Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 1
Chemical constituents
Syllabus : Carbohydrates - The chemical structure of glucose. The types of carbohydrates : monosaccharides (hexose
and pentose), disaccharides (sucrose and maltose) and polysaccharides (cellulose, starch and glycogen).
The formation of glycosidic bond. The function of carbohydrate as an energy source: glucose as an
intermediate energy source, starch and glycogen as storage compounds. The function of carbohydrates as
structural materials : cellulose as component of cell wall. The functions of starch and cellulose in relation to
their molecular structures, with a brief reference to a- and ß- linkages.
Lipids - The basic components of triglycerides. The function of lipids as an energy source : triglycerides
as storage compounds. The function of lipids as structural components : phospholipids as components of
membranes. The function of lipids as regulatory substances, with an awareness of cholesterol as a
precursor of steroid hormones (e.g. sex. Hormones ) and vitamin D.
Proteins - Amino acids as the monomers that make up proteins. The chemical structure of amino acid.
Peptide bonds and polypeptide chains. The 3-dimensional conformation of proteins : its ultimate
dependence upon amino acid sequence and its functional significance. The functions of proteins : as
structural components, e.g. in cell membrane and cytoplasm. The roles of proteins as enzymes, hormones
and antibodies.
Nucleotides and nucleic acids - The basic components of nucleotides. Mononucleotides : ATP
(adenosine triphosphate) as an energy carrier. Dinucleotides : NAD (nicotinamide adenine dinucleotide) as
a coenzyme. Polynucleotides : RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).
Inorganic components - the presence of inorganic ions in cells. The biological significance of water in
relation to its properties.
Carbohydrates
Carbohydrates comprise a large group of organic compounds which contain carbon, hydrogen and
oxygen. The ratio of hydrogen atom to oxygen atom is 2 : 1. Their general formula is C x (H 2 O) y , where
x and y are variable numbers.
A. Types of carbohydrates
The carbohydrates can be divided into three groups :
a) Monosaccharides (simple sugars)
- sweet, soluble crystalline molecules of relatively low molecular mass
- have general formula Cn (H 2 O) n
- all are reducing sugars, the most widely occurring are hexoses (6- carbon compound i.e. n =
6) and pentose (5- carbon compound i.e. n = 5)
- the most important monosaccharides are the glucose, fructose and galactose. Although they
all have a formula of C 6 H12 O 6 , they are different compounds because the arrangement of the
atoms in them are different.
(i) Glucose (6- C)
• occur in all living cells, especially in plant juices and in
the blood of animals
• it is the chief end- product of carbohydrate digestion in
gut
• because of its potentially active aldehyde (- CHO)
group, it is called aldose
(ii) Ribose (5- C)
• it is a constituent of ribonucleic acid (RNA) and
deoxyribonucleic acid (DNA)
Fig. 1 Structural formulae of glucose
b) Disaccharides (double sugar)
- two monosaccharides may join together, by glycosidic bond, to form a disaccharide, the
union involves the loss of a single water molecule and is therefore a condensation reaction :
C 6 H12 O 6 + C 6 H12 O 6 - - - - - - - - - - - > C 12 H22 O 11 + H2 O
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 2
- to split the disaccharide into its constituent monosaccharides, water should be added. This
is called hydrolysis .
