Bio151 - Plant Transport Notes

Chapter 36: Resource Acquisition and Transport in Vascular Plants

1. Introduction

• Plants have various adaptations that aid in the acquisition of resources, including:

  • Water

  • Minerals

  • Carbon dioxide (CO₂)

  • Light

• The success of plants relies on their ability to gather resources from their environment and transport them to necessary locations.

• English ivy effectively acquires light energy for photosynthesis by covering every square centimeter of a wall with foliage.

2. Mechanisms of Movement in Vascular Plants

• Water and minerals:

  • Pulled up from the roots by negative pressure generated by evaporation from leaves.

• Sugars:

  • Pushed by positive pressure from where produced or stored to where needed.

3. Evolution of Resource Acquisition in Vascular Plants

• Adaptations for acquiring resources were crucial in the evolution of vascular plants:

  • Algal ancestors of land plants absorbed water, minerals, and CO₂ directly from surrounding water.

  • Early nonvascular land plants:

  • Lived in shallow water with aerial shoots.

  • Natural selection favored taller plants with:

    • Flat appendages

    • Multicellular branching roots

    • Efficient transport

• Xylem and Phloem Evolution:

  • Evolution of xylem and phloem made long-distance transport of:

  • Water

  • Minerals

  • Products of photosynthesis possible.

  • Xylem: Transports water and minerals from roots to shoots.

  • Phloem: Transports photosynthetic products from their production site to where they are needed.

4. Shoot Architecture and Light Capture

• Functions of Stems:

  • Serve as conduits for water and nutrients.

  • Provide structural support for leaves.

• The length and branching pattern of shoots impact light capture:

  • Trade-off: Growing tall vs. branching—more energy in branching means less for height growth.

• Generally positive correlation between:

  • Water availability

  • Leaf size

• Phyllotaxy:

  • Leaf arrangement on a stem is a species-specific trait crucial for light capture.

  • Most angiosperms have an alternate phyllotaxy with leaves arranged in a spiral; the angle is approximately 137.5º which minimizes shading of lower leaves.

5. Impact of Leaf Area and Orientation

• The total area of leafy portions in a community influences productivity.

• Self-Pruning:

  • Shedding of lower shaded leaves occurs when their respiration exceeds photosynthesis.

• Leaf Area Index (LAI):

  • Ratio of total upper leaf surface area to the land surface area.

  • LAI > 7 leads to shading that results in self-pruning:

  • Example:

    • Plant A: Leaf area = 40% of ground area (LAI = 0.4)

    • Plant B: Leaf area = 80% of ground area (LAI = 0.8)

• Leaf Orientation:

  • In low light: horizontal leaves maximize sunlight capture.

  • In sunny conditions: vertical leaves minimize damage and allow light penetration to lower leaves.

6. Photosynthesis–Water Loss Compromise

• Stomatal Pores:

  • Essential for CO₂ diffusion into photosynthetic tissues of leaves.

  • Over 90% of water loss occurs through evaporation from these pores.

• Adaptations in shoots reflect compromises between:

  • Enhancing photosynthesis.

  • Minimizing water loss.

7. Root Architecture and Resource Acquisition

• Soil contains resources accessed by the root system.

• Root growth is adaptable to local resource conditions:

  • Roots branch into high nitrate availability and move straight through areas of low availability.

• Roots of the same plant are less competitive than roots of different plants.

  • Example: Buffalo grass cuttings have fewer, shorter roots in presence of the same plant’s cuttings.

• Mycorrhizae:

  • Mutualistic association between roots and soil fungi.

  • Fungi enhance surface area for water and mineral absorption, notably phosphates, aiding plant colonization on land.

8. The Apoplast and Symplast: Transport Pathways

• Apoplast:

  • Everything external to the plasma membrane, including:

  • Cell walls

  • Extracellular spaces

  • Interior of dead cells (vessel elements, tracheids).

• Symplast:

  • Consists of the cytosol of all living cells and plasmodesmata.

• Transport Routes:

  • Apoplastic Route: Through cell walls and extracellular spaces.

  • Symplastic Route: Water and solutes cross a plasma membrane once and then navigate through the cytosol.

  • Transmembrane Route: Continuous crossing of plasma membranes as substances move from cell to cell.

9. Short-Distance Solute Transport Across Plasma Membranes

• Plasma membrane permeability governs short-distance substance movement:

  • Active and passive transport occur in plants.

  • Membrane potential in plants is established by proton pumps (H⁺) while in animals, it is through sodium-potassium pumps (Na⁺/K⁺).

  • Plant cells utilize H⁺ gradients and membrane potential for cotransport of other solutes via active transport.

