The Need for Food

This sub-topic establishes animals’ need for food. The major food types and their functions are introduced as a prelude to a study of the need for the process of digestion.  The role of digestive enzymes leads to an appreciation of some detail of absorption and transport of simple molecules.

Food types

Animals need food for several reasons:

• Energy.
• The raw materials for growth and development.
• To help in important chemical reactions.
• To ensure the healthy operation of our digestive organs.

There are several categories of food:

• Carbohydrates – e.g. starch, glycogen, cellulose.
  • Contains the elements carbon, hydrogen and oxygen.
  • Made up from sugar subunits joined together.
  • Used for energy.

• Proteins – e.g. albumen, enzymes:
  • Contain the elements nitrogen, carbon, hydrogen and oxygen.
  • Made up from amino acid sub-units joined together.
  • Used for growth, repair and replacement of tissues.
  • Excess protein broken up into urea and carbohydrate in the liver.
  • Proteins are functional – that is they have a job to do in the body – when they are broken down into shorter, non-functional chains of amino acids we refer to them as peptides.

Fats: Contains the elements carbon, hydrogen and oxygen Made up from one glycerol and three fatty acid subunits. Used for energy. Excess carbohydrates are converted to fats by the body for storage. Minerals: Simple chemical substances required in small quantities for vital chemical reactions.

• Vitamins:
  • Complex chemical substances required in small quantities for vital chemical reactions.

• Roughage:
  • Indigestible by humans.
  • Made up of cellulose from plant cell walls.
  • Helps peristalsis in the gut and keeps the gut healthy.

Digestion and the digestive system

Starch, protein and fat consist of large, insoluble molecules:

• To pass through the gut wall these must be converted into small, soluble molecules
• To do this at body temperature enzymes must be used (see Investigating Cells topic)

Digestion is the process whereby large, insoluble food molecules are converted into small, soluble food particles by enzymes.

The digestive system consists of:

• the teeth.
• the alimentary canal – a tube running from mouth to anus.
• enzyme producing organs.
• bile production and storage organs (see below).

The teeth:

Break up the food by cutting and chewing (mechanical breakdown – as opposed to chemical breakdown – the food is chopped up into smaller parts without being changed chemically).

• Help to mix the food with saliva (see below).

Different teeth have different functions:

• Incisors – sharp and thin for cutting.
• Canines – pointed for tearing.
• Pre-molars and molars – thick and flat topped for grinding.

Dentition is the number and type of teeth possessed by an organism.

Carnivores:

• Incisors small.
• Long pointed canines for killing, holding and tearing.
• Four molars adapted into very large carnassials for cracking bones and cutting through tough tissues.
• Other molars are pointed for chewing meat.

Herbivores:

• Large, well developed incisors for cutting vegetation.
• No canines.
• The space where the canines were is stretched into a gap called the diastema.
• Molars are flat and ridged for grinding plant material.

Omnivores:

• Incisors, canines and molars present.
• All teeth roughly the same size.
• Relatively little difference between them.

The mouth:

• Salivary glands produce saliva:
  • Salivary amylase in the saliva begins breaking up starch into maltose sugar.
  • Water and mucus in the saliva will lubricate the ball of food.
• Teeth mix the food with saliva and begin mechanical breakdown of the food.

The pancreas:

• Produces these enzymes:

	Pancreatic amylase	starch  maltose
	Trypsin	longer peptides shorter peptides
	Lipase	fats fatty acids and glycerol

The wall of the small intestine: Produces these enzymes:

Digestion is completed at this point.

Absorption: The surface of the small intestine is deeply folded to provide a large surface area for absorbing food into the blood.
The folds are covered in villi to further increase the surface area. It has an excellent blood supply. The short distance between the blood and the food improves diffusion. A villus is about 1 mm long and the entire surface of the small intestine is covered with them. Amino acids and sugars diffuse into the blood capillaries Products of fat digestion diffuse into the lacteal. The lymphatic system carries the products of fat digestion to the blood.

The appendix:

• Between the small and the large intestine.
• A legacy organ – it no longer performs any useful function in humans.

The large intestine:

• Removes water and salt from the waste material after digestion and absorption to form faeces.

The rectum and anus:

• The rectum stores faeces.
• The anus is the opening at the end of the digestive system where food is egested (opposite of ingested).

Digestive Enzymes – Summary

Enzymes are used in digestion to:

• Break down large, insoluble food molecules into smaller, soluble food molecules.
• So that they can be absorbed through the villi of the small intestine into the blood.
• The chemical an enzyme acts on is its substrate.
• The chemical(s) produced by the enzyme are the products.

The enzymes involved in digestion are:

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Reproduction

Pupils should be aware that selection favours those individuals that leave most surviving offspring.  This sub-topic provides opportunities for pupils to investigate ways in which animals achieve this through sexual reproduction. Aspects to be considered are the achievement of fertilisation, protection of the developing embryo and care of the young.

