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IB Biology Topic 6: HumanPhysiology
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IB TOPIC 6 | HUMAN PHYSIOLOGY
2016 | SYJ0014
Topic 6.1: Human physiology – Digestion and absorption
The structure of the wall of the small intestine allows it to move, digest and absorb food.
• Understanding: The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the
gut.
Smooth muscles: muscles that exerts continuous moderate force that consists of relatively short cells not elongated fibres
Circular muscle: constricts the gut to prevent it from being pushed back towards the mouth
Longitudinal muscle: moves the food along the gut
Peristalsis: wave of muscle contraction that occurs in one direction
Function: churns the semi-digested food with the enzyme for digestion
Purpose: moves the food along the gut to initiate digestion process
Food moves very slowly in the intestine via peristalsis to allow time for digestion
• Understanding: The pancreas secretes enzymes into the lumen of the small intestine.
Function of the pancreas:
Secretion of hormone: insulin and glucagon
Secretion of digestive enzymes: amylase (starch), lipase (lipids), proteases (proteins)
Secretion of digestive enzymes is controlled by hormones secreted by the stomach
Ducts within pancreas merge into larger ducts to form one large pancreatic duct for secretion
• Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.
The wall of the small intestine produce enzymes (both immobilized enzymes and as intestinal juice)
Types of enzymes produced by intestine: nuclease, maltase (maltose), lactase (lactose), sucrase (sucrose)
Substance that cannot be digested: cellulose
• Understanding: Villi increase the surface area of epithelium over which absorption is carried out.
Absorption: process of taking substance into cells and the blood
Assimilation: conversion of nutrients into solid or liquid parts of the organism
Digested materials need to be absorbed to be used in cell processes
Rate of absorption is proportional to the surface area of the epithelium
Villi: small finger-like projection of the mucosa on the inside in intestinal wall
Surface area: increases surface area of the absorption surface (by about a factor of 10)
Size: between 0.5mm and 1.5mm long
• Understanding: Villi absorb monomers formed by digestion as well as mineral ions and vitamins.
Absorbed products of digestion by t he villi:
Monosaccharides: glucose, fructose, galactose
Amino acids: broken down from twenty; any of the twenty
Lipids: fatty acids, glycerides and glycerol
Nucleic bases: from digestion of nucleotides
Absorbed products required by the body by the villi:
Mineral ions: calcium, potassium, sodium etc.
Vitamins: ascorbic acid (vitamin C) etc.
• Understanding: Different methods of membrane transport are required to absorb different nutrients.
Molecule Transport mechanism Transported
Monoglyceride Simple diffusion Lacteal
Long chain (units forms triglyceride
fatty acid in epithelial cells and is
transported)
Short chain Simple diffusion Blood capillary
fatty acids
Glucose and Secondary active transport Blood capillary
galactose (facilitated coupled with active
building of sodium gradient)
Amino acids Secondary active transport
(facilitated transport coupled with
active building of sodium gradient)
Dipeptides and Active transport
tripeptides (facilitated transport coupled with
active building of hydrogen gradient)
Fructose Facilitated diffusion
Lipoprotein complex: forms in the epithelium when phospholipids and proteins covers reformed triglyceride and cholesterol
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IB TOPIC 6 | HUMAN PHYSIOLOGY
2016 | SYJ0014
• Application: Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the
liver.
Starch: a long chain of α-glucose molecules that consists of 1,4 bonds and 1,6 bonds (specific in amylopectin)
Enzyme Target bond Target reactant Product
Amylase 1,4 α-glucose bonds Starch (chain of at least four α-glucose molecules) Maltase, maltotriase, dextrinase
Maltase 1,4 α-glucose bonds Maltose (fragment two α-glucose) α-glucose
Maltotriase 1,4 α-glucose bonds Maltotriose (fragment of three α-glucose) α-glucose
Dextrinase 1,6 α-glucose bonds Dextrin (fragment of amylopectin containing 1,6 bonds) α-glucose
Process of glucose transport and storage in liver:
Villus epithelium: glucose is absorbed into villus epithelium by co-transport with sodium ions
Blood capillary: glucose enters the porous blood capillary through diffusion
Venules: glucose in blood capillary flows through to the venules in the sub-mucosa
Hepatic portal vein: venules then flows to the hepatic portal vein to the liver
Liver: excess glucose is absorbed and stored by glycogen or vice versa
• Application: Use of dialysis tubing to model absorption of digested food in the intestine.
Dialysis tube: semi-permeable tube that allows molecules of smaller size than the pore to pass through
Method of observation in function of intestine through dialysis tubes:
Dialysis tube filling: dialysis tube is filled with solution of starch and glucose
Dialysis tube immersion: dialysis tube is then immersed into a beaker of distilled water
Observation and record: after one hour, only glucose is present in water
• Skill: Production of an annotated diagram of the digestive system.
Structure Function
Mouth Control of eating and swallowing; mechanical digestion of food and initiating chemical
digestion by mixing food with saliva
Esophagus Movement of food from mouth to stomach through peristalsis
Stomach Killing of foreign bacteria by mixing with water and acid and initial stage of protein digestion
Pancreas Secretion of enzymes (amylase, protease, lipases)
Liver Secretion of surfactants bile to digest lipids and for neutralization
Gall bladder Storage and regulation of bile release
Small intestine Production of enzymes, final stages of digestion (of carbohydrates, lipids, proteins and
nucleic acid) and stages of absorption of nutrients
Large intestine Re-absorption of water, further digestion by symbiotic bacteria
• Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.
Structure Function
Epithelial layer Selective absorption of nutrients
Mucosa Lining of the small intestine that produces mucus that protects cells
Submucosa Tissue layer containing blood and lymph vessels
Circular muscle Cause waves of contraction; peristalsis
Longitudinal muscle Moves food along the gut and aids peristalsis
• Nature of science: Use models as representations of the real world—dialysis tubing can be used to
model absorption in the intestine.
• Utilization: Some hydrolytic enzymes have economic importance, for example amylase in production of sugars from starch and in the brewing of beer.
• Guidance: Students should know that amylase, lipase and an endopeptidase are secreted by the pancreas. The name trypsin and the method used to
activate it are not required.
• Guidance: Students should know that starch, glycogen, lipids and nucleic acids are digested into monomers and that cellulose remains undigested.
• Guidance: Tissue layers should include longitudinal and circular muscles, mucosa and epithelium.
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