Maintaining a Balance: Part 3 - Regulation of Substances
9.2.3 Plants and animals regulate the concentration of gases, water and waste
products of metabolism in cells and interstitial fluid.
 Explain why the concentration of water in cells should be maintained within a narrow range
for optimal function.
 Explain why the removal of wastes is essential for continued metabolic activity.
 Identify the role of the kidney in the excretory system of fish and mammals.
 Distinguish between the processes of active and passive transport and relate these to
processes occurring in the mammalian kidney.
 Explain how the processes of filtration and reabsorption in mammalian nephron regulate
body fluid composition.
 Outline the role of the hormones aldosterone and ADH (anti-diuretic hormone), in the
regulation of water and salt levels in the blood.
 Define enantiostasis as the maintenance of metabolic and physiological functions in response
to variations in the environment and discuss its importance to estuarine organisms in
maintaining appropriate salt concentrations.
 Describe adaptations of a range of terrestrial Australian plants that assist in minimizing
water loss.
Students:
o
Use available evidence to explain the relationship between the conservation of
water and the production of concentrated nitrogenous wastes in a range of
Australian insects and terrestrial mammals.
o
Analyse information from secondary sources to compare and explain the differences
in urine concentration of terrestrial mammals, marine fish and freshwater fish.
o
Perform a first –hand investigation of the structure of a mammalian kidney by
dissection, use of a model or visual resource and identify the regions involved in the
excretion of waste products.
o
Gather, process and analyse information from secondary sources to compare the
process of renal dialysis with the function of the kidney.
o
Present information to outline the general use of hormone replacement therapy in
people who cannot secrete aldosterone.
o
Process and analyse information from secondary sources and use available evidence
to discuss processes used by different plants for salt regulation in saline
environments.
o
Perform a first hand investigation to gather information about structures in plants
that assist in the conservation of water.
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Background:
The role of the Respiratory System:

Animals have respiratory systems to exchange gases with the external
environment to enable cellular respiration to occur.

In mammals the respiratory surfaces are the lungs. In fish they are the gills and in
insects they are the tracheae.

Oxygen is required by cells for respiration and carbon dioxide must be removed.

Gas exchange requires special respiratory surfaces that must be thin, moist and
have a large surface area for the diffusion of gases.

On land, organisms usually keep their respiratory surfaces inside the body, with
only a small opening to prevent the surface drying out.

In multicellular organism a circulatory system transports gases between cells and
the respiratory surface. Gas exchange is often aided by active movement of an
organism. This is called ventilation.

Aquatic mammals such as whales have lungs for gas exchange and go to the
surface regularly to obtain air. However they can take in large volumes of air
quickly in each breath because their lungs are particularly elastic and they have
valves to close their nostrils when they dive.

Fish regulate their rate of ventilation according to the amount of oxygen in their
blood.

Mammals regulate their ventilation according to the amount of carbon dioxide in
their blood. In addition the rate of blood flow can increase as the heart pumps
faster to deliver the extra oxygen required by tissues when an organism is active.
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Role of the Kidneys:

Cellular metabolism involves the production of waste products. These need to be
removed from the body.

This process is called excretion.

Carbon dioxide is an excretory product removed by the lungs.

Nitrogenous wastes are removed as ammonia in fish, as uric acid in insects
and as urea in mammals. Humans excrete urea as urine from the kidneys.
 Explain why the concentration of water in cells should be maintained within a
narrow range for optimal function.
The Role of Water:

Water is essential for life.

Living organisms are composed of between 70 and 90% water.

Water is the solvent for all the metabolic reactions that occur in living cells.

It is the solvent in which most substances dissolve and is the transport medium for
distributing them.

Water takes part in many metabolic reactions and is formed as a product in many
others, including respiration.

Living cells work best in an ISOTONIC environment – one in which the solute
concentration is the same both inside and outside the cell. It is critical for the
proper functioning of these reactions that the amount and concentration of
water in the cell is kept constant.

Cells are very sensitive to changes in solute concentration and may loose or take
in a large amount of water by osmosis if the concentration in their external
environment changes too much. See Diagram1.

