Plant structures and their functions

• Photosynthesis

  Word Equation:

  Carbon dioxide + Water → Glucose + Oxygen

  Symbol Equation:

  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Cross-Section of a Leaf

  Waxy Cuticle

  Upper Epidermis

  Palisade Mesophyll

  Spongy Mesophyll (with air spaces)

  Lower Epidermis (with stomata and guard cells)
• Thin waxy cuticle

  Allows light to penetrate through to all cells quickly and reduces water loss
• Stomata

  Allows gas exchange of O₂ and CO₂ into and out of the leaf.
• Air spaces

  Rapid gas exchange, allowing CO₂ to diffuse in and O₂ to diffuse out.
• No chloroplasts in upper epidermis

  Allows light to penetrate to the mesophyll tissue below
• Large surface area

  Increases light absorption and CO₂ absorption
• Chloroplasts containing chlorophyll

  Absorb light energy for photosynthesis
• Xylem

  Supplies water to leaf cells, which is used in photosynthesis
• Phloem

  Transports sucrose (produced in photosynthesis) to other parts of the plant
• Supported by stem and petiole (leaf stalk)

  Exposes more leaf surface to sunlight and air
• Palisade mesophyll cells arranged upright

  fewer cell walls in the way of light, maximizing absorption
• Light intensity

  More light means a faster rate of photosynthesis (until another factor limits it)
• CO₂ concentration

  Higher CO₂ levels increase the rate of photosynthesis
• Temperature

  Enzyme activity increases with temperature, but too high a temperature denatures them
• Lignin embedded in walls

  Strengthens xylem vessels, preventing collapse (e.g., in windy conditions)
• Xylem has no cell contents (it is dead)

  Reduces friction, allowing mass flow of water
• Pits in walls

  Allow water and minerals to move sideways into another xylem if a blockage occurs
• No end walls between cells

  Creates a continuous tube for water and minerals to move by transpiration
• Lignin waterproofs the walls

  Prevents water loss
• Sieve tube elements

  Transport sucrose and other nutrients
• Companion cells

  Provide ATP for active transport in translocation.
• Sieve plates (perforated end walls)

  Allow flow of sap between phloem cells
• Alive cells

  Unlike xylem, phloem cells are living and require energy for transport.
• Nitrates

  Used for production of DNA bases and amino acids.
• Phosphates

  Used for production of DNA bases and cell membranes.
• Magnesium

  Required for the production of chlorophyll.
• Higher air temperature

  Increases evaporation of water
• Lower humidity

  Maintains a diffusion gradient for water loss
• Increased wind speed

  Blows away water vapor, maintaining the diffusion gradient.
• Larger leaf surface area

  More stomata and increased transpiration.
• More stomata

  Greater gas exchange leads to higher transpiration rates
• Stomata closing

  Prevents transpiration, stopping the plant from wilting.
• More leaves

  Greater surface area for transpiration
• Quicker uptake of water in roots

  Increases the rate of transpiration
• Light Intensity Formula

  Light Intensity ∝ 1 / Distance²
• Phototropism

  Growth response to light.

  Positive phototropism → Growth towards light (e.g., shoots).

  Negative phototropism → Growth away from light (e.g., roots).
• Geotropism

  Growth response to gravity.

  Positive geotropism → Growth towards gravity (e.g., roots grow downward).

  Negative geotropism → Growth away from gravity (e.g., shoots grow upward).