U3 - Digestive System & Enzymes

What is digestion?

The hydrolysis of large, insoluble macromolecules into small, soluble molecules

Why do we digest?

So the products of digestion can be absorbed and then used (assimilated) to make the macromolecules we need.

What are the stages of digestion?

Ingestion > digestion > absorption > assimilation > egestion (not excretion - largely cellulose / fibre (indigestible))

What is physical breakdown?

• Chewing - teeth and tongue break up food to increase surface area

• Churning - the stomach mixes the food (done by the muscles in the stomach lining)

What is chemical digestion?

ENZYMES

E.g. Salivary amylase digests starch to maltose and pancreatic amylase in the small intestine wall are then immobilised maltose enzymes.

Maltose > alpha glucose

What does the liver do?

Make bile, which is stored in the gall bladder

How do villi increse te efficiency of absorption?

• Increased surface area

• Good blood supply (capillaries and lacteal)

• Wall is once cell thik (short diffusion distance)

• Microvilli on the epithelial cell

• Epithelial cells have many mitochondria

ALL OF THESE ADPATATIONS ARE ABOUT DIFFUSION AND ACTIVE TRANSPORT (CO-TRANSPORT)

What do carbohydrases hydrolyse?

Carbohydrases hydrolyse complex carbohydrates into monosaccharides (especially glucose)

What do proteases hydrolyse?

Proteases hydrolyse proteins into amino acids

What do lipases hydrolyse?

Lipases hydrolyse lipids into fatty acids and glycerol (maybe a phosphate)

How is starch digested?

• Salivary amylase (salicary glands) - starch > maltose

• Pancreatic amylase (pancreas) - starch > maltose

How is maltose digested?

Membrane-bound maltase in epithelial membranes of duodenum - maltose > glucose

What does sucrase help to digest?

Hydrolyses the glycosidic bond in sucrose to form glucose + fructose

What does lactase help to digest?

Hydrolyses the glycosidic bond in lactose to form glucose + galactose

Where does protein digestion take place?

In the stomach and small intestine

What are the three types of proteases / peptidases?

• Endopeptidases

• Exopeptidases

• Dipeptidases

What do the 3 different peptidases do?

• Endopeptidase - Cut polypeptides inside the chain

• Exopeptidase - Cut 2 amino acids at a time from the ends of a polypeptide

• Dipeptidase - Dipeptides to individual amino acids

What is the point of having all the different types of enzymes?

To make the process more efficient:

• Endopeptidases (mainly in the stomach) cut the chain up

• Exopeptidases (mainly in the pancreatic duct) then have more ‘ends’ to work on

• Mebrane bound dipeptides in small intestine wall then have many dipeptides to hydrolyse

Overall, hydrolysis is much faster with these enzymes in this order

Are lipids and their enzymes soluble or insoluble?

• Lipids - Insoludble in water

• Lipases - soluble in water

What is it called when lipids are split into very tiny droplets by bile salts?

Micelles

How do lipids digest?

• Lipids are split into very tiny droplets (micelles) by bile salts = emulsion

• This increases the surface area of the lipids so lipases can work quicker

• Lipases produced in the pancreas hydrolyse the ester bond found in triglycerides

• to form fatty acids and monoglycerides (glycerol with a single fatty acid attached)

What is the role of diffusion in the absorption of glucose and amino acids?

When there is a higher concentration of glucose / amino acid in the ileum than the blood, these molecules can move down a concentration gradient into the blood via facilitated diffusion (using a carrier protein in the membrane)

What is the role of active transport in the absorption of glucose and amino acids?

Means that ALL glucose and amino acids should be absorbed into the blood. This is against a concentration gradient, using co-transport

What is the role of co-transport in the absorption of glucose and amino acids?

Glucose / amino acids are drawn into the cells along with sodium ions that have been actively transported out of the cell by the sodium potassium pump:

1. Na+ / K+ pump - Using ATP to pump Na+ out and K+ in

2. Due to the Na+ / K+ pump, the concentration of Na+ in the cell is very low

3. Co-transport of both glucose and Na+ into the cell down the Na+ concentration gradient. This works even though glucose moves against its concentration gradient

4. Glucose diffuses out of the cell into the blood stream down a concentration gradiet through a carrier protein via facilitated diffusion

Where is bile made and stored?

• Made in the liver from breaking down old red blood cells

• Stored in the gall bladder

What are the jobs of bile?

1. To neutralise the stomach acids to get the correct pH for hydrolytic enzymes from the pancreas and small intestine wall

2. For emulsifying lipids to increase the surface area

What are micelles?

Tiny structures (4-7nm in diameter). They are formed from monoglycerides and fatty acids and are in association with bile salts.

What are chylomicrons?

Special particles adapted for the transport of triglycerides

What is the first stage of triglyceride absorption?

In the ileum, the micelles come into contact with the epithelial cells lining the villi. The micelles break down, releasing monoglycerides and fatty acids. These are at high concentration and diffuse across the membrane into the epithelial cells (simple diffusion, no membrane proteins needed)

What is the second stage of triglyceride absorption?

In the cells, the monoglycerides and fatty acids are transported to the smooth endoplasmic reticulum, where they recombine to form triglycerides. Starting in the SER and continuing in the vesicles, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons

What is the third stage of triglyceride absorption?

Chylomicrons move out of the epithelial cells by exocytosis and enter lymphatic capillaries called lacteal (found at the centre of each villus). Chylomicrons pass via lymphatic vessels into the lymphatic system (then into the blood stream). The triglycerides in the chylomicrons are hydrolysed (products diffuse into cells) by an enzyme in the endothelial cells of the blood capillaries