PLANTS 🌿

• The leaf is the plant organ where gas exchange occurs
• Gases involved: co2, o2 and water vapour
• The top and bottom of the leaf are covered  by a single layer of cells, the epidermis
• inner part of the leaf contains mesophyll tissue and vascular tissue in a system of vascular bundles.

  The vascular bundles in leaves  are referred to as veins.
• Palisade mesophyll is located in the upper portion of the leaf, where light is most available. The cells of this region are chloroplast rich, to allow maximum photosynthesis.
• Veins are distributed throughout the leaf to transport raw materials and products for photosynthesis. The veins occur roughly in the middle of the leaf so that they are near all the leaf cells.
• The spongy mesophyll is located just superior to the stomata, to allow continuous channels of gas exchange.
• The stomatal pores are on the bottom surface of the leaf, this area receives less light, so as a result, the temp is lower than on the upper side.
• This minimises water loss from the pores and the plant, so the lower epidermis has a thinner cuticle than the upper epidermis.
• Leaves are the primary organ for photosynthesis and it involves synthesis of carbohydrates using light energy.
• Carbon dioxide is the raw material and oxygen is one of the products. Exchange of the two gases must take place to sustain photosynthesis.
• The epidermis of the leaf has a waxy cuticle and it has a very low permeability to carbon dioxide so pores called stomata( stoma)  are found in the lower epidermis of the leaf.
• The problem for plants is that the stoma that lets carbon dioxide in will usually allow water vapour to escape.
• The loss of water vapour from the leaves and stems of plants is called transpiration.
• Plants minimise water losses through stomata using guard cells. These are the cells that are in found in pairs, on one either side of a stoma.
• Transpiration is a consequence of gas exchange in leaves.
• • When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
  • Abscisic acid triggers the flowing out of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
  • A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening.
  • When water enters the guard cells, the turgor pressure increases and the stomatal pore opens.
• Function of the guard cells.

  • To control the aperture of the stoma.
  • Can open from wide open to fully closed.
• Water leaving through stomata by transpiration is replaced by water from xylem.
• Water in the xylem climbs the stem through the pull of transpiration combined with the forces of adhesion and cohesion.
• Factors affecting transpiration

  1. Light
  2. temp
  3. wind
  4. humidity
• LIGHT AFFECTING TRANSPIRATION

  Guard cells close the stomata in darkness

  transpiration is much greater in light , open stomata increase the rate of diffusion of CO2 increasing transpirational water loss through stomata
• TEMP AFFECTING TRANSPIRATION

  Rate of transpiration (water loss) through stomata is doubled for every 10°C increase in temperature.
• WIND AFFECTING TRANSPIRATION

  Removes water vapor from leaf, reducing water potential around leaf thus increasing the water potential gradient between the leaf and its surroundings and therefore increasing the rate of transpirational water loss
• HUMIDITY AFFECTING TRANSPIRATION

  As humidity increases, water potential around leaf is increased thus decreasing the water potential gradient between the leaf and its surroundings and therefore decreasing the rate of transpirational water loss
• transverse section of the stem of a dicotyledonous plant

  A) cortex

  B) pith

  C) phloem

  D) cambium

  E) xylem

  F) epidermis

  
• Xylem cells are larger than phloem cells.
• Within one vascular bundle, phloem cells tend to be closer  to the outside of the plants in stems and roots.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  All the vascular tissue is grouped in the centre of the roots, with xylem in a star shaped area and the phloem between the points of the star.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  The xylem vessels can be identified by their large size, thick walls and rounded shape in transverse section.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  Xylem walls may be stained red in microscope images because they are lignified.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  Other cells in the root are unlignified and are usually stained blue.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  Phloem cells are smaller than xylem with thinner wall.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  The outer layer of cells in the root is epidermis with small cells that may have root hairs protruding.
• The distribution of tissues in stems and roots of dicotyledonous plants is different.

  
  

  Between the vascular tissue and the epidermis there is cortex with relatively large and thin walled cells.
• Distribution of tissues in a transverse section of the root of a dicotyledonous plant.

  A) epidermis

  B) ground tissue

  C) xylem

  D) phloem

  E) vascular tissue