AQA Biology Unit Three (higher) revision notes

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  • Created on: 17-05-10 18:04
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Unit Three
Dissolved substances move by diffusion.
Active transport: substances are sometimes absorbed against a concentration gradient.
This requires the use of energy from respiration.
It enables cells to absorb ions from very dilute solutions.
Other substances, such as sugar and ions, can also pass through cell membranes.
Many organ systems are specialised for
exchanging materials.
The lungs are in the upper part of the body,
(thorax), protected by the ribcage and
separated from the lower part of the body,
(abdomen), by the diaphragm.
The breathing system takes air into and out of
the body so that oxygen from the air can
diffuse into the bloodstream and carbon
dioxide can diffuse out of the bloodstream into
the air.
The alveoli provide a very large,
(spherical shape), moist surface, richly
supplied with blood capillaries,
(maintains concentration gradient by high carbon dioxide levels
and low oxygen levels), so that gases can readily diffuse into and
out of the blood.
Atmospheric gas Air Air breathed out
Nitrogen 80% 80%
Oxygen 20% 16%
Carbon dioxide 0.04% 4%
Gas exchange in other organisms:
Frogs: tadpoles have frilly external gills with a large
surface area and a rich blood supply. The tadpoles
get all their oxygen in and carbon dioxide out by
diffusion from the water through these gills.
In adult frogs these gills disappear as an adult frog
has very moist skin with a rich blood supply for use
in water and also has a simple pair of lungs for use
on land.
Fish: gills made up of many thin
layers of tissue, (short diffusion distance),
with a rich blood supply. They are also always
moist as they work in water. Water is
pumped over them constantly to maintain
the concentration gradient.

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Insects: along the side of an insect there are spiracles which open when plenty of oxygen is
needed and close when it isn't, (preventing water loss). The spiracles lead to a tube system,
(containing tracheoles where most gas exchange takes place), which runs right into the cells
of the insect. The tracheoles are very moist and air is pumped in and out of them by the
insect to maintain a concentration gradient.…read more

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Valves to stop the backflow of blood
Walls one cell thick
Low pressure and speed (allows exchange of materials)
Blood plasma transports:
Carbon dioxide: organs to lungs
Soluble products of digestion: small intestine to other organs
Urea: liver to kidneys
The energy released during respiration is used to enable muscles
to contract, (increased demand during exercise).…read more

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The kidney
People whose kidneys do not function properly die because toxic substances accumulate in
their blood.
A healthy kidney produces urine by:
Ultra filtration: small molecules, (water, glucose, urea pass from the blood
capillaries under high pressure into the kidney tubules.…read more

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Microorganisms in food production
Biogenesis: living matter arises only from living matter
Spontaneous generation (abiogenesis): the theory supposed the production of living
organisms from non-living mater.
The development of the theory of biogenesis:
Boiled two sets of broth to kill the microbes.
Sealed one flask and left the other open, only the
open one went off.
This showed that microbes got into the food from
the air.
But opponents thought it meant air from outside
the flask was necessary.…read more

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Wine filtered to remove yeast
In the production of yoghurt:
1. A starter of bacteria is added to warm milk
2. The bacteria ferment the milk sugar (lactose) producing lactic
acid, (giving yoghurt its tangy taste)
3. The lactic acid causes the milk to clot and solidify into yoghurt
In the production of cheese:
1. A different starter of bacteria is added to warm milk
2. The bacteria ferment the milk sugar (lactose) producing
much more lactic acid than in yoghurt production
3.…read more

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Main constituents in biogas: Percentage in the mixture by volume:
Methane 50-80
Carbon dioxide 15-45
Water 5
On a large scale, waste from sugar factories or sewage works can be used. On a small scale,
biogas generators are used to supply the energy needs of individual families or farms.…read more

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They work best at around 30°C, so biogas generators work best in hot countries. However
the process is exothermic, so if the generators are well insulated, (e.g. underground), they
can work anywhere.
Ethanol-based fuels can be produced by the anaerobic fermentation of:
Sugar cane ==(fermentation)==> Ethanol
Maize ==> Starch broken down to sugars by carbohydrases ==(fermentation)==> Ethanol
The ethanol is distilled from the products of the fermentation and can be used in motor
vehicle fuels.…read more

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The lid of the Petri dish should be taped down to prevent microorganisms from the
air contaminating the culture
In school and college laboratories, cultures should be incubated at a maximum temperature
of 25 °C which greatly reduces the likelihood of pathogens growing that might be harmful to
humans. In industrial conditions higher temperatures can produce more rapid growth.…read more


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