Topic 9 - Plant Biology

• The majority of photosynthesis takes place in the leaves of plants

  
• Some plants are able to carry out photosynthesis in the cells of their stems
• During photosynthesis


  1. Carbon dioxide is taken in by the leaf

  2. Oxygen is released
• Stomata


  Pores in the epidermis of the leaf through which gas exchange takes place

  
• The stomata need to be open all the time in order for gas exchange, and therefore photosynthesis, to continue
• Transpiration


  Water loss in the form of water vapour through the stomata

  
• Most plants can use cells called guard cells to close their stomata in order to reduce water loss, but this will also reduce gas exchange and therefore their rate of photosynthesis
• Transpiration


  • It provides a means of cooling the plant via evaporation
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells, due to the presence of water as it moves up the plant, provides support to the leaves and to the stems of non-woody plants
  • Leaves with high turgor pressure do not wilt and therefore have an increased surface area for photosynthesis

  
• The loss of water vapour from leaves by evaporation through the stomata is unavoidable as gas exchange for photosynthesis can only occur when the stomata are open

  
• Xylem vessels


  One of the vascular tissues found within plants that transport water

  
• Cohesion between water molecules

  • Within water molecules the oxygen atom has a slight negative charge while the hydrogen atoms have a slight positive charge; this difference in charge across the molecule means that water is a polar molecule
  • As a result of the polarity of water, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules
  • This force of attraction between water molecules is known as cohesion
  • Water molecules are also attracted to the hydrophilic surface of the cell walls on the interior of xylem vessels
  • This attraction between molecules of a different type is known as adhesion

  
• Xylem vessels


  • Formed from long lines of cells that are connected at each end
  • As the xylem vessels develop, the cell walls between the connected cells degrade and the cell contents are broken down
  • This forms mature xylem vessels that are long, continuous, hollow tubes
  • Mature xylem vessels are non-living cells
  • The walls of xylem vessels are thickened with cellulose and strengthened with a polymer called lignin
  • This means xylem vessels are extremely tough and can withstand very low internal pressures, i.e. negative pressure caused by suction, without collapsing in on themselves

  
• The transpiration stream


  1. When water evaporates from the surfaces of cells inside a leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation

  2. Water molecules adhere to the cell walls of plant cells in the leaf, enabling water to move through the cell walls

  3. The loss of water from the xylem vessels generates low pressure within the xylem

  4. The low pressure generated in the xylem when water moves into the cells in the leaves creates a pulling force throughout the xylem vessels that is transmitted, via cohesion between water molecules, all the way down the stem of the plant and to the ends of the xylem in the roots

  5. This is known as transpiration-pull and it allows water to be moved upwards through the plant, against the force of gravity

  6. At the same time, forces of adhesion between the water molecules and the walls of the xylem vessels ensure that there are no air bubbles or spaces inside the xylem vessels

  
• Transpiration stream


  The continuous upwards flow of water in the xylem vessels of plants

  
• Blotting paper or filter paper


  Can be used to model the movement of water up a xylem vessel as it has the ability to absorb water and the adhesive and cohesive forces cause water to be drawn into and through the paper

  
• Porous pot


  A partially permeable container made from a material full of microscopic pores through which water can pass, used to model the evaporation of water that occurs from the leaves of a plant

  
• Capillary tubing


  When a very narrow glass tube is dipped into water, water can flow up the tube due to the adhesion of water molecules with the inner surface of the tube and cohesion between water molecules, demonstrating the importance of adhesion and cohesion in the transport of water up xylem vessels

  
• Active transport of minerals in the roots

  1. Mineral ions are actively transported into root cells by the action of specific transporter proteins in their cell surface membranes

  2. The movement of water in the surrounding soil and into the spaces within the cell walls of the root cells brings mineral ions into contact with their specific pump proteins

  3. Some plants have a mutualistic relationship with soil fungi in which the fungus grows on, and sometimes into, the roots of the plant and absorbs mineral ions that the plant's roots may not be able to access, passing them on to the plant

  
• Water movement across the root

  1. Apoplast pathway: Water moves through the series of spaces running through the cellulose cell walls of the root cells, reaching the casparian strip which forces the water out of the cell walls and into the interiors of the cells

  2. Symplast pathway: Water moves by osmosis into and out of the cells, this pathway is slower than the apoplast pathway

  
• Water movement across the leaves

  1. As transpiration occurs at the stomata, water vapour is removed from air spaces surrounding the leaf cells, creating a water vapour concentration gradient between the air spaces and nearby cells

  2. The water within the cell walls of the leaf cells lining the air spaces evaporates into the air spaces, generating transpiration pull that draws water into these cells from neighbouring cells

  3. Water moving across the leaf can move via either the apoplast or symplast pathways

  
• Movement of water across the leaves

  1. Transpiration occurs at the stomata, removing water vapour from air spaces

  2. Water within the cell walls of the leaf cells lining the air spaces evaporates into the air spaces, generating transpiration pull that draws water into these cells from neighbouring cells

