B4

A quadrat is a square frame enclosing a known area. You can study a small area within a quadrat and scale up your findings to make estimates for larger areas:

Count all the organisms in the 1m quadrat then multiply the number of organisms by the total area of the habitat.

The sample size affects the accuracy of the estimate - the bigger your sample then the more accurate your estimate of the total population is likely to be.

• Capture a sample of the population and mark the animals in a harmless way
• Release them back into the environment
• Recapture another sample of the populatoin. Count how many of this sample are marked
• Estimate population size: Population size = no. in first sample x no. in second sample
•                                                                       no. in second sample previously marked

When using this method you must assume:

• There have been no changes in the population size due to deaths, immigration or emigration
• The sampling methods for the capture and recapture were identical
• The marking hasn't affected the individuals' chance of survival

An ecosystem is all the organisms in a particular area, as well as the non-living (abiotic) conditions e.g. temperature, salinity and soil quantity

An ecosystem isn't the same as a habitat - a habitat is just the place when an organism lives

Ecosystems are self-supporting - they contain (almost) everything they need to maintain themselves. Water, nutrients and essential elements like carbon all get recycled within the ecosystem

The only thing that's needed from outisde the ecosystem is an energy source - this is normally the sun.

Distribution is where organisms are found within a particular area.

You can investigate distribution using lines called transects.

To do a transect, you mark out a ling using a tape measure and place quadrats next to each other all the way along the line. You then count and record the organisms you find in the quadrats.

If it's difficult to count all the individual organisms (e.g. grass) you can calculate the percentage cover.You can plot the results of a transect in a kite diagram, allowing you to map the distribution of organisms in the area.

• The abundance of each organism is shown by the thickness of the kite shap. The abundance is plotted above and below a central line to make the shape symmetrical.
• The x-axis shows the distance along the transect line.

Abiotic factors are all the non-living, physical factors in an environment e.g. light, temperature, water, oxygen, salinity and salt quantity.

The distribution of organisms is affected by abiotic factors because:

• Organisms are adapted to live in certain physical conditions. This means they're more likely to survive and reproduce in areas with these conditions.
• Many organisms can only survive in the conditions they're adapted to.

Zonation is the gradual change in the distribution of species across a habitat.

A gradual change in abiotic factors can lead to the zonation of organisms in a habitat.

Biodiversity includes:

• The amount of variation between individuals of the same species in an area
• The number of different species in an area
• The number of different habitats in an area

Biodiversity is important - ecosystems with a high level of biodiversity are healthier than those without. This is because more diverse ecosystems are better able to cope with changes in the environment

• Natural ecosystems maintain themselves without any major interference from humans e.g. native woodlands and natural lakes
• Artificial ecoysystems are created and maintained by humans e.g. forestry plantations and fish farms.

• Photosynthesis uses energy from the Sun to change carbon dioxide and water into glucose and oxygen.
• It takes place in chloroplasts in plant cells - they contain pigments like chlorophyll that absorb light energy.
• First, light energy is used to split water into oxygen gas and hydrogen ions
• Carbon dioxide gas then combines with the hydrogen ions to make glucose and water
• Water isn't one of the overall products of photosynthesis a smore gets used up in the first stage than is made in the second stage

For respiration - plants use some of the glucose for respiration. This releases energy so they can convert the rest of the glucose into various other useful substances

Stored in seeds - glucose is turned into lipids for storing in seeds. Sunflower seeds, for example, contain a lot of oil that we use for cooking oil and margarine

Making proteins - glucose is combined with nitrates (collected from the soil) to make amino acids, which are then made into proteinds. These are used for growth and repair.

Making cell walls - glucose is converted into cellulose for making cell walls, especially in a rapidly growing plant

Stored as starch -  glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening, like at night. Starch is insoluble which makes it good for storing:

• It can't dissolve in water and more away from storage areas in solution
• It doesn't affect the water concentration inside cells - soluble substances would bloat the storage cells by drawing in water.

In 1648 Jan Van Helmont set up this experiment:

• He dried some soils, weighed it and put it in a pot.
• He planted a willow tree weighing 2.2kg in the soil.
• He added rainwater to the pot whenever it was pot.
• 5 years later Van Helmont removed the tree from the pot
• The tree weighed 76.7kg - so it had gained 74.5kg of mass
• He dried the soil and weighed it - its mass had changed very little.
• Van Helmont concluded that because the weight of the soil had changed so little, the tree must have gained mass from another source. Because he only added water to the tree, he concluded that the tree must have gained mass by taking in water.

Today we known that plants also gain mass using carbon dioxide from the air - this experiment was still important as it introduced the idea that plants don't just gain mass by taking in minerals from the soil.

