Previous Topic :Osmosis
Turgidity And Plasmolysis
A cell is said to be turgid, when the cell contents press against the cell wall as a result of the water absorbs by the cell through osmosis in all directions causing the cell to be very firm and rigid.
Tugor pressure : is the outward pressure acting on the cell wall as a result of the entry of water molecules by osmosis.
The movement of water into and out of cells of an organism is brought about by osmosis, hence, osmosis is very vital to many biological processes.top↑
Dissolved substances are present in the cytoplasm of living animal cells and in both the cytoplasm and cell sap of living plant cells.
These dissolved substances often inside the vacuoles make the osmotic potential of the cell normally higher than that of the surrounding water medium.
I believe you still remember what osmotic pressure is? But if you don’t, probably maybe you are just joining us, “well welcome” here is the link to the session Osmosis
So back to surrounding water medium.
For example, amoeba and spirogyra, the surrounding medium is normally water in fresh water pond, while for red blood corpuscle or erythrocyte, the surrounding medium is the plasma.
When the osmotic potential of a cell is higher than that of the surrounding water medium, water molecules pass into the cell by osmosis. As a plant cell absorbs water by osmosis, its vacuoles and the protoplasm increase in volume.
So when an animal absorbs water by osmosis, it becomes turgid and the cell membranes become firmer.top↑
Flaccidity : a cell is said to be Flaccid, when the osmotic potential of the surrounding water medium is higher than that of the cell, which leads to, water diffusing out of the cell.
Flaccid can also be defined as when the cell loses its turgidity.
So note that, a cell that is flaccid, is not rigid or firm.
For example spirogyra .
In a Living plant cell, when the cells become flaccid, the plant wilts, that is, they collapse.
But in living cells e.g spirogyra as mentioned above, if living cells are put into concentrated salt solution, a series of changes are observed which go beyond merely becoming flaccid. The vacuoles and the cytoplasm collapse or shrink away from the cell wall, this state is called Plasmolysis . If the outflow of water continues, plasmolyis reaches its last stage in which the protoplasm shrivels into a mass inside the cell.top↑
Types Of Plasmolysis
Mild plasmolysis is reversible if the cell is again placed in water, this is observed in animal cells.
Several plasmolysis damages the cell and may be irreversible. Since water is the largest component of the protoplasm of the cell, hence, plasmolysis is bound to have a great effect on all the life activities of the cell.
Demonstration Of Plasmolysis In Plant Cells
Materials needed for the experiment are : Fresh, healthy filaments of spirogyra in its own pond water, microscope, water, common salt solution, glass slide, filter paper and dropping pipette.
1) Mount two filaments of fresh spirogyra on a glass slide in a drop of the pond water from which they were collected.top↑
2) observe the state of the turgidity of the cells under the microscope.
3) Prepare a slightly concentrated common salt solution and replace the pond water in your preparation with a drop of slightly concentrated common salt solution. This is done by sucking away the pond water from one end of the glass slide using filter paper, whilst introducing the common salt solution from the opposite side with a dropping pipette.
4) After the third procedure, observe the cells of the filaments. That is, observe and record its turgidity.
5) Now, replace the common salt solution with the pond water (make sure that the preparation is well flooded with the pond water). Observe the specimen again and record your observations.top↑
If you carry out you demonstration accurately, at the last observation, you will observe a recovery as osmosis take place (in the 5th procedure).
After the plasmolysis observed in the fifth procedure, it will be observed that, with the pond water surrounding the cells, water flows back into the cells as a result that, the solution have higher osmotic potential than the water.
The stem of non-woody plants are kept upright by the turgor pressure exerted by the individual cells.
On dry and hot days, the cells lose water more rapidly than the root due to the high temperature and transport the water to the shoot. Hence, there is a net loss of water, this process is called Partial Plasmolysis of cells which leads to the plant to wilts or droops.
Demonstration Of Turgor Pressure In Plants Tissues
Materials needed for the experiment are : concentrated common salt solution, inflorescence stalk of harmattan lily, troughs, pen knife and distilled water.top↑
1) Cut the lily stalk lengthwise into strips and place one strip in a dry trough (make sure that the strip tends curl outwards.) The strip will be able to curl outwards because of the turgor pressure in its cortical cells that is, there are turgor pressure in the cells of the inner layer of the stalk.
2) Place another strip in a trough of distilled water and observe the behavior of the strip when it get in contact with distilled water.
3) Transfer the strip in the distilled water very quickly but carefully, into a concentrated common salt solution.
Procedure (ii), in distilled water, the strip will curled even further outwards because more water had entered the cortical cells by osmosis and so therefore, there will be an increase in turgor pressure in the cortical cells.top↑
In the 3rd procedure, the cells of the strip placed in the salt solution, will have lost water to the salt solution.
The cells of the strip will have lost its turgidity and so the strip straightened out, this effect would have been seen as a dropping effect in the full plant.
Demonstration Of Osmosis And Plasmolysis In Animal Cells
Materials needed for the experiment are : Methylated spirit, cotton wool, mounted needle or sterile lancet, fine sterile pipette, bunsen burner, slides, concentrated salt solution, cover slips, distilled water and two drops of blood ( from you own thumb).
