S-Cool Revision Summary

S-Cool Revision Summary

The whole point of nutrition is to obtain a source of energy and of carbon . This allows cell processes to continue and for growth and repair to take place. Obtaining this energy and carbon is done in a variety of ways by different organisms.

The following is a selection of the different types of nutrition which living things use:

Autotrophic means self-feeder, i.e. they produce their own food out of raw materials.

Heterotrophic means that they feed on other organisms that have made their own food.

Photoautotrophic: e.g. plants and algae. Light is the primary source of energy. Carbon dioxide is the primary source of carbon.
Chemoautotrophic: e.g. some bacteria. A chemical is the primary source of energy and the carbon source may be organic or inorganic.
Chemoheterotrophic: e.g. animals, fungi, and most protoctista. Their energy and carbon source is often glucose.

Within the chemoheterotrophs there may be:

Saprotrophic nutrition: e.g. fungi. They feed on the soluble organic matter from dead organisms. Enzymes are secreted onto the dead organism to digest the large molecules so digestion is external. The small molecules are absorbed and then transported within the fungus.
Parasitic nutrition: e.g. tapeworm. They feed on already digested food (digested by the host of the endoparasite). The parasite therefore produces no enzymes and needs no gut system. Small molecules are absorbed over the body surface.
Holozoic nutrition: e.g. many animals and carnivorous plants. They feed on solid organic matter from a living or dead organism. They therefore will need to be able to catch or obtain and produce enzymes to digest their food.

Food must be:

Ingested

Masticated

Digested

Absorbed

Egested

Tissue layers of the gut wall

The submucosa contains nerves, blood and lymph vessels, collagen and elastic fibres.

The nerves regulate:

The gut movements by muscle contractions to force the food along or to mix the food with secretions in a particular region.

The digestive secretions into the lumen of the gut.

Even before food enters the mouth the sight, smell and thought of the food stimulates a conditional reflex that results in the release of saliva into the mouth.

When food enters the mouth, the stimulation of the taste buds results in an unconditional reflex, whereby impulses are relayed to the brain via sensory neurones and then via motor neurones to the salivary glands. Again, releasing saliva.

1 - 1.5 litres of saliva is released each day.

The saliva contains mucus, which lubricates the food, mineral salts to activate enzymes, lysozyme which kills bacteria entering with the food, and amylase, an enzyme that breaks down starch into shorter polysaccharides and then into maltose.

Chewing mechanically breaks up the food so that there is a larger surface over which the amylase can work. The food and saliva mixture is pushed into a ball called a bolus and swallowed.

The oesophagus is a muscular tube with a squamous epithelium lining and mucus glands to lubricate the passageway down to the stomach.

Peristalsismoves the food down and when the food reaches the lower portion of the tube, the circular muscle making up the sphincter (a muscular ring controlling the passage of food between consecutive sections of the gut) relaxes and opens.

With no food present, the sphincter remains closed so that no acid can enter and burn the oesophagus.

Most chemical digestion by enzymes takes place in the duodenum.

The mucosais 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.