Revision notes to support OCR GCSE Biology module B4

1. Year 11 Module B4 Revision notes
Photosynthesis:
Light
Carbon dioxide + water glucose + oxygen
Chlorophyll
Leaves
• Designed for making food by photosynthesis.
Leaves are adapted for efficient photosynthesis:
• They are broad – large surface area exposed to light
• Thin- carbon dioxide and water vapour only have a short distance to reach palisade cells for
photosynthesis.
• There are air spaces in the spongy Mesophyll layer – allows carbon dioxide and oxygen to diffuse
easily between cells, large surface area for gas exchange. There is a big internal surface area to
volume ratio.
• Leaves contain lots of chlorophyll, found in lots of chloroplasts, found in the palisade layer
• Upper epidermis is transparent – light can pass through it into the palisade layer.
• Lower surface of leaf is full of little holes called stomata; let carbon dioxide and oxygen in and out;
also let water escape (transpiration).
• Leaves have a network of veins – deliver water and nutrients to every part of the leaf and take food
produced by the leaf away; they also help to support leaf structure.
Palisade cells
Packed with chloroplasts; tall shape means a lot of surface area is exposed down the side for absorbing
carbon dioxide from the air in the leaf; shape also means good chance of light hitting a chloroplast.

2. Plants exchange gases by diffusion.
• Diffusion is the passive movement of particles from an area of higher concentration to an area of
lower concentration.
Osmosis
- A special kind of diffusion.
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.
A partially permeable membrane is one with very small holes in it – water can pass through but larger
molecules can’t.
The water molecules can pass through both ways – they move randomly all the time. Because there are
more water molecules on one side than the other, there is a steady net flow of water into the region with
fewer water molecules, i.e. water moves into the stronger sucrose solution. This means the concentrated
sucrose solution gets more dilute.
Turgor pressure – supports plant tissues
When a plant is well watered, all its cells will draw water in by osmosis, they become plump and swollen.
When cells are like this they are said to be turgid. The contents of the cell push against the cell wall – this is
called turgor pressure. Turgor pressure helps to support the plant tissues.
If there is no water in the soil a plant starts to wilt. The cells start to lose water and they lose their turgor
pressure. The cells are flaccid.
If a plant is really short of water the cytoplasm inside the cell starts to shrink and the membrane pulls away
from the cell wall. The cell is now plasmolysed; however the plant doesn’t totally lose shape because the
inelastic cell wall keeps things in position.
Animal cells do not have a cell wall. If an animal cell takes in too much water it bursts - this is known as
lysis. If an animal cell loses too much water it shrivels up – this is known as crenation.
Water flow through plants
Root hairs take in water by osmosis
Root hairs give the plant a large surface area for absorbing water from the soil.
There is usually a higher concentration of water in the soil than the plant so water is drawn into the root
hair cell by osmosis.
Transpiration is the loss of water from the plant.
Transpiration is caused by the evaporation and diffusion of water from inside the leaves.
This creates a slight shortage of water in the leaf; more water is drawn up from the rest of the plant
through the xylem vessels.

3. More water is then drawn up from the roots. There is a constant transpiration stream of water through the
plant.
Transpiration is a “side-effect” of the way leaves are adapted for photosynthesis. The stomata are essential
for gas exchange, but this means that because there is more water inside the plant than in the surrounding
air, water escapes from the leaves through the stomata.
Benefits of transpiration
• The constant stream of water from the plant helps keep the plant cool
• The plant is provided with a constant supply of water for photosynthesis
• The water creates turgor pressure in the plant cells – this helps support the plant and prevents
wilting.
• Minerals needed by the plant can be brought in from the soil along with the water.
What affects transpiration?
• Light intensity – the brighter the light the greater the transpiration rate. Stomata begin to close as
it gets darker. Photosynthesis can’t happen in the dark so they don’t need to let carbon dioxide in.
This means water can’t escape.
• Temperature – the warmer it is the greater the transpiration rate. This is because the water
particles have more energy to evaporate and diffuse out of the stomata.
• Air movement – lots of air movement (wind) around a leaf means faster transpiration. If the air is
still the water vapour surrounding the leaf doesn’t move away, this means there is a higher
concentration of water outside of the leaf as well as inside it, hence diffusion doesn’t happen as
quickly. If it’s windy the water vapour is quickly moved away, this means there is a greater
concentration difference and diffusion will happen more quickly.
• Air humidity – if the air around the leaf is very dry transpiration happens more quickly. If the air is
humid there is a lot of water vapour already in it so there is less of a concentration difference
between the surrounding air and that in the leaf – hence a slower rate of diffusion.
Achieving the balance
Plants have adaptations to help reduce water loss
• Leaves have a waxy cuticle covering their upper epidermis; this means the upper surface of the leaf
is waterproof.
• Most stomata are on the underside of the leaf where it is darker and cooler. This helps slow down
the diffusion of water out of the leaf.
• Bigger stomata and lots of stomata mean more water loss. In hot climates plants have fewer,
smaller stomata on the underside of the leaf and no stomata on the upper epidermis.

