Edexcel IGCSE Biology

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  • Created on: 06-05-22 11:10

Plant Transport - Transport Systems

Multicellular organisms need transport systems.

They need a variety of substances to live e.g. water, minerals and sugars

They need transport systems to move substances to and from individual cells quickly.

They also need to get rid of waste substances.

The xylem and phloem are arranged throughout the root, stem and leaves in groups called vascular bundles

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Plant Transport - Xylem


The xylem tube transports water and minerals.

The xylem carry water and minerals salts from the roots up the shoot to the leaves in the transpiration stream.


It is composed of dead cells which form hollow tubes

Xylem cells are strengthened by lignin and so are adapted for the transport of water in the transpiration stream

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Plant Transport - Phloem


The phloem transports sugars, like sucrose, and amino acids where they're made in the leaves to other parts in the plant.

This movement of food substances around the plant is known as translocation.


The cells are living cells and are not hollow

Substances move from cell to cell through pores in the end walls of each cell

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Plant Transport - Root Hair Cells


Root hair cells are adapted for the efficient uptake of water (by osmosis) and mineral ions (by active transport)

Provide anchorage for the plant


Root hairs increase the surface area to volume ratio significantly

This increases the rate of the absorption of mineral ions by active transport

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Plant Transport - Water Uptake and Movement

Root hair --> Root cortex cells --> Xylem of root --> Xylem of stem --> Xylem of leaves --> Mesophyll cells

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Plant Transport - Transpiration

Transpiration is the loss of water from the plant.

Transpiration is caused by the evaporation and diffusion of water from a plant's surface. Most transpiration happens at the leaves.

This evaporation creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it. 

This in turn means more water is drawn up from the roots, and so there's a constant transpiration stream of water through the plant.

Note - Transpiration is just a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata by diffusion.

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Plant Transport - Factors that Affect Transpiratio

Light Intensity - The brighter the light, the greater the transpiration rate. Stomata begin to close as it gets darker. Photosynthesis can't happen in the dark, so they don't need to be open to let CO2 in. When the stomata are closed, very little water can escape.

Temperature - The warmer it is, the faster transpiration happens. When it's warm the water particles have more energy to evaporate and diffuse out of the stomata.

Wind Speed - The higher the wind speed around the leaf, the greater the transpiration rate. If wind speed around a leaf is low, the water vapour just surrounds the leaf and doesn't move away. This means there's a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn't happen as quickly. If it is windy, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly, from an area of high concentration to an area of low concentration.

Humidity - The drier the air around the leaf, the faster transpiration happens. This is like what happens with air movement. If the air is humid, there's a lot of water in it already, so there's not much of a difference between the inside and the outside of the leaf. Diffusion happens fastest if there's a really high concentration in one place and a really low concentration in the other.

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Plant Transport - Measuring Transpiration Diagram

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Plant Transport - Measuring Transpiration Notes

Rate of transpiration = distance moved by bubble/time

You can see how environemental conditions affect transpiration rates:

Airflow: Set up a fan or hairdryer

Humidity: Spray water in a plastic bag and wrap around the plant

Temperature: Temperature of room (cold room or warm room)

Light intensity: Lamp to increase or cupboard to decrease

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Tropisms - Growth Hormones

Auxins are plant hormones which control growth at the tips (meristem) of shoots (plumule) and roots (radicule). They move through the plant in solution (dissolved in water).

Auxin promotes growth in the shoot but inhibits growth in the root.

Auxins are involved in growth responses of plants to light (phototropism) and gravity (geotropism).

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Tropisms - Direction of Shoot Growth

Shoots are positively phototropic (grow towards light).

When a shoot tip is exposed to light, it accumulates more auxin on the side that's in the shade than teh side that's in the light.

This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light. 

Shoots are negatively geotropic (grow away from gravity).

When a shoot is growing sideways, gravity produces an unequal distrubution of auxin in the tip, with more auxin on the lower side.

This causes the lower side to grow faster, bending the shoot upwards.

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Tropisms - Direction of Root Growth

Roots are positively geotropic (grow towards gravity).

A root growing sideways will also have more auxin on its lower side.

