bio4

The Chemicals of Living Cells
©The Wellcome Trust

2
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.

Carbohydrates
3
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

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

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

Proteins
6
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

Protein structure and shape
7
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-Cyst-Ser-Ala-Ser-Cyst-Gly
Val
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.

Lipids
8
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
H 2
C
O
stearic acid
H
C
O
oleic acid
A simple lipid
H 2
C
O
palmitic acid
glycerol
fatty acids

Salts and water
9
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

Enzymes
10
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

Enzyme action (1)
11
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

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

13

14
substrates combine temporarily with enzyme
enzyme joins substrates together

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

Different types of enzyme reaction
16
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

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

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

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

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

Properties of enzymes
21
Enzymes can act on only one type of substrate
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
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)

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

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

ENZYME ACTION 24
1
E
glucose
molecules
1. A glucose molecule combines
with the active site on an enzyme

25
E
2 A region of the active site is still available

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

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

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

29
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

30
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

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

32
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

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

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

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

36
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

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

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

39
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

40
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

41
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

42
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

43
ANSWER
Correct

44
ANSWER
Incorrect