As 2.1.1 Biological Molecules

7/29/2019 As 2.1.1 Biological Molecules

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2.1.1 Biological Molecules

Water

Water is the most abundant component of all organisms.One human cell on average contains about 80% water.

Life originated in water. Much life still depends upon it as a habitat.All biochemical reactions take place in water.

Structure and polar ity

Hydrogen bonding

A water molecule is said to be dipolar:

A single molecule consists of two hydrogenatoms and one oxygen atom joined

covalently such that the single electron of

each of the hydrogen atoms is shared withelectrons in the outer shell of the oxygen

atom.

The nucleus of the oxygen atom has a strong

positive charge which tends to pull electrons

away from the smaller hydrogen nuclei. Theend of the molecule where the hydrogen

nuclei are located is left positively charged

while the other end where the oxygen atom

is found is negatively charged.

Such a molecule is dipolar.

The polarity of water molecules accounts formany of the properties of water which are

important in living organisms.The negative pole of a water molecule may be

attracted to the positive pole of other water

molecules.

Hence weakhydrogen bonds are formed andwater exists as molecular clusters rather than

individual molecules and is a liquid rather than a

gas at room temperature.


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The polarity of water molecules makes water a good solvent.The reason for this is that water molecules can form hydrogen bonds with molecules of

the dissolving substance orsolute.For example, sodium chloride is virtually insoluble in non-polar liquids (e.g.

chloroform) but dissolves easily in water.

The water molecules cluster round the Na+ and Cl-ions and form hydrogen bonds with

them. The electrostatic attraction which holds the salt crystal lattice together isovercome. Each ion goes into solution surrounded by a shell of water molecules.

[The hydrogen bond in general:

Chemical groups involving hydrogen atoms may sometimes demonstrate an electrical

polarity.The positive charge of the small hydrogen nucleus is relatively weak and the electrons

involved in any covalent bond involving hydrogen atoms may tend to be pulled towardslarger nuclei with stronger positive charges e.g. oxygen atoms.

Hence an electrical dipole is created.The hydrogen part of the chemical group is slightly positive whereas the rest of the

group is slightly negative.Groups with such a polar nature may be attracted to one anotherpositive poles

towards negative poles. The weak bonds thus formed are called hydrogen bonds].

Thermal properties of water

Due to the hydrogen bonds, more energy is required to separate water molecules. It

therefore requires more heat than expected to convert liquid water into vapour.i.e. the specific latent heat of vaporisation of water is high.

(The specific latent heat of vaporisation of a substance is the quantity of heat neededto change unit mass from liquid to vapour without a change in temperature).

This means that evaporation of sweat is an effective means of cooling inmammals.

Another consequence of hydrogen bonding is that the specific heatcapacity ofwater is abnormally high. A relatively large amount of heat is needed to raise the

temperature of a given mass of water. Water heats up and cools down more slowlythan expectedin effect it buffers sharp temperature changes.

At temperatures of 0C and below, water forms the crystalline substance ice. The

arrangement of the water molecules in ice makes it less dense than liquid water. Asa result, ice will form at the surface of a body of water such as a pond or lake. This

insulates the water beneath and thus prevents the whole of the body of water

freezing solid. Living organisms can therefore survive beneath the ice providing

there is sufficient food and oxygen available.


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Cohesion and surface tension

Cohesion is the tendency of molecules of a substance to attract one another.Hydrogen bonding increases the cohesive forces between water molecules.

One effect of these large cohesive forces in water is that the molecules are pulled

inwards towards each other, so forming spherical drops rather than spreading out in alayer. The inward pull of the water molecules creates a skin-like layer at the surface.

This force is called surface tension.It is surface tension which allows insects like pond-skaters to walk on the surface ofthe water.

The cohesive forces between molecules accounts for the upward pull of water inxylem when evaporation occurs at the leaves.

Note that the term adhesion refers to the tendency of molecules to be attracted to onesof a different type. Adhesive forces exist between water molecules and the walls of

xylem vessels.Capillarity, the passive movement of water through narrow tubes, e.g. through xylem

vessels and through soil, is the result of cohesive and adhesive forces.


