Transcript Transport in Plants

Transport in
Vascular Plants
Transport in Plants Overview

movement of
materials from one
part of a plant to
another

involves 2 specialized
tissues:
1.
2.
Xylem
Phloem
Xylem Tissue
Structure:
 made of thick cell
walls
 most cells are dead
 cells are stacked up
on top of another to
form a hollow tube
through the plant
Function:
 provides structural
support to the plant
 carries water and
minerals from roots to
leaves
Phloem Tissue
Structure
 cells are living
 cell walls are porous,
allowing exchange of
materials with
neighbouring cells
Function
 carries nutrients
(sugars and
hormones) throughout
the plant
Water Transport in Xylem (1 of 2)
http://www.youtube.com/watch?v=w6f2BiFiXiM&feature=related



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Roots take in water through the
root hairs & epidermal cells by
osmosis
Water flows through the cell
walls, into the intercellular
spaces within the root, and
enter the xylem
Water is then transported in
the xylem tissue up through
the root into the stem
Within the stem, water and
minerals move by diffusion
(and, to a lesser extent, active
transport) into the other tissues
of the plant
Water Transport in Xylem (2 of 2)


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As the xylem tissue carrying
water and other minerals enters
the leaf, the conducting vessels
branch and rebranch into the
numerous veins visible in the leaf
From the end of each vein, water
and minerals can diffuse into the
cells of the leaf
About 99% of the water that
reaches the leaf is lost through
transpiration

water vaporizes from the leaf
through the stomata

this draws (pulls) water up the
rest of the plant
How does a plant move water up from
its roots to its leaves?

3 main theories that have been
developed to account for the movement
of water in plants

1.
2.
3.
these mechanisms are not mutually exclusive; any
or all of these may be at work in a single plant at
any one time
Root pressure: because of a concentration
gradient through osmosis, water enters the
root
Capillary action: water is drawn into the
tiny diameter of the root
Cohesion-tension: water likes to stick
together and travels up the xylem
1. Root Pressure


Plant roots build
up pressure that
forces water
upward
Occurs normally
in shorter plants

Pressure built in
roots of tall trees
won’t really push
the water that
far up
2. Capillary Action


takes place because of
the adhesion of
water molecules and
walls of xylem turn
upwards
the finer the capillary
or vessel, the higher
water will climb up
3. Cohesion-tension




Also known as
“transpiration pull”
http://www.youtube.com/watch?v=At1BJJDcXhk
&feature=related
transpiration is the
evaporation of water
vapour from the leaves
through the stomata
depends on the cohesive
forces between water
molecules and adhesive
forces between water and
the walls of the vessel
Mineral Transport
By testing the concentration of minerals in
the roots, scientists know that the amount
exceeds that of diffusion
 Thus roots are actively pumping minerals
and ions from the surroundings
 These minerals are dissolved in the water
in the xylem or roots, which makes it
more concentrated


This increases the uptake of water into the
plant
Translocation:
Transport in Phloem


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Scientists still unsure
of how exactly
transport happens in
phloem
Phloem transport is bidirectional
Most accepted theory:
mass-flow theory


Moves from a sugar
source a place
where sugar is
produced by
photosynthesis or by
the breakdown of
sugars) to a sugar
sink (an organ which
consumes or stores
sugar)
“borrows” water from
xylem to help
transport sugars
Transport in Xylem vs. Phloem