Digestion

Question 1

Explain complete lipid digestion + absorption

Answer 1

Lipid digestion

Lipid digestion involves the action of:
lipase enzymes
bile salts
The process of lipid digestion is as follows:
emulsification
Partially digested food arrives in the small intestine and mixes with bile
Bile salts bind to large lipid droplets and breaks them into smaller droplets; this is emulsification
The resulting small lipid droplets have a large surface area on which lipase enzymes can act
Lipase enzymes in the lumen of the small intestine break down lipids to glycerol, monoglycerides and fatty acids
Lipid absorption:
The products of lipid digestion are:
Fatty acids
Monoglycerides
Monoglycerides and fatty acids associate with bile salts to form micelles, which transport these insoluble molecules to the cell surface membranes of the epithelial cells
Micelles constantly break up and reform; when they break apart their lipid-soluble contents can cross the membrane by diffusion
The contents of micelles are non-polar so can diffuse through the phospholipid bilayer of the cell membrane

Short fatty acid chains within the epithelial cells can move directly into the blood via diffusion
Longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum
The triglycerides are packaged into chylomicrons which eventually enter the bloodstream

Question 2

Explain complete carbohydrate digestion + absorption

Answer 2

Carbohydrase enzymes are a group of enzymes involved in carbohydrate digestion; examples include:
amylase
maltase
lactase
The process of digesting starch into simple carbohydrates is as follows:
Amylase hydrolyses starch into the disaccharide maltose
Amylase is made in the salivary glands, the pancreas and the small intestine
Maltose is then hydrolysed into the monosaccharide glucose by maltase

Maltase is a membrane-bound disaccharidase, meaning that it:
is attached to the cell-surface membranes of the epithelial cells lining the small intestine
breaks down disaccharides into monosaccharides
To absorb monosaccharides (i.e. glucose in this case):
Is by co-transport, the co-transport of glucose uses the same mechanism as that of amino acids:
active transport of sodium ions into the blood
facilitated diffusion of sodium and glucose into the epithelial cell, via a glucose co-transporter protein
facilitated diffusion of glucose into the blood

Question 3

Explain complete protein digestion + absorption

Answer 3

Protein digestion involves the action of different types of protease enzymes:
endopeptidases
exopeptidases, including dipeptidases
Protein digestion involves the following:
Endopeptidase enzymes in the stomach and small intestine hydrolyse peptide bonds within polypeptides, creating shorter polypeptide chains
Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, producing single amino acids
Dipeptidases are a type of exopeptidase that break down dipeptides into individual amino acids
In the absorption of amino acids:

Co-transporter proteins are found within the cell-surface membranes of the epithelial cells in the small intestine
The process of cotransport occurs as follows:
Sodium ions are actively transported from the epithelial cell into the blood via a sodium-potassium pump, decreasing the concentration of sodium ions in the epithelial cell
This stage maintains the sodium ion gradient that is essential to the next part of the process
Sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying an amino acid is transported at the same time by the co-transporter protein
This is a form of facilitated diffusion
The concentration of amino acids in the epithelial cell increases, and amino acids diffuse down their concentration gradient into the blood
While the action of the co-transporter protein is passive, energy is required to create the sodium ion gradient, so the process of co-transport is considered, overall, to be active transport

Question 4

Explain the function of each part of the digestive system

Answer 4

Structure

Function

Mouth and salivary glands

Food is ingested and teeth break it down into smaller pieces

Saliva is secreted into the mouth

The enzyme amylase in begins to digest starch into maltose

Stomach

Protease enzymes begin protein digestion

Hydrochloric acid provides a suitable pH for enzymes and destroys any pathogens in food

Liver

Bile salts are produced here

Bile salts aid the digestion of lipids, as well as neutralising stomach acid as it exits the stomach

Pancreas

Amylase, protease and lipase enzymes are produced here before being released into the duodenum

Small intestine: duodenum

The acidic stomach contents are neutralised by bile and become slightly alkaline

Enzymes complete chemical digestion here

Small intestine: ileum

Food and water are absorbed into the blood via villi in the lining of the ileum