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A. Introduction Transport Systems:
For a cell to be able to live, there must be communication (=transport) between the intracellular fluid (=the cytoplasm) and the extracellular fluid (= interstitial fluid).
These are the four major passive transport systems:
- diffusion
- facilitated diffusion
- osmosis
- filtration
These are the three major active transport systems:
- pumps (co-transporters, etc.)
- exocytosis / endocytosis
- phagocytosis
Diffusion is the process whereby something soluble that is placed in water is ‘automatically’ distributed, dissolved, in that water.
4.
Diffusion takes place because of the Brownian movements of the particles. (Brownian movements are caused by the kinetic energy stored in every particle. (What is kinetic?)
In the body, diffusion occurs all the time across the cell membranes. This will occur if the concentration of something (=the solute) is higher at one side of the membrane than at the other side, AND if the membrane is permeable for that solute.
The same thing happens in the opposite direction with the combustion of oxygen; carbon dioxide. This molecule is formed inside the cell (by converting oxygen) and will diffuse out of the cell into the extracellular space.
In this diagram, a beaker is divided by a membrane. On the left, a solute (blue) has been added to the fluid.
As the molecules can pass through the membrane, with time, more and more molecules will pass the membrane to the other side.
Eventually, the concentration will be the same in both compartments, left and right of the membrane (light blue in both halves).
The factors that determine the speed of diffusion are:
- temperature
- the difference in concentration
- the diffusion distance
- the size of the area of diffusion
Increasing the temperature will increase the movements of the particles and this will speed up the diffusion.
The higher the difference in concentration, the quicker the diffusion takes place.
The longer the distance between the two compartments (thicker membrane), the longer the diffusion will take place.
8.
The graph shows what happens when the diffusion is started. After some time, the concentration has become equal on both sides and net diffusion will be stopped (why ‘net’?)
So, the membrane, in principle, is not permeable for these molecules but still, they are carried through the membrane by these special carriers.
But usually, glucose is used (=metabolized) in the cell so the concentration inside is lower, so the transport is usually from outside into the cell.
Similarities:
- no energy required
- concentration dependent
- bi-directional
Differences:
- saturation
- specificity
Saturation means that there is a maximum to the amount that can be transported. This is reached when all the available carrier molecules are occupied.
In normal diffusion, there is no saturation, and the process can go on as long as possible, as long as there is a concentration gradient.
Specificity means that a particular carrier can only transport a particular molecule, such as glucose, and not something else.
Example of facilitated diffusion is transport of glucose, as discussed, in all body cells essentially, but especially in the brain. Also, amino acids can be transported in this way through the cellular membrane.
Another example of facilitated diffusion occurs in the kidney when sodium is reabsorbed. This is also done by facilitated diffusion.
A special type of diffusion, which also does not require energy, is diffusion of water, a process that is called osmosis.
Osmosis takes place across a membrane. This membrane is permeable for water (as usual) but NOT permeable for a solute (a substance that is dissolved in water). We call this a semi-permeable membrane (not permeable for the solute but permeable for water).
In the diagram (A), there is more solute (blue) to the right then to the left of the membrane. But this solute can not pass the membrane whereas water can.
Since there is (a little) more water at the left side of the membrane, then there is a concentration difference for water and the water molecule will cross to the right side (blue arrow).
This will increase the amount of water at the right side! As indicated in B, the water level will raise on the right side (and decrease on the left).
As with any diffusion, this process will in principle continue until the concentration difference across the membrane has become the same. But this is not always possible.
As indicated in the diagram, the increase in water will also increase the column of water (and thereby also increase the pressure) to the right of the membrane.
This pressure increase will cause the water molecules to move to the left (red arrow).
In other words, a concentration gradient to the right is causing an opposite pressure gradient towards the left. As soon as the pressure gradient is equal (but opposite) to the concentration gradient, this system (osmosis) has become stable (indicated by the blue and red arrows).
Like all other cells, the red blood cells (=erythrocytes) have a plasma membrane that is permeable for water but not permeable for many molecules (such as salt, hemoglobin etc.).
If the red blood cell is placed in a solution that contains less non-permeable particles than in the cell, then water will go into the cell; the cell will expand.
The solution that contains less non-permeable solute is called: ‘hypotonic’. If the solution contains the same amount of solute, then the solution is called ‘isotonic’ (iso=equal; tonic=tone).
If the difference is too big, for example if there is no solute at all in the environment, then the cell may blow itself up, break and burst (=lysis).
The opposite can also happen. If the red blood cell is place in a solution that has too much salt (=hypertonic), then water will move out of the cell; the red blood cell will then shrink!
In filtration, there is a pressure difference between one side of the membrane and the other side.
Because of this pressure difference, water will flow from the region of high pressure to that of low pressure, assuming that the barrier is permeable to water.
Filtration occurs in many parts of the body, but especially in the transport of plasma through capillaries, in the kidneys (formation of urine) and in the lungs.
Sub-notes:
Kinetic energy (from the Greek word ‘Kinein’ = to move) is present in all dissolved particles and molecules. This energy will ‘shake’ these particles all the time. The shaking intensity increases with higher temperatures and slows down at lower temperatures. The shaking stops completely at the absolute zero (zero Kelvin = -273 oC = -459 oF).
D.8. Why ‘net’?
In reality, because the molecules are still moving according to their kinetic energy, some molecules will accidentally cross the membrane and move to the other side. But other molecules that are moving accidentally in the opposite direction will offset this. In other words, the molecules are still moving left and right but the net effect is zero.