Topic 6.1 Digestion and Absorption Flashcards
Alimentary Canal
Oesophagus
• A hollow tube connecting the oral cavity to the stomach (separated from the trachea by the epiglottis)
Stomach
• A temporary storage tank where food is mixed by churning and protein digestion begins
Small Intestine
• Consists of three sections – the duodenum, jejunum and ileum
Large Intestine
• The final section of the alimentary canal, where water and dissolved minerals (i.e. ions) are absorbed
• Consists of the ascending / transverse / descending / sigmoidal colon, as well as the rectum
Accessory Organs
Salivary Glands
- Release saliva to moisten food and contains enzymes (e.g. amylase) to initiate starch breakdown
Pancreas
- Produces a broad spectrum of enzymes that are released into the small intestine via the duodenum
Liver
- Its role includes detoxification, storage, metabolism, bile production and hemoglobin breakdown
Gall Bladder
- The gall bladder stores the bile produced by the liver (bile salts are used to emulsify fats) Bile stored in the gall bladder is released into the small intestine via the common bile duct
Mechanical digestion - Chewing (mouth)
- Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
- The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
- The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity
Mechanical digestion - Churning (stomach)
- The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices (‘churning’)
- Food is digested within the stomach for several hours and is turned into a creamy paste called chyme
- Eventually the chyme enters the small intestine (duodenum) where absorption will occur
Peristalsis
- Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
- Continuous segments of longitudinal smooth muscle rhythmically contract and relax
- Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
Segmentation
- Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
- Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices
- While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement
Stomach Acids
- The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
- The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
- The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
- The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine
Bile
- The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
- Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
- The emulsification of fats increases the total surface area available for enzyme activity (lipase)
Enzymes
- Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
- Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
- Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations
Carbohydrate
- Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
- Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
- Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
- Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested
Proteins
- Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
- Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
- These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine
Lipids
- Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
- The smaller fat droplets are then digested by lipases released from the pancreas
Nucleic Acids
The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides
Structure of Small Intestine
- Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
- Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
- Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
- Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen
Membrane transport mechanisms
Co-transport (active): for glucose, amino acids
Facilitated diffusion: monosaccharides
Osmosis: water
Simple diffusion: triglycerides
