the
nitrogen bases from the two strands form weak hydrogen bonds by complimentary base
pairing i.e. A with T, C with G.
4.
32
ADAPTATIONS OF DNA
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
(ix)
(x)
(xi)
Sugar-phosphate backbone is held together by strong covalent phosphodiester bonds
to provide stability.
The two sugar-phosphate backbones are antiparallel which enables purine and
pyrimidine nitrogen bases to project towards each other for complimentary pairing.
Sugar-phosphate backbones are two / it is double stranded to provide stability.
The two sugar-phosphate backbones form a double helix to protect bases/hydrogen
bonds.
Long/large molecule for storage of much information.
Double helical structure makes the molecule compact to fit in the nucleus.
Base sequence allows information to be stored.
Double stranded for replication to occur semi- conservatively/ strands can act as
templates.
There are many hydrogen bonds which increase stability of DNA molecule.
There is complementary base pairing / A-T and G-C for accurate replication/identical
copies can be made;
Weak hydrogen bonds enable unzipping /separation of strands to occur readily.
Theories of DNA replication
DNA replication is the process by which the parent DNA molecule makes another copy of itself.
1. Fragmentation hypothesis (Dispersive hypothesis)
The parent DNA molecule breaks into segments and new nucleotides fill in the gaps precisely.
2. Conservative hypothesis
This suggests that the DNA strands remain intact but in some way initiate the synthesis of new
but exact copies of DNA to the parent DNA.
3. Semi-conservative hypothesis
The parent DNA molecule separates into its two component strands, each of which acts as a
template for the formation of a new complementary strand. The two daughter molecules
therefore contain half the parent DNA and half new DNA. The semi conservative hypothesis
was shown to be the true mechanism by the work of Meselsohn and Stahl (1958) in their
experiment on bacterium E.coli using radioactive 15N.
Illustration of three possible theories of DNA replication
33
Parent DNA
Old strand
New strand
Dispersive
conservative
Semi-conservative
Necessities of DNA replication
1.
2.
3.
4.
Free nucleotides to bond with complementary bases on the separated DNA strands.
Energy source in form of ATP
Complementary DNA strand
Enzymes such as DNA polymerase, DNA helicase and DNA lingaze.
Steps of DNA Replication
1. DNA unwinds and then split open by a Helicase enzyme to expose the bases on either
strand. The initiation point is a place rich in A-T probably because these are held by two
hydrogen bonds as oppose to the three hydrogen bonds between C and G. The structure
that is created is known as "Replication Fork".
34
2. An enzyme RNA Primase bind to DNA at the initiation point and attracts the first
nucleotide of the 3‟-5‟ strand
3. The elongation process involves the alignment of complementary nucleotides against the
bases on open stands and then joined into new strands by the DNA polymerase. However, it
occurs differently for the 5'-3' and 3'-5' template.
a. 5'-3' Template: The 3'-5' proceeding daughter strand -that uses a 5'-3' template- is
called leading strand because DNA Polymerase ä can "read" the template and
continuously adds nucleotides (complementary to the nucleotides of the template, for
example Adenine opposite to Thymine etc.).
b. 3'-5'Template: The 3'-5' template cannot be "read" by DNA Polymerase ä. The
replication of this template is complicated and the new strand is called lagging strand. In
the lagging strand the RNA Primase adds more RNA Primers. DNA polymerase å reads
the template and lengthens the bursts. The gap between two RNA primers is called
"Okazaki Fragments".
The RNA Primers are necessary for DNA Polymerase å to bind Nucleotides to the 3'-5‟
end of them. The daughter strand is elongated with the binding of more DNA
nucleotides.
35
4. In the lagging strand the DNA Pol I -exonuclease- reads the fragments and removes the RNA
Primers. The gaps are closed with the action of DNA Polymerase (adds complementary
nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the
phosphate - sugar backbone).
Each new double helix is consisted of one old and one new chain. This is what we call
semiconservative replication.
5. The last step of DNA Replication is the Termination. This process happens when the DNA
Polymerase reaches to an end of the strands.
6. The DNA Replication is not completed before a mechanism of repair fixes possible errors
caused during the replication. Enzymes like nucleases remove the wrong nucleotides and
the DNA Polymerase fills the gaps.
36
General structure of RNA
RNA molecules are small/short, single stranded (rRNA and mRNA) but may be
coiled around such that bases of the same strand pair with each other. b
RNA
nucleotide is made up of three molecules:
a
(i) Phosphate group
(ii) Ribose sugar
(iii)Nitrogen base - either adenine (A), guanine (G), cytosine (C) or uracil (U) c
The
sugar-phosphate-sugar backbone is held by covalent phosphodiester bonds. d RNA occurs in
three types whose sizes, shapes, amounts/abundance and roles vary:
Types of RNA
1. Ribosomal RNA (rRNA)
Makes the highest percentage of RNA. It is a single strand with a double helical region.
Its function are:
a. integral component to ribosome structure i.e when removed from ribosome, ribosomes
structure collapses
b. it serve as a temperate partner for synthesis of ribosomal proteins
2. Messenger RNA (mRNA)
Forms 3-5% of the total RNA in a cell. mRNA carries coded information from DNA to ribosomes
in the cytoplasm
3. tRNA
is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length,[2] that serves
as the physical link between the mRNA and the amino acid sequence of proteins. tRNA does
this by carrying an amino acid to the protein synthetic machinery of a cell (ribosome) as
directed by a three-nucleotide sequence (codon) in a messenger RNA (mRNA). As such, tRNAs
are a necessary component of translation, the biological synthesis of new proteins in
accordance with the genetic code.
37
Comparison of DNA and RNA
Similarities Both:
(1) are polymers of nucleotides
(2) carry genetic information
(3) have same purine bases adenine and guanine plus pyrimidine bases cytosine
(4) originate from the nucleus
Differences
Aspect
Deoxyribonucleic Acid (DNA)
Ribonucleic Acid (RNA)
Function
Stores genetic information for a long time
and transmits it.
Transfers genetic code needed for the
creation of proteins from the nucleus to the
ribosome.
(i) Double-stranded.
(ii) High molecular mass
(iii) Pentose sugar is deoxyribose
(iv) Quantity is fixed in a cell
(i) Single-stranded.
(ii) Low molecular mass
(iii) Pentose sugar is ribose
(iv) Quantity is variable
Structure
Base
Pairing
(v) Organic bases are guanine, adenine, (v) Organic bases are guanine, adenine,
cytosine, and thymine
(vi) Much of DNA is in the nucleus of a cell,
Location
Stability
Propagation
little in mitochondria and chloroplasts.
cytosine, and uracil
(vi) Much of RNA is in the cytoplasm, little in
the nucleus.
(vii) stable
(vii) less stable
(xiii) DNA is self-replicating.
(xiii) RNA is synthesized from DNA when
needed.
38
Unique
Features
(xiv) DNA is protected in the nucleus, as it is
tightly packed.
(xiv) RNA strands are continually made,
broken down and reused.
