Leaf Structure, Diffusion in Leaves, Osmosis, Water Flow Through Plants, Transport Systems in Plants, Minerals needed for Healthy growth,  Pyramids of Number and Biomass, Energy Transfer and Energy flow,  Biomass and intensive farming, Pesticides and biological Control, Alternatives to Intensive Farming, Decay, The Carbon Cycle and The Nitrogen Cycle 

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  • Created by: Amina
  • Created on: 06-05-12 13:55

Leaf Structure

Leaves are designed for making food by photosynthesis.

A leaf has:  

Upper Epidermis - Transparent so that light can pass through the palliside layer.

Chloroplast - Contains Chlorophyll which is the pigment that absorbs light energy for photosynthesis. Chloroplasts are found in the palliside layer (top of leaf so more light) Palliside layer is tall so large surface area.

Vein - deliver water and other nutrients and take away food produced bu leaf. help support structure 

Stomata - lower surface: let gases like co2 and o2 in and out. Allow water to escape (transpiration).

Air Space Spongy Mesophyll Layer - Air spaces in this layer allow gases like Carbon Dioxide and oxygen to move easily between cells. Also they have a large surface area for gas exchange.

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Diffusion in Leaves

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration. 

When the plant is photosynthesising it uses up a lot of C02, so there's hardly any left inside the leaf. This means that more C02 moves into the leaf my diffusion. O2 is being made as a waste product of photosynthesis. Some in used in respiration and the rest diffuses out through the stomata. 

At night there is no photosynthesis because there is no light. Lots of C02 is made in respiration and lots of oxygen is used up. The C02 diffuses out and the O2 diffuses in

. Water vapour escapes the leaf my diffusion because there's lots of it inside the leaf and less outside. This is known as transpiration. 

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Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region a region of lower water concentration. 

Water molecules pass both ways through the membrane during osmosis because water molecules move about randomly all the time.

Because there are more water molecules on one side than the other, there's a steady net flow of water into the region with fewer molecules i.e into the stronger glucose solution. This means that the strong sugar solution gets more dilute. 

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Turgor Pressure supports plant tissues. When a plant is well watered, all its cells will draw water in by osmosis and become plump and swollen. When the cells are like this, they are called turgid. The contents of the cell push against the cell wall - this is called turgor pressure. Turgor pressure helps support the plant tissues. 

If there's no water in the soil, a plant starts to wilt water and so lose their turgor pressure. The cells are then sad to be flaccid. If the plants really short of water, the cytoplasm inside its cells starts to shrink and the membrane pulls away from the cell wall. The cell is now plasmolysed. The plant doesn't totally lose its shape though because the inelastic cell wall keeps things in position. It just droops a bit.

Animal cells don't have an inelastic cell all. If an animal cell has too much water it bursts - lysis.

If it loses too much water - crenation. 

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Water Flow Through Plants

Root hairs take in water by OSMOSIS. The cells on plant roots grow into long hairs which stick out into the soil. Each branch of a root Will be covered in millions of these microscopic hairs. This gives the plant a big surface area for absorbing water from the soil. There's usually a higher concentration of water in the soil than there is in the plant so the water is drawn into the root hair cell by osmosis.

Transpiration is the LOSS of water from the plant: It is caused by the evaporation and diffusion of water from inside the leaves. This creates a shortage in water in the leaf and so more water is drawn up from the rest of the plant through xylem vessels. This means more water is drawn up from the roots so there's constant transpiration.Transpiration has benefits:

keeps plant cool, provides plant with constant supply of water for photosynthesis.

Water creates turgor pressure in the plant ells which helps support the plant.

Minerals needed by the plant can be brought in from the soil along with the water.

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Water Flow Through Plants

Transpiration Rate is affected by FOUR main things:

Light intensity - brighter=greater transpiration. Stomata close as it gets darker. Photosynthesis can't happen in the dark so they don't need to be open. When the stomata are closed, water can't escape. 

