S.6 Bio notes 09-10 Cytology I -Chem of Life
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CYTOLOGY I: THE CHEMICALS OF LIFE
Cells, tissues and organs are composed of chemicals. The study of chemical compounds found in
living systems, and the reactions in which they take part, is known as bio_________. Studies on the
structure and behaviour of individual molecules constitute mole________ biology.
Chemical compounds are conventionally divided into two groups: organic and inorganic. In 1828
the organic compound u____ was synthesized in the laboratory. Until then it was believed that organic
compounds could only be formed in living organisms.
The principal organic compounds found in
organisms are carbohydrates, fats, proteins
and nucleic acids.
Of the inorganic
constituents, minerals and water are among
the most important.
We shall see that the fun____ of a particular
compound in the cell is often related to its
str________.
1)
Substance
Water
Fat
Protein
Carbohydrate
Other organic sub.
Inorganic sub.
Percentage of body
mass
Woman
Man
57
64
23
15
16
17
2
2
I
I
I
I
INORGANIC COMPONENTS
a) Water and its Biological Significance
Water is by far the most ab______ component of organisms. Most human cells are approximately
80% water. Life probably orig______ in water. Water provides the m_______ in which all
biochemical reactions take place, and it has played a key role in influencing the evol______ of living
things on land and in aquatic env__________.
The importance of water as a medium for life springs from five of its properties: its sol_____
properties, h_____ capacity, s______ tension, fr______ properties and its activities as an rea____.
Solvent properties
Water is an excellent solvent for p_____ substances. These include i_____ substances like salts, and
some non-ionic substances like sugars and simple alcohols which contain ch_____ (polar) groups.
Once a substance is in solution its molecules or ions can move about fr_____, thus making it more
chemically reactive than if it were solid. Thus the majority of the cell's ch_______ reactions take
place in aqueous solutions.
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Non-polar substances, such as lipids, are immiscible with water and can serve to separate aqueous
solutions into comp_______ (with membrane structures), thus allowing meta____ reactions to take
place without affecting one another.
Water's solvent properties also mean that it acts as a tr_______ medium, as in the blood, lymphatic
and excretory systems, the alimentary canal and in xylem and phloem.
High heat capacity
Water has a high heat c_________. This is because much of the energy is used in breaking the
h________ bonds. Temperature changes within water are min______. Biochemical processes proceed
at more con____ rates and are less likely to be inhibited by extremes of temperature. Water also
provides a very constant external env________ for many cells and aqua___ organisms.
High heat of vaporisation
As a result of the hydrogen bonding, water has an unusually high b______ point. There is also a
c_______ effect when water molecules vaporise. This is made use of in s________ and panting of
mammals, the opening of the mouth of some reptiles in sunshine, and may be important in cooling
tran_______ leaves. The high heat of vaporisation means that a large amount of heat can be lost
with minimal l___ of water from the body.
High heat of fusion
Liquid water must lose a relatively large amount of heat energy to freeze. Contents of cells and their
environments are therefore less likely to fr______. Ice cry____ are particularly damaging if they
develop inside cells. WHY?
Density and freezing properties
The density of water decreases below 4°C and ice therefore tends to fl____ It is the only substance
whose solid form is less dense than its liquid form. Since ice floats, it forms at the s______ first and
the bottom last. Ice ins_____ the water below it, thus increasing the chances of survival of organisms
in the water. This is important in cold climates and cold seasons, and must have been particularly so in
the past, such as during I___ Ages.
The fact that water below 4°C tends to rise also helps to maintain cir__________ in large bodies of
water. This may result in nu______ cycling and colonisation of living things to water to greater
depths.
High surface tension and cohesion
Co_______ is the force whereby individual molecules stick together. Water has a higher surface
tension than any other liquid. The high cohesion of water molecules is important in mo______ of
water through xylem. Many small organisms rely on surface tension to settle on water or to skate over
its surface.
Water as a reagent
Water is biologically significant as an essential meta_____ ; it participates in the chemical reactions
of metabolism. In particular, it is used as a source of hydrogen in photo_______ and is used in
hydroly____ reactions.
