Edexcel Biology GCSE - Topic 6: Plant Structures and Their Functions Notes

6.1, 6.2, 6.3 - Photosynthesis

  • Plants and algae are primary food producers, synthesizing it from sunlight via photosynthesis.
  • They are the primary producers of biomass in food webs and chains.
  • Photosynthesis happens in plants and algae.
  • It's an endothermic reaction, absorbing more energy than it releases.
  • Light energy transfers to chloroplasts in leaves.
  • Photosynthesis equation:
    light carbon dioxide+water→glucose+oxygenlight\ carbon\ dioxide + water \rightarrow glucose + oxygen
  • Chemical symbols:
    • Carbon dioxide: CO2CO_2
    • Water: H2OH_2O
    • Oxygen: O2O_2
    • Glucose: C<em>6H</em>12O6C<em>6H</em>{12}O_6
  • The rate is affected by several factors, and any of these can become a limiting factor, restricting photosynthesis rate at low levels, regardless of other factor increases.
  • Factor Effects:
    • Temperature: Rate increases with temperature until enzymes denature, then decreases.
    • Light Intensity: Rate increases with light intensity for most plants.
    • Carbon Dioxide Concentration: Rate increases with concentration.

6.4 Higher Only - Interaction of Limiting Factors in Photosynthesis

  • Experiment to measure the rate of photosynthesis by measuring oxygen production of a plant.
    • Pondweed in a test tube with water, sealed with a bung and capillary tube connected to a syringe.
    • Lamp at a measured distance.
    • Oxygen produced forms a gas bubble in the capillary tube.
    • Measure bubble distance to calculate oxygen volume.
    • Variables: temperature (water bath), time, light intensity (lamp distance).
  • Control all factors except the independent variable for a valid experiment.
  • Limiting factor: restricts rate of photosynthesis even when other factors increase, causing the curve to level off on a graph.
  • Graph Interpretation:
    • One limiting factor: one line levels off.
    • Two limiting factors: two lines at different environmental conditions (e.g., temperatures).
    • Three limiting factors: Similar to two, but another factor is stated on each line, which is the same in each.
  • Light intensity is measured in lux.
  • Farmers use limiting factors to enhance greenhouse conditions for increased photosynthesis, growth, and profits.

6.5 - Core Practical: Light Intensity and Rate of Photosynthesis

  • Experiment setup to measure photosynthesis rate:
    • Sealed 100ml flask with water, gas syringe, pondweed, lamp, 1m ruler.
    1. Place flask and pondweed 15cm from lamp.
    2. Wait 10 minutes for adjustment.
    3. Connect gas syringe and record volume change after 5 minutes.
    4. Move lamp 10cm further and repeat.
  • Graph results: distance from lamp (x-axis) vs. change in gas volume (y-axis).

6.6 Higher Only - Inverse Square Law: Rate of Photosynthesis

  • Light intensity is directly proportional to photosynthesis rate.
  • More light, more photons hit chloroplasts, increasing photosynthesis.
  • Inverse proportion: distance from light source increases, the light intensity decreases.
  • Inverse square law: Light intensity ∝ 1/distance21/distance^2
  • Example: Lamp 2 meters away has ¼ the light intensity ( 1/22=¼1/2^2 = ¼ ).

6.7 and 6.8 - Structure Adaptations

  • Cells adapted for specific functions:
    • Root Hair Cells:
      • Take up water by osmosis and mineral ions by active transport.
      • Large surface area (root hairs) for more water intake.
      • Large permanent vacuole affects water movement speed.
      • Mitochondria provide energy for active transport of mineral ions.
    • Xylem Cells:
      • Transport water and mineral ions from roots to shoots.
      • Lignin is deposited, causing cell death and lignification.
      • Hollow, joined end-to-end to form a continuous tube.
      • Lignin deposited in spirals to withstand water movement pressure.
    • Phloem Cells:
      • Carry photosynthesis products (food) to all parts of the plant.
      • Cell walls form sieve plates for substance movement between cells.
      • Cells are alive.
      • Companion cells provide energy (mitochondria) for sucrose transport.

6.9 - Transpiration and the Stomata

  • Transpiration: water loss from leaves and stems, a consequence of gaseous exchange via open stomata.
    • Water evaporates at open stomata on leaf surfaces.
    • Water molecules are attracted; when some leave, the rest are pulled up through the xylem.
    • This results in a continuous transpiration stream, increasing water uptake from the soil.
  • Guard cells control stomata opening and closing.
    • Kidney-shaped with thin outer walls and thick inner walls.
    • When water is abundant, cells fill and open stomata (also light-sensitive).
    • This allows gas exchange and water evaporation.
    • More stomata underside of leaf to minimize water loss due to shading and cooler temperatures.

