EXCHANGE AND TRANSPORT SYSTEMS

• EXCHANGE OF SUBSTANCES WITH THE ENVIRONMENT

  • the exchange if substances between the internal and external environments takes place at exchange surfaces
  • to truly enter or leave an organism, most substances cross cell plasma membranes
  • the environment around the cells of multicellular organisms is called tissue fluid
  • every organism, whatever its size, need to exchange things with its environment
• EXCHANGE OF SUBSTANCES WITH THE ENVIRONMENT

  • cells need to take in oxygen (for aerobic respiration) and nutrients
  • cells also need to excrete waste products like carbon dioxide and urea
  • most organisms need to stay at roughly the same temperature, so heat needs to be exchanged too
• SURFACE AREA TO VOLUME RATIO

  An organisms surface area:volume ratio affects how quickly substances are exchanged. For exchange to effective, the exchange surface of the organism must be large compared with its volume

  • smaller organisms have a high surface area : volume ratio
  • larger organisms have a low surface area : volume ratio
• WHAT DOES SURFACE AREA : VOLUME AFFECT?

  how quickly substances are exchanged
• HOW CAN EXCHANGE BE EFFECTIVE

  the exchange surface of the organism must be large compared with its volume
• CALCULATING SURFACE AREA TO VOLUME RATIO

  SURFACE AREA = area of one face * number of faces

  VOLUME = length * width * depth

  SA / V = surface area to volume ratio in the form x : 1
• EXCHANGE IN SINGLE CELLED ORGANISMS

  • in single celled organisms substances can diffuse directly into or out of the cell across the cell surface membrane
  • diffusion rate is quick because of the small distances the substances have to travel
  • single celled organisms are small and therefore have a large SA:V ratio
• WHY IS THERE A FAST DIFFUSION RATE IN SINGLE CELLED ORGANISMS?

  Because of the small distances substances have to travel
• EXCHANGE IN MULTICELLULAR ORGANISMS

  in multicellular organisms, diffusion across the outer membrane is too slow because:

  1. Some cells are deep within the body so there’s a big distance between them and the outside environment
  2. larger animals have a low SA:V ratio so it’s difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface

  • so rather than using straightforward diffusion to absorb and extreme substances, multicellular organisms need specialised exchange organs
• MASS TRANSPORT

  Mass transport is an efficient system to carry substances to and from their individual cells

  • in mammals, ’mass transport’ normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body
  • it also carries hormones, antibodies and waste like carbon dioxide
  • mass transport in plants involves the transport of water and solutes in the xylem and phloem
• METABOLIC RATE AND SA:V

  METABOLIC RATE: the amount of energy expended by an animal over a specific period of time

  • it may be measured in joules, calories, or kilocalories per unit time. It may also be given as oxygen consumed or co2 produced per unit time
• METABOLIC RATE AND SA:V

  Organisms with a high metabolic rate exchange more materials and so require a larger surface area to volume ratio.

  • in turn this is reflected in the type of exchange surface and transport system that evolved to meet the requirements of each organism
• METABOLIC RATE AND SA:V

  smaller organisms need a higher metabolic rate in order to generate enough heat to stay warm, this is because they have a larger surface area so heat exchange is easier.
• FICKS LAW OF DIFFUSION

  DIFFUSION a (SURFACE AREA * CONCENTRATION GRADIENT) / LENGTH OF DIFFUSION PATH (THICKNESS)

  OPTIMAL:

  • SA = high
  • CG = high
  • TH = low

  
• FEATURES OF SPECIALISED EXCHANGE SURFACES

  • a large surface area relative to the volume of the organism which increases the rate of exchange
  • very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
  • the organism also maintains a steep concentration gradient of gases across the exchange surface, which increases the rate of diffusion
• EXCHANGE SURFACE: the boundary between the outside environment and the internal environment of an organism
• HEAT EXCHANGE

  As well as creating waste products that need to be transported away, the metabolic activity inside cells creates heat. Staying at the right temperature is difficult, and it’s pretty heavily influenced by:

  1. BODY SIZE
  2. BODY SHAPE
• HEAT EXCHANGE

  BODY SIZE

  The rate of heat loss from an organism depends on its surface area.

  • If an organism has a large volume, e.g. a hippo, its surface area is relatively small. This makes it harder for it to lose heat from its body.
  • If an organism is small, e.g. a mouse, its relative surface area is large, so heat is lost more easily.

