Topic : Maintaining a Balance

Notes for Maintaining a Balance

Below are the syllabus dot points of Maintaining a Balance. Click on the dot point to expand relvant information. These notes were written by; Emalee Callaghan Click here to donate her

1.1 identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to describe their specificity substrates

Role of enzymes in metabolism:

  • A catalyst is a substance that increases the rate of a chemical reaction without being changed itself. Enzymes function by providing an alternative pathway for a chemical reaction, thus allowing for metabolism reactions to occur faster which accommodates the high energy and changing conditions faced by an organism in day to day life
  • Enzymes are biological catalysts which increase the rate of chemical reactions.
  • Without enzymes, metabolism would be to slow to support life

Chemical composition of enzymes:

  • Most enzymes are made up of protein
  • Proteins are composed of long chains of amino acids joined together by peptide bonds
  • These long chains are called polypeptide chains
  • Proteins consist of one or more polypeptide chains

Structure of enzymes:

  • The polypeptide chain is folded into a 3-dimentional globular shape
  • Part of the enzyme is called the active site. This part attaches to the substrate
  • The substrate are the molecules the enzymes acts upon
  • Each protein is specific to a substrate

Specificity of enzymes:

  •  Enzymes are highly specific in their action. This means that each enzyme acts on one substrate only
  • This is because the shape of the active site of the enzyme matches the shape of the substrate material
  • The substrate molecules bind to the active site and a chemical reaction occurs
  • The products are the substances that the substrate(s) become. One substrate can be split or two substrates can be joined

Models to explain specificity:

  • Lock and key model suggests the substrate fits exactly into the active site of the enzyme like a lock and key. It assumes that enzymes have a rigid and unchanging shape
  • Induced fit model states that the binding of the substrate to the enzyme induces a temporary change in shape of the enzyme. The new shape of the enzyme better accommodates the shape of the substrate and a reaction occurs

1.2 identify the pH as a way of describing the acidity of a substance

  • the substance that makes a solution acidic is the hydrogen ions
  • pH is a measure of the acidity or the alkalinity of a substance
  • the pH is a measure of the concentration of hydrogen ions per litre of solution
  • the pH scale is from 0 to 14, a pH of 7 is neutral, above 7 is alkaline and below 7 is acidic

1.3 Explain why the maintenance of a constant internal environment is important for optimal metabolic efficiency

  • All chemical reactions within cells must occur efficiently and be effectively co-ordinated to bring about optimal metabolic efficiency
  • Cells are sensitive to internal changes and any imbalances adversely affects functioning
    • Internal environments must be maintained within a narrow range of conditions
    • e. temperature and chemical content must remain constant to enable enzymes to function
  • metabolic efficiency relies on a constant level of
    • temperature and pH for enzymes functioning’s
    • concentration of reactants
    • water and salt concentration
    • absence of toxins that may inhabit functioning
  • enzymes work beat within a limited range, thus a constant and stable environment Is needed so that the enzymes will always work at optimum rate

pH and temperature

  • since all chemical reactions are controlled by enzymes and enzymes only function in a narrow temperature and pH range, anything outside these ranges causes a decrease in activity or the enzymes to denature and become unreactive


  • reactants must be present in order for a chemical reaction to take place
  • metabolites are chemicals that participate in chemical reactions in cells and a lack of metabolites may slow down or stop the chemical reactions

1.4 Describe homeostasis as the process by which organisms maintain a relatively stable internal environment

  • the mammalian body has best perfected keeping internal functioning constant, regardless of external changes
  • temperature regulation (predominately by skin and leaves)
  • control of chemical substances (blood vessel and vascular tissues)
  • homeostasis à the maintenance by an organism of a constant  internal state, regardless of external environmental change
  • maintains the concentration of metabolites or physical conditions in a narrow range that enables cells to function efficiently
  • all living organisms need careful control and constants to run smoothly, particularly in changing external environments


Homeostasis is defined as: the maintenance by an organism of a constant or almost constant internal state, regardless of external of external environmental change (e.g. temperature).

In order to maintain a constant internal environment, the following two steps are essential:

  • Detect the change
  • Counteract the change

Detecting the change:

Sensory cells or receptors present within the body detect change in temperature and/or chemical composition within the body. This change in the environment is called stimulus (e.g. light is a stimulus)

Counteracting the change:

Effector organs (such as muscles or glands) then work to reverse the change. A response that successfully reverse the change will return the body to homeostasis – its relatively constant state

Homeostatic mechanisms ensure that variables (such as temperature, concentration of chemical substances) in the internal environment of an organism are maintained within a narrow range. If the fluctuation (increase or decrease) is large, a negative feedback mechanisms comes into operation in response to this change. It is termed negative as it counteracts the fluctuation returning it to homeostasis

