Exchange and Transport.

Biology AQA new AS level unit 2, exchange and transport.

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Exchange Between Organisms and Their Environment.

  • Once absorbed, materials must be rapidly distributed to the cells that require them & the waste products removed. This requires a transport system.
  • Size & metabolic rate affect amount of each material that needs to be exchanged.
  • E.g. respiratory gases, nutrients, excretory products & heat.
  • The exchange can only take place passively by diffusion & osmosis or actively by active transport.
  • For exchange to be effective the s.a. of the organism must be large compared with its volume. With larger animals their volume increases faster than surface area so simple diffusion is only useful for relatively inactive organisms.
  • Organisms have evolved to become flat so that no cell is far from the surface or have specialised exchange surfaces with large areas e.g. lungs/gills. To be effective exchange surfaces must:
  • have a large s.a. to volume ratio to increase exchange rate.
  • be very thin for a short diffusion distance.
  • be partially permeable to allow materials to pass without obstruction.
  • have movement of the environmental and internal mediums to maintain gradient.
  • As they are thin, exchange surfaces are easily damaged so are inside & need a way of moving the external medium over the surface e.g. ventilating the lungs.
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Gas Exchange in Single-Celled Organisms and Insect

These are small so have a large s.a. to volume ratio. O2 is absorbed by diffusion across body surface which is only covered by a cell-surface membrane.


  • Most are terrestrial (live on land) so H20 easily evaporates from body surface & they become dehydrated. Gas exchange requires a thin permeable surface which conflicts with the need to conserve H20. To reduce H20 loss they have:
  • 1. Waterproof coverings
  • 2. Small s.a. to volume ratio - to minimise area H20 is lost from.
  • This means they cannot use their body surface for diffusion of gases.
  • They have an internal network of tubes (tracheae) supported by rings. They divide into smaller tubes (tracheoles) with extend through all the body tissues.
  • Respiratory gases move in 2 ways:
  • 1. Along a diffusion gradient - when cells are respiring O2 is used up so concentration at end of tracheoles falls, this creates a gradient so air moves from atmosphere to tracheoles. When cells respire they release CO2 which creates a gradient in the opposite direction so CO2 moves into atmosphere.
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Insects Continued.

  • 2. Ventilation - the movement of muscles creates mass movements of air which speeds up the exchange.
  • Gases enter & leave tracheae through tiny pores - spiracles - on body surface.
  • They can be opened & closed by a valve.
  • When open H20 can evaporate so much of the time they keep them closed.
  • The tracheal system is efficient but relies on diffusion which needs a short pathway to be effective, this limits the size insects can be.
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Gas Exchange in Fish.

  • Fish have a waterproof and therefore gas-tight outer covering.
  • They have a small s.a. to vol ratio as are large so developed exchange surface:
  • The gills are inside the fish & are made of gill filaments which are stacked in a pile.
  • At right angles to the filaments are lamellae which increase s.a.
  • H20 taken in through the mouth, forced over the gills and out through an opening.

Countercurrent Flow:

  • Countercurrent flow - flow of H20 over lamellae & flow of blood within them go in opposite directions.
  • This is important for max possible gas exchange.
  • Blood is already well loaded with O2 when it meets H20, which has its max concentration of O2. So, diffusion of O2 from H20 to blood takes place.
  • Blood with little O2 meets H20 which has had most of its O2 removed. So, it diffuses from H20 to blood.
  • There is a fairly constant rate of diffusion across entire lamellae .
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Gas Exchange in Leaves.

As plants photosynthesise as well as respire the gases produced in 1 can be used in another which reduces the need for gas exchange with the air. When photosynthesis takes place although some CO2 comes from respiration, the majority comes from the air. Most of the O2 diffuses out of the cell. When photosynthesis is not occurring O2 diffuses into the leaf as it is constantly being used in respiration. CO2 produced from respiration diffuses out.

