B4

covers OCR B4 for higher and standard students

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The Leaf

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/addgateway_leafinternal.gif)

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Adaptions of the leaf

The epidermis is thin and transparent, allowing more light to reach the palisade cells (where photosynthesis happens). The palisade layers are at the top of the leaf so they can be closer to sunlight, therefore improving photosynthesis.

The waxy cuticle prevents too much water loss, as does the fact that the stomata are on the underside of the leaves. This reduces water loss because less sunlight can reach them, and sunlight stimulates the opening of the stomata.

Air spaces in the spongey mesophyll layer allow Co2 to diffuse through the leaf, and the fact that the leaf is thin with a large s.a/volume ratio also helps imrpove diffusion in the leaf.

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Movement of water

Osmosis is the movement of water across a semi-permeable membrane from an area of high concentration to low concentration.

When plant cells take water in they swell and become turgid. Turgidity is important because it provides the plant with rigidity. It is important for the cell wall to be inelastic because it prevents too much water being taken in and the cell bursting.

But If too much water leaves plant cells they become flaccid and the plant wilts. Plasmolysis is the shrinking of protoplasm away from the cell wall due to too much osmosis. 

Root hairs increase osmosis by increasing surface area.

Transpiration happens when water evaporates from leaves, creating a vacuum. This means water is then sucked up the roots. Transpiration provides plants with cooling, photosynthesis, support and movement of minerals.

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Phloem, Xylem and Guard Cells

The phloem carries food, is made from living cells, has a 2 directional flow with sieves in the vein.

The xylem carries water, is made from dead cells, has a 1 directional flow and a thick cell wall.

In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

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Rate of Transpiration

An increase in light will increase transpiration because it stimulates the opening of the stomata.

An increase in temperature will increase transpiration because it makes water evaporate quicker.

An increase in wind will increase transpiration because it will replace humid air with drier air, giving a greater concentration gradient.

But an increase in humidity will decrease transpiration because it means there is less of a concentration gradient.

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Plant Minerals

Plants need nitrates because they are converted to proteins for cell growth. A nitrate deficiency will cause poor growth and yellow leaves.

Phosphates are needed for respiration and growth. They make DNA and cell membrane. A lack of phosphates will cause poor root growth and discoloured leaves.

Potassium is needed for respiration and photosynthesis, it helps enzymes. A lack of this will cause poor flower and fruit growth, with discoloured leaves.

Magnesium is needed for photosynthesis because it makes chlorophyll. A deficiency will cause yellow leaves.

Minerals are taken up by active transport. This is when energy from respiration is used to move nutrients from a low to a high concentration, meaning it happens across a concentration gradient.


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Energy in a food chain and Biofuels

Energy flows through food chains via photosynthesis and feeding. It is wasted by heat, respiration, movement etc.

We rarely get food chains longer than five trophic levels because so much energy is lost at each stage.

The efficiency of energy transfer is (energy available at tropic level / the energy available at the previous trophic level) x100

To transfer energy from biomass we can burn fast growing trees or ferment biomass using yeast/bacteria. Biofuels are good because they provide us with energy independancy, less air polution and they are renewable.


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Farming

Intensive farming reduces the energy transferred to competeing pests, plants and movement etc. producing as much as possible.

Organic farming techniques include crop rotation, use of organic fertilisers, biological control (using animals/insects/diseases to get rid of the pest), weeding and varying seed planting times.

Pesticides can be bad because they kill harmless insects, which can damage food chains. But pesticides also can't be excreted, meaning that they will accumulate in the food chain until one level gets a toxic amount.

Organic farming is doesn't use pesticides. This means it is better for the environment and is more ethical in the way it treats animals. But it does take up more land and produce less food. 

Biological control is good because the control organism usually only affects the pest and no chemicals are used so there is less of a health rick. But it is slow, requires a lot of management and the control organisms can become pests.

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Hydropnics

This is when plants are grown out of soil, in nutrient solutions. This is useful for glass house tomatoes or growing plants in areas of barren soil.

It takes up less space and no soil preperation or weeding is needed. It means plants can be grown in areas of poor soil and avoids pests in the soil, also allowing mineral levels to be better controlled.

But it can be expensive to set up and run with speicially formulated nutrients and skilled workers needed. There is also no soil to anchor roots so plants need support.

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Decay

Decay is affected by the temperature and the amount of oxygen and water available.

They affect the rate of decay because they effect microbial respiration, growth and reproduction.

A detrivore is an organism which feeds off dead and decaying material. They increase the rate of decay by breaking food up, increasing surface area.

A saprophyte is an organism which feeds off dead and decaying material by using extracellular digestion. This is when the micro-organism secretes enzymes onto the food, which then breaks it up for the food to be absorbed by the saprophyte.

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Recycling carbon

1) Plants remove CO2 from the air by photosynthesis.

2) Feeding then passes the carbon compounds along the food chain/web.

3) Plants, animals and combustion releases CO2 into the air.

In the sea marina organisms make shells of carbonates. These shells then become limestone. Carbon then returns to the air as CO2 during weathering.

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Recycling Nitrogen

1) Plants take in nitrogen for growth

2) Feeding passes nitrogen along the food chain/web

3) Nitrogen compounds in dead organisms are broken down by decomposers into nitrates and returned to the soil

It is also recycled by bacteria/decomposers;

1) Decomposers convert protein and urea into ammonia

2) Ammonia is then converted to nitrates by nitrifying bacteria

3) The nitrates are then converted to nitrogen gas by de-nitrifying bacteria

4) This nitrogen gas is then fixed in place by nitrogen fixing bacteria in roots of plants

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