  This means smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm.
• HEAT EXCHANGE

  BODY SHAPE

  • animals with a compact shape have a small surface area relative to their volume - minimising heat loss from their surface
  • animals with a less compact shape (those that are a bit gangly or have sticky outy bits) have a larger surface area relative to their volume - this increases heat loss from their surface
• ADAPTATIONS FOR HEAT EXCHANGE - SMALL MULTICELLULAR ANIMAL

  • SPECIALISED ORGANS: no
  • MASS TRANSPORT SYSTEMS: no
  • SA:V RATIO: high
  • HEAT LOSS: easy
  • METABOLIC RATE: high
  • PHYSICAL ADAPTATIONS (COLD): thick fur
  • PHYSICAL ADAPTATIONS (WARM): kidney structure adaptations - less urine, as water evaporates quickly to prevent dehydration
  • BEHAVIOURAL ADAPTATIONS (COLD): eat high energy food, hibernate, burrow, huddle
  • BEHAVIOURAL ADAPTATIONS (WARM): migration
• ADAPTATIONS FOR HEAT EXCHANGE - LARGE MULTICELLULAR ANIMAL

  • SPECIALISED ORGANS: yes
  • MASS TRANSPORT SYSTEMS: yes
  • SA:V RATIO: low
  • HEAT LOSS: hard
  • METABOLIC RATE: low
  • PHYSICAL ADAPTATIONS (COLD): small SA:V helps prevent heat loss
  • PHYSICAL ADAPTATIONS (WARM): large, flat ears to increase surface area and aid heat loss
  • BEHAVIOURAL ADAPTATIONS (COLD): eat high energy food, hibernate, huddle
  • BEHAVIOURAL ADAPTATIONS (WARM): stay in water
• GAS EXCHANGE IN SINGLE CELLED ORGANISMS

  SIMPLE DIFFUSION OF GASES THROUGH CELL SURFACE MEMBRANE

  Single celled organisms absorb and release gases by diffusion through their cell-surface membranes

  • they have a relatively large surface area
  • they have a thin surface
  • they have a short diffusion pathway

  -oxygen can take part in biochemical reactions as soon as it diffuses into the cell

  So there’s no need for a specialised gas exchange system.
• GAS EXCHANGE IN DICOTYLEDONOUS PLANTS

  DIFFUSION OF OXYGEN AND CARBON DIOXIDE DURING PHOTOSYNTHESIS AND RESPIRATION

  • Plants need co2 for photosynthesis which produces o2 as a waste gas
  • they need o2 for respiration, which produces co2 as a waste gas
• GAS EXCHANGE IN DICOTYLEDONOUS PLANTS

  At times the gases produced in one process can be used for the other

  • this reduces gas exchange with the external air
  • overall, this means that the volumes and types of gases that are being exchanged by a plant leaf change
  • this depends on the balance between the rates of photosynthesis and respiration
• GAS EXCHANGE IN DICOTYLEDONOUS PLANTS

  • the main gas exchange surface is the surface of the mesophyll cells in the leaf
  • there’s no specific transport system for gases, which simply move in and through the plant by diffusion
  • diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water
• THE BALANCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION

  • all plant cells carry out respiration in the mitochondria all the time
  • some plant cells photosynthesis as well, using carbon dioxide and releasing oxygen during the daytime
  • There’s always a favourable diffusion gradient for both gases
• THE BALANCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION DAYTIME

  • photosynthesis is happening faster than respiration so all the co2 the plant makes by respiration is used up by the chloroplasts in photosynthesis.
  • The plant also takes extra co2 from the air.
  • Some of the o2 that’s made in photosynthesis is used in respiration, but there’s lots left over so it diffuses out the cell
• THE BALANCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION

  NIGHT

  • chloroplasts stop photosynthesising.
  • The Mitochondria continue to respire so oxygen is used up and co2 diffuses out of the cells
• THE BALANCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION

  DAWN AND DUSK (dim light conditions)

  • photosynthesis and respiration balance each other out, so there’s no diffusion gradient and no net loss or uptake of gases from the leaf - dynamic equilibrium
• PLANT LEAF STRUCTURE

  1. Waxy cuticle
  2. upper epidermis
  3. air space
  4. guard cell
  5. spongy mesophyll
  6. stoma
  7. palisade mesophyll cells
  8. xylem
  9. phloem
  10. vascular bundle
  11. lower epidermis

  
• PLANT LEAF STRUCTURE

  1, WAXY CUTICLE: helps prevent water loss by evaporation as its hydrophobic and made of lipids

  
• PLANT LEAF STRUCTURE

  2. UPPER EPIDERMIS: helps protect the leaf and provides extra layer to prevent water loss
• PLANT LEAF STRUCTURE

  3. AIR SPACE: in spongy mesophyll to increase efficiency of gas exchange

  
• PLANT LEAF STRUCTURE

  4. GUARD CELL: control opening and closing of stomata using water potential

  
• PLANT LEAF STRUCTURE

  5. SPONGY MESOPHYLL CELLS: covered in water to dissolve gases, packed loosely for efficient gas exchange

  
• PLANT LEAF STRUCTURE

  6. STOMA: control water loss and gas exchange

  
• PLANT LEAF STRUCTURE

  7. PALISADE MESOPHYLL CELL: main site of photosynthesis, packed closely together, full of chloroplasts

  
• PLANT LEAF STRUCTURE

  8. XYLEM: transpiration, transport water and minerals from the roots to the rest of the plant, strengthened with lignin

  
• PLANT LEAF STRUCTURE

  9. PHLOEM: translocation, transport food and nutrients from the leaves to the rest of the plant

  
• PLANT LEAF STRUCTURE

  10. VASCULAR BUNDLE: xylem and phloem are in vascular bundles, xylem forms central column, phloem surrounds it