Structure of a Plant Leaf and Gas Exchange - no living cell is far from external air; diffusion takes place in air which is faster than in H20. There is a short, fast diffusion pathway. It also has a very large s.a. compared to vol so no specialised transport system needed. Most exchange occurs in leaves which are thin & flat for large s.a.; have many small pores (stomata) in lower epidermis; and have numerous connecting air spaces throughout mesophyll.

Stomata - minute pores on the underside of leaves. Each stoma is surrounded by a pair of guard cells which open & close the stomatal pore to control the rate of exchange. This is important as terrestrial organisms lose H20 by evaporation, when H20 loss is excessive the fully/partially close the stomata.

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Circulatory System of a Mammal.

Large organisms need a transport system because:

  • Diffusion not efficient over larger distances.
  • They need to exchange materials with the environment.
  • s.a. to volume ratio too small so organisms' needs cannot be met.
  • Specialist exchange surface needed to absorb nutrients & respiratory gases & remove excretory products.
  • Takes materials from cells to exchange surfaces and vice versa.
  • Materials need taking around body, between exchange surfaces & the environment.
  • Transport system occurrence depends on s.a. to vol ratio & activity of organism.

Common Features of Transport Systems:- suitable medium to carry materials, normally liquid/H20 as it dissolves substances easily & can be moved; mass transport for large distances; closed system of tubular vessels containing transport medium, mechanism for moving medium in vessels (requires pressure difference by muscular contraction in animals or passive processes in plants); mechanism to maintain direction of flow; way of controlling flow to suit changing needs.

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Transport Systems in Mammals.

  • Closed blood system.
  • Muscular pump (heart) to circulate blood.
  • Double circulatory system - blood passes twice through heart in 1 circuit as blood loses pressure in lungs so would be too slow if continued on.
  • High body temperature therefore high metabolism so need substances quickly.
  • Final part of journey into cells is by diffusion.
  • Almost all cells are within 1mm of a capillary.

Heaptic portal vein - transports digested food.

Heaptic artery/vein - blood to/from liver.

Pulmonary artery/vein - blood to/from lungs.

Renal artery/vein - blood to/from kidneys.

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Structure of Blood Vessels.

Different types of blood vessel:

  • Arteries - from heart to arterioles.
  • Arterioles- smaller arteries controlling blood flow to capillaries.
  • Capillaries - tiny vessels linking arterioles to veins.
  • Veins - from capillaries to heart.

Arteries, arterioles and veins have the same basic layered structure:

  • Tough, outer layer resisting pressure changes.
  • Muscle layer that contracts to control blood flow.
  • Elastic layer which maintains blood pressure by stretching and springing.
  • Thin inner layer (endothelium) that is smooth to stop friction and allow diffusion.
  • Lumen which is the central cavity through which blood flows.

Arterioles are smaller in diameter and have a relatively larger muscle layer and lumen compared to arteries.

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Artery/Arteriole Structure Related to Function.

Artery Structure:

  • Thick muscular layer so can be constricted/dilated to control volume of blood flow.
  • Thick elastic layer to keep pressure high & ensure blood reaches extremities. Systole = stretches; diastole = recoils to maintain pressure.
  • Large overall thickness stops bursting under pressure.
  • No valves as high pressure stops backflow.

Arteriole Structure:

  • Carry blood under lower pressure and control blood flow.
  • Thicker muscular layer than arteries - contraction allows constriction of lumen to restrict blood flow and control movement to capillaries.
  • Thinner elastic layer as pressure is lower.
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Vein/Capillary Structure Related to Function.

Vein Structure:

  • Transport blood slowly under lower pressure.
  • Thin muscle layer as constriction/dilation can't control flow.
  • Thin elastic layer as won't burst as pressure is lower & there's no recoil.
  • Overall small wall as no burst risk, flattened easily to aid flow.
  • Valves to stop backflow due to low pressure. When muscles contract veins compress & pressurise blood, valves ensure 1 direction of flow.

Capillary Structure:

  • Exchange metabolic materials between blood & cells. Slow flow for exchange.
  • Wall only made of lining layer so short diffusion distance & rapid diffusion.
  • Numerous & highly branched so large s.a. for diffusion.
  • Narrow diameter so permeate tissues (close to all cells).
  • Narrow lumen so red blood cells squeezed flat against side so closer to cells, reducing diffusion distance.
  • Spaces between endothelial cells allowing white blood cells out.
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Formation of Tissue Fluid.

