1. Maintaining a Balance Topic 15: Water Balance
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis

2. DOT POINT
Explain why the concentration of water in cells should be maintained within a narrow range for optimal function.

3. Introduction
The maintenance of relatively constant concentrations of gases, water and waste products in the internal environment of living organisms (cells and body fluids) is an important aspect of homeostasis. The concentration of these substances directly affects metabolism in cells.
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4. Introduction
Excretory wastes are produced by cellular functioning in the body and include carbon dioxide and nitrogenous wastes such as ammonia, urea and uric acid. These products alter the normal internal environment of cells of the body and body fluids and unless they are removed, they can poison the organism, interfere with metabolism and ultimately lead to death.
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5. Introduction
Excretion is the process by which waste products, which have been produced as a result of metabolism, are removed from the body. The excretory system is made up of those systems and organs that carry out the removal of metabolic wastes from the body.
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6. Introduction
Carbon dioxide is excreted via the lungs and nitrogenous wastes are removed along with excess salts and water, via the kidneys. In mammals, sweat glands in the skin may also excrete some excess salts, as well as nitrogenous wastes in a dilute form.
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7. Introduction
In aquatic animals such as tadpoles and fish, gills act as excretory organs to eliminate excess carbon dioxide, while in plants the elimination of carbon dioxide at night occurs through the stomata of leaves.
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8. Introduction
Excretion in animals should not be confused with elimination, which is the removal of unabsorbed food from the body, since this undigested food was never a part of the metabolic functioning of the body.
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9. Introduction
Homeostasis, the maintenance of a steady internal state, is essential for cell functioning. Cell metabolism is controlled by enzymes which catalyse each chemical reaction in living organisms.
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10. Introduction
In addition to being extremely sensitive to temperature and pH, enzymes are substrate specific, and do not function effectively in an environment where there is an accumulation of waste products or the optimal concentrations of water and dissolved substances are not maintained.
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11. Introduction
In the next few weeks, we are going to discuss the role of the excretory system in regulating the levels of water and salts (ions) in the body, as well its role in eliminating wastes which are toxic to cells.
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12. Water Balance
Water makes up at least two-thirds of the body composition of most living organisms, and the internal concentration of water and dissolved substances in cells is similar to that of sea water. (This is not surprising if we consider that all early life forms are thought to have evolved from the sea)
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13. Water Balance
Water is the solvent that forms the basic aquatic medium of cytoplasm in cells and also of body fluids such as blood, interstitial fluid and digestive juices in animals. It is also the transport medium in plants, acting as a medium for the translocation of ions in xylem and sugars in phloem tissue.
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14. Water Balance
Solutes that dissolve in water in living organisms include inorganic dissolved ions (such as sodium, potassium, chloride and hydrogen ions) and organic solutes such as glucose and amino acids (end products of digestion), as well as urea and ammonia (nitrogenous wastes that accumulate as a result of protein breakdown).
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15. Water Balance
Changes in water concentration lead to corresponding changes in solute concentration in cells. The relative concentration of solutes to the water in cells determines the osmotic pressure of cells.
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16. Water Balance
Water enters and leaves cells by the process of osmosis and the net direction of water movement is dependent on the osmotic gradient – water moves from a high to a low water concentration through selectively permeable cell membranes. The movement of water into and out of cells therefore depends directly on the concentration of solutions both inside and outside the cells.
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17. Water Balance
Water provides the necessary medium in which all chemical reactions of metabolism can occur. Chemical reactions in cells can proceed only if the reactants are dissolved in water. Water and solute concentration in cells and fluids in living organs must be maintained at a relatively constant level, within a narrow range so that these cellular reactions can take place.
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18. Water Balance
Water itself may participate as a reactant in some metabolic processes (such as photosynthesis) and may be a product (for example, in cellular respiration).
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19. Water Balance
If the balance of water and solutes in cells is not maintained at an optimal concentration, too much water may move into cells, causing them to burst (if they are animal cells) or too much water may move out, causing the cell contents to shrink and the cytoplasm to become too concentrated for normal cell functioning.
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20. Water Balance
The osmotic pressure of living tissue can also affect the pH in cells. For example, too little water leads to an increase in the concentration of solutes such as carbon dioxide and this in turn lowers pH. Both pH and osmotic pressure must be maintained within a narrow range so that enzymes can function effectively.
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21. Water Balance
Correct fluid concentrations maintain the osmotic pressure of cells and this is responsible for structural support in many living organisms. For example, in plants osmotic pressure maintains cell shape, the outward osmotic pressure of the vacuole is counteracted by the inward pressure of the cell wall, making plant cells turgid.
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22. Water Balance
If water accumulates in high quantities as an end product of metabolism in cells, it may dilute reactants and slow down metabolism, therefore excess water must be removed from living organisms.
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23. Activity
-Students to use ALARM scaffold to answer the DOT Point: Explain why the concentration of water in cells should be maintained within a narrow range for optimal function.