Lecture
#9: Transport in Plants (Fig. 36.1, Fig. 35.2, Fig. 35.19)
I. ABSORPTION OF WATER AND
MINERALS BY ROOTS
1. Root anatomy: apical meristem, stele,
cortex, epidermis, root hairs. (Fig. 35.14)
2. Proton pump in epidermal cells
facilitates intake of many minerals. (Fig. 36.2)
a. the pump actively transports (ATP) H+
from inside to outside cell.
b. this creates an ion gradient that translates
into a membrane potential.
c. the high concn of protons on outside helps
drive selective cations (K+) inside.
d. thru a process called cotransport, the extracellular H+ can couple with and drive anions (A-) against their electrochemical gradient toward the inside of cell.
II. ENTRY OF WATER INTO THE ROOT XYLEM:
1. root anatomy: epidermis, cortex, endodermis,
Casparian strip, xylem. (Fig.
36.6 & 36.7)
2. water and minerals can get into the stele
only by passing thru endothelial cells.
3. Casparian strip is a waxy belt of suberin
that prevents extracellular flow.
4. transit of water (and minerals) across cortex
can be by two routes:
a. symplast pathway is through cells via plasmodesmata
(openings).
b. apoplast pathway is extracellular via intercellular spaces. (and cell wall)
II. TRANSPIRATION AND THE ASCENT OF XYLEM SAP UP
A PLANT (Fig.
36.11)
1. Transpiration is the evaporative loss of
water from a plant thru its leaves. (Fig. 36.10)
2. As the water evaporates, it is replaced by
water movement up the xylem.
a. this is due in part to the cohesion of
water due to hydrogen bonds.
b. this is due in part to the adhesion of
water to the hydrophylic walls of xylem cells.
3. Functions of transpiration:
a. it assists in the transfer of minerals and
other substances from roots up.
b. it maintains a moist surface area in the mesophyll
to facilitate gas exchange.
c. it allows evaporative cooling of the plant by as much as 10-20 C.
IV. STOMATAL FUNCTION
1. Stomatal anatomy: guard cells, cellulose
microfibrils, subsidiary cells. (Fig. 36.13)
2. Stomata open primarily when K+
increases guard cell turgor pressure. (Turgor pressure is the pressure
against a cell wall after an influx of H20.)
3. Factors that stimulate stomata to open at
dawn:
a. light (via a blue-light receptor) causes
guard cells to accumulate K+. (this is probably via a proton
pump-ATP mechanism.)
b. photosynthesis stimulates glucose
production and ¯ CO2 in
guard cells. (this reduction in CO2 somehow increases turgor
pressure in guard cells.)
c. an internal circadian rhythm causes
stomata to open even in a dark closet.
4. Factors that stimulate stomata to close:
a. water deficiency causes guard cells to
lose turgor pressure.
b. the hormone abscisic acid is produced
by mesophyll cells during water deficiency.
c. high temperatures increase cellular
respiration and increase mesophyll CO2.V. TRANSLOCATION OF SUGAR AND OTHER PRODUCTS IN
THE PHLOEM
1. The sap consists primarily of sucrose, but
also minerals, amino acids, and hormones. (Fig. 36.16)
2. Sugar from photosynthesis translocates to
growing roots (and tubers), shoot tips, and fruit. (Fig. 36.17)
3. Translocation occurs in both directions in
the sieve tubes of the aqueous phloem sap.
4. In early spring, translocation is from roots
to budding stems and flowers.