Tissue fluid is a watery liquid containing glucose, amino acids, fatty acids, salts & O2. It supplies substances to tissues & receives CO2 & waste. It is the means for exchange between blood & cells. Formed from blood plasma & controlled by homeostatic systems. Formation:

  • Blood pumped into smaller vessels creating hydrostatic pressure at arterial end of capillaries.
  • Pressure forces fluid out of blood plasma.
  • Outward pressure opposed by:
    • hydrostatic pressure of tissue fluid outside capillaries, stopping outward movement of liquid.
    • Lower water potential of blood due to plasma proteins, pulls H2O back into blood.
    • Combined effect creates overall pressure pushing tissue fluid out of capillaries. Pressure only forces small molecules out, leaving cells & proteins in blood, this filtration under pressure = ultrafiltration.
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Return of Tissue Fluid to Circulatory System.

Returned once it has exchanged metabolic material with cells it bathes. Most return to plasma via capillaries.

  • 1. Loss of tissue fluid from capillaries reduces hydrostatic pressure inside.
  • 2. By the time blood reaches venous end of capillaries its hydorstatic pressure is less than tissue fluid outside it.
  • 3. Therefore, tissue fluid forced into capillaries by higher pressure outside them.
  • 4. Also, osmotic forces (proteins in plasma) pull H2O back in capillaries.

Fluid loses O2 & nutrients by diffusion but gains CO2 & waste. Not all can be returned, remainder carried back by lymphatic system (like capillaries but gradually merge into larger vessels forming network). Larger vessels drain contents into bloodstream via 2 ducts joining veins near heart. Contents moved in 2 ways:

  • 1. Hydrostatic pressure - of tissue fluid that's left capillaries.
  • 2. Contraction of body muscles - squeeze lymph vessels, valves ensure fluid moves away from tissues to heart.
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Movement of Watrer Through Roots.

Waterproof layer means cannot absorb H2O over general body surface, special exchange surface in soil = root hairs.

  • Plants lose H2O by transpiration, replaced through root hairs - long, thin extensions of root epidermal cell that remain functional for a few weeks.
  • Large s.a. as are very long & there are 1000s on each root.
  • Thin surface layer for materials to move across.
  • Arise from epidermal cells behind tips of young roots.
  • Grow into spaces around soil particles.
  • Soil solution has high water potential, roots have much lower do H2O moves by osmosis from soil down concentration gradient.
  • After absorbed H2O continues across root in 2 ways:
    • Apoplastic pathway (apoplast).
    • Symplastic pathway (symplast).
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Apoplastic and Symplastic Pathways.


  • As H2O drawn into endodermal cells it pulls more in behind it due to cohesive property of H2O molecules. This creates tension that draws H2O along cell walls of cells of root cortex.
  • Mesh-like structure of cell walls has water-filled spaces so there is no resistance.


  • Takes place across cytoplasm of cells of the cortex by osmosis.
  • H2O passes through walls along plasmodesmata (tiny openings), each filled with strand of cytoplasm to form a continuous column from root hair to xylem.
  • 1. H2O enters by osmosis & increases water potential of root-hair cell.
  • 2. Cell now has higher water potential than 1st cell in cortex so H2O moves to 1st cell by osmosis down concentration gradient.
  • 3. 1st cell now has higher water potential than neighbour so H2O moves etc.
  • 4. At same time, this loss of H2O decreases water potential of cell causing more water to enter and the process repeats. A water potential gradient is set up.
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Passage of Water into Xylem.

