12.2

• Photosynthesis

  Carbon dioxide + water → (light energy/ chlorophyll) glucose + oxygen
• Photosynthesis

  6CO2 + 6H2O → C6H12O6 + 6O2
• Xylem structure


  • Continuous hollow tubes extending from roots to leaves
  • No cross walls or protoplasm to obstruct water flow
• Structure of xylem

  • Walls are lignified to prevent collapse of vessel and to provide mechanical support
  • Pits in wall allows transverse movement across different vessels
• Adaptation of xylem

  • No cross wall, continuous lumen to reduce resistance to water flow
  • Wall with lignin and cellulose to prevent collapse and provide mechanical support
  • No protoplasm, hollow lumen to reduce resistance to water flow
• Transport in xylem from roots to leaf

  xylem vessels are continuous extending form root to stem to leaf

  Xylem tissues transports water and dissolved minerals from roots to all parts of the plant
• adaptation of root hair cell

  1. Long and narrow root hair increases surface area-to-volume ratio for faster absorption of water molecules (by osmosis) and mineral ions (by diffusion or active transport) from soil solution.
  2. Root hair cell contains many mitochondria to release moreenergy through higher rate of aerobic respiration for increased rate of active transport of mineral ions from soil solution into the root hair cell.
• Entry of water through root hair

  1. Soil solution has higher water potential than root hair cell, water enters by osmosis

  2. Water potential of root hair cell becomes higher than inner cells, water moves into inner cells by osmosis

  3. Water moves from cell to cell and eventually reaches root xylem

  4. Water enters root xylem and moves up the stem xylem
• Transpiration pull (caused by transpiration occurring at leaf)

  Suction force that transports water and dissolved minerals from xylem in roots to xylem in stem to xylem in leaves in a continuous transpiration stream
• Transpiration

  • Loss of water vapour through the aerial parts of a plant, mainly through the stomata of leaves
  • created transpiration pull (force) that draws water up from roots to leaves.
• Transpiration

  Creates transpiration pull that draws water up from roots to leaves
• How water moves out of leaf

  1. Water molecules move from xylem to mesophyll cells by osmosis and move from cell to cell.
  2. Water molecules move out of mesophyll cells to form a thin film of moisture over their surfaces.
  3. Water molecules evaporate from the thin film of moisture and accumulate as water vapour molecules in the intercellular air space.
  4. Water vapour molecules diffuses through stomata to drier air outside the leaf.
• In the leaf:

  Water that entered the leaf xylem can:

  • move into mesophyll cells and use for photosynthesis
  • move out of stomata as water vapour (when stomata are open to take in CO2 for photosynthesis).
• The more humid the environment is,

  the less steep the water vapour concentration gradient between the intercellular air spaces of the leaf and the environment, the lower the rate of transpiration.

  
  

  Note: The intercellular air spaces in the leaf are normally

  saturated with water vapour.
• The stronger the wind,

  the faster the water vapour that accumulates around the leaf gets blown away, the steeper the water vapour concentration gradient between the intercellular air spaces of the leaf and the environment, the higher the rate of transpiration.
• The higher the temperature,

  the higher the rate of evaporation of the thin film of moisture lining the mesophyll cells, higher concentration of water vapour in the intercellular air spaces of the leaf, steeper water vapour concentration gradient between the intercellular air spaces of the leaf and the environment, the higher the rate of transpiration.
• The brighter the light,

  the faster the rate of photosynthesis. As glucose accumulates in the guard cells, water potential of the guard cells decrease. Water molecules move from the epidermal cells into the guard cells by osmosis. Guard cells become turgid due to the entry of more water molecules. Stomata open, Resulting in a higher rate of transpiration.
• What conditions will give the highest rate of transpiration?

  • Humidity (low)
  • Air movement (high)
  • Temperature (high)
  • Light (high)
• Importance of Transpiration

  1. Draws water and mineral salts from roots to leaves so that photosynthesis can occur
  2. Evaporation of water from leaves removes latent heat and cools the plant
  3. Allows water to be transported to all parts of the plant that keeps the cells turgid and replace water loss
• Excessive Transpiration

  Cells lose turgidity and become flaccid.