Types
(xvii) Only two types: intra nuclear and extra (xviii) Three different types: mRNA, tRNA and
nuclear DNA
rRNA
The central dogma of molecular biology It states that DNA makes RNA makes proteins
PROTEIN SYNTHESIS
Protein synthesis is the process by which individual cells construct proteins. Protein synthesis
occurs in separate but interrelated steps as follows:
1. Transcription
Transcription is the process whereby the DNA code of a gene is copied to make messenger RNA
(mRNA).
Importance of transcription
(i)
(ii)
DNA is too large to fit through the nuclear pores, yet mRNA being small can readily exit
the nucleus.
DNA contains many codes that aren‟t always needed at a given time, so m-RNA only
carries that code needed to make specific proteins out of the nucleus to the ribosome.
Stages in transcription
1. A specific region of DNA called a cistron unwinds to expose bases on each strand.
2. Each base on one strand attracts its complementary RNA nucleotide, for example. a dree
guanine base on DNA attracts an RNA nucleotide with cytosine and adenine attracts uracil.
3. The enzyme RNA polymerase moves along the DNA adding one complementary RNA
nucleotide at a time to a newly unwound portion of DNA.
4. The region of base pairing between the DNA and RNA is only around 12 base pairs at a time
as the DNA helix reforms behind the RNA polymerase.
5. The DNA acts as a template against which mRNA is constructed.
39
Translation
This describes how the genetic code is converted into amino acids.
It occurs in three stages
The beginning step is called initiation, a middle step, called elongation, and a final step, called
termination.
Initiation
In initiation, mRNA is attached to tRNA, which is attached to the specified amino acid.
During initiation, the mRNA, the tRNA, and the first amino acid all come together within the
ribosome. The mRNA strand remains continuous, but the true initiation point is the start codon,
AUG that specifies amino acid methionine. So, methionine is first the amino acid that is brought
into the ribosome.
40
Elongation
The next step makes up the bulk of translation. It's called elongation, and it's the addition of
amino acids by the formation of peptide bonds. Elongation is just what it sounds like: a chain of
amino acids grows longer and longer as more amino acids are added on. This will eventually
create the polypeptide.
The mRNA shifts a little through the ribosome so that the next codon exposed to attract the
next tRNA with a matching anticodon. A peptide bond forms between the two amino acids on
the tRNA . one codon is exposed a time to attract activated tRNA with appropriate anticodon
until all the mRNA has moved through the ribosome and each case a peptide bond is formed
between the amino acid on the arriving tRNA and the peptide chain already formed.
Termination occurs when all the codons on mRNA are read. The ribosome reaches one or more
stop codons on mRNA (UAA, UAG, UGA). The ribosome detaches from the mRNA and splits into
its small and large sub-units, while the new protein floats away
Note:
1. Several ribosomes can attach to a molecule of mRNA (to form polysomes/polyribosomes)
one after another so that several proteins of the same type can be made from one mRNA at
the same time.
2. Newly synthesized proteins are packaged and sent to Golgi complex for
modification/processing. This is called post translation processing of the protein.
Comparison between DNA replication and transcription
Similarities Both:
(1) involve unwinding the helix
(2) involve separating the two strands
(3) involve breaking hydrogen bonds between bases
(4) involve complementary base pairing
41
(5) involve C pairing with G
(6) work in a 5` to 3` direction
(7) involve linking/ polymerization of nucleotides
(8) DNA or RNA polymerase require a start signal
Differences
DNA replication
Transcription
Involves DNA nucleotides, where the pentose
sugar is deoxyribose, and the base adenine pairs
with thymine
Involves RNA nucleotides where the pentose
sugar is ribose, and the base adenine pairs with
uracil
Both strands are copied
Only one strand copied not both
Ligase enzyme / no Okazaki fragments are involved
No ligase enzyme / no Okazaki fragments
Has multiple starting points
Has only one starting point
replication gives two DNA molecules
whilst transcription gives mRNA
Compare DNA transcription with translation
Both: (1) Occur in 5‟ to 3‟ direction (2) Require ATP
Differences
(i) DNA is transcribed while mRNA is translated
(ii) Transcription produces RNA while translation produces polypeptides/ protein
(iii) RNA polymerase for transcription while ribosomes for translation/ ribosomes in translation
only
(iv) Transcription occurs in the nucleus (of eukaryotes) while translation occurs in the
cytoplasm/ at ER
(v) tRNA is needed for translation but not transcription
Explain briefly the advantages and disadvantages of the universality of the genetic code to
humans.
(i) Genetic material can be transferred between species/ between humans
(ii) One species could use a useful gene from another species
(iii) Bacteria/ yeasts can be genetically engineered to make a useful product
(1) Viruses can invade cells and take over their genetic
apparatus e.g. HIV (2) Viruses cause disease
GENETIC CODE
The genetic code is the set of rules by which information encoded in genetic material (DNA or
RNA sequences) is translated into proteins (amino acid sequences) by living cells.
42
THE GENETIC CODE CHART / TABLE
MOST IMPORTANT PROPERTIES OF GENETIC CODE
1. The code is a triplet codon
The nucleotides of mRNA are arranged as a linear sequence of codons, each codon
consisting of three successive nitrogenous bases, i.e., the code is a triplet codon.
2. The code is non-overlapping In translating mRNA molecules, the codons do not
overlap but are “read” sequentially.
3. The code is comma less
This means that no codon is reserved for punctuations. After one amino acid is coded, the
second amino acid will be automatically, coded by the next three letters and that no letters
are wasted as the punctuation marks.
43
4. The code is non-ambiguous
A particular codon will always code for the same amino acid. The same codon shall never
code for two different amino acids.
5. The code has polarity
The code is always read in a fixed direction, i.e., in the 5‟→3′ direction.
6. The code is degenerate
More than one codon may specify the same amino acid; For example, except for tryptophan
and methionine, which have a single codon each, all other 18 amino acids have more than
one codon.
Biological advantages of degeneracy
It permits essentially the same complement of enzymes and other proteins to be specified
by microorganisms varying widely in their DNA base composition.
(ii) It provides a mechanism of minimizing mutational lethality. E.g. Substitution of the third
base-U in GUU (for valine) with C/A/G does not change the amino acid coded for.
7. Some codes are start codons
In most organisms, AUG codon is the start or initiation codon, i.e., the polypeptide chain
starts either with methionine (eukaryotes) or N-formylmethionine (prokaryotes).
(i)
8. Some codes are stop codons
Three codons UAG, UAA and UGA are the chain stop or termination codons. They do not
code for any of the amino acids. These codons are also called nonsense codons, since they
do not specify any amino acid.
9. The code is universal
Same genetic code is found valid for all organisms ranging from bacteria to man.
Sample questions
1. If a messenger RNA has a base sequence of CUGACGAGU, which one of the following would
be the possible maximum number of amino acids coded for, if the code is overlapping?
A. 7
C.4
B.6
D.3
2. The two strands of DNA easily separate during replication because of the
44
A. helical nature of the nucleotide
B. the closeness of the base pairs
C. weak hydrogen bonds between the base pairs
D. the week hydrogen bonds between phosphate and sugars.
3. Which one of the following is the mRNA strand that corresponds to the DNA strand
TAGGCT?
