One interface with the environment is the digestive system. The digestive system is composed of the:

• oesophagus
• stomach
• small intestine
• large intestine
• rectum

It’s also composed of two glands:

• the salivary glands
• the pancreas

The process of digestion involves large molecules being broken down into smaller ones in a process known as hydrolysis. These smaller molecules can than then be absorbed by the body and assimilated.

See below for a summary of the key notes on the Digestive System for your As and A level biology revision :

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REMEMBER IT!

• Digestion involves hydrolysis, absorption and assimilation.
• Proteins are composed of amino acids
• They have four structures: primary, secondary, tertiary and quaternary.
• A protein can be either globular or fibrous.
• The biuret test can be used to test for proteins.
• Enzymes are biological catalysts which lower the activation energy required for a reaction to take place.
• Enzymes are composed of an active site which is specific to a substrate. They bond to form an enzyme-substrate (ES) complex which becomes an enzyme-product (EP) once the reaction is complete.
• The lock-and-key hypothesis stated that the enzyme and substrate fit exactly together.
• The induced-fit hypothesis is more widely accepted and states that the enzyme changes shape slightly to allow the substrate to fit.
• An enzyme rate of reaction can be affected by temperature, pH level and substrate concentration.
• Inhibitors slow down the rate of reaction.
• Carbohydrates are composed of monsaccharides which contain carbon, hydrogen and oxygen.
• ?-glucose molecules bond together via glycosidic bonds produced through condensation.
• Someone who’s lactase intolerant cannot produce enough lactase.
• Benedict’s reagent can be used to test for reducing sugars.
• Starch can be tested for with iodine.
• Eukaryotic cells are made up of a number of different organelles.
• Scanning electron microscopes provide high magnification and resolution.
• Cell components can be separated using ultracentrifugation.
• The Fluid Mosaic Model describes the structure of plasma membranes which are composed of phospholipids, proteins and carbohydrates.
• Phospholipids are composed of a hydrophilic, phosphate head and two hydrophobic, fatty acid tails.
• Lipids can be test for using the ethanol emulsion test.
• Proteins have a number of roles: transport, structural, recognition and as enzymes.
• Substances can travel across the membrane by passive transport (simple diffusion, facilitated diffusion or osmosis) or active transport.
• Starch is broken down by enzymes into glucose before being absorbed into the blood stream.
• Glucose is absorbed by coupled active transport.
• Cholera affects the small intestine causes the sufferer to dehydrate by losing too much water through diarrhoea.
• ORS are sugar and salt solutions which can be used to rehydrate cholera sufferers.