Water and evolutionary change
Its shortage appears to have been a major sel______ pressure in the development of species. All
terrestrial organisms are adapted to obtain and cons____ water, and the extreme ad_________ of
xerophytes, desert animals and so on, provide some fascinating examples of biological design.
Give some of the biologically important functions of water :
All organisms
S.6 Bio notes 09-10 Cytology I -Chem of Life
P.3
Plants
Animals
b) Minerals and their General Functions
Q. What are the possible benefits of isotonic drinks?
http://www.planet-science.com/text_only/outthere/foodtech/isotonic.html
 As constituents of large organic molecules. Proteins contain n_______ and often sul____ as well.
Enzymes, which are nearly always proteins, often contain m_____ ions such as copper, iron or
zinc which function as act_____. Phospholipids, as their name suggests, contain p__________.
Nucleic acids contain phosphorus and nitrogen.
 As constituents of smaller molecules. Adenosine triphosphate (ATP) contains phosphorus.
Phosphorus is also often required for the activation of small organic molecules. For instance,
glucose is phosphorylated (has phos_____ added to it) before it is broken down in respiration. The
hormone thyroxine contains i_______.
 As constituents of certain pigments. The two best-known biological pigments are haemoglobin
and chlorophyll, which contain i____ and mag_______ respectively. I_____ is also found in the
cytochromes, a group of pigments of great importance in energy transfer.
 As constituents of structures. Cal____ and ph_________ are found in bones. Calcium is found in
plant c___ walls.
 As determinants of the anion-cation balance in cells. Sodium, potassium and chloride ions are
particularly important in this regard, especially in nerves, muscles and sensory cells where they
are involved in the transmission of im______.

2)
As determinants of water potential. Mineral salts, together with other solutes, determine the water
p_________.
ORGANIC COMPONENTS AND MACROMOLECULES
A macromolecule is a g______ molecule made from many repeating units; it is therefore a poly___.
There are three types of macromolecule, namely polysaccharides, proteins and nucleic acids.
Macromolecules account for over 90% of the dry mass of cells.
Nucleic acids and proteins can be regarded as 'in______________' molecules. This means that the
seq________ of subunits is important in proteins and nucleic acids. Lipids, although generally much
smaller molecules and are not polymers (average Mr 750-2,500), are included since they generally
associate with each other into much larger groups of molecules.
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A) Carbohydrates (saccharides)
Carbohydrates are substances with the general formula Cx (H2O)y. All carbohydrates are ald______
or ket_____ and all contain several hy_______ (-OH) groups. Aldehydes are very easily oxidised
and hence are powerful r_________ agents. Carbohydrates are divided into mono-, di- and
polysaccharides.
a) Monosaccharides
Monosaccharides are single sugar units. They are classified according to the number of carbon atoms
as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). Monosaccharides are
important as e______ sources and as b_________ blocks for the synthesis of larger molecules.
Aldoses and ketoses (Reference)
In monosaccharides, all the carbon atoms except one have a hydroxyl group attached. The remaining carbon
atom is either part of an aldehyde group, in which case the monosaccharide is called an aldose or aldo sugar,
or is part of a keto group, when it is called a ketose or keto sugar. The two simplest monosaccharides are the
trioses glyceraldehyde and dihydroxyacetone.
Chief functions of Monosaccharides
Monosaccharide
Trioses C3H6O3
Example
glyceraldehyde
Functions
Intermediates in resp_________, and photo____________
Pentoses
C5H10O5
ribose, ribulose
Synthesis of nu_____ acids; ribose is a constituent of RNA,
deoxyribose of DNA, Synthesis of some coenzymes, e.g. NAD,
NADP, coenzyme A, FAD, Synthesis of AMP, ADP, and ATP
Ribulose bisphosphate is the CO2 acceptor in photosynthesis
Hexoses C6H12O6
glucose, fructose,
galactose
Source of energy when oxidised in respiration; synthesis of
disaccharides and polysaccharides
Derivatives of Monosaccharides
Chemical
changes
involved
Sugar reduction
Sugar oxidation
Derivatives
sugar alcohols
sugar acids
Removal of O
deoxy sugars
Add NH2
amino sugars
Examples and Functions
glycerol, used in lipid synthesis.
Vitamin C (ascorbic acid) is a sugar acid derived from a hexose.
