WHAT IS HOMEOSTASISDefinition: Homeostasis is the ability to maintain a constant internal environment in response to environmental changes. It is a unifying principle of biology.

The nervous and endocrine systems control homeostasis in the body through feedback mechanisms involving various organs and organ systems. Examples of homeostatic processes in the body include temperature control, pH balance, water and electrolyte balance, blood pressure, and respiration.

WHAT IS OSMOREGULATION

Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is, it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.

Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion (getting rid of metabolic wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood) through organs such as the skin and the kidneys.

WHAT ARE OSMOREGULATORSAn osmoregulator is an organism that can regulate or keep the solutes or salts of its body fluid at a higher or lower concentration than the concentration of solutes in the external medium, although this regulation may be limited at extremely high or extremely low external solute concentrations. If the solutes in the organism's body fluids are kept at a concentration higher than that of the external medium (e.g., lake water), this organism's body fluids are said to be hyperosmotic to the external medium and the organism is a hyperosmotic osmoregulator. If the solutes of the organism's body fluids are kept at a concentration lower than that of the external medium (seawater for example), this organism's body fluids are said to be hypoosmotic to the external medium and the organism is a hypoosmotic osmoregulator. If the solutes of the organism's body fluids are the same concentration as that of the external medium, the organism is isoosmotic and is at its isoosmotic point. An organism could be hyperosmotic in an external medium of low solute concentration and be isoosmotic in an external medium of higher solute concentration. Conversely, an organism could be hypoosmotic in an external medium of high solute concentration and be isoosmotic in an external medium of low solute concentration. Both of these situations could happen, for example, in a tidal marsh or other estuarine habitat because in these environments the solute concentrations of the water change with the tides, salinity being lower at low tide and higher at high tide as ocean water floods the estuary. In either case the organism is still an osmoregulator as long as the solute concentration of its body fluids are different than the solute concentration of the external medium. An organism could also be hyperosmotic in both low and high solute concentrations in the external medium. This is the case with sharks. They are always hyperosmotic because of the high concentration of urea in their bodies.

MENTION THE OSMOREGULATORS

IN ANIMALS

Kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), aldosterone, and angiotensin II. For example, a decrease in water potential of blood is detected by osmoreceptors in hypothalamus, which stimulates ADH release from pituitary gland to increase the permeability of the wall of the collecting ducts in the kidneys. Therefore a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.

A major way animals have evolved to osmoregulate is by controlling the amount of water lost through the excretory system.

IN PLANTS

While there are no specific osmoregulatory organs in higher plants, the stomata are important in regulating water loss through evapotranspiration, and on the cellular level the vacuole is crucial in regulating the concentration of solutes in the cytoplasm. Strong winds, low humidity and high temperatures all increase evapotranspiration from leaves. Abscisic acid is an important hormone in helping plants to conserve water — it causes stomata to close and stimulates root growth so that more water can be absorbed.

Plants share with animals the problems of obtaining water but, unlike in animals, the loss of water in plants is crucial to create a driving force to move nutrients from the soil to tissues. Certain plants have evolved methods of water conservation.

Xerophytes are plants that can survive in dry habitats, such as deserts, and are able to withstand prolonged periods of water shortage. Succulent plants such as the cacti store water in the vacuoles of large parenchyma tissues. Other plants have leaf modifications to reduce water loss, such as needle-shaped leaves, sunken stomata, and thick, waxy cuticles as in the pine. The sand-dune marram grass has rolled leaves with stomata on the inner surface.

Hydrophytes are plants in water habitats. They mostly grow in water or in wet or damp places. In these plants the water absorption occur through the whole surface of the plant, e.g. the water lily.

Halophytes are plants living in marshy areas (close to sea). They have to absorb water from such a soil which has higher salt concentration and therefore lower water potential(higher osmotic pressure). Halophytes cope with this situation by activating salts in their roots. As a consequence, the cells of the roots develop lower water potential which brings in water by osmosis. The excess salt can be stored in cells or excreted out from salt glands on leaves. The salt thus secreted by some species help them to trap water vapours from the air, which is absorbed in liquid by leaf cells. Therefore this is another way of obtaining additional water from air, e.g. glasswort and cord-grass.

Mesophytes are plants living in lands of temperate zone, which grow in well-watered soil. They can easily compensate the water lost by transpiration through absorbing water from the soil. To prevent excessive transpiration they have developed a waterproof external covering called cuticle.