A. AUCCGA
B. UUCCGU
C. CGAAUC
D.UAGGCU
4. Which one of the following bas triplet pair with ACG triplet base?
A. TGC
B. AAT
C. GTG
D. ACC
5. Which one of the following is confined within the nucleus?
A. DNA molecules B. Ribosome C. Messenger
RNA
D. Transfer RNA
6. Which of the following are purines?
A. Adenine and cytosine
B. Thymine and Adenine
C. Thymine and cytosine
D Adenine and guanine
7. Which one of the following statement correctly describes the transcription of DNA?
A. It produces amino acids
B. it results in and increased DNA synthesis
C. It produces messenger RNA
D. It occurs at the surface of the ribosome
8. Which of the following is found in both DNA and messenger RNA?
A. double helix structure
B. ribose
C. sugar-phosphate chain
D Thymine
9. Which base sequence below pair with AGU?
A. TGC
B. TCA
C. GCC
D ATT
10. What is the maximum number of triplets of nucleotides that could code for 20 amino acids?
A. 3
B. 6
C. 48 D. 64
45
11. Which one of the following tRNA anti-codons will correspond to the mRNA base triplets
CAT?
A. GUA B.GUC
C. GTC
D. GTA
12. The synthesis of mRNA may be described as a
A. Replication
B. Transcription
C. translocation
D. transduction
13. Which of the following is the correct sequence of combination forming a double helix of
DNA?
A. Phosphate- sugar- guanine-hydrogen bond- cytosine-sugar - phosphate
B. Thymine-sugar-phosphate-hydrogen bond-adenine-sugar-phosphate
C. Sugar- phosphate-cytosine-hydrogen bond-guanine-sugar- phosphate
D. Phosphate-sugar-guanine-hydrogen bond-thymine-sugar-phosphate
14. Hyroxylamine, a mutagen, converts cytosine to a compound which pairs with adenine. If
DNA is treated with hydroxylamine, the resulting mutation is
A. a deletion
B. a substitution
C. an insertion
D. an inversion
15. Name the bases that may be synthesized of the t-RNA from strand (ii) of the DNA indicated
in the diagram below
16. If the bases on t-RNA are ACU, what would be the corresponding bases on original DNA
coding strand during protein synthesis?
A. ACT
B. UGA
C. TGA
D. ATG
17. When DNA replicate, it thought to unwind and „unzip” along
A. bonds between the deoxyribose and phosphate units
B. Phosphate to phosphate bonds
46
C. hydrogen bonds between base pairs
D. ribose to deoxyribose sugars.
18. Which of the following carries the triplet nucleotide code?
A. ribosome RNA (r-RNA)
B. Transfer RNA (t-RNA)
C. Messenger (mRNA)
D. Nuclear (n-RNA)
19. The nucleotide is attached to another in DNA strand by bridge
A. Base to Base
B. Sugar to Base
C. Phosphate to Base
D. phosphate to Sugar
20. Which one of the following show the correct coding sequence during protein synthesis?
A. DNA, mRNA, tRNA, rRNA, amino acids
B. rRNA, tRNA, mRNA, polypeptide
C. DNA, mRNA, tRNA, amino acids
D. DNA, mRNA, rRNA, tRNA, amino acids
21. Transfer RNA of function in
A. Carrying RNA from the ribosome to mRNA
B. Attaching RNA to ribosome
C. Carrying amino acids to the correct site on mRNA
D. Carrying nucleotide to mRNA on the ribosome
22.The genetic code is most directly related to the sequence of
A. Ribose unit
B. Deoxyribose unit
C. Nitrogen bases
D. Phosphates and pentose sugar
23.
Any change in the sequence of bases of a DNA molecule by addition, deletion or
substation always result in a
A. Lethal gene
B. mutation of the gene
C. visible change in offspring
D. variation in the new individual making it more capable of adaptation
24.
In the Watson-Crick model of double helix, the “rugs” of the “twisted ladder” are
composed of
A. Sugar
B. A purine and a pyrimidine
47
C. Two purines
D. Two pyrimidine
25.
The figure below shows a stage in protein synthesis. A mutation in DNA template strand
involving substitution of the original base G on the mRNA codon was transcribed with
match with a complementary anticodon having the following base triplet code
AG U
A. UUA
B. TCU
C. GCA
D. UCA
Analysis of a sample of DNA showed that 33% of the bases were adenine. The percentage
of guanine bases in the sample was
A. 34
B. 33
C. 17
D. 28
27. The figure below represents the structure of
26.
A. Amino acid
B. Glucose
C. Ribose
D. Fatty acid
28.
Some bacteria when infected with microphages, may make a particular amino acid they
could make before. This is due to
A. Transformation
B. Mutation
48
C. Transduction
D. conversion
29.
In a non-dividing cell, the percentage of guanine is 40%. The percentage of adenine in the
cell is.
A. 30
B. 20
C. 40
D. 10
In an outline for protein synthesis,
DNA
mRNA
polypeptide
30.
Stage 1
stage 2
Stage 2 represents
A. Transcription
B. Translation
C. Transduction
D. transformation
If the triplet of mRNA is AAG, what is the complementary triplet of the base on tRNA
molecule?
A. TTC
B. UUC
C. CCT
D. CCU
32. Which of th following molecules is represented in figure below
31.
A. Fattay acid
B. Deoxyribose
C. Glucose
D. ribose
49
33.
Which two of the following strands of nucleotide would pair with strand X in the figure?
A
C
A
G
A
T
A
U
C
G
C
G
A
T
A
U
G
C
C
C
T
A
U
A
T
50
X
(i)
(ii)
(iii)
(iv)
A. (i) and (iii)
B. (ii) and (iv)
C. (i) and (ii)
D. (iii) and (iv)
A biochemical analysis of a DNA sample showed that 34% of the bases were guanine. The
percentage of adenine bases in the sample is
A. 32
B. 16
C. 17
D. 34
35. The process of changing the information on mRNA into formation of polypeptides is known as
A. Transcription
B. Translation
C. Transduction
D. transformation
36. Which one of the following shows he correct coding sequence during protein synthesis?
A. DNA → mRNA → tRNA → rRNA → amino acid
B. rRNA → tRNA → mRNA → polypeptide
C. RNA → mRNA → tRNA → proteins
D. DNA → mRNA → rRNA → tRNA → amino acid
34.
During protein synthesis, the anticodon base of tRNA is AUG. What is th base sequence on the
template DNA strand?
A. UAU
B. ATG
C. AUG
D. TAC
38. Analysis of a sample of DNA showed that 33% of the bases were adenine. The percentage of
guanine bases in the sample was
A. 34
B. 33
C. 17
D. 28
37.
(a) Biochemical analysis of sample of DNA showed that 33% of the nitrogenous base were
guanine. Calculate the percentage of the bases in the sample which would be adenine. Explain
how you arrived at your answer
(b) What name is given to the triplet of bases which designate individual amino acid?
39.
51
(c) If the triplet of mRNA base which designate the amino acid lysine is AAG, what is the
complementary triplet of bases on the tRNA
40.
(a) state where each of the following is found in a cell
DNA ……………………………………………… RNA
……………………………………………..
(b) give three structural differences between DNA and RNA (3marks)
(c) What is the genetic significance of DNA replication? (2marks)
(d) Give two ways that suggest that DNA is hereditary material (4marks)
41.
Describe the process of protein synthesis (20marks)
42.
