transport in plants

• Why do multicellular plants needs a specialised transport system ?

  High metabolic demand

  Size, bigger organisms have a longer transport distance around the plant

  Small SA:V so diffusion alone is not efficient enough
• cotyledon = embryonic leaf found in a seed
• dicotyledonous plant = a group of flowering plants whose leafs have 2 embryonic leaves
• Vascular bundles are made of xylem and phloem tissue distributed throughput the plant. They provide structural support and transport substances around the plant
• Describe the vascular bundle in a root ?

  Xylem is found at the centre in a cross-like shape, surrounded by the phloem
• What is the benefit to the root of having xylem in the centre ?

  Xylem vessels are mechanically strong so they provide a dfill like structure at the centre allowing the plant to push down inot the roots, stabilising it.
• Describe the vascular bundle in the leaf ?

  Xylem is located on the top with phloem underneath
• Describe the vascular bundle in the stem ?

  Round the edge of the stem, with the phloem on the outside and xylem in the middle. Cambium separates the two vessels.
• What is in cambium ?

  Meristem cells
• What are the benefits of the vascular bundle to the stem ?


  Supports the stem, preventing bending
• Describe 3 structures and explain their functions in the xylem ?

  1. Thick walls of lignin : waterproof and provides structural support to help withstand the hydrostatic pressures. Preventing collapse of the xylem
  2. Pits : allow for lateral movement of water/ions out of the xylem
  3. Hollow tube with no cytoplasm : uninterrupted mass flow of water
• Describe 4 structures and explain their functions in the phloem ?

  1. Sieve tube elements : no nucleus or vacuole to reduce resistance and maximise space for transport of phloem sap.
  2. Sieve plates (pores at the end of sieve tube elements) : allow for phloem sap to move between cells
  3. Companion cells : contain nucleus and lots of mitochondria to provide essential molecules to sieve tube elements
  4. Plasmodesmata (channels between companion cells and sieve tube elements) : allow movement of ATP/proteins ect into the phloem.
• How is a root hair cell adapted to its function ?

  Large SA:V (long projections)

  Thin surface layer for short diffusion pathway

  Lots of mitochondria

  Small enough to penetrate compact soil
• Why may a root hair need lots of mitochondria ?

  For active transport of ions into the cell to lower its water potential. Increasing the movement of water via osmosis
• Transpiration is the loss of water vapour from the leaves of the plant.
• Apoplast pathway = water moves by mass flow through the cell walls
• Symplast pathway = water moves via osmosis between plasmodesmata through cells cytoplasm.
• The apoplast pathway must join the symplast pathway to pass the casparian strip.
• What is the benefit to the plant of the casparian strip ?

  Allows the cell membrane to control what substances enter the xylem
• casparian strip = waterproof band of suberin around the endodermis
• root pressure = active transport of minerals into the xylem to lower the water potential and increase movement of water into xylem via osmosis
• What are the structures found in a leaf ?

  waxy cuticle

  palisade mesophyll

  Spongy mesophyll

  Stomata
• Surface of the cells of the leaf are covered in thin layer of water which evaporates as water vapour
• Transpiration pull = movement of water up the xylem by capillary action to take place of the water lost in transpiration
• Why does water move up the xylem ?

  Moves from area of high hydrostatic pressure in the roots to low hydrostatic pressure in the leaves
• What is the cohesion tension theory ?

  Movement of water by capillary action due to the cohesion as water molecules bind to themselves and the adhesion of water to the walls of the xylem.
• What is the evidence for cohesion tension theory ?

  1. when a stem is cut, air is sucked into the xylem : suggesting xylem is under tension.
  2. Diameter of tree trunk decreases whne transpiration is maximum : supports transpiration pull is generating a negative pressure in the xylem
• Describe a bubble potometer set up ?

  Fine capillary tube filled with water connected to a plant ,with a cut stem, and connected to a syringe filled with water. A bubble is made in the tube using a needle.
• What precautions should be taking when using a potometer ?

  cut the plant stem underwater

  cut shoot at a slant

  no bubbles in the apparatus

  underside of leaf must be dry

  allow time for plant to acclimatise before measuring
• When using a mass potometer what precaution should be taken ?

  Must prevent evaporation of water from soil so cover the soil with plastic
• what is the advantage of using a mass potometer ?

  Directly measures rate of transpiration rather than rate of water uptake like a bubble potometer

  Less disruptive to the plant as stem is not cut
• Name an adaption of guard cells to their function ?

  Inside wall is thicker, this prevents even expansion allowing the stomata to open/close
• Describe the opening of stomata ?

  1. ions diffuse into cell lowering the water potential
  2. Water enters cell via osmosis
  3. Guard cells swell, become turgid and open
• Hydrophytes = plants that live either partially or completely submerged in water
• Xerophytes = plants that live in areas where water lost via transpiration is greater than water taken up by roots
• Why can plants not always close their stomata when water availability is low ?

  They need to keep them open to allow CO2 to diffuse in for photosynthesis
• Give examples of xerophytes ?

  cacti and marrom grass
• Name 5 adaptions of xerophytes ?

  leaf loss

  spines

  thick waxy cuticle

  sunken stomata

  extensive shallow roots
• How does leaf loss aid xerophytes ?

  Reduces surface area so reduces water lost through transpiration.
• How do spines aid xerophytes ?

  They trap moist air near the stem, reducing the rate of transpiration. They provide shade for the stem