Leaf Structure and Function

• Leaves are essential for photosynthesis, which sustains most life on Earth.

Leaf Structure and Function

• The upper epidermis is transparent to allow light to reach mesophyll cells, with few stomata and no chloroplasts.
• A waxy cuticle covers the epidermis to prevent excess water loss and allow sunlight to penetrate.
• Palisade mesophyll cells are long, cylindrical, and packed with chloroplasts for maximum light absorption.
• Spongy mesophyll cells have irregular shapes and large air spaces for efficient gas exchange (CO2 and O2) and contain fewer chloroplasts than palisade cells.
• A thin film of moisture covers spongy mesophyll cells, facilitating carbon dioxide dissolution.
• Vascular bundles contain xylem and phloem for transport.
  • Xylem transports water and mineral salts.
  • Phloem transports sucrose and amino acids.
• The lower epidermis resembles the upper epidermis but has more stomata.
• Stomata are small openings surrounded by guard cells.
• Guard cells regulate stomatal opening and closing for gas exchange.

Guard Cell Mechanism

• In light, guard cells produce glucose via photosynthesis, increasing glucose concentration and decreasing water potential.
• Water enters guard cells via osmosis, making them turgid and causing the stomata to open.
• On hot, sunny days, increased evaporation can cause excessive water loss.
• Guard cells become flaccid, closing the stomata to conserve water.

Xylem Vessels

• Xylem transports water and minerals from roots to stems and leaves.
• Provides mechanical support to the plant.
• Empty lumen, lacking cytoplasm and cross-walls, facilitates water flow.
• Lignin strengthens xylem walls, preventing collapse.

Phloem Vessels

• Phloem transports sugars and amino acids between plant parts.
• Contain porous cross-walls for rapid transport of manufactured food.
• Companion cells have numerous mitochondria to provide energy for active transport needed to load sugars into the pholem.
• Mitochondria releases energy via cellular respiration.

Root Hair Cells

• Root hair cells are long, narrow extensions of epidermal cells.
• Increase surface area for absorption of water and mineral salts.
• Contain many mitochondria providing energy for active mineral transport.
• They have a concentrated cell sap, maintaining a lower water potential for water absorption by osmosis

Photosynthesis Process

• Photosynthesis converts light energy into chemical energy using chlorophyll.
• Water and carbon dioxide are synthesized into glucose, releasing oxygen.
• Most life on Earth is completely dependent on photosynthesis for energy and food.

Carbon Dioxide Intake

• Carbon dioxide diffuses through stomata into intercellular air spaces.
• Then it dissolves in the moisture film around mesophyll cells.
• Dissolved carbon dioxide diffuses into mesophyll cells.

Water Intake

• Xylem transports water and minerals from roots to leaves.
• Water moves from cell to cell via osmosis, minerals via diffusion through mesophyll.

Factors Influencing Photosynthesis Rate

• Light intensity: Increasing light intensity increases rate up to a point where it is no longer limiting.
• Carbon dioxide concentration: Increasing concentration increases rate up to a point where it is no longer limiting.
• Temperature: Increasing temperature increases rate to a peak, then decreases as enzymes denature.

Translocation

• Translocation transports food (sucrose and amino acids) via phloem.
• It is bidirectional (up or down).

Tranpiration

• Transpiration is water vapor loss from plant aerial parts, mainly through stomata.
• Creates transpiration pull, aiding water and mineral transport.
• Evaporation cools the plant to protect from scorching.

Water Entry into Plants

• Root hairs closely contact soil solution.
• Root hair cell sap has lower water potential than soil solution.
• Water moves into root hair cells via osmosis.
• Water moves cell to cell into the xylem.

Water Movement Through Leaves

• Water moves out of mesophyll cells via osmosis into moisture film.
• Water evaporates into air spaces, increasing vapor concentration.
• Water vapor diffuses out of stomata.
• Drawing water from deeper cells creates suction, pulling water up xylem.

Factors affecting transpiration

• Light intensity: High light intensity increase the transpiration rate by opening stomata.
• Temperature: High temperature increase the transpiration rate by increasing the rate of evaporation.
• Relative humidity: High relative humidity decreases transpiration rate due to high saturation of water in the air.
• Air movement: High air movement increase the transpiration rate by removing acumulated water vapour on the leaf surface.

Wilting

• When water loss exceeds absorption, cells lose turgor, causing wilting
• Leaves fold, reducing surface area exposed to sun
• Stomata close, reducing carbon dioxide intake
• Decreased photosynthesis rate as carbon dioxide becomes limiting