AQA Biology GCSE // Topic 2 - Organisation

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  • Created on: 07-10-21 13:10

Topic 2 - Organisation

A group of similar cells are organised into tissues.

A group of different tissues are organised into organs which perform a certain function  together.

A group of organs working together to perform a specific function is an organ system.


Enzymes are biological catalysts produced by living things.

A catalyst is a substance which increases the speed of a reaction without being used up.

Every enzyme has an active form with a unique shape that fits to a specific substance. They can only catalyse one specific reaction. For enzymes to work, the substrate must fit into the active site. The reaction will not be catalysed if the substrate doesn’t fit. 

The lock and key model can be seen on the diagram above. The active site changes shape a little after the substrate binds it to get a tighter fit. This is called the ‘induced fit’ model of enzyme action.

To work well, enzymes need the right temperature and pH. 

Enzymes reach their peak rate of reaction once the optimum temperature and optimum pH is reached. If the temperature is too hot. Bonds holding the enzyme together break apart, changing the shape of the active site, and causing the enzyme to denature. If the pH is too high, the same effect happens. The optimum pH is often pH 7 (not for all enzymes). 

Investigating Enzymatic Reactions Required Practicals

The enzyme amylase catalyses the breakdown of starch. You’ll use iodine to check if starch is present. If starch is present, the solution will turn from browny-orange to blue-black.

  1. Put a drop of iodine solution into every well of a spotting tile.

  2. Place a bunsen on a heat-proof mat, and a tripod and gauze over the bunsen burner. Put a beaker of water on top of the tripod and heat water until it’s 35C.

  3. Use a syringe to add 1cm³ of amylase and 1cm³ of a buffer solution of pH 5 to a boiling tube. Put the tube into a beaker of water and wait for five minutes.

  4. Use another syringe to add 5cm³ of starch solution to the boiling tube.

  5. Mix the contents of the boiling tube. Start stopwatch.

  6. Use continuous supply to record how long it takes for amylase to break down all the starch. Use a dropping pipette to take a fresh sample every 30 secs. Drop it into a well and observe whether the solution remains browny-orange. If it does remain browny-orange, starch is no longer present.

  7. Repeat the whole experiment with buffer solutions of different pH values.

Equation for Rate of Reaction

Rate = 1000 / Time


Digestive enzymes are used to break big molecules down into smaller ones. Examples of big molecules are starch, proteins and fats.  Smaller, soluble molecules can pass easily through the walls of the digestive system u like big molecules

Carbohydrases convert carbohydrates to simple sugars.

Amylase is a carbohydrase which breaks down starch into maltose. It is made in salivary glands, the pancreas and


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