  Advantages:

  1. Leaf folds up to reduce surface area exposed to sunlight; Guard cells are flaccid, stomata closes and transpiration rate is reduced.

  Disadvantages:

  1. Rate of photosynthesis is reduced! WHY??
  2. Less water is drawn up leaf xylem, water becomes a limiting factor.
  3. Stomata is closed, CO2 becomes the limiting factor.
  4. Leaf is folded, reduced surface area exposed to light. Wilting occurs when Rate of transpiration > Rate of water absorption
• Adaptations of plants living in very dry condition

  1. Stomata are sunken and lies in grooves
  2. Tiny hairs in grooves trap water vapour
  3. Increase humidity around stomata to reduce rate of transpiration
• Fate of glucose from photosynthesis

  Used immediately for respiration to release energy for cellular activities or form cellulose cell walls, Excess glucose is converted to starch and stored in leaf, Converted to amino acids in leaves and used to form proteins and new protoplasm. Excess amino acids are transported to other parts of plant for synthesis of proteins and new protoplasm, Converted to sucrose that is transported in phloem to storage organs (stem/root tubers) converted back to glucose for respiration, forms a component of nectar in flowers, Converted into fats for: storage, used in respiration, synthesis of new protoplasm
• Translocation


  Transport of food (sucrose and amino acids) through phloem, Bidirectional (move from leaf to roots or leaf to buds) and requires energy.
• Structure of phloem

  • Consists of sieve tubes (sieve tube cells and sieve plate) and companion cells, sieve tube cell with thin layer of cytoplasm, sieve plate with pores, companion cell
• Adaptations of phloem


  • Sieve tubes: Each sieve tube is made up of columns of sieve tube cells or sieve tube elements arranged end-to-end. Sieve tube cells are lack of nuclei and have thin layers of cytoplasm. Purpose: Reduces the resistance to flow/allows rapid flow of manufactured food substances through the sieve tubes. Sieve plates: Cross walls found in between the sieve tube cells. Purpose: Pores in sieve plates allow manufactured food substances to flow rapidly through the sieve tubes. Companion cells: Narrow, thin-walled cell with cytoplasm, nucleus and numerous mitochondria. Purpose: Provides energy needed for companion cells to "load" sugars from mesophyll cells into the sieve tubes by active transport, Carries out metabolic processes and provides nutrients to the sieve tube cell to keep the sieve tube cell alive.
• Translocation Studies

  Using aphids: Aphids feed on plant saps by using their stylet to penetrate leaf or stem. Analysis of content exuding from cut-end of stylet shows presence of sucrose and amino acids. Stem examined and found stylet inserted into phloem. Conclusion: Translocation occurs in phloem. "Ringing" Experiment: Removal of phloem prevents the translocation of sugars to the region below the ring. Accumulation of sugars in the region just above the ring lowers the water potential of the cells in that region. Water molecules enter the region and results in swelling. Using Radioactive isotope (14CO2): A leaf is provided with carbon dioxide containing the radioactive carbon. When photosynthesis takes place, the sugars formed contain the radioactive carbon. It is found that the radioactive carbon are present in the phloem.
• adaptation of root hair cell

  • Concentrated cell sap that contains sugars, amino acids and salts and has a lower water potential than soil solution, thus allowing water molecules to move into root hair by osmosis.
  • Partially permeable cell membrane prevents cell sap from leaking out
• HOW ARE VASCULAR TISSUES (XYLEM AND PHLOEM) ORGANISED?

  memorise

  
• Comparison of stem and root

  
• Pathway of a water molecule form root to leaf

  1. Water vapour molecules in the intercellular air space diffuse out of the leaf through the stomata into the surrounding air
  2. is replaced by more water evaporating from the thin film of moisture lining the mesophyll cells.
  3. This causes inner mesophyll cells to draw more water from leaf xylem.
  4. Water molecules are drawn from root hair cells, which in turn absorb water molecules from the soil water by osmosis.
  5. The water molecules pass by osmosis from the root hair cells into the inner cells, until the water molecules enter the xylem vessels.
• Sun vs shade plants

  Shade plants: higher rate of photosynthesis at lower light intensity

  Rate of photosynthesis reaches max, plateau at lower light intensity