- they are sweet, soluble and crystalline
- all are reducing sugars except sucrose
(i) Maltose
- glucose + glucose - - - - - - - > maltose
- it polymerizes into starch
- it is hydrolysed back to 2 glucose molecules by the action of the enzyme maltase
(ii) Sucrose
- glucose + fructose - - - - - - - > sucrose
- it also calls cane- sugar, a non-reducing sugar
- it can be hydrolysed by diluted acids, or by the enzyme invertase producing equi- molecular
proportion of glucose and fructose
Fig. 2 Formation of maltose and sucrose
c) Polysaccharides (complex sugar)
- they are compounds of high molecular weight formed by the condensation of large number of
monosaccharides units
- because of the large molecular size, they are insoluble that make them suitable for storage
as they exert no osmotic influence and do not easily diffuse out of the cell
- upon hydrolysis, they can be converted to their constituent monosaccharides
- they are mainly used as food and energy stores (e.g. starch in plants and glycogen in animals)
and as structural materials (e.g. cellulose in plant cell wall)
(i) Starch
- polymers of α - glucose, in which glucose are joined by 1- 4 or 1- 6 glycosidic bond
- it is the major food reserve stored in plants, but absent in animals (where the equivalent
is glycogen), formed from excess glucose produced during photosynthesis
- starch has two components : amylose and amylopectin
amylose : has a straight chain structure consisting of several thousand glucose residues
(1,4 glycosidic linkage), the chain then coils helically into a more compact
shape. A suspension of it in water gives a blue- black colour with iodine
solution.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 3
Amylopectin : has many branches, formed by 1,6 glycosidic bonds, it has up to twice as
many glucose residues as amylose. When tested by iodine solution, red
violet colour observed.
(a)
(b)
Fig. 3 Simplified structure of (a) amylose and (b) amylopectin , each hexagon represents an α–glucose ring
(ii) Glycogen
- the major polysaccharide storage material in animals and fungi and is often called ‘animal
starch’
- stored mainly in liver and muscles
- like starch, it is made up of α - glucose molecules and exists as granules in cytoplasm
- similar to amylopectin in structure but has shorter chains (10- 20 glucose units) and highly
branched
(iii) Cellulose
- comprise up to 50% of a plant cell wall, and in cotton it makes up 90%
- polymer of around 10000 β - glucose units forming a long unbranched chain
- many chains run parallel to each other and have cross linkages between them, this makes
it chemically stable.
- the stability makes it a valuable structural material and also difficult to digest
- it is insoluble in water but dissolves in cold concentrated sulphuric acids; if this solution
is diluted with water and then boiled, completed hydrolysis into glucose units take place
Comparison of cellulose, starch and glycogen :
Monomer residue
Linkage
Cellulose
β - glucose
1- 4
Starch
α - glucose
1- 4, 1- 6
Branch
unbranched
branched
Major functions
Iodine test
structural
material
brownish
yellow
Glycogen
α - glucose
1- 4, 1- 6
highly
branched
storage
storage
dark- blue
red
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 4
Fig. 4 simplified structure of glycogen.
Each circle represents an α–glucose
ring
Fig. 5 Structure of the cellulose
molecule
B. Functions of carbohydrates
a) As respiratory substrate : glucose is the chief respiratory substrate in most organisms.
b) As storage material : starch is the storage form of carbohydrate in green plants while the
glycogen is the form in animals and non- green plants (fungi and bacteria); some plants such
as the sugar cane store sucrose.
c) As structural material : cellulose is the chief component forming the cell wall of the plant cells
and thus giving strength to these cells. Chitin forms the exoskeleton of the insects
C. Tests for carbohydrates
a) Reducing sugars (monosaccharides and disaccharides except sucrose)
(i) Benedict’s test
1. add an equal volume of Benedict’s solution to the test samples,
2. mix and then boil in a water bath,
Positive result >> the blue Benedict’s solution will change to brick red precipitate.
[Note] in quantitative test, complete reaction between the reducing sugar and Benedict’s solution is
needed, to do this, excess volume of Benedict’s solution should be added; the amount of red
precipitate formed is directly proportional to the amount of reducing sugar present in the solution.
(ii) Clinistix paper test
1. Dip the ‘pink end’ of the test paper to the test sample,
2. Observe the colour change of the ‘pink end’,
Positive result >> the ‘pink end’ will change to purple- blue.
