Plant Responses, Photosynthesis, and Nutrient Transport

Plant Responses: Tropisms

Tropisms are directional growth responses by plants to external stimuli (anything that induces a response).

Types of Tropisms

• Phototropism: Response to a light stimulus. Can be positive (+) (towards light) or negative (-) (away from light).
• Geotropism: Response to the stimulus of gravity. Can be positive (+) (towards gravity) or negative (-) (away from gravity).
• Chemotropism: Response to a chemical substance stimulus.

Photosynthesis: The Process

Photosynthesis is a biochemical process unique to chlorophyll-containing organisms that requires light and occurs in chloroplasts. It releases O2 (oxygen) to the environment and consumes CO2 (carbon dioxide) from the environment (gas exchange with the atmosphere).

Nutrient Transport in Plants

Dissolved mineral salts in water (H2O) pass through the plant roots, go up the stem as Raw Sap, and reach the leaves. In the leaves, photosynthesis occurs, producing Processed Sap (food), which travels down again to the root hairs.

Sap Types and Transport Vessels

• Sap: A liquid (water with dissolved substances) transported through capillary tubes.
• Raw Sap: Mineral salts dissolved in soil water, entering the plant through the roots.
• Xylem: Conducting vessels where raw sap travels upwards.
• Processed Sap: Sugars (food) synthesized by leaves during photosynthesis.
• Phloem: Conducting vessels through which the processed sap travels (usually downwards).

Plant Anatomy: Vegetative Organs

The Root: Structure and Function

The root is a vegetative organ, typically subterranean. It exhibits Positive Geotropism and Negative Phototropism. Its functions include:

• Fixing the plant to the soil.
• Absorption of water and nutrients.
• Storage of nutrient reserves.

The Stem: Structure and Function

The stem is a vegetative organ, typically erect and aerial. It exhibits Negative Geotropism and Positive Phototropism. Its consistency can be herbaceous or woody. Internally, it connects roots with leaves. Key parts include:

• Nodes: Parts of the stem where leaves are born.
• Internodes: The distance between two nodes.

Its functions include:

• Transporting raw and processed sap up and down.
• Supporting aerial organs (leaves, flowers, fruits).
• Storing nutrient reserves.
• Performing photosynthesis (if green).
• Facilitating transpiration through pores called lenticels.

The Leaf: Structure and Function

The leaf is a vegetative organ, generally green and aerial. It is the primary photosynthesizing organ (contains chloroplasts) and the primary transpiring organ (contains stomata). It typically has dorsi-ventral symmetry. Its functions include:

• Performing photosynthesis.
• Transpiration (release of water vapor).
• Gas exchange.
• Nutrient reserve storage (in some species).

Leaf Classification

• According to the petiole:
  • Petiolate: Has a petiole (leaf stalk), which can be long or short.
  • Sessile: Has no petiole.
• According to the leaf blade number:
  • Simple: Has a single leaf blade.
  • Compound: Has multiple leaflets (many blades).
• According to the number of main nerves (venation):
  • Uninerved: One main nerve.
  • Plurinerved: Many main nerves.

Photosynthesis Details

Phases of Photosynthesis

• Light-Dependent Phase / Light Reactions: Chemical reactions dependent on light energy.
• Light-Independent Phase / Dark Reactions (Calvin Cycle): Chemical reactions not directly dependent on light (but rely on products from the light phase). Requires CO2 taken from the environment by the plant.

Photosynthetic Pigments

• Primary Photosynthetic Pigment: Chlorophyll (types A and B are most common) directly carries out photosynthesis.
• Accessory Pigments: Help capture light energy and transfer it to chlorophyll. Examples include Carotenes and Xanthophylls.

Factors Affecting Photosynthesis

Environmental factors influencing the rate of photosynthesis include:

• Light intensity
• Carbon dioxide concentration
• Water availability
• Temperature