Biology B4


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Leaf Structure

Leaf Structure

  • leaves are broad, so there's a large surface area exposed to light
  • they're also thin, which means carbon dioxide and water vapour only have to travel a short distance to reach the photosunthesis cells where its needed.
  • There are air spaces in the spongy mesophyll layer. 
  • Leaves contains lots of chlorophyll, which pigment that absorbs light energy for photosynthesis 
  • The upper epidermis is transparent so that light can pass through it to the palisade layer
  • the lower surface is full of little holes called stomata. They let gases like C02 and O2 in and out.
  • Leaves have a network of veins, These deliver nutrients and water to every part of the leaf and take away the food produced by the leaf. Also help to support leaf structure.

Leaf Palisade Cells 

  • packed with chloroplasts for photosynthesis 
  • their tall shape means alot of surface area is exposed down the side for absorbing co2 
  • their tall shape also means theres a good chance of light hitting a chloroplast before it reaches the bottom of the cell.

Stomata open and close through guard cells become flaccid and turgid

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Diffusion In Leaves

Diffusion In Leaves

  • Plants exchange gases by diffusion
  • Diffusion= the passive movement of particles from an area of high to low concentration.

Diffusion of gases in the leaves is vital for photosynthesis

  • When the plant is photosynthesizing it uses up lots of co2. So there's hardly any inside the leaf.
  • So by diffusion more co2 enters the leaf.
  • Water vapor also escapes from the leaf by diffusion, because there's a lot of it inside the lead and less of it in the air outside.
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  • Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
  • The water molecule actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time.

Turgor Pressure supports plant tissues

  • When cells absorb water they become plump and swollen, the contents push against the cell wall and the pressure helps to support the plant tissue.
  • if there's no water in the soil, a plant starts to wilt. this is because the cells loose their turgor pressure and become flaccid.
  • if a plant is really short of water the cytoplasm inside the cells start to shrink and the membrane pulls away from the cell wall. the cell is now plasmolysed.

Animal Cells 

  • Animal Cells takes in too much water it bursts- this is known as lysis. If it loses too much water it gets all shriveled up this is known as crenation. This means a animal cell is less tolerant to a drought. 
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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 inside the plant, so the water is drawn into the root hair cell by osmosis.

Transpiration is the loss of water from the plant

  • Transpiration is the caused by the evaporation and diffusion of water from inside the leaves.
  • This 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.
  • 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.
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Water Flow Through Plants

Transpiration Rate is affected by four main things


  • Light intensity- the brighter the light, the greater the transpiration rate. Stomata begins to close as it gets darker.
  • Temperature- the warmer it is, the faster transpiration happens. When its warm the water particles have more energy to evaporate and diffuse out of stomata.
  • Air Movement- if there's lots of air movement around a leaf, transpiration happens later, if the air movement is quick then the concentration is low so diffusion can occur.
  • Air Humidity- if the air around the leaf is very dry, transpiration happens more quickly.

Plants need to balance water loss with water uptake

  • leaves usually have a waxy cuticle covering a upper epidermis. This helps make the upper surface of the leaf of waterproof.
  • Most stomata are found on the lower surface of a leaf where its darker and cooler. This helps slow down diffusion of water out of the leaf.
  • The bigger the stomata and the move stomata a leaf has, 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 upper epidermis.

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Transport Systems In Plants

Phloem Tubes transport food

  • Made of columns of living cells with perforated end-plates to allow stuff to flow through
  • They transport food substances (mainly sugars) made in the leaves to growing and storage tissues in both directions.
  • This 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 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 stream.

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Minerals Needed For Healthy Growth

Plants Need Three Main Minerals

  • Nitrates- contains nitrogen for making amino acids and proteins. These are needed for cell growth. If a plant cant get enough nitrates it will be stunted and will have yellow older leaves.
  • Phosphates- Contain phosphorus for making DNA and cell membranes and they're needed for respiration and growth. Plants without enough phosphate have poor root growth and purple older leaves.
  • Potassium- To help the enzymes needed for photosynthesis and respiration. If there's not enough potassium in the soil, plants have poor flower and fruit growth and discolored leaves.
  • Magnesium is also needed in small amounts. the three main minerals are needed in fairly large amounts, but there are other elements which are needed in much smaller amounts, as it is needed for making chlorophyll