  • When H2O reaches endodermic by AP pathway the waterproof band that makes up Casparian ***** prevents it progressing further along cell wall.
  • H2O forced into living protoplast of cell where it joins H2O from SP pathway.
  • Active transport of salts is most likely mechanism of H2O entering xylem.
  • Endodermal cells transport salts into xylem, requiring energy.
  • Takes place along carrier proteins in membrane.
  • H2O must 1st enter cytoplasm of endothelial cells to enter xylem.
  • Active transport of mineral ions into xylem creates lower water potential in xylem.
  • H2O moves into xylem by osmosis along water potential gradient.
  • This creates a force to help it move up plant - root pressure.
  • Minimal contribution to H2O movement compared to transpiration pull.
  • Evidence for existence of root pressure due to active pumping:
    • Pressure increases with rise in temp & falls with lower temp.
    • Metabolic inhibitors prevent most energy release by respiration & cause root pressure to cease.
    • Decrease in availability of O2 or respiratory substances leading to fall in root pressure.
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Movement of Water up Stems.

Out Through Stomata - humidity of atmosphere is less than that of air spaces next to stomata. Water vapour molecules diffuse out of air spaces into surroundings. Water lost from the air is replaced by water evaporating from cell walls of mesophyll cells. Control rate by changing size of pores.

Across the Cells of a Leaf - H2O is replaced to mesophyll cells from xylem by AP or SP. In SP, movement occurs because of cells lose H2O to air spaces so have a lower water potential & H2O enters from neighbouring cells, which in turn take H2O from their neighbour by osmosis.

Up the Stem in the Xylem - 2 main factors are root pressure and cohesion-tension. H2O evaporates due to transpiration. Molecules form H bonds so stick together (cohesion). It forms a continuous, unbroken pathway across cells & down xylem. As H2O evaporates more molecules are drawn up because of cohesion. H2O is pulled up due to transpiration (transpiration pull). This puts the xylem under tension (cohesion-tension). Evidence includes: change in the diameter of tree trunks according to rate, at its greatest there is more tension so trunk shrinks; if a xylem vessel is broken then air enters & it can no longer draw up H2O as mols can't stick together. Air is drawn in not out due to tension.

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Transpiration & Factors Affecting it.

Transpiration is the main force that pulls water up the stem in evaporation. Diffusion and photosynthesis result in a huge loss in H2O. Transpiration helps bring H2O to leaves but osmosis also does this. Materials such as ions, sugars & hormones are moved dissolved in H2O carried by the transpiration pull. Without it, it would be slow. Factors:

  • Light - photosynthesis only occurs in light. Stomata open in the light so H2O moves out, increase in light intensity increases rate of transpiration.
  • Temperature - changes affect 2 factors that influence rate: how much H2O the air can hold (i.e. water potential) and speed at which molecules move. A rise in temp increases kinetic energy & so speed of movement of mols, this increases the rate of evaporation & hence the transpiration rate. It also decreases the amount of H2O air can hold which increases rate.
  • Humidity - this is a measure of the numberof H2O mols in air. It affects the water potential grad between air inside & outside leaf. When outside air has high humidity the gradient is reduced and rate is lower.
  • Air movement - as H2O diffuses out it accumulates outside the leaf, reducing the water potential gradient so the rate is reduced. Movement of air will disperse the humid layer & increase the rate of diffusion.
  • Ultimately, it is the sun that drives transpiration.
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Limiting Water Loss in Plants.

Xerophytic Plants - plants that do not have a plentiful water supply develop adaptations to limit water loss through transpiration. Increase water uptake & storage.

  • Thick cuticle - waterproof barrier. The thicker it is the less that is lost e.g. holly.
  • Rolling up leaves - stomata confined largely to lower epidermis. Leaves roll to protect lower epidermis from the outside so trap a region of air. it becomes saturated so there is no water potential gradient & transpiration is reduced. Good in hot/windy conditions e.g. marram grass.
  • Hairy leaves - thick layer, especially on lower epidermis trap moist air so the gradient is reduced e.g. heather.
  • Stomata in pits/grooves - trap moist air to reduce gradient e.g. pine trees.
  • Reduced s.a. to volume ratio - slows diffusion, must be balanced against needs of photosynthesis e.g. pine needles.

Xerophytes found in the desert, sand dunes, places with high rainfall but low temps, salt marshes near the coast and cold regions.

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