Water & Organisms

Plants

Osmosis and plant cells

Although the osmotic principles apply equally to plant and animal cells, a different set of terms is currently applied to the osmotic relationship of plant cells. Water potential is a measure of the tendency of water to leave a solution. Pure water is designated a water potential of zero. As the solute molecules in a solution tend to prevent the water molecules leaving it, the solution will have a lower water potential than pure water. Its value will therefore be less than zero, i.e. negative. The more concentrated the solution, the more negative is its water potential.

For practical purposes a plant cell can be considered as a solution of salts and sugars in the vacuole surrounded by a partially permeable membrane (tonoplast, cytoplasm and plasma membrane) and a slightly elastic but completely permeable cell wall. A plant cell therefore has a more negative water potential than pure water and will draw in water when surrounded by it. This entry of water forces the living part of the cell, known as the protoplast, against the cell wall. In effect, the water in the vacuole is being subjected to a pressure from the cell wall. This pressure is referred to as the pressure potential. In a turgid plant cell this is a positive value, although in the xylem of a transpiring plant it is negative. The water potential of a cell is changed by the presence of solute molecules. This change is referred to as the solute potential. As solute molecules invariably lower the water potential, its value is always negative. The relationship between these three terms is given as:

water potential = solute potential + pressure potential

Water for the support of plants

Water is almost impossible to compress (press into a smaller volume). This feature of water allows it to be used as a form of support by many smaller plants. Although such plants do not have a skeleton, they do have cells with strengthened walls the cell is filled with a gel-like substance called cytoplasm. The cytoplasm contains a lot of water. Plant cells also contain a large vacuole (a fluid-filled structure bound by a membrane). The vacuole holds a lot of water. If the plant cell takes up more water, the cytoplasm and the vacuole fill with water and become swollen. This causes the contents of the cell to push against the cell wall. These cells are described as being turgid, and turgid cells make the plant stiffer and better able to stand up to forces such as wind.

The importance of water is clearly demonstrated by a plant that has not had enough water because it soon starts to wilt. As the cell lose water, the plant loses its support and the leaves begin to collapse. If the plant is quickly given some water, the cell can recover. However, if a plant is allowed to remain without water for too long, the cell will be permanently damaged and the plant will die.

Water is a rare resource in many parts of the world. Deserts are places where there is very little water for almost all of the year. When it eventually does rain, it usually comes all at once in heavy downpours, often causing floods. In many other climatic areas of the world there are rainy seasons and dry seasons. In these places, water has to be used carefully and stored for use in the seasons.

Water for the transport of plants

Plants also rely on water for their internal transport system. They have two main tissues that are responsible for the movement of substances around the plant. One is a tissue called em. This is responsible for moving water and dissolved minerals from the roots to the parts of the plant above ground. The water flows up the tiny tubes called xylem vessels, which are rather like a bundle of drinking straws. When you drink using a straw, water is drawn up the straw because you suck at the top. Plants draw up water from the ground in a similar manner. As water evaporates from the surfaces of the leaves, more water is drawn upward from the root and up the stem to replace it. This creates a continual flow of water from the roots to the leaves. The evaporation of water from the leaves is called transpiration.

Leaves have tiny pores, called stomata. Most of these are found on the under surface of the leaf. The stomata are designed to allow gases to enter and leave the plant for photosynthesis (the process of making food from carbon dioxide and water using light energy) and respiration (the process in which glucose and other food materials are broken down to release energy). When the stomata are open, water vapor can also escape. Water loss is less at night because the stomata are closed. Many leaves have a shiny, wax-like, waterproof covering on the upper surface of their leaves, known as a cuticle. This covering helps to reduce water loss because plants with a waxy cuticle can only lose water through the stomata on the lower surfaces of each leaf.

Desert adaptions of plants

Desert plants have to be able to survive for months, and sometimes even years, without a supply of water. Plants that show special adaptations to conserve or store water are called xerophytes. Cacti and succulents are the most common examples. They have small leaves, so less water evaporates from their surfaces. In extreme cases the leaf has been reduced to a spine. The leaves are usually covered with a thick, waxy cuticle to reduce water loss. Many species of cacti are covered in a layer of white hairs. The white color reflects heat away from the plant, while the hairs trap a thin layer of air around the plant, making it more difficult for water to evaporate. Some plants, such as the giant fig, have evolved incredibly deep roots that are able to reach down to the water table many feet below ground. Others have a shallow but wide root system. This is designed so that, when it rains, the roots can absorb water from as large an area as possible. Many cacti are shaped like a barrel so that liquid can be stored within the stems. Often this liquid is quite acid to the taste, but one or two cacti contain sweet, fresh water that is safe to drink.