(a) Describe the biological function of amino acids
(05marks)
(b) Describe how amino acids form a polypetide
(09marks)
(c) How do inhibitors change the rate of enzyme controlled reaction? (06marks)
(a) Describe the structure of DNA (11marks)
(b) Using examples, explain an effect of gene in humans (6marks)
(c) What is the signifince of mutation in crop hasbandary? 06marks)
43.
44.
(a)Compare DNA and RNA (10marks)
(b) Describe the role of mRNA protein synthesis in a cell(5marks)
(c) How does molecular structure of proteins relate to their functions? (05marks)
Suggested answers
1
2
3
4
5
B
D
A
A
A
6
7
8
9
10
D
C
C
D
D
11
12
13
14
15
B
B
A
B
A
16
17
18
19
20
A
C
B
A
D
21
22
23
24
25
C
B
B
B
D
26
27
28
29
30
C
C
C
D
B
31
32
33
34
35
B
D
B
B
B
36
37
D
B
39. (a) Percentage of adenine = 17%
(b) codon
(c) UUC
40. (a)(i) DNA is found in the cell nucleus
RNA is found in both nucleus and cytoplasm
(b)
52
DNA
Double helix
Has thymine
Has no uracil
Has hydrogen bonds between base pairs
High molecular weight
(c)
RNA
Single
Has no thymine
Contains uracil
Had no hydrogen bond
Low molecular weight
a. it allows maintenance of a constant amount of genetic information within organisms of a
population
b. it allows passing over of genetic information from parents to offsprings in constant amounts,
generation after generation
(d)
c. ability to replicate
d. it the major component of chromosomes believed to transmit genetic material
41. Process of protein synthesis
(i) Protein synthesis occurs in the cells of living organism controlled by DNA which reside in the
nucleus
(ii) (ii) In the nucleus, DNA unwinds to form two separate DNA strands of which one acts as a
template for synthesis of mRNA strand in the process called transcription.
(iii) mRNA molecule leaves the nucleus through nuclear pores to the cytoplasm where it attaches
to ribosome which consists of rRNA and proteins on the rough endoplasmic reticulum.
(iv) The ribosome reads the sequence of codons in mRNA, and molecules of tRNA bring amino
acids to the ribosome in the correct sequence.
(v) rRNA helps bonds peptide bonds form between the amino acids starting with methionine
until the entire genetic code on mRNA is read to form a polypeptide.
(vi) The polypeptide molecule then peels off from the ribosome and released into the cytoplasm
(vii) After a polypeptide chain is synthesized, it may undergo additional processing to form the
finished protein
42. Solution:
(a) (i) Amino acids are used to build up proteins, which have functional roles such as enzymes and
hormones, and structure roles such as keratin and collagen.
(ii) The amino and carboxylic group of amino acids act as buffers, resisting changes in pH in the
body fluids.
(iii) Amino acids can be used substrate in cell respiration to provide energy in cases of starvation.
53
(iv) They are also used to synthesize neurotransmitters. For example, tyrosine is used to
synthesize adrenaline and norepinephrine.
(v) Amino acids are regulators of gene expression. They do so by modulating the translation of
mRNA
(b) The order and nature of amino acids used in the assembly of polypeptide chain is dictated by the
sequence of bases in the mRNA molecule, in a process called translation, it occurs in ribosomes.
-
First, the mRNA attaches to the ribosome.
-
The amino acids are then activated and attached to their specific tRNA molecules in a reaction
catalyzed by the enzymes aminoacyl –RNA synthetase.
-
The first mRNA codon then binds the tRNA molecule having the complementary anticodon and
carrying the first amino acid (usually methionine)
-
The second mRNA codon then also binds the second tRNA molecules bearing the
complementary anticodon and carrying a specific amino acid.
-
The ribosome holds the mRNA, tRNA and associated enzymes controlling the process until a
peptide bond forms between adjacent amino acids.
-
Once the new amino acid has added to the growing polypeptide chain, the ribosome moves
one codon along the mRNA.
-
The tRNA previously attached to the polypeptide chain leaves the ribosome and returns to the
RNA pool in the cytoplasm. another tRNA molecule binds to the new codon.
-
This process continues until the ribosome comes to a terminating codon (no sense codon),
signaling „stop‟
-
At this point, the polypeptide chain detaches from the ribosome and is released into the
cytoplasm.
-
The necessary energy for the synthesis of the polypeptide chain comes from ATP.
-
Note: translation is the mechanism by which the sequence of bases in a mRNA molecule is
converted into a sequence of amino acids in a polypeptide chain. It is the second steps in
protein synthesis
-
The first is transcription where the base sequence of a section of DNA representing a gene
converted into the complementary base sequence of mRNA.
54
this occurs in the nucleus and is influenced by the enzymes RNA polymerase.
-
The last step of protein synthesis involves modification in the protein structure.
This may occur in the Golgi apparatus in the cytoplasm.
(c) Inhibitors reduces the rate of enzyme controlled reactions.
They do so two mechanisms competitive inhibition and non-competitive inhibition
Competitive inhibition
-
Substance inhibitors‟ which are structurally similar to the substrate compete with the substrate
for the active site on the enzyme.
-
This result in fewer enzyme-substrate complex formation and therefore the rate of reaction
reduces.
-
This type of inhibition is reversible.
Non- competitive inhibitions.
-
Substance (inhibitors) bearing no structural resemblance to the substrate bind with the
enzymes or it cannot be released from it.
-
This types of inhibitions can be reversible or irreversible.
Note: in competitive inhibition, an increase in substrate concentration increases the rate of the
reaction. This is because the chances of the substrate molecules binding to the active site are then
higher.
However, in non-competitive inhibition, increasing the substrate concentration has no effect on the
rate of reaction since the inhibitors does not bind to the active site. The degree of inhibition only
depends on the concentration of the inhibitor.
43. Solution
(a) In this question you should outline the structural components of DNA and show how they are
linked to form the DNA molecule.
55
•
A DNA molecule is composed of two antiparallel polynucleotide chains as shown above.
•
Each chain consists of repeated units of deoxyribonucleotides formed by a deoxyribose sugar
phosphate backbone with nitrogenous bases as side branches.
•
The two polynucleotide chains are twisted around a common axis to form a right handed
double helix.
•
These two chains are held together by hydrogen bonds between the nitrogenous bases from
either chain. Adenine in one chain is always pairs with thymine of the other by two hydrogen
bonds while cytosine pairs with Guanine by three hydrogen bonds.
•
The arrangement of these nitrogen bases gives DNA its identity.
•
DNA in eukaryotes is wound around histone proteins to form chromosomes.
•
In prokaryotes, it occurs in a circular shape.
(b) In this questions, first show your understanding of the term gene mutation. Then using suitable
examples outline the effect of this process in humans.
A gene mutation is a sudden irreversible change in the structure of a gene. It is also called point
mutation. It results in loss or change of function of the gene in question. Gene mutation is the cause
of some of the disease called genetic disease. Example is sickle cell anemia;
This is caused by a mutation in the gene responsible for synthesis of β – polypeptide chains in
hemoglobin.
56
This results in replacement of glutamine with valine in position 6 of the chains, leading to formation
of hemoglobin S.
