Plant Biology: Leaf Anatomy and Functions

Guard Cells and Stomata

• Guard cells control the opening and closing of stomata.
• Guard cells change shape due to water movement.
• Turgid guard cells open stomata, allowing for gas exchange.
• Deflated guard cells close stomata, minimizing water loss.
• Guard cells contain chloroplasts, unlike most epidermal cells.

Vascular Bundles in Leaves

• Veins, or vascular bundles, are scattered throughout the leaf mesophyll.
• Xylem transports water and minerals from roots to leaves.
• Phloem distributes sugars and carbohydrates produced during photosynthesis.
• Bundle sheath surrounds vascular tissue and is made of thicker-walled parenchyma cells.

Leaf Skeleton

• Veins provide structural support to the leaf.
• Veins run in various directions, especially in dicots.
• Vein networks facilitate efficient transport.

Monocot Leaf Structure

• Monocots typically have parallel veins.
• Monocots lack differentiation between palisade and spongy mesophyll layers.

Epigynous Flowers (Inferior Ovary)

• Thalamus is fused with the ovary, placing calyx, corolla, and stamens above the gynaecium.
• Ovary is inferior, meaning it is located below other floral parts.
• Examples: Sunflower, Cucumber, Apple.

Free Central Placentation

• Found in polycarpellary syncarpous ovaries that are unilocular.
• Ovules are attached to a central axis and not connected to the ovary wall.
• Examples: Dianthus (carnation), Saponaria, Portulaca.

Basal Placentation

• Characteristic of polycarpellary and unilocular ovaries.
• Only one ovule is present and arises from the base of the ovary.
• Example: Sunflower.

Pollination

• Transfer of pollen grains from the anther to the stigma.

Self-Pollination (Autogamy)

• Pollen from a flower's anther fertilizes its own stigma.
• Homogamy: Anthers and stigmas mature simultaneously.
• Cleistogamy: Flowers do not open, facilitating self-pollination without external agents.

Cross-Pollination (Allogamy)

• Pollen is transferred from the anther of one flower to the stigma of another.
• Facilitated by various agents: insects, wind, water, animals.

Types of Inflorescence

• Racemose or Indefinite Inflorescence
  • Peduncle continues to grow, producing flowers acropetally (younger flowers at the top).
  • Examples:
    • Raceme: Long peduncle with stalked flowers arranged acropetally (e.g., Mustard, Radish).
    • Spike: Similar to raceme but with sessile flowers (e.g., Rangoon creeper).
    • Spadix: Short peduncle with numerous small unisexual flowers covered by a boat-shaped bract called spathe (e.g., Banana, Arum).
    • Catkin: Spike with unisexual sessile flowers on a long peduncle (e.g., Mulberry, Oak).
    • Umbel: Shortened axis with flowers having equal stalks arranged centripetally (e.g., Coriander, Cumin).
    • Spikelet: Characteristic of Gramineae family, small, branched spikes with bracts (e.g., Ixora, Jasmine).
• Cymose or Definite Inflorescence
  • Peduncle ends in a flower, and new flowers are produced laterally.
  • Examples:
    • Solitary Cyme: Single terminal flower (e.g., Hibiscus).
    • Biparous or Dichasial: Main axis terminates in a flower, then two axillary flowers develop (e.g., Jasmine).
    • Multiparous or Polychasial: Main axis ends in a flower with several laterally produced flowers (e.g., Nerium, Calotropis).
• Special Types:
  • Include unique structures like: hypanthodium (e.g., peepal, fig), verticillasters (e.g., sacred basil, mentha), and cymose-umbel (e.g., onion).

Seed Morphology

• Seeds are fertilized ovules and crucial for plant reproduction.
• Seed Components:
  • Endosperm: Nutritive tissue supporting the embryo.
    • Endospermic (Albuminous) Seeds: Endosperm remains during germination and is partially absorbed by the embryo (e.g., wheat, rice).
    • Non-Endospermic (Exalbuminous) Seeds: Endosperm is fully absorbed by the embryo during development (e.g., sunflower, tamarind).
  • Embryo: Develops into a new plant.
    • Contains an apical meristem (plumule - shoot development), radicle (root development), and cotyledons (seed leaves, determining monocot or dicot).

Photosynthesis Structures

• Mesophyll: Middle layer of leaf cells where photosynthesis occurs.
• Stomata: Small openings on the underside of leaves for gas exchange (CO₂ in, O₂ out).
• Chloroplasts: Organelles where photosynthesis happens in autotrophic eukaryotes.
  • Double Membrane Envelope: Outer and inner membranes.
  • Thylakoids: Disc-shaped structures stacked within the chloroplast.

Photosynthesis Steps

• Light-Dependent Reactions:
  • Location: Thylakoid membranes of chloroplasts.
  • Process: Chlorophyll absorbs sunlight, converting it into chemical energy.
  • Inputs: Water (H₂O).
  • Outputs: Oxygen (O₂), ATP, and NADPH.
• Light-Independent Reactions (Calvin Cycle):
  • Location: Stroma of chloroplasts.
  • Functions: Utilize ATP and NADPH from light-dependent reactions to convert carbon dioxide (CO₂) into sugar molecules.
  • Does not directly use light but relies on products from the light-dependent reactions.
  • Enzymes involved are activated by light.
• Products:
  • Glyceraldehyde-3-phosphate (GA3P): A simple carbohydrate that can be further converted into sugars (glucose, sucrose).
  • Oxygen (O₂): Released as a waste product.

Chemiosmosis

• Proton Flow: The proton gradient generated during the electron transport chain generates potential energy.