Sugar acids are important intermediates in carbohydrate metabolism
deoxyribose, formed by removal of oxygen from ribose, and used in
DNA synthesis.
glucosamine, used in synthesis of chitin and joint catilage
Optical isomers (Reference)
If two different compounds have the same molecular formula, they are said to be isomers of each other. Two
types of isomerism occur, structural and stereoisomerism. Structural isomerism is due to different link___ of
the atoms or groups within the molecules. Thus all hexoses C6H12O6 are structural isomers of each other
(glucose, mannose, galactose and fructose).
Stereoisomerism occurs when the same atoms or groups are joined together but are arranged differently in
spa___.
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Certain solid compounds when in solution (and certain liquid compounds) possess the power to rotate the plane
of vibration of plane-polarised light, and are said to be op_______ active. If the substance rotates the plane
of polarisation to the right, it is said to be dextro-rotatory and if to the left laevo-rotatory. Dextro-rotatory
compounds are given the prefix 'd'- or more recently (+) and laevo-rotatory compounds 'l'- or more recently (-)
Optical isomerism is a property of any compound which can exist in two forms whose structures are mirror
images. Like right- and left-handed gloves, such structures cannot be superimposed on each other. In organic
compounds this occurs when a carbon atom has four different atoms or groups attached to it. Such a
carbon atom is called an asymmetric carbon atom. The tetrahedral arrangement of bonds about the central,
asymmetric carbon atom means that there are two possible arrangements of the groups in space, forming
two mirror images.
All naturally occurring monosaccharides are virtually D-isomers. Although D- and L-isomers of
the same substance have the same chemical and physical properties, their three-dimensional
differences have biological significance in one important respect, namely that enzymes, which
depend on recognition by sh____, can distinguish between the mirror images. It is possible that early
in evolution an arbitrary bias to accept the D-isomers of sugars was established since their L-isomers
are rare in nature. Naturally occurring amino acids in proteins, however, are all L-isomers.
Ring structures, -and -isomers
Pentoses and hexoses can exist in r____ forms in addition to straight chain molecules. The ring
structures of pentoses and hexoses are the usual forms, with only a small proportion of the molecules
existing in the 'open chain' form at any one time. The ring structure is the form incorporated into
disaccharides and polysaccharides.
The ring structures results in 2 more isomers. Carbon One has a hydroxyl group (-OH) attached to it.
This may be below the plane of the ring (-isomer), or above the plane of the ring (-isomer). The
existence of -& -isomers is important in the formation of st______ and cell______.
S.6 Bio notes 09-10 Cytology I -Chem of Life
b)
P.6
Disaccharides
They are formed by condensation reactions between two monosaccharides. The bond formed
between two monosaccharides is called a glycosidic bond and it normally forms between carbon
atoms 1 and 4 of neighbouring units (a 1-4 gly______ bond). The most common disaccharides are
maltose, lactose and sucrose:
glucose + glucose = maltose,
glucose + galactose = lactose,
glucose + fructose = sucrose.
Reducing sugars
All mono________ and most di__________, including maltose and lactose, are reducing sugars.
Suc____ is non-reducing. Non-reducing sugars are non-reducing because their aldo and keto groups
are not available since they are linked together in the glycosidic bond. Two common tests for
reducing sugars, Benedict's test and Fehling's test, both contain copper(II) sulphate (CuSO4) which
can be reduced to insoluble copper(I) oxide (Cu2O) ----therefore the observation : b___ solution to
b_____-r____ precipitate)
c) Polysaccharides
The process of linking together monosaccharides can be repeated indefinitely to build up the gi___
molecules of polysaccharides. Polysaccharides function chiefly as food and e______ stores (e.g.
starch and glycogen) and as stru______ materials (e.g. cellulose). They are convenient storage
molecules for several reasons: their large size makes them more or less insoluble in water, so they
exert no os______ or chemical influence in the cell; the solid form can be folded into compact
shapes thus taking up less sp_____ and they are easily converted to sugars by hy_________ when
required.
i) Starch
Starch is a polymer of -glucose (-glycosidic linkage). It is a major fuel storage in plants, but is
absent from animals where the equivalent is glycogen. Starch may have straight chain or branched
structure consisting of several thousand glucose residues. Branches if present are formed by 1-6
glycosidic bonds.