[Note] this can only be used in qualitative test only. But since it is simple to use, so it is often used in
Clinical test for diabetes.
b) Non- reducing sugars (sucrose)
(i) Acid hydrolysis
1. divide the test sample into two portions,
2. add a few drops of HCl to one portion and boil for a few minutes,
3. neutralize this acidic solution with NaOH,
4. carry out the Benedict’s test for both portions,
Positive result >> red precipitate resulted in the acid treated portion and no change in
another if the test sample contains only sucrose.
Explanation >> the HCl hydrolyses the complex sugar into simple sugar, the simple sugar
with its reducing property gives the red precipitate in Benedict’s test .
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 5
(ii) Enzyme action i.e. invertase, sucrase
1. divide the test sample into two portions,
2. add few drops of invertase in one portion, and few drops of water into the other portion,
3. shake well and keep them in water bath at 37 0 C for 10 minutes,
4. carry out Benedict’s test for them
Positive result >> red precipitate is formed in the enzyme treated portion and no change in
another if the test sample contains only sucrose.
Explanation >> The enzyme hydrolysed the sucrose into simple sugars.
[Note] If the test sample contain both reducing and non-reducing sugar, then after Benedict’s test, both
portion will show positive result, however, the portion treated with acid or enzyme will have more
brick-red precipitates than the other.
c) Starch
Iodine test
Add several drops of iodine solution to the test sample
Positive result >> the blue- black colour of the mixture resulted immediately.
Lipids :
Lipids are a large and varied group of organic compounds. Like carbohydrates, they contain
carbon, hydrogen and oxygen, although the proportion of oxygen is much smaller in lipids. They are
insoluble in water but dissolve readily in organic solvents such as acetone, alcohol and ethers. There
are two types of lipids : fats (semi- solid at room temperature) and oils (liquid at room temperature).
Strictly speaking, true lipids are esters of fatty acids and alcohol.
A. Types of lipids
a) Triglycerides
- formed by condensation of three fatty acids with one glycerol
- fatty acids have a general formula RCOOH where R is hydrogen (- H) or an alkyl group
(- CH, - C 2 H5 and so on, usually up to 16 - 18 C and this carbon chain may possess double
bonds that gives unsaturated fatty acids e.g. palmitic acid, or have no double bonds that
forms saturated fatty acids e.g. oleic acid),
- glycerol has three hydroxyl ( - OH) groups and each may esterize with a separate fatty acid
by the condensation reaction
- hydrolysis of the triglyceride will yield a glycerol and three fatty acids
Fig.6 Formation of a triglyceride
b) Phospholipids
- one of the fatty acid groups is replaced by phosphoric acid (H 3 PO 4 )
- the molecule consists of a phosphate head and with two hydrocarbon tails from the two fatty
acids
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 6
-
the phosphate head carries an elecrical charge and is therefore soluble in water
(hydrophilic), however, the tails are still insoluble in water (hydrophobic)
so that one end of it attracting water while the other end repels
it is the main component of plasma membrane in cell
Fig. 7 Structure of a phospholipid
B. Functions of lipids
a) Food reserve : the fats (usually triglycerides) are important food reserve materials, one gram
of it yields 38 KJ of energy (where 1 gm carbohydrate or protein yields
only 17 KJ energy), thus they are a more economic form for storage and
it is compact and insoluble so it will not enter into metabolism.
b) Structural materials : phospholipids are important compounds in the cell membranes,
c) Insulation : fats conduct heat only slowly and so are useful heat insulators, the fat deposited
beneath the skin, the subcutaneous fat, forms an insulating layer to reduce heat
loss from the body; the myelin sheath around the nerve fibres has a lipid content
to act as an insulating materials.
d) Water- proof layer : all terrestrial plants and animals need to conserve water, in plant leaves
and exoskeleton of insects, waxy cuticle is used; in mammals skin,
sebaceous glands secrete oil to keep skin or fur water- proof.
e) Transport medium : lipid can serve as an organic solvent to transport the oil- soluble substances
e.g. vitamin A and D
f) Metabolic regulators : cholesterol, a type of lipid, is a precursor of sex hormones and vitamin
D.
g) Energy source : when carbohydrate storage is used up, fats are often used.
h) Protection : fat surrounding the internal organs e.g. kidney, act as shock absorbance.
i) Others : plant scents are fatty acids or their derivatives and so aid to attraction of insects for
pollination; bees use wax in constructing their honeycombs.
