Topic 3B

Digestion basics and digestive enzymes

Large biological molecules cannot move across cell membranes so they have to be broken down into smaller molecules to move across
Hydrolysis reactions break down the large molecules into smaller ones by breaking the bonds using water.
Carbohydrates are broken down into disaccharides and then monosaccharides
Fats are broken down into fatty acids and monoglycerides
Proteins are broken down into amino acids
Many different digestive enzymes are used to break down foods.
The enzymes are released to mix with the food.

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Amylase is a digestive enzyme that catalyses the breakdown of starch. It works by catalysing the hydrolysis reactions that break the glycosidic bonds in starch to produce maltose.
Membrane-bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells that line the ileum and they also involve the hydrolysis of glycosidic bonds.
The monosaccharides can be transported across the epithelial cell membranes in the ileum via specific transporter proteins.

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Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids
This involves hydrolysis of the ester bonds in lipids
Lipases are mainly made in the pancreas and then secreted into the small intestine.
Bile salts emulsify lipids which causes the lipids to form small droplets. Many small droplets have a higher surface area than a large single droplet this increases the surface area lipases have to work on.
After the lipid has been broken down by lipase, the monoglycerides and fatty acids stick with the bile salts to form micelles and they help to with lipid digestion

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Proteins are broken down by a combination of different peptidases and these are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between the amino acids
Endopeptidases hydrolyse the peptide bonds within the protein
Exopeptidases hydrolyse peptide bonds at the ends of the proteins and only removed single amino acids.
Dipeptidases are exopeptidases that work specifically on dipeptides and they act to separate the two amino acids that make up a dipeptide by hydrolysing the bond between them and are normally found in the cell-surface membranes of epithelial cells in the small intestine

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Monosaccharide- Glucose and galactose is absorbed by active transport with sodium ions via a co-transporter protein. Fructose is absorbed via facilitated diffusion through a different transporter protein
Monoglycerides and fatty acids- Micelles help to move move monoglycerides and fatty acids towards the epithelium. Monoglycerides and fatty acids are lipid soluble, so can diffuse directly across the epithelial cell membrane.
Amino acids- They are absorbed in a similar way to glucose and galactose. Sodium ions are actively transported out of the epithelial cell into the ileum itself. They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them

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The role of haemoflobin is to transport oxygen around the body. It is a large protein with a quaternary structure. Each chain has a haem group which contains an iron ion and gives hameoglobin its red colour. Each molecule of haemoglobin can carry 4 oxygen molecules. In the lungs, oxygen joins with haemoglobin in red blood cells to form oxyhaemoglobin. This reaction is reversible so when oxygen leaves the body it turns back to haemoglobin. When an oxygen molecule joins to haemoglobin its referred to as association or loading, and when oxygen leaves oxyhaemoglobin its referred to as dissociation or unloading

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Affinity for oxygen means the tendency a molecule has to bind with oxygen. Haemoglobins affinity depends on the conditions and one of these conditions is partial pressure of oxygen. Partial pressure of oxygen is a measure of oxygen concentration. The greater the concentration

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