6.10 - Translocation

  • Translocation: movement of food substances (e.g., sucrose) in the phloem, for use or storage.
    • Occurs only in phloem.
    • From sources (where made) to sinks (where used or stored).
    • Source and sink locations vary by season (e.g., spring: root to leaf, summer: leaf to root).

6.11B Biology Only - Adaptations of the Leaf

  • Leaf adaptations for specific functions:
    • Stomata: close to minimize water loss, open for gas exchange.
    • Chlorophyll: green for efficient light absorption.
    • Thinness: short distance for carbon dioxide to enter and oxygen to exit.
    • Large surface area: maximizes light absorption.

6.12 - Environmental Factors and Rate of Water Uptake

  • Factors affecting water uptake and transpiration:
    • Increase in temperature:
      • Molecules move faster, increasing evaporation and transpiration.
      • Increased photosynthesis, opens more stomata, increases transpiration.
    • Increase in relative humidity:
      • Reduces concentration gradient, decreases transpiration.
    • Increased air movement (wind):
      • Lowers water vapor concentration around the leaf, increasing the concentration gradient and transpiration.
    • Increase in light intensity:
      • Increased photosynthesis, opens more stomata, increases transpiration.

6.13 - Rate Calculations for Transpiration

  • Measuring water uptake indicates transpiration rate (water is only taken up when it leaves).
  • Use a potometer: place a plant in a capillary tube in water.
  • Measure the distance the bubble travels in a set time.
  • Greater distance = greater transpiration and water uptake rate.

6.14B Biology Only - Extreme Adaptations

  • Plants in extreme environments have specific adaptations to maximize sunlight and carbon dioxide intake:
    • Leaf shape and size: small or absent leaves to reduce water loss.
    • Waxy cuticle: prevents water evaporation.
    • Stomata: close to prevent evaporation, open for carbon dioxide intake when needed.

6.15B Biology Only - Plant Hormones and Growth

  • Hormones coordinate and control growth; needed for tropisms (phototropism, gravitropism/geotropism).
  • Move from production site to where needed.
  • Auxins:
    • Positive phototropism: Plant grows towards light.
      1. Light on one side.
      2. Auxin moves to the shaded side of the shoot.
      3. Stimulates cell growth.
      4. Shoot bends towards light.
      5. Increased light, faster photosynthesis.
    • Negative gravitropism: Shoot grows away from gravity.
      1. Horizontal shoot.
      2. Auxin moves to the lower side.
      3. Stimulates cells to grow more.
      4. Shoot bends and grows away from ground.
      5. Higher ground = more light.
    • Positive gravitropism: Root grows towards gravity.
      1. Horizontal root.
      2. Auxin moves to the lower side.
      3. Stimulates upper cells to grow.
      4. Root bends and grows downwards.
      5. More ground = Increased water and nutrients, stability.
  • Equal auxin distribution results in straight growth.
  • Investigate effects of light/gravity by:
    • Placing seedlings in a cardboard box with light from one side.
    • Attaching a petri dish with seedlings to a wall.

6.16B Higher and Biology Only - Commercial Uses of Plant Hormones

  • Humans use plant hormones to alter plant growth in agriculture and horticulture:
    • Increase yield, obtain desirable features, lower costs.
    • Auxin:
      1. Weed Killers:
        • Affect broad-leaved plants.
        • Cause rapid cell growth, killing weeds.
      2. Rooting Powders:
        • Clone plants with desirable features.
        • Apply rooting powder containing auxin to cuttings.
        • Roots grow rapidly.
      3. Tissue Culture:
        • Clone plants in a growth medium with nutrients.
        • Add auxins to form roots and shoots.
    • Gibberellins are used in germination, for fruit and flower:
      • Breaks seed dormancy.
      • Allows fruits to grow heavier and larger, increasing yields.
      • Encourages flowering plants to flower at a faster rate.
    • As ethene controls ripening, it is used in the food industry.
      • Fruit picked unripe for transport.
      • Exposed to ethene and warmer temperatures to ripen.
      • Ethene controls cell division and stimulates enzymes.
      • Reduces wastage. Fruit is more suitable to be sold and it does not ripen too early.