Digestion

Digestion

Human can eat a wide variety of foods, containing many different biochemicals. Most of these biochemicals are useful to the body but cannot be absorbed or used in the form they are in when they are eaten-they have to be digested. Proteins, polysaccharides and lipids are all digested. This is essential, because they consist of large molecules, which cannot be absorbed through the wallof the gut into the bloodstream. The membranes of cells in the wall of the gut are impermeable to large molecules. Digestion is a chemical process. The large food substances are converted by chemical reactions into smaller molecules such as monosaccharides and amino acids. These small molecules can pass easily through the gut wall.

There is second reason for digesting food molecules. Plants,fungi or other animals have produced them, to perform functions in those organisms. They are unlikely to be ideally suited to perform function in human body. By breaking down proteins and other macromolecules, the human body can obtain all subunits that it needs to build up its own macromolecules. For examples, there are nine amino acids that human must obtain by digesting proteins in food. Otheramino acids can be synthesized.

Starting points:

• The bulk of carbon compounds that make up the cytoplasm of cell consist of carbohydrates, lipids, and proteins.
• In cells, energy is transferred by breakdown of nutrients, principally carbohydrates such as glucose, by the process of cell respiration.
• Animals obtain nutrients by digestion of food substances, followed byabsorption of the products of digestion – described as heterotrophic nutrition.
• Enzymes are biological catalyst made of protein, produced by cells to bring about chemical changes inside cells, such as cell respiration, or outside cells, such as digestion.

Some mammals eat only plant material (herbivores), some eat only other animals (carnivores), and other eat both animals and plant material(omnivores). Whatever is eaten, a balance of the essential nutrients is required by the body. The sum total of the nutrients eaten is the diet.

The mammalian gut is a long, hollow muscular tube connecting mouth and anus. Along the gut are several glands, and the whole structure is specialized for the movement and digestion of food and absorption of the useful products of digestion.

**Draw andlabel a diagram of the digestive system. The diagram should show the mouth, esophagus, stomach, small intestine, anus, liver, pancreas and gall bladder. The diagram should clearly show the interconnections between these structures.

[pic]

|The five steps of holozonic (feeding like an animals) nutrition |
|Step|Process |
|Ingestion |Food taken into mouth for processing in the gut |
|Digestion |Mechanical digestion by action of teeth and the muscular walls of|
||the gut. Chemical digestion by enzymes, mainly in the stomach and|
| |intestine. |
|Absorption |Soluble products of digestion absorbed into blood circulation |
||system (into lymphatic system if fat droplets) |
|Assimilation |Products of digestion absorbed from blood into body cells (such |
| |as liver and muscle cells) and used or stored. |...