The leaf

• Structure of the leaf


  • Upper epidermis
  • Spongy Mesophyll Layer
  • Lower epidermis
  • Waxy cuticle
  • Guard Cells
  • Stomata
  • Palisade Mesophyll Layer
  • Vascular Bundle
• Plants make their can food using the energy provided by sunlight is a process called photosynthesis
• Photosynthesis occure inside the leaves, inside the leaf cells
• Photosynthesis equation


  carbon dioxide + water -> glucose oxygen
• The upper epidermis is a strong, skin-like layer of dead cells that are virtually empty and so allow sunlight to poss through easily
• The spongy Mesophyll Layer is called spongy because it has mony air spaces that allow for good gas exchange and the cerben dioxide con diffuse mare easily through this layer to reach the pelade cels where mast phatasynthesis wil happen
• The lower epidermis is a strong, skin-like layer of deed cells on the underside of the leaf and this leyer also hosts the guard cells and their stameta
• The waxy cuticle is a waterproof layer the surface of the upper epidermis that prevents wer from evaporating from the exposed upper surface of the leat
• The palisade Mesophyll Layer is a tightly packed layer of cells that are packed with chloroplasts and bunched together near the top of the leaf to trap as much sunlight for photosynthesis
• The vascular bundles are the veins you see along the leaves and consist of xylem vessels carrying water and mineral ions to the leaves and phloem tubes carrying the dissolved glucose made during photosynthesis to the rest of the plent
• Stomata are pores found only on the underside of the leaf where carbon dioxide (CO) diffuses in and water can evaporate and diffuse out
• The structure of the leaf is in 30 layers which is quite difficult to show in a simple diagram
• Transpiration


  The loss of water from a plant
• Translocation


  The transport of food substances (mainly dissolved sugars) made in the leaves to the rest of the plant
• Phloem tubes


  • Made of columns of elongated living cells with small pores at the end to allow cell sap to flow through
  • Transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or storage
  • Transport goes in both directions
• Xylem tubes


  • Made of dead cells joined end to end with no end walls between them and a hole down the middle
  • Strengthened with a material called lignin
  • Carry water and mineral ions from the roots to the stem and leaves
• Transpiration stream


  Movement of water from the roots, through the stem and out of the leaves
• Transpiration


  Caused by the evaporation and diffusion of water from a plant's surface, mostly from the leaves
• Transpiration creates a slight shortage of water in the leaf


  More water is drawn up from the rest of the plant through the xylem vessels to replace it
• More water is drawn up from the roots


  There is a constant transpiration stream of water through the plant
• Transpiration is a side-effect of the way leaves are adapted for photosynthesis
• Leaves have stomata so that gases can be exchanged easily
• There is more water inside the plant than in the air outside


  Water escapes from the leaves through the stomata by diffusion
• Factors affecting transpiration rate


  • Light intensity
  • Temperature
  • Air flow
  • Humidity
• Light intensity


  The brighter the light, the greater the transpiration rate
• Temperature


  The warmer it is, the faster transpiration happens
• Air flow


  The better the air flow around a leaf, the greater the transpiration rate
• Humidity


  The drier the air around a leaf, the faster transpiration happens
• Measuring transpiration rate


  1. Set up apparatus with a potometer

  2. Record distance moved by air bubble per unit time

  3. Keep conditions constant
• Guard cells


  • Have a kidney shape which opens and closes the stomata
  • When the plant has lots of water, the guard cells fill with water and go plump, opening the stomata
  • When the plant is short of water, the guard cells lose water and become flaccid, closing the stomata
  • Thin outer walls and thickened inner walls make the opening and closing work
  • Sensitive to light and close at night to save water without losing out on photosynthesis
  • More stomata on the underside of leaves than the top surface