The Chemicals of Living Cells

©The Wellcome Trust

The chemicals of life

All living organisms are made up of chemical substances

Reactions between these substances keep the cytoplasm(and the organism) alive. They are living processes.

The chemical substances described in the next seriesof slides are carbohydrates, proteins and lipids, but there are hundreds of others.

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Carbohydrates

Carbohydrates contain the elements carbon, hydrogen and oxygen

Familiar carbohydrates are sugar and starch

Glucose, fructose, maltose and sucrose are sugars

Glucose and fructose have the same formula, C6H12O6

Sucrose and maltose have the same formula, C12H22O11

Carbohydrates provide the main source of energy forrespiration in living organisms

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Glucose C6H12O6

C

C

C

C

C

C

HO

HO H

HO H

H OH

H OH

H2OH

A glucose molecule as a straight chain

5 of the carbon atoms maybe arranged in a ring

This molecule is often represented simply as a hexagon

C O

C C

C C

C

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Other carbohydrates2 molecules of glucose canjoin together to form a molecule of maltose

maltose

sucrose is formed whena molecule of glucose anda molecule of fructose combine

Starch and cellulose are formed from hundreds ofglucose molecules joinedto form a long chain

part of a starch molecule

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ProteinsProteins are made up of carbon, hydrogen and oxygenmolecules but with the addition of nitrogen

Carbohydrates are made up of glucose units.Proteins are made up of units called amino acids

There are about 20 different amino acids. Examples are glycine (Gly), alanine (Ala), valine (Val) and cysteine (Cyst)

The amino acids, Gly-Val-Val-Cyst-Ala-Gly-Ala-Valjoined together would make a small protein

Proteins make up the structure of cells; cytoplasm, nucleuscell membranes and enzymes

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Protein structure and shapeThe way the amino acids join up, gives a protein molecule a particular shape, which is different for every protein

Ser-Cyst-Val-Gly-Ser-Cyst Ala Val Val-Cyst-Ser-Ala-Ser-Cyst-Gly

Val- Cyst-Ala-Ala-Ser-Gly

This is a small, imaginary protein molecule showing howit acquires a shape

High temperatures or certain chemicals can cause theprotein molecule to lose its shape and its properties.

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Lipids

Lipids are fats and oils

They are made up from glycerol and fatty acids

Examples of fatty acids are stearic acid, oleic acid and palmitic acid

stearic acid

oleic acid

palmitic acid

C

C

H

H2

H2 C

O

O

O

glycerol fatty acids

A simple lipid

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Salts and water

In addition to proteins, carbohydrates and lipids, cytoplasm contains salts and water

Water makes up the bulk of cytoplasm

All the chemical reactions in cytoplasm take place in solution, i.e. in water

Water itself takes part in many of these chemical reactions

Salts of sodium, potassium and calcium and many othersplay an important part in these reactions

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Enzymes

Enzymes are special proteins

They are present in the cytoplasm of all cells

They help to speed up the chemical reactions in the cell

There are hundreds of different enzymes but each enzymespeeds up only one kind of reaction

For example, glucose and fructose might join up slowly toform sucrose

glucose--fructose

With the right enzyme present, the reaction happens faster

glucose--fructose

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Enzyme action (1)

Enzymes are large protein molecules

Like all proteins, each enzyme molecule has a particular shape

This shape determines which chemical reaction the enzymecan speed up

In speeding up the reaction, the enzyme combines temporarilywith the substances it is acting on

Any substance an enzyme acts on is called a substrate

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enzyme

substrate A

substrate B

The substrate molecules fit the shape of the enzyme12

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substrates combine temporarily with enzyme

enzyme joins substrates together

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new compound released by enzyme

enzyme unchangedand ready fornext reaction

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Different types of enzyme reaction

The last 4 slides show how an enzyme is involved in combining substrates to create a larger molecule

For example, the enzyme could be building up a sucrosemolecule from glucose and fructose

The next sequence shows how an enzyme can help to break a large molecule into smaller molecules

