Section 3A

Biology 1002

Course Information

  • Course Title: Biology 1002

  • Term: Winter 2026

  • Section: 3

Flowering Plants: Transport (Ch.36)

KEY CONCEPTS

  • 36.1: Adaptations for acquiring resources were key steps in the evolution of vascular plants.

  • 36.2: Different mechanisms transport substances over short or long distances.

  • 36.3: Transpiration drives the transport of water and minerals from roots to shoots via the xylem.

  • 36.4: The rate of transpiration is regulated by stomata.

  • 36.5: Sugars are transported from sources to sinks via the phloem.

36.1 Adaptations for Acquiring Resources

  • The algal ancestors of land plants utilized direct absorption for acquiring resources:

    • Water, minerals, and CO₂ were absorbed from their surrounding environment.

  • Early nonvascular land plants lived in shallow waters and developed aerial shoots.

  • Natural selection emphasized traits in taller plants, which included:

    • Broad, flat appendages for light capture.

    • Multicellular branching roots for stability and nutrient uptake.

    • Efficient long-distance transport mechanisms for resource distribution.

Adaptation Mechanisms

  • The evolution of xylem and phloem in land plants enabled long-distance transport of key resources:

    • Xylem: Responsible for transporting water and minerals from roots to shoots.

    • Phloem: Transports photosynthetic products from sources (where they are produced) to sinks (where they are needed).

Resource Acquisition Overview

Essential Resources:

  • Water (H₂O)

  • Carbon Dioxide (CO₂)

  • Minerals (e.g., K⁺, NO₃⁻)

  • Sugars

  • Oxygen (O₂)

  • Light

36.2 Transport Mechanisms

  • There are three transport routes for water and solutes within plant tissue or organs:

    1. Apoplastic Route:

    • Involves movement through cell walls and extracellular spaces.

    1. Symplastic Route:

    • Involves movement through the cytosol.

    • Substances must cross a plasma membrane only once when they first enter the plant.

    • After entering, substances can move from cell to cell via plasmodesmata.

    1. Transmembrane Route:

    • Involves crossing cell walls and repeatedly crossing plasma membranes.

36.3 Long-Distance Fluid Transport

  • Bulk Flow:

    • Defined as the movement of fluid due to a pressure difference between two locations.

    • Long-distance bulk flow occurs in:

    • Tracheids and vessel elements of xylem.

    • Sieve-tube elements of phloem.

    • Efficient movement occurs because:

    • Mature tracheids and vessel elements lack cytoplasm.

    • Sieve-tube elements have minimal organelles.

Transpiration and Water Movement

  • Plants can transport substantial volumes of water from the roots to the shoots.

    • Most absorption occurs near the root tips, where root hairs are located, and epidermis allows permeability to water.

Absorption of Water and Minerals

Root Epidermal Cells

  • Root Hairs:

    • Critical for maximizing surface area for absorption.

    • Responsible for absorbing the soil solution, which contains water and dissolved mineral ions.

  • The absorbed solution then passes to the cortex, where active transport allows for mineral accumulation:

    • Essential minerals can be accumulated up to hundreds of times greater than their concentrations in soil.

Transport into the Xylem

Endodermis

  • Defined as the innermost layer of cells in the root cortex.

  • Surrounds the vascular cylinder and serves as the last checkpoint for selective mineral passage from cortex to vascular cylinder.

Water and Mineral Movement

  • Water and minerals can traverse the cortex utilizing either the symplastic or apoplastic routes:

    • If minerals reach the endodermis through the symplast, they continue through the plasmodesmata of endodermal cells into the vascular cylinder.

    • If minerals arrive via the apoplast, they encounter a barrier consisting of the Casparian strip in the walls of endodermal cells.

Casparian Strip

  • Casparian Strip:

    • A belt made of waxy suberin that is impervious to both water and dissolved minerals.

    • Prevents the crossing of water and minerals into the vascular cylinder via the apoplast.

  • Consequently, water and minerals must pass through the selectively permeable plasma membrane of an endodermal cell to enter the vascular cylinder, necessitating entry through the symplast.

Transport Mechanisms for Minerals

  • Essential mineral ions moving via the apoplast are then transported by specific membrane transporters into the cytosol of endodermal cells.

  • This process ensures that the endodermis selectively collects needed minerals into the xylem while excluding toxic substances.

Summary of Water and Mineral Transport From Root Hairs to the Xylem

  1. Apoplastic Route:

    • Water and minerals diffuse into the cortex following the hydrophilic walls of root hairs and along extracellular spaces.

  2. Symplastic Route:

    • Minerals and water that cross the plasma membrane of root hairs can enter the symplast.

  3. Transmembrane Route:

    • As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of epidermal and cortical cells, moving inward via the symplast.

  4. Endodermis Control:

    • Entry to the vascular cylinder is controlled by the Casparian strip in endodermal cells, blocking passage of substances through the walls.

    • Only minerals in the symplast or those crossing the plasma membrane can bypass the Casparian strip and enter the vascular cylinder.

  5. Transport in Xylem:

    • Endodermal cells and alive cells within the vascular cylinder discharge water and minerals into their walls (apoplast), allowing xylem vessels to then transport water and minerals upward through bulk flow into the shoot system.

Final Passage into Xylem

  • Water and minerals finally move into the tracheids and vessel elements of xylem:

    • These water-conducting cells do not contain protoplasts, thus they are part of the apoplast.

    • The movement from protoplasts of endodermal cells to cell walls involves diffusion and active transport, resulting in water and minerals entering the tracheids and vessel elements.

    • This transport of water and minerals to the shoot system is achieved via bulk flow.

Practice Question:

  1. The primary function of all types of leaves is photosynthesis. True or False?

  • Answer: TRUE