9) Transport in plants

• What is the equation for photosynthesis?

  6CO2 + 6H2O = C6H12O6 (glucose)+ 6O2
• What are the three reasons why multicellular plants need transport systems?

  Metabolic Demands

  Size

  Surface area:volume ratio
• Metabolic demands are when the underground parts need oxygen and minerals and above ground need carbon dioxide and sunlight.
• Size - some plants are very tall so this means they have to have very efficient transport systems to move substances right to the top leaves.
• SA:V ratio = leaves are adapted to have large SA:V ratio for the exchange of gases with the air. But, the size and complexity of multicellular plants means that when the stem etc are taken into account they have a small SA:V ratio which means they can't rely on diffusion alone to supply their cells with everything they need.
• Dicotyledonous plants make seeds that contain two cotyledons, organs that act as food stores for developing the embryo plant and form the first leaves when the plant germinates.
• Dicotyledonous plants have a series of transport vessels running through the stem, roots and leaves. This is known as the vascular system. In herbaceous dicots this is made up of a two main types of transport systems: xylem and phloem.
• herbaceous dicots with soft tissues last a short life cycle and have a woody stem.
• What are the three plant processes?

  Photosynthesis - plant produces glucose, through sunlight

  Respiration - plants supplying energy

  Active transport - energy required to move and transport substances
• The transpiration system:

  • The movement of water molecules and dissolved mineral ions
  • Xylem vessels
  • Passive process
• The translocation system:

  • The movement of sugars (sucrose) and amino acids
  • Phloem vessels (sieve & companion cells)
  • Active process (requires energy)
• What type of vascular bundle?

  Root

  A) ?

  
• What is this vascular bundle?

  Leaf

  
• What is this vascular bundle?

  Stem

  
• In the stem the vascular bundles are around the edge to give strength and support
• In the roots, the vascular bundles are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind.
• In the leaves, the midrib of a dicot leaf is the main vein carrying vascular tissue through the organ. It supports the leaf and many small branching veins spread through the leaf functioning both in transport and support.
• Xylem Structure

  
• Phloem structure

  
• Water potential is the tendency of water to move from one area to another due to osmosis, gravity and mechanical pressure.
• There are fewer free water molecules in a solution than there are in pure water
• Sequestered means attached and no longer free
• Water sequestered by hydration shells are no longer free, only free water molecules contribute to water potential
• A sample of pure water will have the highest possible water potential as there is a maximum amount of free water molecules.
• Pure water = 0kPa

  All other water potentials have a negative value.
• 0kPa = pure

  -10kPa = dilute

  -500kPa = concentrated
• Hypertonic solution = high concentration inside cell

  Hypotonic = low concentration inside cell

  Isotonic = balanced concentration
• Root hair cells:

  • large SA:V ratio
  • Permanent vacuole
  • Many mitochondria
  • Specialised exchange surface for uptake of water & minerals
  • Water moves in via osmosis as lower water potential in plant
  • Mineral ions actively transported in
• Root hair cell adapatations:

  • microscopic size, the can penetrate easily between soil particles
  • Each hair has a large SA:V ratio
  • Thousands of root hairs on each growing root tip
  • Each hair has a thin surface layer so diffusion and osmosis take place more easily
  • The cellulose cell wall is completely permeable to water
• Symplastic pathway = through cytoplasm

  Apoplastic pathway = through cell wall
• Casparian strip is a band of waxy material that runs around each od the endodermal cells forming a waterproof layer.
• The suberin is a axy material that prevents water to pass through
• How does water move up the stem?

  • changes in concentration
  • attraction between water molecules
  • capillary action
  • root pressure
  • transpiration pull
• Capillary action

  • travels up against force of gravity
  • hydrogen bonds
  • cohesion between water molecules
  • adhesion to side of vessel
• Root pressure

  • process independent pf transpiration pull and action potential
  • active pumping of minerals into xylem drives force behind root pressure
  • Make water potential in xylem lower than that in root
• What is the evidence that active transport is necessary for root pressure?

  effect of cyanide - stops mitochondria from working

  effect of temperature - suggestive of an enzyme controlled chemical process

  Guttation - sap and water move out cut stem, suggesting they are actively pumped out.
• Transpiration - evaporation of water through the stomata
• transpiration stream - movement of water through the xylem
• water enters the leaves and passes into the mesophyll cells by osmosis

  water evaporates from mesophyll cells to form water vapour

  large air spaces between mesophyll cells allow water vapour to collect and diffuse through leaf
• Evidence for cohesion-tension theory:

  changes in tree diameter - at high transpiration rates dimeter decreases due to tension, At night, during low transpiration rates the diameter increases

  cut flowers - often they draw air in rather than leaking water out, as water continues to move up the cut stem

  broken xylem