1.5 explain the homeostasis consist of two stages

  • Homeostasis requires huge coordination and control, involving both endocrine and nervous system in mammals
  • Stage 1 – detecting change
    • Sensory cells or receptors present within the body detect change in temperature and chemicals composition
    • Change is call stimulus
  • Stage 2 – counteracting change
    • Effector organs work to reverse the change
    • Response that efficiently reverse the change returns the body to homeostasis
  • These mechanisms ensure that the internal environment Is maintained in a narrow range
  • It is fluctuation exceeds outside normal range and homeostasis is lost, a negative feedback mechanisms comes into operation
    • The term negative suggests that it counteracts the change to return the body back to its normal range

1.6 explain the role of the nervous system in detecting and responding to environmental changes

  • Detecting changes is a process co-ordinated by the nervous system, based on a negative feedback system

Introduction to the nervous system

  • Coordination
    • The function of the nervous system
      1. Detects information about internal and external environments
      2. Transmits information to control centre
      3. Information is processed to generate a response
    • Structure
      • Receptors – sensory cells
      • Control centre – CNS (brain and spinal cord)
      • Effectors – muscle and glands
      • Nerves – link all parts together to pass on messages through electrochemical nerve impulses
    • Stimulus – response pathway
      • Stimulus àreceptorsàCNSàeffectorsàresponse

Role of the nervous system in homeostasis

  • Coordinates a negative feedback system, detecting a stimulus that could alter internal balance and creating a response to counteract this process.
  • Pathway exist whereby a stimulus is detected by a receptor, message carried by nerve to a control centre and a response is triggered
  • Detecting change via stimuli
    • Sensory cells called receptors detect stimuli
    • These consist of simple cells scattered around the organism to complex sense organs eye, ear, tongue)
    • Some receptors are sensitive to internal change, called interoreceptors, these detect changes relating to homeostasis, pH, temperature and chemical composition, called thermoreceptors o chemoreceptors, photoreceptors, mechanoreceptors
  • Processing information
    • CNS made up by brain and spinal cord
    • Peripheral system made up of nerves to carry information in the form of electromechanical impulses
    • Information sent to CNS, develops new information to carry out a response, sent to effector organs by motor nerves
    • CNS designed to analyse and act on information and these which involve homeostasis are carried out without conscious awareness
  • Counteracting change
    • Response –reaction in an organism or tissue as a result of a stimulus
    • Carried out by effector organism, muscle or glands
    • Responses correct deviation, maintaining homeostasis

The role of the nervous system in thermoregulation

  • Detecting change
    • Thermorecpetors present inside and outside the body
    • Central receptors monitor temperature of the blood as it circulated through the brain and the hypothalamus acts as a control centre, detecting very small changes in temperature
  • Processing information
    • Hypothalamus is control centre for detecting any temperature change, so receptors do not have transmit information very far
    • Anterior hypothalamus – sends effectors messages for cooling
    • Posterior hypothalamus – sends effectors messages for heating
  • Counteracting change
    • Skin is the main organ for temperature regulation
    • Goosebumps create a layer attempting to trap heat, Arteries narrow to direct heat to the core and give paler complexion, shivering produces muscle contractions to generate heat and activity in thyroid glands increases to boost metabolism, all heat creating actions
  • Redness suggests blood is pumped closer to the surface to release heat more quickly, sweat glands remove heat by rapid evaporation, and the thyroid gland generates a slower metabolism, suggesting why we might become tired or lethargic on hot clays

1.7 identify the broad range of temperatures over which life is found compared with marrow limits for individuals

  • Chemical reactions is cells only take place in narrow spectrum, due to temperature sensitivity of enzymes
  • Most living things live at stable temperatures between 10 and 35 degrees Celsius, which is the optimal range extended from 0 to 45 degrees Celsius
  • Broad range of temperatures
    • Diverse array of living organisms can survive temperatures between -70 and 350 degrees Celsius
    • While this range overall is huge, individual species cannot manage this much change
    • Enormous variation in the earth’s temperature is catering for diverse species
  • Narrow limit for species
    • Species are comfortable in a range much smaller than all earths variation, as a result of their structural, behavioural and physiological adaptations
    • The temperature range in which a species can survive is called the tolerance range – this may be slightly altered by variation

1.8 compare the responses of a named Australian ectothermic and endothermic organisms to changes in the ambient temperature and explain how these assist temperature regulation

  • Terms ectothermic and endothermic relate to an organisms ability to regulate body temperature
  • Ectothermic – dependant on external sources (environmental) for heat energy, body temperature is the same as the external environment
    • g. fish, amphibians, reptiles, most invertebrates
  • Endothermic – rely on internal sources such as metabolic activity for heat energy, able to regulate temperature using feedback systems
    • g. birds and mammals
  • Ambient temperature – temperature of the environment immediately surrounding an organism
  • Internal heat – produced by cellular respiration and transported by blood
  • External heat – produced and lost by 4 main processes, conduction, convection, radiation and evaporation