For example an enzyme can split a sucrose moleculeinto the smaller glucose and fructose molecules

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A ‘breaking-down’ reaction

the shape of the substrate molecule fits the enzyme shapethis is called

the active siteof the enzyme

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Intermediate stage (1)

substrate combinestemporarily with enzyme

enzyme will breakmolecule here

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Intermediate stage (2)

substrate splits andseparates from enzyme

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Final break-down products

end-products

enzyme ready for next reaction

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Properties of enzymes

They always produce the same end products

Although they take part in the reaction, they are not used up

Because enzymes are proteins, they are denatured by heator some chemicals

Enzymes can act on only one type of substrate

Denaturing involves a change of shape in the enzyme molecule so that it cannot combine with the substrate

Individual enzymes work best at a particular temperatureand pH (acidity or alkalinity)

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Enzymes can act on only one type of substrate

this substrate cannot combine with this enzyme

this substrate cannot combinewith this enzyme

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Because enzymes are proteins, they are denatured by heat or some chemicals

enzymedenaturedby heat

denatured enzyme cannot combine with substrate

enzyme +substrate

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1

glucosemolecules

E

1. A glucose molecule combines with the active site on an enzyme

ENZYME ACTION 24

E

2 A region of the active site is still available

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part of starchmolecule

E

3 One end of a growing starch molecule combines with the glucose molecule at the active site

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E

4 The growing starch molecule breaks free from the enzyme which is now free to repeat the reaction

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Enzyme action

E

E

E 2

E 3

E 4

part of starchmolecule

1

glucosemolecules

E1

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Question 1

The correct formula for glucose is

(a) C12H22O11

(b) C5H10O5

(c) C4H8O4

(d) C6H12O6

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Question 2

Which is the most accurate description of a carbohydrate?

A carbohydrate contains

(a) carbon and oxygen

(b) carbon, oxygen and nitrogen

(c) carbon, hydrogen and oxygen

(d) carbon and hydrogen

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Question 3

When two molecules of glucose combine, they form

(a) maltose

(b) sucrose

(c) fructose

(d) ribose

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Question 4

Which is the most accurate description of a protein

Proteins contain

(a) carbon, hydrogen and oxygen

(b) carbon, hydrogen, oxygen and nitrogen

(c) carbon, hydrogen and nitrogen

(d) carbon, nitrogen and oxygen

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Question 5

Which statements are correct?

Proteins are present in

(a) cell membranes

(b) cell walls

(c) cytoplasm

(d) nucleus

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Question 6

High temperatures damage proteins by

(a) decomposing them

(b) changing their chemical composition

(c) changing their shape

(d) making them soluble

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Question 7

A protein is made up of a series of

(a) glucose units

(b) fatty acids

(c) amino acids

(d) carbohydrates

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Question 8

Lipids are made up of

(a) glycerol and amino acids

(b) glycerol and fatty acids

(c) protein and fatty acids

(d) starch and fatty acids

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Question 9

Enzymes are

(a) proteins

(b) lipids

(c) carbohydrates

(d) a combination of these

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Question 10

An enzyme can

(a) change a reaction

(b) prevent a reaction

(c) slow down a reaction

(d) speed up a reaction

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Question 11

A substrate is a substance which

(a) an enzyme acts on

(b) is produced by an enzyme reaction

(c) is a particular kind of enzyme

(d) is any chemical substance in a cell

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Question 12

An enzyme can

(a) combine with different substrates

(b) form different kinds of end-product

(c) function at temperatures above 90oC

(d) speed up a reaction in the cytoplasm

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Question 13

The part of an enzyme which combines with the substrateis called

(a) the reaction centre

(b) the active site

(c) the action centre

(d) the reaction site

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Question 14

After being exposed to a high temperature an enzyme cannot function because

(a) it has been broken down

(b) its shape has been changed

(c) its composition has been changed

(d) it cannot separate from its substrate

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ANSWER

Correct

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ANSWER

Incorrect

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