Transport in Plants Chapter 36

Cellular Transport A. passive transport Driven by the principles of diffusion Much of the diffusion is facilitated Selective channels are usually gated and regulated

Cellular Transport B. Active Transport Often uses proton pump Generates membrane potential may be used to drive active transport of other molecules (cotransport)

Cellular Transport Water Transport By osmosis water moves from hypotonic to hypertonic solution in cells BUT in plant cell, cell wall provides a pressure component, therefore water potential must be used

Water potential review: What is ψ of a container of distilled water that is open? If ψ inside a cell is -0.7 MPa, and outside of the cell is -0.1 MPa, which direction will water move? If ψS=-0.5MPa and ψP=0.1MPa, will water move into or out of this cell if it is placed in an environment where ψ=-0.2 MPa?

Water Potential Water moves from higher water potential (ψ) to lower water potential Ψ=0 in pure water at atmospheric pressure Ψ and solute concentration have an inverse relationship Ψ and pressure have a direct relationship

Plant Cells and Water Potential Plasmolysis: protoplast pulls away from the cell wall due to loss of water The pressure exerted on the cell wall is 0 Turgor pressure: Pressure exerted on cell wall by protoplast due to influx of water The pressure exerted on the cell wall is positive

Aquaporins 1990’s Peter Agre (John’s Hopkins) discovers water pores in membranes that facilitate diffusion of water 2003 Nobel Prize in Chemistry along with Roderick MacKinnon Water is small and can diffuse through bilayer to some extent despite polarity Aquaporins speed up this diffusion May be gated to regulate water diffusion

3 Cellular Compartments Apoplast: continuum of cell walls and extracellular spaces Symplast: continuum of cytosol linked by plasmodesmata Vacuole: bound by membrane called tonoplast

Lateral Transport Short distance horizontal movement Mechanisms Transmembrane (through membranes from one cell to the next) – slow Symplastic (through one membrane into cell, then through plasmodesmata) Apoplastic (through cell walls)

Absorption by Roots Root hairs increase surface area Mycorrhizae (fungal partners) enhance absorption Endodermis functions as selective region Symplastic continues through plasmodesmata Apoplastic must cross plasma membrane into cytosol due to Casparian strip Casparian strip is waxy and hydrophobic, traps water and minerals in the vascular tissue

Bulk Flow Movement of a fluid driven by pressure In both xylem and phloem

Long Distance Transport in the Xylem Root Pressure: Pushing Transpiration: Pulling

Root Pressure Used by some plants, at night At night, plants still expend energy pumping minerals into xylem Accumulation of nutrients decreases ψ , causing water to flow in by osmosis This upward push is root pressure Guttation: exudation of water droplets seen in the morning

Transpiration Major mechanism of movement Water is adhesive and cohesive As one water droplet moves, the next also moves (water in continuous column in xylem) As water evaporates out of the stomata, water below moves upward

Guard Cells Guard cells regulate water loss. What conditions will promote the closing of guard cells? Buckle outward when turgid Regulate opening and closing by managing K+

Stomata

Guard Cells Pump H+ out, K+ enters due to electric charge, water follows due to osmosis Close when K+ leaves (water follows) Aquaporins may vary permeability to regulate

Some leaf molds, which are fungi that parasitize plants, secrete a chemical that causes guard cells to accumulate potassium ions. How does this adaptation enable the leaf mold to infect the plant?

Guard Cell Cues Light stimulates a blue light receptor in guard cells, activates H+ pumps CO2 depletion when Calvin cycle starts Can trick guard cells by placing in environment without CO2 Internal Clock Open and close even in 24 hour dark Circadian rhythm

What adaptations are seen for arid climates? thick leaves (low SA/vol) Thick cuticle Stomata on lower leaf surface Stomata in pores to shield wind Shed leaves in hot dry season Cacti have no leaves (adapt to spines) CAM photosynthesis

Phloem Transport Translocation Sugar source to sugar sink Sugar loaded into sieve tube members by active transport Water follows by osmosis (increases pressure) At sink, sugar leaves sieve tube members (by diffusion or active transport) Water follows (decreases pressure) Water is recycled by xylem

Oleander: stomata in “cypts”

Old Man cactus