The condition has the following effects.
•
Hemoglobin S crystallizes at low oxygen tensions, making the red blood cells fragile and to
adopt a sickle shape. They can hardly pass through capillaries and the spleen sinusoids.
•
The red blood cells become more easily hemolyzed in the liver and spleen. This reduces the
number of red blood cells causing anaemia.
•
Bilirubin, the bi-product of hemoglobin breakdown, accumulates in the body and causes
yellowing of the body surface membranes especially the sclera (jaundice)
Note: other conditions that occur due to gene mutation include;
•
Alkaptonuria
•
Phenylketonuria
•
Albinism
(c). In this question you should point out how the effect of a mutation is of importance in crop
husbandry.
Mutations can induce polyploidy in plants causing hybrid vigor. This leads to;
•
Increased crop yield
•
Increased pest and disease resistance
•
Faster rates of growth.
Increase size of fruits, leaves and flowers.
43. Solution:
(a) Similarities between DNA and RNA
• Both contain purines adenine and guanine and the pyrimidine base cytosine
Both
contain phosphate groups.
• They both have a basic sugar- phosphate backbone in their chains
Both contain
polynucleotide chains.
57
Differences between DNA and RNA
DNA
RNA
•
Double stranded structure.
•
Single stranded structure
•
Found only in the nucleus.
•
Found in
•
The sugar molecule is deoxyribose.
•
Contains a pyrimidine base thymine
•
Giant molecules with very high
molecular weight
•
Has hydrogen bonds in its structure
both
nucleus
and cytoplasm
•
Found both sugar molecular is
ribose
•
Contains a pyrimidine base uracil
•
Short, small with low molecular weight
•
Has no hydrogen bonds in its
structure
(b) mRNA caries genetic information in form of nitrogenous bases from chromosomal DNA in the
nucleus to the ribosomal RNA in the in the cytoplasm. It acts as a template for protein
synthesis. It has complementary DNA base sequence to part of DNA from which it is
transcribed. As a result, mRNA directs the synthesis of proteins as directed by the base
sequence on the segment of DNA from it is copied, a process called translation
(c) According to molecular structure, proteins can be classified as follows in relation to function;
•
Fibrous proteins from a long, tough, fibre insoluble in water. They therefore function as
structural and supporting proteins and include; keratin, collagen, elastin, e.t.c.
•
Globular proteins have a globular shape but lack contractile properties. They contain
polypeptide chains coiled about them. They therefore act as regulatory molecules and as
food storage molecules in plants. Soluble globular proteins are amphipathic and therefore
act as buffers of body fluid pH.
•
Conjugated proteins consist of a simple protein united with some non-protein sub-stance.
Examples include; glycoproteins which form mucin of saliva that helps in softening food;
58
chromo proteins combined with a pigment such as hemoglobin, an oxygen carrying molecule;
metallo-proteins which act as enzymes.
Enzymes
Specific objectives
The learner should be able to:
a.
b.
c.
d.
Describe the criteria for naming enzymes using type of substrate and type of reaction
Explain the characteristics/properties of enzymes
State factors that affect enzyme action (pH, temperature, inhibitor, substrate concentration)
Explain the mechanism of enzyme action using the lock and key mechanism and induced
fit
e. The role of enzyme in the organism’s life.
Practical
f. Demonstrate properties of enzymes action in specific temperature, pH range, substrate
concentrations
g. Identify enzymes in different parts of the gut based on their different actions on different food
substances
The chemical reaction that occur in cells constitute metabolism and the participating molecules are
called metabolites.
Types of metabolism
1. Anabolism [or synthetic reaction]. Here single molecules are linked together by chemicals bonds to
form more complex compounds.
A + B → AB
A and B are substrate molecules or reaction and AB represent the product.
Generally anabolic reactions require [absorb] energy and are therefore endogenic reaction.
These reactions are concerned with building up structures, storage compounds and complex
metabolites in the cell. Starch, glycogen, lipids and proteins are all products of anabolic pathway.
Plants and other autotrophic organism synthesize these complex organs c molecules from simple
inorganic sources such carbon dioxide and water. They have much greater synthetic power, and their
metabolic pathways and therefore more extensive than heterotrophs.
59
2. Catabolism [or breakdown reaction] The complex compounds are splint into simple molecules
AB
→ A+B
In this case AB is the substrate and A and B are products.
Generally catabolic reaction release energy and are therefore exergonic reaction. These reactions are
mainly concerned with mobilizing food stores and making energy available in cells. Energy is require for
3 main purpose.
(a) For synthesis e.g. the synthesis of proteins, storage compounds
(b) For work e.g., contraction of muscles
(c) for maintenance; such as the constant body temperature.
Small steps gentle reactions
Metabolites are not converted into products in single large reactions. Instead they are converted
gradually, step by step, through a series of small reactions which together comprise a metabolic
pathway, each reaction though small, brings the raw materials closer to the end product.
There are five main reaction why metabolism proceeds in small steps.
(i)
(ii)
(iii)
(iv)
(v)
Large catabolic reaction would create unfavorable conditions, such as very high
temperature, that would be incompatible with life.
Energy can be obtained from small catabolic reaction in a usable form.
Substance can be partially broken down so as to provide raw materials for other.
It is not possible to synthesize, in one step complex organic compound from simple small
raw material in the gentle condition prevailing in cell.
Having small steps in an anabolic pathway increase the cell’s ability to control what
product are made.
Living cells have the unique ability to perform numerous individual reaction in dilute aqueous solution at
low temperatures and with a narrow range of pH.
However, the sheer number of reaction require a fantastic degree of organization in the cell. Much of
this organization is achieved by enzyme which catalysis individual reactions.
Energy
All living organisms may be regarded as working machines which require a continuous supply of energy
in order to keep working and so to stay alive.
60
Energy is defined as the ability [ capacity] to do work. The main source of metabolic energy is the cell in
the oxidative break down of glucose [ respiration] given by the following over simplified reaction
C6H12O6 + 6O2 →
6CO2 + 6H2O + Energy
Activation energy
The speed of biological reaction.
It’s affected the following factors
-
The concentration of the substrate molecule. The higher the concentration the higher the rate
and reaction due to high rate of collision.
-
-
-
The temperature of the reaction mixture: Temperature speeds up biological reaction by
supplying the activation energy for the reaction and increase the rate of collision of the
substrate molecules.
Enzymes are biological catalyst i.e. a catalyst is a substance that increase the rate of the
chemical reaction without taking part in a chemical reaction through lowering the activation
energy.
Difference between enzyme and catalysts.
1.
2.
Enzymes
proteins in nature
catalyze specific reactions Such as
hydrolysis of starch
3.
4.
5.
Denatured by heat above 450C
Very sensitive to pH
Initiate reaction
Catalysts
inorganic chemicals e.g. Pt
may catalyze more than one reaction e.g.
platinum catalyze decomposition of H2O2 and
reduction of benzene
Usually are not affected by heat
Not sensitive to pH
Do not initiate reaction
Enzyme classification
Enzymes are placed into six groups according to the general type of reaction which they catalyze Each
enzyme is given a systematic name, accurately describing the reaction it catalysis. However, since many
of these names are very long and complicated, each enzyme as allocated a trivial name of the substrate
acted on by the enzyme (i)
the name of the substrate acted on by the enzyme (ii) the type of
reaction catalyzed.