C.
Tests for lipids
(i) Grease spot test
1. add a drop of test sample on the filter paper,
2. hold the paper towards light and examine it,
3. allow the paper to dry and examine it again,
4. then dip the paper in ether for 2- 3 minutes,
5. after drying the paper, examine it again.
Positive result >> a translucent spot appear in first and second examination, then disappear
after dipping in ether
(ii) Sudan III test
1. place 1 cm3 of test sample in a test tube with 5 cm3 of water,
2. add a few drops of Sudan III and shake gently,
3. leave the tube stand for 3 minutes,
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 7
Positive result >> the oil layer will settles out on the top of water and is red colour, while the
water beneath remains clear
Proteins :
Proteins are organic compounds of large molecular mass and are polymers of amino acids. They are
not truly soluble in water, but form colloidal suspensions. In addition to carbon, hydrogen and
oxygen, they always contain nitrogen, sometimes sulphur and phosphorus
A. Basic units of protein
- amino acids are the basic units of protein
- there are 20 amino acids that make up all proteins in man
- some amino acids can be made by mammals, those are not essential part of the diet and are
called non- essential amino acids, however, the amino acids which can only be obtained
from diet and cannot be synthesised by us are called essential amino acids
- they always contain a basic group- - - the amino group ( - NH2 ), an acid group- - - the carboxyl
group (- COOH) and a side chain
- as one amino acid possess both basic and acid group, so they are said to be amphoteric i.e.
have both acidic and basic properties
- those amino acids have equal number of both groups are therefore neutral, otherwise they
are considered as acidic (more amino group than carboxyl group) or basic ( more carboxyl
group than amino group) amino acids
Fig. 8 Basic structure of amino acid and some common amino acids
-
they are soluble in water and form ions by losing a hydrogen atom from carboxyl group
(making it negatively charged), then the hydrogen atom associated with amino group
(making it positively charged), the ions so formed are therefore dipolar (having both
positive and negative poles), such ions called zwitterions
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 8
Fig. 9 Formation of zwitterion
B. Formation of polypeptides
- a condensation reaction occurs between the amino group of one amino acid and the
carboxyl group of another to form a dipeptide, they are joined by a peptide bond
- further combinations of the dipeptide gives polypeptides
Fig.10 Formation of a dipeptide
C.
Structure of proteins
- one end of the polypeptide chain where amino group is exposed is called N- terminal while
the other with carboxyl group exposed is C- terminal
- the chains of amino acids which make up a polypeptide have a specific three- dimensional
shape, this is due to four types of bonding which occur between various amino acids in the
chain
1. Disulphide bond : They are bonds between two sulphur- containing amino acids
residues. They are quite strong and remain unchanged when warming up to 80 0 C.
2. Ionic bond : Such bonds are in close range of two charged groups of polypeptide chains.
They are weak and easily affected by heat, organic solvent and pH, etc..
3. Hydrogen bond : This occur between certain hydrogen atoms and certain oxygen atoms
within the polypeptide chain. The hydrogen atoms have a small positive charge on them
(electropositive) and the oxygen atoms a small negative charge (electronegative). The
two charged atoms are attracted together and form a hydrogen bond. They are also
weak bonds.
Fig.11 Types of bond in a polypeptide chain
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 9
(i) Primary structure
- is the sequence of amino acids found in the polypeptide chains, it determines the
properties and shape of the protein
Fig. 12 Sequence of amino acid forming the primary structure of protein
(ii) Secondary structure
- is the shape which the polypeptide chain forms as a result of hydrogen bonding
- it is often a spiral known as the α - helix
- the chain may sometimes be linear but more often it coils to form different shapes
Fig. 13 Secondary structure of protein, the polypeptide chain coiled to form α-helix.
