All animals and plants are made of cells. Animal cells and plant cells have features in common, such as a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells also have a cell wall, and often have chloroplasts and a permanent vacuole. Note that cells may be specialised to carry out a particular function.

Dissolved substances pass into and out of cells by diffusion. Water passes into and out of cells by osmosis.

Animal and plant cells

Function of cells which animal and plant cells have in common

partfunction nucleus contains genetic material, which controls the activities of the cell cytoplasm most chemical processes take place here, controlled by enzymes cell membrane controls the movement of substances into and out of the cell mitochondria most energy is released by respiration here ribosomes protein synthesis happens here

Plant cells also have extra parts:

Extra parts of plant cells

partfunction cell wall strengthens the cell chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis permanent vacuole filled with cell sap to help keep the cell turgid

Make sure you can label diagrams of animal and plant cells, like these:

Generalised animal and plant cell

Specialised cells

Cells may be specialised for a particular function. Their structure will allow them to carry this function out. Here are some examples:

Examples of the functions of cells

CellFunctionAdaption

Leaf cell

Absorbs light energy for photosynthesis Packed with chloroplasts. Regular shaped, closely packed cells form a continuous layer for efficient absorption of sunlight.

Root hair cell

Absorbs water and mineral ions from the soil Long 'finger-like' process with very thin wall, which gives a large surface area.

Sperm cell

Fertilises an egg cell - female gamete The head contains genetic information and an enzyme to help penetrate the egg cell membrane. The middle section is packed with mitochondria for energy. The tail moves the sperm to the egg.

Red blood cells

Contain haemoglobin to carry oxygen to the cells. Thin outer membrane to let oxygen diffuse through easily. Shape increases the surface area to allow more oxygen to be absorbed efficiently. No nucleus, so the whole cell is full of haemoglobin.

You are likely to be given information, perhaps in a diagram, to help you to explain the adaptations of a particular cell type to its function.

Diffusion

Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.

Examples of diffusion

Two examples of diffusion down concentration gradients

locationparticles movefromto gut digested food products gut cavity blood in capillary of villus lungs oxygen alveolar air space blood circulating around the lungs

Remember, particles continue to move from a high to a low concentration while there is a concentration gradient.

In the lungs, the blood will continue to take in oxygen from the alveolar air spaces provided the concentration of oxygen there is greater than in the blood. Oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away.

Water can move across cell membranes because of osmosis. For osmosis to happen you need:

• two solutions with different concentrations
• a partially permeable membrane to separate them

Partially permeable membranes let some substances pass through them, but not others. The animation shows an example of osmosis.

Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.

This is shown in the animation above. Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls. When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. At this point, the net exchange of water is zero and there is no further change in the liquid levels.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid or stiff so that they are able to hold the plant upright.

Green plants absorb light energy using chlorophyll in their leaves. They use it to react carbon dioxide with water to make a sugar called glucose. The glucose is used in respiration, or converted into starch and stored. Oxygen is produced as a by-product.

This process is called photosynthesis. Temperature, carbon dioxide concentration and light intensity are factors that can limit the rate of photosynthesis.

Plants also need mineral ions, including nitrate and magnesium, for healthy growth. They suffer from poor growth in conditions where mineral ions are deficient.

Photosynthesis

Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food - not just for plants but ultimately every animal on the planet.

During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires light energy, which is absorbed by a green substance called chlorophyll.

Photosynthesis takes place in leaf cells. These contain chloroplasts, which are tiny objects containing chlorophyll.

Cross-section through a leaf cell

Here is the equation for photosynthesis:

carbon dioxide + water (+ light energy)    →    glucose + oxygen

'Light energy' is shown in brackets because it is not a substance. You will also see the equation written like this:

Plants absorb water through their roots, and carbon dioxide through their leaves. Some glucose is used for respiration, while some is converted into insoluble starch for storage. The stored starch can later be turned back into glucose and used in respiration. Oxygen is released as a by-product of photosynthesis