plant structures and functions

• photosynthesis

  carbon dioxide + water -> glucose + oxygen
• CO2 + H20 -> C6H12O6 + O2

  carbon dioxide + water -> glucose + oxygen
• limiting factors of photosynthesis

  light intensity, carbon dioxide, temperature

  
• enzymes denature at high temperature
• more CO2 increases the rate of photosynthesis because there is more CO2 to make sugars (glucose)
• phloem - transports sucrose and other nutrients around the plant
• xylem - transports water from the roots
• what are the two specialised plant cells?

  xylem and phloem
• phloem

  sieve tube elements - little cytoplasm so there's more space for nutrients

  companion cells - contain lots of mitochondria (supply energy from respiration for active transport of sucrose in + out of sieve tubes
• phloem

  diagram:

  A) companion cell

  B) sieve plate

  C) sieve tube

  
• sucrose is translocated around the plant in the phloem sieve tubes
• xylem cells are dead cells which have no cell contents so there's more space for water containing mineral ions
• xylem cells have pits in their walls to allow water and mineral ions to move out\
• xylem cell walls are strengthened with lignin rings - prevents them from collapsing
• xylem cells have no end walls so they're a long tube for water to flow easily
• xylem


  diagram:

  
• transpiration is the loss of water by evaporation from the leaves
• the movement of water from the roots to the leaves is called the transpiration stream
• transpiration:

  1. water enter roots by osmosis
  2. water is drawn up the stem through the xylem
  3. water is drawn out of the leaf cells and xylem
  4. water vapour evaporates from leaves through the stomata
• transpiration


  diagram:

  
• stomata are mainly found on the lower surface of the leaf
• stomata are known as the guard cells
• when guard cells take in water by osmosis, they swell and this causes the stoma to open
• when guard cells lose water, they become flaccid and the stoma closes
• stoma is the gap when the stomata opens
• stomata

  
• translocation is the transport of sucrose around a plant
• dissolved sugars are transported around a plant in phloem
• translocation

  
• translocation

  1. sucrose is produced in leaves from glucose in photosynthesis
  2. dissolved sucrose is carried around plant in phloem
  3. dissolved sugars converted to starch and stored in storage organs (e.g. potato) for later
  4. sucrose is needed in growing regions (e.g. bud)
• xylem flows in a one way system
• factors affecting transpiration:

  light intensity - high light intensity cause stomata to open, increasing evaporation so more water is taken up to replace it

  air movement - wind blows moist air away from stomata, keeping diffusion gradient high. so more air movement = higher transpiration rate

  temperature - higher temp means water molecules have more energy, so they move faster, increasing rate
• plants have adaptations to allow them to survive in extreme conditions
• dry conditions -

  waxy cuticle - reduce water loss

  leaf hairs - trap moist air around stomata
• tropical conditions

  large leaves - absorb sunlight

  water runs off leaves
• plant hormones control and coordinate plant growth and development
• tropism - plants response to a stimulus
• positive tropism - plant grows towards stimulus
• positive phototropism - plant grows towards light (shoots)
• positive gravitropism - grow downwards, with gravity (roots)