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

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

Table of Contents

• 6.1, 6.2, 6.3 - Photosynthesis
• 6.4 - Higher Only Interaction of Limiting Factors in Photosynthesis
• 6.5 - Core Practical: Light Intensity and Rate of Photosynthesis
• 6.6 - Higher Only Inverse Square Law: Rate of Photosynthesis
• 6.7 and 6.8 - Structure Adaptations
• 6.9 - Transpiration and the Stomata
• 6.10 - Translocation
• 6.11B - Biology Only Adaptations of the Leaf
• 6.12 - Environmental Factors and Rate of Water Uptake
• 6.13 - Rate Calculations for Transpiration
• 6.14B - Biology Only Extreme Adaptations
• 6.15B - Biology Only Plant Hormones and Growth
• 6.16B - Higher and Biology Only Commercial Uses of Plant Hormones

6.1, 6.2, 6.3 - Photosynthesis

• Photosynthesis is the process by which plants and algae synthesize food using sunlight.
• It is performed by plants and algae and is critical as the primary producer of biomass in all food webs and food chains.
• Photosynthesis is an endothermic reaction, meaning it absorbs more energy than it releases.
• Light energy from the environment is absorbed by chloroplasts in leaves.
• The equation for photosynthesis is:
  ext{light} + ext{carbon dioxide (CO}2 ext{)} + ext{water (H}2 ext{O)}
  ightarrow ext{glucose (C}6 ext{H}{12} ext{O}6 ext{)} + ext{oxygen (O}2 ext{)}
• Each compound's chemical symbol:
  • Carbon dioxide: CO₂
  • Water: H₂O
  • Oxygen: O₂
  • Glucose: C₆H₁₂O₆

Factors Affecting Photosynthesis

• The rate of photosynthesis can be affected by several environmental factors:
  • Temperature:
  • An increase in temperature increases the rate of photosynthesis until enzymes denature, causing a decrease in the reaction rate.
  • Light Intensity:
  • Most plants experience increased photosynthesis rates with higher light intensity up to a saturation point.
  • Carbon Dioxide Concentration:
  • Higher concentrations of CO₂ contribute to higher rates of glucose production.
• A limiting factor exists when an increase in one factor (e.g., CO₂ or temperature) does not further increase the rate of photosynthesis due to another factor being in low supply.

6.4 Higher Only - Interaction of Limiting Factors in Photosynthesis

• To measure the impact of various factors on photosynthesis, an experiment can be set up using pondweed in a sealed test tube filled with water, with a lamp placed at a fixed distance.
• As the pondweed photosynthesizes, it produces oxygen, which can be measured to calculate the rate of photosynthesis.
• Key elements of the experiment include:
  • Setup:
  • Pondweed in water-filled test tube with a capillary tube connected.
  • A light source (lamp) at a predetermined distance.
  • Oxygen production is indicated by the movement of a gas bubble in the capillary tube, which is measured to calculate volume.
  • Variables:
  • Independent Variable: the factor being tested (e.g., light intensity, temperature).
  • Controlled Variables: all other factors such as CO₂ concentration, temperature, etc.
• Graph Interpretation:
  • A graph depicts the relationship between the limiting factor and the rate of photosynthesis, illustrating leveling off at the maximum rate due to limiting factors.
  • For two limiting factors, two lines can represent varying conditions in a controlled experiment.

6.5 - Core Practical: Light Intensity and Rate of Photosynthesis

• A simple experimental setup can help measure the rate of photosynthesis:
  1. Sealed 100 ml flask filled with water at room temperature, gas syringe, pondweed, lamp, 1m ruler.
  2. Position flask and pondweed 15 cm from lamp. Allow time to adjust (10 minutes).
  3. Measure gas volume change using a gas syringe after 5 minutes.
  4. Move lamp further away by 10 cm and repeat measurements.
  5. Graph results with distance from lamp on x-axis and change in gas volume on y-axis.

6.6 Higher Only - Inverse Square Law: Rate of Photosynthesis

• Light intensity is directly proportional to the rate of photosynthesis. As light intensity increases, more photons hit chloroplasts, increasing photosynthesis rates.
• The inverse square law states that light intensity decreases with increasing distance from the light source:
  $ext{Light intensity} ext{ } ext{ } ext{ } ext{ } ext{I} ext{ } \frac{1}{ ext{distance}^2}$
• Example: If the light source is 2 meters away, light intensity is decreased to 1/4 of its original value:
  $ext{I} = \frac{1}{2^2} = \frac{1}{4}$

