OCR B4 Revision



Sampling techniques:

Quadrats - count animals in it to assess percentage cover of plants

Transect lines - count organisms that touch the line

A big sample is needed to make sure results are accurate

When using capture-recapture method, assume no death, immigration, or emigration has occured.

Population size can be estimated by:

Number in first sample x Number in second sample /  Number in second sample marked         

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6CO2 + 6H2O ==> C6H12O6 + 6O2

Photosynthesis occurs in 2 stages:

The first stage uses light energy to split water into waste oxygen gas and hydrogen. The second stage combines the hydrogen ions with carbon dioxide to form glucose.


Used for respiration to release energy

Converted into cellulose for making cell walls

Turned into lipids (fats and oils) for storing in seeds

Turned into starch and stored for future use for photosynthesis

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Rates of Photosynthesis

The availability of light, the amount of carbon dioxide, and a suitable temperature are all factors that affect the rate of photosynthesis (limiting factors).

Not having enough light or carbon dioxide will slow the process down. The temperature has to be just right and cannot be too cold or hot. If the temperature is too high the plant's enzymes will denature and the process will stop.

At daytime, plants photosynthesise and respire they do photosynthesise at a much faster rate than they respire.

At night-time plants respire only - oxygen is taken into the leaf and waste carbon dioxide is released.

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Adaptations of leafs for photosynthesis

-Outer epidermal cells are transparent to allow light through so they do not contain chloroplasts

-Upper palisade layer which receives the most light, contains the most chloroplasts

-Air spaces in the spongy mesophyll layer to allow CO2 to diffuse from the stomata to the palisade cells

-Air spaces inside the leaf give a large surface area to volume ratio which allows maximum absorption of gases

-Thin to provide a short diffusion pathway for CO2 to diffuse to the mesophyll and palisade cells

Order of internal structure of leaf:                                                                                      1) Upper epidermis  2) Palisade mesophyll layer  3) Spongy mesophyll lay  4) Lower epidermis

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Net movement of particles in gas or liquid from an area of high concentration to an area of low concentration through the cell membrane due to constant random movement of particles.

Plants use this to get molecules such as water, oxygen and carbon dioxide into the cells. Cells need a constant supply of some substances and get rid of some.

Adaptations of leaf for diffusion

- Large surface area     - Stomata     - Gaps between mesophyll layers

Factors that affect the rate of diffusion:

1) Distance - the shorter the distance the particles have to move the quicker the rate of diffusion

2) Concentration gradient - substances diffuse faster if there is a big difference in concentration

3) Surface area - the more surface area there is available for move across, the faster they can get from one side to another

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Type of diffusion in a partially permeable membrane that allows the passage of water molecules to pass but not other big molecules such as glucose.

Osmosis in plant cells:

Hypotonic solution: the pressure in the cell increases and the cell becomes turgid.

Isotonic solution: no differences in concentration so there is no net movement of water and the cell stays the same

Hypertonic solution: cell becomes flaccid from the loss of water and becomes plasmolysed.

Osmosis in animal cells:

Hypotonic solution: cell swells up and bursts (lysis)

Isotonic solution: no differences in concentration so there is no net movement of water and the cell stays the same

Hypertonic solution: cell loses water and becomes crenated

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Plant structure

Two major tissues inside plants are the xylem and phloem. They are both found in vascular bundles.Vascular bundles form a continuous transport system from the roots, through the stem, and into the leaves.

Xylem cells are stacked on top of each other to form long hollow tube-like vessels. These cells are dead and have a lumen which transports water and minerals from the roots to the shoots and leaves.They also have thickened, strengthened cellulose cell walls to help support the plant.

Phloem cells are also stacked on top of each other, however, unlike the xylem, they are living cells and transport food substances to all other parts of the plant.

In the root, the vascular bundles are located in the centre of the root. It acts as an anchor.

In the stem, the vascular bundles are located around the outer edge which provides strength to resist bending in the breeze.

In the leaf, the vascular bundles form a network which supports softer leaf tissues.

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The process of water leaving plants through evaporation and diffusion is called transpiration.

This shortage of water causes more water to be drawn up from the rest of the plant through the xylem vessels, so there is a constant transpiration stream.