Cacti and succulents are not the only plants that live in deserts. Annual plants (plants that live for just one year) live there too, sometimes surviving only for a few weeks, just long enough to produce seeds before dying. The landscape in a desert is transformed after rain, for the annual plants have to complete their life cycles very quickly. As soon as the rain falls, the seeds in the ground germinate and within a few weeks they flower and produce seeds. These seeds may have to stay dormant in the ground for several years until it rains again when they, in turn, can germinate.

The Namib Desert in southwest Africa is different from other deserts. Although it rarely rains, the desert is close to the coast and is sometimes covered by fog. On several nights each year, the fog moves over the desert and, as it does so, tiny droplets of water condense in the cool air and fall to the ground. Many animals and plants have developed behavior patterns to make use of this water supply. Darkling beetles, which have very long legs, clamber to the top of sand dunes and align themselves to face the coast. The beetle will raise its abdomen and, as the fog moves past, droplets of water condense on it and roll down into its open mouth. The Namib Desert is also home to an unusual plant called welwitschia. It does not resemble a conventional desert plant because rather than having small leaves, welwitschia has huge leaves, each being several yards long. Running beneath the upper surface of the leaf are absorbent fibers. These fibers are specially adapted to absorb any moisture that condenses on the surface.

Animals

Osmosis and animal cells

If a solution is separated from pure water by a partially permeable membrance,the pressure which must be applied to prevent osmosisis called the osmotic pressure. As this situation is a hypothetical one, and as a solution does not actually exert any pressure in normal circumstances, the term osmotic potential is preferred. As the osmotic potential is the potential of a solution to full in water, its value is always negative. The more concentrated a solution, the more negative is its osmotic potential.

When two solution have the same osmotic potential they are said to be isotonic. Where one solution has a greater osmotic potential (i.e. the less concentrated one) than another it is said to be hypertonic to it. The one with the lower osmotic potential (i.e. the less concentrated one)is said to be hypotonic.

Water for the support of animals

Animals and plants that live in water rely on it for support. Animals such as jellyfish do not have a skeleton but rely on water to support their body organs. There are also large plants living in water. Large land-living plant such as trees has to produce wood to support their trunks. Some of the largest seaweeds are many hundreds of yards in length and they get all the support they need from the water. When they are washed up on to the shore they collapse as they have lost their means of support.

Many animals rely on water for internal support. Worm, such as the marine fireman, have a central cavity that contains water. The worm's muscles push on the fluid cannot compress, so it is forced toward one end of the worm. By controlling the pressure on the fluid and there it is applied, the muscles can cause the worm's body to change shape. By using one set of muscles, the front end of its body extents forwards, while another set cause the back part of its body to contract and move toward the front end. By repeating this process, the worm can move its whole body forward.

Water for the transport of animals

Water makes up almost half of the volume of human blood. A person has about five liters (10.5 pints) of blood, which is made up of two main parts, plasma and cells. The plasma is a sticky, straw-colored liquid made up mostly of water. There are many substances dissolved in the water. Food materials, such as glucose, and amino acids are absorbed from the intestines and carried by the blood around the body to the individual cells. The blood also picks up waste material, such as urea. This diffuses from the cells and dissolves in the water of the plasma. The urea is carried by the blood to the kidneys where it is removed. Another waste product, carbon dioxide, is carried by the blood from the cells to the lungs. Special chemical messengers, called hormones, are also carried in the blood. Hormones have very specific roles in the body. For example, the hormone adrenaline prepares the body for either "fight or flight." If you are frightened, it is released from a tiny gland near the kidney and carried to the organs of the body by the blood. It causes your heart to beat more quickly and makes you breathe more rapidly.

There are three types of cell suspended within the plasma: red blood cells, white blood cells and platelets. The red blood cells are responsible for transporting oxygen. They pick up oxygen in the lungs and carry it to every cell in the body. The white blood cells play an important role in the body's immune system. They destroy bacteria and other disease-causing organisms that gain entry to our bodies. The third type of cell, the platelets, are tiny fragments of cells. Their job is to help the blood to clot when a blood vessel is damaged.

Animals living in salt water

Animals that live in salt water have that contain a weaker salt solution than the surrounding sea. Due to the natural process of osmosis, water would want to leave their cells .If too much water left their bodies; they would dehydrate and eventually die. They also face another problem connected with the salt water .If they drink the sea water, they would consume a lot of salts that their bodies do not need and that could be damaging to them.

Many marine organisms survive in the sea by altering the salt concentration of the fluid in their cells so that it is exactly the same concentration as the surrounding water. This means that there will be no gain or loss of water. Some marine creatures remove unwanted salts from their body by pumping them out through a special adapted gland.

