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

Transport in plants H 2 O (passive) Minerals (active)  transport in xylem  transpiration evaporation, adhesion & cohesion negative pressure Sugars  transport in phloem  bulk flow Calvin cycle in leaves loads sucrose into phloem positive pressure Gas exchange  photosynthesis CO 2 in; O 2 out stomates  respiration O 2 in; CO 2 out roots exchange gases within air spaces in

Transport in Plants Physical forces drive transport at different scales  cellular from environment into plant cells transport of H 2 O & solutes into root hairs  short-distance transport from cell to cell loading of sugar from photosynthetic leaves into phloem sieve tubes  long-distance transport transport in xylem & phloem throughout 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.

Short Distance (cell-to-cell) Transport Compartmentalized plant cells  cell wall  cell membrane cytosol  vacuole

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

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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 plant  An 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

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Control of Transpiration Keeps leaf temperature lower Rate is affected by evaporation:  Sunny, dry, warm, windy

Control of Transpiration Stomata:  Open when turgid  Closed 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 XEROPHYTES 1. small, thick leaves 2. Thick cuticle 3. Stomata in pits 4. Reduced surface area 5. 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