Vascular plant transport

Transpiration and Water Movement

• Definition of Transpiration: Process of drawing water from roots to leaves via evaporation from stomata.

  • Pulling force created by evaporation of water from thin films on leaves.

  • Evaporation causes water potential to decrease, enhancing water flow from roots.

• Cohesive and Adhesive Properties of Water:

  • Cohesion (water to water): Water molecules attract each other, facilitating upward movement.

  • Adhesion (water to surfaces): Water molecules cling to the inner walls of xylem, assisting in transport up the plant.

Stomata and Guard Cells

• Guard Cells: Specialized cells that regulate the opening and closing of stomata, small openings for gas exchange.

  • Allow water vapor to exit and CO2 to enter for photosynthesis.

• Direction of Water and Carbohydrate Movement:

  • Water moves upwards towards leaves.

  • Carbohydrates (sucrose) move both up and down to supply nutrients to cells for energy and growth.

Cellular Mechanisms of Water Movement

• Osmosis: Water movement across cell membranes from areas of high water concentration to low.

  • Aquaporins: Proteins that facilitate rapid water transport across plasma membranes.

    • increase rate of osmosis bc they allow bulk flow

      

• Diffusion Types:

  • Passive Transport: Movement along the concentration gradient without energy (e.g., simple diffusion, osmosis).

  • Facilitated Diffusion: Movement facilitated by transport proteins, also passive and follows concentration gradients but requires specific proteins for molecules to cross membranes.

Transport Mechanisms in Plants

• Plasmodesmata: Channels between plant cells allowing for direct cytoplasmic transport, similar to gap junctions in animals, facilitating intercellular communication.

• Active vs Passive Transport:

  • Active Transport: Movement against concentration gradients, consuming ATP (e.g., proton pumps).

  • Passive Transport: Movement down concentration gradients without energy use.

Water Transport Routes in Plants

1. Apoplastic Route: Movement through cell walls and intercellular spaces, avoiding plasma membranes.

2. Symplastic Route: Movement through the cytoplasm, utilizing plasmodesmata.

3. Transmembrane Route: Movement through cell membranes and vacuoles, most regulated route.

   

Effects of Osmosis on Plant Cells

• Turgidity: When a plant cell is in pure water, water enters via osmosis, causing the cell to swell and become turgid.

• Plasmolysis: Occurs when a cell is placed in a hypertonic solution (high solute concentration), causing water to exit and the cell to shrink.

Regulation of Stomata

• Turgor Pressure: Increased pressure inside guard cells leads to stomatal opening; decreased pressure leads to closure.

• Factors Influencing Stomatal Movement:

  • K+ (Potassium Ions): Active uptake increases turgor in guard cells; water follows by osmosis.

  • Environmental Factors: High temperatures and drought conditions increase transpiration rates, altering stomatal behavior.

Signaling: Abscisic acid (ABA) triggers stomatal closure during drought conditions, preserving water. → regulate stomatal opening and closing

Inward Movement of Water

• molecule reach endodermis → any further passage through cell walls is blocked by casparian strips (root)

Water and Nutrient Transport in Plants

• Xylem: Conducts water and minerals from roots to leaves via transpiration. Main components are:

  • Tracheids: Long, narrow cells effective in water transport.

  • Vessels: Wider cells that carry water more efficiently.

• Phloem: Transports sugars and other metabolites throughout the plant.

  • Sieve-tube elements: Specialized for nutrient transport in living cells.

  • Companion cells: Assist with transport functions of sieve-tube elements.

• Guard Cells → only epidermal cells containing chloroplasts (ground tissue)

• 2 guard cells come together = stomata

• turgor → active uptake of K, Cl, + malate

• Active pumping of sucrose out of guard cells in the evening leads to loss of turgor and closes the guard cell

Rate of Transpiration

• Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere

• At the same time, photosynthesis requires a CO2 supply from the atmosphere

• Closing the stomata can control water loss on a short-term basis

• However, the stomata must be open at least part of the time to allow CO2 entry

• increases with temperature and wind velocity (h2o molecules evaporate more quickly)

Phloem Transport

• carbohydrates produced in leaves are distributed through phloem to rest of plant

• Food moves from a source to a sink

Also transports hormones, mRNA, and other molecules

Pressure-flow Theory

• Pressure-flow theory is a model describing the movement of carbohydrates in phloem

  • Dissolved carbohydrates flow from a source and are released at a sink

  • Sources include photosynthetic tissues

  • Food-storage tissue can be sources or sinks

  • Sinks include growing root and stem tips as well as developing fruits

Phloem-loading

• Phloem-loading occurs at the source

  • Carbohydrates enter the sieve tubes in the smallest veins at the source

  • Sieve cells must be alive to use active transport to load sucrose

  • Water flows into sieve tubes by osmosis

  • Turgor pressure drives fluid throughout plant

  • At sink, sucrose actively removed and water follows by osmosis

    • Water may be recirculated in xylem or lost

      

Summary of Key Concepts

• Water transport in plants relies on physical properties of water (cohesion and adhesion), osmotic processes, and physiological adaptations (guard cells and plasmodesmata).

• Stomatal behavior is crucial for balancing water loss with gas exchange, highlighting the importance of environmental factors on plant physiology.

• Understanding these processes is essential for comprehending plant growth, nutrient distribution, and responses to environmental stress.