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
part | function |
---|---|
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
part | function |
---|---|
cell wall | strengthens the cell |
chloroplasts | contain chlorophyll, which absorbs light energy for photosynthesis |
permanent vacuole |
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
Cell | Function | Adaption |
---|---|---|
Absorbs light energy for photosynthesis | Packed with chloroplasts. Regular shaped, closely packed cells form a continuous layer for efficient absorption of sunlight. | |
Absorbs water and mineral ions from the soil | Long 'finger-like' process with very thin wall, which gives a large surface area. | |
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. | |
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 [diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration ] 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
location | particles move | from | to |
---|---|---|---|
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 [concentration gradient: A difference in concentration between two areas next to each other. Particles will move down the concentration gradient from an area of high concentration to an area of low concentration. ].
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.
Osmosis
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.