AQA GCSE Biology Unit B2 Chapters 1-3

Animal and plant cells all have:

• A nucleus that controls the cell's activities
• Cytoplasm which is where chemical reactions take place
• A cell membrane that controls the movement of materials
• Mitochondria where energy is released during aerobic respiration
• Ribosomes where proteins are made (synthesised)

Plant cells also have:

• A rigid cell wall (made from cellulose) for support
• Chloroplasts that contain chlrorphyll for photosynthesis
• A vacuole containing cell sap

When an egg is fertilised it begins to grow and develop.

At first there is a growing ball of cells but as the organism gets bigger some of the cells change and become specialised.

There are lots of different specialised cells, e.g.

• Plants - root hair cells with a large surface area so they can get as much water as possible
• Animals - sperm cells with lots of mitochondria for energy and a tail so it can swim quickly

Diffusion is the random movement of molecules from an area of high concentration to an area of lower concentration.

The particles move because of the natural vibrations they have.

The larger the difference in concentration, the faster the rate of diffusion.

• Oxygen gets into cells from the blood stram by diffusion
• CO2 getting into photosynthesising plant cells
• Simple sugars and amino acids diffuse from the cust through cell membranes

Osmosis is the net movement of water molecules down a concentration gradient (from an area of high to an area of low concentration) across a partially permeable membrane.

It is very important - if cells recieve too much water they swell up (become turgid) and if they recieve too little they shrink (become flaccid). Also, chemical reactions in cells take place in solution.

The partially permeable membrane means that large molecules like glucose can't get into the cell.

carbon dioxide + water (+ light energy) ---> glucose + oxygen

Carbon dioxide is taken from the air by the leaves and the water is taken in from the soil by the roots.

Chlorophyll in plant leaves traps the energy needed for photosynthesis.

Any excess glucose is stored by the plant as starch

If certain things are in short supply, they will slow down the rate of photosynthesis.

Plant growers use this knowledge because otherwise they could waste money.

A lack of light would slow the rate of photosynthesis because light provides the energy for the process.

The wrong temperature would also slow it because if it's too cold the enzymes don't work effectively and if it's too hot the enzymes get denatured

Too little CO2 would also slow the rate, because there wouldn't be enough to react with water quickly

The product of photosynthesis is glucose which is used to make energy in respiration.

It is also combined with other mineral ions from the soil to produce new materials.

Glucose is stored as starch which is insoluble. This is done so that it has no effect on osmosis.

Plants need minerals for healthy growth.

Nitrates are taken from the soil to produce amino acids which plants use to make proteins for growth. If a plant doesn't take up enough nitrates, it will have stunted growth.

Magnesium ions are needed to make chlorophyll. If a plant doesn't make enough chlorophyll it will have yellow leaves.

A pyramid of biomass represents the mass of the organisms at each stage in a foood chain.

It may be more accurate than a pyramid of numbers.

For example, one tree may have many insects feeding on it but the mass of the tree is far larger than the mass of the insects.

Pyramid of numbers

Energy is lost in:

• Faeces when not all of the food eaten can be digested
• Respiration to release energy for living processes
• Homeostasis to keep the body at a constant temperature

We can use Sankey diagrams to show how energy is transferred in a system

The shorter the food chain, the less energy will be lost.

This means that it is more efficient ofr us to eat plants than it is to eat animals. Also, it would mean that there would be more food for people across the world.

We can artificially produce meat more efficiently by preventing the animal from moving so that it doesn't waste energy on movement and keeping the animal at a warmer temperature so it doesn't use as much energy keeping itself at a constant temperature but this is controversial because it can be seen as cruelty.

• Deritus feeders may start decay by eating dead animals or plants and producing waste materials.
• Decay organisms (that are bacteria and fungi) then break down the waste and and dead plants and animals.
• Decay is faster if it is warm and wet