6.7 and 6.8 - Structure Adaptations

• Specialized plant cells exhibit unique adaptations to facilitate their functions:
  • Root Hair Cells:
  • Designed for water absorption via osmosis and nutrients via active transport.
    • Large surface area due to root hairs allows for maximal absorption.
    • Large permanent vacuole influences water uptake speed.
    • Contains mitochondria for energy via respiration for active transport of minerals.
  • Xylem Cells:
  • Specialized for transporting water and minerals from roots to shoots.
    • Cells are lignified (hollow and dead) for efficient water flow, connected end-to-end.
    • Spiral lignin structure enables them to withstand pressure.
  • Phloem Cells:
  • Carry products of photosynthesis (like sucrose) throughout the plant.
    • Sieve plates formed by breaking down cell walls to facilitate substance movement.
    • These are living cells supported by energy from mitochondria of companion cells.

6.9 - Transpiration and the Stomata

• Transpiration: The process by which water vapor is lost from the leaves and stems of plants.
• Stomata are small pores in the leaf allowing for gas exchange and water evaporation.
• Water loss from stomata creates a negative pressure, pulling water up through xylem from roots.
• Function of Guard Cells:
  • Control the opening and closing of stomata.
  • Kidney-shaped cells with thin outer walls and thick inner walls.
  • When hydrated, they swell and open stomata; when dehydrated, they close stomata to reduce water loss.
  • Stomata are primarily located on the underside of leaves, minimizing water loss via evaporation, being shaded and cooler.

6.10 - Translocation

• Translocation: The movement of nutrients, particularly sucrose, through the phloem from production sites (sources) to storage or utilization sites (sinks).
• Occurs exclusively within phloem, not xylem or other plant tissues.
• Seasonal fluctuations may shift source and sink locations (roots in spring, leaves in summer).

6.11B Biology Only - Adaptations of the Leaf

• Leaves possess adaptations essential for optimizing photosynthesis:
  • Stomata: Control gas exchange; close to minimize water loss and open to enhance evaporation.
  • Chlorophyll: Green pigment optimizing light absorption efficiency.
  • Thinness: Minimizes distance for gas diffusion, facilitating efficient exchange of CO₂ and O₂.
  • Large Surface Area: Enhances light absorption, maximizing photosynthesis rate.

6.12 - Environmental Factors and Rate of Water Uptake

• Factors affecting water uptake and transpiration include:
  • Temperature Increase: Accelerates molecular movement, enhancing evaporation rates leading to more transpiration.
  • Relative Humidity Increase: High humidity diminishes the concentration gradient for diffusion, reducing transpiration rates.
  • Increased Air Movement (Wind): Lowers water vapor concentration near leaves, steepening the gradient and increasing transpiration.
  • Light Intensity Increase: Elevates photosynthesis rates, prompting stomatal opening and increasing water vapor loss through transpiration.

6.13 - Rate Calculations for Transpiration

• Water uptake measurements provide insights into transpiration rates, using a potometer setup:
  • Involves placing a leaf shoot in a capillary tube submerged in water and measuring bubble displacement over a timed interval (e.g., 1 minute).
  • Greater displacement indicates higher transpiration and corresponding water uptake rates.

6.14B Biology Only - Extreme Adaptations

• Plants in extreme environments exhibit special adaptations to enhance survival and resource acquisition:
  • Leaf Structure/Size: Many desert plants have minimal or absent leaves to decrease transpiration rates.
  • Waxy Cuticle Presence: A waxy layer prevents excessive water loss through evaporation.
  • Stomata Adaptability: Small stomata can close to conserve water in arid conditions and open for CO₂ uptake during photosynthesis, adding to their survivability in harsh environments.

6.15B Biology Only - Plant Hormones and Growth

• Plant hormones are crucial for growth regulation and response to environmental stimuli (tropisms).
• Examples of Tropisms:
  • Phototropism: Growth towards light; auxins accumulate on shaded side, promoting growth and causing bending.
  • Gravitropism (Geotropism): Roots grow toward gravity; auxin distribution triggers differential growth responses.
• Auxins impact seedling experimentation through controlled light/angle conditions, affecting growth directions.

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

• Plant hormones have practical applications in agriculture and horticulture, enhancing productivity and reducing costs:
  • Auxin Applications:
  1. Weed Killers: Targeting broad-leaved plants with auxin-based herbicides causing rapid, destructive growth.
  2. Rooting Powders: Enhancing root growth in plant cuttings applied to facilitate cloning.
  3. Tissue Culture: Supporting growth of cells in nutrient-rich mediums with auxin assistance.
  • Gibberellin Functions: Stimulating germination, large fruit development, and accelerating flowering cycles.
  • Ethene Influence: Controlled ripening in the food industry by managing fruit harvesting, transport, and market readiness, minimizing waste and maximizing sales effectiveness.