61
(iii)
the suffix-ase.
The categories of enzyme are;
1. Oxidoreductase; are involved in biological oxidation and reduction reaction. They include
2.
3.
4.
5.
dehydrogenase which catalyze removal of hydrogen atoms from a substrate and oxidase which
formation of water e.g., in respiration.
Hydrolase; catalyze the addition of water to or the removal from a substrate.eg protease.
Transferase; These catalyze transfer of chemical groups or atoms from one substrate to
another. Those that transfer amino groups [NH2] are called transaminase.
Lyase; break chemical bonds other than hydrolysis this creating double bond. They include
carboxylase which remove carboxyl group [ COOH] from intermediates in respiratory pathways.
Isomerase; These enzymes catalyze the transfer of an atom from one part of a molecule to
another.
6. Ligase or synthetase; catalyze joining together of two molecules coupled with the
breakdown of ATP e.g., phosphokinase which catalyze the addition of phosphate group to a
compound.
The properties of enzymes.
1. They catalyze the rate of biological reactions.
2. 2. They are not destroyed by the reaction in which they catalyze.
3. They work in either direction i.e., catalyze both forward and backward reaction.
4. They are inactivated by high temperatures
5. They are sensitive to pH changes
6. They are usually specific to particular reactions
FACTORS AFFECTING THE RATE OF ENZYME REACTION
(a) Enzymes concentration; provided that the substrate concentration is maintained at a high
level, and other conditions such as pH and temperature are kept constant, the rate of reaction
is proportional to enzyme concentration.
A graph showing relationship between enzyme concentration and the rate of enzyme- controlled
reaction
62
Enzyme concentration
Question; How would you design an experiment to show this?
(b) Substrate concentration.
For a given enzyme concentration, the rate of an enzyme reaction increases with increasing substrate
concentration. The theoretical maximum rate [Vmax] is never quite obtained, but there comes a point
when any further increase in substrate concentration produces no signification change in reaction rate.
This is because at high substrate concentration the active sites of the enzyme molecules at any given
moment are virtually saturated with substrate. Thus, any extra substrate has to wait until the enzyme/
substrate complex has dissociate into product and free enzyme before it may itself complex with the
enzyme.
Substrate concentration
(c) Temperature
Up to 400C, the rate of enzyme -controlled reaction increase smoothly with temperature, a ten degree
rise in temperature being accompanied by approximately doubling the of reaction. Above that 400C the
rate begins to fall off and then decline rapidly, ceasing at about 600C, because the enzyme is denatured.
Graph showing the effect of temperature on the activating of enzyme as salivary amylase.
63
37 0
45 0
Temperature
60 0
An experiment to investigate the rate of action of amylase Materials.
- 1% starch solution
- 40 water bath
- White tile
- Test tubes
-_Glass rod
- Iodine solution
Method
1.Prepare 3 test tube each with 2ml of starch solution and 2ml of 1% amylase solution.
Test tube 1; in beaker containing ice
Test tube2; in a water bath at 40 c water bath.
Test tube3; in a beaker of boiling water And
simultaneously start the stop clock.
2.Every after 1 minute withdraw a few drops from every test tube, place them on a white tile and add a
drop of iodine solution.
3.Note the time taken for starch to disappear from each test tube.
NB; The rate of enzyme reaction is inversely proportion to this time.
64
(b) pH
Under conditions of constant temperature, every enzyme function most efficiently over a narrow pH
range e.g. pepsin at pH= 2, Amylase (salivary) at pH 6.8.
Mechanism of enzyme action.
The action of enzymes are specific to a given substrate and this specificity can be explained by two
hypothesis.
1. Lock and key hypothesis
Enzyme are very specific to the substrate they act on because they have particular shape /
configuration into which substrate with complementary shape fit in exactly as the key fit into the
lock, thus the lock (enzyme) and key (substrate) hypothesis.
When an enzyme / substrate complex is formed, the substrate activated into forming the product of
the reaction. Once formed, the product no longer fit into the active site and escape into the
surrounding medium leaving the active site free to receive other substrate molecule.
2.
Enzyme
Substrate
Induced fit hypothesis.
The Substrate fit exactly into
Enzyme active site like a key
in the lock
Enzyme - Substrate
Complex
This hypothesis claim that enzyme and their active site are physically rather more flexible structure
than this described by the lock and key hypothesis, and that the active site of the enzyme is molded
into a precise configuration in presence of a substrate to enable it perform its catalytic functions
more effectively.
Inhibitors of enzymes
Certain substrate inhibit enzyme, thereby slowing down or stopping enzyme – controlled reactions.
These enzyme inhibitors are of specific interest for the following reasons.
a. They give us important information about the shape and properties of active site of enzyme.
b. They can be used to block particular reactions thereby enabling biochemist to reconstruct
metabolic pathways.
65
c. They have important medical and agricultural use as for example drugs and pesticides.
Type of inhibitors
(i)
Competitive reversible inhibitor;
Here a compound is structurally similar to that of the usual substrate associates with the
enzymes active site but it’s unable to react with it. While it remains there, it prevents access of
any molecules of the substrate. This type of inhibition depends on the concentration of the
substrate and that of an inhibitor. At high concentration of the substrate, inhibition is overcome.
This knowledge of competitive inhibitors has been utilized in chemotherapy. Sulphonamides
drugs and antibiotic such as penicillin are competitive inhibitors of essential metabolites for
enzymes in bacteria.
(ii) Noncompetitive reversible inhibitors.
This is a type of inhibition in which the inhibitors attach themselves outside the active site
thereby preventing the enzyme normal catalytic reaction by changing the shape of the enzyme or
allosteric effect. It may be revisable when the inhibitor forms loose attachment to the enzyme
that may be detached when circumstances permit e.g. cyanide or irreversible noncompetitive
inhibition when the inhibitor permanently disorganizes the structure of the enzyme that it may
no longer react with the substrate, e.g., mercury.
Allosteric enzymes and their inhibition
Allosteric enzymes are enzymes which exist in two different form, one active and the other inactive. The
activity of these enzyme is regulated by compounds which are not their substrate and these substances
(allosteric effectors) binds the enzyme at specific sites well away from the active site. They modify
enzyme activity by causing a reversible change in the structure of the enzyme active site. Allosteric
activators speed up enzyme catalyze reaction whereas or allosteric inhibitor slow down the action of
allosteric enzymes.
Enzyme cofactors.
These are non-protein components required by enzymes to function efficiently. Cofactor may vary from
simple inorganic ions to complex organic molecules, and may either remain unchanged to the end of a
reaction or be regenerated by the later processes. The enzyme-cofactor complex is called a halo enzyme
whilst the enzyme portion without its cofactor is called an apoenzyme.
There are three recognized cofactors.
(i) Inorganic ions (activator)
66
These are thought to modify either the enzyme or the substrate such that an enzyme-substrate complex
can be formed, hence increasing the rate of reaction catalyzed by that particular enzyme, salivary
amylase activity is increased in the presence of chloride ions.
