AQA Biology Unit 2: 13 Exchange and Transport

Covers all of topic 13 except transpiration and xerophytic plants which are on separate documents :)

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Exchange between Organisms and Environment

  • Substances need to be interchanged:
    • Respiratory gases (Oxygen and Carbon Dioxide)
    • Nutrients (fatty acids, glucose, amino acids, vitamins and minerals)
    • Excretory products (urea and Carbon Dioxide)
    • Heat
  • The exchange takes place:
    • Passively (no energy required): diffusion and osmosis
    • Actively (energy required): active transport

Surface Area: Volume Ratio

  • Small organisms have surface area large enough for efficient exchange across their body
  • Larger organisms cannot do this so have adaptations:
  • Flattened shape so cells close to surface
  • Specialised exchange surface with large area to increase surface area:volume ratio
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Exchange Cont

Features of Specialised Exchange Surfaces

  • Large surface area to volume ratio: increases rate of exchange
  • Thin: diffusion distance is short so exchange is rapid
  • Partially permeable: allow selected materials to cross without obstruction
  • Movement of environmental medium e.g. air
  • Movement of internal medium e.g blood
  • Ficks Law:
  • Diffusion = Surface Are X Difference in Concentration
  •                              Length of Diffusion Path
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Gas Exchange in Single Celled Organisms/Insects

Single Celled Organisms

  • Are small so have large surface area:volume ratio
  • Oxygen is absorbed by diffusion across the body which is covered by a cell surface membrane
  • Carbon dioxide diffuses out the body surface
  • Living cells with cell walls are completely permeable so no barrier for diffusion

Gas Exchange in Insects

  • Insects are terrestrial so suffer from water evaporation easily making them dehydrated
  • To reduce water lose terrestrial organisms have:
  • Waterproof covering
  • Small surface area to volume ratio: minimise area for water loss
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Gas Exchange Cont

  • Insects cannot use their body surface to diffuse gases like single celled organisms
  • Insects have tracheae (internal network of tubes supported by strengthened rings)
  • Tracheoles (smaller tubes tracheae divide into): Extend through the body so air is brought directly to respiring tissues
  • Resiratory gases move in and out:
    • Along a diffusion gradient: Cell respiration uses oxygen so concentration towards tracheoles end falls creating a diffusion gradient. Gradient causes oxygen to diffuse from the atmosphere along tracheae. Cells respiring produce CO2 is removed into the atmosphere. It is quick as diffusion in air is more rapid than in water
    • Ventilation: Movement of muscles creates mass movements of air in and out of tracheae speeding up exchange of gases
  • Gas enters and leaves through spiracles (tiny pore) on the body surface, the spiracles open and close by a valve
  • Spiracles are usually kept closed to reduce water lose
  • The tracheal system relies on diffusion, which needs a short pathway, this limits the size of an insect
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Gas Exchange in Fish

  • Fish have waterproof, air tight outer covering
  • Have relatively small surface area to volume ratio
  • They have specialised internal gas exchange surface: Gills

Structure of the Gills

  • Made up of gill filaments stacked up in a pile
  • Right angles to the filaments are gill lamellae which increase surface area
  • Water taken in through the mouth and forced over the gills, goes out opening sides of body
  • The flow of blood and the flow of water are in opposite directions: countercurrent flow

The Countercurrent Exchange Principle

  • Blood well loaded with oxygen meets water, water has maximum concentration of oxygen so diffusion to the blood occurs
  • Blood with little oxygen meets water neets water that has most oxygen removed so diffusion occurs
  • If the flow was parallel only 50% of oxygen from the water would diffuse into the blood 
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Gas Exchange in Leaves

  • When photosynthesis occurs, some CO2 comes from respiring cells, most CO2 comes from external air. Oxygen from photosynthesis is used in respiration but most is diffused out
  • When photosynthesis isn't occurring oxygen diffuses into the leaf as needed by the cells during respiration. Carbon dioxide produced during respiration diffuses out

Structure of a Leaf

  • Plant leaves have large surface area and short diffusion path allowing quick diffusion
  • Leaves have many stomata in lower epidermis and air spaces throughout mesophyll


  • Surrounded by guard cells to prevent water loss but also allow gases in and out
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Cirulatory System of a Mammal

How Large Organisms move Substances around their Bodies

  • Specialist exchange surface absorbs nutrients, respiratory gases and removes waste
  • Transport system required to take materials from cells to exchange surfaces and from exchange surfaces to cells
  • Tissues and organs have been specialised for their job
  • The lower the surface area:volume ratio and the more active an organism the greater the need for a specialised transport system with a pump

Features of Transport Systems

  • Suitable medium to carry materials e.g. blood (normally liquid based in water as it readily dissolves substances and moves easily)
  • Form of mass transport
  • Closed system of vessels that form a branching network
  • A mechanism to move the medium which requires pressure (Animals have the heart)
  • Mechanism to maintain the mass flow movement in one direction e.g. valves
  • Means of controlling the flow of medium to suit changes
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Circulatory System in Mammals Cont

