Maintaining A Balance

Most organisms are active in a limited temperature range
Enzymes:
Biological catalysts. A specific enzyme controls every reaction and process within a cell. Enzymes control all the chemical processes of living systems.
Enzymes are proteins made up of amino acids that are linked and then folded to produce a three-dimensional protein structure. Most metabolic processes would not occur at an efficient rate without enzymes.
One model used to illustrate the action of an enzyme is the lock-and-key model.

Enzymes are unique to one substrate
The enzyme combines with the substrate to form an enzyme-substrate molecule. This somehow alters the substrate so that a chemical reaction can occur.
The substrate is then altered and no longer fits’ the enzyme – the enzyme is released.
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– Effect of temperature on enzymes: each has an optimum temperature for activity. High temperatures can denature enzymes.
– Effect of pH on enzymes: each has an optimum pH (acidity level) for activity. Changing the pH from the optimum reduces the enzyme’s activity.
– Effect of substrate concentration on enzymes: substrate concentration means the amount of compound present that the enzymes catalyses. Beyond certain substrate concentrations, the rate of reaction is limited by the amount of enzyme.
pH as a way of describing the acidity of a substance:
A measure of the concentration of hydrogen ions that are released by acids, therefore a way of describing the acidity of a substance.
A pH value of 0-6 indicates an acid solution, a pH value of 8 – 14 indicates a basic (alkaline) solution. A pH of 7 indicates a neutral solution.
Procedures to investigate the activity of an enzyme:
Aim: to demonstrate the effect of increased temperature on the enzyme, rennin
Method: make a rennin solution by dissolving a junket tablet in distilled water. Add the same amount of rennin solution to a number of test tubes of milk. Place test tubes in different water baths with different temperature ranges (0-60), making sure each water bath is kept at its allocated temperature. Time the interval between adding the rennin and curdling of the milk for each temperature.
Results: The optimum temperature for curdling milk is 30-40 degrees. Anything above that optimum temperature will denature the rennin, preventing it from curdling the milk. Anything below the optimum temperature won’t work as effectively.
Maintenance of an optimal internal environment:
Enzymes control all the metabolic processes in the body.
Despite the internal and external changes occurring in the body, organisms need to maintain a constant internal environment for optimal metabolic efficiency.
Enzymes work optimally in an environment where their optimum temperature and pH conditions are met. At temperatures and pH values other than the optimum, the enzymes fail to work as they should or not at all.
The maintenance of an optimal internal environment is important for optimal enzyme efficiency.
Homeostasis:
The process by which organisms maintain their internal environment regardless of the external environmental conditions. Through homeostasis, organisms maintain an internal equilibrium by adjusting their physiological processes. Homeostasis ensures that the organism operates at maximum performance. To maintain homeostasis, this involves:
A receptor: constantly monitors the internal environment, which may reflect the external environment
A control centre: monitors the information passed on from the receptor
An effector: carries the message from the control centre.

Hypothalamus: the control centre for maintaining homeostasis.
Homeostasis as a two-stage process:
Coordination in animals is controlled by two systems; the nervous system and the endocrine system
A feedback mechanism is self-regulating, which maintains balance or homeostasis. For a state of homeostasis to exist, the body must have some way of detecting stimuli that indicate a change in the body’s internal or external environment.
– Stage 1 – Detecting changes from the stable state:
A receptor detects a change in some variable in the organism’s internal environment.

