All living things need to move or transport materials within their cells or throughout their body. Cell fluids consist of water, mineral ions, glucose, fatty acids, glycerol, amino acids and dissolve gasses, oxygen and carbon dioxide. The composition of the cell fluid is kept constant within certain limit. These are homeostatic processes.
Substances pass through biological membranes by two kinds of transport:
- Passive transport, which does not use energy from the cell. Examples of passive transport mechanisms are absorption, diffusion, and osmosis.
- Active transport, which use energy from the cell.
Passive transportAbsorptionThis process is also called
imbibition. Many dry or dehydrated substances can attract water. For example when dried seeds or dried fruits are soaked in water they take some of it and swell up. It is an important process in the germination of seeds.
DiffusionDiffusion is the passive movement of substances from a region of
high concentration to a region of
low concentration. The difference in concentration between the two regions forms a
concentration gradient. When the difference in concentration between the two regions is great, the concentration gradient is steep and the rate of diffusion is fast. As the difference in the concentration between the two regions becomes less, the concentration gradient becomes flatter and the rate of diffusion slows down. Diffusion stops when the concentration of particles in the two regions is the same and the concentration gradient no longer exists.
Diffusion occurs in both liquid and gases and can take place over long distances. The rate of diffusion of oxygen in the air is 100.000 times the diffusion rate of oxygen in water. Diffusion can also take place through a
permeable membrane. This is a membrane which lets all diffusing substances pass through it.
Diffusion of solutes through cell surfaces membranesDissolved sugar and ions of sodium and potassium can pass through cell surface membranes by diffusion. The cell can take in dissolved foods such as sugar molecules from its surrounding fluids when its stock of sugar is low.
Osmosis Osmosis is the passive diffusion of water through a selectively or partially permeable membrane. The water moves from a region of a
high water concentration (a dilute solution) to a region of a
lower water concentration (a concentrated solution). The selectively permeable allows water molecules to pass through it but does not allow any dissolved substances to pass through.
Osmosis is a slow process in which only water is transported. It only take place over a short distance but the cell does not have to use up any energy for it to take place.
The demonstration of osmosis illustrated in Figure 1 uses an artificial, non-living membrane called Visking or cellophane.
Osmosis in plant and animal cells Figure 2 shows how a plant cell change when they are put in a solution which is (b) more dilute and (c) more concentrated than their cytoplasm.
In the plant cell, there are two cell barriers:
- wholly permeable cell wall and
- partially permeable cell surface membrane
In (b), the entry of water into the vacuole by osmosis causes an
increase in cell volume and
turgor pressure which is opposed by the
cell wall pressure. In (c), the loss of water from the vacuole by osmosis causes a decrease in cell volume and withdrawal of the cell surface membrane from the cell wall. This is reffered to as
plasmolysis.
While in (a) is the condition when the liquid within cell is equal to the liquid outside the cell.
In the animal cell (Figure 3) there is only one cell barrier - the cell surface membrane. In Hypo-tonic stage the entry of water into the red blood cell by osmosis causes an
increase in cell volume resulting in bursting of the cell membrane (
haemolysis) with the escape of haemoglobin. In hyper-tonic stage, the loss of water from the red blood cell by osmosis causes a
decrease in cell volume and shrinkage of the cell surface (
crenation).
In plant cells, the wholly permeable cell wall acts as a resistance againts the partially permeable cell surface membrane. Animal cell do not have a cell wall and therefore their cell surface membranes are not restricted.
Active transport This method of transport uses
energy from the cell. Certain selected solute ions, glucose, sucrose, and amino acids, are transported rapidly through a short distance from a region of
low concentration to a region of
high concentration. This movement of substance takes place through a living, partially permeable membrane - the cell surface membrane. About a third of all the energy released in the cell respiration is used to draw vital substances into cell
againts their concentration gradients.
Question:- What is the difference between passive and active transport systems?
- Why does dietary fibre swell up in the gut?
- Which way do particles move when they diffuse?
- What is permeable membrane?
- What substances pass through cell surface membranes by diffusion?
- How is osmosis different from diffusion?
- How is osmosis similar to diffusion?
- What happens to plant and animal cells when they are put in: (a). a more dilute solution than their cell sap; (b) a more concentrated solution than their cell sap.
- How is active transport different from osmosis?
