Chapter Summary
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34.1 Plants Acquire Water and Minerals from the Soil
Water moves through biological membranes by osmosis, always moving toward regions with a more negative water potential. The water potential (Ψ) of a cell or solution is the sum of the solute potential (Ψs) and the pressure potential (Ψp). Review Figure 34.2, Activity 34.1
Turgid plant cells have significant positive pressure potential because the rigid cell wall limits expansion of the cell. This positive pressure (turgor pressure) maintains the physical structure of many plant cells; if the pressure potential drops, the plant wilts.
The movement of a solution due to a difference in pressure potential between two parts of a plant is called bulk flow.
Aquaporins are channel proteins that facilitate movement of water molecules through biological membranes.
Mineral uptake requires transport proteins. Some minerals enter the plant passively by facilitated diffusion; others enter by active transport. A proton pump provides energy for the active transport of many mineral ions across membranes in plants. Review Figure 34.4
Water and minerals pass from the soil into the root by way of the apoplast and symplast, but must pass through the symplast to cross the endodermis and enter the xylem. The Casparian strip in the endodermis blocks movement of water and minerals through the apoplast. Review Figures 34.5, 34.6, Activity 34.2
34.2 Water and Minerals Are Transported in the Xylem
Experiments proved that neither a root pump nor capillary action can alone account for the ascent of xylem sap in trees.
Water transport in the xylem results from the combined effects of transpiration, cohesion, and tension—
the transpiration– cohesion– tension mechanism. Evaporation from the leaf produces tension in the mesophyll cells, which pulls a column of water— held together by cohesion— up through the xylem from the root. Review Focus: Key Figure 34.7, Animation 34.1 Transport in the xylem is passive. It does not require the expenditure of energy by the plant.
Water-
use efficiency is the ratio of plant growth to water uptake.
34.3 Stomata Control the Loss of Water and the Uptake of CO2
The waxy cuticle of plant epidermis is impermeable to both water and carbon dioxide. Stomata allow for carbon dioxide uptake (when open) while minimizing transpirational water loss (when closed).
A pair of guard cells controls the size of the stomatal opening. A light-
activated proton pump moves protons out of the guard cells to the walls of surrounding epidermal cells, setting up an electrochemical gradient that drives the transport of potassium ions into the guard cells. Water follows osmotically, swelling the guard cells and opening the stomata. Review Figure 34.8 When threatened by dehydration, mesophyll cells release abscisic acid, which causes guard cells to close the stomata, even in the light.
34.4 Solutes Are Transported in the Phloem
Products of photosynthesis, as well as some minerals, are translocated through sieve tubes in the phloem by way of living sieve tube elements. Review Figure 34.9
Translocation in the phloem can proceed in both directions in the stem. Translocation requires a supply of metabolic energy from living cells.
Translocation in the phloem is explained by the pressure flow model: the difference in solute concentration between sources and sinks creates a difference in (positive) pressure potential along the sieve tubes, resulting in bulk flow. Review Figure 34.10, Table 34.2, Animation 34.2
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