If our body temperature rises, the temperature rise in the blood stimulates the brain’s anterior hypothalamus.
Alternatively, when a mammal is exposed to cold, skin receptors increase their activity, sending nerve impulses to the posterior hypothalamus.
– Stage 2 – Counteracting changes from the stable state:
An appropriate response occurs that counteracts the changes and thus maintains the stable environment.
After detecting the rise in body temperature, the hypothalamus then stimulates heat loss by blood circulation through the skin, sweating and metabolic activity, thus lowering the body’s temperature.
After detecting a drop in temperature, activity in the posterior hypothalamus detects the nervous system to activate mechanisms to conserve heat.
A model of a feedback system:
The body has some effective mechanisms to alter body temperature.
To reduce temperature, heat can be expelled through sweating or the radiation of heat from the skin. To increase heat, the body can respond by shivering or by contracting the skin. These responses can be activated by heat receptors.
If receptors in the skin detect heat, they relay information via the nerves to the hypothalamus, which also contains receptors sensitive to the heat of passing blood. This triggers the nervous system to activate sweat glands.
When receptors in the skin detect a low temperature, a negative feedback mechanism is activated to stop the original action.
Feedback system model: thermostat
Controls temperature to a set level in a room.
236855011430000Has a device to measure temperature (hypothalamus). If the temperature is too low, then a heating process will be initiated. Eventually, the device will detect that the temperature is at the appropriate level and then will send a message to the heater to cease operations.
This is a negative feedback mechanism. This occurs when an appropriate response has occurred and the increase in some factor has been sensed, resulting in the termination of a further response.
The role of the nervous system:
Provides rapid coordination of internal organ systems, and detects and responds to environmental changes.
The nervous system consists of the central nervous system (CNS) – the brain, spinal cord and peripheral nerves.
Special endings on the sensory nerves detect stimuli such as heat, pressure or chemical conditions. These receptors relay messages that are processed within the CNS and then messages are conveyed to effector organs or muscles that bring about the responses.
Temperature ranges:
Organisms live in environments with ambient temperatures ranging from less than 0 to 100 degrees. Ambient temperatures are the external or environmental temperature.
Individual organisms cannot survive this wide range of temperatures, e.g. mammals can only generally survive temperatures of about 0-45 degrees and can only be normally active in a range of body temperatures between 30-45 degrees.

Ectotherms and endotherms:
Ectotherms: animals whose temperature is determined by an external environment. Desert lizards respond to changes in temperature by burrowing during the warmer parts of the day and increasing body temperature by basking in the sun.
Endotherms: animals that regulate their internal body heat regardless of the external environment. Red kangaroos respond to changes in temperature by sheltering during the heat of the day and licking the inside of their paws to increase heat loss by evaporation of water.
Responses to changes in ambient temperature:
Adaptations are structural, functional or behavioural characteristics that help an organism to survive in certain environments.
Behavioural
Structural and Physiological
Migration: animals move to avoid temperature change (birds)
Insulation: fur, feather, fat layers and/or blubber. Reduces heat exchange with the environment.
Nocturnal: allows animals to escape the daily heat and become active during the cooler night (brown snake)
Evaporation: endotherms keep cool by controlling the rate of water loss (kangaroos)
Hibernation: to survive cold conditions, many animals hibernate during cooler periods of the year (bears)
Metabolic activity:
– cold: activity, heat produced through shivering
– heat: sweating, cooling the body
Responses of plants to temperature change:
Plants need certain temperatures for growth and the germination of seeds.
Responses of plants to temperature change includes:
Reduced surface area, reducing heat absorption and supporting convective cooling
The dropping of leaves in the result of cooler temperatures
Closing stomates in response to high temperatures to reduce water loss
Plants and animals transport dissolved nutrients and gases in a fluid medium
The forms in which substances are carried in mammalian blood:
Mammalian blood consists of cells and cell-like bodies that are carried about in a watery fluid called plasma.
Carbon Dioxide: mostly carried in solution in plasma as bicarbonate ions
Oxygen: carried as an oxygen-haemoglobin combination in red blood cells
Water: carried as blood plasma, which is 90% water
Salts: carried as dissolved ions in the plasma
Lipids: mostly transported in the blood as phospholipids and cholesterol that are associated with plasma proteins
Nitrogenous wastes: mostly carried as urea, with a small amount of ammonia and uric acid
Other products of digestion: carried as substances such as amino acids or glucose and are dissolved or suspended in the plasma.
Estimating the size of red and white blood cells:
Red blood cells contain no nucleus, while white blood cells are larger and contain and large, lobular nucleus
Red blood cells are about 7 microns in diameter and are disc-shaped
White blood cells are about 10 microns in diameter and are more spherical and contain an obvious nucleus.
Haemoglobin:
Globule-shaped protein containing four polypeptide sub-units, enabling red blood cells to carry oxygen.
One haemoglobin molecule can carry four molecules of oxygen, increasing the rate and efficiency of oxygen intake and transport in the molecule.
Adaptive advantage of haemoglobin:
The presence of haemoglobin increases the oxygen carrying capacity of blood by 100 times, giving mammals a considerable survival advantage.
The structure of the haemoglobin molecule is also an advantage as it’s the type of molecule that can combine with oxygen loosely at the respiratory surfaces and then release the oxygen freely in capillaries.

