BBC Science - Cells

Cells

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	filled with cell sap to help keep the cell turgid [turgid: having turgor; enlarged and swollen with water ]

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
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 [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.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid [turgid: having turgor; enlarged and swollen with water ] or stiff so they that able to hold the plant upright.