Temperature: Warmer=Faster. Water particles have more energy to evaporate and diffuses out if the tomata when warm.

Air Movement: More(wind) around leaf=Faster Transpiration. Water vapour is swept away maintaining low concentration of water in the air outside the leaf. 

Air Humidity: Drier=Faster. If the air is humid there's a lot of water in it already so the diffusion happens slow. 

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Water Flow Through Plants

Plants have adaptions to reduce water loss from their leaves in Case of drought:

Leaves have a waxy cuticle which helps the surface of the leaf be waterproof. 

Stomata are found on the lower surface of a leaf where its darker and cooler - this slows down diffusion of water out of the leaf.

The bigger the stomata and the more there are, the more water the plant will lose. Plants in hot climates really need to conserve water so they have fewer and smaller stomata on the underside of the leaf and no stomata on the supper epidermis.

Stomata open and close automatically. The guard cells open and close the stomata as they go turgid or flaccid. Thin outer walls and thickened inner walls make this opening and closing function work properly. Open stomata allow gases in and out for photosynthesis. They are sensitive to light and close at night to conserve water without losing out on photosynthesis.

Cells turgid-Stomata Open. Cells Flaccid-Stomata closes.

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Transport System In Plants

Phloem Tubes:  Made of columns of living cells with perforated end plates to allow stuff to flow through

                          They transport substances, mainly sugars, made in the leaves to growing and storage tissues in both direction.  

                           The movement of food substances around the plant is known as TRANSLOCATION.

Xylem Tubes:     Take water up

                           Made of dead cells joined end to end with no end walls between them and a hole (lumen) down the middle

                           The thick side walls are strong and stiff which gives the plant support. They carry water and minerals from the roots up the shoot to the leaves in the transpiration system. 

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Minerals Needed for Healthy Growth

Plants need three main minerals, they get them from the soil...

1. NITRATES - contain nitrogen for making amino acids and proteins. These are needed for cell growth. Lack of nitrates=Stunted and Yellow leaves.

2. PHOSPHATES - contain phosphorus for making DNA and cell membranes and they're needed for respiration and growth. Lack of Phosphates=Poor root growth and purple leaves.

3. POTASSIUM -to help the enzymes needed for photosynthesis and respiration. Lack of potassium= poor flower and fruit growth and discoloured leaves.

Root Hairs take in minerals using ACTIVE TRANSPORT - Hairs give the plant a big surface area for absorbing minerals. The concentration of minerals in the soil is usually low and higher in the root hair. This does not follow the rules of diffusion so a different process called active transport is responsible. Active transport uses energy from respiration to help the plant pull minerals into the root hair against the concentration gradient. This is essential for growth. 

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Energy Transfer and Energy Flow

Energy from the sun is the source of energy for all life on earth.

Plants use a small percentage of the light energy from the sun to make food during photosynthesis. This energy then works its way through the food web animals eat the plants and each other.

The energy lost at each stage is used for staying alive i.e, respiration.

Most of this energy is eventually lost to the surrounding as hear. Material and energy atre also lost from the food chain in the droppings (egestion). 

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Intensive farming and Bio fuels

Advantages of Bio fuels: Renewable, No acid ran, can be supplied by waste.

Intensive Farming is used to produce more food. Farmers do this by reducing energy losses at each stage in a food chain:

They use herbicides to kill weeds. This means that more of the energy from the sun goes to crops. 

They use pesticides to kill insects that eat the crops. This means that no energy is transferred into a different food chain.

Animals are battery farmed. They're kept close together indoors in small pens so that they're warm and can't move about. This saves them wasting energy as they move and stops them using energy to keep warm. 

Intensive farming means more food from less land.

Intensive farming can destroy the environment by making it polluted, unattractive and devoid of wildlife. Fertilisers pollute rivers. Pesticides disturb food chains.