The chain coils helically into a compact shape. H-bonds point inwards within the coil.
(intramolecular). There is little inter______ force and so starch is brittle.
Starch molecules accumulate to form starch gr______. These are visible in many plant cells, notably
in the chloroplasts of leaves, in storage organs such as the potato tuber, and in seeds of cereals.
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ii) Glycogen
Glycogen is the animal equivalent of starch, being a storage polysaccharide made from glucose;
many fu_____ also store it. It is very similar in structure to starch, but shows more branching.
iii)Cellulose
Cellulose is a polymer of -glucose. It is the most abundant organic molecule on Earth. Its
abundance is a result of its being a structural component of all plant c___ walls, constituting about
20-40% of the wall on average.
 It consists of long chains of glucose residues (-glycosidic linkage) with ~ 10,000 sugars per
chain.
 Hydroxyl groups (-OH) project outwards from each chain in all directions and form hy______
b____ with neighbouring chains, thus establishing a rigid cross-lin____ between chains.
 The chains associate in groups to form micro_____ -- which have tremendous tensile strength.
 Microfibrils are arranged in layers in a cementing matrix of other polymers e.g. lignin, pectic
substances.
 Despite their combined strength, the layers are fully perm_____ to water and solutes.
Apart from being a structural compound, cellulose is an important f____ source for herbivorous
animals, bacteria and fungi. The enzyme cell_____, which catalyses the digestion of cellulose to
glucose, is relatively rare in nature and most animals, including Man, cannot utilise cellulose despite
its being an abundant and potentially valuable source of glucose. Herbivorous mammals like cattle
and rats, however, have micro_________ living sym________ in their guts which digest cellulose.
S.6 Bio notes 09-10 Cytology I -Chem of Life
B)
P.8
Lipids
Lipids are sometimes classified loosely as those water-in_______ organic substances which can be
extracted from cells by org_____ solvents such as ether, chloroform and benzene. They cannot be
defined precisely because their chemistry is so variable, but we could say that true lipids are esters
of fatty acids and an alcohol.
Fatty acids
They have the general formula R.COOH where R is hydrogen or an alkyl group. R usually has many
carbon atoms in lipids. Most fatty acids have an even number of carbon atoms between 14 and 22.
The long chain of C and H atoms forming a h____c_____ tail. Many of the properties of lipids are
determined by these tails, including their insolubility in water. The tails are hydro______.
Fatty acids sometimes contain one or more double bonds –un_________. Fatty acids and lipids
lacking double bonds are said to be saturated. Unsaturated fatty acids melt at much l____
temperatures than saturated fatty acids.
Q. Cells of poikilothermic animals usually have a higher proportion of unsaturated fatty acids than homeothermic animals.
Can you account for this?
Formation of a lipid
G1ycerol has three hydroxyl (OH) groups, all of which can condense with a fatty acid to form an
ester, and the lipid formed is therefore called a tri________.
Triglycerides are classified as fats or oils, according to whether they are solid (fats) or liquid (oils) at
20º C. The higher the proportion of unsaturated fatty acids, the ________ their melting points.
Triglycerides are non-p____ and therefore relatively insoluble in water.
A major function of lipids is to act as e_____ stores. They have a higher calorific value than
carbohydrates, this is because lipids have a higher proportion of hy______ and an almost
insignificant proportion of oxygen compared with carbohydrates.
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Animals store extra fat when hib_______, and fat is also found below the dermis of the skin of
vertebrates where it serves as an in_______. Plants usually store oils rather than fats. Seeds, fruits and
chloroplasts are often rich in oils. When fats are oxidised, water is a product. This meta_____ water
can be very useful to some desert animals.
Q. A camel stores fat in the hump as a water source as well as an energy source. By what metabolic
process would water be made available from fat?
Carbohydrate could also be used as a water source in the same process. What advantage does fat have over
carbohydrate?
Waxes
Waxes are esters of fatty acids with long-chain alcohols.
