Plant organisation

• Epidermal tissue

  • Covers the whole plant
  • Covered with a waxy cuticle, which helps reduce water loss
  • Upper epidermis - transparent so that light can pass through it to the palisade layer
  • Lower epidermis - full of little holes called the stomata, which let carbon dioxide diffuse directly into the leaf
• Palisade mesophyll tissue

  • Where most photosynthesis happens
  • has lots of chloroplasts
  • near the top of the leaf where they can get the most light
• Spongy mesophyll tissue

  • Big air spaces to allow gases to diffuse in and out of cells
• Meristem tissue

  • Found at the growing tips of shoots and roots.
  • Able to differentiate into lots of different types of plant cells, allowing the plant to grow
• Stomata

  • The opening and closing of the stomata is controlled by guard cells
  • They let carbon dioxide diffuse directly into the leaf
  • They are open for as short a time as possible to reduce water loss
  • They are sensitive to light and close at night to save water without losing out on photosynthesis
  • Mostly on the undersides of leaves - lower surface is shaded and cooler so less water is lost through the stomata than if they were on the upper surface
• What is translocation?

  • The movement of food molecules through the phloem tissue
  • Transport goes in both directions
• Phloem tissue

  • Transports dissolved sugars from the leaves to the rest of the plant for immediate use or for storage
  • Composed of tubes of elongated living cells with small pores in the end walls to allow cell sap to flow through
• Xylem tissue

  • Transports water and mineral ions from the roots to the stems and leaves
  • Made up of dead cells joined end to end with no end walls between them and a hole down the middle
  • Strengthened by a material called lignin
• What is transpiration?

  • The evaporation of water from the leaves
• What is the transpiration stream?

  • The movement of water from the roots, through the xylem and out of the leaves
• What factors affect the rate of transpiration?

  • Temperature
  • Light intensity
  • Air movement
  • Humidity
• Light intensity

  • Increases transpiration rate
  • The brighter the light, the greater the rate of photosynthesis - stomata open to let in carbon dioxide → water diffuses out of the leaf
• Temperature

  • Increases transpiration rate
  • The warmer it is, the more energy the particles have - the water particles are more likely to evaporate and diffuse out of the leaf
• Air movement

  • Increases transpiration rate
  • Removes water vapour from leaf surfaces; more water diffuses from the leaf
• Humidity

  • Decreases transpiration rate
  • High humidity will reduce the water vapour concentration gradient → the rate of evaporation will decrease, and so will the rate of transpiration