Plant biology - unit 1

Why do plants need a transport system
1. Need to take substances from environment
  1. E.g. Nitrates, water
  2. Epithelial cells in large multicellular plants can receive supply by simple diffusion
  3. What substances need to be moved?
    1. Water and soluble minerals
      1. Move upwards
      2. Transported in the xylem
    2. Sugars (most commonly sucrose)
      1. Move up or down the plant
      2. Transported in the phloem
2. Need to return waste to the environment
  1. E.g. Carbon dioxide
3. Particular problem in plants
  1. Roots can take up water but not sugars
  2. Leaves can produce sugars, but can't take up water from the air.
  3. So transport system is required to exchange substances all over the plant
Vascular tissue
1. Xylem and phloem are found in vascular bundles
2. In the root
  1. Xylem in the core in an X shape
    1. Arrangement provides strength against pulling and crushing forces which roots experience
  2. Phloem is dotted around each arm of xylem vessel
3. In the stem
  1. Xylem in the inside
  2. Phloem on the outside of the ring
  3. Cambium in the middle separating the xylem and phloem
  4. Cambium are meristem cells that can differentiate to xylem or phloem
  5. Arrangement provides strength and flexibility against bending that stems and branches experience
  6. Complete ring of vascular bundles just under the bark
4. In the leaves
  1. Form the midrib and veins of a plant
  2. Two major groups of flowering plants
    1. Monocotyledons
    2. Depends on how many first leaves they have
      1. Two groups have different patterns of veins
    3. Dicotyledons
      1. Have a branching network of veins
      2. Veins get smaller as they move further from the midrib
  3. Xylem on the inside of the phloem
    1. In a slightly curved V shape in centre of leaf
5. XYLEM
  1. Consists of tubes to carry water and dissolved minerals
  2. Fibres help support the plant and living parenchyma cells
  3. Long thick cell walls of xylem vessel element
    1. Impregnated with lignin
      1. Waterproofs the cells and causes them to decay and die
      2. Leaves a long column With no end plates or contents
  4. Lignin
    1. Strengthens the wall and prevents it from collapse
    2. Keeps vessel open when water supply is low
    3. Can be spiral, annular, reticulate patterns
      1. Prevents vessel from being too rigid
      2. Allows flexibility of stem or branch
    4. Can be gaps in lignin called PITS
      1. Allow lateral movement of water to other vessels
      2. Allow water movement to living parts of the plant
  5. Adaptations of XYLEM
    1. Tubes are end to end to form a continuous column
    2. Tubes are narrow so water column doesn't break easily
    3. Flow of water is not impeded because:
      1. No end walls
      2. No cell contents e.g. RER
      3. No nucleus or cytoplasm
      4. Lignin prevents wall collapsing
    4. Pits allow lateral water movement
    5. Lignin patterns allow flexibility when growing
6. Phloem
  1. Transport sugars and contain two cells: sieve tube elements and companion cells
  2. Sieve tube element
    1. Contain very little cytoplasm and no nucleus
    2. Tubes lined up end to end
    3. Transport sucrose mainly- dissolved in water to firm sap
    4. Has perforated sieve end plates to allow sap to flow through it
  3. Companion cell
    1. Have a large nucleus and dense cytoplasm
    2. Numerous mitochondria to produce ATP for active processes
    3. Companion cell carries out metabolic processes required by sieve tube
    4. ATP used for loading at the source
    5. Plasmodesmata link the cytoplasm of the companion cells and sieve tbe
Water uptake and movement
1. Water uptake from the root
  1. Epidermis of root cell is covered by root hair cells
    1. Increase surface area
    2. Absorb minerals from the soil by active transport
      1. Uses ATP
  2. Minerals reduce water potential of cell cytoplasm
    1. Makes water potential of cell lower than the water potential of soil
    2. Water potential gradient rule
    3. Water is taken up across the plasma membrane by osmosis
2. Movement across the root
  1. Driven by an active process at the endodermis
    1. Endodermis is a layer of cells surrounding the xylem
      1. Also known as the starch sheath
        Due to grains of starch being present - indicates energy use
  2. Casparian strip in the endodermis
    1. Special cells with a waterproof strip
      1. Blocks the apoplast Pathway forcing the symplast pathway
  3. Endodermis cells move minerals by osmosis
    1. From cortex to xylem by active transport
      1. Decreases water potential in xylem
        Water moves into the xylem by osmosis
        Reduces water potential in cells outside of the endodermis
        Creates a gradient across whole cortex
        Enter text here
3. Role of the casparian Strip