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The importance of water to living organisms

Metabolic role of water

1. Hydrolysis - Water is used to lyse (split) many substances, e.g. proteins to aminoacids, polysaccharides to monosaccharides, and fats to fatty acids and glycerol.

2. Medium for chemical reactions - All chemical reactions in living organisms take

place in an aqueous medium.

3. Diffusion and osmosis - Water is essential for the diffusion of materials across

surfaces such as the lungs or alimentary canal.

4. Photosynthetic substrate - Water is a major raw material in photosynthesis.

Water as a solvent

1. Transport - Blood plasma, tissue fluid and lymph are all predominantly water and

are used to dissolve a wide range of substances which can then easily be transported.

2. Removal of wastes - Metabolic wastes like ammonia and urea are removed fromthe body in solution in water.

3. Secretions - Most secretions comprise substances in aqueous solution, e.g.

digestivejuices have salts and enzymes in solution; tears consist largely of water.

Water as a lubricant

Waters viscosity makes it a useful lubricant. Lubricating fluids which are mostly waterinclude:

1. Mucus - This is used externally to aid movement in animals, e.g. snail and

earthworm; or internally, e.g. in the vagina and gut wall.

2. Synovial fluid - This lubricates movement in many vertebrate joints.

3. Pleural fluid - This lubricates movement of the lungs during breathing.

4. Pericardial fluid - This lubricates movement of the heart.

5. Perivisceral fluid - This lubricates movement of internal organs, e.g. the peristaltic

motions of the alimentary canal.


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Supporting role of water

Water is not easily compressed because water molecules lie close together as a result of

the large cohesive forces between them. Water is therefore a useful means of supporting

organisms.

1. Hydrostatic skeleton - Animals like the earthworm are supported by the pressureof

the aqueous medium within them.

2. Turgor pressure - Herbaceous plants and the herbaceous partsof woody ones are

supported by the osmotic influx of water into their cells.

3. Humours of the eye - The shape of the eye in vertebrates is maintained by the

aqueous and vitreous humourswithin them. Both are largely water.

4. Amniotic fluid - This supports and protects the mammalian foetus duringdevelopment.

5. Erection of the penis - The pressure of blood, a largely aqueous fluid, makes the

penis erectso that it can be introducedinto the vagina during copulation.

6. Medium in which to live - Water provides support to the organisms which livewithin it. Very large organisms, e.g. whales, need to live in water as their size makes

movement on land difficult.

Miscellaneous functions of water

1. Temperature control - Evaporation of water during sweating and panting is usedto cool the body.

2. Medium for dispersal - Water may be used to disperse the larval stages of some

terrestrial organisms.In mosses and ferns it is the medium in which the male gameteis transferred. The build-up of osmotic pressurehelps to disperse the seed

of the squirting cucumber.

3. Hearing and balance - In the mammalian ear, the watery endolymph and

perilymphplay arole in hearing and balance.


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Proteins

Proteins are polymers made up of subunits called amino acids.

There are 20 different amino acids which occur naturally in proteins.

The order of these different amino acids in the polymer determines the structure and

function of the protein.

There are two main types of protein:

1. Fibrous proteins

Extended chains of amino acids making long thead-like molecules.Tend to be insoluble and resistant to temperature and pH changes.

Biological significance: Often have structural functions, e.g. collagen and elastin(occur in skin and connective tissue), and keratin (in hair,

horns, nails and wool).

2. Globular proteins

Chains of amino acids are folded to produce fairly compact rounded molecules.They may dissolve or form colloidal suspensions.

May be susceptible to changes in temperature and pH because the weak bondswhich help maintain globular protein shape may be broken.

Biological significance: Often have functions involved in metabolism.e.g. enzymes, some hormones (such as insulin), antibodies,

blood transport pigments (such as haemoglobin), light-sensitive pigments, blood clotting agents.

Some proteins have a storage role, e.g.ovalbumin ineggs, and the aleurone layer in seeds. They serve as a

store of energy and amino acids.

