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Extracellular enzymes

Enzymes made inside cells but secreted to be used outside of the cells that made them.

Intracellular enzymes

Enzymes made and used inside cells.

Cellular respiration

Breaking down glucose (food) without oxygen to provide available energy for the cells. The glucose reacts with oxygen to produce energy in the form of ATP with carbon dioxide and water as waste products

Prosthetic groups

Organic molecules that are very tightly bound to an enzyme to act as a cofactor.


The biochemical process by which the cells in the body releases energy


Complex non-protein organic molecules that are loosely associated with an enzyme and act as cofactors.


Non-protein factors needed by some enzymes for them to function effectively.


Breaking down the large insoluble food molecules into small soluble molecules


Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus


An endocrine gland which produces insulin


Chemical needed in very small amounts as part of a balanced diet to keep the body healthy.

Adenine (A)

The nitrogenous base, adenine, which pairs with T, thymine.


Enzymes which break down lipids into fatty acids and glycerol


Fungi (singular fungus) are either uni-cellular, as in yeasts, or multi-cellular, as in mushrooms, toadstools and moulds. Fungi have a nucleus, cytoplasm and a cell wall

Examples of enzymes

Enzymes are found in all organisms. What types of enzymes are there – and how do we name them?

Intracellular and extracellular enzymes

Intracellular enzymes
  • Intracellular enzymes are found inside cells. The majority of enzymes are intracellular
  • Intracellular enzymes are produced as a result of protein synthesis (see Cell Biology, Page 7)
  • Intracellular enzymes control the metabolism of every cell and catalyse a wide range of reactions. These include photosynthesis and respiration on which all life depends, the production of stunning colours and shapes in feathers, scales and petals, the production of chemicals that act as toxins, forms of communication and control mechanisms and the formation of entire new organisms in reproduction.
Green lizard Fly agaric

Intracellular enzymes are involved in all the reactions within an individual cell or an entire organism that bring about the shape and form of the organism and the production of new organisms in reproduction.

Extracellular enzymes
Extracellular digestive enzymes

Carnivorous plants such as this pitcher plant use the extracellular digestive enzymes of bacteria and fungi as well as some they produce themselves to break down the bodies of the animals they capture.

  • Extracellular enzymes are produced by protein synthesis in glands and then released to work outside of the cells that made them
  • Many extracellular enzymes are involved in digestion – the breakdown of large molecules into smaller molecules that can be absorbed and used by the cell or organism.
  • Human examples include the digestive enzymes produced in the salivary glands, the stomach, the pancreas and the small intestine, the lysozymes produced in tears that break open and destroy bacteria.
  • Examples of extracellular enzymes in other organisms include digestive enzymes produced by fungi and digestive enzymes produced by insects and extruded onto their food (for example by flies)

Naming enzymes

Most enzymes have several names:

  • The recommended name - this is a relatively short name that often includes the substrate of the reaction followed by –ase or kinase e.g. sucrase, lipase and creatine kinase
  • The systematic name - this name is usually longer and describes the type of reaction that is being catalysed e.g. ATP:creatine phosphotransferase and ribulose bisphosphate carboxylase/oxygenase
  • A classification number e.g. EC (creatine kinase) or EC (ribulose-bisphosphate carboxylase)

Some enzymes are still known by old and very unsystematic names that tell you very little about them – for example pepsin and trypsin.

Cofactors, coenzymes and prosthetic groups

Many enzymes need an extra, non-protein part to function properly. These non-protein parts may be ions or molecules and are referred to as cofactors. When inorganic ions act as cofactors they bind to the enzyme-substrate complex making it more efficient. Other cofactors form an integral part of the enzyme molecule and are vital for it to function at all.

An example of this is salivary amylase which works more efficiently in the presence of chloride ions.

Prosthetic groups are organic molecules that are very tightly bound to an enzyme where they act as cofactors. Haem, a flat ring molecule containing iron, is the prosthetic group for a number of enzymes including cytochrome oxidase (cellular respiration) and catalase (breaks down hydrogen peroxide to water and oxygen). Haem is a permanent part of the structure of these molecules.


The enzyme catalase, which contains four haem prosthetic groups (Vossman, CC BY-SA 3.0).

Coenzymes are cofactors that are complex non-protein organic molecules that are loosely associated with an enzyme. They transfer chemical groups, atoms or electrons from one enzyme to another. Many coenzymes are vitamins or vitamin derivatives e.g. nicotinamide adenine dinucleotide (NAD) is a coenzyme that plays an important role in cellular respiration and other metabolic pathways.