definition of digestion
the process whereby large food molecules are broken down into smaller, soluble ones that can be absorbed into the body cells
purpose of physical digestion
increase the surface area-to-volume ratio of digested food particles for digestive enzymes to act on more efficiently
purpose of chemical digestion
break down large food molecules into small soluble ones that can be absorbed into the bloodstream
what is peristalsis
rhythmic, wave-like muscular contractions in the wall of the alimentary canal that enables food to be mixed with digestive juices and be propelled across the gut
which muscles contract and relax to allow wall to constrict and food to move through alimentary canal
circular muscles contract, longitudinal muscles relax, hence gut becomes narrower and longer, squeezing food forward
which muscles contract and relax to allow wall to dilate and food to enter the lumen
longitudinal muscles contract, circular muscles relax, hence wall of gut dilates which widens lumen for food to enter
function of chewing in digestive system
physically break down large pieces of food into smaller ones and increases the surface area of food for digestion by enzymes
digestion process in mouth
- salivary glands release saliva which mixes with food and softens it
- food is broken up into smaller pieces by chewing, increasing the surface area-to-volume ratio of the food for salivary amylase to work on
- salivary amylase digests starch present in food into maltose
function of hydrochloric acid in stomach
- stops action of salivary amylase by denaturing it
- provides a low pH environment for pepsin to digest proteins
- kills potentially harmful microorganisms
function of mucus on stomach walls
- protects stomach wall from being digested by enzymes
- moistens food to allow easy movement within the stomach
process of digestion in stomach
- peristalsis of stomach walls (churning) helps food to mix with gastric juices
- pepsin in stomach breaks down proteins present into smaller, shorter polypeptides
process of digestion in small intestine
- food entering duodenum stimulates the secretion of pancreatic juice, intestinal juice and bile
- protein digestion: undigested proteins are digested by pancreatic proteases into shorter polypeptides, shorter polypeptides are digested by intestinal proteases into amino acids
- carbohydrate digestion: undigested carbohydrates are digested by intestinal and pancreatic amylase into maltose and then by maltase into glucose (simple sugars)
- fat digestion: emulsified fats / triglycerides are digested by intestinal and pancreatic lipases into fatty acids and glycerol
function of bile secreted by liver into small intestine
bile salts lower surface tension of fats and causes them to break down from large fat globules into tiny fat droplets. this increases their surface area-to-volume ratio for digestion by lipases
absorption in small intestine
small intestine contains villi that increase surface area for absorption. glucose and amino acids are absorbed into villi capillaries via diffusion and active transport, while fatty acids and glycerol diffuse into the epithelium and combine to form fat globules that enter the lacteals
adaptations of villi in small intestine
- epithelium of villi is only one cell thick to provide a short diffusion distance for nutrients to pass through
- cells of epithelium have many microvilli to further increase surface area of small intestine
- each villus has many capillaries that allow blood to transport absorbed glucose and amino acids to maintain a diffusion gradient
- each villus contains a lacteal to transport absorbed fats away to maintain a steep diffusion gradient
- epithelial cells contain many mitochondria to provide energy for active transport of nutrients into villi
definition of assimilation
the process whereby absorbed nutrients are converted into new cytoplasm or used to provide energy
how is glucose utilised after absorption
- nutrient-rich blood is transported through hepatic portal vein to the liver
- glucose is absorbed into liver cells and converted into glycogen for storage.
- glucose is assimilated into body cells and broken down during respiration to release energy
- excess glucose returns to liver to be converted to and stored as glycogen (part of lowering blood glucose concentration)
how are amino acids utilised after absorption
- nutrient-rich blood is transported through hepatic portal vein to the liver
- amino acids that enter cells are converted to cytoplasm used for growth and repair
- excess are deaminated in the liver by removing their amino groups to form urea
- remains are converted to glucose and excess glucose is converted into glycogen
how are fats utilised after absorption
- carried to all parts of the body (especially liver)
- used to build protoplasm (e.g. cell membranes) when glucose is in sufficient supply
- broken down to provide energy when glucose is in short supply
- excess are stored in adipose tissues that protect certain organs
function of large intestine
absorb (90% of) water and mineral salts from undigested food material and stores faeces in rectum before it is expelled through the anus
function of liver
- produces and secretes bile
- deaminates excess amino acids
- regulates blood glucose concentration
- detoxification
- breaks down hormones
function of pancreas
- produces pancreatic juice that contains amylase, lipase and protease
- secretes insulin and glucagon that regulate blood sugar levels
