Transport in Plants


Transport in plants occurs on three levels:

Cellular-level Transport

A key component of cellular-level transport is the movement of solutes and ions across the plasma membrane.  We have already covered this, so I won't repeat it.  If you are unsure, review Lecture 9.

Survival of the plant depends on balancing water uptake and water loss.
In an animal cell, water flows from hypotonic to hypertonic solutions, but in a plant cell, there is the added presence of the pressure created by the cell wall
The combination of solute concentration differences and physical pressure are incorporated into water potential, abbreviated with the Greek letter psi ()

Movement of Water Through Cells - Two Routes, the Symplast and the Apoplast

Symplastic Movement

Apoplastic Movement

Absorption of Water and Minerals by Roots

Absorption is a surface area phenomenon - the more surface area there is, the more absorption there will be. The Endodermis - The Root's Border Guard

Water flowing through the apoplast contains many minerals that the plant needs - it may also contains toxins and substances that the plant may not want.  However, since the water is flowing through the apoplast, there is no way to prevent the passive transport of these toxins, until the water hits the endodermis.

Endodermis

Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin).


Transport of Xylem Sap

Xylem sap rises against gravity, without the help of any mechanical pump, to reach heights of more than 100m in the tallest trees.  How can this occur?

Transpiration-Cohesion-Tension: A Mechanism to Pull Xylem Sap up the Plant

Stomata open up during the day to let CO2 in and inadvertently let H2O escape

Root Pressure: A Mechanism to "Push" Xylem Sap Up the Plant

At night, transpiration is almost nil.  However, the root cells continue to actively transport minerals into the stele (the root stele is basically everything surrounded by the endodermis - primarily the xylem and the phloem).


The Control of Transpiration

Water is needed for photosynthesis - it is also lost as a product of obtaining carbon by this very same process.  How does the plant balance is requirement for water with its requirement for carbon in photosynthesis? How Stomates Open and Close

Each stoma is flanked by a pair of guard cells that are capable of changing shape, thereby widening or narrowing the gap between the two cells

The changes in turgor pressure result primarily through the reversible uptake of K+ ions Adaptations to reduce transpiration loss in plants growing in dry conditions (xerophytes)

The Translocation of Phloem

Translocation - the process of moving photosynthetic product through the phloem Phloem Loading and Unloading The Mechanism of Translocation in Angiosperms