Digestion

Mechanical digestion 

• Crushing action of teeth breaks down large pieces of food into smaller pieces to reduce its size and increase its surface area.

 Chemical digestion 

• Salivary glands produce saliva which contains amylase and mucus.
• Amylase hydrolyses starch to glucose and maltose.
• Mucus acts as a lubricant and protects the gut walls.

• The bolus of food moves from the mouth to the stomach.
• The circular muscles contract behind the bolus which pushes it downwards.
• The circular and longitudinal muscles alternatley contract and relax in a series of contractions called peristalsis.
• Goblet cells in the mucosa secrete mucus for lubrication and protection against enzyme action. 

Mechanical digestion 

• The stomach has three layers of muscle which churn food into liquid. 

Chemical digestion - gastric juice

• Pepsin is secreted by Chief cells as an inactive precursor called pepsinogen. 
• Hydrocloric acid is secreted by parietal cells. It kills bacteria, activates pepsinogen and provides the optimum pH for pepsin. 
• Mucus, secreted by goblet cells, protects stomach wall from the acid and enzymes. 

• Brunner's glands in the sub-mucosa secrete mucus and enterokinase which activates the inactive curosor trypsinogen to the active enzyme trypsin. 
• Liver produces bile which enters the duodenum through the bile duct. 
• Bile salts emulsify large lipid droplets into smaller lipid droplets. Also contains bicarbonate salts which are alkaline and help neutralise the stomach acid. 
• Pancreas secretes enzymes into the duodenum via the pancreatic duct. 
• Pancreatic amylase completes the digestion of starch and glycogen to glucose and maltose. 
• Lipases to hydrolyse the lipids to fatty acids and glycerol. 
• Inactive precursor trypsinogen. 
• Exopeptidases. 
• Epithelial cells of the villi in the duodenum and ileum produce enzymes. These may be found in the plasma membrane of epithelial cells, secreted from the epithelial cells, or within the cytoplasm of the epithelial cells. 

• The colon absorbs the remaining water. 
• It also makes and absorbs vitamin K and folic acid. 
• Undigested cellulose, bacteria and rubbed off cells pass out eith the faeces. 
• the cellulose bulks the faeces and stimulates peristalsis.