Transport

Processes

Transport Processes1. Transport is a fundamental aspect of life.

2. Diffusion is the movement of solutes from areas of high solute concentration to areas of low solute concentration.

3. No energy required (passive)

4. If solutes only moved by diffusion……….

Need coordinated transport processes in order to move solutes & water faster and farther.

Active transport1. Solutes move against their gradient (i.e. to a

higher solute concentration)2. Requires energy: driven by breaking ATP’s high–

energy phosphate bonds.3. Facilitated by membrane proteins called proton

pumps – trans-membrane proteins. ATP gives energy to pump H+ out of a cell.A. Creates a membrane potential (+ outside, - inside) that

drives positive ions into the cell (like K+).B. The diffusion of H+ back down the gradient (into cell)

can also co-transport anions & neutral substances against their concentration gradient.

Fig 36.4

Transport Processes

1. Occur at virtually every level of biological organization.

2. Enzymes transport electrons, protons, acetyl groups.

3. Membranes transport material across themselves.4. Cells transport material to and from other cells,

and within themselves.5. Whole organisms transport water, etc from one

organ to another

Transport in Plants

1. Plants adapted to the division of resources in the land environment (soil & air) by the differentiation of the plant body into roots and shoots.

2. But this created a new dilemma, the need to transport materials between roots and shoots.

3. Sometimes up to 100m away, in all kinds of environmental conditions.

4. What’s a poor plant to do?

Three levels of transport in plants:

1. Cellular – uptake of H2O and solutes by individual cells.

2. Short-distance - between cells

3. Long-distance – throughout whole plant (xylem & phloem)

Transport at the Cellular Level

1. Diffusion

2. Osmosis – diffusion of H20 from areas of low solute concentration into areas of high solute concentration (i.e. water always acts to dilute)

3. Facilitated Diffusion - same as diffusion but with through trans-membrane proteins

4. Active Transport

Review of cellular transport:

An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.

a) sucrose

b) glucose

c) fructose

Which solute(s) will exhibit a net diffusion into the cell?

a) sucrose

b) glucose

c) fructose

Which solute(s) will exhibit a net diffusion out of the cell?

Which solution is hypertonic to the other?a) the cell contents

b) the environment

In which direction will there be a net osmotic movement of water?a) out of the cell

b) into the cell

c) neither

After the cell is placed in the beaker, which of the following changes will occur?a) The artificial cell will become more flaccid.

b) The artificial cell will become more turgid.

Directional movement of water is driven by: Water Potential

1. Water potential refers to the free energy of water, its capacity to do work.

2. By definition pure free water has a water potential of zero.

3. Water potential can be increased byheatingpressureelevation

4. Water potential can be decreased by ?

Y = Ys + Yp + Ym

Yš = osmotic potentialYp = pressure potentialYm= matric potential

Components of Y

Ys = osmotic potential

1. Osmotic potential is a measure of the effect that solutes have on water potential.

2. Pure water has a osmotic potential of zero.

3. Adding solutes decreases the osmotic potential because water interacts with solutes. (more solutes = more negative)

4. Thus Ys is always negative (if solute is present)

Yp = pressure potential

1. A measure of the effect that pressure has on water potential.

2. Pressure can be positive (when something is compressed). Pushing

3. Pressure can be negative (when something is stretched or pulled). Called tension.

4. Water can handle large amounts of tension because of cohesion – the tendency of water to stick to itself (H bonding)

Figure 3.1

Ym= matric potential

1. Matric potential is a measure of water’s adhesion to non-dissolved but hydrophilic structures such as cell walls, membranes, soil particles etc.

2. Adhesion can only decrease water’s free energy.

3. So matric potential is always negative

1. Water moves from regions of high Ψ (less negative) to regions of lower Ψ (more negative).

Why would water move to where it is less free?

– Water acts to dilute, hydrate, decrease tension– i.e. water acts to stabilize water potentials– If two Ψ’s are equal, no net movement of

water

Examples

• Ψ = +46MPa Ψ = -22MPa

• Ψ = -15MPa Ψ = -300MPa

Fig 36.5

Cells and water movement1. A flaccid cell (Yp = 0). No pressure is being

exerted against the inside of the cell wall. Cell is not firm. Why? Solute concentration within the cell is

lower than surroundings. Water leaves the cell.2. A turgid cell (Yp = +) is filled with water, exerting

pressure against its cell walls. Cell is firm.Why? Solute concentration within the cell is

higher than surroundings. Water moves into the cell.

Cells and water movement

1. Hypotonic solution – low solute concentration

2. Hypertonic solution – high solute concentration

3. A flaccid cell placed in a hypotonic solution will: ?

4. A turgid cell placed in a hypertonic solution will:?

This causes the cell membrane to shrink away from the cell wall (i.e. plasmolyze)

Fig 36.6

Short Distance (Lateral) Transport – between cells

Three routes for lateral transport:1. Transmembrane – water & solutes move across

the plasma membranes and cell walls of adjacent cells

2. Symplast – movement through a continuum of cytoplasm connected by the plasmodesmata of cell walls.

3. Apoplast – extracellular pathway; movement through the continuous matrix of cell walls

Fig 36.8

Examples of Short Distance Transport

1. Guard cells

2. Motor cells

3. Transfer cells

4. Absorption of water & minerals by roots

1. control stomatal diameter by changing shape.

