6.1 Digestion Flashcards
groups of organs that comprise human digestive system
alimentary canal: consists of organs through which food passes (oesophagus, stomach, small and large intestine)
accessory organs: aid in digestion but do not actually transfer food (salivary glands, pancreas, liver, gall bladder)
Mouth
food is chewed (masticated) and mixed with saliva and swallowed
Oesophagus
a hollow tube connecting the oral cavity to the stomach (separated from the trachea by the epiglottis). Food is moved in a bolus via the action of peristalsis
Stomach
both stores food and begins the process of digestion. Churning and digestion of proteins into polypeptides occurs. Sphincter muscles control the entry and exit of material.
Small intestine
is long and divided into three regions; the duodenum, jejunum and longer ileum. The small intestine is lined with smooth muscle to allow for the mixing and moving of digested food products (via segmentation and peristalsis). When acidic chyme enters the duodenum it is mixed with pancreatic juice, intestinal juice and bile which digests large molecules.
Pancreas
secretes the enzymes amylase, lipase and endopeptidase. Endopeptidase converts into trypsin enzyme in the small intestine. Enzymes digest most macromolecules in food into monomers in the small intestine. Also secretes certain hormones which regulate blood sugar concentrations.
Liver
produces bile which does not contain digestive enzymes but salts that act as an emulsifier and helps in the absorption of fats. It also contains some wastes of the liver.
Gall bladder
stores the bile
Large intestine
consists of the ascending/transverse/descending/sigmoidal colon as well as the rectum. The indigestible parts of the food, such as cellulose fibre, as well as a large volume of water is passed to the large intestine. Water and ions are re-absorbed here leaving solid faeces which passes down the rectum and is egested through the anus.
Peristalsis
is the method of movement in the oesophagus, stomach and small intestine
smooth muscle rhythmically contract and relax
food is moved along the alimentary canal from mouth to anus
one directional
Segmentation
involved with intestines
involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
segmentation contraction in the stomach move chyme in both directions, allowing for a greater mixing of food with digestive juices
occurs in both directions
Chemical digestion - mouth
enzymes produced in salivary glands
salivary amylase
starch into disaccharides
pH = 7
Chemical digestion - stomach
enzymes produced in gastric glands
pepsin + HCl = gastric juice
proteins to polypeptides
pH = 2
Chemical digestion - small intestine
enzymes produced in pancreas and liver
lipase, amylase, maltase, lactase, endopeptidase, bile salts
triglycerides -> fatty acids and glycerol
disaccharides -> monosaccharides
polypeptides -> amino acids
pH = 8
Small intestine - structure for absorption
ileum is lined with villi which contain a network of blood capillaries close to the surface allowing for rapid transport
lacteals (lymph vessels) for rapid transport of lipids
folded into tiny microvilli, hence increasing surface area
the villi are a single layer of epithelial cells
the liquid that passes along the ileum contains products of digestion that are absorbed through the wall of ileum into blood via the hepatic portal vein
Tissue layers of small intestine
serosa - protective outer layer
muscular layer - peristalsis
sub mucosa layer - connected mucosa and muscle
mucosa layer - contains villi and goblet cells
Features of Villi Mnemonic
Microvilli - increases surface area
Rich blood supply - dense capillary network rapidly transports absorbed substances
Single layer epithelium - minimises diffusion distance between lumen and blood
Lacteals - absorbs lipids from the intestine to the lymphatic system
Intestinal glands - exocrine pits release digestive juices
Membrane proteins - facilitates transport of digested materials into epithelial cells
Starch digestion
Starch is a polysaccharide composed of glucose monomers and accounts for ~ 60% of the carbohydrates consumed by humans
Starch can exist in one of two forms – linear chains (amylose) or branched chains (amylopectin)
The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
Starch digestion by amylase does not occur in the stomach as the pH is unsuitable for amylase activity (optimal pH ~ 7)
Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins
Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
The hydrolysis of maltose / dextrin results in the formation of glucose monomers
Glucose can be hydrolysed to produce ATP (cell respiration) or stored in animals as the polysaccharide glycogen
Glucose monomers can also be generated from the breakdown of other disaccharides (such as lactose and sucrose)
