IB DP Biology Topic 6: 6.1 Digestion and absorption: Study Notes

In the Digestion and Absorption unit you will learn how large food particles can not be absorbed through cell membranes that line the small intestine. These large molecules must be broken down into smaller molecules through the mechanical and chemical mechanisms.

This unit will last 3 school days.

​Essential idea:

• The structure of the wall of the small intestine allows it to move, digest and absorb food.

​Nature of science:

​Use models as representations of the real world—dialysis tubing can be used to model absorption in the intestine. (1.10)

• Explain the use of models in physiology research.
• State two examples of model systems used to study digestion.
• State limitations of using model systems in physiology research.

6.1.U1 The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut. 

• Outline the role of peristalsis in the digestive process.​

Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut. Continuous segments of longitudinal smooth muscle rhythmically contract and relax. Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)

• contraction of circular and longitudinal muscle of the small intestine helps mix (mechanical digestion) the food with enzymes and moves the semi-digested food (bolus) along the gut in a process called peristalsis
• These muscles are made up of smooth muscle
• The process by which continual waves of contraction and relaxation (peristalsis) of the circular and longitudinal muscle occur along the muscle layers surrounding the small intestine is controlled by the autonomic nervous system
• Food is transported slowly through the small intestine to allow for maximum digestion and absorption of nutrients

U 6.1.U2 ​The pancreas secretes enzymes into the lumen of the small intestine.  

• List the name and substrate of the three major classes of enzymes secreted by the pancreas.​

• Endopeptidases – example trypsin (breaks apart the peptide bond between amino acids in polypeptides)
• Lipases – catalyzes the hydrolysis of lipids (triglycerides and phospholipids)
• Amylases – digestion of starch.
• Pancreatic juice is alkaline (basic) to allow enzymes to work at an optimal pH (around 7-8 in the small intestine)​

The pancreas is controlled by the enteric nervous system and through hormones produced and released by the stomach. Enzymes are produced by ribosomes in the pancreatic gland cells, excreted by exocytosis into smaller ducts, which converge to form the pancreatic duct. Pancreatic juice flows through the pancreatic duct into the lumen of the small intestine

6.1.U3 ​Enzymes digest most macromolecules in food into monomers in the small intestine. 

• List the name, substrate and product of four pancreatic enzymes that hydrolyze food in the small intestine.
• List the name, substrate and product of six enzymes produced by gland cells in the small intestine wall.
• Describe why enzymes produced by gland cells in the small intestine wall often remain immobilized in the cell membrane.

Enzymes released by the small intestines break down macromolecules into smaller molecules called monomers through catabolic reactions (hydrolysis is the type of specific reaction).

The rate of reaction is actually slow due to the body temperature. Therefore enzymes increase the rate of breakdown by acting as biological catalysts. They work by hydrolysis of bonds to form monomers. Digestion is completed by enzymes located on microvilli and lumen of the small intestine.

The following is a table showing the enzyme, source, substrate, products

• Please note that some carbohydrates such as cellulose cannot be digested by humans as we lack the enzymes needed to break down cellulose
• This is why a number grass eating animals have specialized digestive systems to breakdown cellulose
• Cellulose is insoluble dietary fibre that helps move waste through our digestive system and prevents constipation

6.1.U4 ​Villi increase the surface area of epithelium over which absorption is carried out  

• List three adaptations that increase the surface area for absorption on the small intestine.
• Draw the villi as viewed in cross section.
• Label the following on a diagram of a villi:  capillary, epithelial cell, lacteal, and goblet cell.
• State the function of the following villi structures: capillary, epithelial cell, lacteal, and goblet cell.

​Absorption is the process by which the nutrients of food move from the small intestine into the bloodstream. It requires the digestion of food into monomers that are small enough to pass across cell membranes. The structure of the villus is related to its role in the absorption and the transport of the products of digestion.

• Villi are finger-like projections that make the surface of the small intestine look highly folded. These projections increase the surface area (by about 10X) available for absorption (the process of taking substances into the cells and blood).
• Microvilli are small hair-like projections attached to the villi to further increase surface area. 
• The outermost layer of the villi is thin epithelial cells to allow nutrients to easily move across a short distance into the blood.
• A dense network of capillaries close to the epithelium allow a large amount of nutrients to move into the blood.
• Lacteals, which are a part of the lymphatic system, run up the middle of the villi. The lacteal allows for the absorption of the products of lipid digestion which are not easily absorbed by the capillaries.
• Protein channels and pumps are present in the membranes of epithelium to help with absorptio

6.1.U5 ​Villi absorb monomers formed by digestion as well as mineral ions and vitamins. (Oxford Biology Course Companion page 283)​

• Define absorption.
• List materials absorbed by the villi cells of the small intestine.

Absorption – process where small molecules and nutrients pass into the blood vessels (capillary beds) in the wall of the intestine.

Assimilation – products of digestion that are absorbed into the blood are transported to the various tissues. These molecules are used to build up larger molecules that become part of the structure of the tissue or body.

These products include the following monomers:

• Monosaccharides such as glucose, fructose, and galactose
• Amino acids from the breakdown of proteins
• Nitrogenous bases from the breakdown of nucleotides
• Glycerol and fatty acids, which are the products of lipids are absorbed by the lacteal inside the villi.
• Mineral ions such as sodium, potassium and calcium, and vitamins such as vitamin C, are also absorbed by the villi in the small intestine

6.1.U6 ​Different methods of membrane transport are required to absorb different nutrients. 