In terms of evolution:

• The success of an individual organism is measured by how many of its offspring survive to reproduce.
• In this way an organism’s genes are passed on to future generations.
• The fittest organisms survive and breed and in this way the best genes are passed on to the next generation.
• This is the first step in evolution.

Gametes In animals the male sex cells are sperm and the female sex cells are ova (singular ovum).  It is just about acceptable to refer to these as ‘eggs’ at this stage though ‘ova’ is preferable. The drawing is not to scale. the ovum is much bigger than the sperm. The ova possess a food supply.

The sperm can move by using its tail.

Fertilisation

When the ovum and sperm meet:

• The sperm burrows into the ovum.
• The nucleus of the ovum fuses with that of the sperm to produce a single nucleus with genetic information. from both parents.
• This process is called fertilisation.

The resulting fertilised cell is called a zygote.

Early in evolution animals were water dwelling or amphibians: 

• Ova were laid by the female in the water.
• The male came later and fertilised the ova.
• The sperm could swim to the ova through the water.
• This meant that amphibians could not move far from sources of water for breeding. This is called external fertilisation.

Later in evolution reptiles, mammals and birds evolved that can live far from sources of water for breeding:

• Ova are kept in the female’s body.
• Sperm is placed inside the female.
• The inside of the female’s reproductive organs are moist so that the sperm can swim to the ovum.
• Fertilisation takes place within the female’s reproductive organs.
• This is called internal fertilisation.

The structure and function of the male and female reproductive system:

Notes on the diagrams: Sperm are produced in the testes (sing. testis). Ova (eggs) are produced in the ovaries Fertilisation occurs when the sperm and ovum meet and fuse together. The new, fertilised cell is called a zygote. Fertilisation occurs in the oviduct (see diagram).

Development of fertilised eggs in fish:

• Fish eggs are covered with a tough, flexible covering.
• The embryo fish gets its food from a yolk sack Generally fish do not protect their eggs.
• Fish produce many eggs at a time to ensure some will survive.
• Most fish emerge from their eggs able to survive without help from their parents.
• Some fish do protect their eggs by:
  • Hiding them Protecting them.
  • Camouflaging them.
• Fish that protect their eggs need to lay fewer.

Development of fertilised eggs in mammals: The egg moves along the oviduct. It embeds in the wall of the uterus. A placenta forms connected to the embryo by an umbilical cord. The placenta carries food and oxygen to the foetus. The placenta carries carbon dioxide and waste to the mother. The foetus is protected from shocks and bumps by the amniotic fluid produced by the amniotic membrane. Humans protect their young for many years after birth. Mammal young are helpless at birth and can’t survive without parental help.

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Water and Waste

This sub-topic focuses on the need to maintain an internal water balance and to remove poisonous wastes from the body.  The structure and functioning of the mammalian kidney in relation to these needs is explored. Some relevant applications to the maintenance of human health are considered.

Mammals need to keep the water concentration of their blood the same as the water concentration of their cells to avoid osmotic problems.

Kidneys can be damaged by disease or accidents

• This causes dangerous levels of urea to build up in the bloodstream causing serious illness or even death.
• The kidney can be replaced by a machine or by a transplant.
• The kidney machine uses dialysis tubing (visking tubing) to filter the urea from the blood:
  • The machine is expensive and slow to work.
  • The patient will need to spend around 12 hours connected to it twice a week There is no operation or problems with rejection.
• A kidney can be transplanted from a donor:
  • There is no need to be connected to the machine.
  • There is a danger of the body rejecting the transplant.
  • Anti rejection drugs will need to be taken and these reduce the body’s ability to fight disease The patient will need to undergo a major operation.

Responding to the Environment

This sub-topic highlights how some organisms respond to environmental stimuli by behaviour which helps to ensure survival.

The environment is filled with stimuli:

• a stimulus is some factor in the environment which can be detected by the organism and changes its behaviour.
• A response is the change in behaviour caused by the stimulus.
• There are many stimuli in the environment e.g.:

• Some example of stimuli and responses are:
  • Blowfly maggots and slaters move away from light – to escape predators.
  • Paramecium move towards acid – because their prey organism, bacteria, produce acid.
  • Slaters move towards moisture – to prevent drying out.

Some behaviour is rhythmical:

• It occurs at regular intervals:
• A stimulus which causes this kind of response is called a trigger stimulus:
• Daily:
  • The trigger stimulus is darkness or daylight.
  • Such as sleep and feeding patterns – to make use of the daylight or the darkness.
  • Examples of night active organisms are bats and owls, and day active organisms are cows and sheep.
• Tidally:
  • The trigger stimulus is the tide rising and falling.
  • Sea anemones come out of their protective jelly-like body at high tide to feed on the plankton.
  • Crabs feed along the shoreline at low tide when there is a lot of debris.
• Annually:
  • Once a year.
  • The trigger stimulus can be the days lengthening or shortening.
  • Geese migrate to Islay in the autumn and to Greenland in the spring.
  • Sheep and cows breed in the autumn to make sure the young are born in the spring when there is plentiful food.

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