Living organisms try to ensure the water balance is maintained in their cells and
the concentration of solutes is kept constant so that the cells can function
properly.

In mammals such as humans, living cells are kept isotonic to the interstitial fluid
that bathes the cells.
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Diagram 1: Water balance in cells
Therefore it is critical for the proper functioning of metabolic reactions that the
amount and concentration of water in a cell is kept constant. Most cells die when
water content is changed significantly.
 Explain why the removal of wastes is essential for continued metabolic activity.
-
Metabolic wastes are the product of metabolism. They are constantly being
formed in cells as a result of metabolic processes.
-
If they are allowed to accumulate in cells and tissues and not removed their
concentration in the cell increases and this inhibits the reactions that produce them
i.e. slows down metabolism and poison the cells.
-
Metabolic wastes particularly nitrogenous wastes are the by-products of the
breakdown of proteins and nucleic acids and are toxic to cells and must be
removed quickly.
-
Ammonia is the nitrogenous waste product of protein metabolism, it is highly
toxic and needs to be either removed quickly or converted to a less harmful form.
-
Nitrogenous wastes have the ability to change pH levels in cells and interfere
with membrane transport functions and may denature proteins.
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Different Animals Secrete Different Waste Products:
A. Aquatic animals, fish and invertebrates mostly excrete ammonia.
B. Terrestrial animals excrete nitrogenous waste as either urea or uric acid.
There is a correlation between the type of waste produced and the animal’s
environment.
Aquatic animals like fish and invertebrates mostly excrete ammonia. This is toxic
but can be released continuously and directly into water and quickly dispersed.
On land, animals usually need to conserve water. So by converting it into less toxic
forms, they can hold it for longer in the body and release it periodically.
The waste is either excreted as urea (e.g. humans) or uric acid (e.g. insects).
a.)Urea is soluble (dissolves) and is released in urine.
- Urea is toxic but 10,000 times less toxic than ammonia therefore can be stored in a
more concentrated solution so requires less water to remove than ammonia.
- It can be safely stored in the body for a limited time.
- The concentration of urine varies according to the regulation of water within an
animal’s body.
- Mammals excrete urea. It is also the waste product of adult amphibians, sharks and
some bony fish.
- Some animals – particularly desert dwelling mammals, can produce small amounts
of highly concentrated urine.
b.) Uric acid is almost insoluble and non-toxic.
- Little water is expended to remove it. This is a great survival advantage.
- It is often excreted by animals as a whitish paste. Many animals get rid of their
excretory products together with the faeces from digestion and in doing so loose very
little water.
- Reptiles, birds and insects secrete uric acid.
- Earthworms secrete both ammonia and urea.
-Tadpoles excrete ammonia but as adult frogs they excrete urea.
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c.) Ammonia
- Very toxic and must be removed immediately either by diffusion or in very dilute
urine.
- Excretory product of most aquatic animals, including many fish and tadpoles.
- Ammonia is highly soluble in water and rapidly diffuses across the cell membrane.
However it needs large quantities of water to be constantly and safely removed.
Assignment 1
o Use available evidence to explain the relationship between the
conservation of water and the production of concentrated nitrogenous
wastes in a range of Australian insects and terrestrial mammals.
Task:
Find out how a range of Australian insects and terrestrial mammals excrete nitrogenous
wastes.
Use available evidence to then examine cause and effect relationships such as the lack of
water and the production of water –efficient waste removal and use this to write an