4. Stomata open and close automatically
When supplies of water from the roots start to dry up stomata will close.
Guard cells, which surround the stomata, have a kidney shape which opens and closes the stomata as the
guard cells go turgid or flaccid.
Thin outer walls and thickened inner walls help this open / close function work properly.
Open stomata allow gases in and out for photosynthesis
Stomata are sensitive to light so they close at night; this conserves water without losing out on
photosynthesis.
Transport systems
Phloem tubes – transport food
• Made of columns of living cells with perforated end plates allowing substances to flow through.
• They transport food substances made in the leaves to growing and storage tissues. This is in both
directions.
• Movement of food around the plant is known as translocation.
Xylem tubes – take water up
• Made of dead cells, these are 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.
Recognising which is the xylem and which is the phloem
Xylem – on the inside – forms “scaffolding”, provides strength

5. Phloem – around the outside of the stem
Plants also need minerals
Nitrates – contain nitrogen for making amino acids and proteins. They are needed for cell growth. Lack of
nitrates means growth will be stunted and the plant will also have yellow older leaves.
Phosphates - Contain phosphorus for making DNA and cell membranes; they are needed for respiration
and growth, lack of phosphates means a plant will have poor root growth and purple older leaves.
Potassium - Helps the enzymes needed for photosynthesis and respiration. Lack of potassium means
plants will have poor flower and fruit growth and discoloured leaves.
Magnesium - also needed in small amounts – it is required for making chlorophyll, this is obviously
necessary for photosynthesis. Plants without enough magnesium have yellow leaves.
How are the minerals absorbed?
By active transport!
Root hairs give the plant a big surface area for absorbing minerals from the soil.
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.
Active transport means the minerals are absorbed against the concentration gradient. They go from a low
concentration gradient to a high concentration gradient. To do this energy (ATP) is required from
respiration.
Pyramids of number
Trophic level is a feeding level
Each bar on a pyramid of numbers shows the number of organisms at each stage of the food chain.
A pyramid of numbers does not have to necessarily look like a typical pyramid.

6. Pyramids of biomass
Each bar on a pyramid of biomass shows the mass of living material at that stage of the food chain – how
much all of the organisms at each level would weigh if you put them all together.
Biomass pyramids are nearly always the right shape (pyramid shape)
Energy transfer and energy flow
Material and energy are both lost at each stage of the food chain.
This helps to explain why we get a pyramid of biomass – most of the biomass is lost and so does not
become biomass in the next level up.
Plants use a small percentage of light energy from the Sun to make food during photosynthesis. The energy
work its way through the food web as animals eat the plants and each other.
Most of the energy is eventually lost to the surroundings as heat. This is true for mammals and birds whose
bodies must be kept at a constant temperature which is usually higher than their surroundings.
Material and energy are also lost from the food chain through egestion (excretion / waste / droppings)
Energy is also lost through movement of animals
This helps to explain why there are hardly ever food chains with more than five trophic levels as there is so
much energy lost at each level there’s not enough to support any more organisms after four or five levels.

7. Interpreting data on energy flow
80 000 kJ 10 000 kJ 900 kJ 40 kJ
The numbers show the amount of energy available to the next level.
e.g. 80 000 kJ energy is available to the rabbit
Energy lost at each trophic level can be calculated.
E.g. energy lost at 1st
trophic level = 80 000 kJ – 10 000 kJ = 70 000 kJ lost
Efficiency of energy transfer can be calculated using the following formula:
Efficiency = energy available to the next level x 100
energy that was available to the previous level
At the first trophic level efficiency of energy transfer = 10 000 KJ / 80 000 kJ x 100
= 12.5 % efficient
Biomass and intensive farming
Energy stored in biomass can be used for other things
Energy can be released by eating it, feeding it to livestock, growing the seeds of plants and using it as a fuel.
For a given area of land a lot more food for humans can be produced by growing crops than by grazing
animals, e.g. only about 10% of the biomass eaten by beef cattle becomes useful meat for people to eat.
However, it is important to get a balanced diet, this is difficult to achieve from just eating crops, also some
land isn’t suitable for growing crops (moorland or fell-sides), here animals like sheep and deer are the best
way of getting food from the land.
Biomass can also be used for fuel.
Two examples:

8. 1. Fast-growing trees – burning trees is ok provided it is balanced by using fast-growing trees planted
especially for the purpose. Each time trees are cut down more are planted to replace them. The
carbon dioxide emissions are balanced as the replacement trees are still removing carbon.
2. Fermenting biomass using bacteria or yeast – fermenting is breaking down by anaerobic
respiration. Micro-organisms can be used to make biogas from plant and animal waste; this is done
in a simple fermenter called a digester. The biogas can be burned to release the energy for heating,
powering turbines etc...
Why the development of biofuels is good:
1. They’re renewable
2. They reduce air pollution – no acid rain gases are produced
3. Theoretically you can be energy self –reliant, all energy needed could be supplied from household
waste.
Intensive farming
This is trying to produce as much food as possible from land and plants.
This is done in different ways but all methods involve reducing the energy losses that happen at each stage
of the food chain.
Examples of how it’s done:
1. Using herbicides to kill weeds. More of the energy from the Sun falls goes to the crops and not any
competing plant.
2. Using pesticides to kill the insects that eat the crops. This means that no energy is transferred into
a different food chain.
3. Animals are battery farmed. They’re kept close together indoors in tiny pens. This means they are
warm and can’t move about. No energy is wasted in movement or in keeping warm.
Intensive farming means a lot of food can be produced from less land. This means a large variety of top
quality food all year at lower prices.
The negatives of intensive farming
1. Removal of hedges to make huge fields for maximum efficiency. This destroys the natural habitats
of wild creatures and can also result in soil erosion.
2. Careless use of fertilisers pollutes rivers and lakes and can lead to eutrophication.
3. Pesticides can disturb food chains
4. Ethics – intensive farming of animals (battery hens and pigs) is considered cruel.
Pesticides – disturb food chains
• They are sprayed onto crops to kill creatures that damage them; the problem is they also kill lots of
harmless animals (bees, beetles etc).

9. • They can cause a shortage of food for animals further up the food chain.
• Pesticides can also be toxic to creatures that aren’t pests; there is also a danger of the poison
passing on through the food chain to other animals. There can even be a risk to humans.
• An example of this can be seen with DDT and otters. Otters were almost wiped out in Southern
England in the 1960s by the pesticide DDT. This can’t be excreted and it accumulates along the
food chain. The otter ended up with most of the DDT that had been “collected” by all the other
animals in the food chain.
Biological control – an alternative to pesticides
Here living things are used to control a pest instead of chemicals.
Examples:
1. Aphids (greenfly) are a pest, they eat roses and vegetables. Ladybirds are predators, by releasing
them into fields and gardens to keep aphids down.
2. Certain types of wasps and flies produce larvae which develop on a host insect. This eventually kills
the insect host.
3. Myxomatosis is a disease that kills rabbits. This virus was released in Australia as a biological
control when the rabbit population grew out of control (the rabbits ruined crops).
Biological control advantages:
1. The predator, parasite or disease usually only affects the pest animal, harmless things aren’t killed.
2. No chemicals are used; this means there is less pollution, disruption of food chains and risk to
people eating the food that’s been sprayed.
Biological control disadvantages:
1. It’s slower – control organism numbers need to build up.
2. Biological control won’t kill all pests; it also only kills one type of pest.
3. It takes more management and planning, workers need training.
4. Control organisms can drive out native species; they might even become pests themselves.
Alternatives to intensive farming
Hydroponics
Plants are grown without soil.
Most commercially grown tomatoes and cucumbers are grown in nutrient solutions (water and fertiliser)
instead of soil.

10. Advantages of hydroponics:
• Takes up less space – less land is required
• No soil preparation or weeding
• Plants can be grown in areas where the soil is poor
• No problems with soil pests
• Mineral levels can be easily controlled
Disadvantages of hydroponics
• It can be expensive to set up and run
• Specially formulated soluble nutrients need to be used
• Growers need to be skilled and properly trained
• The plants need support as there’s no soil for the roots to anchor to.
Organic farming - what makes it different?
1. Organic fertilisers - (animal manure and compost) are used. This re-cycles the nutrients left in plant
and animal waste. It is not as effective as artificial fertilisers but is better for the environment.
2. Crop rotation – a cycle of different crops are grown in a field each year. This stops the pests and
diseases of one crop building up and stops nutrients running out. Most crop rotations involve
growing a legume – these plants help put nitrates back into the soil.
3. Weeding – physically removing the weeds rather than spraying them with an herbicide, this is much
more labour intensive as no chemicals are involved.
4. Varying the planting times – sowing seeds later or earlier in the season avoids the major crops for
that pest and means pesticides aren’t necessary.
5. Biological control is used.
Advantages and disadvantages – an overview
• Organic farming takes up more space than intensive farming; this means more land has to be used
as farmland instead of being set aside for wildlife or other uses.
• It’s more labour intensive, it provides more jobs but makes the food more expensive.
• Not as much food can be grown (however Europe over-produces food anyway)
• Organic farming uses fewer chemicals – less risk of toxic chemical remaining on food.
• It’s better for the environment – less chance of polluting rivers with fertilisers – less chance of
disruption to food chains and wildlife as pesticides aren’t used.

