Water and ions pass to the xylem by way of the apoplast and symplast

Mineral ions enter and move through plants in various ways. Where water is moving by bulk flow, dissolved minerals are carried along in the stream. Both water and minerals also move by diffusion. At certain sites, where plasma membranes are being crossed, some mineral ions are moved by active transport. One such site is the surface of a root hair, where mineral ions first enter the cells of the plant. Later, within the stele, the ions must cross another plasma membrane before entering the nonliving vessels and tracheids of the xylem.

The movement of ions across membranes can also result in the movement of water. Water moves into a root because the root has a more negative water potential than does the soil solution. Water moves from the cortex of the root into the stele (which is where the vascular tissues are located) because the stele has a more negative water potential than does the cortex.

Water and minerals from the soil may pass through the dermal and ground tissues to the stele via two pathways: the apoplast and the symplast. The apoplast (from the Greek, apo-, "away from"; -plast, "living material") consists of the cell walls, which lie outside the plasma membranes, and the intercellular spaces (spaces between cells) that are common to many tissues. The apoplast is a continuous meshwork through which water and dissolved substances can flow or diffuse without ever having to cross a membrane (Figure 36.4). Movement of materials through the apoplast is thus unregulated—until it reaches the endodermis, as we will soon discuss.

36.4 Apoplast and Symplast Plant cell walls and intercellular spaces constitute the apoplast.The symplast comprises the living cells, which are connected by plasmodesmata.To enter the symplast, water and solutes must pass through a plasma membrane. No such selective barrier limits movement through the apoplast.

Water and ions travel through cell walls and intercellular spaces in the apoplast.

Water and ions cross a plasma membrane to enter the symplast path.

The remainder of the plant body is the symplast (from the Greek, sym-, "together with"). The symplast is the portion of the plant body enclosed by membranes—the continuous cytoplasm of the living cells, connected by plasmodesmata (see Figure 36.4). The selectively permeable plasma membranes of the cells control access to the symplast, so movement of water and dissolved substances into the symplast is tightly regulated.

Water and minerals can pass from the soil solution through the apoplast as far as the endodermis, the innermost layer of the root cortex. The endodermis is distinguished from the rest of the ground tissue by the presence of Casparian strips. These waxy, suberin-im-pregnated regions of the endodermal cell wall form a water-repelling (hydrophobic) belt around each endodermal cell where it is in contact with other endodermal cells. The hydrophobic Casparian strips act as a seal that prevents water and ions from moving between the cells (Figure 36.5).

The Casparian strips of the endodermis thus completely separate the apoplast of the cortex from the apoplast of the stele. However, they do not obstruct the outer or inner faces of the endodermal cells. Accordingly, water and ions can enter the stele only by way of the symplast—that is, by entering and passing through the cytoplasm of the endodermal cells. Thus transport proteins in the plasma membranes of these

36.5 Casparian Strips Casparian strips in the endodermis of the cortex are impregnated with the water-repelling substance suberin. These strips separate the apoplast in the cortex from the apoplast in the stele.

Water and ions travel through cell walls and intercellular spaces in the apoplast.

Water and ions cross a plasma membrane to enter the symplast path.

Mineral Ions Plasma

j* Pericycle j* Tracheary elements j* Pericycle j* Tracheary elements cells determine which mineral ions pass into the stele, and at what rates.

Once they have passed the endodermal barrier, water and minerals leave the symplast and enter the apoplast of the stele. Parenchyma cells in the pericycle or xylem can aid this

Lymph Node Parenchyma
To bypass the Casparian strips, water must enter the living cells and access the stele via the symplast.

Casparian strips prevent water in the apoplast from passing between the endodermal cells into the stele.

Casparian strips prevent water in the apoplast from passing between the endodermal cells into the stele.

Symplast Apoplast Pathways

Plasmodesmata

Endodermis

Pericycle * (stele)

Plasmodesmata

Endodermis

Pericycle * (stele)

Wall extensions

Wall extensions

Disease Apoplast
36.6 A Transfer Cell Three walls of this transfer cell in a pea leaf have knobby extensions that face the cells from which the transfer cell imports solutes. A transfer cell exports the solutes to the neighboring sieve tube element.

process. Some of these parenchyma cells, called transfer cells, are structurally modified for transporting mineral ions from their cytoplasm (part of the symplast) into their cell walls (part of the apoplast). The cell wall that receives the transported ions has many knobby extensions projecting into the transfer cell, increasing the surface area of the cell's plasma membrane, the number of transport proteins, and thus the rate of transport (Figure 36.6). Transfer cells also have many mitochondria that produce the ATP needed to power the active transport of mineral ions.

