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 glycogen 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.
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- 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
oesophagus (gullet):
- Food passes down to the stomach by the process of peristalsis.
- Lubricated with saliva.
Peristalsis:
- The muscles of the oesophagus relax around the ball of food
and contract behind it.
- A wave of muscular relaxation is followed by a wave of muscular
contraction and moves the food through the gut.
The stomach:
- Is a muscular sack.
- The muscles contract and relax to churn up the food.
- The stomach lining contains:
- Acid secreting cells help mechanical breakdown of protein.
- Enzyme secreting cells produce pepsin that breaks the protein
down into long peptides.
- Mucus secreting cells protect the stomach itself from the
acid and pepsin.
The small intestine:
- A long tube;
- At the start of the small intestine, just after the stomach,
the common bile duct brings in chemicals for digestion.
The liver:
- Produces bile salts:
- Which are stored in the gall bladder.
- And are added to food as it is released from the stomach.
- Bile salts are not enzymes but help lipase work.
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The pancreas:
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Pancreatic amylase
Trypsin
Lipase |
starch
maltose
longer peptides
shorter peptides
fats
fatty acids and glycerol |
The wall of the small intestine: Produces these enzymes:
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Peptidases |
peptides
amino acids |
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.
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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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Enzyme |
Produced by: |
Affect in: |
Substrate |
Product(s) |
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Salivary Amylase |
salivary glands |
mouth |
starch |
maltose sugar |
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Pepsin |
stomach wall |
stomach |
protein |
peptides |
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Trypsin |
pancreas |
small intestine |
peptides |
shorter peptides |
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Peptidase |
gut wall |
small intestine |
peptides |
amino acids |
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Pancreatic amylase |
pancreas |
small intestine |
starch |
maltose sugar |
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Lipase |
pancreas |
small intestine |
fats |
fatty acids and glycerol |
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.
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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).
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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.
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Water gain |
Water loss |
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drinking |
urine |
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in food |
sweat |
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from respiration |
breathing |
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faeces |
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During the day the water loss must equal the water
gain or cells will lose or gain water by osmosis.
The liver breaks down excess protein into carbohydrates and ammonia.
- The ammonia is changed into urea and released into the blood.
The kidney controls the water concentration of the blood:
- The kidney receives blood in the renal artery and returns it
in the renal vein.
- The kidneys produce the urine a solution of water and urea.
- Which drains through the ureters.
- And is stored in the bladder.
- Finally passing to the outside environment through the urethra.
The kidney is made up of many tiny structures called nephrons:
- each nephron filters the blood and then reabsorbs useful material.
- digestion of protein creates poisonous nitrogen compounds these
are changed to urea by the liver and dumped into the blood plasma.
- water full of salts, food molecules, urea and other chemicals
are forced out of the blood into the glomerulus.
- this is collected in the Bowman’s capsule.
- The kidney tubule reabsorbs the useful chemicals.
- Leaving behind water and urea.
- Several nephrons drain into a collecting tubule and then into
the ureter.
The brain detects the water concentration of the blood:
- If the water concentration is too high:
- The pituitary gland produces less ADH (Anti
Diuretic Hormone).
- The kidney reabsorbs less water from the urine.
- Lots of dilute urine is produced.
- If the water concentration is too low:
- The pituitary gland produces more ADH.
- The kidney reabsorbs more water from the urine.
- Little, concentrated urine is produced.
- In this way the water concentration of the blood is maintained.
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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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