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Pesticides and Biological Control

Pesticides are sprayed onto crops to kill the creatures that damage them but they also kill harmless animals such as bees and beetles. This can cause a shortage of food for animals further up the food chain. Pesticides also tend to be toxic to creatures that aren't pests and there's a danger of the poison passing on through the food chain to other animals. Otters were almost wiped out by a pesticide called DDT. DDT accumulated along the food chain and the otter ended up with ost of the DDT.

BIOLOGICAL CONTROL: this means using living things instead of chemicals to control a pest. You could use a predator a parasite or a disease to kill the pest:

Ladybirds are released into the fields to eat aphids (pests). Wasps and flies produce larvae which develop on a host insect. This kills the insect host. 

ADVANTAGES:The Predator, parasite or disease only affects the pest animal No chemicals used; less pollution.

DISADVANTAGES:slower than pesticides, wont kill all the pests, takes more management time(training), control organisms can become pests.

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Hydroponics: When tomatoes and cucumbers are grown in nutrient solution instead of in soil.


  • Takes up less space so less land required
  • No soil preparation or weeding needed
  • can still grow plants in areas with poor soil
  • many pest species live in soil, so it avoids these
  • mineral levels can be con trolled more accurately


  • It can be expensive to set up and run
  • Need to use specially formulated soluble nutrients
  • Growers need to be skilled and properly trained
  • There's no soil to anchor the roots so plants need support 
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Organic Farming

  • Use of organic fertilisers: This recycles the nutrients left in plant and animal waste (e.g manure and compost). It is better for the environment
  • Crop rotation - growing a cycle of different crops in a  field each year. This stops the pests and diseases of one crop building up and stops nutrients running out. Most crop rotations include a legume plant like peas or beans as they help put nitrates back in to the soul.
  • Weeding- physically removing the weeds rather than spraying them with herbicides. More labour intensive but no chemicals involved.
  • Carrying seed planing times- sowing seeds later or earlier in the season will avoid the major pests for that crop. This means the farmer wont need to use pesticides. 
  • Organic farming takes up More space. 
  • Its more labour intensive - makes food more expensive
  • You cant grow as much food
  • Uses fewer chemicals
  • Better for the environment
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Things decay because of micro-organisms.

When something dies or decomposes,. they realise materials as waste. The elements they contain are returned to the soil or air where they originally came from.

These elements are then used by plants to grow and the whole cycle repeats again.

All the important elements are recycled, including carbon hydrogen oxygen and nitrogen.

Rate of decay depends on three main things: 

  • Temperature- warmer=decay faster because it speeds up respiration in decomposers
  • Moisture- moister=faster decay because decomposers need water
  • Oxygen- decay is faster when there is more oxygen because the decomposers can respire aerobically, providing more energy.
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Food preservation

Canning - putting food in an airtight can.

Cooling- putting food in a fridge. Slows down decay because it slows down respiration in microorganisms. They cant reproduce either.

Freezing- microorganisms cant respire orm reproduce at all in such low temperatures. 

Drying - dried foods last longer because microorganisms need water. 

Adding salt- if there is a high concentration of salt around decomposers, they'll lose water b y osmosis. This damages them and means they cant work properly. (brine).

Adding vinegar- acidic and the low PH inhibits the enzymes inside the micro-organisms and stops them decomposing. 

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The Carbon Cycle

Detritivores feed on dead and decaying material. e.g earthworms, maggots and woodlice. As they feed on it, they break it up into smaller bits. This gives a bigger surface area for smaller composers to work on and so speeds up the decay.

Saprophytes feed on decaying material by extracellular digestion; they feed by secreting digestive enzymes on to the material outside of their cells. The enzymes break down the material into smaller bits which can then be absorbed by the saprophyte. Most saprophytes are bacteria and fungi. 

For carbon cycle, see book.

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Nitrogen cycle

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

 Plants get their nitrogen from the soil, so nitrogen in the air has to be turned into nitrates before plants can use it.

Animals can only get protein's by eating plants.  

Decomposers break down proteins in rotting plants and animals and urea in animal waste, into ammonia.

So the nitrogen in these organisms is recycled. 

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