Plant cell organisation

• epidermal tissue


  covers the whole plant
• spongy mesophyll tissue


  contains big air spaces to allow gasses to diffuse in and out of the cell
• palisade mesophyll tissue


  where most photosynthesis happens
• meristem tissue


  found in tips of shoots and roots - is able to differentiate into different types of plant cell
• the upper epidermal tissues are covered with a waxy cuticle which helps to reduce water loss by evaporation
• the upper epidermis is transparent so that light can pass through
• the palisade layer has lots of chloroplasts
• tissues of leaves are also adapted for efficient gas exchange e.g the lower epidermis is full of stomata which lets CO2 diffuse directly to the leaf
• phloem


  made up of living cells with small pores in the end walls to allow cell sap to flow through. it transports food substances (mainly dissolved sugars) around the plant. the transport goes both directions - this is called translocation
• xylem


  made of dead cells - hollow tubes. lignin in the cell wall strengthens it. transports water and mineral ions
• translocation


  transportation of sugars
• transpiration


  transportation of water
• water is drawn up from the rest of the plant through the xylem vessels to replace water lost. this means more water is drawn up from the roots so there is a constant transpiration stream of water through the plant

  
• why do plants lose water?

  The loss of water vapour from the plant cools the plant down when the weather is very hot.
• transpiration


  loss of water from a plant
• transpiration rate is affected by

  • light intensity - brighter light = faster rate
  • temperature - higher temp = faster rate
  • air flow - more air flow = faster rate
  • humidity - drier leaves = faster rate
• guard cells


  open and close the stomata
• when the plant has lots of water the guard cells fill up and go plump it this makes the stomata open so gases can be exchanged for photosynthesis
• guard cells are sensitive to light so they close at night to save water without losing out on photosynthesis
• guard cells are on the lower side because it is cooler and therefore less water is lost