NOTES: CH 36

- Transport in

Plants

Recall that transport across the cell membrane of

plant cells occurs by:

-diffusion

-facilitated diffusion

-osmosis (diffusion of water)

-active transport (done by transport proteins)

PROTON PUMPS:

● pump out H+ ions,

producing a proton

gradient (more H+

outside cell) and a

membrane potential

(inside is negative

relative to outside)

• this “stored” energy is used to transport other molecules across the membrane:

-K+ ions pulled into cell

-sugar molecules are loaded into companion cells via COTRANSPORT

**These are all examples of CHEMIOSMOSIS

WATER POTENTIAL (): predicts the

direction water will flow

● combines solute concentration (osmotic

potential, s) with differences in pressure

(pressure potential, p)

WATER POTENTIAL ()

● as solute conc. increases, s decreases

● as pressure (hydrostatic) increases, p

increases

= s + p

● water flows from HIGH water potential to

LOW water potential!

● PLASMOLYSIS:

when a plant cell

loses water by

osmosis; protoplast

pulls away from cell

wall

● TURGOR

PRESSURE:

develops when a

plant cell gains water

by osmosis

Recall the 3 major parts of a plant cell:

1) cell wall

2) cytosol / cytoplasm

3) vacuole (surrounded by TONOPLAST)

● SYMPLAST: continuum of cytoplasmic

compartments of neighboring cells; connected

by PLASMODESMATA

● APOPLAST: continuum of adjacent cell walls

and intercellular spaces

Lateral / short-distance transport

can occur:

1) across cell membranes (trans-membrane

route)

2) via the SYMPLASTIC ROUTE (molecules

travel through the plasmodesmata)

3) via the APOPLASTIC ROUTE (molecules

don’t enter cells)

Vertical / long-distance transport occurs by:

1) BULK FLOW: movement due to pressure differences (substances move from regions of higher to lower pressure)

2) TRANSPIRATION: creates tension which“pulls” sap up through the xylem from the roots

3) HYDROSTATIC PRESSURE: builds up at one end of phloem vessels; forces sap to the other end of the tube

Absorption of Water & Minerals by Roots:

● Transport pathway:

*soil epidermis root cortex xylem

-minerals moving through symplastic route move

directly into vascular tissues

-minerals & water moving

through apoplastic route

are blocked at the

endodermis by the

CASPARIAN STRIP (a

ring of waxy substance,

SUBERIN) and must

enter an endodermal cell

**this ensures that all

minerals entering the

STELE pass through at

least one selectively

permeable membrane.

Transport of Xylem Sap

● xylem sap flows upward at 15 m per hour

● is it pushed from below or pulled from

above?

● root pressure builds up when transpiration is

slow (i.e. at night); this causes GUTTATION

● this only accounts for small amt. of xylem

transport

Transport of Xylem Sap

● most xylem sap moves

via the mechanism of:

TRANSPIRATION-

COHESION-TENSION

TRANSPIRATION-COHESION-TENSION:

● TRANSPIRATION: (loss of water from leaf

cells through stomata) creates negative

pressure

TRANSPIRATION-COHESION-TENSION:

● neg. pressure pulls

water from the xylem

● transpiration pull on

xylem sap is transmitted

from one water molecule

to another through

COHESION (due to H-

bonds between water

molecules)

THE CONTROL OF TRANSPIRATION

stomata provide openings in leaf tissue

for the transpiration of water (out of leaf)

and the diffusion of CO2 into the leaf for

photosynthesis

GUARD CELLS surrounding the stomata

regulate the requirements for

photosynthesis with the

need to conserve water

Adaptations to reduce water loss:

more stomata on bottom of leaves

waxy leaf cuticle on rest of leaf surface

Benefits of transpiration:

assists in mineral transfer from roots

shoots

evaporative cooling

Stomatal Opening / Closing: GUARD CELLS: cells that flank the

stomata and control stomatal diameter by

changing shape:

-when TURGID, guard cells

“buckle” and stomata

open

-when FLACCID, guard

cells sag and

stomata close

a change in turgor pressure in guard cells results from the reversible uptake of K+

*when K+ leaves cell,

s increases

H2O is lost

stomata close

*when K+ enters cell,

s decreases H2O

is taken up

stomata open

Studies show that K+ fluxes across guard cell

membrane are likely coupled to membrane

potentials created by PROTON PUMPS

Stomata open at dawn in response to:

1) Light: induces K+ uptake; activates a

blue-light receptor which drives

photosynthesis ATP

2) Decrease of CO2 in air spaces due to

photosynthesis

3) internal clock in guard cells

(CIRCADIAN RHYTHM = 24 hour

cycle)

Guard cells may close during daytime if:

1) there is a water deficiency

flaccid guard cells

2) production of abscisic acid (hormone); in

response to water deficiency; signals guard

cells to close

3) high temperature increases CO2 in air

spaces due to increased respiration

Xerophytes have special adaptations:

small, thick leaves

thick cuticle

stomata are in depressions

on underside of leaves

some shed leaves

during driest time of year

cacti store water in stems

during wet season

Translocation of Phloem Sap

TRANSLOCATION = transport of products

of photosynthesis by phloem to rest of plant

PHLOEM SAP = sucrose, minerals, amino

acids, hormones

phloem sap moves through sieve tubes from a SUGAR SOURCE (production area) to a SUGAR SINK (use or storage area)

Sugars move into sieve tubes via symplastic and/or apoplastic routes

Sucrose is “loaded” into cells at the sourceend by active transport (COTRANSPORT)

Sucrose is “unloaded” at the sink end of sieve tubes

Pressure Flow / Bulk Flow of Phloem Sap

pressure builds up at source end (phloem

loading s decreases water enters

tubes hydrostatic pressure)

pressure is released at sink end (phloem is

unloaded s outside tube decreases

water leaves release of hydrostatic

pressure)