Transport in plants

• Tissues in a dicotyledonous root

  • phloem transports organic solutes
  • xylem transports water
  • endodermis acts as a barrier to the free passage of water and ions via the apoplast pathway
  • piliferous layer is an epidermis modified to produce root hairs
  • cortex is composed of parenchyma
• Root structure

  • piliferous layer contains root hairs which are thin, permeable tubular extensions of epidermal cells
  • cortex consist of parenchyma cells where starch is stored
  • endodermis is one cell thick and possess an impermeable casparian strip of suberin
  • phloem transports the products of photosynthesis. consists of living cells e.g. sieve elements and companion cells
  • xylem transports water and mineral ions from the roots to the leaves and consists of non-living tissue made of vessels and tracheids
• Uptake and transport in plant roots

  • water and ions by root hairs which have a large surface area
  • ions are absorbed into the root by diffusion and active transport
  • water uptake occurs by osmosis along a water potential gradient
  • water continues to move along a water potential gradient from the root hair cells across the cortex to the inner xylem vessels
  • water potential gradient maintained by water continually moving up the xylem and by the dissolved ions in the xylem sap
• Mycorrhizae

  • a mutualistic association between fungus and a plant root
  • fungus absorbs mineral ions and water which can then pass into the plant root
  • fungus receives organic nutrients e.g. carbohydrates and vitamins from the plant
  • hyphae spread through the surrounding soil covering a larger surface area than the roots would
• Ectotrophic mycorrhizae

  • form a sheath around the root and penetrate the air spaces between the cells in the cortex but don't enter cells
  • an extensive intercellular net is formed
  • found in forest trees e.g. conifers, beech
  • involve fungi of mushroom groups
• Endotrophic mycorrhizae

  • form an intracellular network and extend into the soil and appear to penetrate cells
• Transport across the root cells

  • occurs via 3 different pathways

  1. apoplast
  2. symplast
  3. vacuolar
• Apoplast pathway

  • movement of water through cellulose cell walls of adjacent cells through the small intercellular spaces between them
  • cell walls are fully permeable across the root except the endodermis
  • impermeable casparian strip in cell walls of the endodermis prevents the passage of water and ions via the apoplast
  • water and ions are forced to pass through the symplast pathway into the cells
• Symplast pathway

  • movement of water by osmosis through the inter-connecting cytoplasm of adjacent cells
  • water travels through the plasmodesmata, thin strands of protoplasm linking the cytoplasm of the adjacent cells
• Structure of xylem

  • xylem vessels and tracheids found in the inner part of each vascular bundle in the stem
  • xylem tissue is dead so have no cell contents leaving hollow tubes that minimise the resistance of flowing water and ions
  • cross-walls have broken down completely to allow free flow
  • vessels and tracheids have bordered pits that allow water and solutes to move laterally to adjacent vessels
• Transpiration

  • the evaporation of water from a plant's surface through the stomata when they open to allow the entry of carbon dioxide for photosynthesis
  • rate of transpiration is affected by the external environmental factors related to the surrounding environment and by internal factors related to the structure of the plant
• Transpiration environmental factors: light

  • during the daylight stomata open to allow carbon dioxide to enter for photosynthesis
  • this increases rate of transpiration as water evaporated from the mesophyll cells and diffuses out of the leaf
• Transpiration environmental factors: temperature

  • an increase in temperature lowers the relative humidity of air outside the leaf increasing the rate of transpiration
  • an increase in temperature increases the rate of transpiration by providing water molecules with more kinetic energy allowing them to evaporate more readily
• Transpiration environmental factors: humidity

  • increase will decrease the rate of transpiration as the water potential gradient for diffusion of water decreases
• Transpiration environmental factors: air movement

  • air movement removes water vapour from the leaf surface increasing the water potential gradient and rate of transpiration
  • in still air water vapour builds around the leaf reducing the water potential gradient and the rate of transpiration
• Measuring the rate of transpiration

  • a potometer
  • assumes the rate of water uptake is the same as the rate of transpiration
  • not entirely valid as some water taken up is used for turgidity and photosynthesis
  • to calculate the rate the distance the air bubble moves, time taken for air bubble to move a distance and radius of the capillary tube must be measured
• Transpiration stream

  • movement of water and dissolves ions occurs through the xylem as part of the transpiration stream
  • 2 main hypotheses to account for the movement of water and mineral ions from the roots to leaves

  1. root pressure
  2. cohesion-tension theory
• Root pressure

  • involves a positive hydrostatic pressure that can be observed when a freshly cut root stump continues to exude sap
  • process involves active transport of mineral ions by endodermal cells into the xylem of the root lowering its water potential so water moves in by osmosis and along a water potential gradient
  • if the production of ATP by respiration is decreased root pressure should be decreased
  • root pressure cannot account for the transport of water to the top of the trees
• Cohesion-tension theory (steps 1-6)

  • solar heat causes evaporation or transpiration
  • water evaporates from the mesophyll cells and diffuses out through the stomata
  • water potential of the mesophyll cells is reduced compared to inner mesophyll cells
  • water moves from adjacent cells via osmosis along the water potential gradient
  • movement of water is by the apoplast and symplast pathway
  • water potential gradient extends across the leaf mesophyll cells to the xylem vessels; water is drawn up from the xylem creating a tension in the xylem vessels pulling up the water and dissolves ions
• Cohesion-tension theory (steps 7-10)

  • water column is maintained in the xylem by cohesive forces and adhesive forces
  • cohesion is the attraction of water molecules to each other
  • adhesion is the attraction of water molecules to the xylem walls
  • upward movement of water from the xylem in the roots maintains the water potential gradient across the root cortex cells providing the mechanism for water uptake from the soil via osmosis
• Evidence for movement of ions in the xylem