  1. Blocks apoplast pathway between xylem and cortex
  2. Water and dissolved nitrate ions must move through cell membrane
  3. Transporter proteins in cell membranes
  4. Nitrates can be actively transported into xylem
  5. Lowers water potential in xylem, creating a gradient
  6. Blocks water from moving back from the xylem to the cortex again
4. How does way move up the stem?
  1. Root pressure
    1. Active transport in the endodermis drives water into the xylem
      1. Forces the water up the xylem due to the constant water potential gradient
    2. Theory would only force water a few metres up a stem
    3. Wouldn't work with tall trees
  2. Transpiration pull
    1. Loss of water by evaporation must be replaced by water in the xylem
    2. Water molecules stick together due to cohesion
      1. Pulls water in a continuous column
    3. The pull creates tension in the walls, which is why lignin is required for extra strengh
    4. Cohesion-tension theory
    5. If transpiration stream is broken in one xylem vessel it can be controlled by another vessel through pits
  3. Capillary action
    1. Forces attract water molecules to the side of the vessel
      1. Force - adhesion
    2. As xylem vessels are very narrow, pulls water up vessels this way
Movement between cells
1. Water potential
  1. Total potential energy of the water molecules in a system. Measure of how likely water will be lost down a water potential gradient
  2. Water potential of pure water is a,ways zero
    1. Water potential of plant cytoplasm is always negative
      1. Due to solutes reducing water potential
  3. Cellulose cell wall prevents bursting and provides flexibility
    1. Full - turgid
      1. Pressure exerted on walls, creates a pressure potential
        Reduces influx of water
    2. Plasmolysis
      1. Plasma membrane pulls away from cell wall
      2. Occurs when plant cells lose water by osmosis
        Because it is in a solution with a more negative water potential than the cell
2. Apoplast pathway
  1. Cellulose walls have many water filled spaces between cellulose molecules
    1. Water moves through these gaps
  2. Water does not pass through any plasma membranes
    1. Means minerals and ions can be transported
3. Symplast pathway
  1. Waters enters the cytoplasm
    1. Travels through plasmodesmata from one cell to another
  2. Plasmodesmata link cytoplasm s of adjacent cells
4. Vacuolar Pathway
  1. Similar to symplast pathway
    1. Can also travel through vacuoles of cells
Translocation
1. Movement of sucrose along the phloem
  1. At the sink
    1. Sucrose may be converted to starch for storage, or used for metabolic processes e.g. Respiration
      1. Reduces sucrose conc. in cells
    2. Sucrose moves to surrounding cells by diffusion or active transport
      1. Increases water potential in phloem
        Water moves into surrounding cells by osmosis
        Reduces hydrostatic pressure at sink
  2. At the source
    1. Sucrose eating sieve tube reduces water potential
      1. Water moves into the sieve tube by osmosis
        Creates a high hydrostatic pressure
  3. Hydrostatic pressure gradient creates from source to sink
2. The movement of assimilates (mainly sucrose) throughout the plant in the phloem tissue, from the source to sink
3. Mass flow
  1. Movement of assimilates in water
  2. Down a gradient
  3. Up or down a plant
  4. Occurs in different tubes in different directions at the same time
4. Loading into the phloem
  1. ATP is used to actively pump H* ions out of the companion cell to surrounding tissue
    1. Creates a diffusion gradient for ions to return
  2. Co transporter proteins allow hydrogen ions to bring sucrose into cell
    1. Conc. of sucrose builds up in companion cell and diffuses through plasmodesmata into sieve tube
5. Evidence for translocation
  1. Radioactively labelled CO2 can be traced in the phloem
  2. Ringing a tree, causes sugars to collect above the ring
  3. Fluid collected from aphids shows sucrose
    1. When sectioning the stem, the styles is found in the phloem
  4. Evidence for metabolic processes
    1. Companion cells have many mitochondria
    2. Translocation can be stopped
      1. By a metabolic poison inhibiting ATP production
    3. Rate of flow of sugars is so high that energy must be required to drive it
  5. pH of companion cells is higher than surrounding cells
  6. Higher conc of sucrose at source
6. Evidence against transolcation
  1. Not all the solutes in the phloem sap move at the same rate
  2. Sieve plate role is unclear
  3. Sucrose moves at the same rate around the whole plant
    1. Doesn't move quicker to areas of a lower concentration

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Plant biology - unit 1

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	Created by ChloC almost 11 years ago