Some proteins, including some enzymes, cannot function unless joined to anon-protein organic component called a prosthetic group. Such proteins are called

conjugated proteins, e.g. haemoglobin contains four haem groups. Each haem

group involves an iron atom which may form a reversible association withoxygen.


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Amino acids

General formula:variable group substituted differentlyin eachof the 20 naturally occurringamino acids

H R O

N C C

H H OH

basic amino group acidic carboxyl group

central carbon atom

may be known as the -carbon

or: R

H2N C COOH

H

Note that all amino acids contain carbon, hydrogen, oxygen and nitrogen.

The R group has a different structure in each of the 20 important amino acids and itdetermines the chemical properties of each amino acid.

Two amino acids contain sulphur as part of their R groupscysteine and methionine.Four different types of amino acid are recognised according to the R groups they

possess.

(i) Non-polar amino acidsThis is where simple non-polar hydrocarbon groups occur in the R position.

e.g. R =CH3

CH3

H2N

C COOH alanine

H

A large proportion of non-polar amino acids in a proteinmake the protein insoluble

and unreactive. They occur in large proportions in structural proteins.


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(ii) Polar amino acids

R groups may develop partial charges of the type involved in hydrogen bonds.

e.g. R =CH2OH

OH

CH2

H2N C COOH serine

H

Polar amino acids in a protein help increase its solubility and allow hydrogen

bonds to form within and between chains.

(iii) Acidic amino acids

R groups can form negatively charged ions.

e.g. R =CH2CO-

(CH2COO-)

O

CH2COO-

H2N C COOH aspartic acid

H

(iv) Basic amino acids

R groups can form positively charged ions.

e.g. R =(CH2)4NH3+

(CH2)4NH3+

H2N C COOH lysine

HBasic and acidic amino acids are strongly hydrophilic and confersolubility

upon proteins.In globular proteins these charged R groups are important in forming bonds

between different parts of the molecule, helping to hold its correct three-dimensional shape.


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peptide

bond

Theimportance of the amino and carboxyl groups which are common to all amino

acid molecules:

(i) These groups tend to dissociate when dissolved in water, and amino acid molecules

thus become dipolar ions. (They each have a positive pole and a negative pole).

i.e. R

+H3N C COO-

H

This dipolarity helps give a solution of amino acids a buffering effect. It willoppose changes in pH because the charged groups can mop up H+ and OH- ions

which would normally confer acidity or alkalinity to a solution. It is important thata constant pH be maintained in cells because extreme pH values can disrupt

enzyme activity.

(ii) A second important function of amino and carboxyl groups is their ability to formlinkages called peptide bonds.

Peptide bonds join amino acids together to make a chain. Condensation reactionsare involved.

H R1 O H R2 O

N C C + N C C

H H OH H H OH

H R1 O R2 O

N C C N C C + H2O

H H H H OH

Two amino acids joined together form a molecule called a dipeptide; three forma tripeptide, etc.

Amino acids are progressively added to the C - terminal end of the chain(carboxyl group end).

hydrolysis when water is gained to reverse the

reaction and break the bond

condensation when water is

lost to form the peptide bond


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The other end is called the N - terminal end.Long chains ususally of several hundred amino acids are called polypeptides.

To form a functional protein these polypeptides either alone or in combination withother polypeptides and perhaps prosthetic groups are folded and twisted in

particular ways.

The folding of polypeptide chains into a functional protein

(i) The primary structure of a protein is the sequence of amino acids in itsmolecule.

The sequence is vital because predictable interactions between the R groups in

different parts of the chain determine the three-dimensional shape of the protein

molecule. The properties of a protein are determined by its three-dimensionalshape.

(ii) Secondary structure refers to any regular folding which may occur throughout

all of the polypeptide chain or just in particular regions (or not at all).Secondary structure is always maintained by hydrogen bonding between the

C O andN H groups of different peptide bond regions of the polypeptidechain.

The most common type of secondary structure is the - helixwhere parts (or all) of the chain form a regular right hand coil. Thehelix may be further coiled in fibrous proteins to form a very

strong structure.