  • radioactive isotopes can be used to provide evidence that the transport of mineral ions through a plant occurs in the xylem
  • xylem and phloem in a section of the stem of a plant is separates using a wax cylinder to prevent lateral transport
  • roots of the plant are supplied with radioactive potassium ions
  • plant is left for hours and the amount of radioactivity in the xylem and phloem tissues is then measured
  • amount of radioactivity in the xylem is considerably greater indicating transport of ions occurs in this tissue
• Transport in phloem

  • translocation
  • carbohydrates transported as sucrose, proteins as amino acids and lipids as fatty acids and glycerol
  • organic molecules are transported to respiring cells that don't carry out photosynthesis, growing areas and storage areas
• Phloem

  • sieve elements possess perforated end walls or sieve plates
  • sieve elements are joined end to end to form sieve tubes
  • mature sieve elements don't possess a nucleus and the cytoplasm which is around the edge of the cell contains few organelles
  • adjacent to each sieve element is a companion cell with dense cytoplasm and many mitochondria
• Mass flow hypothesis

  • transport of organic substances occurs along a hydrostatic or turgor pressure gradient
  • photosynthetic products are produced in the mesophyll cells in leaves (source)
  • sugars are actively transported into the sieve tubes by transfer cells
  • lowers water potential causing water to enter by osmosis
  • creates a high hydrostatic or turgor pressure in sieve tubes in the leaf
  • growing areas have low hydrostatic or turgor pressure as water moves out the cell (sinks)
  • photosynthetic products are transported along the pressure gradient by mass flow
• Evidence for translocation in the phloem

  • ringing or girdling experiments provided the first evidence
  • ringing involves removing a complete ring of phloem from a part of a plant
  • transport through the phloem is prevented
  • a few weeks after ringing a tree trunk, a slight swelling developed above the girdle
  • swelling was linked to the build up of photosynthetic products from the leaves which were prevented from being transported past the girdle due to the removal of the phloem
• Evidence for translocation in the phloem: radioactive isotopes

  • using radioactive carbon
  • plants of the same species at similar growth stages are taken
  • one plant is girdled and the other is the control
  • both plants are supplied with radioactive carbon-14 and left in sunlight for a few hours
  • the radioactive photosynthetic products are detected using autoradiography
  • the autoradiogram shows that the photosynthetic products are found at sinks and the girdled plant hasn't allow these products past the girdle
• Hydrophytes

  • freshwater aquatic plants that have fewer osmoregulatory problems than any of the other plant types
  • plant cells in fresh water are surrounded by a solution of higher water potential and water enters the cells via osmosis
  • cells become turgid and the water potentials of the surrounding water and plant cells are equal so there is no further net movement of water into the cells
• Mesophytes

  • occupy habitats with adequate water supply
  • problem of water loss by transpiration from all aerial parts
  • waxy cuticle, protected stomata, variable leaf shape and abscission prevent water loss
• Halophytes

  • plants inhabiting areas of high salinity
  • high transpiration rates and a low water potential in root cells enables increased water uptake at the roots
  • extensive root network providing anchorage and a large surface area for water and iron uptake
  • smaller plants are able to store water when it's freely available
  • some species are able to regulate their salt content by excreting salt from glands at the margins of their leaves
• Xerophytes: live in habitats where water is in short supply

  • thickened waxy cuticle reducing the rate of evaporation
  • reduced surface area of leaves
  • curled leaves reduce the surface area for evaporation and increase the humidity in the air around the stomata reducing transpiration
  • hairs on leaf surface trap air reducing the water potential gradient for water loss
  • stomata positioned in 'epidermal pits' beneath leaf surface become saturated with water vapour and reduce water potential gradient for evaporation
  • shallow root network and deep root tap to maximise water uptake
• Bergman's rule

  • total heat production of endotherms depends on volume of the body whilst the rate at which heat is lost depends on surface area
  • volume increases more rapidly than surface area as the size of an animal increases
  • animals living in cold region are larger whilst animals in hotter climates are smaller
  • small mammals in arctic regions have large appetites enabling them to maintain a high metabolic rate
• Allen's rule

  • species living in colder climates have smaller extremities than related species in warmer climates
• Osmoregulation in desert-living rodents

  • animals in hot deserts have adapted to lack of water and extreme hot temperatures
  • kangaroo rat burrows in sand to escape the sun
  • feed on dry seeds and other dry plants so survives on water produced by chemical reaction of respiration and amounts in food
  • nephrons have a long loop of Henle so more water is reabsorbed
  • high levels of ADH
  • water loss by evaporation from the lungs is reduced by exhaling air at a temperature below body temperature
• Depression of freezing points

  • lowering of a solution's freezing point below a pure solvent depends on the number of molecules of solute per unit volume of solution
  • animals in arctic conditions produce natural 'antifreeze' and staying active
  • some species manufacture proteins to prevent cellular damage as a result of the formation of ice crystals
  • antifreeze proteins depress the freezing point of blood and body fluids. bind to the surface of small ice crystals and inhibit their growth
  • presence of increased concentrations of glycerol in haemolymph of invertebrates reduce freezing
• Migration

  • periodic long-distance movement from one location to another
  • birds fly north in summer to breed then south in winter to find food
  • control of migration is mostly endogenous though exogenous factors (photoperiod) also affect migration
• Physical model demonstrating mass flow

  • mass-flow eventually stops as sugars aren't being continually produced at the source or used up at the sink
  • a scheme shows the stages in mass flow in a plant