Water can be absorbed in two ways by root hairs an ...
  • 情報
    すべての機能を使うには「保存して編集」をクリック
    エクスポート
    学習する
    編集
    管理
公開
保存して編集
検索中...
このデッキのフラッシュカード（11）

  • What are the three main routes for water movement across a root cortex?

    • Apoplast route
    • Symplast route
    • Vacuolar route
    roots water

  • How does water move in the apoplast route?

    Water soaks into cellulose cell walls and moves between cells by diffusion through cell walls; the cell walls act as a sponge and movement is passive by cohesion.

    apoplast water

  • Why is the apoplast route the fastest route across the cortex?

    There is less resistance because membranes are not crossed and the cellulose cell wall is fully permeable to water.

    apoplast transport

  • What prevents the apoplast route from continuing at the endodermis?

    A water-proof layer called the Casparian strip, made of suberin, in the endodermal cell walls prevents apoplast flow.

    endodermis casparian

  • What happens to water at the endodermis when the apoplast route is blocked?

    Water is forced into the symplast route, entering cytoplasm and moving between cells via plasmodesmata.

    endodermis symplast

  • How does water move in the symplast route?

    Water enters the cytoplasm by osmosis and moves passively through the cytoplasm and between cells via plasmodesmata.

    symplast osmosis

  • How does the vacuolar route differ from the symplast route?

    Water moves between cytoplasm and the central vacuole and between cells via plasmodesmata; it is the slowest route because membranes must be crossed.

    vacuolar transport

  • Why is ion concentration in soil usually lower than inside plant cells?

    Clay and other soil particles attract charged ions and effectively reduce their available concentration in the soil.

    soil ions

  • How are most mineral ions taken up at root hairs?

    Most ions enter via the symplast route using active transport at root hairs; some ions move passively with water along the apoplast route.

    ions uptake

  • Which active transport mechanisms move ions into root cells and give examples for cations and anions?

    Cations are taken up by ion exchange using a proton pump; anions like NO3- are taken up by co-transport with H+ ions.

    active ions

  • How does active uptake of ions affect water movement into the xylem?

    Active ion uptake raises ion concentration in the stele, lowering its water potential, which establishes a water potential gradient that increases water absorption into the xylem.

    xylem water
学習ノート

Overview

  • Roots absorb water and mineral ions through root hairs and epidermal cells by passive and active processes.
  • Water must cross the cortex to reach the xylem; ions are actively taken up to build concentration gradients that drive water uptake.

Three pathways for water movement across the root cortex

1. Apoplast route

  • Water moves through cell walls and intercellular spaces (cellulose matrix) without crossing membranes.
  • Movement is passive and driven by cohesion and diffusion; this is the fastest pathway because there are no membrane resistances.
  • The apoplast route is blocked at the endodermis by the Casparian strip (suberin), forcing water into the symplast.

2. Symplast route

  • Water enters cells via osmosis into the cytoplasm and moves between cells through plasmodesmata.
  • This route requires crossing the plasma membrane once (at the root hair) and is slower than the apoplast route but still efficient.

3. Vacuolar route

  • Water moves into and out of the central vacuole and between cells via plasmodesmata.
  • This route involves multiple membrane crossings and is the slowest due to higher resistance at each membrane.

Endodermis and the Casparian strip

  • The Casparian strip is a band of suberin in endodermal cell walls that is impermeable to water and solutes.
  • It prevents continued apoplastic flow into the stele, ensuring all water and dissolved ions must enter the symplast before reaching the xylem.
  • This control allows selective uptake (via membrane transport proteins) and prevents uncontrolled loss of solutes.

Mineral ion uptake (key mechanisms)

  • Most mineral ions enter at root hairs and then move inward via the symplast.
  • Active transport is used to concentrate ions in root cells, making internal ion levels higher than in soil.
  • Cations (e.g., Ca^{2+}, K^{+}): often taken up by mechanisms involving ion exchange and are facilitated by proton pumps that create favorable electrochemical gradients.
  • Anions (e.g., NO_3^{-}): typically taken up by co-transport (symport) with H^{+} ions driven by the proton gradient.
  • At the endodermis, apoplastic ion movement stops and ions must cross membranes using active transport or co-transport to enter the stele.

Role of proton pumps and co-transport

  • Plasma membrane H^{+}-ATPases pump protons out of the cell using ATP, creating a proton gradient and membrane potential.
  • The proton gradient does work: H^{+}-coupled co-transporters import anions or neutral solutes against their gradients.

Water potential and driving forces

  • Water moves from regions of higher water potential to lower water potential; water potential is often written as:
\[\Psi = \Psi_s + \Psi_p\]

where \(\Psi_s\) is solute potential and \(\Psi_p\) is pressure potential.

  • Active accumulation of ions in the stele lowers its \(\Psi\) (more negative), establishing a gradient across the cortex that draws water into the xylem.

Summary of relative speeds and resistances

  • Apoplast: fastest, low resistance, blocked at endodermis.
  • Symplast: intermediate speed, requires membrane crossing at entry.
  • Vacuolar: slowest, highest resistance due to multiple membrane crossings.

Why this matters (physiological consequences)

  • Selective ion uptake protects the plant from harmful solutes and allows concentration of essential nutrients.
  • Control at the endodermis ensures regulated movement into the vascular system and maintains root/soil homeostasis.

Quick facts / memory aids

  • Casparian strip = suberin barrier in endodermis; forces symplastic entry.
  • Proton pump (H^{+}-ATPase) = central to ion uptake and co-transport.
  • Water follows ions: raising solute concentration in the stele → water uptake into xylem.