AQA GCSE Biology Unit B2 Chapters 4-6

Enzymes are like biological catalysts - they speed up reactions

They are large proteins with a particular shape with an area where other molecules can fit in. This area is called the active site

Too high a temperature will change the shape of the enzyme, so that the molecules it's trying to break down (substrates) no longer fit in the active site. We say it has been destroyed or denatured.

Enzymes lower the amount of energy necessary for a reaction to take place - the activation energy

• Reactions take place faster when it is warmer because at higher temperatures the molecules move around more quickly so collide more often and with more energy.
• However, if the temperature gets too hot the strings of proteins in the enzyme start to unravel causing the shape of the active site to change. The enzyme becomes denatured.
• The main factors affecting enzymes are pH and temperature
• Different enzymes work best at different temperatures. For example, a protease enzyme in the stomach will work best in very acidic conditions.

glucose + oxygen ---> carbon dioxide + water (+ energy)

This process takes place in mitochondria usually.

The energy is used for 3 main things:

• Build larger molecules from smaller ones
• Enable muscle contraction
• Maintain a constant body temperature

Digestion involves the breakdown of large, insoluble molecules into smaller, soluble molecules.

All three sorts of digestive enzymes (carbohydrases, proteases and lipases) are made in the pancreas and the small intestine. In addition to this, amylase (a carbohydrase) is produced by the salivary glands too and protease is produced by the stomach.

• Amylase catalyses the digestion of starch into sugars
• Protease catalyses the breakdown of proteins into amino acids
• Lipase catalyses the breakdown of lipids (fats and oils) to fatty acids and glycerol

• Protease enzymes in the stomach work best in acidic conditions because glands in the stomach wall produce hydrochloric acid to create very acidic conditions.

• Amylase and lipase work in the small intestine. They work best when the conditions are slightly alkaline.

• The liver produces bile that is stored in the gall bladder. Bile is squirted into the small intestine and neutralises the stomach acid. It makes the conditions slightly alkaline.

• Enzymes are used in biological washing powders to digest food stains. An advantage of using biological washing powder is that it works at lower temperatures than non-bio so can save us money.
• Proteases are used in baby food to pre-digest the food
• Carbohydrases are used to convert starch into sugar syrup for use in foods

The thermoregulatory centre of the brain and receptors in the skin detect changes in temperature. The thermoregulatory centre controls the body's response to a change in temperature.

If the core temperature rises:

• Blood vessels near the surface of the skin dilate meaning that more heat is lost by radiation.
• Sweat glands produce more sweat which evaporates from the skin's surface taking energy away from the skin

If the core temperature falls:

• Blood vessels near the surface of the skin constrict meaning that less heat is lost
• We 'shiver'. Muscles contract quickly which requires respiration and some of the energy produced is released as heat

The pancreas monitors and controls the level of sugar in the blood.

Too much sugar:

Pancreas produces insulin that stimulates the liver to store excess sugar as glugogen

If the pancreas isn't producing enough insulin, this is known as diabetes. It can be controlled by diet or insulin injections.

Cell division is necessary for the growth of an organism or for repair of tissues.

Mitosis results in two identical cells being produced from the original cell

In mitosis, a copy of each chromosome is made before the cell divides and one of each chromosome goes to each new cell.

• Gametes (sex cells) are produces by meiosis from cells in the reproductive organs.
• Before division, a copy of each chromosome is made. The cell then divides twice to form four gametes.
• Each gamete has only one chromosome from the original pair. All of the cells are different from each other and the parent cell.

Sexual reproduction results in variation as the gametes from each parent fuse.