Chemical digestion: is the breaking down of macromolecules by hydrolysis, often
catalysed by enzymes
Mechanical Digestion: where large pieces of food are broken down into smaller ones,
e.g. matication and churning
Endopeptidases: catalyses the hydrolysis of peptide bonds within a protein
exopeptidases:catalse the hydrolysis of the peptide bonds at the end of a molecule
resulting in a single amino acid being seperated from the chain.
All digestive enzymes are hydrolases (catalyse hydrolysis)
Protease/peptidases break down Proteins
Carbohydrase break down Carboyhdrates
Lipase break downlipids
The alimentary canal is made of thefour main layers described below:
1) The mucosa is made of an epithelium containing goblet ceels to secret mucus to lubricate and protect the cells. Connective tissue and muscle. The musclecontracts involuntarly rythmically for long periods of time
2)The submucosa is made up of mainly fibrous proteins and contains blood vessels
3) The muscularis externa is made up of two layers of smooth muscle, the first is longitudinal, the second; circular.
4) The serosa is a thin layer of connective tissue
The Mouth and Oesphagus
Food is ingested using the teeth and lips, mastication then takes place (mechanical digestion) whichincreases the foods surface area.
Saliva is the secreted from 3 salivary glands. Slaiva is made up of:
1) water: helps to disolve any soluble components thus allowing interaction
with receptors on the taste buds
2) mucus: helps the tongue to mold food into a bolus
3) salivary amylase: an enzyme to break down starch
sawllowing pushes the bolus into the oesphagus and a wave of contraction and relaxation of the musclaris externa pushes the food towards the stomah
The stomach is a sac with a capacity of 5dm^3. at each end are muscles called Sphincters. The Cardiac Sphincter relaxes to allow food into the stomach and the Phloric Sphincter relaxes to allow partially digested food called chyme.
Gastric pits(or glands) are large folds in the stomach's mucosa which secrete gastric juice. The epithelium of the mucosa is made up of columular cells
Gastric juice is made up of:
2) Hydrocholric acid
6) Hydrogencarbonate ions
Oxyntic (or Parietal) cells secret hydrocholoric acid. These cells have many mitochondria and deep invaginations on their surface. The hydrochloric acid gives the gastric juice a pH of 1.0 which kills most bacteria.
The stomach continued
Chief cells secrete pepsinogen (inactive pepsin) which is activated when hydrocholirc acid or pepsin removes a length of amino acid from it.
Gastric lipase and pepsin are adapted to work well in these acidic conditions.
The acidity and high level of enzymes found in gastric juice can be very harmful to the stomach's walls so the goblet cellssecrete mucus containing hydrogen carbonate ions (the ions make it alkaline).
The stomach is not adapted for absoption hoever small lipid soluble molecules (e.g. alcohol) which can easily pass through the plasma membrane are absorbed.
Medicinal drugs can often also be absorbed however frequent use of these drugs can lead to ulcers.
The liver and Pancreas
Chyme from the stomach moves into the small intestine, at the same time fluid from the liver and pancrease also enter the small intestine.
The Liver produces bile, which is stored in the gall bladder, travels down the bile duct to the duodenum.
2) Bile Salts e.g. sodium glycocholate and sodium taurocholate
3) Hydrogencarbonate Ions
The salts help to emulsify fats (turning large droplets into tiny globules), they are absorbed into the blood and later returned to the bile - recycled.
The hyrdogencarbonate ions help to neutralise the chyme
The Pancreas is both an endocrine and exocrine gland
Endocrine: secrets hormones into the blood
Exocrine: secrets substances into a duct
The pancreases endocrine function is the secrete insulin and glucagon which control blood-glucose levels. Its exocrine funstion is the secrete pancreatic juice into the
Which contains: enzymes, enzyme precursors and hydrogencarbonate ions.
Enzymes found in the pancreatic duct are:
(1, 2 & 3 are all precursors)
Cells in the wall of the duodenum secrete enerokinase which activates the percursors.
The hydrogencarbonate ions help to neutralise the chyme as all the enzymes which work in the duodenum have an optimum pH of neutral.
The Small Intestine
The samll intestine can be up to 5m long and is split into 3 sections:
1) Duodenum - first 25cm
2) Jejunum - next 2m
3) Ileum - final 2.75m
The walls of the ileum have lots of tiny folds called villi (made of the mucosa layer). Each villus is approx. 1mm tall and supply a large surface area for absorption, (often described as the brush border.) The smooth muscle in the mucosa can contract to make the villi sway causing increased contact with food. Each villus has a good blood supply and lympatic system.
Each vili are glands called: the Crypts of Liberkuhn, these crypts contain goblet cells that secrete mucus and Paneth cells which may destroy pathogens (their function is not clear).