As mineral ions move into the solution in the cell walls, the water potential in the apoplast becomes more negative; thus water moves out of the cells and into the apoplast by osmosis. In other words, active transport of ions moves the ions directly, and water follows passively. The end result is that water and minerals end up in the xylem, where they constitute the xylem sap. How do the water and materials move on from the xylem of the root system?

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Responses

  • Duenna Hayward
    How does water in apoplastic pathway from the cell wall of the pericycle cells enter xylem vessel?
    7 years ago
  • MUSTAFA MASSAWA
    How water passes through the cell membrane?
    7 years ago
  • Kimmo
    Why minerals can not pass directly thru apoplast?
    7 years ago
  • MATTIA ZETTICCI
    How do mineral ions enter the body?
    7 years ago
  • alesha
    How do ions move to xylem?
    7 years ago
  • Marco
    How do mineral ions enter a plant?
    7 years ago
  • alfredo
    How water move from cortex to stele?
    7 years ago
  • NOORA UPPA
    How does water enter the xylem from the endodermis?
    7 years ago
  • Asmait
    Do ions move by apoplast or symplast?
    7 years ago
  • BIBIANA
    How mineral ions end up in xylem?
    7 years ago
  • Haddas Abraham
    Can an ion move through the root cortex without passing through a plasma membrane?
    7 years ago
  • june
    Which ion travels through the cell membrane?
    7 years ago
  • Ralph
    How does water pass the casparian strip?
    7 years ago
  • bellisima
    How do mineral ions enter the xylem?
    7 years ago
  • v
    Which part of the cortex cells forms the apoplastic pathway?
    7 years ago
  • Ruby
    How does water pass from the root hair through the cortex to the xylem?
    7 years ago
  • Lisa Schwing
    How does water move through cell wall?
    7 years ago
  • Stephen
    How is water and mineral ions transported through the xylem vessel?
    7 years ago
  • Tapio
    How ions move through plants?
    7 years ago
  • Frank
    How is water passed into the xylum?
    7 years ago
  • ilenia
    What is the apoplastic and symplastic movement of water in flowering plant?
    7 years ago
  • ahti
    How do materials move from apoplast and symplast?
    7 years ago
  • Mariam
    How does water and mineral ions get moved through xylem?
    7 years ago
  • ELINA
    Can water diffuse from xylem to endodermis?
    7 years ago
  • j
    How do mineral ions enter into the xylem cells?
    7 years ago
  • Mathias
    When water and minerals move through the apoplast they pass through the plasmodesmata.?
    7 years ago
  • azeglio
    What ways can a mineral ion travel from the soil into the xylem?
    7 years ago
  • nazario
    Can a mineral ion enter an xylem of a plant?
    7 years ago
  • aatifa
    What prevents ions from leaving a cell of the lung?
    7 years ago
  • Topias
    Which way can substances travel in the xylem?
    6 years ago
  • tom
    When mineral ions pass between the cell walls in a root, they do so?
    6 years ago
  • Eija
    Which part of the cortex cells forms the apoplast membrane?
    6 years ago
  • fnan
    How does water and nutrients pass through the cortex via plasmodesmata?
    6 years ago
  • nibs
    What prevents ions from leaving lung cells?
    6 years ago
  • isengar
    What would prevent ions from leaving a cell in the lung?
    6 years ago
  • Aziz Tesfalem
    How does water and minerals move through xylem?
    6 years ago
  • kirsi
    Can water enter the stele without entering the symplast?
    6 years ago
  • J Gottlieb
    How is water moved into the xylem from pericycle?
    6 years ago
  • CHICA
    Does water travel through membranes in the xylem?
    6 years ago
  • trahand
    Why can't ions transport in apoplast pathway?
    5 months ago
  • bowman
    What is the barrier causing water only to move through the endodermis cells of roots?
    5 months ago

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