Ectothermic organisms

  • Body temperatures are influenced by the ambient temperature, the organism can only change behaviour to regulate body temperature
  • Eastern brown snake
    • Found in hot, dry Australian areas
    • Usually active during the day but will become active at night if the temperature is too hot
    • If ambient temperature rises above tolerance level it seeks out shade during the day and/or increases activity during cooler nights
    • Gain additional heat basking in the sunlight
    • Reduce heat by becoming less active and slowing metabolism
    • Hibernates during prolonged periods of cold
  • Central netted dragon
    • Desert adapted lizard
    • Lies in sunlight and changes body positions in low ambient temperature to expose more body to sun rays
    • Retreats to share or burrow to decrease activity if ambient temperature is too high

Endothermic organisms

  • Body temperature tends to remain stable despite changes in ambient temperature
  • Able to adjust metabolic rate to control heat loss
  • Size is important Is important as smaller animals loss heat more quickly and therefore need faster metabolism
  • Bent-wing bat
    • Produced fat in warm conditions, storing it to be metabolised during periods of cold
    • Brown fat is used as needed to increase body temperature In the cold
    • Alter the flow of blood at surface, using sweating and evaporation to remove heat more quickly
  • Fairy penguin
    • Found along South Australian coastline
    • Have feathers to provide an insulating layer to reduce heat loss
    • Feathers lift off during cold conditions to provide greater area of insulation, and lies flat during the heat to help the penguin keep cool
    • Behavioural mechanisms – moves to water for cooling and huddle together for warmth

1.9 identify some responses of plants to temperature change

  • Changes in temperature effecting plant ability for functioning and growth
  • Important to maintain a stable internal environment for plants as well as animals
  • Responses to change in light, water availability and temperature are all linked to heat

Plant responses to high temerpatures

  • Since plant cannot move from the sun to the shade, their responses to high temperatures are mostly structural and physiological
  • Evaporative cooling (transpiration)
    • Stomata’s open, leading to water loss that decreases the plant temperature by evaporative cooling
  • Turgor responses (wilting)
    • Reduce the exposure of their surface area to sun and heat
    • In extreme heat, plants lose turgor in leaf cells, causing wilting to reduce area exposed to the sun
  • Leaf orientation
    • Some plants (eucalypts) are able to change the orientation of their leaves so they hang vertically downwards in hot weather – to reduce exposure to sunlight during the hottest periods of the day
    • Leafs also regulate stomates opening and closing
  • Leaf fall
    • Trees such as eucalyptus drop their leaves during hot climates to reduce the surface area able to absorb heat
  • Reseeding and re-sprouting in responses to heat
    • To ensure survival through harsh climates like bushfires, plants have come up with methods for using fire as means of sprouting o spreading seeds
    • Trees such as eucalyptus have epicormic buds that sprout after fires, and banksias have seed pods that need fire to prompt their release
  • Thermogenic plants
    • Flowers produce heat by altering metabolic rates when ambient temperatures drop

Plant responses to cold temperatures

  • Organic ‘anti-freeze’
    • Production of compounds that prevent water between plant cells from freezing, by reducing the freezing temperature of cytoplasm, cell sap and vacuoles
  • Dormancy
    • Responding to cold temperatures, deciduous trees lose leaves and undergo a period of dormancy, allowing them to survive low temperatures and little sunlight
    • Sometimes the pant parts above the ground … off, while parts below the ground remain dormant, waiting to grow in warmer weather
    • g. alpine ash uses dormancy to withstand cold
  • Vernalisation
    • Plants flower in response to low temperatures
    • g. tulip bulbs must undergo long exposure to cold before flowering

2.1 Identify the forms in which each of the following is carried in mammalian blood ; Carbon dioxide, Oxygen, Water, Salts, Lipids, Nitrogenous waste, Other products of digestion

  • Blood plays an important role in homeostasis, distributing heat and acting as a buffer to maintain pH levels
  • Extremely complex body tissue
  • Functions in the transport of a wide variety of chemical substances
  • To maintain homeostasis, chemicals must be transported in a specific form at a particular concentration
  • Four main functions of the mammalian circulatory system
    • Transport gases, nutrients, water and waste
    • Blood clotting helps with damage repair and sealing wounds
    • Defence against disease through white blood cells and immune system
    • Temperature regulation, related primarily to homeostasis

Composition of Blood

  • Plasma makes up 55% of blood
    • Stikcy, slightly yellow and salty liquid
  • Red blood cells – erythrocytes
    • Contain haemoglobin
    • 5-6 million in every mL of blood
    • Produced in bone marrow
  • White blood cells – leucocytes
    • Built to fight disease
    • 4-12 thousand in every mL of blood
    • Produced in lymph nodes and glands
  • Platelets
    • Fragments of old cells
    • Function is to clot blood
    • Made in bone marrow