How Blood is Circulated in Mammals

  • There is a closed blood system
  • A muscular pump (heart) circulates the blood around the body
  • Mammals have a double circulatory system
  • Blood passes through the heart twice since:
    • After passing through the lungs its pressure is reduced, this would make circulation slow if passed around rest of the body
  • Blood is returned to the heart to boost pressure and then passed through the body
  • This is needed to keep the body temperature and metabolism in safe conditions
  • The blood is passed into the cells be diffusion over a large surface area with a short distance and steep diffusion gradient
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Blood Vessels Structure

Structure of Blood Vessels

  • Arteries: Carry blood away from the heart into arterioles
  • Arterioles: Smaller arteries that control brood flow from arteries to capillaries
  • Capillaries: Tiny vessels that link arterioles to veins
  • Veins: Carry blood from capillaries back to the heart
  • Arteries, arterioles and veins have same basic structure:
    • Tough outer layer: Resist pressure changes from both within and outside
    • Muscle layer: Can contract and control blood flow
    • Elastic layer: Help maintain blood pressure by stretching and springing back
    • Thin inner lining (Endothelium): Smooth to prevent friction and thin to allow diffusion
    • Lumen: Central cavity of the blood vessel
  • The vein's lumen is larger than the arteries
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Structure and Function: Artery

  • Function: Transport blood rapidly under high pressure from heart to tissues
  • Thick muscle layer: Smaller arteries can constrict and dilate to control volume of blood passing through them
  • Thick elastic layer: To keep blood pressure high to reach body extremities. It can then strech and recoil according to when the heart beats allowing it to maintain a high pressure
  • Thickness of wall is large: Resists the vessel bursting under pressure
  • No Valves: Blood under constant high pressure so doesn't flow backwards

Arteriole Structure and Function

  • Function: Carry blood under lower pressure than arteries to the capillaries
  •  Thicker muscle layer: Contractions of the muscle layer allow constriction of the lumen, this restricts the flow of blood and controls movement
  • Thinner elastic layer: Blood is under lower pressure
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Structure and Function: Veins

  • Function: Transport blood under low pressure to the heart
  • Thin muscle layer: Veins carry blood away from tissues so don't need to control flow to them
  • Thin elastic layer: Low pressure stops the bursting
  • Small thickness of wall: No need for thick wall as the pressure of the blood is too low to burst the vein, it also allows them to flatten easily
  • Valves: Ensure blood flows in the right way, when body muscles contract they compress the veins and pressurise them, the valves stop backwards flow

Capillary Structure and Function

  • Function: Exchange metabolic materials like oxygen, carbon dioxide and glucose
  • Walls with lining layer: Allow short diffusion distance for rapid diffusion of materials between blood and cells
  • Branched: Many of them providing large surface area
  • Narrow diameter: Permeate tissues
  • Narrow Lumen: Red blood cells flattened so closer to cells
  • Spaces between endothelial cells: Allows white blood cells to escape and help infection
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Tissue Fluid

  • Capillaries cannot reach every cell directly so tissue fluid bathes the tissues
  • It contains glucose, amino acids, fatty acids, salts and oxygen
  • It supplies these substances to the tissues in return for CO2 and waste materials
  • It is formed from blood plasma and is controlled by homeostatic systems


  • Blood pumped by the heart passes along arteries, then the arterioles and then the capillaries which causes hydrostatic pressure at the arterial end of the capillaries
  • This pressure forces tissue fluid out of the blood plasma
  • The outward pressure is opposed by two forces:
    • Hydrostatic pressure of tissue fluid outside the capillaries prevents outward movement of liquid
    • The lower water potential of the blood due to the plasma porteins pulls water back into the blood within the capillaries
  • Ultrafiltration occurs as the pressure is only enough to force small molecules out of the capillaries leaving cells and proteins in the blood
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Tissue Fluid Return to the Circulatory System

  • Once it exchanges metabolic materials it must return to the circulatory system
  • Most tissue fluid returns to the blood plasma directly via capillaries:
    • The loss of tissue fluid from the capillaries reduces hydrostatic pressure inside them
    • By the time blood reaches the venous end of the capillary network its hydrostatic pressure is less than tissue fluid outside of it
    • Tissue fluid is then forceds back into the capillaries by high hydrostatic pressure outside them
    • Osmotic forces resulting from the proteins in the blood plasma pulls water back into the capillaries
  • The remainder tissue fluid is carried back via the lymphatic system; a system of vessels than begin in the tissues that resemble capillaries but then merge into larger vessels
  • The larger vessels drain their contents back to the bloodstream via two ducts that join veins close to the heart
  • The contents of the lymphatic system are moved by;
    • Hydrostatic pressure of the tissue fluid that left the capillaries
    • Contraction of body muscles squeeze the lymph vessels, valves in the lymph vessels ensure fluid inside them moves away from the tissues in the direction of the heart
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Movement of Water through Roots