(ii) Prosthetic group (for example FAD, Biotin, Haem)
The organic molecule is integrated in such a way that it effectively assists the catalytic function of its
enzyme, as in Flavin adenine dinucleotide [FAD]. This contain riboflavin [vitamin B2] which is the
hydrogen accepting part of FAD. It’s concerned with cell oxidative pathways such as part of the
respiratory chain in respiration.
AH 2
Hydrogen donor
A
Oxidized
substrate
Enzyme - FAD
BH 2
Reduced substrate
Enzy me - FADH 2
B
Hydrogen acceptor
Net effect; 2H are transferred from A to B. one halo enzyme acts as a link between A and B.
(iii) Co enzyme (NAD, NADP, Co enzyme A, ATP]
Nicotinamide adenine dinucleotide (NAD) is derived from the vitamin nicotinic acid and can exist
in both a reduced and an oxidized form. In the oxidized state, it functions in catalysis as a
hydrogen acceptor.
AH 2
Hydrogen donor
e1
A
Oxidized
substrate
BH 2
Reduced substrate
NAD
e2
NADH 2
B
Hydrogen acceptor
Where e1 and e2 are two different dehydrogenase enzyme.
Exercise
67
1. Chloride ions are vital for efficient functioning of salivary amylase because ion
A. Activator
B. Are coenzyme
C. Are co factor
D. For alkaline medium
2. Enzyme that catalyze the removal of water molecules from a substrate are known as
A. Reductase
B. Dehydrase
C. Hydrolase
D. hydrase
3. What name is given to a chemical reaction in which two or more hexose sugars combine to form
larger units
A. condensation
B. hydrolysis
C. dehydrogenation
D. isomerization
4. C6H12O6 + C6H12O6
Glucose
Energy from ATP
(Enzyme) C12H22O11 + H2O
Fructose
What is the above called?
A. Dehydration synthesis
B. Hydrolysis
C. Dehydration process
D. Condensation reaction
5. The activity of an enzyme in a
A. Molecukar weight of the enzyme
B. Protein nature of the enzyme
C. Activation energy of the enzyme
D. Surface configuration of the enzyme
6. The reaction rate of salivary amylase with starch decrease as the concentration of chloride ions.
A. Co enzymes
B. Competitive inhibitor
C. Co factor
D. Allosteric inhibitor
7. Which one of the following characteristic of enzyme distinguishes than from inorganic catalyst
A. Initiation and speed up the rate of reaction
B. Remain the same at the end of the reaction
C. May promote reversible reaction
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D. Exert their effect when present even in smal quanties
8. Use the figure below, the part of reaction of the Krebs cycle to answer 8 and 9
α- keto
glutaric acid
Enzyme 1
Succinic
acid
Enzyme 2
NAD +
Fumaric
acid
NADH 2
Enzyme 3
Maleic acid
Succinic acid acculates when mallonic acid is added to the reaction medium. Which one of the following
statement best describes the role of mallonic acid?
A. Malonic acid is an inhibitor of enzyme 1
B. Malonic acid reacts with α-keto glutaric acidto form succinic acid
C. Malonic acid is an inhibitor of enzyme 2
D. Molonic acid acts as coenzyme of enzyme 1
9. Enzyme 2 is a
A. Dehydrogenase
B. Decarboxylase
C. Dehydrase D. Reductase
10. An enzyme which catalyze the conversion of dipeptide into separate amino acid is example of
A. Dehydrogenase
B. Hydrolase
C. Transferase
D. Oxidase
11. An enzymatic reaction of the type
ATP + hexose
ADP+ hexose-6-phosphate is an example of
A. A hydrolysis
B. An isomerase
C. Transfer
D. A synthesis
12. When a piece of liver is dropped into containing H2O2, there is vigorous reaction. this is due to
enzyme
A. Catalyze
B. Amylase
C. Trypsin
D. Carbonic anhydrase
13. In the lock and key hypothesis for the mechanism of enzyme the mechanism of enzyme action,
how does inhibitor substance stop enzyme action? By
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A. Raising activation energy
B. Distorting substrate molecule
C. Destroying coenzyme
D. Occupying active sites on substrate and enzyme
14. When the extent of inhibition in an enzyme controlled reaction depends entirely on the
concentration of the inhibitor, it indicates that the inhibition is
A. Competitive
B. Reversible
C. Non-competitive
D. Irreversible.
15. Which one of the following is not true about the lock and key theory in enzyme-catalyzed
reaction?
A. A small change in the active site alter the enzyme
B. The substrate and active site are complementary
C. Enzyme catalyzed action go through the enzyme-substrate complex stage
D. A molecule which fits in the active site is a substrate
16. Which one of the following does not have an effect on a non-competitive inhibition?
A. Temperature change
B. pH change
C. enzyme concentration
D. substrate concentration
17. Which one of the following describes the turnover number of an enzyme?
A. Number of molecules affected by the enzyme
B. Number of substrate molecules turned into its product per minute
C. Number of product molecules formed
D. Number of substrate molecules catalyzed per minute
18. Which of the following is true about non-competitive inhibition of enzyme catalyzed reaction?
A. The degree of inhibition decreases with increase in substrate concentration
B. The inhibitor has a similar structure and chemical composition with the substrate
C. The degree of inhibition is independent of the substrate concentration D. The shape of the
enzyme is not affected by the inhibitor
19. When the extent of inhibition of an enzyme-controlled reaction depends entirely on the
concentration of the inhibitor, it indicates that the inhibition is
A. Competitive
B. Reversible
C. Non-competitive
D. irreversible
20. Chloride ion are vital for efficient functioning of salivary amylase because the ions
A. are activator
B. are co-enzyme
C. are co-factors
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D. form alkaline medium
21. The activity of an enzyme in a chemical reaction depends on the
A. Molecular weight of the enzyme
B. Protein nature of the enzyme
C. Activation energy of the enzyme
D. Surface configuration of the enzyme
22. Which one of the following correctly represents the effect of increasing substrate concentration
on the degree of inhibition in a competitive and noncompetitive inhibition reaction?
competitive
noncompetitive
A
Decrease
increased
B
Decrease
No change
C
increased
Decrease
D
No change
23.
A
B
C
XEnzyme 1 Enzyme 2
Enzyme 3
If an excess X controls the metabolic pathway of the reaction, the control mechanism is known as,
A. Multi-enzyme control
B. Excess inhibition
C. End product inhibition
D. Negative feedback
24. An adaptation by plant to obtain nitrogen include all the following except.
A. Mycorrhiza on plant roots
B. Bacteria in root nodules
C. Possession of aerial roots
D. Being insectivorous
25. Which one of the following environmental factors has a direct effect on all organisms? A. Light.
B. Humidity.
C. Temperature.
D. Rainfall.
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26. Figure 4 show the effect of increasing the concentration of a substrate on the rate of an enzyme
controlled reaction in presence of inhibitors A and B, in relation to the control experiment without
an inhibitor.
Enzyme only
( control )
Enzyme + inhibito r
A
Enzyme + inhibitor
B
Substrate
concentration
(a) Describe the effect of each inhibitor on the rate of reaction.
(i)
Inhibitor A
(ii)
Inhibitor B
(b) Explain the difference in the effect of inhibitors A and B on the rate of reaction
27. Explain the following
(i)
Competitive inhibition
(b) In what way do enzymes differ from catalyst?
