B2 Biology - Enzymes

A set of cards relating in detail to the enzyme section in B2 Biology AQA GCSE.

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  • Created by: Claire
  • Created on: 02-05-10 14:58

Enzyme Structure:

In the body, particles need to collide with enough energy to overcome the activation energy. A reaction is more likely if the particles collide more often, if the particles collide with greater energy and if the activation energy is lowered. (Both enzymes and inorganic catalysts)

Controlling the rate of reactions:

Catalysts change the rate of a reaction, usually speeding it up. Catalysts are not used up and don’t change the reaction. Catalysts bring reacting particles together and lower the activation energy.

Enzymes control chemical reactions in the body. Each enzyme catalyses a specific type of reaction. Enzymes are involved in building larger molecules for smaller ones, changing one molecule to another and breaking down large molecules into smaller ones.

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Enzymes are large protein molecules. They are made up of long chains of amino acids, which are folded and coiled. The molecule has a special shape which lets other molecules fit into the enzyme, the active site.

How do enzymes work?

The substrate is the reactant (molecule effected by the enzyme). The substrate of the reaction fits into the active site of the enzyme. Once in place, the enzyme and the substrate bind together, forming an enzyme-substrate complex.

The reaction is rapid and the products are released from the surface of the enzyme. The substrate then splits into products which leave the active site. Enzymes work under specific conditions of temperature and pH. (Temp. only up to a certain point)

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Enzymes are affected by concentration, temperature and particle size. Temperature – chemical reactions in living cells take place at relatively low temperatures, yet the rate of reaction increases with an increase in temperature. The enzyme and substrate particles move faster so they are more likely to collide and overcome the activation energy.

After about 40 degrees, the protein structure of the enzyme is affected by temperature. The long chains of the amino acid unravel and the enzyme is denatured. The rate of the reaction drops. Most enzymes work best from about 20 degrees to 40 degrees.

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The effect of pH on enzyme action:

Temperature and pH can both change the shape of the active site. The shape of an enzyme is due to forces between the different parts of the protein within the molecule and these forces hold the chains in place. A change in pH affects these force sand so changes the shape of the molecule. So the active site is lost and the enzyme can’t catalyse the reaction any more. Different enzymes prefer different pH levels.

The role of enzymes:

Enzymes catalyse reactions and are needed for respiration, photosynthesis and protein synthesis (building up protein from amino acids on the surface of a ribosome).

Enzymes need the right pH and temperature, which is why your body enzymes start to become denatured when u run a fever and your body temperature goes above 41 degrees.

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Aerobic Respiration:

Aerobic respiration is controlled by enzymes. The digestive system, lungs and circulatory system provide cells with glucose and oxygen for respiration.

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

Glucose is produced in digestion, oxygen comes from the air. Carbon dioxide and water are waste products. Energy is used by cells. To test if an organism respires you can deprive it of oxygen/glucose or test for waste products being produced (this is the humane one). Carbon dioxide will turn lime water cloudy and energy will cause a rise in temperature.

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Respiration happens in the mitochondria…

The chemical reactions (controlled by enzymes) or respiration happen in the mitochondria of the cells. They are tiny rod-shaped bodies, called organelles. They are in almost all plant and animal cells. They have a large surface area for the enzymes by having a folded inner membrane.

Cells which need a lot of energy have lots of mitochondria (like muscles and sperm), whereas fat cells have very few.

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Reasons for respiration:

- It releases energy from food for cells to use.

- Plant and animal cells need energy to … Build up large molecules from smaller ones and to make new cell material. These are synthesis reactions (building). In plants, sugars, nitrates, etc, are built up into amino acids and then proteins

- Needed for muscles to contract in animals. Even heart beating and gut churning.

- Needed for homeostasis. Mammals and birds are warm blooded, so bodies stay same temp in different conditions. On cold days energy is used to stay warm and on hot days we sweat to keep cool.

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Enzymes in digestion:

Food is made of insoluble molecules your body can’t absorb. Digestion is the break-down of these large insoluble molecules into smaller, soluble molecules. These can be absorbed and used by your cells. Digestion is controlled by digestive enzymes.

Most enzymes work inside cells, but digestive enzymes work outside of cells. They are produced by specialized cells in glands, like salivary glands and the pancreas, as well as in the lining of your gut.The enzymes pass out of these specialized cells into the gut itself. They break down food molecules in the gut.

The gut is a hollow muscular tube which squeezes food. It helps to break up food into small pieces and give them a large surface area for the enzymes to work on. It mixes food with digestive juices so enzymes come into contact with as much as the food as possible. It uses its muscles to move food along the area of the gut.

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- Enzymes which break down carbohydrates = carbohydrases

Starch is the most common carbohydrate eaten. It is broken down into sugars like glucose. - Carbohydrases which turns starch to glucose = amylase

Amylase is produced in the salivary glands. So starch is digested a bit in the mouth. Amylase is also made in the pancreas and small intestine. No digestion happens in the pancreas. The enzymes made in the small intestine flow into the small intestine. Most of the starch is digested in the small intestine.