But in a root the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.

Roots are negatively phototropic (grow away from light).

If a root starts being exposed to some light, more auxin accumulates on the more shaded side. 

The auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground.

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Nutrient Cycles - Recycled

Materials that organisms need to survive, such as carbon and nitrogen are recycled through both the biotic and abiotic components of ecosystems.

This means they pass through both living organisms (the biotic components of an ecosystem) and things like the air, rocks and soil (abiotic components of an ecosystem) in a continuous cycle.

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Nutrient Cycles - The Carbon Cycle

1) CO2 in the air to plants. The whole thing is powered by photosynthesis. Green plants use the carbon from CO2 to make carbohydrates, lipids and proteins.

2) Eating passes the carbon compounds in the plant along to the animals in a food chain or web.

3) Respiration by living plants and animals releases CO2 back into the air.

4) Plants and animals eventually die and decompose, or are killed and turned into useful products.

5) When plants and animals decompose, they're broken down by microorganisms, such as bacteria and fungi. These microorganisms are known as decomposers and they release enzymes, which catalyse the breakdown of dead material into smaller molecules. Decomposers release CO2 back into the air by respiration as they break down the material. 

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Nutrient Cycles - The Carbon Cycle

6) Some useful plant and animal products, e.g. wood and fossil fuels, and burned (combustion). This also releases CO2 back into the air.

7) Decomposition of metrials means that habitats can be maintained for the organisms that live there, e.g. nutrients are returned to the soil and waste material (such as dead leaves) doesn't just pile up.

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Nutrient Cycles - The Nitrogen Cycle

1) The atmosphere contains about 78% nitrogen gas, N2. This is very unreactive and so it can't be used directly by plants or animals.

2) Nitrogen is needed for making proteins for growth, so living organisms have to get it somehow. 

3) Plants get their nitrogen from the soil, so nitrogen in the air has to be turned into nitrogen compounds (such as nitrates) before plants can use it. Animals can only get proteins by eating plants (or each other).

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Nutrient Cycles - Nitrogen Fixing

Nitrogen fixation is the process of turning N2 from the air into nitrogen compounds in the soil which plants can use. There are two main ways that this happens:

a) Lightning - there's so much energy in a bolt of lightning that it's enough to make nitrogen react with oxygen in the air to give nitrates. 

b) Nitrogen-fixing bacteria in the soil and the roots of some plants.

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Nutrient Cycles - The Nitrogen Cycle

There are four different types of bacteria involved in the nitrogen cycle:

1) Decomposers - break down proteins (in rotting plants and animals) and urea (in animal waste) and turn them into ammonia (a nitrogen compound). This forms ammonium ions in the soil.

2) Nitrifying bacteria - turn ammonium ions in decaying matter into nitrates (nitrification). 

3) Nitrogen-fixing bacteria - turn atmospheric N2 into nitrogen compounds that plants can use.

4) Denitrifying bacteria - turn nitrates back into N2 gas. This is of no benefit to living organisms.

Some of these bacteria live in the soil and some of them live in nodules on plant roots.

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Microorganisms - Bacteria


  • Single-celled
  • Microscopic
  • No nucleus
  • Circular chromosone of DNA
  • Some can photosynthesise
  • Most bacteria feed off other organisms

Example: Lactobacillus bulgaricus

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Microorganisms - Viruses


  • These are particles, rather than cells, and are smaller than bacteria
  • They can only reproduce inside living cells
  • Example of a parasite - depends on another organism to grow and reproduce.
  • They infect all types of living organisms
  • They come in loads of different shapes and sizes
  • They don't have a cellular structure - they have a protein coat around some genetic material. 

Example: HIV

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Microorganisms - Fungi


  • Some are single-celled
  • Others have a body called a mycelium, which is made up of hyphae (thread-like structures). Hyphae contain lots of nuclei.
  • They can't photosynthesise
  • Their cell walls are made from chitin

Example: Yeast

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Microorganisms - Yeast

1) Bread dough is made by mixing yeast with flour, water and bit of sugar.

2) The dough is then left in a warm place to rise - this happens with the help of the yeast.