Phospholipids
Phospholipids are lipids containing a ph_____ group. They are formed when one of the alcohol groups
of glycerol forms an ester with phosphoric acid instead of a fatty acid. The molecule consists of a
p_______ head with two h____________ tails (the fatty acids).
Steroids
Steroids do not contain fatty acids. They are
derived from hydrocarbon building blocks. All
steroids contain a nucleus composed of 17 carbon
atoms.
In Man, the steroid present in largest amounts is chol________, a key intermediate in the synthesis of
related steroids, and an important constituent of cell mem_______.
Summary of major structural, biochemical
and physiological roles of lipids
Waxes
Mainly used as waterproofing material by plants and animals as in: additional protective layer on cuticle of
epidermis of some plant organs, e.g. leaves, fruits, seeds (particularly xerophytes), skin, fur and feathers of
animals. exoskeleton of insects (see chitin). Beeswax.
Phospholipids
Constituents of membranes.
Steroids
Constituents of bile, forming part of bile salts which emulsify and dissolve lipids during digestion
Steroid hormones e.g. aldosterone
Sex hormones, e.g. oestrogen, progesterone, testosterone
Cholesterol (absent from plants)
Vitamin D - rickets occurs if deficient.
Glycolipids
Components of cell membranes.
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C) Proteins and Amino acids
Over 170 amino acids are currently known to occur in cells and tissues. Of these, 20 occurring
commonly in protein structure. Plants are able to make all the amino acids they require, animals
however, are unable to synthesise all that they need, and therefore must obtain some 'ready-made'
amino acids directly from their diet. These are termed essential amino acids.
a) Structure and range of amino acids
Almost all amino acids are -amino acids, that is the amino
(NH2) group is attached to the -carbon of the carboxylic (-COOH) group. The general formula of an amino acid is shown
as follows:
The majority of amino acids possess one acidic car_______
group and one basic am____ group and are termed 'neutral'
amino acids.
No. of acidic
groups
1
>1
1
No. of basic
groups
1
1
>1
Type
neutral amino acids
acidic amino acids
basic amino acids
The R group varies considerably in different amino acids and is responsible for the unique
properties they display. The simplest amino acid, glycine is formed when R is a H.
All amino acids but glycine are optically active (Why ?) and may exist in either the D or L forms.
Amino acids as found in the proteins of most organisms are found in their ___ forms.
b) Properties of amino acids
Amino acids are colourless, crystalline solids. They are generally soluble in water, but insoluble in
organic solvents. In neutral aqueous solutions they exist as di_____ ions (zwitterions) and are
amp________, possessing both basic and acidic properties.
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Each amino acid has its own specific pH at which it will exist in its neu____ z__________ form and
will be strongly dipolar.
If it is placed in an electric field at this pH, the amino acid will not m____ either to the cathode or to
the anode. The pH causing this electrical neutrality is called the iso_________ point of the amino
acid. Thus each amino acid possesses its own specific isoelectric point.
The amphoteric nature of amino acids is useful biologically as it means that they can act as bu____ in
solutions, resisting changes in pH. They do this by don______ H+ ions as pH increases and acc______
H+ ions as pH decreases.
c) Peptide bond
This is formed when a w_____ molecule is eliminated during interaction between the amino group of
one amino acid and the carboxylic group of another. (condensation)
The linkage formed is a covalent C-N bond, called a p_______ bond. The compound formed is a
dipeptide. It possesses a free amino group at one end, and a free carboxylic group at the other.
This enables further combination between the dipeptide and other amino acids. If many amino acids
are joined together in this way, a poly_______ is formed.
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Size of protein molecules
Protein
molecular
mass
No. of amino
acid residues
Lysozyme
13 930
129
No. of
polypeptide
chains
1
Myoglobin
16 890
153
1
Haemoglobin
64 500
574
4
~ 40 000 000
~ 336 500
2 130
TMV (Tobacco
mosaic virus)
d) Proteins
Proteins are complex organic compounds always containing the elements carbon, hydrogen, oxygen
and nit______, and in some cases sul____. Some proteins form complexes with other molecules
containing phosphorus, iron, zinc and copper. Proteins are macro__________ of high molecular
mass, between several thousands and several millions, consisting of one or more poly_______
chains.
Protein classification according to function.