Guard cells

A. Take up water, become turgid, buckle => stomata are open

B. Lose water, become flaccid, stomata close

Guard cellsC. Opening Mechanism:

a. K+ is pumped into GC by active transport. Proton pump creates membrane potential that drives K+ in.

b. Thus Ψ inside cell is lower than outside cell.

c. Water diffuses into the guard cell.

d. Sunlight, circadian rhythms, & low CO2

concentration in leaf air spaces stimulate the proton pumps & thus stomatal opening

Guard cells

D. Closing mechanism:a. Proton pumps no longer active (darkness)

b. K+ is lost from the GC, creating lower water potential outside cell.

c. Water flows out of GC and cells become flaccid

d. Stomatal closure during the day stimulated by water stress – not enough water to keep GCs turgid

Fig 36.15

Motor Cells

1. Examples: prayer plant Oxalis, Venus’ flytrap

2. Leaves of these plants can flex & fold in response to stimuli

3. Motor cells are the “joints” where this flexing occurs.

4. Accumulate or expel potassium to adjust their Ψ & thus turgidity.

5. Oxalis – leaves fold in sunlight to minimize transpiration; open in shade

6. Transpiration = loss of water vapor from the stomata

http://www.uccs.edu/~ppbotany/Colo_family/Oxalid/oxalis_stricta_P.htm

Transfer Cells

1. Cell walls have many finger–like projections on the inner surface.

2. The plasma membrane is pressed firmly against these convolutions, creating an increase in surface area

3. Greater surface area means more molecular pumps & thus high – volume solute transport

4. Found in areas of rapid, high volume transport: salt-excreting glands, sugar loading into phloem

Absorption of water & minerals by roots1. Soil particles coated with water, minerals; adhere

to hydrophyllic epidermal cells of root hair2. Soil solution moves freely through epidermal cells

& cortex via symplast and apoplast pathways3. Endodermis – selective barrier to soil solution

between cortex & stele. Sealed together by the waxy Casparian strip – forces soil solution in apoplast to pass through the selectively permeable membrane of the endodermis.

4. Once through the endodermis, soil solution freely enters the xylem

Fig 36.9

Long-Distance Transport

Mechanisms of Long Distance Transport

1. Xylem: Transpiration (evaporation from leaves) creates a tension which pulls sap up from the roots, in direction of lower Ψ.

2. Phloem: Hydrostatic pressure at one end of the sieve tube forces sap to the other end of the tube (= bulk flow).

Transport of xylem sap

1. Xylem sap both pushed & pulled up the stem

2. Pushed by root pressureA. Stele has high concentration of minerals,

decreasing Ψ. Water flows in, creating pushing pressure

B. Guttation – exudation of water droplets by leaves during the night when transpiration is low. Caused by root pressure

Pulling xylem sap

1. Transpiration – cohesion – tension mechanism

2. Transpirational pull: Ψ of air typically << than Ψ of leaf, thus evaporation.

3. Water remaining in leaf is tightly adhered to cell walls in the mesophyll

4. This adhesion & surface tension of the water creates negative pressure – the pulling force

Figure 36.12

Ascent of xylem sap against gravity1. Aided by:

A. Cohesiveness of water – allows water to be pulled up in a continuous column without breaking

B. Adhesion of water to hydrophyllic cell walls of the xylem, creates tension (negative pressure/ pull)

C. Diameters of tracheids & vessel elements are small, so lots of surface area for adhesion

2. Since movement is by bulk flow (i.e. no membranes to pass through), Ψs is not involved in the overall process

Fig 36.13

Speed of Xylem Sap Translocation

•Examples Max. Speed (cm/hr) •Conifers 120•Angiosperm Trees 600 - 4400•Herbs 6,000•Vines 15,000 (0.6 mi/hr)

Why is xylem transport in trees much slower than in herbs?

Forces that act against transpirational pull:1. Gravity

2. Hydraulic resistance

Control of Transpiration

1. Guard cells! – balance two contrasting needs of the plant:– Conserve water– CO2 for photosynthesis

2. Water use efficiency (WUE) = g H2O lost / g CO2 assimilated by photosynthesis

average = 600/1

3. Desert plants have adaptations to increase their WUE:A. Small thick leaves (less SA for water loss)

B. Thick cuticle

C. Stomata only on bottom of leaves

D. High-volume water storage (cacti)

E. Crassulacean Acid Metabolism (CAM) – plants take in CO2 only at night, so that stomata only have to be open at night.

Wilting1. Transpirational pull is greater than the

delivery of water by the xylem. Cells lose turgor pressure & stomata close

Trans-stomatal and Transcuticular Transpiration (gH20/dm2/hr)

Trans-stomatal

Trans-cuticular

Herbs:Coronilla varia 1810 190Stachys recta 1620 180Oxytropis pilosa 1600 100Trees:Pinus sylvestris (pine) 527 13Picea abies (spruce) 465 15Fagus sylvaticus (birch) 330 90

Translocation of Phloem Sap

1. Phloem sap = sucrose, amino acids, hormones

2. Sieve tubes carry sap from sugar source (e.g. leaves) to sugar sink (e.g. growing roots, shoot tips, stems, flowers, fruits)

3. Thus not unidirectionale.g. tubers can be source in spring and sink in fall

Phloem loading

1. Sap moves into sieve tubes via companion cells by symplast and apoplast pathways

2. Loading is by active transport only – why?Sap in sieve tubes is highly concentrated with

solutes

Thus phloem unloading is passive (source to sink)

Fig 36.17

Mechanism of phloem translocation1. Pressure-flow hypothesis:

A. Phloem loading creates high sugar concentration at source, decreasing Ψ.

B. Thus water flows into sieve tubes, creating hydrostatic pressure (pushing pressure: positive).

C. Less pressure at sink end, where sugar is leaving sieve tube for consumption

D. Thus movement from source to sink

Fig 36.18

Speed of Phloem Sap Translocation•Examples Maximum Speed (cm/hr)•conifer stem 13-48•angiosperm tree 48•vine 72•grass 168•sunflower 240•corn 660

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