B2.5 Proteins

B2.5 Proteins


Proteins have many functions, both inside and outside the cells of living organisms. Proteins, as enzymes, are now used widely in the home and in industry.

Protein structure

Protein molecules are made up of long chains of amino acids.


These long chains are folded to produce a specific shape that enables other molecules to fit into the protein.

Proteins act as:

  • Structural components of tissues such as muscles: o Muscle cells contain interlinking protein fibres. o These interact when the muscle contract.

  • Hormones:

o Some hormones are proteins. o Example: insulin

o    Hormones are released into the blood from glands.

  • They have specific shapes, and attach to molecules on the cell membrane of specific target cells.
  • Eg when the blood sugar levels increase, insulin is released from the pancreas. It attaches to the cell membrane of cells in the liver, and this causes them to increase rate of glucose uptake.

  • Antibodies:

o    Antibodies are made of proteins.

o    They are released by white blood cells

o    They have a specific shape that attaches to antigens.

o    Antigens are chemicals that pathogens carry or release.

o    The antibody prevents the pathogens from damaging our own cells.


  • Catalysts:

o    Proteins act as biological catalysts called enzymes.

  • These control and sequence all of the reactions that occur inside and outside cells in all living organisms.

Enzymes structure and function

  • Enzymes are biological catalysts.
  • Catalysts increase the rate of chemical reactions.
  • Enzymes are protein molecules made up of long chains of amino acids.
  • These long chains are folded to produce a special shape which enables other molecules to fit into the enzyme.
  • This shape is vital for the enzymes function.
  • Normally only one type of molecule (the substrate) will fit into the enzyme.
  • The active site is the part of the enzyme which the substrate fits into.

Activation Energy

  • In order for a chemical reaction to take place, energy is required.
  • This is called the activation energy.
  • Enzymes reduce the activation energy of a reaction.


Effect of temperature on enzymes

  • Like most chemical reactions, the rate of enzyme-controlled reactions increases as the temperature increases.
  • The enzyme and substrates move around faster so they collide more often.
  • The temperature when the enzyme is working fastest is called the optimum.
  • This is true up to approximately 40˚C, higher than this and the structure of the enzyme changes.
  • As a result, the active site becomes a different shape and the substrate no longer fits.
  • It is then described as denatured.

The effect of pH on enzymes

  • pH can also affect the shape of the active site.
  • It does this by affecting the forces that hold the enzyme molecule together.
  • A change in pH denature the enzyme.
  • Different enzymes work best at different pH values.
  • Stomach enzymes work best in acidic conditions.
  • Mouth enzymes work best in neutral conditions.


  • Some enzymes work outside the body cells.
  • These are called extracellular enzymes.
  • The digestive enzymes are produced by specialised cells in glands and in the lining of the gut.
  • The enzymes then pass out of the cells into the gut where they come into contact with food molecules.
  • They catalyse the breakdown of large molecules into smaller molecules.
  • Digestion is the process where food is broken down into substances the body can absorb.
  • Nutrition is the process of taking in and using food.

The Human Digestive System


Digestion in the mouth

  • Food is chewed to create a larger surface area for the action of enzymes.
  • Saliva is released which contains amylase.
  • Amylase digests starch into smaller sugars (maltose).
  • Further chewing enables swallowing.
  • The food enters the oesophagus.

Digestion in the stomach

  • Food enters the stomach from the oesophagus.
  • The walls of our stomach produce juice.
  • This juice contains:
  • A protease enzyme (called pepsin).
    • This digests proteins into amino acids.

o    Hydrochloric acid – this kills bacteria in our food. It creates pH3.

  • Mucus – this protects the wall of our stomach from acid and pepsin.
  • The wall of our stomach is muscular, and churns our food.
  • The food remains in our stomach for a few hours. The proteins are digested.
  • Food leaves our stomach in small squirts into the small intestine.

Digestion and absorption in the small intestine

The small intestine has 2 main jobs:

  • To complete the digestion of the food
  • To absorb the soluble products of digestion into the blood.


Digestion in the small intestine

3 juices are released:

  1. Bile
  • Produced by the liver.
  • Stored in the gall bladder.
  • Released into the small intestine.
  • 2 main things in bile:

o    Alkali to neutralise the stomach acid

  • Bile salts which convert large fat droplets to small fat droplets – for a large surface area for the enzymes to act on.
    • There are no enzymes in bile.
  1. Pancreatic juice and
  2. Intestinal juice
  • Both are released into the small intestine.
  • Both contain 3 main enzymes:

o    Amylase to complete the digestion of starch into sugars.

o Protease to complete the digestion of proteins into amino acids. o Lipase to break down fats into fatty acids and glycerol.

Making use of enzymes

  • Some microorganisms produce enzymes which pass out of the cells.
  • These enzymes have many uses in the home and in industry.
  • In the home, biological detergents may contain protein-digesting and fat-digesting enzymes (proteases and lipases).

  • In industry:
  • Proteases are used to pre-digest the protein in some baby foods.
  • This reduces how much the baby needs to digest the food. o Carbohydrases are used to convert starch into sugar syrup.
  • This is cheaper than extracting sugar from sugar cane.
  • Isomerase is used to convert glucose syrup into fructose syrup:
  • This is much sweeter
  • It therefore can be used in smaller quantities in slimming foods.