Type
Examples
Structural
component
Trypsin
Ribulose bisphosphate
carboxylase
Insulin, Glucagon
Component of connective tissue, bone. tendons,
cartilage
Skin, feathers, nails, hair, horn
~Wraps up' nucleic acid of virus
Catalyses hydrolysis of protein
Catalyses carboxylation (addition of C02) of
ribulose bisphosphate in photosynthesis
Help to regulate glucose metabolism
Haemoglobin
Haemocyanin
Myoglobin
Serum albumin
Antibodies
Fibrinogen
Transports 02 in vertebrate blood
Transports 02 in some invertebrate blood
Transports 02 in muscles
Transport in blood, e.g. fatty acids, lipids
Form complexes with foreign proteins
Precursor of fibrin in blood clotting
Myosin
Actin
Moving filaments in myofibril of sarcomere
Stationary filaments in myofibril of sarcomere
Storage
Ovalbumin
Casein
Egg white protein
Milk protein
Toxins
Snake venom
Diphtheria toxin
in biological membranes
Enzymes
Toxin made by diphtheria bacteria
Where they function as enzymes, receptor sites
and transport sites.
Enzymes
Hormones
Transport
Protective
Contractile
Membrane proteins
Collagen
keratin
Viral coat proteins
Occurrence/function
Twenty different amino acids are commonly found in naturally occurring proteins. The potential
variety of proteins is unl______ because the seq______ of amino acids (primary structure) in each
protein is specific for that protein. The p______ structure of a protein is genetically determined by the
D___ of the cell.
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Proteins are the most abundant organic molecules to be found in cells and comprise over 50% of
their total dry mass. Their structural div______ enables them to display a great range of structural
and metabolic activities within the organism.
I) Structure of proteins
Each protein possesses a characteristic 3-dim________ shape-- its confug_______. In describing the
3-D dimensional structure of proteins it is usual to refer to four separate levels of organisation as
follows.
i) Primary structure
It is the seq______ of amino acids in a polypeptide chain. This dictates its biological function. In turn,
this sequence is controlled by the g____.
ii) Secondary structure
It usually takes the form of an extended spiral spring, the -h____,
whose structure is maintained by many h_______ bonds which are
formed between adjacent CO and NH groups.
A protein which is entirely -helical. and hence fibrous is keratin. It
is the structural protein of hair, wool, nails, claws, beaks, feathers and
horn, as well as being found in vertebrate skin. Its hardness and
stretchability vary with the degree of cross-linking by dis_______
bridges between neighbouring chains.
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iii)Tertiary structure
Usually the polypeptide chain bends and fo___ extensively, forming a precise, compact 'glo_____'
shape. This is the protein's tertiary conformation and it is maintained by the interaction of the 4 types
of bonds, namely i____, h_______ and di_________ bonds as well as hydro______ interactions. *
iv) Quaternary structure
Many highly complex proteins
consist of an aggregation of
several p___________ chains
held together by hydro_____ interactions, hyd____ and i____ bonds.
Their precise arrangement is the
quaternary structure.
Haemoglobin consists of four separate polypeptide chains.
eg. The protein coats of some
viruses, are composed of many
polypeptide chains arranged in a
highly ordered fashion.
Classification of proteins according to structure
Fibrous proteins
Globular proteins
Level of
Organization
Secondary structure most important (little
or no tertiary structure)
Tertiary structure most important
Solubility in
water
Structure and
conformation
Insoluble in water
Easily soluble to form a colloidal
suspension
Polypeptide chains tightly folded to form
spherical shape
Examples and
Functions
Components of connective tissue, collagen
(tendons, bone matrix), myosin (in muscle
sarcomere), silk (spiders' webs), keratin
(hair, horn, nails, feathers)
Physically tough, Long parallel polypeptide
chains cross-linked at intervals forming
long fibres or sheets Perform structural
functions in cells and organisms,
-
-
*The
Enzymes
Plasma protein in blood serum is
important as they hold water –colloidal
osmotic potential
Immunity --antibodies
Some hormones e.g. insulin.
latter are quantitatively the most important and occur when the protein folds so as to shield hydrophobic
side-groups from the aqueous surroundings, at the same time exposing hydrophilic side-chains.
