Biology

• Dicotyledonous leaf


  Leaf with two seed leaves
• Cellular and tissue structure of a dicotyledonous leaf

  • Upper epidermis
  • Palisade mesophyll layer
  • Spongy mesophyll layer
  • Lower epidermis
  • Vascular bundle
• Upper epidermis


  Protects leaf cells from injury and excessive water loss
• Palisade mesophyll layer


  • Carries out photosynthesis
  • Densely packed cells, near surface
  • Many chloroplasts for maximum light absorption
• Spongy mesophyll layer


  • Allows gaseous exchange with the atmosphere
  • Loosely packed cells with air spaces for gases to reach other cells easily
• Lower epidermis


  Contains guard cells that controls the size of the stomata for gaseous exchange
• Vascular bundle


  • Transports substances to and from the leaf
  • Centrally located, branching network of tube-like structures to reach all cells
• Xylem


  • Transports water and mineral ions from roots to all parts of the plant
  • Long, continuous, hollow tube from roots to leaves
  • Thick wall strengthened with lignin to keep xylem vessel rigid and lumen open
• Phloem


  • Transports manufactured sugars (mainly sucrose) from leaves to all parts of the plant
  • Sieve tube elements made of continuous series of cells with perforated ends to form a tube
  • Minimal cell contents, e.g. no nuclei, no mitochondria
  • Companion cells contain many mitochondria to provide energy to sieve tubes to transport food
• Root hair cell


  • Long and narrow extension – Increase surface area-to-volume ratio for increased rate of absorption of water and mineral ions
  • Maintains a lower water potential in vacuole – Allow water to enter the root hair cell via osmosis
• Chlorophyll absorbs light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent uses
• Photosynthesis enables plants to make food from carbon dioxide and water
• Photosynthesis carried out by plants releases oxygen into the atmosphere
• Photosynthesis carried out by plants removes carbon dioxide from the atmosphere
• Photosynthesis


  1. Light energy absorbed by chlorophyll

  2. Chemical energy formed

  3. Carbohydrates formed from carbon dioxide and water
• How carbon dioxide reaches mesophyll cells in a leaf

  1. CO2 from the atmosphere diffuses into the leaf through the stomata

  2. CO2 moves though the intercellular air spaces in the spongy mesophyll cells towards the palisade mesophyll cells

  3. CO2 dissolves in the moisture layer on the surface of the palisade mesophyll cells and diffuses into the cell
• Limiting factors affecting rate of photosynthesis

  • Light intensity
  • CO2 concentration
  • Temperature
• Transpiration is the loss of water vapour from the stomata
• Transpiration


  The loss of water vapour from the stomata
• An open stomata is necessary for gaseous exchange with the environment (CO2 for photosynthesis and O2 for respiration). Transpiration is the consequence of an opened stomata. Stomata close to prevent excessive transpiration.
• Plants transport water from the roots to the leaves to replace water loss from transpiration.
• Water potential


  The measure of the free energy of water, which determines the direction of water movement
• How water moves through the root

  1. Water potential is higher in soil solution than the root hair cell

  2. Water enters root hair cells, moves through successive cells in the root cortex to the xylem by osmosis, down a water potential gradient
• How water moves through the leaf

  1. Water moves from xylem to leaf cells – osmosis

  2. Water moves from cell surface to air space in spongy mesophyll layer – evaporation

  3. Water vapour exits the leaf through the stomata – diffusion (resulting in transpiration)
• How water moves through the stem

  1. Water moves in through root hairs by osmosis

  2. Water moves up through xylem by transpiration pull

  3. Water moves out through stomata by transpiration
• Transpiration pull


  • Transpiration creates a force which 'pulls' the water upwards through the xylem to replace the water lost from the leaves
• Transpiration stream is the continuous flow of water through the xylem from the root, up the stem to the leaves.
• Investigating transpiration rate


  1. Potometer measures movement of air bubble/meniscus (more movement represents increased transpiration rate)

  2. Weighing method measures change in mass (greater mass loss represents increased transpiration rate)
• Factors affecting transpiration rate

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


  In moving air, the water vapour will be swept away from the leaf, increasing the diffusion of water vapour out of the leaf. In still air, the water vapour accumulates around the leaf, reducing the diffusion of water vapour out of the leaf.
• Humidity


  More humid = more water vapour in the air, reducing the diffusion of water vapour out of the leaf. Less humid = less water vapour in air, increasing the diffusion of water vapour out of the leaf.
• Temperature


  At higher temperature, moisture on the mesophyll cells evaporates into the intercellular air space more quickly, increasing diffusion of water vapour out of the leaf. At lower temperature, moisture on mesophyll cells evaporates into the intercellular air space more slowly, decreasing diffusion of water vapour out of the leaf.
• Light intensity


  At high light intensity, stomata open, allowing more water vapour to diffuse out of the leaf. At low light intensity, stomata close, reducing the diffusion of water vapour out of the leaf.
• How wilting occurs


  1. Water loss from transpiration exceeds water uptake at root

  2. Cells are plasmolysed, guard cells become flaccid and stomata close

  3. Plant wilts, leaves become soft and folded, plant may collapse due to loss of turgor
• Translocation


  The transport of manufactured food (mainly sucrose) in the phloem tissue
• How food moves through the stem

  1. Phloem loading - Sugar is actively transported from photosynthesising leaf cells to the phloem, and water follows by osmosis

  2. Mass flow - Higher water pressure at the source (photosynthesising leaves) forces the phloem contents to move towards the sink (growing regions)

  3. Phloem unloading - Sugar is unloaded at the sink, and water returns to the xylem
• Evidence of translocation


  • Aphid Stylet penetration
  • Radioactive carbon-14 tracing
  • Ringing (Girdling)
• Measuring rate of translocation


  Translocation rate = Distance between colony A and colony B / Time taken for the radioactively-labelled sugars to move from colony A to colony B
• The removal of bark containing phloem (girdling) improves the quality and size of the fruits.
• The droplet of liquid contains mostly sucrose and amino acids.