In the early 1770s Joseph Priestley did this experiment:

• He placed a burning candle in a sealed container and observed that the flame went out after a short time. The candle couldn't be re-lit while in the container.
• He then placed a burning candle and a living plant in the container. The flame went out after a short time, but after a few weeks the candle could be re-lit

He decided that the burning candle used up something in the container and that this made the flame go out. He thought that the living plant 'restored the air' so the candle could burn again. He did this experiment too:

• He filled a sealed container with exhaled air. He put a mouse in the container, and observed that it only survived for a few seconds.
• He filled another sealed container with exhaled air. He put a living plant in the container and waited a few days. He then put a mouse in the container - this time it survived for several minutes.

From these experiments, Priestley concluded that plants restore something to the air that burning and breathing take out.

Scientists realised that plants release oxygen during photosynthesis. To find out where the oxygen came from, a scientist supplied plants with water containing an isotope of oxygen (oxygen-18). The carbon dioxide the plants received contained ordinary oxygen-16.

It was found that when the plants photosynthesised, they released oxygen-18

This showed that oxygen came from the water that was supplied to the plant, not the carbon dioxide.

Not enough light slows down the rate of photosynthesis:

• If the light level is raised, the rate of photosynthesis will increase, but only to a certain point
• Beyond that, it won't make any difference because it'll be either the temperature or carbon dioxide leverl which is now the limiting factor

Too little carbon dioxide also slows it down:

• As with light intensity, the amount of carbon dioxide will only increase the rate of photosynthesis up to a point. After this the graph flattens out, showing that carbon dioxide is no longer the limiting reactant.
• As long as light and carbon dioxide are in plentiful supply then the factor limiting photosynthesis must be temperature

The temperate has to be just right:

• As the temperature increases, so does the rate of photosynthesis. But if the temperature is too high, the plant's enzymes will be denatured, so the rate rapidly decreases

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

Diffusion happens in both liquids and gases - this is because the individual particles in these substances are free to mover about randomly. 

• Just like diffusion in air, particles flow through the cell membrane from where there's a higher concentration to a lower concentration.
• They're moving about randomly so they go both ways - but if there's a lot more particles in one side of the membrane, there's an overall movement from that side.

The rate of diffusion depends on three main things:

• Distance - substances diffuse more quickly over a shorter distance
• Concentration gradient - substances diffuse faster is there's a big difference in concentration. If there's lots of particles on one side, there are more to move across
• Surface area - the more surface there is available for molecules to move across, the faster they can get from one side to the other.

Photosynthesis and respiration are opposite processes.

Photosynthesis only happens during the day, but plants must respire all the time to get the energy they need to live.

During the day, plants make more oxygen in photosynthesis than they use in respiration. So in daylight, they release oxygen and take in carbon dioxide.

At night plants only respire - there's no light for photosynthesis. This means that they take in oxygen and release carbon dioxide.

Photosynthesis  

• When the plant is photosynthesising it uses up lots of carbon dioxide, so there's hardly any inside the leaf.
• This makes more carbon dioxide move into the leaf by diffusion
• At the same time lots of oxygen is being made as a waste product of photosynthesis
• Some is used in respiration, and the rest diffuses out of the leaf

Respiration  

• At night there's no photosynthesis so lots of carbon dioxide is made and lots of oxygen is used up
• There's a lot of carbon dioxide in the lead and not a lot of oxygen, so now it's mainly carbon dioxide diffusing out and oxygen diffusing in.

• Leaves are broad so there's a large surface area for gases to diffuse
• They're thin which means that carbon dioxide and water vapour only have to diffuse a short distance to reach the photosynthesising cells where they're needed.
• The lower surface is full of little holes called stomata. They're there to let gases like carbon dioxide and oxygen in and out. They also allow water to escape (transpiration).
• Leaves have guard cells surrounding each stoma to control when the stoma opens and closes. This allows the guard cells to control gas exchange.
• There are air spaces in the spongy mesophyll layer. This allows gases like carbon dioxide and oxygen to move between the stomata and the photosynthesising cells. This also means there's a large surface area for gas exchange - they have a very big internal surface area to volume ratio.

