[BIO] Chapter 4 - Nutrition in Human

© Hee Xin Wei (Copyrighted) Topic 4 : Nutrition in Human

Chapter Analysis FOCUS EXAM WEIGHTAGE •heavy content but straightforward •commonly tested in MCQ and structured questions •Constitute to around 5% in Paper 2 in the past 5 years

Digestive System Digestion Key Concept Ingestion Digestion Absorption Assimilation Egestion Carbohydrate, proteins and fats are broken down into soluble glucose, amino acids, fatty acids and glycerol

Overview of digestive system 1.Mouth and buccal cavity (a)Teeth: chewing, breakdown large pieces of food to smaller size. •Increase surface area to volume ratio of the food that enzyme can act on more efficiently. (b)Salivary glands: Secrete saliva into mouth via salivary ducts. •saliva contains salivary amylase that digest starch to maltose (c)Tongue: mix food with saliva into bolus 2.Oesophagus -A narrow muscular tube that connects the buccal cavity and stomach -It is made up of two layers of muscle. The external layer is the longitudinal muscle and the inner layer is the circular muscle.

Overview of digestive system Peristalsis -Rhythmic wave-like contractions of the muscles -Peristalsis allows mixing of food and digestive juices and propel the contents of the alimentary canal -The circular and longitudinal muscles are antagonistic muscles. -When the circular muscles contract, the longitudinal muscles relax. As a result, the wall of the gut constricts, and the gut becomes narrower and longer. The food is pushed forward. -When the longitudinal muscles contract, the circular muscles relax. The gut dilates, and it becomes wider and shorter. This widens the lumen for food to enter. -These muscles found along much of the entire length of the alimentary canal (oesophagus, stomach, small intestine, large intestine) Peristalsis

Overview of digestive system Stomach wall has pits that lead to gastric gland, which secretes (1) mucus which protects the stomach walls (2) gastric acid/dilute hydrochloric acid •Gastric acid is pH 2, denatures salivary amylase thus stop its activity •changes the inactive pepsinogen into the active form, pepsin •provides an acidic environment as pepsin optimum pH is 2 •kills germs and bacteria. (3) pepsinogen (inactive form) •dilute HCl activates pepsinogen to pepsin •pepsin digest proteins to polypeptides -Food normally remains in the stomach for about three to four hours. The partly digested food becomes liquefied, forming chyme. -The pyloric sphincter is located at the place where the stomach joins the small intestine. When the muscle contracts, the entrance to the small intestine closes. When the ring relaxes, chyme passes in small amounts into the duodenum Stomach Stomach Oesophagus Oesophagus sphincter Pyloric sphincter Duodenum

Overview of digestive system BILE •Bile is produced by the liver and stored in the gall bladder. •It passes into the small intestine through the bile duct. •Bile emulsifies large fat