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 organism alive. They are living processes. The sum of all the chemical reactions in an organism is called it metabolism 2

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 substrate A substrate B The 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 An enzyme can also 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 molecule 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

pH = % hydrogen ions The pH of a substance is the measure of its acidity. pH is measured on a scale from 1 to 14.

pH of substances o To measure pH an indicator is used. Indicators change colour depending on the pH of the substance they are mixed with

pH in food production  Yeast and bacteria grow and reproduce within specific ranges of pH  In cheese-making sometimes bacteria are used to help change the milk into a more solid form (coagulation).  The bacteria produce lactic acid  The bacteria can tolerate slightly acidic conditions  In bread-making yeast produce carbon dioxide (CO 2 ) during the process of fermentation. CO 2 dissolves into water to form carbonic acid.  Yeast cells prefer neutral environments to grow and reproduce