Technologies to measure oxygen saturation and carbon dioxide concentrations in blood:
ABG (arterial blood gas) analysis measures the amounts of oxygen and carbon dioxide in the blood. This analysis evaluates how effectively the lungs are delivering oxygen and how well the lungs are getting rid of carbon dioxide.
A blood gas analyser measures the partial pressure of oxygen and carbon dioxide, the oxygen content, oxygen saturation, bicarbonate content and blood pH. Oxygen saturation compares the amount of oxygen actually combined with haemoglobin to the total amount of oxygen the haemoglobin is capable of combining with. Arterial blood is collected for this analysis.
A pulse oximeter can be used for monitoring oxygen saturation. It is a device attached to the finger and uses the absorption of light to measure oxygen saturation. It has the advantage of being non-invasive and can provide continuous monitoring for patients undergoing anaesthesia or mechanical ventilation.
The conditions under which blood gas studies are used are to assess respiratory disease and other conditions that may affect the lungs, as well as to manage patient receiving oxygen therapy, mechanical ventilation or anaesthesia.
The structure and function of arteries, capillaries and veins
Arteries
Veins
Capillaries
– Thick, muscular walls
– No valves present
– Carry blood away from the heart
– Carry oxygenated blood (except for the pulmonary artery)
– Blood is arteries is pumped under high pressure
– Thin walled
– Valves are present to prevent back-flow of blood
– Carry blood back to the heart
– Carry deoxygenated blood (except for the pulmonary vein)
– Blood is under low pressure; movement is assisted by body muscles
– Thin walled, often only one cell thick
– Carry blood between arteries and veins
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Chemical composition of blood in the body:
-Blood flow through the heart:
Pulmonary system: Deoxygenated blood from the body enters the right atrium, is squeezed into the right ventricle and then pumped into the lungs. Carbon dioxide is decreased and oxygen levels increased.
Systemic system: Oxygenated blood from the lungs enters the left atrium, is squeezed into the left ventricle and then pumped to the body tissues through the aorta. Deoxygenated blood then returns to the heart via the vena cava.
-Changes in the chemical composition of blood:
Chemical composition of the blood as it moves around the body
Tissues in which these changes occur
Blood receives oxygen and carbon dioxide is released
Lung tissue
Blood receives carbon dioxide and oxygen is released
General body tissues, such as skin tissues
Water diffuses into blood
Stomach tissue
Digested foods diffuse into the blood and go straight to the liver
Small intestinal tissue
Glucose is added or removed
Liver tissue
Water, salts and vitamins are absorbed in the large intestine and pass into the blood
Large intestinal tissue
Urea, excess water and salts are removed from the blood to be excreted
Kidney tissue
Hormones are secreted directly into the blood stream
Endocrine tissue
Products extracted from donated blood and their uses:
Red blood cells are used to increase the amount of oxygen that can be carried to the body’s tissues. They are given to people who have anaemia.
Platelets are essential for blood clotting. Platelets are given to people who have cancer of the blood or lymph such as leukemia.
Plasma is the liquid portion of the blood and is used to treat people with clotting disorders such as haemophilia. It is also used to adjust the osmotic pressure of blood and to pull fluids out of tissues.
White blood cells are an infection-fighting component of the blood. Are only used occasionally to treat life-threatening infections when the cell count is very low or the white blood cells are not working properly.
The importance of research into the production of artificial blood:
Up until the HIV crisis in the 1980s, there was little interest in artificial blood, as there did not seem like a great need.
Artificial blood is currently only designed to increase plasma volume and carry oxygen (and carbon dioxide).
No substitutes have yet been developed that can replace other function – coagulation and immune defence.
– Two types of oxygen-carrying artificial blood have been produced:
Perflurochemicals are synthetic materials that can be dissolved about fifty times more oxygen than blood plasma. They are cheap to produce and, because they are synthetic, there is no risk of the material being infected by disease.
More research is needed because perflurochemicals must combine with other substances in order to mix in the blood stream. Research has included mixing them with lipids and more recently, lecithin
Haemoglobin-based oxygen carriers are made from haemoglobin extracted from red blood cells. They are not contained in a membrane and therefore do not require blood typing and cross matching of blood. More research is needed because haemoglobin must be modified before it can be used.
Current blood substitutes do not have the enzymes that prevent haemoglobin from oxidizing. Once haemoglobin is oxidized, it cannot carry oxygen.
– Some advantages of artificial blood include:
Pasteurisation could be used to remove all pathogens
No need for cross matching and blood typing
Storage benefits – artificial blood can be stored for more than one year, compared with donor blood only lasting for one month
Oxygen and carbon dioxide in cells:
– The need for oxygen:
Cells require oxygen in the process of respiration: glucose + oxygen carbon dioxide + water + energy (in the form of ATP)
A constant supply of oxygen to cells and tissues is essential. If oxygen is not available, the cell dies.
– Why the removal of carbon dioxide is essential:
Carbon dioxide is a waste product and must be removed to maintain the normal pH balance of the blood.
274320028130500By removing excess carbon dioxide, it prevents a build up of carbonic acid, which causes the lowering of the pH, and therefore increasing breathing rate and depth.
The movement of materials through plants in xylem and phloem tissue:
Xylem tissue transports water in plants. Phloem tissue transports sugars.
– Processes responsible for the movement of materials in xylem
The current mechanism, which transports materials in xylem, is the transpiration-cohesion-tension mechanism, which accounts for the ascent of xylem sap.
The transpiration-cohesion-tension mechanism is passive transport.