  • Ion channels in membranes selectively permit ion passage.

10. Short-Distance Water Transport Across Plasma Membranes

• Osmosis: The diffusion of water influenced by solute concentration and pressure.

• Water Potential (Ψ): A quantity that combines the effects of solute concentration and physical pressure, determining the water movement direction (higher Ψ to lower Ψ).

  • Water’s capacity to perform work is referred to as potential.

  • Pure water's water potential at sea level and room temperature is Ψ = 0 MPa.

11. Water Potential Influence Factors

• Water Potential Equation: $Ψ = Ψ<em>S + Ψ</em>P$

  • Solute Potential (Ψ_S):

  • Proportional to molarity

  • Also termed osmotic potential.

  • Pressure Potential (Ψ_P): The physical pressure on a solution, which can be positive or negative.

  • Example: Syringe example—negative when withdrawn, positive when expelled.

  • Turgor Pressure: Positive pressure from plasma membrane against the cell wall, crucial for cell structure.

• Plasmolysis: Occurs when flaccid cells lose water and shrink from cell walls.

  • Example Scenario:

  • Environment: Flaccid cell in a 0.4 M sucrose solution = ΨP = 0 and ΨS = -0.7; Final state = Ψ = -0.9 MPa.

12. Water Movement Dynamics

• Water uptake and loss in plant cells are affected by water potential:

  • Upon entering a lower solute concentration environment, a flaccid cell gains water and becomes turgid.

• Aquaporins:

  • Transport proteins that facilitate water passage across cell membranes.

  • Their opening and closing regulate osmotic water movement rates.

13. Long-Distance Transport: Bulk Flow

• Bulk Flow: Requires pressure gradients for efficient long-distance fluid transport; transports both water and solutes via xylem and phloem.

• Transpiration drives water and minerals from roots to shoots via xylem.

14. Bulk Flow and Structural Adaptations of Xylem and Phloem

• Structural adaptations enhance bulk flow:

  • Mature tracheids and vessel elements lack cytoplasm, sieve-tube elements have few organelles.

  • Ensures rapid fluid movement with minimal resistance.

• Cohesion and Adhesion:

  • Cohesive properties of water molecules maintain column integrity through xylem.

  • Adhesive properties assist in counteracting gravity, especially with thick-walled xylem cells.

15. Water Transport Mechanisms During Transpiration

• Transpirational Pull:

  • Water vapor diffuses from leaf air spaces through stomatal pores creating negative pressure.

  • As water evaporates, the air-water interface retreats, pulling from mesophyll cell walls.

16. Cohesion-Tension Mechanism Overview

• The cohesion-tension mechanism describes:

  • Bulk flow against gravity based on tension differences in water potential.

  • Cohesion: Water molecules are attracted to each other; as one exits, adjacent molecules follow.

  • Cavitation: Occurs when drought or freezing introduces air bubbles disrupting flow.

17. Regulating Transpiration via Stomata

• Stomatal Structure:

  • Each stoma has guard cells regulating pore diameter controlling water vapor loss (95% of total loss).

• Stomatal Regulation Mechanism:

  • Changes in turgor pressure alter guard cell shape:

  • Turgid = opened.

  • Flaccid = closed.

• Environmental and internal stimuli regulate stomatal opening and closing, balancing photosynthesis, water loss, and gas exchange.

18. Adaptations in Arid Conditions

• Xerophytes:

  • Plants adapted to arid environments, utilizing various strategies like movement from sugar sources to sinks to minimize water loss.

• Modified Water Conservation Methods:

  • Some drought-tolerant plants employ Crassulacean Acid Metabolism (CAM) for respiration during nighttime.

19. Phloem Transport Overview

• Sugars are transported from sources to sinks via phloem through a process called translocation:

  • Sieve-tube elements as conduits for translocating phloem sap (high in sucrose).

  • Sugar Source: Organs (e.g., mature leaves) producing more sugar than consumed.

  • Sugar Sink: Organs (e.g., roots, fruits) consuming/store sugar.

20. Mechanisms of Phloem Loading and Translocation

• Sugar Loading into Sieve-Tube Elements:

  • Utilizes active transport, with proton pumping facilitating cotransport of sucrose and H⁺.

  • Bulk Flow via Pressure Flow: Occurs driven by regions of high pressure (sources) to low pressure (sinks).

• Companion cells assist in transporting solutes between apoplast and symplast, enhancing loading efficiency.

21. Plasmodesmatal Dynamics

• Symplast Dynamics:

  • Living tissue responds dynamically, impacting transport processes.

  • Changes in the number and pore size of plasmodesmata influence solute movement and can even promote viral movement between cells.