• The leaves are broad so there's a large surface area exposed to light
• Leaves contain lots of chloroplasts. Chloroplasts contain chlorophyll and other photosynthetic pigments to absorb light energy.
• Different pigments absorb different wavelengths of light, so plants cells can make the most of the Sun's energy by absorbing as much of it as possible.
• The cells that contain chloroplasts are arranged in the palisade layer near the top of the leaf where they get the most light
• The upper epidermis is transparent so that light can pass through it to the palisade layer

Vascular bundles - the vascular bundles are the transport vessels, xylem and phloem. They deliver water and other nutrients to every part of the leaf and take away the glucose produced by photosynthesis. They also help to support the leaf structure,

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

• A partially permeable membrane means that tiny molecules like water can pass through them and bigger molecules (sucrose) can't.
• The water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules mover about randomly all the time.
• But because there are more water molecules on one side than the other, there's steady net flow of water into the region with fewer water molecules.
• This means the concentrated sucrose solution gets more dilute.

• When a plant is well watered, all its cells will draw water in by osmosis and become plump and swollen - they are turgid.
• The contents of the cell push against the inelastic cell wall - this is turgor pressure. Turgor pressure helps support the plant tissues.
• If there's no water in the soil, a plant starts to wilt. This is because the cells start to lose water and so lose their turgor pressure - they are flaccid.
• If the plant's really short of water, the cytoplasm inside its cells start to strink and the membrane pulls away from the cell wall. The cell is now plasmolysed. The plant does not totally lose its shape though, because the inelastic cell wall keeps things in position.

Animal cells don't have a cell wall. If an animal cell takes in too much water, it bursts - this is known as lysis. If it loses too much water it gets all shrivelled up - this is known as crenation.

Animals have to keep the amount of water in their cells pretty constant.

Phloem tubes transport food 

• Made of columns of living cells with perforated end-plates
• They transport food substances both up and down the stem to growing and storage tissues
• This movement of food substances around the plant is known as translocation

Xylem Vessels take water up 

• Made of dead cells joined end to end with no end walls between them and a lumen down the middle
• The thick side walls are made of cellulose. They're strong and stiff, giving the plant good support
• They carry water and minerals from the roots up the shoot to the leaves in the transpiration stream.

They usually run alongside each other in vascular bundles

• 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 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 caused by evaporation and diffusion of water vapour 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 theplant 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 adapated for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there' more water inside the plant than in the air outside, the water escapes from the leaves through the stomata. The transpiration stream does have some benefits for the plants, however:

• The constant stream of water from the ground helps to keep the plant cool
• It provides the plant with a constant supply of water for photosynthesis
• The water creates turgor pressure in the plant cells, which helps support the plant and stops it wilting
• Minerals needed by the plant can be brough in from the soil along with the water.

1) An increase in light intensity -  the brighter the light, the greater the transpiration rate. Stomata begin so close as it gets darker. Photosynthesis can't happen in the dark, so they don't need to opent to let carbon dioxide in. When the stomata are closed, water can't escape

2) An increase in 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

3) An increase in air movement - if there's lots of air movement around a leaf, transpiration happens faster. If the air around the leaf is still, 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.

4) A decrease in air humidity - if the air around the leaf is very dry, transpiration happens more quickly. This is like what heppens 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.

• Leaves usually have a waxy cuticle covering the upper epidermis. This helps make the upper surface of the leaf waterproof.
• Most stomata are found on the lower surface of a leaf where it's darker and cooler. This helps slow down diffusion of water out of the leaf.
• The bigger the stomata and the more stomata a leaf has, the more water the plant will lose. Plants in really hot climates really need to conserve water, so they have fewer and smaller stomata on the underside of the leaf and none on the upper epidermis.

• Stomata close automatically when supplies of water from the roots start to dry up.
• The guard cells have a special kidney shape which open and closes the stomata as the guard cells for turgid or flaccid.
• Thin outer wall and thickened innner walls make this opening and closing function work properly
• Open stomata allow gases in and out for photosynthesis
• They're sensitive to light, so they open during the day and close at night. This allows them to conserve water without losing out on photosynthesis.

1) Nitrates - contain nitrogen for making amino acids and proteins. These are needed for cell growth. If a plant can't get enough nitrates, its growth will be poor and it'll have yellow leaves.

2) Phosphates - needed for respiration and growth. Contain phosphorus for making DNA and cell membranes. Plants without enough phosphate have poor root growth and discoloured leaves.

3) 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 discoloured leaves.

Magnesiums is also needed in small amounts. It's required for making chlorophyll. Plants without enough magnesium have yellow leaves.

• 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. It's normally higher in the root hair cell than 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 (from low to high concentrations).

• Living things are made of materials they take from the world around then
• When they die and decompose, or release material as waste, the elements they contain are returned to the soil or air where they originally come from.
• Nearly all decomposition is done by microorganisms like soil bacteria and fungi (decomposers).

The rate of decay depends on:

• Temperature - a warm temperature makes things decay faster because it speeds up respiration in microorganisms
• Amount of water - things decay faster when they're moist because microorganisms need water
• Amount of oxygen - decay is faster when there'e oxygen available. The microorganisms respire aerobically providing more energy.