Digesting proteins: (meat, fish, cheese, etc) - Protein foods are broken down into amino acids, catalysed by protease enzymes. Proteases are produced in the stomach, the small intestine and the pancreas. The breakdown of proteins into amino acids happens in the stomach and the small intestine.

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Digesting fats:

Fats and oils are called lipids. Lipids are broken down into fatty acids and glycerol in the small intestine, catalysed by lipase enzymes. Lipase enzymes are made in the pancreas and the small intestine. These enzymes are made in the pancreas but no digestion happens here as they flow into the small intestine.

Once food molecules have been digested (into soluble glucose, amino acids, fatty acids, glycerol) they leave the small intestine. They pass into the blood supply and are carried around the body to the cells which need them.

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Using the digested food:

Glucose – used by cells for respiration

Fatty acids and glycerol – used as a source of energy, to build cell membranes, to make hormones or as fat stores.

Amino acids – built up into proteins needed once in the cells (protein synthesis). These synthesis reactions are catalysed by enzymes (protein molecules).

Protein synthesis takes places in the ribosomes.

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Speeding up digestion:

Digestive enzymes are sensitive to pH and temperature. Average body temperature is 37˚C. Different enzymes work better at different pH levels, where they can catalyse reactions as fast as possible.

The protease enzyme in the stomach works best in acidic conditions.

The protease made in the pancreas works best in alkaline conditions.

Pepsin = protease from the stomach (acidic)

Trypsin = protease made in the pancreas and used in the small intestine (alkaline)

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Changing pH in the gut - Around 35 million glands in the lining of the stomach secrete protease enzymes to digest proteins. These protease enzymes prefer acidic conditions, so from the same glands the stomach produces a concentrated hydrochloric acid solution (around 3 litres of acid every day).

The acid lets the enzymes work effectively and kills most of the bacteria taken in with the food.

The stomach also produces a thick layer of mucus. This coats the stomach walls so that they aren’t digested by the acid and the enzymes.

When food leaves the stomach it flows to the small intestine. Some enzymes that act as catalysts in the small intestine are made in the pancreas and some are made in the small intestine. These prefer an alkaline environment.

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The liver produces bile. Bile is a greenish-yellow, alkaline liquid which is stored in the gall bladder. Bile is squirted on the food as it comes into the small intestine from the stomach. Bile neutralises the stomach acid and makes the food semi-alkaline to comply with what the enzymes in the small intestine prefer.

Altering the surface area: The enzymes in the gut work best when the food has a large surface area. Carbohydrates and proteins can be broken down easily, but fats eaten don’t mix with the watery liquids in the gut. Fats stay as large globules, which are hard for lipase enzymes to act on.

Bile emulsifies the fat in food. It breaks up large drops into smaller droplets. This gives lipase a much larger surface area to act on, so they can chemically break down the fats more quickly into fatty acids and glycerol.

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Using enzymes - Some microorganisms produce enzymes which pass out of the cells (easy to use) or we have to use the whole microorganism.

Enzymes in the home: Biological detergents – remove stains from clothes (like grass, sweat, food and blood). These detergents contain protease and lipase enzymes which break down the proteins and fats in the stains. The enzymes work best at lower temperatures so they save electricity and money.

Enzymes in industry: Pure enzymes (have passed out of cells)…

Protease – used in baby food. Pre- digest some of the protein in food. This makes the food easier for a baby’s digestive system to cope with and is easier for them to get the needed amino acids.

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Carbohydrases – used to convert starch into sugar (glucose) syrup. Sugar syrup used in food production.

Starch is made from plants (corn) so is very cheap. The conversion to sugar syrup saves money. Starch is also used to make the fuel ethanol from plants.

The glucose syrup from starch can be converted into fructose syrup by isomerase enzymes.

They contain the same amount of energy (1700 kJ/100g) but fructose is sweeter, so less is needed for the same sweetness in food.

Fructose is used in slimming foods as the food tastes sweet but has fewer calories.

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Advantages and Disadvantages of using enzymes:

In industrial processes… Most reactions need high pressures and temperatures to be fast enough. Building chemical plants which can withstand high pressures and supplying heat is expensive.

Enzymes don’t have this problem. They catalyse reactions at (relatively) low temperatures and normal pressures. So the processes are cheaper to run.

But… enzymes are very sensitive to their environment. The temperature must be kept down (below about 45˚C) and the pH within carefully monitored limits of what the enzyme works best at. It is expensive to control these conditions.

Whole microorganisms are (relatively) cheap but they need to be given food and oxygen and have their waste products removed. They use some of the substrate to grow more microbes, so are less efficient. Pure enzymes use the substrate more efficiently, but are more expensive to produce.

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This is below average .......... sorry ! :)

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