The mechanism is summarized as three processes:
Cohesion: water molecules stick together within a continuous network of liquid columns, which have the ability to instantaneously transfer pressure or tension.

Transpiration: water is evaporated through the stomates and is replaced by water from cells and xylem tissue.

Tension: water moves up the xylem like a wire being pulled up, due to cohesion. This helps resist the formation of bubbles within the stream.

– Processes responsible for the movement of materials in phloem:
Movement of materials in phloem moves both up and down the stem.
The pressure-flow mechanism (Source to Sink) is the model for phloem transport.
In this mechanism, sieve elements accumulate solutes such as sugars from the leaves (source), which is a process that requires metabolic energy. Companion cells also accumulate solutes and deluver them to sieve elements. At these sites, the sugar concentration is high and this causes the entry of water by osmosis from surrounding cells and xylem. The resulting pressure causes water and dissolved solutes to flow along under the force of turgor pressure to the places where sugar is being removed (sink).

The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of phloem sap.

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Longitudinal sections show relatively long, narrow cells in both phloem and xylem. Transverse sections show the relatively thick cell walls of xylem tubes and the perforated sieve plates in phloem cells.
Plants and animals regulate the concentration of gases, water and waste products of metabolism in cells and in interstitial fluid
The concentration of water in cells:
Water is the solvent for metabolic reactions in living cells. Many molecules and all ions important for the life of the cell are carried in an aqueous solution and these diffuse to reaction sites through the water in the cell.
Metabolic reactions within the cell can occur only in solution where water is the solvent. It is critical for proper functioning of these reactions that the amount and concentration of water in the cell be kept constant.
Most cells die when the water content is changed significantly.
The removal of wastes:
Metabolic wastes, particularly nitrogenous wastes, are toxic to cells and must therefore be removed quickly. Nitrogenous wastes have the ability to change the pH of cells, interfere with membrane transport functions and may denature enzymes.
The excretory system is concerned with the removal of metabolic waste products from the body. In humans, the organ systems for excretion are the kidneys, the lungs and the skin.
The excretory system is also responsible for maintaining a constant blood composition and therefore maintaining a constant internal environment in cells.
The role of the kidney in the excretory system:
The kidney is an organ of the excretory system of both fish and mammals. It has a dual role of excreting nitrogenous wastes and maintaining the water balance in mammals and fish. It is an organ of filtration, reabsorption and secretion.
In mammals, it plays a central role in homeostasis, forming and excreting urine while regulating water and salt concentration in the blood. It maintains the precise balance between waste disposal and the animal’s needs for water and salt.
The role of the kidney in fish is dependent on their environment:
In marine (saltwater) environments, the kidneys excrete small quantities of isotonic urine, helping to converse water and excrete salt they gain from their environment.
In freshwater environments, the kidneys work continuously to excrete copious quantities of dilute urine with very low salt concentration. This helps to remove excess water gained from their environment.
The processes of diffusion and osmosis:
Diffusion: the movement of molecules from an area of low concentration to an area of high concentration divided by a concentration gradient.
Osmosis: movement of water from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration) across a semi permeable membrane.
The processes of diffusion and osmosis are inadequate in removing dissolved nitrogenous wastes because diffusion is too slow and non-selective of solutes and osmosis would mean that waste would stay in the body and water would leave it. These problems are resolved by having a kidney that dumps everything outside’ the body and selectively reabsorbs the still-useful materials.
Active and passive transport in the mammalian kidney:
Active transport involves an expenditure of energy. Passive transport involves no expenditure of energy.
In the mammalian kidney, both active and passive transport processes occur.
Passive transport: once filtration has occurred in the Bowman’s capsule, water returns via the interstitial fluid from the tubule to the capillary in the process of osmosis. This occurs along the length of the tubule.
Active transport: depending on their concentration, the ions in the blood can be transported to cells in the nephron tubule and then secreted by the cells into the tubule. Some poisons and certain drugs are eliminated from the body in this manner.
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The processes of filtration and reabsorption in the mammalian nephron:
Filtration of all the blood occurs in the Bowman’s capsule where high blood pressure in the glomerulus forces all small molecules out of the blood into the capsule. The structure of the glomerulus means that it acts as an ultra-filter.
Water, urea, ions, glucose, amino acids and vitamins are all small enough to be moved into the glomerulus filtrate. Blood cells and proteins are too large to be removed. This filtering process is non-selective and therefore many valuable components of the blood must be recovered by reabsorption.
Reabsorption takes place selectively at various points along the proximal tubule, loop of Henle and the distal tubule. All glucose molecules, amino acids and most vitamins are recovered, although the kidneys do not regulate their concentrations. The reabsorption of the ions occurs at different rates depending on feedback from the body. In some cases, active transport is required.
Water is reabsorbed in all parts of the tubule except the ascending loop of Henle. The amount of water reabsorbed depends on feedback from the hypothalamus. If no water were reabsorbed, humans would soon dehydrate. The chemical composition of the body fluids is precisely regulated by the control of solute reabsorption from the glomerulus filtrate.