The insoluble, fibrous protein collagen is found in mammalianconnective tissues including tendons, cartilage and bones. A

molecule of collagen consists of three polypeptide chains. Theprimary structure of each chain is largely a repeat of the

tripeptide sequence, glycine-proline-alanine.The secondary structure of each polypeptide chain is a left-handed

helix. Three helices wind together into a right-handed triple helix

to form a collagen molecule. Hydrogen bonds both maintain the

helical structure of each individual polypeptide and form cross

bridges linking the three unbranched chains, the latter providingadditional structural support.Collagen fibrils are formed when many triple helices lie side by

side linked to each other by strong covalent cross links between thecarboxyl end of one molecule and the amino end of another.

Collagen cannot be stretched but is flexible.


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Another type of secondary structure which may occupy part or all of a protein is

called the - pleated sheet. This occurs in fibroin - part of the silk threads madeby caterpillars to spin cocoons. Here, polypeptide chains lie parallel with hydrogenbonding between chains. A single chain may fold back on itself and form a region

of - pleated sheet.

The variable R groups of a protein are not involved in its secondary structure.

(iii) Tertiary structure refers to the folding up into a compact shape of the secondary

configuration.The actual shape attained depends upon the properties of the different R groups in

the polypeptide chain(s).

Four types ofinteraction between R groups are involved in maintaining tertiary

structure:

(a) Disulphide bonds (or bridges)

Cysteines R group isCH2SH.

SH groups of two cysteine units can be linked to form a strong covalent

SS(disulphide) bond.

(b) Hydrogen bondsWeak bonds between the hydrogen and oxygen atoms of polar R groups.

(c) Ionic bonds

Weak bonds between the NH3+

and COO-

groups of basic and acidic aminoacid residues respectively, and also between the NH3

+

and COO-

groups at the

ends of the polypeptide chain.

(d) Hydrophobic interactions

These are interactions involving non-polar R groups which cause the protein

to fold as the non-polar R groups are shielded from water.

Hydrophilic amino acids not otherwise involved in forming hydrogen bonds or weak

ionic bonds between R groups as described above may be found on the outside of aglobular protein forming hydrogen bonds with water molecules.


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(iv) Quaternary structure describes the situation when two or more polypeptide

chains are involved in producing a functional protein.

For example, the globular protein, insulin has two chains, an A-chain containing

21 amino acid residues and a B-chain of 30 amino acid residues, linked by

disulphide bridges.

S SA chain N C

S S

S S

B chain N C

Note that the weak non-covalent bonds involved in protein structure can easily bebroken by high temperatures and conditions of extreme pH values. The protein isthen said to be denaturedit loses its shape and usually becomes non-functional.

The soluble, globular proteinhaemoglobin consists oftwo each of

two different polypeptide chains

two and two chains. The -globin

chains each contain 141 amino acid

residues. The -globin chains each

contain 146 amino acid residues.

Each type of polypeptide involves

lengths of - helix secondary structure

within its folded tertiary structure.

Each chain is associated with an iron-containing haem group. Each iron ion(Fe2+) is capable of carrying a single

oxygen molecule. Hence one moleculeof haemoglobin, having four haem

groups, can carry a maximum of fouroxygen molecules. The four chains fit

tightly together into a globularconfiguration due to much weak

bonding but no disulphide bridges orother covalent links.


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Carbohydrates

Carbohydrates usually have the general formula Cx (H2O)y.

Simple carbohydrates can be broken down to release energy.

More complex carbohydrates are used as energy stores.

The latter are also involved in the composition of various structures, e.g. the cellulose

cell wall of plant cells.

1. Monosaccharides

Simple sugars.

Sweet, soluble in water and can crystallise.Monosaccharides are usually classified according to the number of carbon atoms

they contain. Most important are the trioses (3C), pentoses (5C) and hexoses (6C).Within each group the atoms can be bonded together in different ways so that even

when the number of carbon, hydrogen and oxygen atoms does not change, e.g.C6H12O6, different chemical structures are possible.

