AS/A-level Biology - The Digestion System

Ingestion

• Food enters through the mouth and down the oesophagus.

Digestion

• Food gets broken down with enzymes in the stomach and the colon.

Absorption

• Food gets absorbed through the epithelial cells all around the digestive system.

Egestion

• Waste material gets ejected from the anus after being stored in the rectum.

Salivary Glands

• Saliva is produced which contains amylase.
• Amylase breaks down starch into maltose.
• Saliva lubricates food making it easier to pass through the oesophagus.
• Saliva is pH7 for the optimum pH of amylase.

Oesophagus

• Relatively large circular and longitudinal muscle layer as peristalsis (the squeezing movement of the oesophagus to push food) is needed to push food down.
• Used only for transportation.
• Only mucus producing cells to protect the lining.

Stomach

• Muscular walls and enzymes help with digestion.
• Pepsin is secreted here.
• There are no villi.
• Mucus layer to prevent self digestion.

Pancreas

• Secretes enzymes: protease, lipase, amylase.
• Pancreatic juice also contains pancreatic salts to neutralise acidity from the stomach.

• The lining is folded into villi with many microvilli to increase surface area for absorption.
• Ileum.
• Villi with microvilli.
• Epithelial cells have many mitochondria to provide energy for active transport.
• Rich blood supply to remove absorbed molecules away quickly, maintaining a diffusion gradient.
• Each villus contains a lymph vessel for removal of absorbed lipids.
• Thin epithelial layer for short diffusion path.
• Bile is entered as soon as the stomach ends to neutralise the stomach acid.
• Bile is made in the liver and stored in the gall bladder.

• Absorbs water.
• The food therefore becomes drier and forms faeces.

• Faeces are stored here.
• They are then removed via the anus.

• Mastication (chewing by the teeth)
• Churning (in the stomach)
• Food is broken down which provides a larger surface area for chemical digestion.

• Breaks down large insoluble molecules into smaller soluble ones by hydrolysis.
• All digestive enzymes function by hydrolysis.
• The large food molecules are first hydrolysed into monosaccharide's and are then absorbed into the blood.
• Once absorbed, assimilation takes place.

• Proteins contain the elements carbon, hydrogen, oxygen and nitrogen.
• They are major structural components in tissues making them vital for growth.

• Proteins are made up from 20 different amino acids which can be assembled in any order.

• Amino acids join together by a condensation reaction to form Dipeptides.
• A peptide bond is formed.

• Made by adding more amino acids.
• Each polypeptide has its own unique sequence of amino acids. This is known as the primary structure of the protein.
• By changing just one amino acid in a polypeptide chain the complete properties of the molecule can be changed.

Do you still remember the structure of proteins in AS/A-level Biology?

1. Primary structure

• The sequence of amino acids that make up a polypeptide chain.

2. Secondary structure

• The polypeptide chain twists into a shape called the secondary structure
• There are two main secondary structures: α-helix & β-pleated sheet
• Structure are held in place by hydrogen bonds

3. Tertiary structure

• The secondary structure is folded into a 3D shape which is held by weak chemical bonds
• In some proteins, the tertiary structure forms fibrous proteins which are insoluble
• In other proteins it can form a globular protein which is more spherical, like enzymes and hormones.

4. Quaternary structure

• Two or more polypeptide chains and non-protein groups combine.

• If the bonds which hold a protein in shape are broken, the structure of said protein will be changed.
• Denaturation can be caused by high temperature, extremes of pH and reducing agents.

• The biuret test is a test for proteins.
• E.g. Sodium hydroxide and copper sulphate are added to the sample
  - if the test turns lilac, protein is present.

References:

1. http://www.astrochem.org/sci/Amino_Acids.php

That's the end of the topic!

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