explanation of the relationship between the conservation of water and the production of
concentrated nitrogenous waste in a range of Australian insects and terrestrial mammals.
Example:
Organism
Terrestrial or
Waste
aquatic
product (s)
Terrestrial
Urea in a
The animal lives in a very arid
Hopping
concentrated
environment; it drinks very little
Mouse of
form
water and excretes urea in a very
Spinifex
Explanation
Central
concentrated form so that water can
Australia
be conserved.
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The following might be useful starting points:
The Euro (Wallaroo)
Mulgara – a dasyurid
The red kangaroo
Kowari
Bilby
Australian Insects
Assignment 2
o Analyse information from secondary sources to compare and explain
the differences in urine concentration of terrestrial mammals, marine
fish and freshwater fish.
Present information through the use of an information organising devise such as a table.
Analyse the information by making generalizations about urine concentration of
terrestrial mammals, marine fish and freshwater fish.
\
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 Identify the role of the kidney in the excretory system of fish and mammals.
The Role of the Kidney
The kidney is an organ of excretion of both fish and mammals. It plays a central role
in homeostasis, forming and excreting urine while regulating water and salt
concentration of the blood.
Function: The primary role of the Kidneys is osmoregulation – the regulation of
water and salt concentrations in the body.
Role 2: Excretion of all nitrogenous wastes mainly in the form of urine (humans).
Excretory System in Fish:
The role of the kidney in fish is dependent on the environment of the fish.
-
In fish, excretion of nitrogenous waste products i.e. ammonia as NH , occurs
across the gills.
-
The kidneys adjust the levels of water and mineral ions in the fish’s body in
order to maintain a constant concentration of internal fluid for the cells.
Freshwater Fish
-
Fish (bony) living in freshwater are hypertonic to their surroundings.
-
Hypertonic – they maintain a higher concentration of solutes in their body than
the concentration of water outside.
-
Water therefore tends to diffuse into their bodies and so the fish need to
continuously get rid of the excess water.
-
Their kidneys produce copious amounts of very dilute urine in an almost
continuous stream in order to achieve this.
-
As the fresh water has a lower concentration of ions than the fish do they actively
reabsorb salts to prevent this loss. See diagram 2
8
Saltwater Fish
-
Bony salt water fish have the opposite problem.
-
Their internal body fluids are less concentrated (more dilute) than the surrounding
water. To avoid loss of water from their body they drink saltwater continuously,
-
They absorb the water and salts.
-
The water is retained and the salts are actively excreted, some via the gills and
some via the kidneys.
-
Saltwater bony fish excrete very little urine.
-
Marine cartilaginous fish (sharks and rays) have their tissues isotonic with the
seawater so that there is no net movement of water in or out. In this way they
avoid the osmoregulation problems of bony fish.
Diagram 2: Osmoregulation in freshwater and saltwater fish.
Figure 5.43 pg 257 Heinemann
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Mammalian Excretory System
Main Function: the kidneys of mammals regulate the internal water and salt
concentrations in the body and excrete urea, the nitrogenous waste produced by
mammals.
Deamination:
Proteins are made up of amino acids. They are made used and broken down by cell
metabolism. However the body cannot store excess amino acids so any excess
becomes nitrogenous waste to be removed.
Deamination: This is the process by which excess amino acids are broken down in the
liver. This process involves removing the part containing nitrogen (the amino group, NH ) to form urea. The remainder is converted to carbohydrate which may be stored
(as glycogen) or used immediately.
Urea is transported by the blood to the kidneys and excreted as urine.
The kidneys make urine. It is an organ of filtration, reabsorption and secretion.
Kidney Structure

