as biology unit 1 biomolecules & enzymes · staight chain form ring forms alpha ... stomach?...

AQA AS Biology Revision notes Trevor Chilton

Topic 1. Biomolecules and enzymes

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AS Biology Unit 1

Biomolecules & Enzymes



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Biological molecules

Chemical issues:

Different types of bonds

Bond Description example

Covalent

Hydrogen

Ionic

Disulphide

Distinguish between:

Polar and non-polar molecules

Monomers and polymers

Carbohydrates

Carbohydrates are molecules made of the three elements, carbon, hydrogen and

oxygen in the proportion Cn(H2O)n

Simple sugars - monosaccharides and disaccharides

Properties: soluble, sweet, crystalline, name ends with ‘-ose’

examples: monosaccharides disaccharides

Glucose structure (draw chemical structures below)

Staight chain form ring forms

Alpha glucose beta glucose



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Ring structure simplified diagrams

Alpha glucose beta glucose

How the monomers join up to form disaccharides and polysaccharides

Show how these two molecules of alpha glucose bond together to form the

disaccharide maltose. This kind of reaction called condensation.

A Condensation reaction is …

Hydrolysis is …

The disaccharide below is sucrose. Draw molecular diagrams below it to show how

it is hydrolysed by the enzyme sucrase to glucose and fructose

(Glucose) (fructose)



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Polysaccharides

Are polymers made up of monosaccharide sub units. e.g. amylase is made up of alpha

glucose sub units as shown below.

Why does it twist up in this way?

Why does amylose (a major component of starch) cause iodine to turn from yellow

brown to blue black?

Starch is a mixture of amylose and amylopectin (a similar molecule to amylose, made

of alpha glucose sub units but with side branches). It is used as a thickening agent in

soups and casseroles (corn flour is the most commonly used for of it). What happens

chemically to make it occupy more space and thicken a soup when heated?



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Digestion of carbohydrates

In the mouth:

In the small intestine

What is lactose intolerance?

Test for reducing and non-reducing sugars

Proteins

make up about 50% of the organic matter of the cell

very large molecules

made of carbon, hydrogen, oxygen and nitrogen

polymers made up from sub-units called amino acids

there are 20 different types of amino acid so proteins are infinitely variable in

structure and properties.

It is the proteins which make our bodies different to each other

The plan for making proteins is the genetic code in DNA molecules

One gene codes for one protein

Examples of proteins

Membrane proteins control transport of materials in and out of cells

Enzymes control all metabolic reactions

Structural component of body tissues: elastin, collagen, actin, keratin.

Structural materials such as silk and wool

Some hormones e.g. insulin

Antibodies



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Amino acids

This molecule is a dipeptide

Q: What are ‘essential’ and ‘non-essential’ amino acids?

Polypeptides

Polypeptides are polymers formed from amino acids. Their structure and function

depends on the amino acid sequence from which they are made.

mRNA substitutes T for U.

Primary Structure of proteins

This is the sequence of amino acids.

The R groups

give each amino acid different properties. R groups may be

Hydrophobic – water hating. They will always turn inwards away from

surrounding water

Hydrophilic – water loving. They will always turn outwards and have a polar

effect, forming H bonds with water.

The R group of the amino acid cysteine contains Sulphur (S) which can form

strong covalent bonds with other cysteine molecules (Disulphide bonds

Ionic - capable of forming ionic bonds with charged ions



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Secondary structure of proteins

This is the coiling or pleating of parts of the polypeptide chain due to the formation of

H bonds.

(NOTE: this is similar to the coiling of starch but do not confuse the two.)

Notes

Tertiary structure

The coils or pleats are folded into more complex shapes by

H bonds

Ionic bonds

Disulphide bonds



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Denaturation

What happens when you heat a protein?

What happens when a protein is subjected to changes in pH?

Quaternary structure of proteins

Proteins made up of more than one polypeptide chain or are joined to an inorganic

prosthetic group. E.g. Haemoglobin, collagen

Structure of Haemoglobin

notes

Chemical test for proteins



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Globular and fibrous proteins

Globular proteins

Fold up into a ball shape

Usually soluble (hydrophilic R groups point outwards and hydrophobic ones

‘hide’ inside the ball)

Control metabolism, e.g. enzymes, antibodies, plasma proteins

Fibrous proteins

Form strong fibres

Insoluble in water

Structural functions e.g. collagen, keratin.

Lipids (fats and oils)

Lipids make up about 5% of all living cells.

Roles of lipids in living organisms Energy , membranes, insulation, protection,

hormones

Glycerol and fatty acids



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Saturated and unsaturated fats. diagrams

Saturated fats have single bonds

Unsaturated fats have double bonds

Note: The double bonds in unsaturated fats tend to make more space within the

molecule which becomes more fluid and flexible. Unsaturated fats are

normally liquid oils at room temperature.

Unsaturated fats are beneficial in the diet because they make the high density

lipoproteins which tend to prevent cholesterol deposition in artery walls. (see

later

Emulsion test for fats



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Enzymes

Enzymes are globular proteins which act as biological catalysts. They catalyse

metabolic reactions.

What does a catalyst do? What is activation energy?

Enzymes are specific each different reaction

requires a different enzyme

Explain how the tertiary structure of globular proteins enables enzyme specificity

Enzyme structure

The tertiary structure of an enzyme generally includes hydrophobic R groups

on the inside and hydrophilic R groups on the outside.

They are soluble in water

An enzyme has a special area where the reaction occurs called an active site

Substrates and products

The substrate is the substance which is used up in the catalysed reaction

The products are the substances produced as a result of the catalysed reaction

Examples:



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Mechanism of enzyme action

Lock and key model

Explain

diagram

Induced fit

model

The lock and

key model illustrated above is not accurate because the active site is not fixed and

rigid like a jig saw piece. It flexes and bends to accommodate the substrate rather like

a glove as you put your hand into it.

Factors affecting enzyme action 1. Temperature

Explain how an increase in temperature causes an increase in the rate of collisions

between enzyme and substrate. (key term: kinetic energy)

Explain the effect of increasing temperature on the bonds which hold the enzyme’s

tertiary structure together

Define the term denaturation



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2. pH

How can pH affect the hydrogen and ionic bonds of a protein?

How may pH changes affect the active site?

What is meant by ‘optimum pH?

What is the optimum pH for pepsin, the enzyme

which breaks proteins down to peptides in the

stomach?

What is the optimum pH for amylase, an enzyme

in saliva which breaks down starch to maltose?

Does a change in pH always denature an enzyme?



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Concentration of enzyme and substrate

3. Substrate concentration

Study the graph and explain

a) why the rate of reaction initially rises

proportionately to the amount of substrate.

b) why the reaction rate levels off after a

certain substrate concentration is reached.

4. Enzyme concentration

Draw a line on the graph to illustrate what would

happen if you increased the enzyme

concentration. Explain your answer.



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5. Inhibitors

An inhibitor is a substance or molecule which slows down the rate of an enzyme

catalysed reaction. Competitive inhibitors bind to the active site, non-competitive

inhibitors affect the overall structure of the enzyme by binding to an allosteric site.

Study the diagrams and write an explanation below.

Competitive inhibition

Non-competitive inhibition



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Describe and explain the action of competitive and non-competitive inhibitors

illustrated in this graph.

End product inhibition

A way of controlling a metabolic process where a number of enzymes are involved in

a chain reaction. The final product may inhibit (non-competitively) the enzyme at the

beginning of the chain. This shuts off the production line when there is sufficient

product.

as biology unit 1 biomolecules & enzymes · staight chain form ring forms alpha ... stomach?...

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