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 series of slides are carbohydrates, proteins and lipids, but there are hundreds of others. 2

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, C 6 H 12 O 6 Sucrose and maltose have the same formula, C 12 H 22 O 11 Carbohydrates provide the main source of energy for respiration in living organisms 3

Glucose C 6 H 12 O 6 C C C C C C HO H H HOH H H 2 OH A glucose molecule as a straight chain 5 of the carbon atoms may be arranged in a ring This molecule is often represented simply as a hexagon CO CC CC C 4

Other carbohydrates 2 molecules of glucose can join together to form a molecule of maltose maltose sucrose is formed when a molecule of glucose and a molecule of fructose combine Starch and cellulose are formed from hundreds of glucose molecules joined to form a long chain part of a starch molecule 5

Proteins Proteins are made up of carbon, hydrogen and oxygen molecules 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-Val joined together would make a small protein Proteins make up the structure of cells; cytoplasm, nucleus cell membranes and enzymes 6

Protein structure and shape The 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 how it acquires a shape High temperatures or certain chemicals can cause the protein molecule to lose its shape and its properties. 7

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 H2H2 H2H2 C O O O glycerol fatty acids A simple lipid 8

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 others play an important part in these reactions 9

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 enzyme speeds up only one kind of reaction For example, glucose and fructose might join up slowly to form sucrose glucose--fructose With the right enzyme present, the reaction happens faster glucose--fructose 10

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 enzyme can speed up In speeding up the reaction, the enzyme combines temporarily with the substances it is acting on Any substance an enzyme acts on is called a substrate 11

enzyme substrate A substrate B The substrate molecules fit the shape of the enzyme 12

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substrates combine temporarily with enzyme enzyme joins substrates together 14

new compound released by enzyme enzyme unchanged and ready for next reaction 15

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 sucrose molecule 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 molecule into the smaller glucose and fructose molecules 16

A ‘breaking-down’ reaction the shape of the substrate molecule fits the enzyme shape this is called the active site of the enzyme 17

Intermediate stage (1) substrate combines temporarily with enzyme enzyme will break molecule here 18

Intermediate stage (2) substrate splits and separates from enzyme 19

Final break-down products end-products enzyme ready for next reaction 20

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 heat or 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 temperature and pH (acidity or alkalinity) 21

Enzymes can act on only one type of substrate this substratecannot combine with this enzyme this substrate cannot combine with this enzyme 22

Because enzymes are proteins, they are denatured by heat or some chemicals enzyme denatured by heat denatured enzyme cannot combine with substrate enzyme + substrate 23

1 glucose molecules 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 25

part of starch molecule E 3 One end of a growing starch molecule combines with the glucose molecule at the active site 26

E 4 The growing starch molecule breaks free from the enzyme which is now free to repeat the reaction 27

Enzyme action EE E 2 E 3 E 4 part of starch molecule 1 glucose molecules E 1 28

Question 1 The correct formula for glucose is (a) C 12 H 22 O 11 (b) C 5 H 10 O 5 (c) C 4 H 8 O 4 (d) C 6 H 12 O 6 29

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 30

Question 3 When two molecules of glucose combine, they form (a) maltose (b) sucrose (c) fructose (d) ribose 31

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 32

Question 5 Which statements are correct? Proteins are present in (a) cell membranes (b) cell walls (c) cytoplasm (d) nucleus 33

Question 6 High temperatures damage proteins by (a) decomposing them (b) changing their chemical composition (c) changing their shape (d) making them soluble 34

Question 7 A protein is made up of a series of (a) glucose units (b) fatty acids (c) amino acids (d) carbohydrates 35

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 36

Question 9 Enzymes are (a) proteins (b) lipids (c) carbohydrates (d) a combination of these 37

Question 10 An enzyme can (a) change a reaction (b) prevent a reaction (c) slow down a reaction (d) speed up a reaction 38

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 39

Question 12 An enzyme can (a) combine with different substrates (b) form different kinds of end-product (c) function at temperatures above 90 o C (d) speed up a reaction in the cytoplasm 40

Question 13 The part of an enzyme which combines with the substrate is called (a) the reaction centre (b) the active site (c) the action centre (d) the reaction site 41

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 42

ANSWER Correct 43

ANSWER Incorrect 44