3) Enzymes break down the carbohydrates in the flour into sugars.

4) The yeast then uses these sugars in aerobic respiration, producing carbon dioxide.

5) When the oxygen runs out, the yeast switches to anaerobic respiration. This is also known as fermentation, and produces carbon dioxide and alcohol (ethanol).

6) The carbon dioxide produced is trapped in bubbles in the dough.

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Microorganisms - Yeast

7) These pockets of gas expand, and the dough begins to rise.

8) The dough is then baked in an oven, where the yeast continues to ferment until the temperature of the dough rises enough to kill the yeast. Any alcohol produced during anaerobic respiration is boiled away.

9) As the yeast dies, the bread stops rising, but pockets are left in the bread where the carbon dioxide was trapped.

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Microorganisms - Yeast Rate of Respiration

Respiration is controlled by enzymes - so as temperature increases, so should the rate of respiration up until the optimum temperature.

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Microorganisms - Making Yoghurt

Fermentation is when microorganisms break sugars down to release energy - usually by anaerobic respiration. Yoghurt is basically fermented milk. How it is made:

1) The equipment is sterilised to kill off any unwanted microorganisms.

2) The milk is pasturised (heated up to 72C for 15 secs) - again to kill any harmful microorganisms. Then the milk is cooled.

3) Lactobacillus bulgaricus bacteria are added, and the mixture is incubated (heated to about 40C) in a vessel called a fermenter.

4) The bacteria ferment the lactose sugar in the milk to form lactic acid.

5) Lactic acid causes the milk to clot, and solidify into yoghurt.

6) Finally, flavours and colours are sometimes added and the yoghurt is packaged.

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Microorganisms - Fermenters

Microorganisms (like bacteria) can be used to make really useful stuff e.g. penicillin or insulin.

In industry, microorganisms are grown in large containers called fermenters. The fermenter is full of liquid 'culture medium' in which microorganisms can grow and reproduce.

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Microorganisms - Fermenter Optimal Conditions

The conditions inside the fermentation vessels are kept at the optimum levels for growth - this means the yield of products from the microorganisms can be as big as possible.

1) Nutrients needed by the microorganisms for growth are provided in the liquid culture medium.

2) The pH is monitored and kept at the optimum level for the microorganisms' enzymes to work efficiently. This keeps the rate of reaction and product yield as high as possible.

3) The temperature is also monitored and kept at an optimum level. A water-cooled jacket makes sure it doesn't get so hot that the enzymes denature.

4) Vessels are sterilised between uses with superheated steam that kills unwanted microbes. Having asecptic conditions increases the product yield because the microorganisms aren't competing with other organisms. It also means that the product doesn't get contaminated.

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Microorganisms - Fermenter Optimal Conditions

5) If the microorganisms need oxygen for respiration, it's added by pumping in sterile air. This increases the product yield because microorganisms can always respire to provide the energy for growth.

6) Microorganisms are kept in contact with fresh medium by paddles that circulate (or agitate) the medium around the vessel. This increases the product yield because microorganisms can always access the nutrients for growth.

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Microorganisms - Growth Phases

1. Lag phase - numbers of individuals is low. Start to grow.

2. Exponential phase - bacteria dividing every 20 mins in ideal conditions.

3. Stationary phase - death of bacteria now equals rate of dividing.

4. Death phase - bacteria are dying faster than they dividing.

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Human Impacts on the Environment - CO and SO2 poll

Carbon Monoxide (CO)

  • When fossil fuels are burnt without enough air supply they produce the gas carbon monoxide. 
  • It's a poisonous gas. If it combines with haemoglobin in red blood cells, it prevents them from carrying oxygen.
  • Carbon monoxide's mostly released in car emissions.
  • Most modern cars are fitted with catalytic converters that turn the carbon monoxide into carbon dioxide, decreasing the amount of CO that's realeased into the atmosphere.