These alternative structures are known as structural isomers.Your syllabus states that you should be able to state the structural difference

between the ring forms of alpha and beta glucose. The other examples given beloware to help you familiarise yourself with some other important monosaccharides

which you will come across later in the course.

(i) Glucose C6H12O6Three isomers of C6H12O6 are known as forms of glucose.

- glucose straight chain - glucose

glucose

- glucose and - glucose differ only in the position of the -H and -OH (hydroxyl)

groups attached to carbon atom no. 1.

Ring structures are adopted when glucose dissolves in water.Glucose has reducing properties as do all monosaccharides and most disaccharides.


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Biological significance: Respiratory substrate - i.e. an energy source.Synthesis of disaccharides and polysaccharides.Constituent ofnectar.

(ii) Ribose C5H10O5

Properties: Sweet and soluble

Biological significance: A component of ribonucleic acid (RNA) which isinvolved in protein synthesis.Constituent ofhydrogen carriers such as NAD, NADP

and FAD.Constituent of ATP.

(iii) Deoxyribose C5H10O4

Properties: Sweet and solubleBiological significance: A component of deoxyribonucleic acid which contains

the genetic information of cells.

(iv) Fructose C6H12O6

Biological significance: Respiratory substrate.Synthesis of inulin - a storage polysaccharide in some

plants.

Synthesis of sucrose - a disaccharide and the form inwhich most carbohydrate is transported in plants.Constituent ofnectar.Sweetens fruits to attract animals to aid seed dispersal.

(v) Galactose C6H12O6

Biological significance: Respiratory substrate.Synthesis of lactose - the sugar in mammalian milk.


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2. Disaccharides

Disaccharides consist of two monosaccharide units joined to make a single moleculea condensationreaction (water is lost).

Sucrose, maltose and lactose all have the general formula C12H22O11.

maltose = - glucose + - glucose;

sucrose = - glucose + - fructose;lactose = - galactose + - glucose

The bond formed between the monosaccharide monomers is known as a

glycosidic bond.

We shall consider the formation ofmaltose as a typical example of a condensation

reaction between two monosaccharides to form a disaccharide plus water:

- glucose - glucose

maltose

1-4 glycosidic bond

Disaccharides often occur as intermediates either in the breaking down or buildingup of polysaccharides. Maltose is found in the alimentary canal as the first

product of starch digestion. It is further broken down to glucose before absorption

into the body.

It consists oftwo molecules of - glucose joined by a glycosidic linkage.It is a reducing sugar.

Biological significance: Respiratory substrate.

condensation when water is

lost to form the glycosidic bondhydrolysis when water is gained to reverse th

reaction and break the bond

+ H2O


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3. Polysaccharides

Polysaccharides are complex carbohydrates - chains of monosaccharide units

joined by glycosidic bonds.

Not sweet.

Insoluble in water or form colloidal suspensions.Widely used as storage and structural materials.

(i) Starch

Starch is not a pure substance. It has two components:Unbranched chain molecules of a polysaccharide called amylose andbranched chain molecules of a second polysaccharide called amylopectin.

Amylose is a polymer (chain of repeating units) of - glucose linked by 1 - 4glycosidic bonds. The amylose molecule coils into a helical configuration.Hydrogen bonds hold the helix together.

Amylopectin is also a polymer of - glucose but has a branching structure.It is the same as amylose but every 24 - 30 glucose units there is a side chain of

glucose units.

Biological significance: Major storage carbohydrate in plants.

It is stored in the form of large aggregations of molecules

called starch grains.Starch is a good storage material because:

it is too big to diffuse out of the cell;

it has little osmotic effectbecause it is relatively

insoluble and has little effect on the water potential ofthe cell;

it is fairly unreactive;

furtherglucose molecules can easily be added to the

side chains or taken away - the enzyme amylase is

involved.Starch is used as an energy store in plants rather than

triglycerides which have a greater energy content per unit

mass because mass does not matter to the immobile plant.

(Triglycerides are used as an energy store in seeds whichusually have to be light).

