Digestion & Absorption

• Peristalsis is the ability to move food through the digestive tract via muscle contraction. It is the involuntary contraction and relaxation of longitudinal and circular muscles throughout the digestive tract, allowing for the propulsion of contents beginning in the pharynx and ending in the anus.
• There are three types of muscles involved in peristalsis:

  • circular
  • longitudinal
  • oblique
• The stomach lining has rugae, defined as a series of ridges produced by the folding of the wall, to increase surface area and allow the stomach to expand.
• Mechanical digestion occurs in the stomach as the contraction of muscles in the stomach walls churns the food.
• Mucous cells secrete mucus to protect the stomach lining from gastric juices.
• Mucous cells secrete mucus to protect the stomach lining from being digested by its own enzymes.
• Mucus secreted by glands in the stomach protects the epithelial cells from being digested by the strong acids present in the stomach.
• The duodenum is the first bend out of the stomach into the small intestine.
• The ileum has folds to increase its surface area for absorption, and the folds are lined with billions and billions of villi on the surface.
• A villus is made up of many cells, e.g. epithelial cells (not ciliated) in the villus walls, and is approximately 500μm tall.

  Villus walls must be one cell thick for a short diffusion pathway into the capillaries.

  The epithelial cells have microvilli to massively increase surface area, despite each microvillus being only 1μm tall.
• 𝙁𝙀𝘼𝙏𝙐𝙍𝙀𝙎 𝙊𝙁 𝙏𝙃𝙀 𝙎𝙈𝘼𝙇𝙇 𝙄𝙉𝙏𝙀𝙎𝙏𝙄𝙉𝙀:

  • folded internal walls (villi);

  = gives a large surface area

  • thin epithelium in villi walls

  = short diffusion pathway

  • villi have dense capillaries

  = maintains the concentration gradient

  • villi contain muscles and can move

  = mixes the contents of the ileum
• The mouth and tongue break down large pieces of food into smaller pieces for easy ingestion. Breakdown is performed mechanically, with the teeth and tongue (mastication), or chemically, with amylase.
• The oesophagus carries food from the mouth to the stomach.
• The stomach secretes hydrochloric acid which lowers the pH of the contents inside it. This creates an environment where enzymes such as pepsin can work effectively.
• The stomach secretes hydrochloric acid which lowers the pH of the contents inside it. This creates an environment where enzymes such as pepsin can work effectively.
• The liver processes nutrients absorbed in the small intestine and also secretes bile (first stored in the gall bladder) into the ileum.
• The rectum stores faecal matter until defaecation occurs.
• The large intestine reabsorbs water and electrolytes from undigested material.
• The small intestine absorbs most of the digested products.
• The gall bladder stores and concentrates bile.
• The anus egests faeces.
• The stomach is a muscular sac that stores and digests food. Its walls and glands produce digestive enzymes for chemical breakdown, such as pepsin in the stomach acid (HCl). It also performs mechanical digestion as the oblique muscles in the walls contract.
• The pancreas is a large gland producing pancreatic juice which contains hydrolytic enzymes such as amylase, lipase, and protease.
• The intestines perform chemical breakdown and absorption using membrane-bound enzymes in the epithelial cells, such as disaccharidases. It also contains bile, amylase, lipase, and protease from the duodenum.
• During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes
• Digestion in mammals includes:

  • Carbohydrates by amylases and membrane-bound disaccharidases
  • Lipids by lipase, including the action of bile salts
  • Proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases
• Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals include:

  • Co-transport mechanisms for the absorption of amino acids and of monosaccharides
  • The role of micelles in the absorption of lipids
• Digestion is the process by which complex food materials are broken down into smaller water-soluble molecules
• Hydrolases are enzymes that help in splitting up molecules by adding water, known as hydrolysis
• Examples of Hydrolases include digestive enzymes like amylase, lipase, protease, etc.
• Mechanisms for the absorption of the products of digestion include:

  • Active transport
  • Passive diffusion
  • Facilitated diffusion
  • Co-transport (secondary active transport)
  • Endocytosis
• Digestion is a process where large, insoluble biological molecules in food (such as starch, proteins) are hydrolysed into smaller, soluble molecules that can be absorbed across cell membranes into the bloodstream and delivered to cells in the body
• Enzymes are essential for digestion as they catalyse the hydrolysis process
• Small soluble molecules produced during digestion, like glucose and amino acids, are used to provide cells with energy through respiration or to build other molecules for cell growth, repair, and function
• Proteins are hydrolysed into amino acids, carbohydrates into simple sugars, and lipids into a mixture of glycerol and fatty acids
• The human digestive system includes glands that produce digestive juices, the stomach and small intestine as sites of digestion, the liver that produces bile, the small intestine as the site of absorption, and the large intestine as the site of water reabsorption
• Villi are finger-like projections of the small intestine lining that increase the surface area for absorption.
• Microvilli are microscopic extensions of epithelial cells lining the small intestine that provide more surface area for digestion.