Plant Nutrition Notes

PLANT NUTRITION

Part 1: Structure of a Plant and Leaf

Plants as Photoautotrophs

• Plants are photoautotrophic organisms, meaning they can produce glucose using light energy.
• Photosynthesis is the process where cells synthesize organic compounds (e.g., glucose) from inorganic molecules (CO2 and H2O) using sunlight.
• This process requires a photosynthetic pigment, chlorophyll, and occurs only in certain organisms (plants, some bacteria).
• Photosynthetic organisms convert light energy from the sun into chemical energy (ATP).
• This chemical energy can be used directly or to synthesize organic compounds like glucose.

Chloroplast

• Chloroplasts are specialized double-membrane organelles responsible for photosynthesis.
• Chloroplasts absorb light energy but reflect green light (due to chlorophyll), giving plants their green color.
• Chloroplasts store glucose in the form of starch.

Photosynthesis and Respiration

• Photosynthesis uses carbon dioxide and water in the presence of light and chlorophyll to produce sugar (glucose) and oxygen. The chemical equation is: 6CO2 + 6H2O \xrightarrow{\text{Light}} C6H{12}O6 + 6O2
  • Carbon dioxide + Water yields Sugar + Oxygen

Ecosystem Energy Flow

• Light energy drives photosynthesis in chloroplasts.
• Photosynthesis produces oxygen and organic compounds.
• Carbon dioxide and water are inputs for photosynthesis.
• ATP is generated in the process.
• Cell respiration occurs in mitochondria, utilizing oxygen and organic compounds to produce cellular energy.

Uses of Glucose

• Glucose is used in respiration to produce energy (ATP) for growth and active transport.
• Glucose can be stored as starch for later use.
• It can be used to synthesize cellulose, a sugar that forms the cell wall.
• Glucose can be converted into fats and proteins.

Adaptations and Functions of a Leaf

• Thin Structure: Provides a short diffusion distance for carbon dioxide.
• Chlorophyll: Absorbs light energy for photosynthesis.
• Stomata: Allow carbon dioxide to diffuse into the leaf.
• Guard Cells: Control the opening and closing of stomata based on environmental conditions.
• Network of Tubes (Xylem and Phloem): Xylem transports water, and phloem transports food.

Leaf Structure

• The leaf is an organ composed of tissues arranged in layers.
• Each tissue consists of independent cells serving a specific purpose.

Layers of a Leaf

• The layers of a leaf include:
  • Cuticle
  • Upper Epidermis
  • Mesophyll (Palisade and Spongy)
  • Lower Epidermis
  • Guard Cells
  • Stoma
  • Chloroplast
  • Vacuole
  • Nucleus
  • Cell Wall
  • Cytoplasm

Micrograph of Leaf Layers

• Shows the arrangement of:
  • Guard cells
  • Vascular bundle (Xylem and Phloem)
  • Stoma
  • Upper epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Lower epidermis

Adaptations and Functions of Leaf Tissues

• Cuticle:
  • Function: Protects the leaf from water loss and pathogen invasion; provides a protective barrier without obstructing light.
  • Adaptations: Waxy, transparent layer that reduces water loss while allowing light to pass through.
• Upper Epidermis:
  • Function: Primary site of photosynthesis.
  • Adaptations: Transparent layer allows maximum light penetration; cuticle coating reduces water loss.
• Palisade Mesophyll:
  • Function: Facilitates gas exchange and photosynthesis.
  • Adaptations: Tightly packed cells with many chloroplasts to maximize light absorption.
• Spongy Mesophyll:
  • Function: Facilitates gas exchange
  • Adaptations: Loosely packed cells with air spaces, allowing gases CO2, O2 to diffuse easily within the leaf.
• Lower Epidermis:
  • Function: Protects the leaf and contains stomata for gas exchange.
  • Adaptations: Contains stomata regulated by guard cells to control gas exchange and minimize water loss.
• Stomata:
  • Function: Allow gas exchange (CO2 in, O2 out) and transpiration.
  • Adaptations: Can open and close to regulate gas exchange and reduce water loss.
• Guard Cells:
  • Function: Regulate the opening and closing of stomata.
  • Adaptations: Change shape to open or close stomata in response to environmental conditions (light, water availability).
• Xylem:
  • Function: Transports water and minerals from roots to leaves.
  • Adaptations: Narrow tubes with strong walls to support water transport under tension.
• Phloem:
  • Function: Transports sugars produced in photosynthesis to other parts of the plant.
  • Adaptations: Sieve tubes adapted to transport food efficiently, with companion cells providing metabolic support.

Guard Cells and Stomata

• Stomata are microscopic pores used by plants for gas exchange.
• The opening and closing of stomata are controlled by guard cells.

Mechanism of the Guard Cells

• When light is present:
  • Photosynthesis occurs, producing glucose and lowering water potential.
  • Water enters the guard cells by osmosis, causing them to swell and become turgid.
  • Stomata open, allowing gas exchange.
• When light decreases or during water stress:
  • Photosynthesis stops, raising water potential.
  • Water leaves the guard cells, making them flaccid.
  • Stomata close to reduce water loss.

Mineral Ions

• Nitrates:
  • Use: Used to make amino acids and proteins.
  • Function: Necessary for growth and production of essential enzymes.
  • Deficiency: Reduces growth and height (stunted growth).
• Magnesium:
  • Use: Used to make chlorophyll.
  • Deficiency: Results in chlorosis (plants turn yellow).