Sulphur Dioxide (SO2)

  • Burning fossil fuels releases harmful gases like CO2 and sulphur dioxide (SO2).
  • The sulphur dioxide comes from sulphur impurities in the fossil fuels.
  • When this gas mixes with rain clouds it forms dilute sulphuric acid.
  • This then falls as acid rain.
  • Internal combustion engines in cars and power stations are the main cause of acid rain.
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Human Impacts on the Environment - CO and SO2 poll

Acid Rain

  • Acid rain can cause a lake to become more acidic. This has a severe effect on the lake's ecosystem. Many organisms are sensitive to changes in pH and can't survive in more acidic conditions. Many plants and animals die.
  • Acid rain can kill trees. The acid damages leaves and releases toxic substances from the soil, making it hard for the trees to take up nutrients.
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Human Impacts on the Environment - Greenhouse Gase

Carbon Dioxide

  • Comes from car exhausts, industrial processes and the burning of fossil fuels.
  • People around the world are cutting down large area of forest (deforestation) for timber and to clear land for farming - and this activity affect the level of carbon dioxide in the atmosphere.


  • Methane gas is produced naturally from various sources e.g. rotting plants in marshland.
  • However two 'man-made' sources of methane are on the increase: rice growing and cattle rearing. 

Nitrous Oxide

  • Nitrous oxide is released naturally by bacteria in soils and the ocean.
  • A lot more is released from soils after fertiliser is used. 
  • It's also released from vehicle engines and industry.
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Human Impacts on the Environment - Greenhouse Gase


  • CFCs are man-made chemicals that were once used in aerosol sprays (e.g. deoderant) and fridges.
  • They're really powerful greenhouse gases.
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Human Impacts on the Environment - Greenhouse Effe

Green house gases trap energy from the sun.

  • The temperature of the Earth is a balance between the energy it gets from the Sun and the energy it radiates back out into space.
  • Gases in the atmosphere absorb most of the heat that would normally be radiated out into space, and re-radiate it in all directions (including back towards Earth). This is the greenhouse effect.
  • If this didn't happen, then at night there'd be nothing to keep any energy in, and we'd quickly get very cold indeed.
  • There are several different gases in the atmosphere that help keep the energy in. They're called 'greenhouse gases' and they include water vapour, carbon dioxide and methane.
  • Human beings are increasing the amount of carbon dioxide in the atmosphere. We're also increasing levels of other gases that can act as greenhouse gases e.g. CFCs and nitrous oxide.
  • As a result of this, the Earth is heating up - this is global warming. Global warming is a type of climate change and causes other types of climate change e.g. changing rainfall patterns. Climate change could lead to things like extreme weather, rising sea levels and flooding due to the polar ice caps melting. This could cause habitat loss, and could affect food webs and crop growth.
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Human Impacts on the Environment - Eutrophication

  • Nitrates and phosphates are put onto fields as mineral fertilisers.
  • If too much fertiliser is applied and it rains afterwards, nitrates are easily leached (washed through the soil) into rivers and lakes.
  • The result is eutrophication, which can cause serious damage to river and lake ecosystems.
  • Another cause of eutrophication is pollution by sewage. Sewage contains lots of phospahtes from detergents e.g. washing powder. It also contains nitrates from urine and faeces.
  • These extra nutrients cause eutrophication in the same way fertilisers do.
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Human Impacts on the Environment - Eutrophication

1) Fertilisers enter the water, adding extra nutrients (nitrates and phosphates).

2) The extra nutrients cause algae to grow fast and block out the light.

3) Plants can't photosynthesise due to lack of light and start to die.

4) With more food available, microorganisms that feed on dead plants increase in number and deplete (use up) all the oxygen in the water. 

5) Organisms that need oxygen (e.g. fish) die.

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Human Impacts on the Environment - Effects of Defo

Leaching - Trees take up nutrients from the soil before they can be washed away (leached) by rain, but return them to the soil when leaves die. When trees are removed, nutrients get leached away, but don't get replaced, leaving infertile soil.

Soil erosion - Tree roots hold the soil together. When trees are removed, soil can be washed away by the rain (eroded) leaving infertile ground.