Starch is used as an energy store in plants rather thanglycogen because starch is less soluble than glycogen.


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(ii) Glycogen

The molecular structure of glycogen is similar to that of amylopectin but there are

more frequent branches about every 8 - 12 - glucose units.It is found in the liver and muscle cells as granules.

Biological significance: Major storage carbohydrate in animals, fungiandbacteria.

It is well suited to this function for the reasons given for

starch.(Animals also store energy as triglycerides which have

less mass for the same amount of energy stored.Triglycerides are a better long term solution to storing

energy because they are insoluble in water and so do notdiffuse into the tissue fluid).

In muscles glycogen is used as an energy store because

it is quickly converted to glucose.


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Cellulose is a polymer of- glucose units attached by 1 - 4glycosidic bonds.

When two - glucose molecules

condense together forming 1,4glycosidic bonds, alternatemolecules are rotated through 180

allowing the appropriateOHgroups to react.

(iii) Cellulose

This has two effects:

The unbranched molecules of cellulose (300 - 15000 glucose units long) cannot

coil into a helix as the bulky side groups have to be accommodated on opposite

sides alternately. Consequently, they lie as flat ribbons.

Hydrogen bonds are formed between theOH groups of adjacent straight

chains. Long threads called microfibrils are formed when 60-70 cellulosemolecules become cross-linked by hydrogen bonds.

More hydrogen bonds form between the microfibrils and they cluster intomacrofibrils. These have a very high tensile strength. They are arranged in a gel-

like matrix of other smaller polysaccharides called pectins in plant cell walls.

Cellulose is completely insoluble.

Biological significance: Gives structural support to plant cell walls.

Summary of differences

between starch,

glycogen and cellulose


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Lipids

Lipids are a large and varied group of organic compounds.

All contain a high proportion of - CH2 groups.

This gives them a low solubility in water but a high solubility in non-polar solvents

such as ethanol and chloroform.

All lipids are esters (see below) offatty acids and an alcohol.

(i) Triglycerides (fats and oils)

Fats at room temperature are solids.

Oils at room temperature are liquids.Fats and oils contain carbon, hydrogen and oxygen.

The relative amounts are different to those present in carbohydrates. There is farless oxygen present in fats and oils. This reduces the number of polar - OH groups

and explains the low solubility of fats and oils in water.Fats and oils are built up from two types of subunitfatty acids and glycerol.

Glycerol: C3H8O3

H

H C OH

H C OH

H C OH

HGlycerol does not vary.

Fatty acids show great variation.

General formula of a saturated fatty acid: CnH2nO2

More useful: CH3(CH2)nCOOH carboxyl groupn varies and is usually around 16 (in the range 4 - 24).

e.g. If n = 16 CH3(CH2)16COOH is stearic acid

H H H H H H H H H H H H H H H H H O

H C C C C C C C C C C C C C C C C C C



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Normally fatty acids are unbranched.However they may be saturated or unsaturated.

Saturated chains contain repeatingCH2 groups joined by single carbon

carbon bonds as in stearic acid.

Unsaturated chains contain double carboncarbon bonds (i.e. when two pairs ofelectrons are shared between carbon atoms) in the form of- CH = CH - groups as

in oleic acid.H H

Oleic acid: CH3 (CH2)7 C C (CH2)7 COOH


H C C C C C C C C C C C C C C C C C C

H H H H H H H H H H H H H H H O

Monounsaturated fatty acids contain one double bond.Polyunsaturated fatty acids contain more than one double bond.

Structures called mono- , di- , and triglycerides are formed when a single glycerol

molecule is joined to one, two or three fatty acid subunits, respectively.

The triglyceride structure is typical of fats and oils.It is formed when condensation reactions take place between the - OH (alcohol

or hydroxyl) groups of the glycerol molecule and the - COOH (carboxyl) groupsof the three fatty acid molecules.

Water is removed at each bond and the bond established between each fatty acid

and the glycerol molecule is called an ester bond.

The particular fat or oil which results depends upon the fatty acids involved.They may all be the same or differ.

We say that fats and oils are esters of glycerol and fatty acids.