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Classification of proteins according to composition (reference)
i) Simple proteins
Only amino acids form their structure.
ii) Conjugated proteins
Complex compounds consisting of globular proteins and non-protein material (a prosthetic group).
Example of simple proteins
Albumins
Globulins
Histones
Keratin, collagen, elastin
Location
Egg albumen,
Serum albumin of blood
Antibodies in blood,
Blood fibrinogen
Associated with nucleic acids, in nucleo-proteins of cell
Keratin of hair, skin, feathers; collagen of bone matrix and tendon; elastin
of ligament
Examples of
Conjugated proteins
Glycoprotein
Prosthetic
group
Carbohydrate
Chromoprotein
Pigment
Lipoprotein
Lipid
Metal proteins
Metal
II)
Location
Blood plasma
Mucin (component of saliva)
Haemoglobin - haem (iron-containing pigment)
Phytochrome (plant pigment) Cytochrome (respiratory pigment)
Membrane structure Lipid transported in blood as lipoprotein
E.g. nitrate reductase, the enzyme in plants which converts nitrate
to nitrite
Electrical properties of proteins
A considerable number of positive and negative electrical ch______ are carried by protein
molecules. Accordingly, proteins demonstrate ampho_____ properties similar to those of amino acids,
and also possess their own specific isoelectric points. At its isoelectric point a protein has no net
c_______.
At pHs below its isoelectric point the protein is ___________ charged, and above its isoelectric point
it is ____________ charged. In either case the net charge which the protein bears is the s_____
(either all positive or all negative) for all its molecules. The net electrostatic effect that this causes is
repulsion between adjacent protein molecules, thus preventing their aggregation. At its isoelectric
point the protein is at its least soluble due to the absence of electrostatic repulsion which would
otherwise keep the molecules apart.
Q. Give an example of protein precipitation caused by change in pH .
Q. Explain why most cytoplasmic proteins carry negative charges ? (Hint: most cytoplasmic proteins has an isoelectric point
at about pH 6)
III) Denaturation and renaturation of proteins
Denaturation
It is the loss of the specific 3-D structure (tertiary structure)of a protein molecule. The change may be
temporary or permanent, but the amino acid sequence (primary structure) of the protein could
remain un________. If denaturation occurs, the molecule unfolds and can no longer perform its
normal biological function.
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Renaturation
Sometimes a protein will spontaneously refold into its original structure after denaturation, providing
conditions are suitable. This is called renaturation, and is good evidence that tertiary structure can be
determined purely by the p
structure.
Factors causing protein denaturation
Factor
Heat or
radiation
(UV, infraRed)
Explanation
High energy causes the atoms of the protein to
vibrate more (increased kinetic energy), thus
breaking the weak hy_______ and I_____
bonds
Example
Coagulation of albumen (boiling eggs makes
the white more fibrous and less soluble)
Acids
Additional H+ ions in acids combine with COOgroups on amino acids and form COOH. I____
bonds are hence broken
The souring of milk by acid (e.g. Lactobacillus
bacterium, produces lactic acid, lowering pH and
causing it to denature the casein, making it
insoluble and thus forming curds)
Alkalis
Reduced number of H+ ions causes NH3+
groups to lose H+ ions and form NH2. Ionic
bonds are hence broken
Inorganic
chemicals
The ions of heavy metals such as mercury and
silver are highly electro_______. They
combine with COO- groups and disrupt ionic
bonds. Heavy metals ion also combine with
the free sulphydryl groups (SH) of cell
enzymes and cause their coagulation.
Similarly, highly electronegative ions, e.g.
cyanide (CN-), combine with NH3+ groups and
disrupt I_____ bonds
Many enzymes are inhibited by being denatured
in the presence of certain ions, e.g. cytochrome
oxidase (resp______ enzyme) is inhibited by
cyanide
Organic
chemicals
Organic solvents have an affinity for the
hydro______ surfaces of the protein and this
results in denaturation of the protein along with
precipitation
Alcohol denatures certain bacterial proteins.
This is what makes it useful for ster__________.
(alcohol kills bacteria also by destroying cell
membrane)
Mechanical
force
Physical movement may break hydrogen
bonds
On stretching a hair, the hy_______ 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
End