Transport in Vascular Plants

Chapter 36

Transport in Plants Occurs on three levels:

the uptake and loss of water and solutes by individual cells

short-distance transport of substances from cell to cell at the level of tissues or organs

long-distance transport of sap within xylem and phloem at the level of the whole plant

Cellular Transport Most solutes have to be transported

via a transport proteinnad with the use of energy active transport

Proton pumps provide this energy Cotransport

proton pumps

Cotransport

Water Potential / Uptake of Water Uptake of water

osmosis Water moves from

high water potential to low water potential

Ψ= Ψs+ Ψp

Movement of Water in Plants

Water potential impacts the uptake and loss of water in plant cells

In a flaccid cell, Ψp=0 and the cell is not firm the cell lower water

potential (presence of solutes) than the surrounding solution water will enter the cell.

As the cell begins to swell, it will push against the wall, producing a turgor pressure

Aquaporins Transport proteins that facilitate the

passive movement of water across a membrane

Do not affect the water potential gradient or the direction of water flow, but rather the rate at which water diffuses down its water potential gradient

Aquaporins are gated channels open and close in response to variables, such as turgor pressure, in the cell.

Routes from cell to cell Moving water & solutes between cells

transmembrane route repeated crossing of plasma membranes slowest route but offers more control

symplast route move from cell to cell within cytosol

apoplast route move through connected cell wall without crossing cell

membrane fastest route but never enter cell

Roots Absorb Water and Minerals Water and mineral salts from soil

enter the plant through the epidermis of roots, cross the root cortex, pass into the stele, and then flow up xylem vessels to the shoot system

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 36.7

Water Route through the Root Water uptake by root hairs

a lot of flow can be through cell wall route

apoplasty

Controlling the route of water Endodermis

cell layer surrounding vascular cylinder of root lined with impervious Casparian strip forces fluid through

selective cell membrane & into symplast

filtered & forced into xylem vessels

Transpiration The loss of water vapor from leaves

and other aerial parts of the plantAn average-sized maple tree losses

more than 200 L of water per hour during the summer

The flow of water transported up from the xylem replaces the water lost in transpiration and also carries minerals to the shoot system.

Mechanisms of Transpiration Root Presssure Cohesion –Adhesion pull

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 36.11

Control of Transpiration Keeps leaf temperature lower Rate is affected by evaporation:

Sunny, dry, warm, windy

Control of Transpiration Stomata:

Open when turgidClosed when flaccid

Figure 36.13a The mechanism of stomatal opening and closing

The role of potassium in stomatal opening and closing K+ actively moved into guard cells Water follows

Triggers: Blue light receptors Low CO2 levels Circadian rhythms

Adaptations to Reduce Transpiration

XEROPHYTES1. small, thick leaves2. Thick cuticle3. Stomata in pits4. Reduced surface area5. CAM plants

Movement of Sugars Translocation process that transports

the organic products of photosynthesis throughout the plant

In general, sieve tubes carry food from a sugar source to a sugar sink

A sugar source (mature leaves) in which sugar is being produced by either photosynthesis or the breakdown of starch

A sugar sink (growing roots, shoots, or fruit) that is a net consumer or storer of sugar