11. • Organic farms follow guidelines on the ethical treatment of animals – no battery farming.
Decay
Decay occurs because of micro-organisms. Living things are made from materials they take from the world
around them. When they die and decompose or release 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 – the cycle
repeats itself.
Nearly all decomposition is done by soil bacteria and fungi, it occurs everywhere in nature, including in
compost heaps and sewage works.
The key elements recycled include: CARBON, HYDROGEN, OXYGEN AND NITROGEN.
The rate of decay depends on three main things:
1. Temperature – warm temperatures increase the rate of decay as it speeds up respiration in
decomposers.
2. Moisture – things decay faster when moist because decomposers need water.
3. Oxygen (air) decay is faster when oxygen is present, decomposers respire aerobically providing
more energy.
Food preservation – reducing the rate of decay
Canning – food is put in an air-tight can. This keeps the decomposers out. After canning the tin and its
contents are heated to high temperatures to kill any micro-organisms already in there.
Cooling – keep food in a fridge. Slows down decay by slowing respiration of micro-organism down and also
the rate at which they reproduce.
Freezing – stops micro-organisms respiring or reproducing at all. Some (but not all) are killed when the
water inside them expands and freezes.
Drying – micro-organisms need water, dried food has no water. Fruit and some meat can be preserved by
drying them out.
Adding salt – high concentration of salt around decomposers means they will lose water by osmosis. This
damages them and they can’t work properly. Tuna and olives are often stored this way (in brine)
Adding vinegar – vinegar is acidic, the low pH inhibits the enzymes inside the micro-organisms. This stops
them decomposing
Detritivores and Saprophytes
Detritivores feed on dead and decaying material (detritus). Examples of detritivores include earthworms,
maggots and woodlice. They break the decaying material up into smaller pieces. This gives a bigger surface
area for the smaller decomposers to work on and speeds up decay.
Saprophytes feed on decaying material by extracellular digestion, mostly they are bacteria and fungi. They
secrete their enzymes onto the material outside of their cells. The enzymes break down the material into
smaller pieces which can then be absorbed by the saprophyte.

12. The Carbon Cycle
Carbon is constantly moving between the atmosphere, the soil and living things.
The whole cycle is powered by photosynthesis. Plants convert the carbon from carbon dioxide in the air
into sugars. Plants incorporate this carbon into carbohydrates, fats and proteins. Eating passes the carbon
compounds in the plant along to animals in the food chain or food web. Plants and animals respire (while
they are alive); this releases carbon dioxide back into the air. Plants and animals eventually die and decay;
they are then turned back into useful products. When plants and animals decay the bacteria and fungi
doing the decaying respire and release carbon dioxide back into the air as they break down the material.
Some useful plant and animal products (wood and fossil fuels) are burned (combustion). This releases
carbon dioxide back into the air.
The other major carbon recycling pathway occurs in the sea. Marine organisms make shells made of
carbonates. When they die the shells fall to the ocean floor and eventually form limestone rocks. The
carbon in these rocks may return back to the atmosphere as carbon dioxide during weathering or volcanic
eruptions.
The Nitrogen Cycle
The atmosphere contains 78% Nitrogen gas; this is very unreactive, it can’t be used directly by plants or
animals.
Nitrogen is needed to make proteins for growth.
Plants get their nitrogen from the soil; nitrogen in the air has to be turned into nitrates before the plants
can use it. Animals get their proteins by eating the plants and each other.

13. Decomposers break down the proteins in rotting plants and animals and urea in animal waste into
ammonia (NH3). The nitrogen in these organisms is recycled.
Nitrogen fixation is the process of turning nitrogen from the air into nitrogen compounds in the soil which
plants can then use. This happens through two main ways:
1. Lightning – the energy in a bolt of lightning makes the nitrogen in the air with the
oxygen in the air to give nitrates.
2. Nitrogen – fixing bacteria in the roots and soil.
There are four different types of bacteria involved in this cycle:
a) Decomposers – decompose proteins and urea and turn them into ammonia
b) Nitrifying bacteria – turn ammonia in decaying matter into nitrates
c) Nitrogen-fixing bacteria – turn atmospheric nitrogen into nitrogen compounds that
plants can use.
d) Denitrifying bacteria – turn nitrates back into nitrogen gas
Some nitrogen-fixing bacteria live in the soil. Others live in the nodules on the roots of legume plants
(remember they’re often used as a crop in crop rotations as legume plants are good at putting nitrogen
back into the soil). The plants have a mutualistic relationship with the bacteria, this means the bacteria get
food (sugars) from the plant and the plant gets nitrogen compounds from the bacteria to make into
proteins – it is a relationship that benefits them both.