Oxygen Transport

  • TWO main things happen:
    • Most oxygen combines with haemoglobin in the red blood cells as it diffuses from the respiratory surface of the lungs into the blood
    • A very small proportion may dissolve and travel in plasma
  • Red blood cells are ideally adapted to carrying oxygen, providing ample space for the carrying of many haemoglobin
    • Haemoglobin has an affinity for oxygen
    • Each red blood cell has around 250 haemoglobin molecules, resulting in a very high oxygen carrying capacity
  • The slightly flattened, biconcave shape of red blood cells gives them a larger SA: vol ratio for easy diffusion of oxygen across the surface
  • The binding of haemoglobin and oxygen forms oxyhaemoglobin, giving blood its bright red colour as it is oxygenated, as opposed to the dark red colour of deoxygenated blood
    • Oxygenated blood is carried by arteries
    • Deoxygenated blood is carried by veins

Carbon Dioxide Transport

  • THREE main things happen:
    • Most carbon dioxide is carried as carbonic acid, which form hydrogen carbonate ions in red blood cells to regulate the pH of the bloodstream
    • Remaining carbon dioxide is dissolved in the plasma
    • Also carried by combining with haemoglobin
  • Carbon dioxide is produced as a waste product of respiration, diffuse from cells into the bloodstream, which transports it to the respiratory surface of lungs for diffusion out of the system
  • A large proportion of carbon dioxide enters red blood cells
    • Most form hydrogen carbonate ions which move out of the red blood cells into the plasma
    • Some binds to haemoglobin, forming carbaminohaemoglobin

Water and Salts

  • Water is the transport medium for all substances in the body – forms the basis of cytoplasm, tissue fluids, and blood
  • Water is the solvent in plasma, forming about 90% of it
  • Salts are carried in blood as ions, charged particles, dissolved in plasma
  • Substances that become ions in these solutions are often referred to as electrolytes, as a result of their ability to conduct electricity, and they are essential for normal body functioning

Lipids and other products of digestion

  • Digestions aims to break large molecules down to a size small enough for absorption through the intestine wall and into the bloodstream, so they can be transported easily to cells where they are required
  • Glucose and Amino Acid Transport
    • Products are water soluble, transported in the bloodstream dissolved in plasma
    • Other substances such as vitamins and glycerol and transported in a similarly dissolved state
  • Lipid Transport
    • Most end products are not water soluble and so cannot be dissolved in plasma
    • Digested lipids are converted into triglycerides in the small intestines
    • Triglycerides, along with phospholipids and cholesterol, are wrapped in a coat of protein to form a package called chylomicron. This occurs during the absorption process in the villi of the small intestine
    • These are released into the lymphatic circulation system, and eventually join the main blood supply by emptying into veins

Nitrogenous Wastes

  • Nitrogenous wastes are harmful substances produced in the body as a result of the breakdown of proteins (e.g. ammonia, which is converted to urea as ammonia is toxic and cannot be carried by the body)
  • Transported by dissolving in plasma

2.2: explain the adaptive advantages of haemoglobin

Structure of haemoglobin:

Protein made up of 4 pay-peptide chains bonded to form a ‘haem – containing iron group.

Necessary iron for haemoglobin is obtained to form diet, creating red pigment molecules

Adaptive advantages

  • Increases the oxygen carrying capacity of blood
    • One haemoglobin molecule has the ability to bond with 4 oxygen molecules
    • Far more oxygen can be carried in the blood by haemoglobins than by dissolving in plasma
  • Ability to bond oxygen increases once the first molecule bonds
    • Bonding of each oxygen molecule causes haemoglobin to change shape slightly and make it easier for subsequent molecules to bond
    • Increases rate of oxygen uptake to increase efficiency
    • Small oxygen increase in the lungs causes a rapid in blood oxygen saturation
  • Capacity to release oxygen increases in presence of carbon dioxide
    • It is important for oxygen to be freed to cells for respiration when the concentration is low, and thus the alternative ability of haemoglobins to release oxygen is crucial
    • Metabolising cells release carbon dioxide which creates carbonic acid and lowers the pH of blood. Haemoglobins release oxygen to these metabolising sites for respiration

2.3 Compare the structure of arteries, capillaries and veins in relation to their function

2.4 describe the main changes in the chemical composition of the blood as it moves around the body and identify tissues in which these changes occur

  • The circulator system in mammals is like a road system within a city
  • Arteries and veins carry blood away and to the heart
    • Arteries carry blood away – as t is oxygenated
    • Veins carry blood to the heart as it is deoxygenated
  • Like roads have two sides – arteries and veins running to and from the same organ are usually side by side
  • The transport system within the body is involved in moving four basic groups of chemical
    • Gases – carbon dioxide and oxygen
    • Nutrients
    • Wastes
    • Hormones