Desert adaptions of animals

Animals also need water to survive, so those that live in deserts have to be specially adapted to conserve water. The camel saves water by not sweating and by allowing its body temperature to rise. The kangaroo rat never needs to drink water since it gains all the water it needs from its food. Water is released as food substances are respired, or broken down, within each cell. This water, called metabolic water, is just sufficient to enable the kangaroo rat to survive. Desert predators, such as the fennec fox and the jackal, obtain the water they require from the bodies of the animals they kill and eat.

Microrganisms

Animals that live in fresh water are faced with a problem. They are surrounded by a lot of water. The call of their body are full of salts, so there is less water inside their cell membranes, water will always try to move into their body from the outside. These animals have to continually get rid of the excess water. The amoeba is one of the simplest living organisms. Unicellular (only have one cell). Since it live in fresh water, water is always passing across its cell membrane into the cell. The amoeba has to pump out this excess water, almost as though it was baling out a leaky boat! It has a special structure called a contractile vacuole that repeatedly fills with water, moves to the surface of the cell and pumps water out. Many larger animals have developed impermeable surfaces that reduce the amount of water entering their body. For example, fish are covered with scales that are impermeable to water. Even the human skin is relatively waterproof. This means that, if you sit in the both for a long time, thankfully your body will not swell up with water!

Advanced knowledge

State of water in the plant

The water content of a leaf or other plant organ is measured as the relative water content (R) which is the water content stated as a percentage of the maximum water content that the tissue is capable of holding.

R = 100 (M_f - M_d) / (M_t - M_d)

When M_f is the mass of the plant material fresh from the plant, M_t is the mass when the material is fully hydrated by being placed in water in the dark until no further water can be absorbed(such a leaf is said to be fully turgid) and Md is the mass after drying by removing all water in an oven at a reference temperature, often 80¢X C.This index of tissue hydration generally more useful than the water content stated as a percentage of the dry mass, as the latter is more sensitive to the varying amount of structural tissue, and the transient nature of storage materials such as starch. R was originally devised for reporting the water content of leaves, but can also be used to report the water content of woody tissues.

The state of water in the plant is measured as the water potential(£Z),which is the difference in free energy (Jmol^-1 or Jm^-3) between the water under consideration and that of pure water at sea level. It is the work that would be required to move water from where it is, to the pure state at the sea level. The water potential tells us about the tendency of the water to move one direction or another. Water always moves from high potential to low potential. For historical reasons, the units used are those of pressure, pascals (Pa), which are dimensionally the same as Jm^-3.Water potential of pure water at sea level is arbitrarily set to zero, and the water potential in plant leaves are nearly always negative, often by as much as -1 or -2MPa.In leaves, the water potential tends to be reduced by the presence of solutes, and increased by the force of the cell walls tending to squeeze the water from the cells. The cellulose walls are not rigid but elastic, and they exert their greatest pressure on the protoplast when the tissue is fully hydrated, and a declining pressure as water is lost from the system. Total water potential £Zt is the sum of the solute potential £Zs and the pressure potential £Zp brought about by the wall pressure : £Z_t=£Z_s+£Z_p

The relationship between the water potential and the water content is very important. As the leaf loses water, the cells reduce in volume and the solutes become more concentrated (£Z_s declines).At the same time, the pressure exerted by the walls declines( £Z_p declines).

The relationship between the water potential and the water content differs markedly between species, and may influence the ecological range of the species. For example, a tomato plant (an example of a mesophyte, a plant unable to grow in dry places) may show a small decline in water potential for a particular decline in relative water content, but an acacia (a xerophyte, normally growing in dry places) shows a relatively large decline whilst still maintaining a positive turgor. Thus, the xerophyte is more able to extract water from the soil, by virtue of its highly negative water potential, and thus is well suited for survival in dry soils.

Water potentials are routinely measured using a pressure chamber. Leaves are cut from the plant with a sharp blade and placed inside a pressurized vessel with their cut petioles protruding. On cutting, the water meniscus retreats into the cut end of the xylem and the cut surface appears very dry when viewed with a hand-lens. Pressure is applied by adding nitrogen gas to the chamber, squeezing the leaf until water begins to exude from the cut surface of the petiole. This pressure (the balancing pressure)is equal and opposite to the water potential. An alternative technique using a thermocouple psychrometer gives very similar readings, and both support the classical view (the cohesion theory of water transport) that water in the stem is under considerable tension when the plant is actively transpiring.

The range of water potentials usually found in plant varies on a diurnal cycle. Immediately before dawn, vascular plants are in a relatively hydrated state, and typically £Z_t falls to a minimal value soon Afternoon. The relationship between transpiration rate and water potential is usually almost linear. Minimum water potentials recorded in vascular plants vary from-1.0MPa in wetland herbs to-6.0MPa in some desert shrubs.

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