Resource Acquisition and Transport in Vascular Plants Chapter 36 Resource Acquisition and Transport in Vascular Plants

What you need to know: The role of passive transport, active transport, and cotransport in plant transport. The role of diffusion, active transport, and bulk flow in the movement of water and nutrients in plants. How the transpiration cohesion-tension mechanism explain water movement in plants. How pressure flow explains translocation.

What does a plant need?

Review: Selectively permeable membrane: osmosis, transport proteins, selective channels Proton pump: active transport; uses E to pump H+ out of cell  proton gradient Cotransport: couple H+ diffusion with sucrose transport Aquaporin: transport protein which controls H2O uptake/loss

Solute transport across plant cell plasma membranes Transport begins with the movement of water and solutes across a cell membrane. Solutes diffuse down their electrochemical gradients. If no energy is required to move a substance across the membrane, then the movement is termed passive transport. Diffusion is an example of passive transport. If energy is required to move solutes across the membrane, it is termed active transport. As most solutes cannot move across the phospholipid barrier of the membrane, a transport protein is required . The most important transport protein is plants is the proton pump. A proton pump creates an electrochemical gradient by using the energy of ATP to pump hydrogen ions across the membrane. This coupling of the steep gradient of one solute (hydrogen in our example) with a solute like sucrose. The drop in potential energy experiences by the hydrogen ion pays for the transport of the sucrose.

Osmosis **Water potential (ψ): H2O moves from high ψ  low ψ potential, solute conc. & pressure Water potential equation: ψ = ψS + ψP Solute potential (ψS) – osmotic potential Pressure potential (ψP) – physical pressure on solution Pure water: ψS = 0 Mpa Ψ is always negative! Turgor pressure = force on cell wall Bulk flow: move H2O in plant from regions of high  low pressure ** Review AP Bio Investigation 4

Flaccid: limp (wilting) Plasmolyze: shrink, pull away from cell wall (kills most plant cells) due to H2O loss Turgid: firm (healthy plant) Turgid Plant Cell Plasmolysis

A watered impatiens plant regains its turgor.

Vascular Tissues: conduct molecules Xylem Phloem Nonliving functional Living functional Xylem sap = H2O & minerals Phloem sap = sucrose, minerals, amino acids, hormones Source to sink (sugar made) to (sugar consumed/stored)

Transport of H2O and minerals into xylem: Root epidermis  cortex  [Casparian Strip]  vascular cylinder  xylem tissue  shoot system Water and minerals from the soil enter the plant through the root epidermis, cross the cortex, pass into the vascular cylinder, and then flow up the xylem. 1) Apoplastic route: the movement between cells. 2) Symplastic route occurs only after the solution crosses a plasma membrane.

At Root Epidermis Root hairs: increase surface area of absorption at root tips Mycorrhizae: symbiotic relationship between fungus + roots Increase H2O/mineral absorption The white mycelium of the fungus ensheathes these roots of a pine tree.

Transport pathways across Cortex: Apoplast = materials travel between cells Symplast = materials cross cell membrane, move through cytosol & plasmodesmata

Entry into Vascular Cylinder: Endodermis (inner layer of cortex) sealed by Casparian strip (waxy material) Blocks passage of H2O and minerals All materials absorbed from roots enter xylem through selectively permeable membrane Symplast entry only! Water can move inward by either route until reaching the innermost layer of the cortex, the endodermis. The endodermal material that blocks the passage of water and dissolved materials. This is a critical control point for materials moving into the plant, because at the Casparian strip the soil solution must cross the plasma membrane. The plasma membrane determines what can cross into the xylem tissue and gain entrance to the rest of the plant.

How does material move vertically (against gravity)? Transpiration: loss of H2O via evaporation from leaves into air Root pressure (least important) Diffusion into root pushes sap up Cohesion-tension hypothesis Transpiration provides pull Cohesion of H2O transmits pull from rootsshoots Root Pressure occurs when water diffusing in from the root cortex generates a positive pressure that pushes sap up. Root pressure does not have the force to push water to the tops of trees. Cohesion-tension hypothesis describes how transpiration provides the pull for the ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots.

Water is lost through transpiration from the leaves of the plants due to the lower water potential of the air. The cohesion of water due to the hydrogen bonding plus adhesion of the water to the plant cell walls enables the water to form a water column. Water is drawn up through the xylem as water evaporates from the leaves, each evaporating water molecule pulling on the one beneath it through the attraction of hydrogen bonds.

Guttation: exudation of water droplets seen in morning (not dew), caused by root pressure

Stomata regulate rate of transpiration Stomata – pores in epidermis of leaves/stems, allow gas exchange and transpiration Guard cells – open/close stoma by changing shape Take up K+  lower ψ  take up H2O  pore opens Lose K+  lose H2O  cells less bowed  pore closes -

Cells stimulated open by: light, loss of CO2 in leaf, circadian rhythms Stomata closure: drought, high temperature, wind

BIOFLIX: WATER TRANSPORT IN PLANTS http://www.pearsonhighered.com/mybiology/bioflix.html

Sugar Transport Translocation: transport of sugars into phloem by pressure flow Source  Sink Source = produce sugar (photosynthesis) Sink = consume/store sugar (fruit, roots) Via sieve-tube elements Active transport of sucrose Phloem transports organic products of photosynthesis from the leaves throughout the plant, a process called translocation. The mechanism for translocation is pressure flow. Sieve tubes, a specialized cell type in phloem tissue, always carry sugars from a sugar source to a sugar sink. A sugar source is an organ that is a net producer of sugar, such as leaves. A sugar sink is an organ that is a net consumer or storer of sugar, such as fruit, or roots during the summer.

Bulk flow in a sieve tube Sugar Transport Animation Loading of sugar (dots) into the sieve tube at the source reduces water potential inside the sieve-tube elements. This causes the tube to take up water by osmosis. This uptake of water generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water at the sink. In leaf-to-root translocation, xylem recycles water from sink to source. Sucrose is loaded into the sieve tubes at the sugar source. Proton pumps are used to create an electrochemical gradient that is utilized to load sucrose. This decreases water potential and causes the uptake of water, creating positive pressure. The pressure is relieved at the sugar sink by unloading of sucrose followed by the loss of water. In leaf-to-root translocation, xylem recycles the water back to the sugar source. Translocation via pressure flow is a second example of bulk flow. https://www.youtube.com/watch?v=MxwI63rQubU

Symplast is dynamic Plasmodesmata allows movement of RNA & proteins between cells Phloem can carry rapid, long-distance electrical signaling Nerve-like function Swift communication Changes in gene expression, respiration, photosynthesis This may lead to changes in gene transcription, respiration, photosynthesis, and other cellular functions in widely spaced organs. This is a nerve-like function, allowing for swift communication.