The importance of the transport system in assisting the metabolic functioning

  • The chemical functioning of cells (metabolism) relies on the correct balance of chemical reactants being brought to cells and the removal of waste products
  • Energy is the basis of all metabolic functioning
    • Usually by means of cellular respiration
    • Depends on the correct balance of nutrients –glucose and oxygen
  • Once the reactants reach the cells, cellular respiration occurs to produce energy in the form of ATP
  • Carbon dioxide is a toxic waste product that must be removed to prevent a change in the pH of bodily fluids which could in turn effect enzyme functioning
  • Nitrogenous wastes are the end products of protein breakdown that occurs during metabolic functioning
  • All wastes are carried from their sites of production to a site of removal
    • Carbon dioxide is taken to the lungs
  • The blood vessels are responsible for the transportation of wastes
  • The transport of hormones – chemical messenger molecules produces by endocrine glands
    • Ductless glands and therefore pour their secretions directly into the bloodstream – which transports them to the target organs
  • Hormones that control water and salt balance are essential to assist homeostasis

The changing chemical composition of blood

  • The difference in chemical concentration of blood leaving or entering that organ depends on the function of that organ
    • External gaseous exchange occurs in the lungs
      • CO2 is released and oxygen is picked up
    • Internal gaseous exchange occurs in all organs of the body and is the result of cellular respiration
    • Absorption of nutrients into the bloodstream takes place in the digestive tract
    • Nitrogenous waste products are produced in the liver and excreted by the kidneys
    • Hormones are secreted into the blood by glands and they then travel to where they are required and used by target tissue

Change in carbon dioxide and oxygen content of blood

  • The lungs are organs of external gaseous exchange in body the body
  • Deoxygenated blood arrives at the lungs are drops off carbon dioxide and picks up oxygen
  • The haemoglobin in red blood cells binds with 4 oxygen molecules and is carried in the form of oxyhaemoglobin
    • A very small proportion less than 1.5% can dissolve in plasma
  • Internal gaseous exchange occurs in the issues of the body as a result of cellular respiration
  • Cells release carbon dioxide which diffuses into the blood capillaries in the tissues
  • When carbon dioxide enters the blood some of it dissolves in the plasma and some is carried by haemoglobins and the rest is transported in form of bicarbonate ions
    • All of which make up deoxygenated blood travelling back to the heart in veins

Changes in other chemicals in blood

  • An increase in oxygen and a decrease in carbon dioxide concentrations are evident in blood that has passed through the lungs
  • A decrease in oxygen and an increase in carbon dioxide is evident in blood that has passed through any organ other than the lungs
  • An increase in digestive end products is evident in blood that has passed through an organ involved in absorbing digested food. These products of digestion travel in the bloodstream from the digestive tract directly to the liver
  • The liver is the centre of the food metabolism. A decrease in digestive end products is evident in blood that has passed through the liver
  • An increase in nitrogenous wastes is evident In blood that passes through the liver
  • A decrease in nitrogenous wastes is evident in blood that has passed through the kidneys

Pulmonary circuit – heart and lungs

  • Blood flows from the heart to lungs and then back to the heart
  • Blood is under lower pressure then in the systemic circuit
  • However the rate of blood flow is faster
  • The blood, having just returned from the body, contains high CO2 levels and low oxygen levels
  • In the lungs blood exchanges CO2 for oxygen

Systemic circuit

  • Blood flows from the heart to the body (except the lungs) and returns back to the heart
  • Blood is under higher pressure due to contractions of the left ventricle of the heart, but pressure gradually lessens the further away from the heart it moves
  • In the tissues
    • Blood gives up oxygen and other ions and nutrients
    • Waste products, e.g. urea, CO2 entre the blood


  • Blood loses urea and has the composition of water and salt balanced


  • Blood collects the products of digestion
  • Levels of glucose, lipids and amino acids rise


  • Regulates the level of glucose in the blood
  • Excess glucose is converted to glycogen and is stored
  • Converts excess amino acids to urea

Recall that the transport system in the mammalian body is involved in moving four main groups of substances

  • Gases
  • Nutrients
  • Wastes
  • Hormones

Hormones are secreted into the blood by glands. They then travel to the target organ.

Internal and external gaseous exchange

  • External gaseous exchange at the lings. Deoxygenated blood arrives with a high CO2 Oxygenated blood then travels to the rest of the body. Here internal gaseous exchange occurs as a result of cellular respiration
  • Remember O2 + C6H12O6 à CO2 + ATP + H2O
  • Cells release CO2 when diffuses into the blood capillaries in the tissues

What changes in levels represent

  • Decreased CO2, increased O2 à blood has travelled through the lungs
  • Decreased O2, increased CO2 à blood has travelled and respiration has occurred
  • Increased lipids, glucose and amino acids à blood has passed through the digestive tract
  • Decreased lipids, glucose and amino acids à blood has been through the liver – metabolism has occurred
  • Increased nitrogenous wastes à blood has passed through the liver
  • Decreased nitrogenous wastes à blood has passed through the kidneys

2.5 outline he need for oxygen in living cells and explain why removal of carbon dioxide from cells is essential


  • is needed in living cells for respiration. Oxygen is needed in respiration to breakdown glucose in order to produce energy in the form of ATP