Disturbing the balance of carbon dioxide and oxygen - Forests take up CO2 by photosynthesis, store it in wood, and slowly release it when they decompose (microorganisms feeding on bits of dead wood release CO2 as a waste product of respiration). When trees are cut down and burnt, the stored carbon is released at once as CO2. This disturbs the carbon cycle and contributes to global warming. Fewer trees in the forest also means that less photosynthesis takes place, releasing less oxygen. This causes the oxygen level in the atmosphere to drop.

Disturbing evapotranspiration - Evapotranspiration includes both the processes of water evaporating from the Earth's surface and from plant transpiration. This water falls back to earth as rain. So, when trees are cut down, evapotranspiration is reduced, which can make the local climate drier.

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Food Production - Factors Affecting Photosynthesis

A plant's rate of photosynthesis is affected by the amount of light, the amount of carbon dioxide (CO2) and the temperature. Since plants have to photoynthesise in order to make food for themselves and grow, these three factirs need to be carefully controlled in order to maximise crop yield.

CO2 - As CO2 concentration increases, the rate of photosynthesis increases.This is because there is more CO2 available to the plant and it is a reactant of photosynthesis. As CO2 increases even more, the rate of photosynthesis is constant. Because other limiting factor prevents more photosynthesis from happening. Same for light intensity.

Temperature - As temperature increases, the rate of photosynthesis increases. This is because more heat gives particles more energy which causes them to move. This increases the rate of collision between them and speeds up the reaction. As the temperature increases further, the rate of photosynthesis reaches an apex, the optimum temperature. As the temperature increases even more, the rate of photosynthesis decreases. This is because the heat makes the enzymes dennature and decreases photosynthesis. 

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Food Production - Polytunnels and Glasshouses

Photosynthesis can be helped along by artificially creating the ideal conditions in glasshouses (big greenhouses) or polytunnels (big tube-like structures made from polythene).

  • Keeping plants enclosed in a glasshouse makes it easier to keep them free from pests and diseases.
  • It also helps farmers to control the water supplied to their crops.
  • Commercial farmers often supply artificial light after the Sun goes down to give their plants more time to photosynthesise.
  • Glasshouses trap the Sun's heat to keep the plants warm. In Winter, a farmer might also use a heater to help keep the temperature at the ideal level.
  • Farmers can also increase the level of carbon dioxide in glasshouses e.g. by using a paraffin heater to heat the place. As the paraffin burns, it makes carbon dioxide as a by-product.

By increasing the temperature and CO2 concentration, as well as the amount of light available, a farmer can increase the rate of photosynthesis for his or her plants. This means the plants will grow bigger and faster - and crop yields will be higher.

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Food Production - Use of Fertilisers

  • Plants need certain minerals e.g. nitrogen, potassium and phosphurus, so they can make important compunds like proteins.
  • If plants don't get enougn of these minerals, their growth and life processes are affected.
  • Sometimes these minerals are missing from the soil because they've been used up by a previous crop.
  • Farmers use fertilisers to replace these missing minerals or provide more of them. This helps to increase crop yield.
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Food Production - Pesticides and Biological Contro

Pests include microorganisms, insects and mammals (e.g. rats). Pests that feed on crops are killed using various methods of pest control. This means fewer plants are damaged or destroyed, increasing crop yield.

Pesticides are a form of chemical pest control. They're often poisonous to humans, so they must be used carefully to keep the amount of pesticide in food below a safe level. Some pesticides also harm other wildlife.

Biological control is an alternative to using pesticides. It means using other organisms to reduce the numbers of pests, either by encouraging wild organisms or adding new ones.

The helpful organisms could be predators (e.g. ladybirds eat aphids), parasites (e.g. some flies lay their eggs on slugs, eventually killing them), or disease-causing (e.g. bacteria that affect caterpillars).

Biological control can have a longer-lasting effect than spraying pesticides, and be less harmful to wildlife.

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Food Production - Fish Farming

Cages in the sea

  • Cages in the sea to stop them using as much energy swimming about.
  • The cage also protects them from interspecific predation (being eaten by other animals).
  • Fed a diet of food pellets that's carefully controlled to maximise the amount of energy they get. The better the quality the food is, the quicker and bigger the fish will grow (good protein). 
  • Separate young fish from bigger fish, and provide regular food to stop intraspecific predation (big fish eating the little ones). 
  • Fish kept in cages are more prone to disease and parasites. Pesticides and biological control are used to kill pests and parasites.
  • The fish can be selectively bred to produce less aggressive, faster-growing fish.
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Food Production - Fish Farming


Freshwater fish can be farmed in ponds or indoors in tanks where conditions can be controlled.