(iii) Tertiary structure
- it is formed due to the bending and twisting of the polypeptide helix into a compact
structure, usually globular shape
- all three types of bondings involved in the maintenance of this structure
- it is responsible for the biological properties of the protein which depends on a specific
surface for its reactions, examples of these reactions are found in their action as
catalysts i.e. enzymes and antibodies
(iv) Quaternary structure
- arise from the combination of a number of different polypeptide chains, and associated
non- protein groups, into a large complex protein complex, e.g. insulin and
haemoglobin
Fig. 14 The tertiary structure of protein
Fig. 15 Quaternary structure of protein
Exercise : (98 II 3c)
In general, what additional processes are necessary for the formation of the three- dimensional
structure of proteins after polypeptide synthesis ?
[2 marks]
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 10
D. Denaturation and renaturation of proteins
- denaturation is the loss of the specific three dimensional conformation of a protein
molecule, but the amino acids sequence of the protein remains unaffected
- this lead to the loss of biological activities, i.e. inactivation of enzymes of the protein
molecules
- factors causing denaturation :
(i) Heat or radiation : heat energy, infra- red, ultra- violet cause the atoms vibrate violently
since they supply a lot of kinetic energy to the protein. So disrupting the weak
hydrogen and ionic bonds. Coagulation of the protein then occurs.
(ii) Extreme pH : strong acids or alkalis break the ionic bonds and the protein is
coagulated. For a long period of treatment, breakage of peptide bonds may occur and
the protein is degraded into amino acid residues.
(iii) Inorganic chemicals : the ions of heavy metals such as mercury and silver are highly
electropositive. They combine with COO - groups and disrupt ionic bonds. Similarity,
highly electronegative ions, e.g. cyanide (CN- ), combine with NH3 + groups and
disrupt ionic bonds. On the other hand, this also reduce the protein’s electrical
polarity and thus increase its insolubility that causes the protein to precipitate out of
solution.
(iv) Organic chemicals : organic solvents alter hydrogen bonding within a protein, e.g.
alcohol is used as a disinfectant because it denatures certain bacterial proteins.
(v) Mechanical force: physical movement may break hydrogen bonds. For example, on
stretching a hair, the hydrogen bonds in the keratin helix are broken. The helix is
extended and the hair stretches. If released, the hair returns to its normal length. If,
however, it is wetted and then dried under tension, it maintains its new length - - - the
basis of hair styling.
(vi) Strong hydrogen bond former i.e. NaF : the strong H- bond former has a higher ability
to form hydrogen bond with O or N atom. Then it leads the disruption of the
pre- existing H- bonds in the protein and denaturation occurs.
- sometimes the protein may resume its natural configuration and regain its normal activity,
providing conditions are suitable. This is called renaturation.
E. Functions of proteins
a) Structural materials : proteins are the most important component of the protoplasm and are thus
necessary for growth and repair, examples are
Collagen - Component of connective tissues, tendons
Keratin - Skin, nails, hair, feathers and horn
Elastin - Elastic connective tissues and ligaments
Mucoproteins - Synovial fluid, mucous secretions
Actin and myosin - Contractile fibres of muscle
b) Biocatalyst : all enzymes are made up of protein, examples are
Trypsin - hydrolysis protein
Amylase - hydrolysis starch
c) Metabolic regulators : some hormones are protein in nature, examples are
Insulin - regulate blood glucose level
Glucagon - reverse the action of insulin
d) Transportation :
Haemoglobin - transport oxygen in vertebrate blood
Myoglobin - transport oxygen in muscles
Serum albumin - transport substances in blood, eg. lipids,
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 11
e) Protection : some protein molecules can kill bacteria and some may just function to prevent
invasion of bacteria, examples are
Antibodies - form complexes with foreign proteins
Thrombin - involved in blood clotting
f) Muscle contraction : myosin and actin are filaments in myofibrils of sacromere, both of them
play an important role in muscle contraction
g) Storage : some animal products, such as egg white and milk, are rich in protein
h) Toxins : some organisms will secrete toxic substances, that are protein in nature, to attack
enemies, for example, snake venom.
