Plant and animal cells are made up of a cell membrane. The plant cells have the cell membrane line in the inside of the cell wall. The cell wall provides the turgidity to the cell. The cell wall is dead and allows all substances to pass freely through it whereas the cell membrane is alive and selective. This is the case also for the cell membrane of the animal cells. The only difference is however that the cell membrane is the only layer around the cell unlike the cell wall that is part of the plant cells. The cell membrane is alive and selective as we know. This means that it is semi-permeable.
This is called a semi permeable membrane. Semi permeable means that it is a layer or a surface that allows only certain particles to pass through but not any others. A completely permeable membrane means that it is a membrane that allows any particles to pass through like is the case for the dead cell wall. This cell membrane is a thin layer of cytoplasm and allows the cell to control the products that come through and go out. For example the cell membrane allows only substances like water, glucose and water are allowed to enter and waste products are removed.
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However the other way round is not possible because the membrane does not allow the wastes to enter and the vital particles to exit. The membrane is very important to organisms. The movement of gas molecules is called diffusion. The diffusion of other substances such as liquids across a semi permeable membrane is called osmosis. The process of osmosis will be studied in detail here. There is another type of movement and it is called active transport. To understand both of these it is important to know what the concentration gradient is.
It is also called diffusion gradient because it also applies to gases. Diffusion or concentration gradient shows the molecules or the particles as from less concentrated to more concentrate. This is demonstrated below. A- higher concentration B- molecules moving C- Less concentration From this diagram we can see that there are more molecules on the left and the molecules are moving from section A through the section B and moves to C where there are fewer molecules. This way we can see that the molecules are diffusing through the empty space.
The arrow points towards the diffusion gradient. For the active transport it will be the other way which shows it is against the concentration gradient. This shows the movement of particles for gases. The difference for osmosis is that in osmosis there is a semi permeable membrane. This semi permeable membrane has tiny pores that allow only certain products to pass through. Active transport is different from osmosis because osmosis is according to the concentration gradient and active transport is against the concentration gradient. Factors of osmosis
Osmosis depends on the concentration and the temperature of the surroundings in which it takes place. The concentration of the solution is very important because then it will be according to the number of molecules in the water. If we take a sugar solution we know that there is sugar dissolved in it but also it is important to know what is the concentration of the solution. The higher the concentration, the more the amount of sugar molecules and so the amount of water is less. Therefore the amount of water that moves towards it will be more.
If we take the above diagram and we place the semi permeable membrane in the middle or have a semi permeable membrane full of the solution we will see this change. This is demonstrated below. For the sake of simplicity, the water molecule has been shown as small and monoatomic where its actual formula is H20 and the sugar molecule is also shown bigger so that it is easier to distinguish and its actual formula is C6H12O6. As we can see form the above diagram that the reason that the sugar molecule cannot pass through is that it is too big to fit through the pore.
Even if the gap is large enough the sugar molecule passes though very slowly. The arrows in between show the movement of the particles. As we can see that the reason that the water molecules are moving is that there are less water molecules on the left and so the solution is concentrated and the solution on the right is dilute. The solution on the left is more concentrated because the sugar molecules are now hydrated and so have stopped the water molecules from moving freely and thus there are less water molecules present in the solution.
This is usually called water of crystallisation. So we can define osmosis as the movement of water molecules from a place of higher concentration to a place of lower concentration. As we will observe the level of water on the right will decrease and the level of water on the left increases until both will be the same. So they will be in balance and proportion. In plants the following occurs so that the plants may take in fresh water and if seawater were added the reverse would happen, the cells would lose water. This we will see later.
Osmosis can take place in any solution, whether it is a salt solution or even a sugar solution. The water potential is the measure of whether the solution is likely to gain or lose water and by how much. So it is concluded that pure water has the highest water potential because it has the highest number of free molecules and can diffuse from one solution to the other no matter how concentrated or dilute it may be. This again repeats the fact that the water moves from a high potential to a low potential. Once again it is important that the substance that is added to the water is soluble in water.
Osmosis is also used industrially, mainly in the process of agriculture. Sometimes in industry, quite different from agriculture, osmosis may also be used. In this case the tubing would be made of cellulose acetate in sheets or tubes used for dialysis. The size of the pores can be adjusted according to the size of the molecules or the particles of the substances and used accordingly for different purposes. In an animal cell the water is taken by osmosis. The cell swells up and can burst if the extra water is not removed. The tissue fluid does this.
The tissue fluid has the same concentration as the cell and so the substances or rather water can pass freely between the cell and the tissue fluid. As we can see form the above representation the cell has expanded and will burst if it does not lose the extra water. That is why the tissue fluid plays an important part. The solution outside is called the hypotonic solution because it has a low concentration of the solute compared to the solution in the inside, which is called the hypertonic solution. As for the solutions between the cytoplasm and the tissue fluid have the same concentration so that they can freely exchange.
These solutions are called isotonic solutions. For the plant cells, too the same thing happens. But when the above change takes place, the cells are now turgid and this property is usually used to check the end of the osmosis in experiments and the cells that have lost water by osmosis until they are flaccid, which means that they have wilted or a starting to wilt. This is important for us to know because we will be experimenting about the plant cells when we investigate osmosis. Plant Cells Plant Cells have vacuoles. This makes all the difference to the turgidity.
When the plant cell is surrounded by dilute solution then the water will enter the vacuole and enlarge it. The vacuole will become larger and also push against the sides of the cytoplasm. This will stretch the cell. This will happen till a limit because there is the cell wall that does not allow the cell to expand too much. This means that the cell is turgid and is exerting turgor pressure on the walls of the cell. When the water content in the vacuole decreases either due to lack of water or the loss of water due to transpiration, the cells become flaccid.
The vacuole becomes smaller. The cells are said to be plasmolysed. The vacuoles have pulled the cytoplasm away from the cell walls and thus allowed the cells to become smaller. As we have already discussed the temperature is an important factor in osmosis. As the temperature increases the rate of osmosis also increase. This is because the when the temperature increases the water molecules move faster from one solution to the other. As we proceed to the experiment, we will see that what other factors are to be kept constant and what changes occur during the procedure of the experiment.
Osmosis is important in nature. Osmosis is important in every living creature because it controls the distribution of water and nutrients. Osmosis occurs only when there is a semi permeable membrane so it keeps waste products out of the cell and provides the cell with the substances required in appropriate amounts. If there were no osmosis, red blood cells would get filled with water and burst just like a balloon. This is called hemolysis. Salty water can cause the tissues to dehydrate and osmosis may prevent this. This is because it is a salty solution.
Osmosis maintains the regularity of concentration in the cells of the living organisms. Even so in plants osmosis helps to maintain the balance of water and the turgidity. Without this plants would wilt and die. The same for humans. If there were too much or too little water in cells, we may also die but osmosis prevents this. In plants this process is partially responsible for drawing water up to the stems where it can be used for respiration. Then again if the water in the soil is salty or has too much fertiliser in it, it can be harmful for the plants because the solution is then hypertonic.