What you need to know...
Transport across cell membranes:
The cell membrane consists of lipids and proteins and is selectively permeable.
Passive transport is with the concentration gradient and does not require energy.
The importance of diffusion in cells as the movement of molecules along a concentration gradient.
Osmosis as the movement of water molecules across a membrane in terms of water concentration.
Animal cells can burst or shrink and plant cells can become turgid or plasmolysed in different solutions.
Active transport requires energy for membrane proteins to move molecules against the concentration gradient.
You know by now that all cells are surrounded by a membrane which contain the cell and controls what can enter and leave. But what are these membranes made of and how do they work? Cells need to be able to exchange substances with the surrounding environment for all sorts of reasons. This includes getting the resources needed for energy and growth, to get rid of waste and also to communicate with other cells. In this topic we'll discuss the different ways substances can get across a membrane, but first you need to know what a membrane is made of.
Cell membranes are made up of two layers of lipids (fats) and are peppered with protein molecules. If you were to zoom in on a cell membrane it would look something like this.
Remember though, in real life this would all be in three dimensions and moving.
Cell membranes are often described as being selectively permeable and this is a result of their structure. The two layers of lipids will only allow small molecules or other fatty molecules to pass through easily. If anything else is to get across it will need another method. For most molecules this will involve the proteins in the membrane. These proteins allow the cell to control which of these molecules can pass across and when.
The simplest way for substances to cross the membrane is by diffusion. All particles have a certain amount of energy. Gas particles have more energy than liquid particles, which in turn have more energy than solid particles. Gas and liquid particles are therefore constantly moving at random. As a result, gas or liquid particles will always diffuse from a region of high concentration to a region of low concentration until an equilibrium is reached. You know this already. If someone sprays some perfume in a room, to begin with there is a high concentration of the perfume particles next to the sprayer and they can smell it strongly, but no one else in the room will be able to smell it as there will be a low concentration of the perfume everywhere else. The perfume particles will be moving at random in the air and will diffuse from the area of high concentration to the areas of low concentration until there is an equal concentration throughout the room and everyone will be able to smell the perfume equally. No one had to do anything to make this happen, it just happens. It is a passive process.
Diffusion occurs across cell membranes also. If there is a concentration gradient, or in other words a higher concentration on one side compared to the other, and if the particles are small enough to actually cross the membrane, they will diffuse across. The cell doesn't need to use any of its own energy to make this happen. Oxygen (O2) is a good example of this. Oxygen molecules are small, and all cells need it for respiration to release energy from food molecules.
Because there's a higher concentration of oxygen molecules outside the cell compared to the cell's cytoplasm, you'll see that oxygen diffuses into the cell along this concentration gradient. You may notice that there are some arrows in the opposite direction. Because the molecules are moving at random, some will move out of the cell, but most will be moving in in this case. The perfume in our example above eventually reached an equilibrium but this won't happen in our cell above for two reasons. Oxygen molecules in the cell will constantly be used up in respiration which will keep the concentration inside the cell low. Also, the concentration of oxygen molecules outside the cell will be kept high with a fresh supply of oxygen. For example, oxygen is continually brought to your body tissues from the lungs by the bloodstream.
What other substances do you think will diffuse across the cell membrane and in what directions?
A molecule which is crucial to all cells which we've not mentioned yet is water (H2O). Water is also small enough to diffuse into and out of cells across the cell membrane. However, we call the diffusion of water osmosis. As osmosis is a form of diffusion, it requires no input of energy from the cell. Because of the way we discuss concentrations and solutions (which you'll understand if you're also studying chemistry) we have to be very careful how we discuss the concentrations of water in osmosis. We need to discuss the movement of water in osmosis as being from a region of high water concentration to a region of low water concentration.
Just as in diffusion, as the water molecules are moving randomly, there will be movement in both directions, but the overall net movement is from high water concentration to low water concentration.
The concentration of water molecules which are able to move in or out of a cell is affected by the presence or absence of other dissolved molecules (solutes) such as salt or sugar. Obviously, a cell's cytoplasm will have a considerable quantity of solutes dissolved in the water of the cytoplasm. The presence of a solute lowers the water concentration of the solution.
To demonstrate this, in the diagram above there are an equal number of water molecules on either side of the cell membrane (20). However, as there is a solute dissolved in the cytoplasm, this stops some of the water molecules inside the cell from being able to cross the membrane. This effectively lowers the water concentration inside the cell compared to the outside of the cell so the net movement of water in this case would be into the cell.
There's more on osmosis on BBC Bitesize.
Effects of Osmosis on Animal & Plant Cells
Osmosis has different effects on animal and plant cells because of their different structures. Plants' cell walls allow them to respond to changes in water concentration in ways animal cells cannot. The following diagram shows the effect of placing an animal cell in solutions of low water concentration and high water concentration.
The next diagram shows what would happen to plant cells treated in the same way.
Animal cells shrink when placed in a solution of low water concentration whereas a plant cell becomes plasmolysed. This is evident from the cell membrane having pulled away from the cell wall.
Animal cells burst when placed in a solution of high water concentration whereas a plant cell does not. The cell wall prevents the cell from bursting and allows the cell to become turgid. This turgidity of a plant's cells provides the plant with support.
This BBC video clip discusses the effects of osmosis on cells.
So, diffusion and osmosis are great. Substances move into and out a cell, and the cell doesn't have to use up any of its energy to make it happen. But it does mean that all the movement has to be from high to low concentration. What happens if a cell needs to move a substance against a concentration gradient? What if it needs to move molecules from a low concentration to a high one? This is actually something cells need to do quite a lot. One example is in nerve cells, which we'll come back to in Unit 2. Nerve cells need to pump ions against their concentration gradient in order to prepare for a nerve impulse. But how can they do this? The answer is active transport.
The energy available in the cell from respiration is in the form of a molecule called ATP, you'll learn more about this in Respiration. ATP is used to do many things in the cell which require energy, one of which is active transport. Protein molecules in the membrane use energy from ATP to transport molecules across the membrane from low to high concentration by the process of active transport.