Fats can be classified as either saturated or unsaturated depending on the structure of the long carbon-carbon chains in the fatty . Fats that contain no double bonds in their fatty chains are referred to as saturated fats. These fats tend to be solid at room temperature, such as butter or animal fat. The consumption of saturated fats carries some health risks in that they have been linked to arteriosclerosis (hardening of the arteries) and heart disease. Unsaturated fats contain some number of double bonds in their structure. These fats are generally liquids at room temperature (fats that are liquid at room temperature are referred to as oils). Unsaturated fats can be either polyunsaturated (many double bonds) or monounsaturated fats (one or few double bonds). Recent research suggests that the healthiest of the fats in the human diet are the monounsaturated fats, such as olive oil and canola oil, because they appear to be beneficial in the fight against heart disease.
Give an account of enzyme activity in the gut indicate conditions under which they function
Once food enters the stomach the muscular wall contracts and relaxes to churn and mix the food. To make this really efficient, the stomach lining has an extra layer of oblique (diagonal) muscle on the lumen side of the circular muscle.
The mucosa contains mucus-secreting goblet cells in the columnar epithelium layer. This mucus creates a barrier and prevents autodigestion (digestion of an organisms own tissue by its own enzymes).In the wall of the stomach are pits (gastric glands) created by infoldings of the epithelium. Some cells lining the pits are called chief cells. These release an inactive enzyme called pepsinogen. Other cells called parietal cells (or oxyntic cells) release HCl (hydrochloric acid).
The acid has several beneficial effects:
- It creates the right pH for the enzymes to work efficiently.
- It kills bacteria that have escaped the lysozyme in the mouth.
- It removes a portion of the pepsinogen protein so that it becomes active pepsin.
It is important to have the pepsin enzyme since it begins the digestion of proteins into shorter polypeptides. The pepsinogen and acid together are called the gastric juice. They are released because the presence of food in the mouth, the smell of the food etc. triggers off impulses to the brain which then send impulses to the gastric glands in the stomach to release these substances.The physical stretching of the stomach when food is in it stimulates the glands themselves to secrete a hormone gastrin. The hormone is released into the blood and its target cells are those of the gastric glands so this too causes the release of the gastric juice. Gastrin also stimulates the pyloric sphincter (between the stomach and first part of the small intestine) to relax. Acidic, churned, partially digested food known as chyme is slowly released into the duodenum.
Most chemical digestion by enzymes takes place in the duodenum.The mucosa is folded and the millions of microscopic projections created by this folding of the inner surface of the wall are called villi. In between the villi are intestinal glands (or crypts of Leiberkuhn) which secrete intestinal juice. The possession of the villi and the folds in the cell surface membranes of the epithelial cells lining the villi (microvilli) massively increases the surface area. The epithelial cells produce various enzymes that are not released into the lumen but are bound to the cell surface of the microvilli (also called brush border because of its appearance). Other enzymes, found free in the lumen of the duodenum have been produced and released by the pancreas. As well as these enzymes, sodium bicarbonate solution is secreted into the duodenum to provide the optimal pH for the enzymes found in this region of the gut.The liver also secretes a substance called bile into the duodenum. (Bile may be stored in the gall bladder before release). Bile consists of bile salts that emulsify fats. The large globules of fat are mechanically dispersed into many smaller globules. This increases the surface are over which lipases can act.
The release of these substances, as in the stomach, is controlled:
1. When the acidic chyme touches the mucosa of the duodenum impulses are sent to the brain. The brain then sends impulses to the gastric glands to slow down the release of gastric juice (since the food has moved on). The acid also stimulates the release of a hormone called secretin.
Secretin affects the pancreas and liver and brings about:
a) the release of sodium bicarbonate solution from the pancreas and,
b) the production of bile from the liver.
2. The presence of the food in the duodenum stimulates the glands to produce their enzymes. It also stimulates the release of a hormone called either CCK (cholecystokinin) or pancreozymin. It brings about:
a) the release of bile from the liver and,
b) the production and release of pancreatic juice (containing enzymes).
Briefly describe absorption of products of digestion
Large molecules have now been chemically digested into their constituent subunits by enzymes. They now need to be absorbed through the intestine wall, along with mucus and the digestive juices. Absorption takes place by simple diffusion, facilitated diffusion and active transport. Monosaccharides, amino acids, dipeptides and tripeptides are actively transported into the epithelial cells lining the wall.
The fatty acids, glycerol and monoglycerides form complexes with bile salts called micelles. The micelles come into contact with epithelium cells and the fat-soluble fat components diffuse into the epithelium cell, leaving behind the fat insoluble bile salts.
In the epithelium, the fatty acids and glycerol are reformed by the endoplasmic reticulum to make triglycerides. They are packaged into globules with cholesterol and phospholipids and then coated with protein to be passed out from the epithelial cell into the lacteal.
Eventually these packages, called chylomicrons, are emptied into the blood.