This is useful because of the cooling effect it has on the plant. Also the support it provides by creating turgor pressure in the cells. Minerals needed by the plant can be brought along with water.

Leafs are adapted for photosynthesis and thus a lot of water is lost and so it has mechanisms to prevent this. Some include:

-Waxy cuticle on upper and lower surfaces

-Very few stomata on the upper side of the leaf

-Stomata be opened or closed - when there is plenty of light and water guard cells take up water and become turgid causing them to bend over and open the stomata

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Rates of transpiration

Transpiration is increased by 4 main things. First is an increase in light intensity. Second is an increase in temperature. The third is an increase in air movement. Fourth is a decrease in air humidity. 

1) Stomata open in the light and so if there is more light more stomata will open. (steady-curve-no change)

2) The higher the temperature the faster the particles will move so the water molecules will evaporate off the leaf faster. (positive line)

3) When air moves over the leaf it moves evaporated water molecules away from the leaf so there's no build up in the air outside the leaf. There's a greater concentration gradient. (curve)

4) The less humid the air the less water there is outside the leaf and makes for a greater concentration gradient. (decrease line)

A bubble potometer used to measure the rate of transpiration. This works by changing a factor and measuring how fast a bubble moves along the glass tube. This shows how quickly water is moving through the plant.

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Plant minerals

Plants need minerals to remain healthy. Without them, they show deficiency symptoms.

Nitrogen is needed to make amino acids for cell growth. Deficiency symptoms show poor growth and yellowing leaves.

Phosphorus is needed to make ATP for growth and to make DNA molecules in the cell membrane. Deficiency symptoms show poor root growth and discoloured purple leaves.

Potassium is needed for enzymes involved in photosynthesis and respiration. Deficiency symptoms show poor flower and fruit growth also yellowed leaves with brown spots.

Magnesium is needed to make chlorophyll. Deficiency symptoms show yellow leaves mostly at the bottom                                                                                           

Active transport:  A method to move molecules across the cell membrane. Low concentration to a high concentration and therefore needs energy from respiration to occur. This is done by a carrier protein in the membrane.This happens against a concentration gradient.

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A type of natural recycling. Three conditions increase the rate decay:

Temperature- at warmer temperatures microbes are able to respire faster, and will grow and reproduce quicker.

Amount of oxygen- the more oxygen available, the faster the microbes will be able to respire which allows them to grow quicker and reproduce more

Amount of water- Microbes need water to remain healthy. In moist conditions, the microbes grow faster and reproduce more

Detrivores eat and digest dead material and release them as waste. This increases the surface area of the dead material for the decomposers to act on.

Decomposers (bacteria & fungi) chemically break down dead material, releasing ammonium compounds.

Most decomposers are saprophytes. They feed my releasing enzymes onto the dead material in a process called extracellular digestion. Then they absorb it back.

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Food preservation

Canning- sealed in a metal container and then heated to kill any microbes.The can prevent entry of oxygen and decomposer microbes

Cooling- placed in low temperatures that slow reproduction of decomposers

Freezing- placed in below 0 temperatures that stop decomposers from reproducing

Adding salt or sugar- causes water to be drawn out by osmosis which kills them

Adding vinegar- acid kills decomposers

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Intensive farming

Producing the maximum amount of food per hectare of land.

Advantages of intensive farming:

-High yield    -Low cost of production    -Less labour intensive 

Disadvantages of intensive farming:

-Use of pesticides   -Use of fertilisers   -Battery rearing of animals in small enclosures


Use of pesticides reduces damage to crop or herd. Pests are removed to stop competing with crops or herds for resources. At every link in the food chain, less energy is lost to the pests.

Advantages of battery farming include animals not moving around and are kept warm saving energy. However, this technique is less humane and as a result to this their behaviour changes as a result

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In hydroponics plants are grown with no soil and are suspended with their roots exposed and sprayed with the correct concentration of minerals dissolved in water.

Plants can be stacked above each other which means that they can be grown in the same place. Any mineral solution that is not absorbed by the plant can be collected and recycled reducing waste.

This technique mans that there is a better use of space and gives better control of minerals needed and diseases can be controlled inside the glasshouse.

However, because there is no soil to store minerals fertilisers need to be added constantly and there is no support for the plant so a frame or tray is needed.

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Thank you so much. Really helpful.

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