Download 15.1.1 Water Balance

Maintaining a Balance
Topic 15: Water Balance
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Explain why the concentration of water in cells should be
maintained within a narrow range for optimal function.
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.