The kidneys are a pair of bean shaped organs found on either side of the
abdomen.

Kidneys produce urine.

The main components of the mammalian urinary system can be seen in
diagram 3 below. It is composed of two kidneys, two ureters, a bladder and
the urethra.

Urine leaves the kidneys via the ureters and is stored in a muscular bag, the
bladder. The bladder expands as it fills with urine.

At a certain point this expansion stimulated nerve endings in the bladder
which send a message to the brain. The brain sends a message to the sphincter
muscles surrounding the base of the bladder which relaxes so urine can pass
through the urethra out of the body.
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
The mammalian urinary system
11
The Internal Structure of the Kidney

Renal Artery: supplies the kidney with oxygenated blood. This blood also
carries the urea with it to the kidneys.

Renal vein: drains the blood from the kidneys and empties it into the inferior
vena cava.

Each kidney is made up of about one million small filtering units called
nephrons.

It is in these structures that urine is produced.

Each nephron is a convoluted tubule measuring up to 4.5 cm in length.

The nephrons are surrounded by a dense network of capillaries.

Diagram 5 shows the structure of a nephron.
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Diagram 5: A nephron.
13
The starting point of a nephron is a Bowman’s capsule which is a small cup shaped
structure situated in the cortex. This leads into a narrow convoluted tubule that makes a
loop (the loop of Henle) in the medulla back up to the cortex and then joins with a
collecting duct. The collecting duct transports urine to the pelvis of the kidney which
leads to the ureter.
 Explain how the processes of filtration and reabsorption in mammalian
nephron regulate body fluid composition.
The Formation of Urine:
The kidneys continuously process a large volume of blood to form a small volume of
urine. This involves three processes: filtration, reabsorption and secretion.
Filtration:
Blood is brought to the kidney by the renal artery. This divides into smaller vessels
which form a network of capillaries called a glamorous outside the Bowman’s
capsule.
The pressure is very high in the glomerulus and this causes some fluid to be forced
out through the walls of the blood vessels into the Bowman’s capsule. This liquid
consists of urea, glucose, amino acids, some hormones, vitamins, salts and water (no
plasma proteins or blood cells). Small soluble molecules pass through by a process of
passive filtration. This liquid is known as glomerular filtrate.
Filtration is a non- selective process. The filtrate contains some substances that the
body can re-use and some that are wastes.
They are all forced into the first part of the nephron tubules – the proximal tubule.
Along the length of the tubule the composition of the filtrate is adjusted carefully
until it contains only unwanted substances. It is then called urine.
14
Filtration in the Bowman’s capsule:
Reabsorption:

Surrounding each nephron is a large capillary network.

As the filtrate travels down the capsule the materials that the body can reuse
are reabsorbed into the blood.

These materials include glucose, amino acids, some vitamins, minerals,
bicarbonate and water.

It occurs in the proximal and distal parts of the tubule and the Loop of Henle.

In the proximal tubule about 80% of water is reabsorbed by osmosis.

All the glucose, amino acids and vitamins pass from the filtrate into the
capillaries by a combination of diffusion and active transport.

Most salts are reabsorbed by a combination of active transport and diffusion.

Reabsorption is an active process that requires energy.
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Secretion:
Secretion is a selective process where by the body actively transports substances from
the blood into the nephron.
This occurs in both the proximal and distal parts of the tubule.
**Regulation of Body Fluid Composition
The nephron is a regulatory unit – it selectively reabsorbs materials required to maintain
homeostasis.
The readjustments occur as substances are moved in either direction – reabsorption back
into the blood or secretion back into the nephron.
The reabsorption of ions ( Na , Cl , K and HNO ) occurs at different rates depending on
feedback from the body.
This regulation helps to maintain the constant composition of the blood and
interstitial fluid.

In the proximal tubule – most of the bicarbonate ions are reabsorbed and there
may be some secretion of hydrogen ions.

This helps to maintain a constant pH of the blood and body fluids.

Drugs such as aspirin and penicillin and poisons identified by the liver are
actively secreted into the tubule.

Nutrients such as glucose and amino acids are actively transported from the tubule
back to the blood. (reabsorbed)

Regulation of salts also occurs. Sodium salts Na are actively transported back
into the blood (reabsorbed). Chlorine ions Cl follow passively. As the salt moves
out, water by osmosis passes back into the blood. Potassium ions are also
reabsorbed here.
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
In the Loop of Henle – in the descending part, the walls are permeable to water
but not to salt.

Water passes across by osmosis.

In the ascending part the walls are permeable to salt not to water.

Salt passes out passively across a thin walled section and then actively across a
thick walled section. The salt passing out makes the interstitial fluid of the
medulla area of the kidney quite concentrated. (This hypertonic medulla helps
remove water by osmosis from the descending part and collecting duct).

In the distal tubule – selective reabsorption and secretion again occur to adjust
pH of the blood and level of salts particularly sodium and potassium.