Note : For your own safety, you must only handle your own blood. Your supervisor will provide you with special containers into which you can place needles and lancets after the experiment.top↑
1) To obtain the blood, first sterilise the skin on the inside surface of your left thumb (if you are right handed) just below the nail, using a piece of cotton wool moistened with methylated spirit. Make sure that you do not touch this area until the blood sample has been taken.
2) Sterilise a mounted needle by passing it several times through the hot part of a bunsen flame. If it is a sterile lancet, there is no need to pass it through bunsen flame.
3) Swing you arm around two or three times in order to increase the blood flow into your finger.
Now bend the top joint of the sterilised needle or lancet firmly and carefully into the skin of your thumb.
Note Don’t inject the needle or the lancet into the skin of your thumb more than ones to avoid injuries. Inject the needle ones and firmly.top↑
4) Take a couple of drops of blood from the needle prick with the sterilised pipette and then wash the thumb in water and sterilise it with alcohol, also put a little cotton wool over the needle prick point in order to stop further bleeding.
5) From the pipette, place a drop of blood on each two glass slides. Mix the blood on the first glass slides with a drop of concentrated salt solution. use the edge of the free glass slide to make a thin smear of the blood solution and place a cover slip over the top.
6) View the preparation under the microscope both under the low and high power magnification, identify and draw the red blood cells.
7) To the second drop of the blood, add a drop of distilled water and make a thin smear blood solution. View under microscope and draw the red blood cell.top↑
In the first preparation, the red blood cells become plasmolysed while in the second preparation, the red blood cells gain water by osmosis, and swell up to the extent that, the red blood cells burst. Such bursting of red blood cells is called haemolysis.
If there is any change in the osmotic potential of plasma, in which blood cells float, the blood may be harmful to the body of the organism.
Hence, in healthy body for example human body, both the fluid and the red blood cells maintain a constant osmotic potential by osmo-regulation.
So therefore, the osmotic potential of the body cells and fluids must be kept constant.top↑
Processes In Which Osmosis Is Involved In Animals
1) Osmosis is involved in animal during maintenance of turgor in animal cells
2) In reabsorption of water from glomerular filtrate in the kidney tubules.
3) In the absorption of water by the colon.
4) In loss of water through sweating.
5) And in the movement of water from one living cell into another.
Processes In Which Osmosis Is Involved In Plants
1) In maintenance of turgor.
2) In absorption of water from the soil by the root hairs.
3) In the movement of water into and out of guard cells of stomata which leads to opening and closing of the stomata.
4) And also in the movement of water from one living cell into another.top↑
Active transport is when solutes pass through a membrane from a region of lower concentration to a region of higher concentration. It can also be defined as when solutes transport against the concentration gradient.
Active transport is not simply a physical process, rather, it seems that there are molecules or groups of molecules across the membrane passing into or out of the cell, using up energy in the process.
Active transport of potassium ions sodium ions and calcium ions is an essential part of the process by which nerve impulses are propagated along the length of a nerve and by which muscles are induced to contract.top↑
For example, cells and tissues contain much less sodium ion than the extra-cellular fluid. That is, there must be a kind of “ sodium pump” or mechanism by which the plasma membrane selectively drives ions from inside the cell to the outside. But if there were no such selective mechanism, the physical process of diffusion world cause sodium ion to enter the cells through the cell membrane until the sodium concentration was the same inside and outside the cells.
There are other selective ion-pumps in some cells, which are specific for hydrogen, potassium and chloride ions.
The transportation of potassium and sodium ions across the red blood cell membrane are found to involve the expenditure of energy.
Cells also have a mechanism for the active transport of amino acids such as its movement through the intestinal epithelium and the kidney tubules.
Note : there are distinct systems of transport for at least four different amino acids.top↑
Another example of active transport is, the selective reabsorption of substances back into the blood stream during the excretory process in the kidney.
During this process, the cells lining the kidney tubules expend energy to move the substances back into the blood from the urine by active transport which is against the concentration gradient because, it has been found that a given amount of kidney tissue consumes more oxygen per hour than an equivalent weight of heart muscle, indicating how hard-working” the kidneys are.
When deprived (poor) of oxygen, re-absorption is the first function to cease in the kidney.
Sodium chloride is the greatest amount of substance that is reabsorbed by the kidney which are actively reabsorbed by the sodium pump in the kidney tubules.
Other substances that are also reabsorbed by selective active transport of the membranes are, Amino acids and glucose.top↑
Examples Of Active Absorption In Plants And Animals
1) In the movement of sodium ions out of the axon during the transmission of nerve impulse.
Axon : is the nerve which conducts nerve impulses away from the body of the cell to a synapse.
2) In reabsorbtion of glucose, amino acids and sodium ions in the kidney tubules.
3) In absorption of high concentration of salts or ions in some plants.
Till next class guys, our next topic is functions and properties of cell
Next Topic :Functions And Properties Of Cell
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