Digestion- Some Definitions
Heterotrophic nutrition- Organisms that need to obtain their food from organic sources of carbon/organic molecules
Ingestion- Entry of food into the alimentary canal
Digestion- The breakdown of large insoluble molecules into smaller soluble molecules
Absorption- Taking digested food from the lumen of the alimentary canal and into cells and blood plasma
Egestion- Removal of undigested food as faeces from the anus
Mechanical Digestion- Physical breakdown of food by chewing and churning produced by muscles in the alimentary canal
Chemical Digestion- Breakdown of food by hydrolysis reactions catalysed by enzymes
Enzymes used in Digestion
All enzymes break down molecules via hydrolysis reactions. They are therefore hydrolases.
Key: Enzyme- Where found- Produced by- What it does
Proteases- Break down proteins such as;
- Pepsin- Stomach- Gastric Juice- Proteins to Peptides
- Trypsin- Duodenum- Pancreatic Juice- Proteins to Peptides
- Chymotrypsin- Duodenum- Pancreatice Juice- Proteins to Peptides
- Carboxypeptidase- Duodenum- Pancreatic Juice- Peptides to amino acids
Carbohydrases- Break down carbohydrates such as;
- Amylase- Mouth- Saliva- Starch to Maltose
- Amylase- Duodenum- Pancreatic Juice- Starch to Maltose
- Maltase- Ileum- Cells covering villi- Maltose to Glucose
- Sucrase- Ileum- Cells covering villi- Sucrose to Glucose/Fructose
- Lactase- Ileum- Cells covering villi- Lactose to Glucose/Galactose
Enzymes used in Digestion 2
Lipases- Breaks down lipids, such as;
- Lipase- Stomach- Gastric Juice- Lipids to fatty acids and glycerol
- Lipase- Duodenum- Pancreatic Juice- Lipids to fatty acids and glycerol
Bile- Emulsification of lipids
- Enters the alimentary canal in the duodenum. Bile contains no enzymes.
- Contains Bile salts that disperse large fat molecules into smaller fat droplets that mix with watery liquids present in lumen of the canal.
- Emulsificaion increases the surface area of lipids at which lipases can act on.
Structure of the Alimentary Canal
The structure of alimentary canal is the same throughout apart from the mouth. Made up from 4 different layers.
Mucosa: Layer nearest to the lumen. On inner surface is thin epithelium. Epithelium contains goblet cells which secrete mucus. Around epithelium is a layer of connective tissue and around this is smooth muscle called the muscularis mucosa.
Submucosa: Connective tissue (with blood vessels/nerves). Contains alot of collagen and elastin (fibrous proteins).
Muscularis Externa: Smooth muscle. Arranged in 2 bands- longitudinal muscle (lengthways) and circular muscle (around wall). Contraction and relaxation of muscle moves food by Peristalsis.
Serosa: Connective tissue, outer layer of wall.
The Mouth and Oesophagus
Food is ingested into mouth using teeth and lips
Mastication using molars and pre-molars breaks food into smaller pieces, increasing its surface area.
Saliva secreted into mouth via salivary glands. Saliva dissolves soluble components in food allowing them to be tasted. Saliva contains mucus which allows the formation of bolus's and amylase to break down amylose (starch).
Swallowing pushes bolus into top of oesophagus and a wave of contraction and relaxation pushes food down.
A sac containing HCl to liquefy food.
Has Sphincter muscles at either end- Cardiac sphincter is always open to allow entry and Pyloric sphincter iscontracted so that food stays in the stomach for digestion. The chyme then passes into Duodenum.
Pits in the mucosa called gastric glands- secret gastric juices
Gastric juices- water and HCl (secreted by oxyntic cells). HCl is used to kill the bacteria and liquefy food for digestion
Chief cells in epithelium secrete pepsinogen which is converted to active pepsin. Another enzyme is lipase. These enzymes are adapted to the low pH conditions.
Alkaline mucus lines the walls of the stomach to prevent digestion of the walls by HCl. This mucus contains hydrogen carbonate ions.
The Liver and the Pancreas
- Produces Bile which is stored in the gall bladder and carried along bile duct into duodenum
- Bile contains salts to help emulsify fats producing small fat droplets which are carried along by water in small intestine
- Bile salts are absorbed into blood and transported back to liver to be re-secreted
- Bile also contains hydrogen carbonate ions to neutralise stomach acid
- Exocrine function concerned with digestion. Secretion of pancreatic juice into the duodenum. Trypsinogen and Chymotrypsinogen turned into active forms via enterokinase
- Other enzymes include carboxypeptidase, lipase and amylase
Digestion in Herbivores and Carnivores
The Dog- A Carnivore
Incisors: Used to scrape meat off of bones and grooming fur
Canines: Pointy, round teeth that protude across the entire jaw. Used for piercing prey and killing
Carnassials: Sharp jagged teeth. Scissor like action as jaw is closed. Used for crushing bone and cutting meat.