The walls of the collecting ducts are permeable to water but not to salt. Water
passes out by osmosis and the final filtrate or urine is formed. (due to high salt
concentration in the medulla)
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Diagram 6 – The function of the nephron and collecting duct
Nephron of the kidney. The labelled parts are 1. Glomerulus, 2. Efferent arteriole, 3. Bowman's
capsule, 4. Proximal convoluted tubule, 5. Cortical collecting duct, 6. Distal convoluted tubule, 7.
Loop of Henle, 8. Duct of Bellini, 9. Peritubular capillaries, 10. Arcuate vein, 11. Arcuate artery,
12. Afferent arteriole, 13. Juxtaglomerular apparatus
Nephron is the basic structural and functional unit of the kidney. Its chief function is
to regulate the concentration of water and soluble substances like sodium salts by
filtering the blood, reabsorbing what is needed and excreting the rest as urine. A
nephron eliminates wastes from the body, regulates blood volume and blood pressure,
controls levels of electrolytes and metabolites, and regulates blood pH. Its functions
are vital to life and are regulated by the endocrine system by hormones such as
antidiuretic hormone, aldosterone, and parathyroid hormone. In humans, a normal
kidney contains 800,000 to 1.5 million nephrons.
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In the mammalian kidney:
1. Water re-absorption is a passive process.
2. Re-absorption of sodium salts is an active process
3. Glucose and amino acids are actively re-absorbed
4. Many drugs are selectively secreted by the kidney.
 Distinguish between the processes of active and passive transport and relate
these to processes occurring in the mammalian kidney.
 Diffusion and osmosis are passive forms of transport that do not require the
expenditure of energy.
 Diffusion and osmosis involve the movement of substances with the concentration
gradient – that is from where there are many particles to where there are few.
 Movement of substances against a concentration gradient requires energy. This is
called active transport.
 In the kidneys both forms of transport are use in the regulation of the body fluid
composition.
 Passive transport occurs in filtration and the osmosis of water back into the blood.
 Active transport occurs in the secretion of substances into the nephron, the active
transport of nutrients back into the blood and the selective reabsorption of salts
required by the body.
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Background:
Diffusion – the movement of particles fro ma n area of high concentration to an area
of low concentration.
Osmosis – The movement of water molecules to an area of high water concentration
(hypotonic) to an area of low water concentration (hypertonic) across a selectively
permeable membrane.
Active Transport – uses energy to pump or carry materials across a membrane
which otherwise might be block by the diffusion gradient or their own properties.
For example: molecules cannot cross a membrane if they are too large or if they
carry electric charges or fatty sections that bind them to the membrane.
(look up Endocytosis, Pinocytosis, Phagocytosis and Receptor – modified
endocytosis
 Explain why the processes of diffusion and osmosis are inadequate in removing
dissolved nitrogenous wastes in some organisms.
Diffusion and osmosis are both examples of passive transport, relying on the
random movement of molecules. Diffusion is too slow for the normal functioning
of the body and does not select for useful solutes.
Osmosis only deals with the movement of water and thus would only allow water
to move out of the body not nitrogenous waste.
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Assignment 3
 Gather, process and analyse information from secondary sources to
compare the process of renal dialysis with the function of the kidney.
-
Gather information on renal dialysis using books and internet sources.
-
Process the information by comparing the dialysis machine with the kidney and
matching the parts of the dialysis machine to the structure of the kidney. You
could include a table like the following in your comparison:
Dialysis Machine
e.g. Artificial Tubing
-
Kidney
Nephron
Analyse the information by determining the outcomes of the dialysis process and
show whether the kidney is more efficient at osmoregulation and excretion than
the dialysis machine.
The Endocrine System and Hormones
- Endocrine glands are ductless glands in the body that secrete hormones.
Examples: pituitary gland, adrenal glands, thyroid gland
-
Hormones are chemical messengers that travel in the blood to a target organ
where they produce an effect. (only certain target cells in organs respond to each
hormone)
-
Hormones bring about changes in the metabolic activity of the body.
-
Hormones are kept at a fairly constant level in the blood by feedback systems.
21
Diagram 7: The location and functions of the endocrine glands
Pg 262 figure5.50 Heinemann
The Roles of Hormones:
Table 5.6 pg 261
Background:
When the glomerular filtrate passes through the nephron tubule, the amount of water and
salts that is reabsorbed matches the body’s needs.
For example if you drink a large amount of liquid then the excess water is excreted as
large amounts of dilute urine. But if the body is little excess water and is losing water
rabidly as sweat on a hot day. Then the kidneys reabsorb the maximum possible amount
of water from the filtrate. The result is that only a small amount of concentrated urine is
produced.
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Two hormones – antidiuretic hormone (ADH) and aldosterone help to regulate salt and
water concentrations as well as blood pressure and volume.
The blood that leaves the kidney in the renal vein has had its nitrogenous wastes removed
and its water and salt composition balanced.
Endocrine glands make chemicals called hormones and pass them straight into the
bloodstream. Hormones can be thought of as chemical messages.
From the blood stream, the hormones communicate with the body by heading towards
their target cell to bring about a particular change or effect to that cell. The hormone can
also create changes in the cells of surrounding tissues (paracrine effect). The endocrine
system works with the nervous system and the immune system to help the body cope
with different events and stresses.
 Outline the role of hormones, aldosterone and ADH (anti –diuretic hormone), in
the regulation of water and salt levels in the blood.