  • Oxygen + glucose à water + ATP + carbon dioxide
  • All living things that metabolise glucose require oxygen à which is supplied via the lungs and carried by the haemoglobin in red blood cells
  • As a result of respiration carbon dioxide is produced
  • When carbon dioxide dissolves in water carbonic acid is produced. If CO2 were to accumulate the it would affect the overall pH of the blood à remember the enzymes require a narrow optimal range
  • Likewise a change in pH would reduce he binding ability of haemoglobin to CO2
  • Hence oxygen must constantly be supplied and CO2 constantly removed for cells to respire efficiently

2.6 describe current theories about the processes responsible for the movement of materials through plants in xylem and phloem tissue


  • The transpiration stream in xylem occurs due to physical forces that result from water being moved by passive transport
  • A column of water is sucked up by the evaporative pull – known as the transpiration stream
  • Once absorbed into the roots the water/mineral ions moves across to the root to the xylem
  • A small amount of root pressure occurs due to the continual influx of water forcing the solutions already present in the xylem upwards
  • This pressure however is not sufficient enough to move the water solution high enough
  • Most movement is caused by the transpiration stream – drawn up to replace water loss in leaves by transpiration
    • Xylem are hallow and narrow – allowing very little resistance of water flow
    • The physical properties of water contribute to the formation of a continuous stream: Adhesive forces lead to capillarity and cohesive forces
  • A concentration gradient consists across the leaf
    • At leaf surface osmotic pressure is high (low water concentration) due to continual loss of water due to evaporation
    • In the centre of the leaf (xylem, tissue, veins) the osmotic pressure is low
  • Water loss at the leaf surfaces results in the osmotic movement of water across an adjacent internal cells into those that have just lost water. This osmotic flow continues across the leaf right into the xylem tissue. When the molecules leave the xylem this creates tension in the column of water rising up the xylem. Because of adhesion and cohesion the water column does not break but the whole column is pulled upwards. This creates a transpiration stream


  • Moves products of photosynthesis by active transport
  • Up to 90% of dissolved substances in the sap is sucrose – when sucrose reaches the cells it may be converted back into glucose for respiration or to starch storage
  • The mechanism of flow is driven by an osmotic pressure gradient – generated by differences in sugar and water concentrations
  • Involves active loading of sugar into phloem at one end (the source) followed by active unloading from phloem into surrounding tissues at the other end (the sink)
  • The loading of sugar attracts water due to difference in osmotic pressure and offloading at the sink causes water to flow out also – this is known as the pressure flow
  • Loading at the source
    • Amino acids, sucrose and other minerals are loaded at the source. Two theories as to how this occurs is
      • Symplastic loading – sugars and nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata
      • Apoplastic loading – sugar and nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube – these sugars pass into the tube through active transport
    • As sugars enter the phloem the sap becomes more concentrated making the osmotic pressure at the source end high drawing in water from the adjacent xylem tissue
  • Offloading at the sink
    • Materials flow to the sink.
    • At the sink sugars and materials are removed by active transport
    • As sugars move out they draw water with them by osmosis resulting in lower osmotic pressure (due to high water concentration) at the sink
  • Pressure flow – along the path
    • The difference in osmotic pressure between the source and the sink drives the sap flow.
    • The direction of flow depends on where the sink areas (roots or flowers) of the plant are in relation to the source (leaves)
    • Water can move into the phloem through osmosis at any point of the gradient
    • The flow is continuous because sucrose is continuously added at one end and removed at the other

3.1 explain why the concentration of water in cells should be maintained within a narrow range for optimal function

  • water makes up around 70-90% of living things, it is essential for life
  • water is the solvent that forms the basic aquatic medium of cytoplasm in cells and also of the body fluids like blood, interstitial fluids and digestive fluids
  • Water is also the transport medium in plants. Acting as a medium for the translocation of ions in xylem and sugars in phloem
  • water is the solvent of all metabolic reactions in living cells and sometimes directly takes part in it e.g. respiration, osmosis
  • recall
    • Isotonic: concentration of solutes outside the cell is the same as inside the cell. No overall movement of water
    • Hypertonic: concentration of solutes is greater outside the cell than inside. Water tends to move out of the cell
    • Hypotonic: concentration of solutes is greater inside the cell than outside. Water tends to move inside the cell
  • Living cells work best in an isotonic environment
  • Any change in the concentration of solutes will result in a change in the levels of water in cells
  • This usually results in death
  • This is why the concentration of water must be kept constant: to ensure the proper functioning in living cells

3.2 explain why the removal of wastes is essential for continued metabolic activity

  • any accumulation of wastes can be toxic to cells and therefore must be removed to maintain homeostasis
  • if not removed the waste will accumulate and alter the conditions of the internal environment
  • This in turn inhabits the enzyme functioning and prevents cells from undergoing normal metabolic functioning. For example
    • Builded up of nitrogenous wastes such as ammonia, which causes increase in pH in cells making them more alkaline
    • Carbon dioxide accumulation which lowers pH resulting in a move acidic internal environment
  • These changes slow down or alter enzyme functioning

