Adaptations for Transport in Plants

Roots
1. Roots absorb water through root hairs. Then transported through plant to xylem. In leaves, either used in photosynthesis or evaporates away in transpiration
2. Water moves into root hair cell (large SA:V, through thin cell wall, freely permeable) from soil down a water potential gradient by osmosis. Soil very dilute and high water potential. Cytoplasm, with many diddolved substances eg. sugars had a low water potential
3. Ions absorbed into root hair cell by active transport, also lowers water potential
4. After entering root hair cell, water moves through the cortex
5. Apoplast pathway through cortex moving from cell wall to cell wall
6. Symplast pathway through membranes/cytoplasm/plasmodesmata by osmosis
7. Vacuolar pathway: water can also travel through the cell cytoplasm and vaculoes
8. Endodermis has waterproof band of suberin called the casparian strip which blocks the apoplast pathway so water moves into symplast pathway
9. Selective active transport of ions into pericycle eg. nitrate by endodermal cells into cytoplasm (symplast pathway) bypassing the casparian strip and lowering water potential in xylem. Water follows by osmosis. Water and minerals then move into xylem. This is root pressure (positive hydrostatic pressure).
Xylem
1. Consists of dead, lignified tracheids and vessels with pits, supporting fibres and living parenchyma
2. Tracheids and vessels form a continuous system of channels for water transport
3. Tracheids have tapered ends that fit together
4. Columns of water in xylem are held up by the cohesive force between water molecules and the adhesive forces between the water molecules and the hydrophilic lining of the xylem vessles
5. Also capillarity may help as water rises up thin tubes
6. Structure: hollow with no cell contents
  1. Function: Less resistance to water
7. Lack of end walls
  1. Creates a continuous column of water
8. Large lumen (0.1-0.2mm)
  1. Less resistance/ wide to carry plenty of water
9. Thick walls with lignin
  1. Strengthens vessel so stops inward collapse when water sucked along it. Waterproofing: stops water entering/leaving. Hydrophilic lining helps water adhesion (stick) to the walls and so move upwards
10. Pits
  1. Allows transfer of water between cells
Stems
1. Water passes through the root to the xylem, up through the stem to the leaves where most evaporates and moves out of the stomata
Transpiration
1. The loss of water vapour by evaporation from the leaves through stomata, powered by sunlight
2. Water follows a water potential gradient
3. Gives rise to the transpiration stream, pulling water up
4. The continued removal of water molecules from the top of the xylem vessels results in a tension causing an upward force on the xylem water column
5. So water moves by 'cohesion tension'
Factors affecting rate of transpiration
1. Temperature
  1. More kinetic energy, more evaporation from mesophyll cells, more diffusion of water vapour through stomata. A higher air temperature can hold more moisture so it has lower water potential, so gradient is steeper
2. Humidity
  1. Air inside leaf is saturated, whereas external air has lower water potential. Increases in external humidity would lower gradient
3. Air movement
  1. Wind removes the layer of saturated air just outside the stomata which reduces the water potential so water potential gradient steeper, transpiration increases
4. Light intensity
  1. Light opens the stomata, so transpiration increases
Plant adaptations to the amount of available water:
1. Plants must allow the exchange of gases, so must open stomata. But this means water is lost through transpiration. Plants can be classified on the bases of structure in relation to the prevailing water supply
Translocation
1. Plants must transport the organic products of photosynthesis:
  1. Transported as soluble sucrose
    1. From the leaves source
      1. To other parts where they are used for energy requiring processes such as growth eg. ATP for active transport or storage 'sink'
        Phloem consists of sieve tubes and companion cells linked by plasmodesmata, with fibre cells and parenchyma cells also present
2. Sieve tube element:
  1. Structure/property: living cell
    1. Allows active processes
  2. Few organelles at the edge
    1. More space for transport with little resistance
  3. Elongated
    1. Less resistance
  4. Sieve plate with pores
    1. Connects elements and lets material through
  5. Joined end to end
    1. For continuous long distance transport
  6. Bi-directional flow
    1. So sugar can move up and down
3. Mass flow theory of translocation:
  1. Passive mass flow of sugars from the leaf where they are produced and exported (source, high concentration) to growing tissues where the sugars are used (sink, lower concentration)
    1. But this does not explain the presence of sieve plates which act as barriers to this
      1. Sucrose and amino acids have been observed to move at different rates and in different directions in the same tissue at the same time
        Phloem tissue has a high rate of oxygen consumption, companion cells with many mitochondira and translocation is slowedby respiratory inhibitors so maybe not a passive process
4. Companion cell:
  1. Structure/property: many mitochondira
    1. Metabolically active to provide ATP energy
  2. Nucleus
    1. Controls the functions of both cells
  3. Plasmodesmata
    1. Allows exchange with sieve tube element in cytoplasmic strands
  4. Hydrogen pump
    1. Allows co-transport for sucrose loading
5. Other theories for translocation:
  1. Diffusion
  2. Cytoplasmic streaming

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	Criado por Emily Sutton quase 9 anos atrás