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Renal dialysis compared with the function of the kidney:
The artificial kidney does not match the complexity of a natural kidney and has limits as a long-term substitute for the kidney. The artificial kidney regulates the concentration of the patient’s blood by removing substances (such as urea and other toxins) and selectively adding substances. The basic process is called dialysis.
The fluid used in dialysis promotes diffusion of the appropriate substances into and out of the blood. Two healthy kidneys filter the blood volume about once every half-hour. Dialysis is a much slower and less efficient process than the natural processes found in a healthy kidney, but it is a lifesaver for those people with damaged kidneys.
Aldosterone and ADH (anti-diuretic hormone):
Aldosterone is a steroid hormone produced by the adrenal cortex of the kidney. Its role is to maintain the balance of water and salts in the body. It stimulates the nephrons to decrease reabsorption of potassium and increase reabsorption of sodium into the blood, leading to an increased reabsorption of chloride ions and water. The reabsorption of these substances causes a rise in blood volume and blood pressure.
ADH is a hormone produced by the hypothalamus and stored in the posterior pituitary of the brain. ADH stimulates the nephrons to reabsorb more water. It acts to decrease urine volume, increase urine concentration and increase blood volume.
Hormone replacement of aldosterone:
The replacement hormone is called fludrocortisone, used to treat people with Addison’s disease mostly caused by the destructing or shrinking of the adrenal cortex.
The adrenal cortex produces two hormones, cortisol and aldosterone. If the body cannot secrete aldosterone, water and salt balance cannot be maintained. When this balance is upset, the volume of blood falls dangerously low; there is a drop in blood pressure and severe dehydration. When levels of both cortisol and aldosterone drop, many functions throughout the body are disrupted.
Enantiostasis and salt concentrations:
The maintenance of normal metabolic and physiological functioning, in the absence of homeostasis, in an organism experiencing variations in its environment. It is particularly important for organisms living in an estuarine environment where salinity varies greatly.
Organisms that must tolerate wide fluctuations of salinity are said to be euryhaline.
Marine mammals and most fish maintain homeostasis in estuarine environments in a process called osmoregulation. They carry out a range of activities that maintain a constant body fluid composition, despite the changing environment. Activities such as excreting salt or concentrating urine are essential to their survival.
Many plants and marine invertebrates are osmoconformers and so rely on enantiostasis to survive. This means that the composition of their body fluids varies along with the environment. Adjustments are made in their physiology. Their enzymes keep functioning despite changes to conditions or metabolic pathways so that their needs are met.
Both homeostasis and enantiostasis are essential to maintaining the diversity of estuarine ecosystems.
Comparison of the urine concentration of terrestrial mammals, marine fish and freshwater fish:
Urine concentration
Reason for the difference
Terrestrial mammal
Concentration and volume varies
Terrestrial organisms face the issue of conserving water and at the same time removing nitrogenous wastes.
Marine fish
Highly concentrated
Aquatic organisms face the problem of osmosis. Marine fish drink large amounts of seawater to replace water loss.
Freshwater fish
Dilute urine
In fresh water, water will tend to move into the organism by osmosis. Freshwater fish must rid themselves of excess water.
Processes used by different plants for salt regulation in saline environments:
