[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 droplets into smaller fat droplets, increasing the surface area to volume ratio of the fats for lipase to work on and speeds up fat digestion Small Intestine •The small intestine is divided into three parts: the duodenum, jejunum and ileum. •In the duodenum, chyme from the stomach mixes with 1.Pancreatic juice secreted from pancreas 2.Intestinal juice secreted by epithelial cells of small intestine 3.Bile from gall bladder •all three juices are alkaline, thus they neutralise the acidic chyme from stomach and provide an optimum alkaline environment for pancreatic and intestinal enzymes

Overview of digestive system •Enzymes found in pancreatic juice: trypsinogen, pancreatic amylase and pancreatic lipase •Enzymes found in intestinal Juice: enterokinase, erepsin, maltase, lactase, sucrase, intestinal lipase Small Intestine Proteins Trypsinogen (inactive) Trypsin (active) Proteins Polypeptides Polypeptides Amino acids enterokinase erepsin / peptidases trypsin Carbohydrate Starch Maltose Maltose Glucose + Glucose Lactose Glucose + Lactose Sucrose Glucose + Fructose pancreatic amylase maltase lactase sucrase Fats Fats Fatty acids + glycerol pancreatic lipase/ intestinal lipase

Absorption Key Concept Ingestion Digestion Absorption Assimilation Egestion Glucose, amino acids, fatty acids and glycerol are absorbed into body cells

Absorption •Nutrients are absorbed across the wall of the small intestine and into the capillaries or lacteal. •Glycerol and fatty acids are absorbed by the epithelial cells of the villi and recombined within those cells to form fats, which are transported into a lacteal. •Glucose and amino acids are absorbed by diffusion or active transport into the blood capillaries depending on the concentration gradient. •Water is absorbed by passive diffusion throughout the length of the small intestine and mineral salts are absorbed in the ileum.