In the lumen enzymes digest food:
Protein -------- Peptides ----- amino acids
(by trypsin, chymotrypsin & carboxypeptidase)
Lipids -------- fatty acids and glycerol (bylipases)
starch -------- maltose (by amylase)
Some of the pancreatic enzyems (e.g. amylase) are absorbed onto the surface of epithelial of vili cells, they get tangled in the carbohydrate chains of glycoproteins. Also the vili which produce digestive enzymes hold them in their epithethelial with their active sites exposed.
This causes a high concentration of nutrients next to the cells that will absorb them.
All of the final products can cross the plasma membrane to blood and lymph vessels. Absobtion happens by; diffusion, facilitated diffusion and active transport.
Gluocse is transported by cotransport (active transport.) Epethial cells pump out sodium ions against their concentration gradient. These then diffuse back into the cell, carrying with them glucose.
Fatty acids and glycerol are lipid soluble and thus diffuse into the cells where they are converted back to tryglycerides on the SER and coated with protein in the golgi body to form a Chylomicron (lipo protein.) Which enters the lymph system to form a white emulsion.
Water, inorganic ions and vitamins are also absorbed in the ileum. Fat soluble vitamins diffuse . Others and inorganic molecules pass through transport proteins = by active transport or facillitated diffusion. Water moves into the epethelium by osmosis.
The colon, caecum, appendix and rectum make up the large intestine.
The colon is approx. 150cm long and is folded to increase surface area (but no villi!) The epethelium is made of columnar cells with micro villi (for arsorption) and a large number of goblet cells which secrete large amounts of alkaline mucus.
The colon's job is the absorb inorganis ions and water.
(Ions mainly active transport - water osmosis)
In humans the caecum and appendix are vestigial organs (have no function).
The material which remains in the colon is indigestable (eg cellulose) as well as shed cells and mucus. It is passed periodically to the rectum and out the anus as faeces.
Control of Digestion
Secretion of digestive fluids is controlled by the nervous system and hormones.
The Nervous System
Sensing or chewing food can trigger the secretion of saliva (reflex action) because action potentials travel from sense organs to the CNS and then along motor neurones to the salivary glands.
Sensing food can also stimiulate the gastric pits as nerve impules=ses carried to the stomach from the brain along the vagus nerve, stimulating the secretion of gastric juice and the hormone gastrin.
The Endocrine System
In the Stomach
Gastrin is released into the blood stream which causes the gastric pits to secrete large quantities of gastric juice. This ensures that there is food inside the stomach ready for when the food arrives and the amount of juice increases when the food does arrive.
Cells in the duodenunm wall react to acidic substances (chyme) by secreting secretin into the blood which stimultes the exocrine cells in the pancrease to release a juice rich in hydrogencrabonate ions into the pancreatic duct.
The cells of the dundenums walls also react to the presence of partially digested fat and protein causing them to secrete cholecystokinin (CCK) which causes the exocrine cells of the pancreas to secrete a juice rich in enzymes and stimulates smooth muscle in the gall bladder forcing it to contract and push bile along the bile duct to the duodenum.
Digestion in Herbivores and Carnivores
Dog - Carnivore
A dog's canines thrust past each other, whent eh jaw closes. This allows the dog to the pierce the body of it's prey with a large force.
The premolars and molars have sharp edges and are sometimes called carnassials. They slide past each other allowing the dog to crush bones, and cut meat into smaller pieces.
The incissors are used only for grooming.
Dogs's teeth have closed roots (no blood flow).
In a dog no chemical digestion takes place in the mouth yet the stomach holds a higher concentration of acid allowing dogs to eat food which we would deem rotten and dangerous.
The rest of a dog's digestive system is very similar to a humans.
Cow - Herbivore
A sow has a diastema (a large gap)instead of canines, which allows the cow's long flexible tongue enough room to move grass around, ensuring it is chewed thoroughly.
The molars and premolars responsible for this chewing have ridges and cusps. The riges of the upper teeth fit the cusps of the lower teeth and vice versa.
The cow only has incisors on it's lower jaw, these point forwards and are shaped like chissels. On it's upper jaw it has a horny pad which the incissors push against to cut grass.
Cow's teeth have open roots (blood flow) which means that they're teeth continue to grow through out life because they are continuously being worn down by the grinding action.
A cow has a 4-chambered stomach which has evolved to help them gain nutrients from plant materials.
The largest chamber is the Rumen and the Reticulum where there are many microorganisms which are able to produce enzymes which breakdown cellulose to cellubiose and glucose.Other enzymes convert these to fatty acids and release CO2 and methane. The fatty acids are absorbed through the walls of the rumen which is often used as a cows main repiratory substance.
This only works if the plant material is well chewed so periodically some of the rumen and reticulum's contents is rechwed and reswallowed.
Material from the rumen and reticulum pass ito the omasum and abomasm. The abomasum secretes hydrochloric acid and proteases, which digest the protein in the microorganisms.
A cow's saliva contains urea which the microrganisms mix with carbohydrates to form proteins, most of the urea is later reconverted to amino acids and then proteins by the bacteria in the rumen.