LB

5 Transport in plants

Transport in Plants

Essential Components for Transport

  • Light, CO2, H2O, Minerals, Sugars, Plant Growth Regulators (PGRs)

  • Transport occurs through leaves, stems, and roots.

Movement Around Plants

Types of Movement

  • Water

  • Minerals

  • Sugars and other nutrients

  • Other chemicals

  • PGRs (signaling chemicals)

Mechanisms of Movement

Processes Involved

  • DiffusionDefinition: Spontaneous intermingling of particles from two or more substances due to random thermal motion.

  • OsmosisDefinition: Diffusion of fluid through a semi-permeable membrane from a low solute concentration solution to a higher solute concentration solution until equilibrium is achieved.

Osmotic Pressure

  • Mechanism: Difference in conductivity (salt content) between soil and plant cells.

  • Water is drawn into areas of higher solute concentration in roots due to semi-permeable membranes.

  • Greater solute differences yield greater osmotic pressure.

Water Assimilation

Process

  • Higher solute concentration in roots results in osmotic pressure drawing water into the root.

  • Root hairs act as semi-permeable membranes for absorption.

Water Absorption into Plants

Methods

  • Most water and nutrients absorbed by roots via root hairs.

  • Movement can occur:

    • Between cell walls

    • Into and between cells via membranes

    • Through plasmodesmata

From Root to Xylem

Transport Mechanism

  • Water crosses the endodermis into the xylem tissue, contributing to the transpiration stream.

Root Pressure

Characteristics

  • Active transport of water across roots.

  • Involves changes in solute potential and utilizes energy.

Turgor Pressure in Plant Cells

Cell Wall Function

  • Rigid cellulose walls prevent bursting from excess water uptake (unlike animal cells).

  • Turgor pressure is created by water entering the cells, analogous to air pressure in a tire.

Stomata and Turgor Pressure

Stomatal Function

  • Absence of turgor pressure leads to wilting.

  • Guard cells control stomatal actions based on water availability.

Opening of Stomata

Mechanism

  • Environmental triggers like light and temperature initiate opening.

  • Receptors on guard cells pump out H+ ions, making cell contents negative and attracting K+ ions, increasing solute potential.

  • Water enters the cells by osmosis, increasing turgor pressure and opening stomata.

Abscisic Acid (ABA)

Role

  • A plant growth regulator causing stomatal closure during water stress.

Capillary Action

Definition

  • Movement of liquid in narrow spaces without external forces.

  • Water's cohesive and adhesive properties contribute.

Factors Influencing Capillary Action

  • Surface Tension: Water forms a meniscus due to surface tension.

  • Attraction to vessel material enhances capillary action.

Transpiration Process

Overview

  • Movement of moisture from roots to stomata where it evaporates.

  • Includes guttation (loss of liquid water through leaf pores).

Guttation Mechanism

Causes

  • Root pressure forces water through leaf pores, often confused with dew.

Xylem Structure

Composition

  • Comprised of tracheids and vessel elements with rigid cellulose walls for water transport.

Transpiration Stream

Flow Dynamics

  • Water flows from roots to leaves, assisted by capillary action and evaporation.

  • The transpiration-cohesion-tension mechanism drives water upward.

Evaporation

Process

  • Transition of water from liquid to vapor, essential for water cycling and cooling the surface.

Evaporation Conditions

Factors Influencing Rate

  • Heat is essential for evaporation, with higher temperatures accelerating the process.

  • Evaporation creates a cooling effect on the plant.

Role of Vapour Pressure

Humidity Influence

  • Relative humidity affects evaporation and water loss from leaves.

Control of Water Flow in Plants

Stomatal Regulation

  • Stomata adjust opening and closing to manage water loss and cooling.

External Factors Affecting Transpiration

Conditions

  • High temperatures and low humidity increase transpiration.

  • Air movement enhances transpiration by removing humid air.

Impact of Soil Water

Transpiration and Water Loss

  • If soil water absorption cannot keep up with transpiration, plants may wilt.

  • Permanent wilting occurs when the plant can no longer recover.

Implications of Poor Transpiration

Effect on Growth

  • Reduced growth and mineral transport; immobile nutrients like Calcium may show deficiency.

Phloem Function

Movement of Photosynthesis Products

  • Transports products from leaves to other parts of the plant via living cells, mainly downward through active processes.