Topic7B Plant Growth and Transport: Physiology and Cell Biology Study Notes

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Topic7B Plant Growth and Transport

Introduction to Plant Physiology

Plant physiology explores the functional processes that allow plants to grow, develop, and respond to their environment. Central to this field are the mechanisms of resource acquisition, transport, and utilization within the plant body.

Vascular vs. Non-Vascular Plants

Structural Differences and Growth Potential

Vascular plants possess specialized tissues (xylem and phloem) for the transport of water, minerals, and sugars, enabling them to grow tall and complex. Non-vascular plants, such as mosses, lack these tissues and remain small due to limited transport capabilities.

Vascular plants: Have xylem (water/mineral transport) and phloem (sugar transport).
Non-vascular plants: Lack true vascular tissues; rely on diffusion and osmosis for transport.
Example: Mosses are non-vascular and do not grow tall because they lack xylem and phloem.

Labeled diagram of moss showing sporophyte, gametophyte, and rhizoids

Resource Acquisition and Transport in Vascular Plants

Overview of Transport Pathways

Vascular plants acquire resources from their environment and distribute them internally through specialized tissues. Water and minerals are absorbed by roots, transported upward via xylem, while sugars produced in leaves are distributed by phloem.

Transpiration: Loss of water vapor from leaves drives upward movement of water in xylem.
Photosynthesis: Leaves produce sugars, which are transported to other plant parts via phloem.
Root exchange: Roots absorb water and minerals, and exchange gases with the soil.

Diagram of resource acquisition and transport in a vascular plant

Short-Distance Transport in Plants

Cellular Pathways for Water and Nutrient Movement

Within plant tissues, water and solutes move by three main pathways: apoplastic, symplastic, and transmembrane. These routes differ in their use of cell walls, membranes, and cytoplasm.

Apoplastic route: Movement through cell walls and intercellular spaces (does not cross membranes).
Symplastic route: Movement through the cytoplasm via plasmodesmata (cell-to-cell connections).
Transmembrane route: Movement across cell membranes, entering and exiting cells repeatedly.

Diagram of apoplastic, symplastic, and transmembrane routes in plant cells

Transport of Solutes Across Membranes

Solute transport across plant cell membranes is driven by membrane potential, established primarily by proton pumps. Co-transporters and ion channels facilitate the movement of specific molecules and ions.

Proton pumps: Use ATP to export H+, creating an electrochemical gradient.
Co-transporters: Use the H+ gradient to import neutral solutes (e.g., sucrose) or ions (e.g., nitrate).
Ion channels: Allow passive movement of ions such as K+.

Proton pump establishing membrane potential H+ and cotransport of neutral solutes (sucrose) H+ and cotransport of ions (nitrate) Ion channel for potassium ions

Osmosis and Water Transport

Water moves across membranes by osmosis, from regions of higher free water concentration (lower solute concentration) to lower free water concentration (higher solute concentration). Aquaporins facilitate rapid water movement across membranes.

Osmosis: Passive movement of water through a selectively permeable membrane.
Aquaporins: Channel proteins that allow water to cross membranes efficiently.

Diagram of osmosis across a selectively permeable membrane Aquaporin channel in a membrane

Water Potential and Its Components

Water potential (Ψ) predicts the direction of water movement and is measured in megapascals (MPa). It is determined by solute potential (ΨS) and pressure potential (ΨP):

Water potential equation:
Solute potential (ΨS): Always negative; decreases as solute concentration increases.
Pressure potential (ΨP): Positive (turgor pressure) or negative (tension).

Effect of solutes on water potential Effect of positive pressure on water potential Combined effects of solutes and pressure on water potential Effect of negative pressure (tension) on water potential

Solute Potential Formula

The solute potential (osmotic potential) is calculated as:

i: Ionization constant (1 for sucrose, 2 for NaCl)
C: Molarity (mol L-1)
R: Ideal gas constant (8.32 J K-1 mol-1)
T: Temperature (K)

Water Movement in Plant Cells

Water moves from regions of higher water potential to lower water potential. The balance of solute and pressure potentials determines cell states such as turgid, flaccid, or plasmolyzed.

Turgid cell: Water enters, cell is firm (normal for plants).
Flaccid cell: No net water movement.
Plasmolyzed cell: Water leaves, cell membrane pulls away from wall.

Plasmolyzed cell in sucrose solution Turgid cell in pure water

Long-Distance Transport in Plants

Xylem: Water and Mineral Transport

Long-distance transport of water and minerals occurs in the xylem, driven by bulk flow resulting from transpiration. Water moves from roots to leaves, powered by evaporation at the leaf surface.

Bulk flow: Movement of water due to pressure differences, not diffusion.
Transpiration: Evaporation of water from leaves creates negative pressure, pulling water upward.
Cohesion-adhesion theory: Water molecules stick together (cohesion) and to cell walls (adhesion), forming a continuous column.

Transport of water and minerals from root hairs to xylem Transpiration-driven water movement in leaves Cohesion-adhesion hypothesis for xylem transport

Stomata: Regulation of Water Loss

Stomata are pores on the leaf surface that regulate gas exchange and water loss. Guard cells control the opening and closing of stomata in response to environmental cues and internal signals, such as potassium ion concentration.

Open stomata: Guard cells are turgid due to K+ influx and water uptake.
Closed stomata: Guard cells are flaccid due to K+ efflux and water loss.
Regulation: Stomata usually open during the day for photosynthesis and close at night or under drought stress.

Open and closed stomata under electron microscope

Phloem: Transport of Sugars

Source-to-Sink Movement

Sugars produced in photosynthetic tissues (sources) are transported to non-photosynthetic tissues (sinks) via the phloem. This movement is driven by pressure flow, resulting from active loading and unloading of sugars.

Source: Leaves or storage organs releasing sugars.
Sink: Growing tissues, roots, or storage organs absorbing sugars.
Phloem loading: Active transport of sucrose into sieve tube elements increases osmotic pressure, drawing in water and generating flow.
Phloem unloading: Sucrose is removed at the sink, lowering osmotic pressure and allowing water to exit.

Leaf cross section showing phloem veins Loading of sucrose into phloem Bulk flow by positive pressure in sieve tube

Summary Table: Comparison of Xylem and Phloem Transport

Feature	Xylem	Phloem
Transported Substances	Water, minerals	Sugars, amino acids, hormones
Direction of Flow	Roots to leaves (unidirectional)	Source to sink (bidirectional)
Driving Force	Transpiration (negative pressure)	Pressure flow (positive pressure)
Cell Type	Dead (tracheids, vessel elements)	Living (sieve tube elements, companion cells)

Key Equations

Water potential:
Solute potential:

Additional info:

Plant cells are normally turgid due to water uptake, while animal cells are normally isotonic with their environment.
Bulk flow in xylem does not require living cells or ATP; it is a passive process driven by evaporation.