i) Respiratory substrate : dietary protein can be used for building and repairing , all excess amino
acids will be deaminated in liver, the amino group will be converted
to urea while the carboxyl group can be used as a source of energy;
however, for the protein that has already been incorporated into
tissue, tissue protein, are
normally not used as energy source unless after prolonged starvation.
F. Test for proteins
(i) Biuret test :
1. add excess diluted NaOH solution to dissolve the protein completely,
2. add diluted copper sulphate drop by drop,
3. shaking all the time until sharp colour develops, no heating is involved,
Positive result >> the solution turns violet.
(ii) Albustix paper test
1. Dip the ‘yellow end’ of the test paper to the test sample,
2. Observe the colour change of the ‘yellow end’,
Positive result >> the ‘yellow end’ will change to green.
Exercise : (94 II 2d)
Explain why protein is less efficient for energy production when compared with
lipid and carbohydrate.
[2 marks]
Nucleotides
Each nucleotide consists of three parts :
i) Phosphoric acid (phosphate H3 PO 4 ). This has the same structure in all nucleotides.
ii) Pentose sugar (5- carbon sugar). Two types occur, ribose and deoxyribose.
iii) Organic base. There are five different bases which are divided into two groups :
• Pyrimidines - examples found in nucleic acids are cytosine, thymine and uracil.
• Purines - two examples found in nucleic acids are adenine and guanine.
The three components are combined by condensation reactions to give a nucleotide.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 12
Fig. 16 A nucleotide
A. Types of nucleotides
1. Mononucleotides :
- consists only one nucleotide unit
- example : adenosine triphosphate (ATP)
• it is the short- term energy store of all cells
• it is formed from the nucleotide adenosine monophosphate by the addition of two further
phosphate molecules
• its function depends on its formation and breaking of the terminal phosphate bond
- when the bond breaks, a large amount of energy is released
• when forming the bond, it required energy of 30.6 kJ per mole and this energy is derived
from the oxidation of glucose
• the terminal phosphate bond, on account of the high energy it contains, is known as
energy rich bond or high energy bond.
• if a rapid supply of energy is needed, the ADP (adenosine diphosphate)
• can be broken down to form AMP (adenosine monophosphate), with the release of
energy of over 30 kJ per mole
ATP → ADP + P + 30.6 KJ
ADP → AMP + P + 30.6 KJ
2. Dinucleotides :
- they are formed by the condensation reaction between the sugar and phosphate groups of
two nucleotides
- example : Nicotinamide adenine dinucleotide (NAD)
• it is an electron (hydrogen) carrier important in respiration, act as coenzyme, in
transferring hydrogen atoms from the Kreb’s cycle along the respiratory chain
3. Polynucleotides :
- repeated condensation of nucleotides form a long polynucleotide chain which consists of
backbone that composed of alternation sugar and phosphate groups with the bases
projecting inwards from the sugars
- examples are :
i) Ribonucleic acid (RNA)
• it is a single- stranded polymer of nucleotide where the pentose sugar is always
ribose and the organic bases are adenine, guanine, cytosine and uracil but never
thymine
• there are three types of RNA found in cells :
Ribosomal RNA (rRNA), Transfer RNA (tRNA) and Messenger RNA (mRNA)
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 13
ii) Deoxyribonucleic acid (DNA) :
• it is a double- strand polymer of nucleotides where the pentose sugar is deoxyribose and
the organic bases are adenine, guanine, cytosine and thymine, but never uracil
• the two parallel polynucleotide chains running in opposite direction (anti- parallel) and
are twisted to form a double helix. (Watson and Crick Model)
- difference between RNA and DNA
Secondary structure
Base
Sugar
Size
DNA
double helix stranded
A, T, C and G
pentose is deoxyribose
usually long and large
Major functions
carry genetic information
Stability
alkali-stable
found almost entirely in the
nucleus
Occurrence
RNA
single stranded
A, U, C and G
pentose is ribose
usually short and small
used for protein synthesis and deals
with carrying of genetic information
from the DNA
unstable
manuf actured in the nucleus but found
throughout the cell
Exercise :
(96 II 4)
Describe the functions of lipids, proteins and nucleotides in living organisms. Illustrate your answer
with examples and provide explanations where appropriate.
