Digestion + Liver Test

Small Intestine Function

-The small intestine is where usuable food substances (e.g. nutrients) are absorbed into the bloodstream -The pancreas and gall bladder (via the bile duct) both secrete substances into the small intestine to aid in digestion -The small intestine is lined with smooth muscle to allow for the mixing and moving of digested food products (via segmentation and peristalsis) -It also contains small pits (crypts of lieberkuhn) that secrete intestinal juices -The small intestine contain infoldings called villi, to increase surface area and optimise the rate of absorption

Stomach Function

-The stomach acts as a temporary storage tank and is where protein digestion begins -The stomach contains gastric glands which secrete digestive juices for chemical digestion -Acids create a low pH environment (pH~1-2) that denatures proteins, while proteases like pepsin hydrolyse large proteins -The stomach also releases a hormone (gastrin) that regulates stomach secretions -The mechanical action of the stomach (churning) also promotes digestion by mixing the food -The stomach turns food into a creamy paste called chyme

Explain the need for enzymes in digestion.

-Enzymes are biological catalysts which speed up the rate of a chemical reaction (e.g. digestion) by lowering the activation energy -Enzymes allow digestive processes to occur at body temperature and at sufficient speed to meet the organism's survival requirements -Enzymes are specific for a given substrate and so can allow digestion of certain molecules to occur independently of others

Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion

-Some digestive enzymes are immobilised on the plasma membrane of the epithelial cells of the small intestine, serving two main benefits: ----The enzyme is fixed in place and does not pass through the digestive system, meaning it can be reused ----The enzyme can be linked to secondary functions (e.g. membrane transport)

Large Intestine Function

-The large intestine absorbs water and dissolved minerals from the indigestible food residues, and by doing so converts what remains from a fluid state into a semi-solid feces -The faeces is stored in the rectum and eliminated out the anus

There are two major groups of organs that comprise the human digestive system

Alimentary Canal: Contains organs through which the food actually passes (esophagus, stomach, small intestine, large intestine, etc.) Accessory Organs: Organs that assist in digestion but no food passes through them (liver, pancreas, gall bladder, salivary glands, etc.)

State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas, and the wall of the small intestine.

Chemical digestion in the alimentary canal involves the secretion of digestive juices capable of breaking down complex macromolecules: Salivary glands - secretes saliva which contains amylase Gastric glands - secretes gastric juices which include hydrochloric acid and pepsinogen Pancreas - secretes pancreatic juices containing lipase, trypsinogen and hydrogen Intestinal wall - The small intestine contains the crypts of Lieberkuhn, which secrete a variety of substances as part of the intestinal juice

Explain the structural features of exocrine gland cells.

Exocrine glands have ducts through which they secrete their product (these ducts may arise from the convergence of smaller ductules) The ducts / ductules arise from a cluster of cells called an acinus (plural = acini), surrounded by a basement membrane Acini are lined by a single layer of secretory cells which release the exocrine product into the lumen of the duct via secretory vesicles Secretory cells are held together by tight junctions, and possess a highly developed rough ER and golgi network for material secretion

Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this

Lipids are hydrophobic and hence insoluble within the aqueous environment of the body They will group together (coalesce) to form large droplets of fat (fat globules) The enzyme responsible for lipid digestion (lipase) is water soluble and can only bind the lipids to its active site at the lipid-water interface (i.e. the surface of the fat globule) As the interior of the fat globule is inaccessible to lipase in this form, this would make lipid digestion normally very slow

Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Microvilli: Greatly increase the surface area of the villus, allowing for a greater rate of absorption Rich capillary networks: Help to maintain a concentration gradient for absorption by rapidly transporting absorbed products away Single epithelial layer: Ensures minimal diffusion distance between the intestinal lumen and capillary network Lacteals: Absorb lipids from the intestine into the lymphatic system (which are later reabsorbed back into normal circulation) Intestinal crypts: Located between villi and release juices that act as a carrier fluid for nutrients Membrane proteins / mitochondria: High amounts to enable active transport into cells (contents then passively diffuse into bloodstream)

Explain why digestion of large food molecules is essential.

Most food is solid and in the form of large complex molecules which are insoluble and chemically inert (not readily usable) -As food was synthesised by other organisms, it contains materials not suitable for human tissue - these need to be separated and removed -Large molecules need to be broken down into smaller molecules that can be readily absorbed across membranes and into cells -Small molecules can be reassembled into new products (e.g. amino acids can be reassembled to make new proteins)

Outline the control of digestive juice secretion by nerves and hormones, using the example of secretion of gastric juice

Pre-Ingestion -The sight and smell of food triggers a reflex response in which gastric juice is secreted from gastric pits in the stomach wall -This ensures that gastric juice is in the stomach by the time the food is consumed Post-Ingestion -Food entering the stomach causes distension, which is detected by stretch receptors in the stomach lining Impulses are sent to the brain, which triggers the secretion of gastrin from the pits lining the stomach wall -Gastrin causes the sustained release of gastric juice, particularly its acid component -When the pH drops too low, gastrin secretion is inhibited by hormones (secretin and somatostatin)

Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers

Stomach ulcers are inflammed and damaged areas in the stomach wall There is a strong correlation between H. pylori infection and the development of stomach ulcers H. pylori is a bacterium that can survive the acid conditions of the stomach. It secretes urease which neutralises the gastric acid to lower the acidity of the stomach for further colonisation It also secretes proteases (e.g. mucinase) to degrade the mucosal lining of the stomach wall, allowing it to burrow into this lining The degradation of this protective lining by H. pylori allows for damage to the stomach wall by gastric acids (causing ulcers) There is a correlation between chronic H. pylori infection over a number of years (20 - 30 years) and the development of stomach cancer

Assimilation

The conversion of nutrients into fluid or solid parts of an organism

Absorption

The movement of a fluid or dissolved substances across a membrane

Outline the reasons for cellulose not being digested in the alimentary canal (the whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines.)

While humans can digest polymers of a-glucose (e.g. starch, glycogen), they cannot digest the polymer of ß-glucose (cellulose) This is because they do not produce the necessary enzyme (cellulase) and lack bacteria in their gut capable of digesting cellulose Cellulose is a component of plant cell walls and the main source of dietary roughage (undigested, it creates bulk which stimulates peristalsis)