Notes on IB Biology SL - Digestion

6.1 Digestion

6.1.1 Explain why digestion of large food molecules is essential

·      Food molecules are too large (insoluble)

·      Unable to pass through the walls of the villi

·      Useful substances cannot be absorbed

6.1.2 Explain the need for enzymes in digestion

·      Work as biological catalysts – globular proteins that increase the rate of reaction by lowering activation energy (This reaction does not require high temperatures so it is able to take place at body temperature)

·      Speed up the digestive process

·      (chemical) break down of large food molecules

·      make soluble products to be absorbed

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.

	Amylase	Protease	Lipase
Enzyme	Salivary Amylase	Pepsin	Pancreatic Lipase
Source	Salivary glands	Stomach	Pancreas
Substrate	Carbohydrates/starch	Proteins	Lipids/triglycerides
Product	Maltose	Polypeptides	Fatty acids and glycerol
Optimum pH	pH 7-7.8	pH 2	pH 7

6.1.4 Draw and label a diagram of the digestive system

6.1.5 Outline the functions of the stomach, small intestine and large intestine.

Function	Stomach	Small intestine	Large intestine
1	Secretes Hydrochloric Acid to kill microorganisms	Neutralizes the chime (food after leaving the stomach) to provide an optimum pH for enzymes to work in and to protect the walls of the small intestine. (Since Bile and Pancreatic juice are both alkali)	Moves material that has not been digested along (by contracting and relaxing: peristalsis)
2	HCl provides an optimum pH for pepsin to work in	Pancreatic enzymes are released	Absorbs water
3	Mechanical digestion – Churning (mixes food with enzyme)	Duodenum: To digest starch into maltase (Maltase à glucose), proteins and fats	Forms faeces
4	Secretes pepsin to digest proteins into amino acids (polypeptides).	Ileum: To absorb digested molecules into the villi

6.1.6 Distinguish between absorption and assimilation.

Absorption: Digested food molecules pass into blood

Assimilation: Enzymes to synthesize new molecules – incorporate substances into new membranes

6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Structure	Function
Villi	Increase S.A. for absorption - Surrounded by a network of capillaries to absorb useful substances such as glucose, amino acids and minerals (short diffusion distance – maintain concentration gradient) Large surface area to volume ratio
Lacteal	To absorb fats or fat soluble vitamins (A and D)
Epithelium is one cell thick	Short diffusion path from lumen into blood à increases the efficiency
Mitochondria	Provides ATP energy during aerobic respiration
Microvilli	Provides a larger S.A. allowing faster diffusion/ active transport (Often facilitated diffusion – needs a channel protein – no energy required)
Protein channels in microvilli	Channels: Facilitated diffusion Pumps (carrier proteins): Active transport

the villus has a large surface area to volume ratio;

microvilli increase surface area for absorption;

 thin surface (epithelial) layer so products of digestion can pass easily through;

channel proteins located in plasma membrane used for facilitated diffusion;

network of capillaries inside each villus (so only short distance) for movement of absorbed products;

capillaries transport absorbed nutrients/sugars and amino acids away from small intestine;

blood flow in capillaries maintains concentration gradient;

central lymph vessel/lacteal to transport absorbed fats/fatty acids away from small intestine;

large number of mitochondria provide ATP needed for active transport;

 protein pumps in membrane (of microvilli) carry out active transport;

 pinocytosis occurs at surface (epithelial) layer;