Ch. 36 Transport in Plants
Occurs at three levels:
Uptake by cell
Cell to cell
Long distance from root to leaves and
Passive transport-movement across
membranes slow without:
Transport proteins-in cell membrane.
May bind to molecule and transport.(carrier protein)
May create passageway (channel protein)
Moving solutes against conc. Gradient.
Uses carrier proteins and energy (ATP)
Ex. Proton pump-uses energy released to
move Hydrogen ions to establish high
conc. (remember electron
Water moves by osmosis
Moves from hypotonic to hypertonic
In plants, cell wall has an effect as well
as solute conc. Combined, they equal
Water potential basics
Movement from high to low.
Called water potential because moving
water can do work-potential energy.
Water potential of pure water open in
Solutes lower water potential.
Increase in physical pressure increases
Water potential cont.
As solute increases, water potential
decreases. As pressure increases, so
does water potential.
Explains water movement across
See p.751 and 752
Transport across membranes
Water is polar; inside cell membrane is
Selective channels called aquaporins aid in
passive transport of water across membrane.
Cell walls of plant cells allow continuous
Absorption by root
Root hairs-grow from epidermal cells near tip;
Soil sticks to hairs; water and minerals stick
Mycorrhizae-fungus fibers; symbiotic
relationship with root hairs.
Soil solution moves into epidermal walls
(hydrophilic) and moves into root cortex.
Connections between cytoplasm area of plant cells
allows continuous flow. (symplast)
Movement of materials from outer to inner
Transmembrane movement-across plasma
membrane after plasma membrane.
Apoplastic-movement through cell walls.
Symplastic-movement through cytoplasm
connected by plasmodesmata.
Root structure and water
Water moves through cortex
Reaches inner layer of cortex known as
endodermis. Protects inner area of root
known as stele.
Stele contains xylem and phloem
Endodermal cells have waxy layer called
Water moving through symplast goes
Water moving apoplastically has barrier.
Water must move across endodermis
plasma membrane and then in
Water moves into tracheids and vessel
element cells of xylem.
Loss of water vapor through leaves and
aerial parts of plants.
Water moves out through pores called
Stomata are connected to air spaces in
spongy mesophyll of leaf tissue.
Water found in air spaces because
space in contact with moist cells.
Xylem contains minerals as well as water.
Transpiration low at night; minerals still
pumped in causing decrease in water
Water moves in creating a pressure or push.
Root pressure causes guttation-water
droplets on leaf margins.
Root pressure NOT major force moving water
Water molecules exhibit hydrogen bonding.
Hydrogen bonding results in cohesion and
Transpiration causes negative pressure in
xylem; water sticks together.
Air dry--- water exits stomata—water from
cells moves into air space as replacement---
pulls on water in xylem.
Water sticks to hydrophilic cell walls of
tracheids and vessel elements.
Water chain must remain continuous.
Bulk flow of water due to solar energy;
no energy of plant used.
Stomata open to allow CO2 in. Water
Oxygen from photosynthesis leaves
Stoma surrounded by two guard cells.
Guard cells change shape to control size
Guard cell function
Guard cells take in or lose water due to
As this ion conc. increases, water
moves in; guard cells become
turgid;cells are thicker along outer edge
so they bow. (p.760)
Stomata typically open during day and
closed at night.
Guard cells cont.
K+ can be stimulated by light; depletion
of CO2 can cause stoma to open;
internal clock-circadian rhythms-can
High temp. can cause closing of
stomata along with high transpiration.
Abscisic acid made due to low water
can cause guard cells to shrink.
food in plants
Occurs in phloem.
Phloem sap may move up or down.
Movement of food from “source” (high
conc.) to “sink” (low conc.)
Fruit and areas of plant growth are
Cells of phloem tissue called sieve
Phloem also has companion cells-have
nucleus and organelles-help control what
occurs in sieve tube cells.
Food moves from source to sink due to
High solute conc. causes water to move into
sieve tube; creates pressure.
As solutes leave sink, water follows which
causes loss of pressure.
Sugar movement into sieve tubes and
companion cells accomplished by active
transport; mechanism too fast by simple
Sucrose too big to cross membranes on own.
In spring, roots of trees and other perennials
are source and stem/leaf cells coming out of
dormancy are sink area. Sap moves up.