Digestion and absorption

digestion

• During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes

  • Proteins are hydrolysed into amino acids

  • Carbohydrates are hydrolysed into simple sugars

  • Lipids are hydrolysed into glycerol and fatty acids

• The resulting small molecules are used by the cells to:

  • release energy via respiration

  • build new molecules for cell growth, repair and function

human digestive system - 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

human digestive system - stomach

Protease enzymes begin protein digestion

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

human digestive system - liver

Bile salts are produced here

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

human digestive system - pancreas

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

human digestive system - small intestine: duodenum

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

Enzymes complete chemical digestion here

human digestive system - small intestine: ileum

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

enzymes in digestion

• Digestive enzymes are extracellular enzymes, meaning that they function outside the body cells

• There are three main types of digestive enzymes:

  • carbohydrases

  • lipases

  • proteases

carbohydrate digestion

• 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:

  1. Amylase hydrolyses starch into the disaccharide maltose

     • Amylase is made in the salivary glands, the pancreas and the small intestine

  2. 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

lipid digestion

• Lipid digestion involves the action of:

  • lipase enzymes

  • bile salts

• The process of lipid digestion is as follows:

  1. 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

  2. Lipase enzymes in the lumen of the small intestine break down lipids to glycerol, monoglycerides and fatty acids

protein digestion

• 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

• Membrane-bound dipeptidases are attached to the cell surface membrane of epithelial cells in the small intestine

mechanisms of absorption

• The products of digestion are absorbed through the intestinal lining

• Molecules pass into the intestinal epithelial cells, from which they can move into the blood

• Absorption of the major biological molecules occurs by different mechanisms:

  • Amino acids and monosaccharides are absorbed via co-transport

  • Lipid absorption involves micelles

absorption of co-transport - 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:

  1. 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

  2. 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

  3. 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

absorption of co-transport - monosaccharides

• The co-transport of glucose uses the same mechanism as that of amino acids:

  1. active transport of sodium ions into the blood

  2. facilitated diffusion of sodium and glucose into the epithelial cell, via a glucose co-transporter protein

  3. facilitated diffusion of glucose into the blood

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