IB SL BIOLOGY TOPIC 6 HUMAN PHYSIOLOGY COMPLETE STUDY GUIDE 2023

IB BIOLOGY TOPIC 6 HUMAN PHYSIOLOGY COMPLETE STUDY GUIDE 2023 HKEXCEL LTD. 1 Topic 6.1 – Digestion and absorption The human digestive system  The parts of the human digestive system are as follows:  Mouth: Ingestion and chewing of food particles. Saliva is secreted in the mouth which contains the enzyme salivary amylase. This enzymes breaks down amylose (starch).  Esophagus: Swallowing the chewed food.  Stomach: Firstly, the stomach secretes pepsin which starts the digestion of proteins into polypeptides and amino acids. Secondly, it secretes hydrochloric acid which kills bacteria and other harmful organisms preventing food poisoning and it also provides the optimum pH for the enzyme pepsin to work in (pH 1.5 - 2). Mucus is also secreted to protect the linings of the stomach wall against the acid.  Liver: It secretes bile.  Gall bladder: Stored the bile produced by the liver.  Pancreas: Secretes digestive enzymes which are released in the small intestine.  Small intestine: The final stages of digestion occur here. The intestinal wall secretes enzymes and it also receives enzymes from the pancreas. The main function of the small intestine is the absorption of the digested food particles. It possesses many villi which increase the surface area for absorption of these food particles.  Large intestine: The material that has not been digested from the small intestine moves into the large intestine, and water is absorbed. This produces solid faeces which are then egested through the anus. lOMoARcPSD| HKEXCEL LTD. 2 Structure of the small intestine  The parts of the tissue layers in transverse sections of the small intestine are as follows: Digestion in the small intestine  The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.  The muscle contraction occurs in the form of waves as the food passes along the intestine.  The contraction of circular and longitudinal muscle (smooth muscles) of the small intestine helps mechanically mix the food with enzymes and moves the semi-digested food along the gut in a process known as peristalsis.  The food passes through the small intestine very slowly to allow for maximum digestion and absorption of nutrients into the bloodstream.  The pancreas secretes enzymes into the lumen of the small intestine.  Enzymes are also known as biological catalysts that speed up the rate of reaction in chemical digestion.  They catalyze hydrolysis (catabolic) reactions.  A variety of macromolecules are acted upon by these enzymes, including starch, proteins, glycogen and lipids.  The pancreas secretes 3 types of enzymes as part of its pancreatic juice.  The optimal pH for these enzymes is around 7-8 as the pancreatic juice is slightly alkaline (basic).  The three enzymes are; 1. Endopeptidases – breaks the peptide bond between amino acids in polypeptides.