Transport and Osmosis: In all living organisms’ plants and animals, physiological processes are continually taking place in their bodies. In order to sustain life, these processes must be kept going on for which the materials required, must be constantly transported to and from all parts of the body right down to the individual cells.
Materials are also to be transported between the cell organism and external environment. In unicellular and simple multicultural organisms, the distribution of materials can be adequately brought about by diffusion and streaming movements of the cytoplasm (fig. 12.1).
However, the evolution of more and more complex body structures resuscitated the development of proper transport system, and more complex the organisms are, the more elaborate transport system they have. The complexity of transport system is related to the size and the metabolic rate of the living organism.
The materials to be transported are taken close to tissues be the transport system so that diffusion can occur efficiently into the cells. The primary function of the transport system is to maintain a link between all cells of the body and the external environment.
It transports the nutrients to the points where they are to be used, facilitates the elimination of metabolic wastes of each cell and transports surplus substances to the specialized storage tissues or to outside their bodies.
In the living organisms, each cell is bounded by cell membrane. The cell membranes are able to regulate the movement of materials across them.
The cells constantly exchange materials with their environment through the cell membranes. But the movement of materials, in and out of the cells, across the cell membranes is not an uncontrolled traffic, because certain materials can pass through the membrane, others cannot. Thus the primary function of the cell membranes is to regulate the passage of substances between the interior of the cell and its external environment.
Many different molecules can move across the cell membrane. Of all these, water is the most important one. Many inorganic salts can move across the cell membrane in solution i.e. dissolved in water. One of the processes by which the exchange of materials between the cells and their environment occurs is called diffusion. For instance, water and inorganic salts dissolved in it pass into the plants by diffusion through the root hairs into the cells of the roots.
Diffusion
It is a process in which the random movement of the molecules or ions takes place down a concentration gradient i.e. from region of higher concentration to a region of lower concentration. Water, CO2, O2 and some other simple molecules can diffuse across the cell-membranes in this way. It is also the main process by which other substances move within the cells. During photosynthesis and respiration in the plants, the exchange of carbon dioxide and oxygen gas, between the cell and the atmosphere also takes place by diffusion. Although diffusion is a slow process, yet it is efficient and rapid enough to fulfill all the gaseous requirements of the plant.
“The molecules of some substance because of their size or polarity cannot pass readily, into and out of the cell through the cell membrane. This is brought about by certain proteins called carrier proteins. But here too the movement would be according to the principle of concentration area. This type of diffusion which takes place with the help of carrier proteins, is called facilitated diffusion. Like simple diffusion, in this case also expenditure of energy is not involved.”
Diffusion is a well known phenomenon. It can be clearly demonstrated by a simple example i.e. when a colored crystal like (KMn O4) is dropped into a bowl of water, initially the color or pigment will remain concentrated at its molecules will diffuse evenly in all directions throughout the water, resulting in uniform distribution of color in all regions of the water in the bowl (fig. 12.2). similarly, if we spray perfume (air-freshener) in one corner of a room, initially its fragrance will be strongest in that corner and will gradually spread by diffusion in all areas of the room.
The efficiency of diffusion depends on two factors, (a) a distance (b) a concentration gradient difference. The speed of diffusion also depends on the kind of molecules, e.g., diffusion of gases is faster than of liquids; and similarly the diffusion of solids is the slowest.
Diffusion plays a key role in the transport of substances into and out of cell in all living organisms.
Active Transport – Sometimes the movement of substances across the cell-membrane takes place against the concentration gradient (uphill movement) i.e. from a region of lower concentration to a region of higher concentration. This type of uphill movement of substances requires the expenditure of energy and is called active transport.
Osmosis
As you all know that a living cell is enclosed in the cell membrane, which allows some substances to pass through it but prevents the passage of others. Such membrane, which is selective in the passage of substances, is called a selectively permeable or differentially permeable membrane. The solvent molecules, which in most cases are water molecules, can readily pass through the cell membrane. The water molecules can easily diffuse through the cell membrane. The movement of water (solvent) through a differentially permeable membrane is a special kind of diffusion called osmosis.
Practical work: to demonstrate osmosis
Take a thistle funnel and tie a cellophane or egg membrane (both are differentially permeable) around the wide end or the mouth of the funnel. These two membranes are permeable to water molecules but not to dissolved solute molecules such as source (sugar).
Fill the thistle funnel, upto a marked level with concentrated sugar solution and place the funnel, with its mouth end immersed in water. You will see that the water would move both into and out of the funnel. But because of the grater water concentration outside the funnel, the movement of water into the funnel would be more rapid than its movement to the outside of the funnel into the beaker. Consequently, the level of sugar solution will rise in the tube of the funnel. As the movement of water into the funnel goes on, the sugar solution in the funnel will become more and more dilute.
The movement of water molecules into the funnel was the quickest at the time when it was first immersed into the beaker because at that time there was a concentration gradient greater difference between the water in the beaker and the sugar solution is the funnel. At that time, the movement of water out of the funnel was the slowest. The movement of water into and out of the funnel went on until the equilibrium of water molecules on both sides of the membrane was reached.
Turgor
If a cell is surrounded by water or a solution more dilute than its contents (i.e. having lesser concentration of solutes and higher concentration of water), the water tends to move into a cell vacuole by osmosis. As the water enters the vacuole, it increases in size and pushes the cell contents against the wall. The plant cell does not burst as the cell wall is fairly strong and relatively inelastic. However, the cell wall cannot expand beyond a certain limit. The cell wall, thus prevents over-expansion of the cell by exerting an inwardly directed or an opposing pressure which prevents the entry of more water into the cell. The cell in this state becomes turgid. The condition thus produced in the cell is called turgor and the internal pressure exerted on the cell wall is called turgor pressure (fig. 12.4a).
When all the cells in a leaf and stem are turgid, the plant structures like leaves and young branches will be stiff, firm and upright. If the cells some how lose water, the cell will lose turgor and will become flaccid (soft and limp). The leaves with flaccid cells become limp and the stem droops. Thus plant cells with reduced turgor due to loss of water is said to be wilting (fig. 12.4b).
Importance of turgor in plants
Turgor plays an important role in maintaining the shape of the soft tissues in plants. The movements of certain plant parts also result from the changes in their turgor e.g., changes in the turgor of the guard cells of the stomata cause the opening and closing of the stomata. Some flowers open during the day time and close at night or vice versa. In these the bending movements of their petals is due to changes in turgidity of the cells. The closing of leaves of some plants e.g. touch-me-not, is also due to turgor changes in the leaf-lets.