  • The water is monitored to check that the temperature, pH and oxygen level is ok.
  • It's easy to control how much food is supplied and give exactly the right sort of food.
  • The water can be removed and filtered to get rid of waste food and fish poo. This keeps the water clean for the fish and avoids pollution wherever the water ends up.
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Homeostasis - Responding to Stimuli

Animals increase their chance of survival by responding to changes in their external environement e.g. by avoiding places that are too hot or too cold.

They also respond to changes in their internal environenment to make sure that the conditions are always right for their metabolism.

Any change in the internal or external environement is called a stimulus.

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Homeostasis - Role of Homeostasis

Homeostasis is the maintenance of a constant internal environement.

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Homeostasis - Coordinated Response

1. Stimuli

2. Receptor - receptors detect stimuli. Receptors in the sense organs are groups of cells that detect external stimuli.

3. Effector - effectors are cells that bring about a response to stimuli. Effectors respond in different ways - muscle cells contract, whereas glands secrete hormones.

4. Response - Restored optimum conditions.

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Homeostasis - Hormones

Hormones are chemicals realeased directly into the blood. Hormones control things in organs and cells that need constant adjustment.

Hormones are produced in gland. They travel quite slowly and tend to have relatively long-lasting effects.

Adrenaline - Produced in adrenal glands. Readies body for 'fight or flight' response which increases heart rate, increases blood floe to muscles and blood sugar level.

Insulin - Produced in pancreas. Helps control the blood sugar level - stimulates the liver to turn glucose into glycogen for storage.

Testosterone - Produced in testes. Main male sex hormone - promotes male secondary sexual characteristics.

Progesterone - Produced in ovaries. Supports pregnancy - maintains uterus lining.

Oestrogen - Produced in ovaries. Main female sex hormone - controls menstrual cycle and promotes female secondary characteristics.

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Homeostasis - Hormones

ADH - Produced in pituritary gland. Controls water content - increases permeability of the kidney tubules to water + this leads to more water being reabsorbed from the collecting ducts.

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Homeostasis - CNS & Endocrine

In the nervous system, electrical impulses carry messages to different organs of the body. The endocrine system uses hormones, chemical signals, to carry commands to the destined organs and cells. Nerve or electrical impulses transmit through neurons. Hormones travel through bloodstreams.

Nervous System

Made from neurones (nerve cells).

CNS is brain and spinal cord only.

Receptors detect stimulus and send electrical impulses along sensory neurones to CNS.

CNS sends electrical impulses to an effector along a motor neurone. The effector then responds accordingly. 

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Homeostasis - CNS & Endocrine

Nerves: 1) Very fast message 2) Act for a short time 3) Act on a very precise area.

Hormones: 1) Slower message 2) Act for a long time 3) Act in a more general way.

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Homeostasis - Temperature Regulation

All enzymes work best at a certain optimum temperature. In the human body, this is about 37 degrees C. 

The hypothalamus is sensitive to the blood temoerature in the brain, and it receives messages from temperature receptors in the skin that provide information about skin tempertaure.

Based on signals from these receptors, your CNS can activate the necessary effectors to make sure your body temperature stays just right.

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Homeostasis - Temperature Skin Response

Too hot

  • Lots of sweat - when it evaporates, it transfers energy from skin to environement, cooling you down.
  • Blood vessels close to the surface of the skin widen - vasodilation. More blood flows near surface so it can transfer more energy to surroundings, which cools you down.
  • Hairs lie flat - not trapping insulating air.

Too cold

  • Very little sweat produced.
  • Blood vessels near surface of skin vasoconstrict. This means less blood flow near surface so less energy is transferred to the surroundings.
  • Shiver - increases respiration which tranfers more energy to warm the body.
  • Hairs (arrector pili) stand on end to trap an insulating layer of air, which helps keep you warm.
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