The effects of varying the concentration of liquid surrounding animal cells
In animal cells, the outer layer is the cell membrane , which is said to be partially permeable, separating the
inner cytoplasm from the fluid outside the cell (extracellular fluid).
Depending on the concentration of the liquid it is placed in, an animal cell may change size and shape. For example, in a concentrated solution, such as seawater, individual animal cells will shrink and perhaps become crinkled as water is "drawn" out of them, but in a dilute solution, or even in distilled water, they will swell, perhaps to the point of bursting .
In reality, most animal cells are bathed by a solution which is equal in concentration to their cytoplasm, so these harmful changes do not occur. Such a liquid is sometimes described as isotonic to the cytoplasm - i.e they are at the same solute concentration. This liquid is usually derived from the blood, and in the more advanced animals there are special osmoregulatory mechanisms to stop it from varying. This is one part of a general process called homeostasis. This is is a major part of the work of the kidneys, for instance.
In the kidneys, solutes such as salt, and water (originating in the diet), which are in excess to the requirements of the body are excreted in the urine. In this way, the concentration of the urine is automatically
varied so that the concentration of the blood (and other body fluids) stays constant.
A concentrated solution - a liquid containing a high solute concentration (a "strong" solution) has a lower concentration of water than a more dilute solution (a "weak" solution) - or pure water itself. An explanation for this may be that, in the formation of a solution, the dissolved solute dilutes the solvent water.
In osmosis, it is only water that moves, in the following direction:
- from a region of high water concentration to a region of lower water concentration,
- from a more dilute solution to a more concentrated one
- from a "weaker" solution to a "stronger" one
- from a hypotonic to a hypertonic solution.
OSMOSIS IN OPERATION in PLANT CELLS
The effects of varying the concentration of liquid surrounding plant cells
In plant cells, a different situation applies. They also have a CELL MEMBRANE which is PARTIALLY PERMEABLE, but on the outside of the cell is the cell wall which is fully permeable, i.e. it allows anything
(dissolved) to pass through. This prevents bursting, because it is made of cellulose and strong, so it is able to withstand the osmotic pressure.
Osmotic pressure is largely responsible for the way stems of herbaceous plants normally remain upright but wilt if water is not available, and for the opening and closing of guard cells surrounding stomata in leaves.
In each of these cases, the osmotic pressure acts against the rigid cellulose cell wall (on the outside of the cell membrane), which exerts a pressure resulting in cell turgidity .
Plant cells are normally bathed in a very weak/dilute solution, consisting mainly of water drawn up from the soil by the roots, so this pressure is used by plants to keep them firm and upright. They do not therefore need to regulate the strength of this liquid.
Plant cells are also closely crowded together, so that if some cells lose water and as a consequence their cytoplasm becomes more concentrated, they may attract water from neighbouring cells until they all have an
equal water content. This equalling-out process is how water passes from cell to cell across a root, or inside a leaf, for instance.
If a plant does not get enough water, then its cells may become PLASMOLYSED due to water leaving the vacuole. When the cell contents peel away from the cell wall, the cell membrane becomes visible (under the microscope), and the plant wilts.
The principle of osmosis is used in the preservation of food, kept in strong solutions of salt (brine), or sugar (syrup), Any bacteria which gain access to the food become plasmolysed as above, and are effectively killed by dehydration!
These examples should remind us that osmosis is only a type of diffusion, and also that it is not even a
special property of living organisms. It is, however, important in biological systems because every cell of all living organisms contains a certain amount of water, and is bounded by a partially permeable membrane, through which exchange of water (and other substances) may occur with its surroundings.
Osmosis and dialysis (next section) are purely physical processes. No energy is put in to power them, and energy cannot be continually obtained from them. All sorts of physical principles apply here (thermodynamics, conservation of energy, etc.).
However, in some organs, e.g. the kidney, the body has to move solutes such as sodium ions against their concentration gradients, effectively reversing the flow of water. Similarly, roots of plants take in mineral ions e.g. nitrates, from the soil, and concentrate them within the plant.
This selective action - called active transport - requires metabolic energy in the form of the active chemical ATP which is produced as a result of respiration. Active transport is thus effectively powered by aerobic respiration, using up oxygen and glucose as fuel.
Root hair cells absorb water by osmosis, but mineral ions are absorbed by a combination of simple diffusion and active transport.
Dialysis involves the movement of some, but not all, of the dissolved substances in a solution. The substance that moves has small molecules, so these can pass through the pores in the membrane, but other substances, with larger molecules, cannot escape.
This process occurs normally in the kidney. Substances with small molecules, such as salts, glucose and urea, continuously pass out of the blood through a membrane under pressure, but useful substances are later reabsorbed. Waste substances are then excreted as urine.
The same principle is applied in the artificial kidney, but there is no reabsorption, so steps must be taken to prevent loss of useful substances. See later notes.
Experiment to demonstrate dialysis
Visking tubing may also be used to perform dialysis.
A section of visking tubing has been knotted at one end to form a "sausage", and to this has been added a mixture of glucose solution (small molecules) and starch sol (larger molecules).
The "sausage" containing the mixture was placed into a wide test-tube containing water, ensuring the mouth of the sausage was folded over the mouth of the tube, and secured with a rubber band, so as to prevent leakage of contents.
The following procedures can be carried out by each bench, acting as a team to pool results and check technique.
Firstly, familiarise yourself with the test procedures to be used, and double-check the identities of the
See A, B, C, D, alongside.
Finally, different samples of the (surrounding) liquid in the tube can be tested for the presence of glucose
(Benedict's test for reducing sugars) and starch (iodine test).
See E, F alongside
RESULTS and CONCLUSIONS:
Which substance has been able to pass out of the "sausage"?
Which substance did not pass out of the "sausage"?