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Chapter 2: Leaf Structure and Function

Form 5 Biology Bab 2: Leaf Structure and Function

2.1 Structure of a Leaf

Leaves are the primary photosynthetic organs in most vascular plants, adapted to absorb sunlight and facilitate gas exchange.

1. External Structure of a Leaf

  • Lamina: Flat, thin, and broad blade structure. Provides a large surface area to absorb maximum sunlight and $CO_2$. Being thin allows light and gases to diffuse rapidly into inner tissues.
  • Petiole: Leaf stalk connecting the lamina to the stem, positioning the lamina for optimal sunlight exposure.

2. Internal Structure of a Lamina

  • Upper Epidermis: Single cell layer transparent to sunlight, covered by a waxy cuticle that minimizes water loss (transpiration). Lacks chloroplasts.
  • Palisade Mesophyll: Located directly under the upper epidermis. Densely packed, vertically elongated cylindrical cells containing high concentrations of chloroplasts for maximum light absorption during photosynthesis.
  • Spongy Mesophyll: Loosely packed, irregularly shaped cells with fewer chloroplasts and extensive intercellular air spaces. Air spaces allow rapid diffusion of $CO_2$ and $O_2$ throughout the leaf interior.
  • Vascular Bundle:
    • Xylem: Located on the upper side of the vascular bundle; transports water and inorganic mineral ions from roots to leaves.
    • Phloem: Located on the lower side of the vascular bundle; transports synthesized organic solutes (sucrose) from leaves to photosynthetic sinks.
  • Lower Epidermis: Single cell layer containing numerous stomata (singular: stoma), each flanked by a pair of guard cells.

2.2 Main Organ of Gaseous Exchange

1. Mechanism of Stomatal Opening and Closing

Stomatal movements are driven by turgor pressure changes within guard cells, regulated by $K^+$ ion transport and photosynthetic activity.

A. Stomatal Opening (Daytime / Light):

  1. Photosynthesis in guard cells generates ATP.
  2. Guard cells actively transport potassium ions ($K^+$) into the cell from adjacent epidermal cells.
  3. The solute potential inside guard cells becomes more negative (hypertonic environment).
  4. Water enters guard cells by osmosis, increasing their turgor pressure.
  5. Guard cells expand. Because the inner cell wall is thicker and less flexible than the outer cell wall, the guard cells curve outward, opening the stoma.

B. Stomatal Closing (Nighttime / Dark / Water Stress):

  1. Photosynthesis stops or water stress triggers the release of abscisic acid (ABA).
  2. $K^+$ ions diffuse or are transported out of guard cells.
  3. Water potential inside guard cells increases relative to surrounding cells.
  4. Water leaves guard cells by osmosis, causing them to become flaccid.
  5. The inner walls straighten, closing the stoma.

2. Environmental Factors Affecting Transpiration Rate

  • Light Intensity: Increased light opens stomata via $K^+$ uptake, accelerating transpiration until a maximum rate is reached.
  • Temperature: Higher temperatures increase the kinetic energy of water molecules, increasing the rate of water evaporation from mesophyll surfaces.
  • Relative Air Humidity: Lower atmospheric humidity increases the water vapor concentration gradient between leaf air spaces and the atmosphere, increasing transpiration rate.
  • Air Movement (Wind): Moving air sweeps away saturated water vapor layers around stomata, maintaining a steep water vapor concentration gradient.

2.3 Main Organ of Photosynthesis

1. Structure of Chloroplast

  • Double Membrane: Outer and inner membranes enclosing the organelle.
  • Thylakoids: Disc-like sac structures containing chlorophyll pigments, electron transport chains, and ATP synthase.
  • Grana (singular: Granum): Stacks of thylakoids that maximize surface area for light absorption.
  • Stroma: Fluid-filled matrix surrounding grana, containing Calvin cycle enzymes (e.g., RuBisCO), chloroplast DNA, and ribosomes.

2. Light-Dependent and Light-Independent Reactions

A. Light-Dependent Reaction (Occurs in Thylakoid Membrane):

  1. Chlorophyll pigments absorb light energy, exciting electrons to higher energy levels.
  2. Photolysis of Water: Light energy splits water molecules ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$). Oxygen gas is released as a byproduct.
  3. Excited electrons pass along the Electron Transport Chain (ETC), synthesizing ATP via photophosphorylation and reducing $NADP^+$ to form NADPH.

B. Light-Independent Reaction / Calvin Cycle (Occurs in Stroma):

  1. Carbon Fixation: $CO_2$ gas combines with Ribulose 1,5-bisphosphate (RuBP) catalyzed by the RuBisCO enzyme to form 3-carbon intermediates.
  2. Reduction: ATP and NADPH from the light-dependent stage reduce these intermediates to Glyceraldehyde-3-phosphate (G3P) / triose phosphate.
  3. Regeneration: ATP is used to regenerate RuBP, while triose phosphate molecules condense to yield glucose.

2.4 Compensation Point

  • Definition: The specific light intensity at which the rate of photosynthesis equals the rate of cellular respiration.
  • At compensation point: Net exchange of $CO_2$ and $O_2$ is zero ($CO_2\text{ produced by respiration} = CO_2\text{ absorbed by photosynthesis}$).
  • Significance:
    • Light intensity below compensation point: Respiration exceeds photosynthesis; plant consumes stored starch reserves, leading to death over prolonged periods.
    • Light intensity above compensation point: Photosynthesis exceeds respiration; net organic matter (glucose) accumulates for growth, development, and storage.
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