Dogs do not chew food. No amylase is secreted as meat does not contain amylose. Stomach has even more concentrated acid than humans. Pepsin in stomach breaks down proteins. Same secretions from gall bladder and pancreas as humans.
Digestion in Herbivores and Carnivores continued
The Cow- A Herbivore
No canines. But does have a diastema- gap in jaw to allow movability of tongue. Chewing is done by molars and pre-molars. Insisors on lower jaw, but horny pad on top jaw. The roots remain open to allow for continued growth.
4 stomachs- abamasum, omasum, rumen and reticulum
Rumen/reticulum- anaerobic organisms which produce enzymes to break down cellulose into cellobiose and glucose. Then converted to fatty acids (absorbed through walls), releasing CO2 and Methane.
The Cow- A Herbivore continued
Contents of the rumen and reticulum regurgetated back up oesophagus for re-chewing (chewing the cud).
Material from Rumen and reticulum then pass onto abamasum and omasum. The abamasum secretes HCl and proteases to digest proteins in microorganisms. They make these proteins using carbohydrates and urea, which they get from the cow's saliva. It is formed in the liver by deamination of amino acids.
This urea is reconverted to amino acids and then proteins by bacteria in the rumen. The bacterial proteins are digested to amino acids in the abamasum and absorbed by the cow.
The Control of Digestion
The Nervous System
Release of saliva triggered by sight/smell of food. This is a reflex action. Action potentials travel from sense organs to the CNS and along motor neurones to salivary glands. This involves the medulla oblongata (brain stem). Involves nerves in autonomic system (both types).
Secretion of gastric juice also triggered by sight/smell of food. Nerve impulses carried from stomach to brain along the vegus nerve. Part of parasympathetic system.
The control of digestion continued
The Endocrine System
Gastrin: Released by cells in mucosa of stomach. Caused by action potentials along vegus nerve. Released into the blood and causes gastric glands to release gastric juice into the stomach
Secretin: When the wall of the duodenum comes into contact with acidic substances (food), causes walls to secrete hormone secretin. Released into blood, causing exocrine cells in pancreas to release pancreatic juices into the pancreatic duct.
CCK: When the wall of the duodenum comes into contact with fats/proteins. Walls secrete cholecystokenin (CCK). This acts on exocrine cells in pancreas to release pancreatic juice. Acts on gall bladder to release bile into the bile duct.
The Small Intestine
Ileum wall has folds = villi. Villi are made up of mucosa layer and provide a L.S.A for absorption. Each epithelial cell on villus has own microvilli, which increases S.A even more = brush border.
Smooth muscle inside muscularis mucosa can contract and relax to make the villi sway around, bringing them into close contact with food. Between the villi are glands, called the crypts of Lieberkuhn. The crypts contain goblet cells and Paneth cells (phagocytosis).
The Small Intestine continued
Digestion in the Small Intestine
Enzymes from pancreatic juice in the duodenum continue to act on food as it passes into the ileum. However, not all digestion happens in the lumen of the ileum.
Many enzymes are attached to epithelial cells of the villi. Some enzymes absorbed onto epithelial cells, entangled with glycoproteins in the plasma membrane. This is an efficient way of ensuring the products of digestion are in close contact with cells in which they will be absorbed.
Plasma membranes of epithelial cells also contain enzymes themselves, with active sites exposed to the outside of the cell.
The Small Intestine continued
Absorption in the Small Intestine
Glucose is absorbed via active transport. Na+ ions pumped out of epithelial cells (using ATP) into the tissue fluid surrounding them, against the conc. gradient. Conc. of Na+ ions outside the cell is greater than inside. Na+ ions then flow back into the cells from the lumen of the ileum, taking glucose with them via a protein channel. This is called Co-transport. Amino acids also absorbed in this way.
Fatty acids and glycerol are lipid soluble thus diffuse through the plasma membrane. Once inside the epithelial cell, they are converted back to triglycerides on the S.E.R, transferred to Golgi apparatus and covered in a protein coat to form a chylomicron. Chylomicrons then leave epithelial cells and enter the lymphatic capillaries (form milky emulsion, lacteals).
The Colon, Caecum, Appendix and the Rectum all part of the large intestine.
Wall of the colon has no villi, but does have a folded inner surface for absorption. The epithelium contains columnar cells with microvilli and large numbers of goblet cells.
The function of the colon is to absorb inorganic ions and water. Ions are absorbed by active transport, the water diffuses.
The caecum and appendix has no function = Vestigial organs.
Any indigestable material that still remains in the colon is passed onto the rectum and excreted through the anus as faeces.