Digestive & nervous system

• Ingested- large molecules are taken into the mouth. Masticated- chewed and mixed with saliva. Digested- molecules are chewed, crushed and churned. Absorbed- only small molecules and ions can be absorbed through the alimentary canal. Egested- defaecation.
• Mechanical digestion- increasing the surface area where enzymes can act on food particles increases the rate of digestion. Chemical digestion- monomers are broken down by hydrolysis reactions, enzymes are used to catalyse reactions.
• mouth- ingestion, digestion of starch. Oesophagus- carriage of food to the stomach. Stomach- digestion of protein. Duodenum- digestion of carbohydrates, fats and proteins. Ileum(small)- digestion of carbohydrates, fats and proteins and absorption of digested food. Colon(large)- absorption of water. Rectum- storage of faeces. Anus- egestion.

  
• Serosa- outer layer, thin, protects gut wall and reduces friction between organs. Longitudinal muscle-relaxes to move organs, outer layer. Circular muscle- inner layer, contracts. Submucosa- connective tissue(collagen&elastin), removes absorbed products. Mucosa- contains goblet cells and secretes mucus.

  
• Carbohydrates- they are digested in the mouth, stomach and small intestine. They are used for energy storage and structural reasons. Carbohydrase enzymes breaks down starch into sugars.
• Fats- digestion of fat in the small intestine is helped by bile, breaking it into smaller pieces. Bile emulsifies fats, increasing surface area of the fats.
• Proteins- digested in the stomach and small intestine, this is helped by stomach acid. Endopeptidase- enzymes that hydrolyse peptide bonds within the protein molecules. Exopeptidase- hydrolyse peptide bonds at the end of shorter polypeptides to release amino acids.
• The mouth- saliva dissolves soluble parts of food. Mucus helps food move down the oesophagus. Salivary amylase- begins starch breakdown. Peristalsis- wave of contraction and relaxation.
• The oesophagus- has no role in digestion, carries food to the stomach.
• The stomach- muscular wall contracts when food enters and relaxes to churn(mix) food. A sphincter is at each end of the stomach to keep food inside.
• Structure of the stomach- there are gastric pits created by folds in the epithelium, this secretes gastric juice. There are chief cells which release pepsinogen. Parietal cells secrete hydrochloric acid(lowering pH). Mucosa secretes mucus which coats the stomach lining.
• Duodenum- pancreatic duct and bile duct enter here. There are Brunner Glands that produce intestinal juice (neutralising contents).
• Bile- neutralises acidity of gastric acid, providing the optimum pH. Bile salts are derived from cholesterol. Bile emulsifies lipids by lowering surface tension and surface area. It contains hydrogen carbonate ions to neutralise acidity from the stomach.
• Pancreatic juice- contains enzymes that break down carbohydrates, lipids and proteins. The endocrine glands secretes hormones, with a group of cells called Islets of Langerhans. Exocrine glands secrete substances into a duct, producing pancreatic juice.
• Ileum- each cells has microvilli (tiny folds to increase surface area). The ileum contains goblet cells(secrete mucus) and paneth cells that destroy pathogens.
• Endopeptidases and exopeptidases are secreted by cells at the villus tips into the gut lumen and continue the digestion of polypeptides. Disaccharides are absorbed into the epithelial cells of the villi.
• Adaptations of the small intestine- peristalsis ensures food moves through the gut(increases concentration). The circulation of blood helps maintain the diffusion gradient and decreases concentration. Columnar epithelial cells mean there's a short distance between the gut lumen and blood. Epithelial cells have a lot of mitochondria.
• Amino acids- absorbed into the epithelial cells by active transport, requires ATP, passes into the capillaries by facilitated diffusion.
• Glucose- passes into the epithelial cells with sodium ions, they move into the capillaries by active transport(sodium) and facilitated diffusion(glucose).
• Fatty acids and glycerol- they diffuse into the epithelial cells, ending lymph capillaries in the villi. The lacteals transport fat.
• Colon- undigested food, mucus, bacteria and water pass into the colon and is absorbed into inorganic ions and water. At the end of the colon the material is semi-solid, dietary fibre is indigestible and is passed out of the body.