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Resource Acquisition and Transport in Vascular Plants (Different…
Resource Acquisition and Transport in Vascular Plants
Adaptations for acquiring resources were key steps in the evolution of vascular plants
Evolutionary adaptations to life on land involved the specialization of roots to absorb water and minerals, taller shoots with broad leaves to absorb CO2 and light, and vascular tissue consisting of xylem and phloem to transport materials between roots and shoots
Phyllotaxy, the arrangement of leaves on the stem of a plant, may be one leaf per node, two leaves per node, or multiple
Mycorrhizae, symbiotic associations between roots and fungi, greatly increase the surface area for water and mineral absorption
Different mechanisms transport substance over short or long distances
The apoplast consists of plant cell walls, extracellular spaces, and interiors of dead cells, such as xylem vessels
The protoplasts of plant cells are connected by plasmodesmata, forming a cytoplasmic continuum called the symplast
The diffusion of free water across a membrane is called osmosis
Water potential is a useful measurement for predicting the direction hat water will move, and it takes into account both solute concentration and physical pressure
Water potential, designated psi, is measured in megapascals (MPa); 1 MPa is equal to about 10 atmospheres of pressure
Solute potential, also called osmotic potential, is proportional to the molarity of a solution
Pressure potential measures the physical pressure on a solution and can have a positive or negative value
A plant protoplast that is expanding due to the osmotic uptake of water pushes against the cell wall, and the rigid cell wall exerts pressure against the protoplast
The positive pressure is called turgor pressure
A flaccid plant cell bathed in solution more concentrated than the cell will lose water by osmosis because the solution has a lower water potential than the solute potential of the cell
Plant cells are usually turgid; they have a greater solute concentration than their extracellular environment and turgor pressure keeps them firm
Long-distance transport in plants occurs by bulk flow, the movement of fluid driven by a pressure gradient
Transpiration drives the transport of water and minerals from roots to shoots via the xylem
A ring of suberin around each endodermal cell, called the Casparian strip, prevents water and minerals from the apoplast from entering the stele without passing through a selectively permeable plasma membrane
The water dissolved minerals of xylem sap move by bulk flow from the vascular cylinder of a root to the branching veins of leaves
Through the evaporartive loss of water vapor from leaves, called transpiration, the plant loses a tremendous amount of water that must be replaced by water transported up from the roots
Water flowing from the cortex into this area of lower water potential produces root pressure, which pushes xylem sap upward
According to the cohesion-tension hypothesis, transpiration creates a negative-pressure pull on xylem sap, which is transmitted from shoots to roots by the cohesion of water molecules within the xylem
The rate of transpiration is regulated by stomata
Cycles that have intervals of approximately 24 hours are called circadian rhythms
Abscisic acid (ABA), a hormone produced in roots and leaves in response to a lack of water, signals guard cells to close stomata
Many xerophytes, plants adapted to arid climates, have leaves that are highly reduced; photosynthesis is carried out in thick, water-storing stems
Sugars are transported from sources to sinks via the phloem
Translocation, the transport of photosynthetic products throughout the plant, occurs in the sievetube elements of phloem
Phloem sap may have a sucrose concentration as high as 30% and may also contain minerals, amino acids, and hormones
Phloem sap flows from a sugar source, where it is produced by photosynthesis or the breakdown of starch, to sugar sink, an organ that consumes or stores sugar
The symplast is highly dynamic
Plant viruses produce viral movement proteins that mimic the cell's regulatory machinery that dilates plasmodesmata
Communication during plant development may occur within interconnected groups of cells called symplastic domains