[20 marks]
(99 II 5)
Compare and contrast the molecular structures of proteins and nucleic acids. Discuss the roles played
by each of these molecules in living organisms.
[20 marks]
Inorganic components :
A. Mineral salts and ions :Mineral salts dissociated into anions (e.g. Cl- ) and cations (e.g. Na+ ) are important in maintaining
osmotic potential and the acid- base equilibrium of the cell.The table below summarises the
functions of some essential minerals in human :
Minerals / ions
2+
Calcium (Ca )
Chlorine (Cl- )
Phosphate (PO 4 3 + )
-
Fluorine (F )
-
Iodine(I )
Iron (Fe 2 + )
Function
Constituent of bones and teeth
Needed in blood clotting and muscle contraction.
Deficiency disease : Rickets
Maintenance of anion/ cation balance.
Formation of hydrochloric acid.
Constituent of nucleic acids, ATP, phospholipids, bones and teeth.
Improves resistance to tooth decay.
Component of the growth hormone, thyroxine.
Deficiency disease : Simple Goiter.
Constituent of many enzymes, electron carriers, haemoglobin and
myoglobin.
Deficiency disease : Anaemia
B. Water :The living cells of all organisms contain a high proportion (65- 95%) of water and it is in aqueous
solution that metabolic reactions take place.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 14
1. Physical properties of water :
(i) Thermal properties of water :
- water has high specific heat i.e. the high capacity of absorbing heat causes it to have small
temperature fluctuation
- the high heat conductivity of water enables even distribution of heat throughout the body
quickly
- the high latent heat of evaporation of water helps to lower the body temperature efficiently
(ii) Hydrogen bond of water
- water molecule is a dipolar molecule, there exists electrostatic attraction
- the positive charged portion will be attracted by the negatively charged part of another
molecule, thus H- bonds form
- this allows the solid state of water, ice, float on the liquid state of water, so protect the
aquatic lives in the two poles.
(iii) As a solvent
- water is the best solvent to dissolve the organic or inorganic matters
(iv) High tensile strength and high viscosity
- the ‘dragging effect’ that moves the water along transport system (xylem vessel) swiftly
and smoothly is due to the cohesive and adhesive characteristics of water
(v) High surface tension
- water rises by capillary action, facilitate absorption of water in dicot. root.
- some insects can move on the surface of water, e.g. Water skier.
2. Significance of water to life
(i) Component of protoplasm
- water comprises 65- 95% of protoplasm
(ii) Universal solvent
- many solutes are soluble in water
- it acts as a medium for many chemical reactions , esp. enzymatic reactions
(iii) Participating in metabolic processed
- water takes part in many chemical reactions such as hydrolysis, photosynthesis, respiration
and condensation
(iv) Cell turgidity
- the turgidity of the sell can be maintained by water since water is relatively incompressible
(v) Surface film
- it covers the whole organism, i.e. facilitates communication between cells
- this may facilitate transportation of substances
(vi) Temperature stability
- water has a high latent heat which helps the maintenance of body temperature
(vii) Translocation medium
- water soluble substances can be transported easily within the body
Exercise : (99 II 2a)
State FOUR properties of water which make it essential for life and explain why they are essential.
[6 marks]