  3. Transpiration pull occurs because of cohesive forces between water molecules

  4. Water moving across the leaf can move via either the apoplast or symplast pathways

  
• Water movement up the stem

  1. Water moves up the stem in the xylem vessels to replace the water that is lost at the leaves by transpiration

  2. The transpiration pull generated in the leaves is transmitted down the xylem due to the forces of cohesion and adhesion acting on water molecules

  3. The evaporation of water vapour from the leaves together with the cohesive and adhesive properties exhibited by water molecules result in water being continuously drawn up through xylem vessels within the plant

  
• The loss of water vapour from the leaves of plants by transpiration results in a transpiration pull that causes water to move upwards through the xylem vessels of the plant

  
• Xerophytes


  • Plants that have evolved effective adaptations to conserve water
  • They have features such as sunken stomata and a thickened waxy cuticle

  
• Xerophytes


  • Cacti
  • Marram grass
• Xerophytic adaptations


  • Very few stomata
  • Sunken stomata
  • Hairs surrounding stomata
  • Needle-shaped or small leaves
  • Thickened waxy cuticle
• Cacti


  • Leaves are reduced to spines that can no longer photosynthesise, reducing leaf surface area and water loss
  • Photosynthesis occurs in the green stem which possesses chloroplasts
  • Stomata are located on the stem and they are more sparsely distributed than they would be on a regular leaf
  • The stem has a thick cuticle and is very large in diameter which allows it to store water
  • The stem can expand to take on water, enabling water storage when it is available
  • Cacti carry out a specialised form of photosynthesis known as CAM photosynthesis that enables them to keep their stomata closed during the day, reducing water loss by evaporation
  • There are both shallow and deep penetrating roots which allow access to all available water

  
• Marram grass


  • Leaves are rolled up to reduce the exposure of surfaces to the wind and so reduce water loss by evaporation
  • The stomata are sunken in pits to reduce water loss by evaporation
  • The inner surface of the leaf possesses a large number of hairs which shield the stomata, again reducing water loss
  • The exposed surface has a thick waxy cuticle to reduce evaporation

  
• Halophytes


  Plants that are adapted to saline, or salty, conditions
• Halophytes


  • Have the ability to sequester, or store away, salts within their cell wall or vacuoles
  • Some halophytes can concentrate the salts they absorb in certain leaves, which then fall off the plant
  • Halophytes can shed their leaves to reduce water loss
  • In such conditions the stem is able to take over the role of photosynthesis
  • Water loss reducing adaptations such as reduced leaf surface area and sunken stomata can be found in some halophytes
  • Some halophytes have salt glands that actively excrete salt to stop it from building up
  • Halophytes have deep roots to reach fresh water underground

  
• Drawing Xylem Vessels


  1. Transverse sections can be taken of different parts of a plant to examine the internal structure

  2. Tissue maps can be drawn to show the relative positions of structures within the different parts of a plant

  3. Primary xylem vessels form from meristem tissue called cambium

  4. The xylem vessels form on the inside of the cambium tissue, towards the centre of plant stems and roots

  5. The lignin in primary xylem forms rings or spirals in the xylem walls, allowing the vessels to continue to grow in length as the plant grows taller

  6. Primary xylem vessels have thinner walls than the secondary xylem tissue that forms later in plant growth

  
• Investigating the effect of light intensity on the rate of transpiration

  1. Cut a shoot underwater to prevent air from entering the xylem

  2. Set up the apparatus ensuring it is airtight, using vaseline to seal any possible gaps

  3. Dry the leaves of the shoot to prevent water on the leaves affecting the rate of transpiration

  4. Remove the capillary tube from the beaker of water to allow a single air bubble to form and then place the tube back into the water

  5. Set up a light source from which the light intensity can be varied

  6. Allow the plant to adapt to the new environment for 5 minutes

  7. Record the starting location of the air bubble, leave for a set period of time, and then record the end location of the air bubble

  8. Change the light intensity by a measurable amount and repeat the experiment

  
• Investigating the effect of temperature and humidity on transpiration rates


  1. Use a fan on different settings to vary the flow of air around a plant shoot

  2. Enclose the plant shoot in a plastic bag to increase the humidity

  3. Use a humidifier or dehumidifier to give a measurable variation in humidity

  4. Use a lamp at different distances or with different types of light bulb to vary light intensity

  5. Use a thermometer or temperature probe to find surroundings with different air temperatures

  6. Use a heater or air conditioner to give a measurable variation in temperature
• Air movement


  More air movement leads to increased rates of transpiration
• Temperature


  Higher temperatures lead to higher rates of transpiration, up to a point at which transpiration rates will slow
• Light intensity


  Higher light intensity leads to higher rates of transpiration
• Stomata


  Pores on the leaf surface that allow gas exchange

  
• When the air is relatively still


  Water molecules can accumulate just outside the stomata, creating a local area of high humidity

  
• With less water vapour diffusion out into the air

  Due to the reduced concentration gradient
• Air currents, or wind


  Can carry water molecules away from the leaf surface, increasing the concentration gradient and causing more water vapour to diffuse out