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.