|Permeable: allowing liquids or gases to pass through it.|
All molecules have energy and are vibrating all the time. Molecules in liquids have more energy than those in solids. Gases have more energy than liquids. Matter is made up of atoms and molecules. Let us consider two containers, each containing a certain gas but contains different levels of gas. The container with more gas has more energy and the molecules
will tend to move to an area with fewer molecules, i.e. less energy. Movement of molecules from a region of high concentration to a region of low concentration is called diffusion. This can be visualised by imagining a container partitioned into two compartments, one containing a colored gas and the other containing air. When the partition is removed, the two gases mix – this is an example of diffusion. When further mixing occurs a state of equilibrium is reached.
|Flaccid: drooping or inelastic through lack of water. Turgid: swollen and distended or congested.|
This term refers to the movement of water from a region of high water concentration to a low water concentration by passing through a selectively permeable membrane. Let us again, consider the partitioned container. If one compartment is filled with pure water the concentration of water is said to be very high, i.e. 100%. However, if in the other compartment there was a Salt solution, the amount of water would be less as the salt would be occupying the space normally occupied by water. The natural tendency on removing the partition is for the salt solution to mix with the pure water until a weak salt solution occupies both chambers. The movement of water is
from the compartment of pure water to the compartment of salt water. If there was a semi- permeable membrane between the two compartments, then the salt would be kept in the one chamber. However, water would move freely and would enter the salt water chamber until no further water could enter. This process is called Osmosis. See Figure 1 on the following page.
In biological systems, osmosis has come to mean the movement of water into and out of cells. The inside of cells is a solution, in fact a very concentrated solution. It seems natural therefore, that pure water will tend to enter the cell, and because the cell membrane is semi-permeable, none of the salts and sugars can pass out of the cell. Water will continue to flow into the cell, until the pressure in the cell is so high that no more water can be absorbed. At this point, the cell ‘membrane is pressed firmly against the cell wall, and the cell is said to be turgid. If the same cell is placed in a salt solution that is more concentrated than the cellular solution, then water will tend to leave the cell until the pressure has dropped so much that the membrane is no longer touching the cell wall, and the protoplasm has formed a small ball in the middle of the cell. The cell is said to be flaccid. See Figure 2.
Figure 1: Demonstration of osmosis in a tank divided by a semi-permeable membrane.
Figure 2: Demonstration of biological osmosis in plant cells.
This is the process by which roots take up water from the soil. The process occurs osmotically, i.e. by osmosis. The water is almost pure and therefore flows into the root hairs whose internal solution is more concentrated. Absorption also refers to the uptake of salts by the root hairs. This cannot occur like osmosis, because the natural tendency is for the salts to pass out of the cell. The salts therefore have to be absorbed actively, i.e. by using extra energy. This phenomenon is called absorption against a concentration gradient. A concentration gradient is like a hill; the top of the hill is the high concentration and the bottom of the hill, the low concentration. Naturally, everything flows downhill, the absorption of salts into a cell is like having to force the salts up hill and to do this energy is needed. Absorption by osmosis is passive absorption and against a gradient is active absorption.
3. FUNCTIONS OF WATER
Water is the reaction medium of the cells. This means that all chemical reactions occurring in the cell occur in solution, so without water they could not occur.
Water is involved in some of the chemical reactions. Water is part of the protoplasm existing in an organised molecular structure. Most plant tissue contains 70% – 90% Water Concentration.
Some plants can withstand severe water loss and this ability is termed drought resistance. However, when such plants are dry they will be alive but completely inactive. Some plants can live in dry conditions because they have evolved methods of holding their moisture; these plants are not, in fact, drought resistant, but drought adapted. Plants adapted to dry conditions often cannot tolerate “drying out”.
Plant cells can only grow when they are turgid and even small decreases in turgidity will result in decreased growth rates. Water is continuously absorbed by the roots, travels up the system, and is lost through the stomata of the leaves. This flow is termed Transpiration and its speed depends upon the speed with which water is lost from the leaves. This is, in turn, affected by temperature and wind. On a hot day, more water is lost than on a cold day. More water is lost on a windy day than on a still day. The flow of water must be maintained otherwise water is lost faster than it is absorbed and the cells of tissues lose their turgidity, thus becoming floppy and flaccid and the plant wilts. Wilting occurs in very dry soil conditions. Normally, a plant will recover after rain. However, in some cases, they do not recover. Permanent wilting is then said to have been reached.
Life on earth is carbon-based and all biological compounds such as proteins and sugars are made of carbon. The ultimate source of carbon is the carbon dioxide gas in the atmosphere. Plants take this gas and convert it into sugars which can then be used to make proteins. To do this, they use energy from the sun. The carbon is also combined with water. The overall reaction of photosynthesis is as follows:
Oxygen is given off.
6 CO² + 12 H²O C6H12O6 + 6H 2O + o²
Carbon Dioxide + Water Glucose + Water + Oxygen
For a plant to live normally, it needs a source of energy. Plants obtain energy by “burning’ fuel. The fuel they burn is glucose. By burning one does not mean setting fire to the sugar, but merely converting it back to CO2 and water, by using up oxygen. When this occurs, vast quantities of energy are released and can be used by the plant.
The overall reaction for respiration is:
C6 H12 O6 + 6 O² CO² + 6 H²O
During the day, plants give off O² because of photosynthesis. At night, they give off CO² owing to respiration.