CHAPTER 36

Transport in Plants

I) Transport at the Cellular Level

A) Selective Permeability1) Diffusion: movement from [high] to [low]2) Passive transport: movement without the use of ATP

3) Transport proteins: channels that increase the rate at which solutes can diffuse across the membrane

4) Active transport: use of ATP to pump solutes against the electrochemical gradient.

5) Gated channelsa) open or close based on environmental stimuli

B) Central Role of Proton Pumps

1) These are transport proteins that pumps H+ ions out of the cella) creates a H+ gradientb) creates a membrane potential

b1) now more + outside of the cell

b2) – charge in the cell helps K+ diffuse into the cell.

2) Cotransport: diffusion of H back into the cell through t-port protein helps bring others in against their gradienta) NO3-

b)Sucrose

C) Water transport

1) Water potential(ψ)a) combined effects of solute concentration and pressure from presence of cell wall

a1) water moves from higher potential to lower potential

b) cell wall creates a pressure that lets cell have solutes in it but not keep taking in water.

c) flaccid: low pressure, cell is not firm

d) plasmolyze: water leaves cell, cell shrinks

e) turgor pressure: water goes in cell and pushes against cell wall

f) turgid: cell is full of water and is firm

D) Aquaporins

1) transport proteins that allow the passive movement of water across the cell membranea) affect the rate at which water can flow

E) Role of Vacuole and other compartments

1) Tonoplast: membrane of vacuole that controls movement of materials between vacuole(cell sap) and cytoplasma) proton pumps draw H+ into vacuole to help keep H+ low in the cytosol

2) Symplast

a) connection of cells by plasmodesmata

a1) creates one continuous cytoplasm

3) apoplast

a) connection of cell walls that create channels between cells

II) Absorption by Roots

A)surface area1) root hairs: extensions of membrane of epidermisa) water diffuses in, travels along symplast or apoplast to stele

b) minerals actively transported in, helping to keep water concentration low in root

2) mycorrhizae

b) symbiosis between roots and hyphae (the “body” of a fungi)b1) root gets nutrients, water, and other mineralsb2) fungus gets carbs

III) Transport in Xylem

1) water moves in and out of cells by osmosis, but requires pressure (both positive and negative)to move up a plant = bulk flow

A) Pushing xylem sap

1) Root pressure:a) endodermis and casparian strip keeps minerals in steleb) causes high water potential to cause water to diffuse into stelec) this pushes water up plant

d) guttation

d1) droplets formed when water is forced out of leaves at night from root pressure – only works over short distances

B) Pulling Xylem Sap: the transpiration-cohesion-tension model

1) Transpirational pulla) water evaporates from stomateb) this creates an area of lower water potential, allowing water to be pulled out of the xylem

2) cohesion and adhesiona) water sticks to sides of xylem, keeping it from sliding down xylemb) cohesion helps pull water up length of xylem

IV) Control of Transpiration

A) The photosynthesis-transpiration compromise1) Need CO2 in through stomata, but water is also lost.

transpiration video

2) transpiration to photosynthesis ratioa) C3 plants have a 600g of water to 1g of CO2 ratiob) C4 plants have a 300g of water to 1g of CO2 ration

3) transpiration also brings minerals to leaves and cools them off

A) How stomata open and close

1) when guard cells are more turgid, the stomate is more open. This happens when they actively take in K+.

a) this causes a lower water potential inside, and water then diffuses in.

2) Stomates open in morning when blue light receptors in the membrane are activated and start the H+ pump.

3) Stomates open from lack of CO2 because of increase in photosynthesis

4) 3rd cue for opening in the morning is based on circadian rhythms.(intervals of about 24hrs)

5) closing during the day caused by stressa) loss of turgorb) abscisic acid (hormone) is released by mesophyll cells when there is a lack of water

B) Xerophyte adaptations

1) plants living in arid climates have to severely limit loss of water by transpirationa) small thick leavesb) thick cuticlec) stomata on bottom of leafd) store water in stem(cacti)

e) Succulents: do photosynthesis by crassulacean acid metabolism(CAM)e1) incorporate CO2 into organic acids at night. During day stomates close and organic acids are broken down to put CO2 into C3 pathway for photosynthesis

V) Translocation of Phloem Sap

A) Move sap from sugar source to sugar sink1) Source: sugar is being made by photosynthesis or starch breakdown.2) Sink: part of plant using or storing sugar

B) Phloem loading and unloading

1) Depending on plant, will move along apoplast and/or the symplast.

2) transfer cells: modified companion cells with ingrowths in walls to increase surface area for transfer between symplast and apoplast

C) Pressure flow moves phloem sap

Steps:1) high [sugar] at source causes water to come in by osmosis2) transport of sugar out of sieve tube at sink causes water to diffuse out of sieve tube.3) this creates a difference in water potential, driving sap toward sink

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