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Biology · Enzymes and Digestion

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Biology · Edexcel Paper 1

Enzymes and Digestion

Levels of Organisation and the Digestive System

An organ is made of different tissues, and an organ system is made of different organs, all working together to do a job.

  • Tissue A group of similar cells that work together to do a particular job.
  • Organ A group of different tissues that work together to do a particular job.
  • Organ system A group of organs that work together to do a particular job.
Five levels of organisation from a single muscle cell to a whole human, each in its own panel, with a summary box.
Each level is made from the one before it, so a muscle cell is one small part of a whole human.

The journey of a meal

  • Mouth Teeth cut and grind the food. Saliva wets it and starts to digest starch.
  • Oesophagus Muscles squeeze the food down to the stomach.
  • Stomach The food is churned with acid and protease, which starts to digest protein.
  • Small intestine Bile from the liver and enzymes from the pancreas and the small…

Enzymes

An enzyme is a biological catalyst. Its active site fits only its own substrate, and a denatured enzyme cannot catalyse the reaction.

  • Enzyme A biological catalyst, made of protein, that speeds up a reaction in a living thing.
  • Catalyst A substance that speeds up a chemical reaction and is not used up.
  • Substrate The substance that an enzyme acts on.
Four stages of the lock and key model: the substrate approaches, fits the active site, is split into two products, and the products leave while the enzyme is unchanged.
The substrate fits the active site like a key in a lock. After the reaction the enzyme is unchanged and can be used again.

Below and above the optimum temperature

  • Below the optimum The rate rises as the temperature rises; The particles move faster and collide more often; The enzyme keeps its shape
  • Above the optimum The rate falls as the temperature rises; Bonds in the enzyme break; The active site changes shape: the enzyme is denatured

Digestive Enzymes and Bile

Carbohydrases break down carbohydrates into simple sugars, proteases break down proteins into amino acids.

  • Carbohydrase An enzyme that breaks down carbohydrates into simple sugars.
  • Amylase A carbohydrase that breaks down starch into simple sugars.
  • Protease An enzyme that breaks down proteins into amino acids.
Starch is broken into simple sugars by amylase, protein into amino acids by protease, and fat into glycerol and fatty acids by lipase.
Each enzyme breaks one kind of large molecule into its small building blocks.

Digestive enzymes and bile

  • Digestive enzymes Made by glands and by the lining of the gut; Are catalysts that break down large molecules chemically; Specific: each one works on one type of molecule
  • Bile Made in the liver and stored in the gall bladder; Is not an enzyme, and does not break down molecules; Alkaline, so it neutralises the acid from the stomach
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Biology · Edexcel Paper 1

Enzymes and Digestion continued

Food Tests (Separate only)

Iodine turns blue-black with starch. Benedict's reagent, heated, turns from blue to brick red with sugar.

  • Reagent A chemical used to detect another substance because it changes colour or appearance.
  • Qualitative test A test that shows whether a substance is present, but does not measure how much.
  • Iodine solution The reagent that turns blue-black when starch is present.
Four food tests showing a test tube before and after: starch turns blue-black with iodine, sugar turns brick red with Benedict's reagent, protein turns purple with Biuret reagent.
A positive result is a change from the original colour. A negative result is no change.

Carrying out Benedict's test for sugar

  • Add the sample Put about 2 cm³ of the food sample into a test tube.
  • Add the reagent Add about 10 drops of Benedict's reagent. The mixture is blue.
  • Heat it safely Put the test tube in a hot water bath, at about 75 °C.
  • Watch for a colour change If there is sugar the mixture goes green, yellow, orange or…

Measuring the Energy in Food (Separate only)

How much energy is stored in a peanut or a slice of bread?

  • Calorimetry A method of measuring the energy released by a reaction, by finding out how much it heats a known mass of water.
  • Calorimeter The apparatus used to measure the energy released when a substance is burned.
  • Joule The unit of energy, abbreviated J. There are 1000 J in a kilojoule.
A boiling tube of water with a thermometer, held in a clamp above a burning piece of food on a mounting needle.
The energy released by the burning food heats the water, and the temperature rise shows how much energy there was.

The calorimetry method

  • Measure the water Use a measuring cylinder to put a known volume of water in a…
  • Record the starting temperature Clamp the tube and measure the temperature of the water with a…
  • Weigh the food Find the mass of a small piece of dry food, such as a nut, then…
  • Set the food alight Light the food in a Bunsen flame, then quickly hold it directly…

Investigating the Effect of pH on Amylase

The rate of an enzyme-controlled reaction is fastest at the optimum pH.

  • Continuous sampling Taking a small sample at regular intervals during a reaction and testing each one.
  • Buffer solution A solution that keeps the pH constant at a particular value.
  • Spotting tile A tile with small wells, used to test many small drops one after the other.
A water bath with a test tube and thermometer, a dropping pipette above a spotting tile with eight iodine drops that change from blue-black to orange-brown, and a stopwatch.
A drop of the mixture is tested every 30 seconds. The time at which the iodine stays orange-brown is the time taken for the starch to be digested.

The variables in this investigation

  • Independent variable What you change; The pH of the buffer solution; Five values: pH 5, 6, 7, 8 and 9
  • Dependent variable What you measure; The time for the starch to be digested; Used to work out the rate of reaction

The method

  • Prepare the tile Put a drop of iodine solution into each well of a spotting tile.
  • Set up the tubes Put 2 cm³ of amylase solution and 2 cm³ of the pH 5 buffer into…
  • Start the reaction Pour the starch into the amylase and buffer, mix.
  • Sample every 30 seconds Use a dropping pipette to take a drop of the mixture and add it…