(c) Briefly describe the lock and key hypothesis of enzyme action
28. The figure below shows the effect of varying substrate concentration on an enzyme catalyzed
reaction, in absence and presence of compound A
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Compound A
absent
Compound A
absent
Substrate concentration (moll - 1)
(a) Explain the relationship between the reaction rate and substrate concentration (i) In
absence of compound A (3marks)
(ii)
In presence of compound A(4marks)
(b) State two factors which would have kept constant in this experiment (1mark)
(c) What would be the effect of increasing the concentration of compound A in this
experiment? (2marks)
29. (a) Explain the difference between coenzyme and activators (b) Giving example explain what is
meant by enzyme inhibitors (c) How do enzyme bring about their effects.
30. (a) Describe the working of an enzyme using the Lock and Key hypothesis
(11mark)
(b) Explain the
(i)
(ii)
Effect of excessive heat on the enzyme action (3marks)
Non-competitive inhibition in an enzyme controlled reaction
(6mark)
31. (a) Classifies enzymes basing on the type of reaction they catalyze. (06marks)
(b) Explain how competitive and noncompetitive inhibitions of enzymes occur. (10marks) (c) What
is the importance of enzyme in enzyme catalyzed reaction?
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Suggested answers
1
A
6
C
11
D
16
D
21
D
2
C
7
A
12
A
17
B
22
B
3
A
8
C
13
D
18
C
23
D
4
D
9
D
14
B
19
A
24
C
5
D
10
B
15
D
20
A
25
C
26. Solution:
(a) Inhibitors A reduced of reaction initially but as substrate concentration increase the rate of
reaction also increase at a lower rate than the control. It attains a lower maximum but at a higher
substrate concentration than the control.
Inhibitors B greatly reduces rate of reaction, however, rate increase gradually with substrate
concentration to a very low maximum at a lowest substrate concentration than all.
(b) Inhibitor A is reversible competitive inhibitors of the enzyme. It competes with the substrate to
bind to the active site of the enzyme, being structurally similar to the substrate.
At low substrate concentration, the inhibitors out compete the substrate for the active site. Fewer
than normal substrate molecules combine with the enzyme and the reaction is slower than the
control.
As substrate concentration increase, the substrate competes more favorable for the active site and the
reaction increase with the substrate concentration up to saturation.
Saturation is slightly lower than control since some enzyme active sites still occupied by inhibitors A
molecules.
Inhibitor B is a non-competitive inhibitor. It binds with the enzyme permanently and prevents the
substrate from binding to the enzyme. This reduces effective enzyme molecules and the reaction proceeds
as it would occur if the enzyme concentration was lower than the control.
27. (a)(i) competitive inhibition
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- Substances structurally similar to enzyme substrate compete with enzyme substrate for active site on
the enzyme molecule where both reversibly bind.
- fewer true substrate bind to the enzyme and so the rate is reduced or inhibited.
- this is a competitive inhibition whose degree depends on the relative amount of the substrate and
inhibitor.
(ii) Noncompetitive inhibitor
-
A substance with the structure different from that of the substrate irreversibly binds on the
enzyme at a point different from the active site stops or reduces the action of the enzyme. The
degree of inhibition depends on the concentration of inhibitor and not the substrate.
(b) Differences between enzyme and catalysts
1.
2.
Enzymes
proteins in nature
catalyze specific reactions Such as
hydrolysis of starch
3.
4.
5.
Denatured by heat above 450C
Very sensitive to pH
Initiate reaction
Catalysts
inorganic chemicals e.g. Pt
may catalyze more than one reaction e.g.
platinum catalyze decomposition of H2O2 and
reduction of benzene
Usually are not affected by heat
Not sensitive to pH
Do not initiate reaction
(c) Lock and key hypothesis
-
The enzyme’s active side and its substrate have complementary structure.
-
The enzyme randomly binds with the substrate to form enzyme-substrate complex like a key
fits in the lock.
-
The enzyme acts on the substrate to form products.
-
The products do not fit in the active site, leave the active site for another substrate.
28. Solution
(a) (i) Reaction rate increase rapidly with substance concentration. Rate then increase gradually
before it becomes constant at higher substrate concentration
Explanation
Initially, the number of enzymes molecules available for reaction is higher than the number of
substrate molecules; any available substance will react to form a product. As the substrate
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concentration increase, the number of enzymes molecules occupied by substrate at a given time
increase, hence more amount of product is formed per unit time.
This continues until all the enzymes molecules are occupied so that a substrate has to wait for
the active site of the enzymes to be freed before it can bind the enzymes, in order to react. the
rate of formation of products therefore becomes constant.
(ii). Relationship
Rate of reaction increase slowly with substrate concentration and reaches a lower maximum at a higher
substrate concentration.
Explanation:
Compound A competes with the substrate for the active sites of the enzyme molecules.
As a result, the number of enzymes molecules available for binding the substrate at a given time is
lower.
As the substrate concentration increase, the substrate (out-competes compound A and the reaction rate
increase, through at a lower rate. He enzymes molecules become saturated at a higher substrate
concentration. However, at this point, the output is lower because some enzyme molecules are occupied
by compound A and cannot react with substrate.
(b)
Temperature of the reaction medium
PH of the reaction medium Others:
The amount of the enzyme.
(c) The rate of reaction would decrease with increase in amount of compound A added until the reaction
stops altogether at higher concentrations of compound A.
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29. (a) Cofactors/activators serve the same purpose as coenzymes, as they regulate, control,
and adjust how fast these chemical reactions would respond and take
effect in our body. The big difference is that coenzymes are organic substances, while
cofactors/activators are inorganic.
(b) Enzyme inhibitors are molecules that interact in some way with the enzyme to
prevent it from working in the normal manner. There are a variety of types of
inhibitors including: nonspecific, irreversible, reversible - competitive and
noncompetitive. Poisons and drugs are examples of enzyme inhibitors.
Competitive reversible inhibitor;
Here a compound is structurally similar to that of the usual substrate associates with the
enzymes active site but it’s unable to react with it. While it remains there, it prevents access of
any molecules of the substrate. This type of inhibition depends on the concentration of the
substrate and that of an inhibitor. At high concentration of the substrate, inhibition is overcome.
(i)
This knowledge of competitive inhibitors has been utilized in chemotherapy. Sulphonamides
drugs and antibiotic such as penicillin are competitive inhibitors of essential metabolites for
enzymes in bacteria.
(ii)
Noncompetitive reversible inhibitors.
This is a type of inhibition in which the inhibitors attach themselves outside the active site
thereby preventing the enzyme normal catalytic reaction by changing the shape of the enzyme or
allosteric effect. It may be revisable when the inhibitor forms loose attachment to the enzyme
that may be detached when circumstances permit e.g. cyanide or irreversible noncompetitive
inhibition when the inhibitor permanently disorganizes the structure of the enzyme that it may
no longer react with the substrate, e.g., mercury.
Chemical or substance added or applied to another substance, to slowdown a reaction or to
prevent an unwanted chemical change. For example, anti-oxidants are added as inhibitors
to food to retard its spoilage from exposure to air (oxygen
77