So far in the exchange and transport series we have mainly been looking at exchange and transport of gases. However, organisms also need to absorb nutrients such as glucose and amino acids so they can carry out processes like respiration and protein synthesis. In this article we will cover how large food molecules are broken down during digestion (mostly by hydrolysis) and absorbed into the blood for transport.
Digestion and absorption of carbohydrates
Large molecules such as starch are too big to be absorbed through the gut wall. Remember that starch (found in plants) is a polymer of glucose molecules joined together with glycosidic bonds. In order for large carbohydrates to be absorbed, they must be broken down by enzymes into small monosaccharides.
The enzyme that hydrolyses many of the glycosidic bonds in starch is called amylase. Amylase is secreted by the salivary glands and the pancreas. However, amylase only breaks starch down into the disaccharide maltose. The final step is the action of membrane-bound disaccharidases found in the membranes of epithelial cells in the ileum (the last part of the small intestine). There are different disaccharidases that are specific for different disaccharides:
- Maltase hydrolyses maltose to two glucose molecules
- Lactase hydrolyses lactose to glucose and galactose
- Sucrase hydrolyses sucrose to glucose and fructose
The monosaccharides can now be absorbed in the ileum. We have already covered absorption of glucose in the active transport article, so head over there to see a diagram. Galactose actually uses the same co-transporter protein as glucose in the epithelial cell membrane. Fructose is absorbed by facilitated diffusion down a concentration gradient using a different membrane protein.
Digestion and absorption of proteins
Proteins are also large molecules and need breaking down into their monomers, amino acids, before they can be absorbed. Protein digestion begins in the stomach with an enzyme called pepsin. There are several protease enzymes involved in protein digestion:
- Endopeptidases hydrolyse peptide bonds within the polypeptide chain. Pepsin is an example – it is secreted by the lining of the stomach. Chymotrypsin and trypsin are two other examples – these are secreted by the pancreas and work in the small intestine.
- Exopeptidases only hydrolyse peptide bonds at the very ends of polypeptide chains, releasing one amino acid. Dipeptidases (a type of exopeptidase) only work on dipeptides to hydrolyse them into two amino acids. Some dipeptidases are membrane-bound in the epithelial cells of the small intestine.
Absorption of amino acids in the ileum is actually very similar to glucose (and galactose). They are transported into epithelial cells with sodium via a co-transporter protein. This can happen because a sodium ion concentration gradient is being maintained by the sodium-potassium pump (see this article).
Digestion and absorption of lipids
Large lipid droplets entering the intestine are quite difficult for lipase enzymes to act on because there isn’t a lot of surface area. Luckily, the liver is on hand to help with this. It produces bile salts which emulsify the large lipid droplets i.e break them up into smaller droplets. Now there is more surface area for the lipase enzyme (secreted by the pancreas) to act on. Lipase hydrolyses the ester bonds between glycerol and fatty acids in triglycerides. However, normally only two fatty acids are removed, so the final products are fatty acids and monoglycerides (glycerol with one fatty acid still attached). These products stay associated with the bile salts to form little structures called micelles.
Micelles can released fatty acids and monoglycerides, which then diffuse across the cell membrane of epithelial cells. They don’t need a transporter protein because they are lipid soluble and the phospholipid bilayer is happy to let them pass through.
Summary
Here is a brief summary table of digestion.
| Carbohydrates | Proteins | Lipids | |
| Enzymes | Amylase (starch) and disaccharidases | Proteases (exopeptidases and endopeptidases) | Lipase (after emulsification by bile salts) |
| Location | Mouth and small intestine | Stomach and small intestine | Small intestine |
| Absorbed products | Monosaccharides | Amino acids | Fatty acids and monoglycerides |