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Root Hairs Take In Minerals Using Active Transport

  • Root Hairs give the plant a big surface area for absorbing minerals from the soil.
  • But the concentration of minerals in the soil is usually pretty low. Its normally higher in the root hair cell then in the soil around it.
  • So normal diffusion doesn't explain how minerals are taken up into the root hair cell.
  • They should go the other way if they followed the rules of diffusion. 
  • The answer is that 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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Pyramids of Number And Biomass

Constructing Pyramids of Numbers

  • Each bar on a pyramid of numbers shows the the number of organisms at that stage of the food chain.
  • The producer would go at the bottom of the food chain because they are the producer.
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Energy Transfer and Energy Flow

  • Energy from the sun is the source of all energy
  • Plants use a small percentage of the light energy from the sun to make food during the photosynthesis process.
  • The energy lost at each stage is used for staying alive i.e. in respiration which powers all life processes.
  • Most of this energy is eventually lost to the surroundings as heat. 
  • Material and energy is lost through excrement 
  • food chains normally only have about 5 levels as there is not a lot of energy after that point.

Interpret Data on Energy Flow

  • efficiency= energy available in the next level / energy that was available to the previous level x 100
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Biomass and Intensive Farming

Energy Stored in biomass can be used for other things

  • Only 10% of food given to cattle actually becomes meat
  • fast growing trees are a great bio- fuel as they only burn the carbon they absorb in the first place and work well when the tree is fast growing.
  • Fermenting Biomass using bacteria or yeast to make fuel

Pros of Biofuel

  • unlike coal, oil ect biofuel is renewable 
  • using biofuel reduces air pollution 
  • you can be self reliant on energy

Intensive Farming is used to produce more food

  • They use herbicides to kill weeds
  • They use pesticides to kill insects
  • Animals are battery farmed
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Intensive Farming

Intensive Farming can destroy the environment 

  • Removal of hedges to make huge great fields for maximum efficiency. This destroys the natural habitat of wild creatures and can lead to serious soil erosion
  • Careless use of fertilizers pollutes rivers and lakes
  • Pesticides disturb food chains
  • Lots of people think that intensive farming of animals such as battery hens is cruel
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Pesticides and Biological Control

Pesticides disturb food chains  

  • Insecticides seeps into the river
  • small water plants take up a little insecticide
  • each little tiny animal eats lots of small plants
  • each small fish eats a lot of tiny animals
  • each eel eats lots of small fish
  • each otter eats lots of eels
  • You can use biological control instead of pesticides 
  • Aphids are a pest because they eat roses and vegetables- Ladybirds eat Aphids
  • Wasps and flies can leave larvae which develop on or in a host insect
  • Myxomatosis is a disease which kills Rabbits, started in Australia to cull the population of Rabbits

 Advantages: only affects pest animal, NO chemicals used

Disadvantages: its slower than pesticides, wont kill all the pests, takes management and planning, control predators could become pests themselves.

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Alternatives to Intensive Farming

Hydroponics is where the plant is grown without soil (a solution of monitored water and fertilizer)  

Organic Farming

  • Use of organic fertilizers (manure and compost) this recycles nutrients left in plant and animal waste.
  • Crop rotation- growing a different crop each year, plant nitrogen fixing plants
  • weeding- physically removing weeds
  • vary seed planting to avoid seasonal pests
  • biological control


  • organic farming takes more space
  • more labour intensive 
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Things decay because of micro- organisms

  • living things are made from the things around them
  • when they die the materials they used are returned
  • these elements are then used by plants to grow
  • nearly all decomposition is done by soil bacteria and fungi
  • all the important elements are thus recycled including carbon hrydogen oxygen and nitrogen
  • rate of decay depends on:
  • temperature
  • moisture
  • oxygen

Food Preservation methods reduce rate of decay

  • canning- sealed container
  • cooling- fridge
  • freezing
  • drying
  • adding salt/ vinegar  
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The Carbon Cycle

Detritivores and Saprophytes feed on decaying material

  • Detritivores feed on dead and decaying material- maggots, woodlice and earthworms
  • Saprophytes feed on decaying material by secreting digestive enzymes onto the material outside of their cells. 
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