When these factors are at optimum levels microorganisms grow and reproduce more quickly.

Detritivores feed on dead and decaying material (detritus). Examples of detritivores include earthworms, maggots and woodlice. As these detritivores feed on the decaying material, they break it up into smaller bits. This gives a bigger surface area for smaller decomposers to worl on and so speeds up decay.

Saprophytes also feed on decaying material, but they do so by extracellular digestion - they feed by secreting digestive enzymes on to the material outside of their cells. The enzymes break down the material into small bits, which can then be absorbed by the saprophyte. Many saprophytes are fungi.

• Canning - put food into an airtight can to keep decomposers out
• Cooling - put food in the fridge to slow down the decomposer's reproduction rate
• Freezing - food lasts longer in the freezer than in the fridge because the decomposers can't reproduce at all
• Drying - dried food lasts longer because decomposers need water to carry out cell reactions.
• Adding salt/sugar - if there's a high concentration of salt/sugar around decomposers, they'll lost water by osmosis. This damages them and they can't work properly.
• Adding vinegar - vinger is acidic and the acid kills the decomposers.

Intensive farming means trying to produce as much food as possible from your land, animals and plants:

• Using herbicides to kill weeds. This means that more of the energy from the sun falling on fields goes to crops and not to any other competing plants that aren't wanted.
• Using pesticides to kill insects that eat the crops. This makes sure no energy is transferred into a different food chain.
• Battery farming animals. The animals are kept close together indoors in small pens, so that they're kept warm and can't move about. This saves them wasting energy as they move around, and stop them using up so much energy keeping warm.

Intensive farming allows us to produce a lots of food from less and less land, which means a huge variety of top quality foods, all year round, at cheap prices.

Intensive farming methods are efficient, but they sometimes raise ethical dilemmas.

• Removal of hedges to make huge fields destroys the natural habitat of wild creatures. It can also lead to serious soil erosion.
• Careless use of fertilisers can pollute rivers and lake (eutrophication)
• Pesticides disturb food chains
• Lots of people think that intensive farming of animals such as battery-hens is cruel because they have very little space or freedoms to move around.

• Pesticides are sprayed onto crops to kill the creatures that damage them, but unforunately they can also kill organisms that aren't pests, like bees and ladybirds.
• This can cause a shortage of food for animals further up the food chain.
• Some pesticides are persistent - they tend to stick around in ecosystems and are hard to get rid of.
• There's a danger of pesticides being passed along the food chain and killing the animals further up.

Biological control 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. For example:

• Aphids are a pest because they eat roses and vegetables. Ladybirds are aphid predators, so people release them into their fields and gardens to keep aphid numbers down.
• Certain types of wasps and flies produce larvae which develop on (or in) a host insect. This eventually kills the insect host. Lots of insect pests have parasites like this.
• Myxomatosis is a disease which kills rabbits. The mxyoma virus was released in Australia as a biological control when the rabbit population there grew out of control and ruined crops.

Advantages - No chemicals are used, so there's less pollution, disruption of food chains and risk to people eating that food that's been sprayed. There's no need to keep repeating the treatment (like you would with chemical pesticides).

Disadvantages - The predator you introduce might not eat the pest, making it useless. The predator could eat useful species like pollinators. The predator's population might increase and get out of control. The predator might not stay in the area where it's needed.

Organic fertilisers - recycles the nutrients left in plant and animal waste. It doesn't work as well as artificial fertilisers, but 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 (as each crop has slightly different needs). Most crop rotations include a nitrogen-fixing crops, such as legume plants. These help put nitrates back in the soil.

Weeding - this means physically removing the weeds, rather than just spraying them with a herbicide. Obviously it's a lot more labour intensive, but there are no chemicals involved.

Varying seed planting times - sowing seeds later or earlier in the season will avoid the major pests for that crop. This means the farmer won't need to use pesticides.

Biological control

Advantages  

• Organic farming uses fewer chemicals, so there's less risk of toxic chemicals remaining on food.
• It's better for the environment. There's less chance of polluting rivers with fertiliser. Organic farmers also avoid using pesticides, so don't disrupt food chains and harm wildlife.
• For a farm to  be classed as organic, it will usually have to follow guidelines on the ethical treatment of animals. This means no battery farming.

Disadvantages  

• Organic farming takes up more space than intensive farming - so more land has to be farmland, rather than being set aside for wildlife or for other uses.
• It's more labour intensive. This provides more jobs, but it also makes food more expensive.
• You can't grow as much food. But on the other hand, Europe over produces food these days anyway.