Aldosterone is a steroid hormone secreted by the adrenal gland.

Function: Its function is to regulate the transfer of sodium and
potassium ions in the kidney.

When sodium levels are low, aldosterone is released into the blood
causing more sodium ions to pass from the nephron to the blood.

Water then flows from the nephron into the blood by osmosis. This
results in the homeostatic balance of blood pressure.

If there is an increase in blood volume and pressure (resulting from
high salt concentrations, which causes water retention), the out put of
aldosterone is reduced. Less salt and water is reabsorbed by the
nephron tubules and increased amounts of water and salts are lost in
the urine.
23

Antidiuretic hormone (ADH or vasopressin) controls water
reabsorption in the nephron. Made in the hypothalamus.

When levels of fluid in the blood drop, the hypothalamus causes the
pituitary gland to release ADH.

ADH increases the permeability of the collecting ducts and distal
tubules allowing more water to be reabsorbed from the urine into the
blood.

The resulting urine is more concentrated.

When there is too much fluid in the blood, sensors in the heart cause
the hypothalamus to reduce the production of ADH in the pituitary,
decreasing the amount of water reabsorbed in the kidney.

This results in a lower blood volume and larger quantities of more
dilute urine.
Diagram 8: The role of the Antidiuretic Hormone
Pg 262 Heinemann figure 5.51
ADH and the water balance of the body
The amount of water in the blood must be kept more or less the same all the time to avoid
cell damage as a result of osmosis. There has to be a balance between the amount of
water gained (from your diet though drinks and food and the water produced by cellular
respiration) and the amount of water lost by the body (in sweating, evaporation, faeces
and urine).
This is achieved by the action of the hormone ADH (anti-diuretic hormone). How does it
work?
Perhaps you have not drunk anything for a while or you have been sweating a lot. Part of
the brain, the hypothalamus, detects that there is not enough water in the blood. The
hypothalamus sends a message to the pituitary gland which releases ADH. This travels in
the blood to your kidneys and affects the tubules so more water is reabsorbed into your
blood. As a result you make a smaller volume of more concentrated urine. The level of
water in your blood increases until it is back to normal.
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Sometimes the level of water in your blood goes up because, for example, it is cold and
you have not been losing any water through sweating or because you have had a lot to
drink. The hypothalamus detects the change and sends a message to the pituitary. The
release of ADH into the blood is slowed down or even stopped. Without ADH the
kidneys will not save as much water and you produce large volumes of dilute urine. The
level of water in the blood falls back to the normal level.
This is an example of negative feedback. As the level of water in the blood falls, negative
feedback ensures that the amount of ADH rises. As the level of water in the blood rises
negative feedback ensures that the amount of ADH falls.
Assignment 4:
 Present information to outline the general use of hormone replacement
therapy in people who cannot secrete aldosterone.
Task :
Present the information as a discussion, with clearly identified issues and or points
provided for or against the use of the therapy.
Here is a key word to get you started in your web search:
Addison’s disease: Nation Institute of Diabetes and Digestive and Kidney Diseases,
USA
Background:
Hypoaldosteronism is a condition where people fail to secrete aldosterone. Addison’s
disease is the name of a disease with these symptoms which include high urine output
with a resulting low blood volume. Eventually as blood pressure falls, this can result in
heart failure. A replacement hormone, Fludrocortisone is used to treat this condition.
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 Define enantiostasis as the maintenance of metabolic and physiological
functions in response to variations in the environment and discuss its
importance to estuarine organisms in maintaining appropriate salt
concentrations.
Enantiostasis: is the maintenance of metabolic and physiological functions in the
absence of homeostasis, in an organism experiencing variations in its
environment.
For organisms living in an estuary this ability is tested as they all experience large
changes in salt concentration in their environment over a short period of time,
with tidal movement and mixing of fresh and salt water.
Estuary – an estuary is formed when a river meets the sea. In this environment
fresh water draining form the land mixes with saline water from the sea. On the
flood tide the sea invades the estuary. On the ebb tide the fresh water invades, the
water is shallower, and areas of land such as mudflats may be exposed.
Organisms living in an estuary that must tolerate wide fluctuations of salinity
are said to be euryhaline.