Uptake of Water by Root Hairs

  • Plants lose water by transpiration so the water is replaced through the root hairs
  • Root hairs are long, thin extensions of a root epidermal cell
  • Efficient surface for exchange of water and minerals:
    • Provide large surface area
    • Thin surface layer
  • The soil the root hairs go into is mostly water so has a high water potential
  • The root hair cells have sugars and amino acids dissolved in them
  • This causes water to move by osmosis into the root hair cells
  • The water travels via:
    • The Apoplastic pathway
    • The Symplastic pathway
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The Apoplastic Pathway

  • Water drawn into the endodermal cells
  • It pulls more water in due to its cohesive property
  • This creates tension that draws water along the cell walls of the cells of the root cortex
  • The mesh like structure of the cellulose cell walls contains many water filled spaces
  • These water filled spaces provide little resistance to the pull of water along the cell walls
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The Symplastic Pathway: Through the cells

  • Takes place across the cytoplasm of the cells of the cortex due to osmosis
  • Water passes through the cell walls along tiny openings called plasmodesmata
  • Each plasmodesmata is filled with thin strand of cytoplasm
  • There is a continuous column of cytoplasm extending through the root hair cell to the xylem at the centre of the root
  • Water moves along the column:
  • Water enters by osmosis increasing the water potential of the root hair cell
  • The root hair cell has a higher water potential than the first cell in the cortex
  • Water then movers from the root hair cell to the first cell in the cortex
  • This is then repeated through the cortex
  • Water is then pulled in as the water potential of the first cortical cell is lowered again making the water travel by osmosis from the root hair cell to the first cell of the cortex
  • A water potential gradient is set up along the cells
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Passage of Water into the Xylem

  • The waterproof band that makes up the casparian ***** prevents water from passing further through the cell wall due to the apoplastic pathway
  • Water is forced into the living protoplast of the cell
  • It joins the water from the symplastic pathway
  • Active transport of mineral salts can take the water into the xylem
  • Endodermal cells actively transport salts into the xylem
  • As the process requires energy it can only occur in living cells
  • It takes place along carrier proteins in the cell surface membrane
  • The active transport of mineral ions into the xylem by the endodermal cells creates a lower water potential so water can move into the xylem by osmosis along a water potential gradient
  • The movement of the mineral ions creates a force that helps to move water up the plant: The is Root Pressure
  • Evidence root pressure is due to the pumping of salts into the xylem:
  • Pressure increases with a rise in temperature
  • Metabolic inhibitors e.g. cyanide prevent most energy release by respiration and stop root pressure
  • Decrease in availability of oxygen causes a reduction in root pressure
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Movement of Water up Stems

  • The main force that pulls water up the stem is transpiration
  • Transpiration is the evaporation of water from the leaves

How Water moves through the Leaf

  • When stomata are open water vapour molecules diffuse out of the air spaces
  • The water is replaced by water evaporating from the cell walls of the mesophyll cells
  • Water from the mesophyll cells is then replaced by water in the xylem by the apoplastic or symplastic pathways
  • In symplastic pathways it occurs:
  • Mesophyll cells lose water to air spaces
  • Cells now have lower water potential so water enters by osmosis
  • The neighbouring cells lose water lowering their water potential
  • The neighbouring cells then take water from the cells next to them
  • This establishes a water potential gradient to pull the water from the xylem
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Movement of Water up Stems Cont

How Water moves up the Xylem

  • The two main factors that cause water movement up the xylem are cohesion tension and root pressure
  • Cohesion Tension operates:
  • Water evaporates from leaves due to transpiration
  • Water molecules form hydrogen bonds between them this is cohesion
  • Water form continuous pathway across the mesophyll cells and down the xylem
  • As water evaporates from the mesophyll cells in the cells in leaves into the air spaces beneath the stomata molecules of water are brought up
  • Water is pulled up the xylem due to transpiration pull
  • Transpiration pull put the xylem under tension
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Evidence of the Cohesion Tension Theory

  • Change in diameter of tree trunks
    • During the day transpiration is at its greatest so more tension in the xylem
    • This causes the trunk to shrink
    • At night transpiration is at its lowest so little tension in the xylem
    • Diameter then increases at night
  • If Xylem vessel is broken and air enters it
    • The tree can no longer draw up water as no continuous column
    • If broken air is drawn in

Transpiration pull is a passive process so doesn't require metabolic energy

As the xylem is dead it can form series of continuous unbroken tubes from root to leaves

These tubes are essential to the cohesion tension theory

Energy is needed for transpiration and it comes from the heat of the sun

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