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We have in general:

H

H C OH HO C (CH2)nCH3

OH C OH + HO C (CH2)nCH3

OH C OH HO C (CH2)nCH3

OH

H a triglyceride

H C O C (CH2)nCH3

OH C O C (CH2)nCH3 + 3 H2O

OH C O C (CH2)nCH3

OH ester bond

Fats often contain only saturated fatty acids.

Oils usually contain unsaturated fatty acids.

(for the purpose ofillustration, all fatty

acids are saturated)

condensation

when water islost to form ester bonds

hydrolysis

when water is gained to reverse thereaction and break the bonds


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Biological significance: 1. An energy source.

2. Energy storage because of:(i) Limited solubility (- therefore fats dont affect the

water potential of the cell)

(ii) Numerous - C - C - and - C - H bonds in their

structure which represent a large pool of storedenergy.

Upon breakdown fats yield more than twice as muchenergy per gram than carbohydrates. This makes

them especially useful foranimals where

locomotion requires mass to be kept to aminimum. In plants fats are useful in seeds wheredispersal by wind makes small mass important.

Note that more water is released from the oxidationof fats than from the oxidation of carbohydrates.

This is known as metabolic water and is importantto organisms living in dry climates.

3. Thermal insulation.

In endotherms fat is stored beneath the skin(subcutaneous fat) where it helps to retain body heat.

4. Buoyancy of aquatic animals.

5. Protection.Fat surrounding delicate organs, e.g. kidneys, protectsthem from mechanical damage.

6. Waterproofing.

Terrestrial plants and animals need to conserve water.

Animal skins produce oil secretions, e.g. from the

sebaceous glands in mammals, which waterproof thebody. Insects and plants both have a waxy cuticle

to reduce water loss.

7. Cell membranes.

Fatty acids may be precursors of cell membrane

materials including phospholipids and cholesterol.

8. Other functions.

Fatty acids areprecursors of steroid hormones and

plant scents.


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The phosphate group is strongly polar and

projects in the opposite direction from thetwo fatty acid side chains.

The polar end containing the phosphate is

hydrophilic and readily soluble in water.

The other end with the non-polar fatty acidchains is hydrophobic and insoluble in

water.These properties are important in the

formation and function of the plasma (cellsurface) membrane.

Phospholipids form a bilayer structure in cell

membranes where the hydrophilic heads

point outwards forming hydrogen bonds withsurrounding water molecules.

The mutually attracting hydrophobic tailspoint inwards.

[Note that the phosphate group abbreviatedas a P above has the following structure:

OH

IO P OH

I ]

(ii) Phospholipids

Phospholipidscomprise two molecules of fatty acids linked to a molecule ofglycerol.

The third position on the glycerol is occupied by a phosphate group which links

the glycerol to an additional moleculeusually a complex, nitrogen-containing

alcohol.

i.e. aminoalcohol

P

glycerol

I IC O C O

I ICH2 CH2

I ICH2 CH2

I I

I II I

I I

I I

I II I

I II I

I II I

I II I

I ICH

2CH

2

I ICH3 CH3 fatty acid chains

polar

hydrophilic

head

non-polar

hydrophobic

tail


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(iii) Cholesterol

Cholesterol is a steroid.

Steroids are lipids whose molecules containfour rings of carbon and hydrogen atoms

with various side chains protruding from

them. Often just one of these side chains,attached to the carbon atom at position 17,determines the steroid molecules

physiological role.

The human sex hormones are steroids.

They are synthesised from cholesterol.

Steroid hormones are hydrophobic and cantherefore pass directly through the

phospholipid bilayer of the plasma

membrane. They then usually act by movinginto the nucleus where they activateparticular genes.

Cholesterol is also an important constituentof plasma membranes. Its small narrow

structure and hydrophobic nature allow it to

sit between the fatty acid tails of thephospholipids. Cholesterol serves to regulate

the fluidity and strength of the plasmamembrane.


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Summary of biochemical tests for the presence of biological molecules

Documents

As 2.1.1 Biological Molecules