One strategy to withstand such changes in salt concentration is to allow
the body’s osmotic pressure to vary with that of the environment.
Organisms that do this therefore do not maintain homeostasis and are said
to be osmoconformers.

Most marine invertebrates are osmoconformers.

In contrast most marine mammals are fish are osmoregulators,
maintaining homeostasis regardless of the osmotic pressure of the
environment.

As the salt concentration of body fluids in an osmoconformer changes,
various functions are affected, such as enzyme activity. For normal
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functioning to be maintained another body function must be changes in a
way that compensates for the change in enzyme activity.

One example of enantiostasis is when a change in salt concentration in
body fluid which reduces the efficiency of an enzyme, is compensated for
by a change in pH, which increases the efficiency of the same enzyme.

Salmon, trout and eels that move from sea to rivers must have adaptations
to deal with salt and water problems experienced in freshwater and marine
environments. Eels for example have special cells in their gills that can act
as salt absorbers and salt secretors.
 Process and analyse information from secondary sources and use available
evidence to discuss processes used by different plants for salt regulation in
saline environments.
Maintaining Salt Concentrations in Plants
Most plants cannot tolerate high salt concentrations in the root zone as it leads to
water stress. The salt accumulates in its leaves and is toxic. Enzymes are inhibited by
Na+ ions.
Halophytes are plants adapted to living in salty environments. They are able to
tolerate higher levels of salt than any other plants or they have special mechanisms to
control their levels of salt.
Three different mechanisms are salt exclusion, salt excretion and salt accumulation.
1. Salt excluders – prevent the entry of salt into their root systems by filtration.
This is a passive process that does not use energy and relies on the transpiration
stream. It can be very successful. Example: The grey mangrove Avicenna marina can
exclude 95% of their salt via the filtration system in its roots and lower stems.
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Other mangroves that rely on this system are the red mangrove Rhizophora stylosa
and the orange mangrove Bruguiera gymnorrhiza.
2. Salt excretors – have special salt glands usually in their leaves. Salt is
concentrated there and then actively secreted from the plant.
The salt can often be seen and tasted as salt crystals on the leaves. Rain washes the
salt off.
Examples: The grey mangrove Avicenna marina and river mangrove Aegiceras
corniculatum have salt glands as do salt bushes (genus Atriplex). Sporobolus
viriginicus has salt glands on its leaves (salt marsh plant)
Atriplex is a genus of the salt bush family that produces a covering of bladder-like
hairs into which salt is excreted at extraordinary high concentrations.
Diagram 9: Atriplex
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3. Salt accumulators – these concentrate or accumulate salt in a part of the plant
usually the bark or older leaves, which is then shed.
Example: The milky mangrove Exoecaria agallocha sheds old leaves full of salt. The
succulent samphire plant Sarcocornia quinqueflora, found on salt marshes
accumulates salts in swollen leaf bases which then drop from the plant, thus removing
excess salt. Another form of salt stress occurs can occur in salt laden air such as in
coastal environments. Some coastal plants such as Norfolk Island pine have a mesh of
cuticle over their stomates, which prevent small water droplets from entering the leaf.
 Describe adaptations of a range of Australian plants that assist in minimizing
water loss.
Leaves of plants contain stomates or small pores that allow the exchange of gases
essential for respiration and photosynthesis. These gases include water vapour as well as
oxygen and carbon dioxide. If stomates are open there will be a loss of water by
transpiration and evaporation. Plants in arid areas have to balance the need for CO2 with
the need to conserve water.
Xerophytes – are plants adapted to arid or dry condition.
Many Australian terrestrial plants show a variety of adaptations to conserve water and
minimize water loss.

