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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…

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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.…

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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…

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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.
Selective re-absorption of all the sugar
Selective re-absorption…

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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…

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3. 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…

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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.


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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…

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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…


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