7, Transport in Plants - Chapter 36 Detailed Notes

Transport in Plants - Chapter 36 Notes

Transport Mechanisms

• Key Process: Water movement in plants occurs through a series of steps:
  • Entry through Roots: Water and minerals enter the roots first.
  • Movement to Xylem: Water then moves to the xylem, which is the innermost vascular tissue responsible for water transport.
  • Stomatal Excretion: Most of the water exits through the stomata located on the leaves.

Long-Distance Movement

• Driving Forces:
  • Local changes can result in long-distance transport of materials within plants.
  • The primary force behind this is transpiration, the evaporation of water through stomata.

Transport of Water

• Mechanisms:
  • Cohesion: Water molecules stick to each other.
  • Adhesion: Water adheres to the walls of tracheids or vessels, aiding upward movement.

Cellular Water Movement

• Water can diffuse through plasma membranes via osmosis, moving down its concentration gradient.

Osmotic Concentration

• Definitions:
  • Hypertonic Solution: Higher solute concentration than another solution.
  • Hypotonic Solution: Lower solute concentration.
  • Isotonic Solution: Equal solute concentrations.

Osmosis Effects

• Cell Behavior:
  • In pure water, plant cells swell (turgid) due to water influx.
  • In hypertonic solutions (e.g. high sucrose), cells lose water (plasmolysis) and shrink.

Osmotic Pressure

• Concept: The force required to stop osmotic flow, balancing pressure from cell walls.
  • A flaccid or plasmolyzed cell is unable to support the plant’s weight.

Water Potential (A_w)

• Importance: Represents the free energy of water and helps in predicting water movement.
  • Affected by:
  • Solute concentration (osmolarity): Higher solute = lower A_w.
  • Pressure (turgor pressure): Higher pressure = higher A_w.

Water and Mineral Absorption

• Absorption Site: Most water is absorbed at root hairs, where surface area is increased by mycorrhizal fungi.
  • Water and ions move through cell layers to reach vascular tissues, ultimately entering the xylem.

Transport Routes through Cells

• Types of Routes:
  1. Apoplast Route: Movement through cell walls and intercellular spaces (avoids membranes).
  2. Symplast Route: Movement through the cytoplasm connected via plasmodesmata.
  3. Transmembrane Route: Movement across membranes, allows for cellular control.

Regulation of Water Movement

• Water potential drives the movement of water through plants:
  • Water moves from soil into roots only if soil water potential is greater than root potential.
  • Movement follows a gradient from soil to leaves.

Xylem Transport

• The solution that moves into the xylem decreases water potential, creating a hypertonic environment, which facilitates water inflow by osmosis.

Root Pressure

• Occurs when ions accumulate in roots during low transpiration (e.g., at night), pushing water upward even without transpiration.
  • Guttation: Loss of water from leaves occurs under high root pressure.

Cohesive Water Forces

• Water's cohesive properties give it tensile strength, aiding in transport, particularly in narrow vessels (tracheids and vessels).

Effects of Cavitation

• Cavitation: A gas-filled bubble can block water movement, weakening column strength. Adaptations minimize damage (e.g., alternative pathways).

Mineral Transport

• Xylem also transports essential minerals (e.g., phosphorus, potassium) from roots to other plant parts.

Rate of Transpiration

• Over 90% of water absorbed by roots is lost through transpiration, necessary for photosynthesis (CO2 supply).
  • Stomatal Regulation: Stomata control water loss but must remain partially open for gas exchange.

Guard Cells

• Unique epidermal cells with chloroplasts that control stomatal openings:
  • Thicker inner walls cause them to bulge outward and open stomata when turgid.

Mechanism of Stomatal Opening

• Process: Active uptake of potassium, chloride, and malate leads to water influx via osmosis, causing guard cells to swell.

Stomatal Regulation

• Stomata close when CO2 is high or temperature exceeds thresholds and can reopen under favorable light and moisture conditions.

Responses to Water Stress

• Morphological adaptations to drought include:
  • Dormancy, leaf loss (deciduousness), cuticle development, and reduced stomatal numbers.

Responses to Flooding

• Plants adapt to flooding (which depletes oxygen) by developing aerenchyma for gas collection and transportation.

Adaptations to Saline Conditions

• Halophytes: These plants deal with high salt concentrations through various mechanisms, such as altering water potential to facilitate uptake.

Phloem Transport

• Transports carbohydrates (e.g., sugars, hormones) from leaves to other plant areas (translocation).

Pressure-Flow Hypothesis

• Describes how carbohydrates move from sources (photosynthetic tissues) to sinks (active growth regions).

Phloem Loading and Unloading

• Process: Sucrose is actively loaded into sieve tubes at the source, with water following by osmosis. At the sink, sucrose is removed and water recycles or is lost to the xylem.