Water storage – storage of water in soft fibrous trunks such as in the bottle tree
(Brachychiton rupestris) and the boab tree (Adamsonia gregorii). Succulent plants
such as the noonflowers or pigfaces (e.g. Karella – Carpobrotus rossii and
rounded noonflower Disphyma crassifolia store water in fleshy leaves and stems.
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The boab tree (Adamsonia gregorii)
Gouty stem tree, Adansonia Gregorii, 58 feet circumference, near a creek south-east of
Stokes Range, Victoria River

Extensive root systems – for water collection. The ability to collect as much
water as possible is exemplified by an extensive rot system. These include deep
tap root systems to reach deep underground water and wide shallow roots to soak
up surface moisture.
Desert plants are often widely spaced because of root competition below
ground.
Example: The Mulga (Acacia aneura) – its branches are also arranges so that any
rain falling is channeled directly to the roots.
Structural Adaptations: these include features that help minimize water loos
from transpiration.

Leaves with a waxy cuticle e.g. leaves of eucalypts and mangroves.

Small leaves with a reduced surface area, for example the needle like leaves of the
Hakea plants or the replacement of leaves with photosynthetic stems such as in
she-oaks (Casuarina and Allocasuarina) or flattened leaf stem called phyllodes
of some Wattle (Acacia Species)
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Diagram 10: Phyllodes
Pg 165 fig 4.20

Wattle Acacia
Reflective leaf surfaces: these may be pale, shiny, hairy or crystalline. Some salt
bushes change the reflectiveness of their leaves during leaf development so that
they have highly reflective leaves during the summer.

Hairy leaves that reduce airflow across the leaf surface thus reducing evaporation:
often underside of the leaves or growing buds are covered in hairs.
Diagram 11: Water Loss Adaptations – Example 1: Ptilotus species (hairy leaves and
flowers) Example 2 – A liverwort Riccia crystallina (a reflective crystalline surface)
Example 1
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Example 2
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
Stomates sunken into pits or grooves and / or reduced number of stomates for
example Hakea and Eucalyptus leaves.

Thick bark as in Mulga or extra thickening of cell walls to prevent wilting; many
Australian xerophytes have leaves that do not wilt.

Rolled up leaves to minimize water loss for example in the porcupine grass ans
spinifex Triodia. (Leaves rolled inwards)
Physiological Adaptations: alter a plants metabolic activities

Leaves hanging vertically that change their orientation during the day to ensure
that only the edges not the full surface of the leaves are exposed to the sun. This
reduces both heat absorption and water loss.
Diagram 11: Eucalypts – leaves hang vertically
Pg 224 figure 5.18

Closing stomates during the hottest part of the day. This is usually associated with
plants such as succulents that open their stomates at night to take in carbon
dioxide.
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
Dormancy periods when leaves or all above ground parts die off during hot dry
conditions. Mallee eucalypts for example die back and regenerate when
favourable conditions return from swollen underground lignotubers.
Diagram 12: Mallee eucalypt showing lignotuber
Pg 194 fig 4.64

Tough hard seeds that can survive long dry periods as well as accelerated life
cycles may be shown by plants in response to a short wet season in arid
environments.
Example: Sturt’s desert pea (Clianthus formosus), germinate grow flower and
produce many seeds within six to eight weeks of heavy rains.

Tolerance to drying out or dessication. Example: the leaves of the resurrection
plant can be dry and shriveled for four to five months then become green and
swollen again after heavy rain.
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