3.3 identify the role of the kidney in the excretory system of fish and mammals

  • in fish and mammals the kidneys are excretory and osmoregulatory organs
    • marine fish: kidneys conserve water, excrete excess salts and nitrogenous wastes
    • freshwater fish: kidneys excrete excess water and nitrogenous wastes and conserve salt
    • mammals: kidneys conserve water and salt when required, excrete excess water and salts and nitrogenous wastes
  • the concentration of water in the immediate surroundings determine the need to conserve water or lose it
  • marine fish
    • urinate less and lose body water to osmosis
    • excess salt tends to accumulate in their bodies – moving in by diffusion from surrounding sea water
    • the kidneys therefore kidneys conserve water, excrete excess salts and nitrogenous wastes
    • marine fish tend to drink salt water, remove salt and use water for metabolic functions
    • kidneys has small glomeruli
  • freshwater fish
    • live in rivers and lakes where water potential is high – contain very few dissolved salts and therefore water is available freely
    • freshwater fish urine frequently as water tends to accumulate in there tissue as a result of passive movement by osmosis
    • these fish are faced with problem of having too much water present in their bodies
    • therefore the kidneys excrete excess water and nitrogenous wastes and conserve salt
    • kidney has large glomeruli
  • terrestrial mammals
    • lose water and solutes as a result of evaporation form the lung surface during respiration and as a result of excretion
    • the kidney excrete urine that is composed of nitrogenous wastes (urea), water, and some excess salts
    • the kidney can adjust the reabsorption of nitrogenous wastes, water and salts
    • mammals have a complex control mechanism to ensure that balance is maintained between the amounts of sweat and urine excreted
    • in hot weather more water is excreted as sweat and less urine and vice versa in cold weather
    • a large amount of salt is lost during sweating and therefore must be replaces to maintain stable osmotic pressure
    • any adjustment to water and salt levels in urine is brought about by the actions of hormones ADH and aldosterone on the kidney tubes

3.4 explain why the process of diffusion and osmosis are inadequate in removing dissolved nitrogenous wastes in some organisms

  • both diffusion and osmosis are types of passive transport and therefore require no energy
  • problems with relying on diffusion
    • the rate of movement is too slow. Nitrogenous wastes and toxins must be dissolved in water when they are removed and therefore the wastes would only be able to move if the concentration inside the bloodstream and cells was higher than the concentration outside. As it equalised it would slow and eventually stop
    • not all wastes can be removed by diffusion e.g. nitrogenous wastes require active transport to remove the wastes against a concentration gradient
  • problems with relying on osmosis
    • too much water may be lost in urine. Urine contains a large amount of nitrogenous wastes in solution. Water will be drawn into the urine by osmosis to dilute the wastes and try to equalise concentration of urine and surrounding kidney leading to a large loss of water
    • movement of water may make wastes too dilute for excretion by diffusion. Osmosis results in water moving into the body tissues, this dilutes the toxic wastes in the body it also slows down their excretion by diffusion (as it lowers the concentration gradient). Therefore active transport is required to move against a concentration gradient
  • solution – combined active transport and osmosis
    • active transport requires energy but is much quicker and more effective at removing wastes against a concentration gradient
    • it can also be used to pump salts from the urine back into the kidney tissues and these will in turn draw water into the kidneys through osmosis and therefore ensuring the water lost in urine does not affect the bodies water balance

3.5 distinguish between active and passive transport and relate to these processes occurring in the mammalian kidney

  • Passive movement includes the process of diffusion or osmosis. These require no energy as the molecules move along a concentration gradient
  • active transport moves against a concentration gradient and therefore requires energy
  • passive transport in the mammalian kidney
    • diffusion and osmosis result from the random movement of particles called Brownian motion – whereby particles continuously collided and move randomly
    • the main limitations of passive movement is it relies on the presence of a difference in concentration of substances between two regions and that It is relatively slow especially when the concentration gradient is not steep
    • Diffusion is the movement of any molecules from a region of high concentration to a region of low concentration until equilibrium is reached. With no energy input
    • Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane. No energy input is required
    • within the kidney the movement of substances between the bloodstream and the excretory fluid in the microscopic tubules called nephrons involves both active and passive transport
    • a balance in the optimal concentrations of blood chemicals is maintained by a selective excretion of wastes as wells as any excess water and salts in urine
    • therefore the ability of the kidney to alter the urine concentration plays a vital role in homeostasis
    • within the kidney tubules there is a two way movement of substances
      • wastes substances pass from the bloodstream into the kidney tubules to be excreted in urine (filtration and secretion)
      • substances required by the body are removed from the urine in the kidney tubules and returned to the bloodstream (reabsorption)
    • active transport in the mammalian kidney
      • active transport is the movement of molecules from an area of low concentration to an area of high concentration against a concentration gradient requiring an input of energy
      • active transport involves a carrier protein that spans the membrane and this carrier molecule can actively move chemicals from low concentration to high using cellular energy
      • active transport mainly moves sodium ions, glucose, amino acids and hydrogen ions across the wall of the nephron
        • glucose and amino acids are reabsorbed by kidney cells so they are not lost in the urine (move against concentration gradient)
        • additional nitrogenous wastes and hydrogen ions are added to urine from blood capillaries in the kidney tubule

