Chap 36

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Bulk flow is the movement of fluid in vessels.

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The plasma membrane and vacuolar membrane provide transmembrane transport.
The cytoplasmic continuum, connected by plasmodesmata , is the symplast route.
The continuum of cell walls plus extracellular spaces provide the apoplast route.

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Guttation.
Root pressure is forcing excess water from this strawberry leaf.

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Mycorrhizae, symbiotic associations of fungi and roots.
The white mycelium of the fungus ensheathes these roots of a pine tree. The fungal hyphae provide an extensive surface area for the absorption of water and minerals.

pressure_flow.html: 36_18PressureFlow.jpg
Pressure flow in a sieve tube. Sap moves through a sieve tube by bulk flow driven by positive pressure .

The building of pressure at the source end and reduction of that pressure at the sink end cause water to flow from source to sink, carrying the sugar along.

proton_pump.html: 36_03ProtonPump.jpg
Proton pumps provide energy for solute transport. By pumping H⁺ out of the cell with the hydrolysis of ATP, proton pumps produce an H⁺ gradient and a charge separation called a membrane potential. These two forms of potential energy can be used to drive the transport of solutes.

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Solute transport in plant cells.

The role of proton pumps in transport is an application of chemiosmosis, using a transmembrane proton gradient to link energy–releasing processes to energy–consuming processes in cells.

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root_transport.html: 36_09RootLateralTransport.jpg
Lateral transport of minerals and water in roots.

Root hairs absorb water and minerals, which move along the walls comprsing the apoplast.
Minerals and water that cross the plasma membranes of root hairs enter the symplast.
Along the apoplast some water and minerals are transported into the cytosols of cells and then move via the symplast.
The Casparian strip, a belt of waxy material, allows only minerals in the symplast to pass into the vascular cylinder.
Cells within the vascular cylinder discharge water and minerals into their walls (apoplast).

stomata.html: 36_15GuardCellFunction.jpg
The mechanism of stomatal opening and closing.

Cellulose microfibrils in the walls resist stretching, causing the cells to increase in length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling.
The transport of K⁺ (potassium ions) across the plasma membrane causes the turgor changes of guard cells by changing the water potential.

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Loading of sucrose into phloem.

Sucrose made in the mesophyll travel the symplast. Some sucrose exit the symplast and accumulate in sieve-tube members and companion cells via the apoplast. (b) Proton pumps generate an H⁺ gradient, driving sucrose accumulation in the cell by chemiosmosis .

Loading of sucrose into phloem.
Sucrose made in the mesophyll travel the symplast. Some sucrose exit the symplast and accumulate in sieve-tube members and companion cells via the apoplast.	(b)	Proton pumps generate an H⁺ gradient, driving sucrose accumulation in the cell by chemiosmosis .

transpirational_pull.html: 36_12Transpiration.jpg
The generation of transpirational pull in a leaf. The negative pressure at the air–water interface in the leaf is the physical basis of transpirational pull, which draws water out of the xylem.

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Turgor loss in plants causes wilting, and can be reversed when the plant is watered.
Video: Elodea in differential interference contrast microscopy.

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The vascular system allows plants to distribute specialized funstions to different parts and grow to great heights.

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In a hypertonic (low external Ψ) environment, the cell loses water and plasmolyzes. In a hypotonic (high external Ψ) environment, there is net uptake of water by osmosis, the cell becomes turgid which helps it ertain its shape.

In a hypertonic (low external Ψ) environment, the cell loses water and plasmolyzes.		In a hypotonic (high external Ψ) environment, there is net uptake of water by osmosis, the cell becomes turgid which helps it ertain its shape.

water_potential.html: 36_05WaterPotential.jpg
Water moves across a selectively permeable membrane from higher water potential to lower.

Water potential (Ψ) is the sum of pressure potential (Ψ_P) and solute potential (Ψ_S): Ψ = Ψ_P + Ψ_S.

Ψ_P = 0 MegaPascal at at sea level and room temperature.

(a) The addition of solutes reduces water potential (to a negative value).
(b, c) Application of physical pressure increases water potential.
(d) A negative pressure (tension) decreases water potential.

xylem_sap.html: 36_13XylemSapFlow_L.jpg
Ascent of xylem sap. Hydrogen bonding forms an unbroken chain of water molecules from leaves to the soil, by cohesion among water molecules and adhesion of water molecules to cellulose. The force that drives the ascent of xylem sap is a gradient of water potential (Ψ), mainly pressure potential ( Ψ_P). Transpiration results in the Ψ_P at the leaf end of the xylem being lower than the Ψ_P at the root end.