1. uptake and release of solutes and water by cells

2. short-distance transport of substances within tissues

3. long distance via the xylem and phloem in the vascular system

· carrier proteins selectively bind to a specific solute

· selective channels eg. one for K+ passage

· may be gated to open and close (stomatal K+) in guard cells

1. C. active transport via energy expenditure (ATP to ADP) eg. pumping solutes against a concentration gradient

· one important example: proton pumps--pump H+ out of cells

· stores energy in form of proton gradient (higher outside cell)

· also generates a membrane potential--voltage due to separation of + and - charges across a membrane

· form of potential (stored) energy

· used to drive transport of many solutes with or against conc. gradients (K+ with gradient)

· NO3- cotransported with H+ against gradient

· sucrose cotransported out with H+

2. Short Distance (Lateral) Transport--along radial axis of plant (Fig. 36.5. 36.6)

three routes possible (can change back and forth):

1. cell to cell by crossing cell walls, repeatedly crossing plasma membranes

2. via symplast (continuum of cytoplasm)--move from cell to cell via plasmadesmata (cytoplasmic channels)

3. via apoplast--extracellular matrix between cell walls

governed either by diffusion (conc. gradients) or bulk flow (pressure gradients)

bulk flow = movement of a fluid under pressure

· xylem transport due to negative pressure (= tension) --Fig. 36.8

· reduction in pressure in leaves via transpiration yields tension which pulls water upward in xylem from roots

epidermal cells and root hair extensions make up most of absorptive surface area of root (plus mycorrhizae)

walls are hydrophilic, causing flow along apoplast to cortex, entering and exiting symplast

cells selectively concentrate K+, exclude Na+, allow essential ions to keep flowing in

endodermis (inner layer of cells in cortex) has Casparian strip (ring of wax-like suberin) barring apoplastic route into stele(central cylinder of vascular tissue)--also prevents backflow out of stele

solutes must be in symplast to enter stele

A. Fig. 35.10, 35.11 Xylem (tracheids and vessels elements) are apoplast only, no living protoplast

xylem flow 15 m/hour or faster, driven by transpiration (avg. maple tree loses 200 L/hr)

rises against gravity, driven by:

a) root pressure -- upward force, leads to guttation in early AM, when transpiration has been slow

cannot keep pace with transpiration after sunrise

b) transpiration-cohesion-tension mechanism--Fig. 36.10

· transpiration provides the pull (tension)

· cohesion of water molecules due to hydrogen bonds transmits the pull along the column of water on xylem

· adhesion of water with hydrophilic walls of xylem

tension actually causes the xylem to pull inward (can measure the decrease in diameter of tree trunk)

transpirational pull transmitted all the way to the roots, allows water to flow inward via passive methods

guard cells of the stomata control size of stomatal openings, balance the need to conserve water, take in CO2 needed for photosynthesis (PSN)--Fig. 36.11

internal surface area of leaves 10-30X greater than the flat surface--amplifies PSN by increasing exposure of mesophyll cells to CO2, but also allows more H2O to evaporate

ratio transpiration/PSN is a measure of a plant's efficiency (g H20 lost/g CO2 fixed)

often 600/1 for average C₃ plant eg. rice, wheat, soybeans (CO₂ first converted to 3-phosphoglycerate via rubisco)

C₄ pathway plants (corn, sugarcane etc) may have ratios of 300/1 or less (CO₂ converted first to 4-carbon oxaloacetate in mesophyll cell, later converted to malate (4C) and exported to bundle sheath cells--Fig. 10.17

adaptations to reduce transpiration:

1. xerophytes: small thick leaves (reduced surface); thick cuticle; stomata in pits (Fig 36.12)

a form of bulk transport from sugar source to sugar sinks (sucrose, minerals, amino acids, hormones)

angiosperms have sieve tube members + companion cells (Fig. 35.10)

sugar sinks = organs that use or store carbohydrates manufactured by sources (leaves) : root and shoot tips, fruits, storage organs (modified roots, shoots, leaves)

phloem transport can be bidirectional within same vascular bundle (one sieve tube up, a neighboring one down)

phloem loading/unloading:

· may move the whole way via the symplast eg. plants in mint and squash families

· or transfer cells (companion cells with numerous ingrowths in cell walls to increase surface areas, enhance transfer from apoplast to symplast--selective)

· active transport usually required (2-3X higher conc. in phloem than in mesophyll)

· proton pump + cotransport (membrane protein transports both H⁺ and sucrose in together)

at sink end--may be active or passive (diffusion)

metabolism of sucrose or conversion to starch may keep actual sucrose conc. low, favoring diffusion out of sieve tube at sink end

phloem sap moves by bulk flow--pressure driven

high solute concentration in phloem causes intake of water due to high osmolarity, hydrostatic pressure, greatest at source end of tube

water then recycled back to source via xylem