Plants in mangroves and coastal marshes live in the boundary between saltwater and freshwater. These plants use three main processes for keeping the growing stems and leaves mostly free of salt:
Salt barriers – special tissues in the roots and lower stems stop salt from entering the plant but allow water uptake.
Secretion – some plants are able to concentrate salt and get rid of it through special glands on the leaves (grey mangrove). The salt is then washed off by rain.
Salt deposits – some plants deposit salt in older tissues, which are then discarded, e.g. the mangrove Rhizophora concentrates salt in its old leaves, which it sheds.
The relationship between the conservation of water and the production and excretion of concentrated nitrogenous wastes:
Terrestrial organisms face the problem of conserving water and at the same time removing nitrogenous wastes in a form that is concentrated but not toxic.
Ammonia is very toxic and must be removed immediately, either by diffusion or in very dilute urine. It is the waste product of most aquatic animals. Including many fish and tadpoles.
Urea is toxic, but less toxic than ammonia, so it can be safely stored in the body for a limited time. It is the waste product of mammals, and some other terrestrial animals, but also of adult amphibians, sharks and some bony fish.
Uric acid is less toxic than ammonia or urea, so can be safely stored in or on the body for extended periods of time. It is the waste product of terrestrial animals such as birds, many reptiles, insects and land snails.
Spinifex hopping mouse
Terrestrial
Urea in a concentrated form
Lives in a very arid environment. Conserves water by excreting concentrated urea. Drinks very little water.
Wallaroo
Terrestrial
Concentrated urine
Very efficient excretory system that recycles nitrogen and urea to make very concentrated urine, allowing them to survive in very arid environments.
Insects
Terrestrial
Uric acid
Insects are covered with a cuticle impervious to water. They conserve water by producing a dry paste of uric acid.
Mammals: excrete concentrated urine, receive water from metabolism and excrete solid faeces.
051054000Grasshopper: passes waste into the Malpighian tubules, which pass it back into the intestine to pass out of the body.
Adaptations of Australian plants to minimize water loss:
Australian terrestrial plants have a range of adaptations that assist in minimizing water loss and at the same time allowing for gas exchange.
– Adaptations of Australian xerophytes include:
Needle-like leaves, which reduce surface area and water loss (acacias)
Waxy leaves, which reduce water loss as cuticles prevent evaporation but also reflect infrared radiation from the sun, reducing heat gain (atriplex)
Sunken stomates, which result in humid air being concentrated above the stomate, which reduces water loss (hakeas)
Hanging leaves, which reduces exposure to the sun (eucalypts)
Hairy or shiny leaves, which reduce air movement and increase humidity over stomates, reducing water loss, as well as reflecting radiation from the sun reducing heat gain (banksias)
Structures in plants that assist in the conservation of water:
This investigation included structures such as waxy leaf cuticle, hairy leaves, sunken stomata, few stomates on leaves, leaves rolled forward and leaves reduced to spikes.
Most acacia (wattle) plants do not have true leaves but flattened, green leaf stalks called phyllodes. These carry out photosynthesis but contain fewer stomata than leaves so that water loss is minimized.
Casuarinas (she-oaks) have jointed needle-like growths that are actually green stems. The leaves are reduced to tiny, teeth-like structures along the joints of the needles. These growths are called cladodes. They also carry out photosynthesis with minimal water loss.
There is a wide variety of structural adaptations such as phyllodes, cladodes, waxy cuticles and hairy leaves that reduce transpiration and so conserve water in plants.