B4.1 How do chemicals reactions take place in living things

B4.1 How do chemicals reactions take place in living things

All living cells are made from basic units – these are called CELLS.

The processes of life carried out by all living cells depend on chemical reactions within cells – these reactions need energy released by RESPIRATION (the release energy from food). RESPIRATION is one of seven major life processes:

PHOTOSYNTHESIS – makes food molecules and energy available to living organisms through food chains.

Photosynthesis can be summarised by saying that carbon dioxide and water are combined to produce sugar and oxygen in the presence of light and chlorophyll.

Photosynthesis uses sunlight to build large molecules in plant cells and some microorganisms (e.g. phytoplankton)

Enzymes are biological catalyst; they are proteins that speed up chemical reactions. Cells make enzymes according to the instructions carried in genes. Enzymes are specific so only molecules with the correct shape can fit into the enzyme – this is called the lock and key model.


Once the enzyme and molecule (substrate) are linked the reaction takes place, the products are released and the process is able to start again.



For enzymes to work to their optimum they need a specific and constant temperature:

Different enzymes have different optimum working temperatures. For example, in the human body enzymes work best at 37˚C . Below this temperature their rate of action slows down however, if the temperature gets too high the enzyme is DENATURED and stops working.



The biological name for the process of permanent change in an enzyme’s shape is denaturing. The place where the substrate fits to the enzyme is called the ACTIVE SITE. When subjected to high temperatures, the shape of the active site changes irreversibly. This means molecules can no longer fit and the reaction stops


Enzyme activity at different temperatures in a balance between:

  • Increased rates of reaction as temperature increase
  • Changes to the active site at higher temperatures, including denaturing

The active site is influenced by pH.

Changes in pH in the enzyme’s environment can make and break intra- and intermolecular bonds in the enzyme. This changes the shape of the active site and its effectiveness.