OSMOSIS- OSMOSIS IN PLANTS AND ANIMALS
Studies will not be complete talking about diffusion and leaving out the world of osmosis. In my last post, I talked about the world of Diffusion and today we will be looking at what do we really mean by OSMOSIS
OSMOSIS
Diffusion takes place where particles can spread freely from one place to another. However, the solutions inside a cell are separated from those outside the cell by the cell membrane, which does not let all types of particles through.
It is always the smallest particles that can pass through. Because it only lets some types of particles through, it is said to be partially permeable.
Partially permeable cell membrane will allow water to move across them. It is important to remember that a dilute solution of (for example) sugar contains a high concentration of water (the solvent) and a low concentration of sugar (the dilute).
A concentrated sugar solution contains a relatively low concentration of water and a high concentration of sugar. Osmosis is a special type of diffusion, where only water moves across a partially permeable membrane, from an area of high concentration of water to an area of lower concentration of water.
A cell basically consist of some chemicals in solution in water inside a partially permeable bag of cell membrane. The cell contents contain a fairly concentrated solution of salts and sugars. Water will move from a high concentration of water particles (a dilute solution) to a less concentrated solution of water particles (a concentrated solution) across the membrane of the cell. I'm other words, osmosis takes place.
Take care with when you define osmosis. Make it clear to people when you are explaining osmosis that it is only water that is moving across the membrane and get your concentrations right as it always confused people.
Cell membrane aren't the only partially permeable membranes. There are many natural and artificial membranes that can be used to make a model cell.
By changing the concentration of the solution inside and outside your model cell. The internal concentration of your cells needs to stay the same all the time for the reactions of life to take place. Yet, animal and plant cells are bathed in liquid that can be at very different concentrations to the inside of the cells.
This can make water move into or out of the cells by osmosis. So, osmosis is very important for all living organisms including human beings.
OSMOSIS IN ANIMALS
Now let us talk about osmosis in animals. Osmosis is an important way of moving water in and out of the cell when needed. If a cell used up water in it's chemicals reactions, the cytoplasm becomes more Concentrated and more water will immediately move in by osmosis. Similarly, if the cytoplasm becomes too dilute because water is produced during chemicals reactions, water will leave the cell by osmosis, restoring the balance.
However, osmosis can also cause some very serious problems in animal cells. If the solution outside the cell is more dilute than the cell contents, then water will move into the cell by osmosis, diluting the cytoplasm. The cell will swell and may eventually burst.
If this happens in the blood cells, it is known as haemolysis. Once the blood cells have burst, they can no longer carry oxygen around your body so the function of the blood is lost.
However if the solution outside the cell is more concentrated than the cell contents, then water will move out of the cell by osmosis, the cytoplasm will become too concentrated, and the cell will shrivel up. This is called crenulation I'm red blood cells.
OSMOSIS IN PLANTS
Plants rely on well- regulated osmosis to support their stems and leaves. Water moves into plant cells by osmosis, making the cytoplasm well and press against the plant cell walls. The pressure builds up until no more water can physically enter the cell, which is hard and rigid. This swollen state(known as turgor) keeps the leaves and stems of the plant rigid and firm.
So, for plants it is important that the fluid surrounding the cells always has a higher concentration of water (a more dilute solution of chemicals) than the cytoplasm of the cells to keep osmosis working in the right direction.
To understand the difference between animal and plant cells when it comes to water moving in by osmosis, imagine blowing up a balloon. As more and more water moves in, the balloons gets bigger and bigger and eventually bursts. This models an animal cell placed in pure water or in a very dilute solution of salts.
Now imagine a balloon sealed into a cardboard shoe box. As the balloon inflates, it fills the box and then presses out against the box walls.
Eventually, you simply cannot force any more air into the balloon. The box feels very rigid and the balloon does not burst. This models a plant cell placed in pure water or in a very dilute solution of salts.
If the surrounding fluid becomes more concentrated than the contents of the plant cells, then water will leave the cells by osmosis. The vacuole shrinks and the cell becomes much less rigid. It is flaccid.
If water continues to leave the cell by osmosis, eventually the cytoplasm pulls away from the cell walls and the cell goes into a state known as pladmolysis.
As you have seen, in normal conditions water moves into plant cells by osmosis and keeps them rigid. This in turn helps to keep the plant upright.
But, if conditions are very dry, the plant cannot take enough water up through the roots from the soil.
The cells are no longer rigid and the plant wilts. Many of the chemicals reactions slow down, and so the plant survives until more water is available. But, only for so long. If the osmotic situation isn't put right fairly quickly, most plants will die. So if your house plants wilts, take note- and give them some water! OSMOSIS is also very important for moving water around within the plant itself.
Water moves into the plant root cells by diffusion across the cell membrane like I have discussed in my last post. The roots are covered in special root hair cells that have tiny hair-like extensions that increase the surface area for osmosis to take place.
The cytoplasm of these root hair cells is now more dilute than the cytoplasm of the surrounding cells. Water moves into those cells by osmosis. These cells now have more dilute cytoplasm than the cells next to them, and the water moves on by osmosis until it reaches the xylem and the transpiration stream.
OSMOMETERS USING LIVING MATERIALS
An instrument that measures osmosis is known as Osmometers. You can use living material as Osmometers in several different ways. You can mount onion cells in a microscope slide in water. Red onion or any other lead with a coloured epidermis I'd good because the cells have coloured cytoplasm and you can see the effects of osmosis more easily. If the cells are examined under the microscope, the cytoplasm is pushing against the cell walls and the cells are full and turgid because water is entering the cell by osmosis, until the point when the pressure of the plant cell walls is such that there can be no more net movement of water into the cell.
If the sucrose (sugar) solution is then drawn across the slide, you will see that the cytoplasm shrinks away from the cell walls as the cells become plasmolysed. This is because water is leaving the cytoplasm and vacuole by osmosis.
You can also use slices, cylinders or chips of plant material as Osmometers. If you cut equal sized prices of yam, weer potato or potato and leave them in containers with different concentrations of sugar for at least 30 minutes, the size and texture of the pieces of plant material will change. Of the solution is water or lower in concentration than the cell sap, water will move into the plant cells and the tissues will swell and become very firm. You can measure an increase in length of diameter of the prices and also of the mass. If the concentration of the solution is the same as the cells, there will be no change in the plant peices.
If the concentration of the external liquid is higher than the cell sap, then water will leave the cells by osmosis and the plant material will become smaller and it will also become soft and floppy.
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