a ‘sodium pump’ mechanism operates in the tubules, actively transporting salt ions from the urine back into the kidney cells. Besides conserving salts this process also brings about the conservation of water within the body as the active transport of salts draw water out of the urine by osmosis

3.6 explain how the process of filtration and reabsorption in the mammalian nephron regulate body fluid composition

  • The nephron is a regularly unit, it absorbs and secrets substances in order to maintain homeostasis – this regulation maintains the constant composition of body fluids

Proximal tubule

  • Bicarbonate ions are reabsorbed into the blood, H+ ions are secreted out – remember this maintains constant ph
  • Drugs e.g. penicillin and other poisons are secreted out
  • Regulation of salts occurs here – sodium ions(Na+) actively reabsorbed along with chlorine ions (Cl-) and potassium ions (K+)

Loop of henle – descending and ascending limb

  • Descending is permeable to H2O, not salt
  • H2O Passes out of nephrons by osmosis
  • Ascending is permeable to salt not h2o

distal tubule

  • selectively reabsorption of sodium ions and potassium ions

collective duct

  • end of nephron, connect to ureter
  • walls are permeable to H2O only à final filtrate is called urine

3.7 outline the role of the hormones aldosterone and ADH in the regulation of water and salt levels in blood

  • hormones are chemical control substances that are secreted by endocrine glands directly into the blood stream.
  • The passage of filtrate from bowman’s capsule to the distal parts of the nephron occur by travel through general circulation to parts of the body until the required target is reached
  • Adjustments to the concentration of water and salts within the urine takes place mainly in the distal parts of the tubules by alterations to the permeability of the membranes of cells lining the nephron walls
  • Aldosterone
    • Brings about retention (conservation) of salts within the body
    • A change in the concentration of the sodium levels in the bloodstream stimulates the required response from the cortex of the adrenal gland (above the kidney) to release aldosterone
    • When aldosterone reaches the kidney it increases or decreases the salt reabsorption to the required levels e.g.
    • High Salt Levels:
      • High blood volume and blood pressure due to water diffusing in.
      • Levels of aldosterone decreased.
      • Less salt reabsorbed, less water diffusing in
      • Salt level decreased, blood volume and pressure decreases
    • Low salt levels
      • Low blood volume and blood pressure due to water diffusing out.
      • Levels of aldosterone increased.
      • More salt reabsorbed, more water diffusing in
      • Salt levels increase, blood volume and pressure increase
    • ADH – anti-diuretic hormone
      • Brings about water reabsorption within the body
      • Also called vasopressin
      • Controls the reabsorption of water by adjusting the permeability of the collecting ducts and the distal tubules.
      • It is made in the hypothalamus in the brain, but stored in the pituitary gland
      • Receptors in the hypothalamus monitor the concentration of the blood:
      • High Salt Concentration:
        • ADH levels increased, collecting ducts and distal tubules become more permeable to water, more water reabsorbed, concentration returns to normal. (Concentrated urine)
      • Low Salt Concentration:
        • ADH levels increased, collecting ducts and distal tubules less permeable, less water absorbed, concentration returns to stable state. (Dilute urine)
      • ADH does not control the levels of salt in the blood. It only controls the concentration of salt through water retention.

3.8 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 response to variations in the environment – extreme variations
  • this occurs specifically in estuarine environments where the salt concentration fluctuates
  • the survival of species that live in environments such as an estuary, where salt and water concentrations fluctuate broadly on a daily basis depends on their ability to avoid these changes or to tolerate them
  • organisms that move between salt water and freshwater experience the same fluctuations in environmental conditions
  • maintaining water and salt balance
    • at high tide – sea water flows into river mouth – creating high salt concentration than the cytoplasm of the cells and body fluids in organisms. This salt increase tends to draw out water from cells by osmosis
    • at low tide – sea water flows out of the river mouth and freshwater flows out of the estuary. Plants and animals in the estuary face the challenge of water moving into living tissue
  • living organisms employ one of two strategies in enantiostasis
    • osmoconformers – are organisms that tolerate the changes by altering the concentration of their internal solutes to match the external environment
    • osmoregulators – are organisms that avoid changes in their internal environment and have the ability to keep the solutes at the optimal level regardless of the changing external environment. These organisms are unable to tolerate a range of salt concentrations in their body fuilds and cells and therefore have mechanisms to exclude salt to keep the internal fluid concentration constant