• ​List four methods of membrane transport required to absorb nutrients.
• Describe the absorption of triglycerides.
• Describe the absorption of glucose.

• A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)
• Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na+)

Facilitated Diffusion

• Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
• Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)
• Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion

Osmosis

• Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)
• The absorption of water and dissolved ions occurs in both the small and large intestine

Simple Diffusion

• Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane
• Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood

​
​During absorption nutrients from food must pass from the lumen of the small intestine to the cells in the capillaries or lacteals in the villi.

• Many types of transport are used to move different nutrients into and out of the epithelium cells of the villi

These modes of transport will be outlined using two products of digestion.

Glucose

Since glucose has many hydroxyl groups it is a polar molecule and cannot pass through the cell membrane by simple diffusion and therefore relies on different types of facilitated diffusion in order to move into and out of the epithelial cells of the villi. (The numbers 1-4 here, match with numbers on the two diagrams above and below).

1)    As seen above Na+ is pumped out of the cytoplasm of the epithelial cells into the interstitial space inside the interstitial by sodium/potassium pumps.

• This creates a concentration gradient between the lumen and the cytoplasm of the epithetical cells.
• This means Na+ ions want to diffuse into the epithelial cells.
• Co-transport proteins in the membrane of the microvilli, allow a sodium ion and a glucose molecule to be transported together into the epithelial cell.

This type of facilitated diffusion is passive, but requires active transport of the Na+ ions out of the cell to create the concentration gradient.

2)    Specific glucose channels allow glucose to diffuse from the epithelial cells into the blood cells of the capillaries

3)    Fatty acids, glycerol and monoglycerides, which are products of lipid digestion can diffuse into epithelial cells from the lumen by passive diffusion

4)    Inside the epithelial cells, fatty acids and monoglycerides reform into triglycerides, and therefore can’t move back out into the lumen because of their size. These lipids combine together with proteins and phospholipids to form lipoproteins. The lipoproteins are then excreted by exocytosis, enter the lacteal and are carried away by the lymph

​6.1.A1 Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver. (Oxford Biology Course Companion page 285)​

• Describe the structure of starch.
• Outline the source, function and specificity of amylase.
• Outline the digestion of maltose, maltotriose and dextrins into glucose.
• Describe absorption of glucose by villus epithelial cells.
• Describe transport of glucose into and through villi capillaries.

​​Starch is a polysaccharide composed of glucose monomers and accounts for ~ 60% of the carbohydrates consumed by humans. Starch can exist in one of two forms – linear chains (amylose) or branched chains (amylopectin)

• Many catabolic reactions take place in the small intestine
• These reactions are catalyzed by a number of enzymes that break down starch into smaller disaccharides and trisaccharides, which are further broken down into monosaccharides.
• These reactions need to occur since the starch molecule is much too large to pass through the membranes of the small intestine
• Starch-a long chain of α (alpha) glucose molecules used as a glucose storage by plants
• Starch consists of two types of molecules, amylose which linear and amylopectinwhich is branched (look at your notes from topic 2)
• Amylose 1,4 bonds can be broken apart by amylase to form the disaccharide maltose and the trisaccharide maltotriose
• Amylose cannot break the 1,6 bond seen in amylopectin
• These larger fragments containing this 1,6 bond of amylopectin are called dextrins
• These dextrins are further broken down by another enzyme into maltose
• Finally, all the maltose is hydrolyzed into glucose by maltase in order for it to be transported from the lumen of the small intestine into the blood in the capillaries surround the small intestine

6.1.A2 ​Use of dialysis tubing to model absorption of digested food in the intestine. 

• ​Explain the use of dialysis tubing as a model for the small intestine.​

• It breaks down insoluble molecules into smaller subunits which can be readily absorbed into body tissues
• It breaks down inert molecules into usable subunits which can be assimilated by cells and reassembled into new products

The size-specific permeability of cell membranes can be modelled using dialysis tubing (Visking tubing)

• Dialysis tubing contains pores typically ranging from 1 – 10 nm in diameter and is semi-permeable according to molecular size
• Large molecules such as starch cannot pass through the tubing, however smaller molecules (such as maltose) can cross
• Unlike the membranes of living cells, dialysis tubing is not selectively permeable based on charge (ions can freely cross)

How dialysis tubing demonstrates absorption

• models are ideas used to explain things observed in the world
• models share imprortant characteristics with the phenomenon being demonstrated

S 6.1.1 ​Production of an annotated diagram of the digestive system. 

• State the role of the digestive system.
• Draw a diagram of the human digestive system.
• Outline the function of the following digestive system structures:  mouth, esophagus, stomach, small intestine, pancreas, liver, gall bladder, and large intestine.

The human digestive system consists of an alimentary canal and associated accessory glands. The main parts to identify in the figure below include the mouth, esophagus, stomach, small intestine, large intestine and anus along with the digestive glands – liver and pancreas. Also include the gall bladder. Make note of the interconnections between these structures the functions of the stomach, small intestine and large intestine.

S 6.1.2 ​Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph. (Oxford Biology Course Companion page 281​

• Outline the function of the four layers of tissue found in the wall of the small intestine.
• Label the four layers of tissue found in the wall of the small intestine as viewed with a microscope or in a micrograph.

• Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
• Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
• Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
• Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen