Digestion and Absorption

Food is Broken Down into Smaller Molecules During Digestion The large biological molecules (e.g. starch, proteins) in food are too big to cross cell membranes. this means they can't be absorbed from the gut into the blood. During digestion, these large molecules are broken down into smaller molecules (e.g. glucose and amino acids), which can move across cell membranes. This means they can be easily absorbed from the gut into the blood, to be transported around the body for use by the body cells. Most large molecules are polymers, which can be broken down into monomers using hydrolysis reactions. Hydrolysis reaction break bonds by adding water. During hydrolysis, 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.

Digestive Enzymes are Used to Break Down Biological Molecules in Food A variety of different digestive enzymes are produced by specialised cells in the digestive systems of mammals. These enzymes are then released into the gut to mix with food. Since enzymes only work with specific substrates, different enzymes are needed to catalyse the breakdown of different food molecules. CARBOHYDRATES ARE BROKEN DOWN BY AMYLASE AND MEMBRANE-BOUND DISACCHARIDASES Amylase is a digestive enzyme that catalyses the conversion of starch into the smaller sugar maltose. This involves the hydrolysis of the glycosidic bonds in starch. Amylase is produced by the salivary glands and also by the pancreas. Membrane-bound disaccharidases are enzymes that are attached to the cell membranes of epithelial cells lining the ileum. They help to break down disaccharides into monosaccharides. This also involves the hydrolysis of glycosidic bonds. Monosacchardies can be transported across the cell membranes of the ileum epithelial cells via specific transporter proteins. Maltase breaks maltose down into molecules of glucose. Sucrase breaks sucrose down into glucose and fructose. Lactase breaks lactose down into glucose and galactose. LIPIDS ARE BROKEN DOWN BY LIPASE AND BILE SALTS Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are made in the pancreas. They work in the small intestine. Bile salts are produced by the liver and emulsify lipids - this means they cause the lipids to form small droplets. Bile salts are really important in the process of lipid digestion. Several small lipid droplets have a bigger surface area than a single large droplet. So, the formation of small droplets greatly increases the surface area of lipid that's available for lipases to work on. Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles. PROTEINS ARE BROKEN DOWN BY ENDOPEPTIDASES AND EXOPEPTIDASESproteins are broken down by a combination of different proteases (or peptidases). These are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids.-ENDOPEPTIDASES Endopeptidases act to hydrolyse peptide bonds within a protein. Trypsin and chymotrypsin are two examples. They're synthesised in the pancreas and secreted into the small intestine. Pepsin is another endopeptidase. It's released into the stomach by cells in the stomach lining. Pepsin only works in acidic conditions - these are provided by the HCl in the stomach. -EXOPEPTIDASES Exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules. They remove single amino acids from proteins. Dipeptidases are exopeptidases that work specifically on dipeptides. They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them. Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.

The Products of Digestion are Absorbed Across Cell MembranesThe products of digestion are absorbed across the ileum epithelium into the bloodstream.MONOSACCHARIDES Glucose is absorbed by active transport with sodium ions via a co-transporter protein. Galactose is absorbed in the same way using the same co-transporter protein. Fructose is absorbed via facilitated diffusion through a different transporter protein. MONOGLYCERIDES AND FATTY ACIDS Micelles help move monoglycerides and fatty acids towards the epithelium. Because micelles constantly break up and reform they can 'release' monoglycerides and fatty acids, allowing them o be absorbed - whole micelles are not taken up across the epithelium. Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane. AMINO ACIDSAmino acids are absorbed in a similar way to glucose and galactose. Sodium ions are actively transported out of the epithelial cells 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.