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A-level
Biology A-level: Enzymes
Nutrition

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.

Types of nutrition
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.
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Food must be:

   1. Ingested: Large molecules are taken into the mouth.
   2. Masticated: Chewed and mixed with saliva. This increases the surface
      area of the food over which the enzymes can act and makes the food
      easier to swallow
   3. Digested: Begins in the mouth. It involves mechanical digestion -
      mixing and churning - and chemical digestion - using enzymes.
   4. Absorbed: Products of digestion are passed across the gut lining.
   5. Egested: The undigested waste is eliminated by defaecation.

Herbivores
Herbivores eat only plants or algae. Terrestrial plants in particular have a lot
of cellulose present in their cell walls for support. Animals cannot digest
cellulose and since it surrounds the cell contents, this can present a problem.

Various adaptations to solve this problem are found in the animal kingdom.
Insects, for example, may have piercing mouthparts that are used to pierce the
cells and suck out the cell contents or the sap in the phloem tubes.

Elephants deal with it in a much more basic way - they eat virtually all the time!
If they eat enough they will receive enough nutrients to keep them alive.

Some herbivores such as sheep have good jaws and teeth to really grind the
food. They regurgitate and re-chew the food (chewing the cud). They also
possess a long gut so the food spends longer in there.

Many herbivores have symbiotic (mutually beneficial) bacteria in their
digestive tracts. These produce cellulase, an enzyme that digests cellulose
into simple carbohydrates. These carbohydrates are used by the herbivores and
by the microorganisms.

Digestion of the microorganisms themselves by the herbivore gives it a protein
supply - useful due to the fact that their diet provides very limited amounts of
protein.

				
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