Water and Sugar Transport in Plants (Ch. 35)

Vocabulary flashcards covering water potential, xylem and phloem transport mechanisms, and plant adaptations for water conservation according to Chapter 35.

Water potential ($\Psi$)

The potential energy of water in a location compared to pure water at room temperature and atmospheric pressure; measured in megapascals ($MPa$).

Solute potential ($\Psi_S$)

The tendency of water to move by osmosis due to dissolved solutes; always $\le 0\,MPa$ relative to pure water.

Pressure potential ($\Psi_P$)

The tendency of water to move in response to physical pressure; can be positive or negative.

Tension

Negative pressure potential; the force that draws liquid up through a drinking straw.

Turgor pressure

The pressure inside a cell created as water moves in and pushes the plasma membrane against the cell wall.

Wall pressure

The force the rigid cell wall exerts back against the membrane, resisting expansion from turgor pressure.

Turgid

A condition where a cell is firm, experiencing wall pressure and positive turgor pressure due to water uptake.

Flaccid

A condition where a cell has no turgor pressure ($\Psi_P = 0$).

Isotonic solution

A solution with the same solute concentration inside and outside a cell, resulting in no net water movement.

Hypotonic solution

A solution with a lower solute concentration than the cell, causing water to move into the cell by osmosis.

Transpiration

The evaporation of water from leaf stomata; requires open stomata and air outside the leaf to be drier than air inside.

Cohesion

The attraction between like molecules (water-water) via hydrogen bonding; transmits pulling force down the water column.

Adhesion

The attraction between unlike molecules, such as water and the cell wall of xylem; pulls water up along the sides of a tube.

Surface tension

The enhanced attraction among water molecules at an air-water interface that minimizes surface area.

Meniscus

The concave water surface formed by adhesion, cohesion, and gravity at an air-water interface.

Bulk flow

Mass movement of fluid molecules along a pressure gradient; how water moves within dead xylem cells.

Cohesion-tension theory

The theory that water transport is solar powered by evaporation at the leaf surface, creating a pulling force transmitted down the xylem.

Lignin

A substance that reinforces secondary cell walls in tracheids and vessel elements, allowing xylem to withstand extremely negative pressures.

Photosynthesis-transpiration compromise

The tradeoff where open stomata are needed for $CO_2$ uptake but also cause water loss via transpiration.

Cuticle

A waxy layer on the upper leaf surface that minimizes water loss from cells exposed to sunlight.

Trichomes

Hair-like epidermal extensions that shield stomata, creating a layer of still, humid air to slow water vapour loss.

CAM (crassulacean acid metabolism)

An adaptation where stomata open at night to fix and store $CO_2$, staying closed during the day to reduce water loss.

$C_4$ photosynthesis

A process where $CO_2$ is fixed in mesophyll cells and transferred to bundle-sheath cells to concentrate it, allowing stomata to remain partially closed.

Translocation

The bulk-flow movement of sugars from sources to sinks throughout the phloem.

Source (sugar transport)

A tissue where sugar enters the phloem, such as mature photosynthesizing leaves or storage tissue early in the season.

Sink (sugar transport)

A tissue where sugar exits the phloem, such as meristems, developing fruit, or storage roots.

Sieve-tube element

The conducting tube of phloem; alive at maturity but lacks a nucleus and most organelles; connected by sieve plates.

Companion cell

The support cell for sieve-tube elements, containing high concentrations of organelles and connected via plasmodesmata.

Sieve plate

The perforated wall connecting sieve-tube elements, made of enlarged plasmodesmata pores.

Pressure-flow hypothesis

The model where events at source and sink tissues create a pressure potential gradient that drives the bulk flow of phloem sap.

Honeydew

Sugary phloem sap excreted by aphids after it is forced through their stylets by phloem pressure.

Proton pump ($H^+\text{-ATPase}$)

A pump that uses $ATP$ to move $H^+$ out of a cell, creating an electrochemical gradient used for phloem loading.

Passive transport

Movement of ions or molecules along their electrochemical gradient; requires no energy.

Active transport

Movement of ions or molecules against their electrochemical gradient; requires energy in the form of $ATP$.

Symporter

A cotransporter that moves two solutes in the same direction, using the gradient of one to power the other against its gradient.

Antiporter

A cotransporter that moves two solutes in opposite directions.

Secondary active transport

Transport via cotransporters powered indirectly by a gradient established by a pump.

Tonoplast

The membrane surrounding the vacuole of a plant cell.