Transcript Chapter 42 - Plant Nutrient Transport

Plant Transport
Moving water, minerals, and sugars
Chapter #42 – Plant Anatomy
& Nutrient Transport
Quiz this Thursday!
42.1 - How Are Plant Bodies Organized; How Do They Grow? p. 860
42.2 - The Tissues and Cell Types of Plants? p. 862
42.3 - The Structures, Functions of Leaves, Roots, & Stems? p. 865
42.4 - How Do Plants Acquire Mineral Nutrients? p. 873
42.5 - How Do Plants Move Water Upward from Roots to Leaves? p. 876
Chapters on Reproduction and Hormones (plant responses)
Vascular Tissue
Source and Sink
• Source: where the
sugar starts its
journey (either
where it is produced
or stored).
• Sink: where sugar
ends up (either
where it is needed
or will be stored).
Xylem
• Xylem tissue
transports water
from roots to
leaves.
• Xylem vessels are
dead at maturity.
Phloem
• Phloem tissue
transports sap
(water and sugar)
from “source” to
“sink.”
• Phloem vessels are
live at maturity, but
need companion
cells.
Transpiration
Water transport in 3 parts
• Transpiration (or evapo-transpiration) is
the transport of water and minerals from
roots to leaves. It involves three basic
steps:
• Absorption at the roots.
• Capillary action in the xylem vessels.
• Evaporation at the leaf.
Part 1: Roots
• Roots absorb water and minerals in a 4step process:
• Active transport of minerals into root
hairs.
• Diffusion to the pericycle.
• Active transport into the vascular cylinder.
• Diffusion into the xylem.
Mineral and water uptake
Casparian Strip
• The Casparian strip
controls water
movement into the
vascular cylinder of
the root.
• Water cannot move
between cells. It
must move through
the cells by
osmosis.
Thinking question:
• What would happen in a root that had no
Casparian strip? Why would this be a
problem?
Microbial helpers
• Microbes in the
soil help plants
absorb nutrients:
• Mycorrhizal
fungi help
absorb minerals
by extending the
surface area
over which
minerals are
absorbed.
Microbial helpers
• Nitrogen-fixing
bacteria in root
nodules help
plants acquire
nitrogen.
• N-fixing bacteria
are associated
mostly with
legumes and
alder trees.
Thinking Question
• Suppose a gardener notices that several
of her favorite flowering plants are
infected with a fungal disease. She
decides to spray not only the plants, but
the soil all over her garden to prevent the
disease. What negative effect could this
have on her plants?
Step 2: Capillary action
• Cohesion: polar
water molecules
tend to stick
together with
hydrogen bonds.
• Adhesion: water
molecules tend to
stick to polar
surfaces.
Capillary action
• Cohesion and
adhesion cause
water to “crawl” up
narrow tubes. The
narrower the tube
the higher the same
mass of water can
climb.
• Maximum height:
32 feet.
Cohesion-tension theory
• Cohesion between water molecules creates a “water
chain” effect.
• As molecules are removed from the column by
evaporation in the leaf, more are drawn up.
Thinking question
• If the forces of cohesion-tension theory
move water up a stem, what happens to
water pressure at the roots? Will that
affect water moving into the roots?
Back to the roots...
• Pressure differences created by transpiration draws
water out of the roots and up the stems.
• This creates lower water pressure in the roots, which
draws in more water.
Part 3: Evaporation
• Evaporation at the surface of the leaf keeps the water
column moving.
• This is the strongest force involved in transpiration.
Stomata control
• Guard cells around
the stomata are
sensitive to light,
CO2, and water
loss.
• Cells expand in
response to light
and low CO2 levels,
and collapse in
response to water
loss.
Stomata
• When stomata are open, evaporation
draws water out of the leaf. Gas
exchange can also occur to keep
photosynthesis and respiration running.
• When stomata are closed, evaporation
cannot occur, nor can gas exchange.
Photosynthesis and transpiration slow
down.
Thinking question
• Suppose you were testing transpiration
in trees on a warm, humid summer day,
and again on a windy day when the air is
dry. How would transpiration on these
two days differ?
Sugar Transport
The trouble with phloem
• Phloem tissue is
living tissue, unlike
xylem. When
scientists studying
how it works cut
into it, the plants
responded by
plugging up the
phloem.
Aphid helpers
• But aphids can
pierce phloem
tissue and suck out
sap without any
problem.
• Scientists used
aphids to study the
flow of sap in
phloem.
Sap
• Sap consists of sugar dissolved in water
at high concentrations: usually between
10% and 25%.
• Since this is highly concentrated, plants
have to use active transport to work
against a diffusion gradient as part of the
sap-moving process.
Pressure-flow theory
• The pressure-flow theory explains how
sap moves in a plant from source to sink:
• Sugars begin at a source and are
pumped into phloem tube cells.
• Osmosis moves water into the cells
and raises pressure.
• Pressure moves the sap.
Pressure flow 1
• The leaf is a source
of sugar, since it
makes sugar by
photosynthesis.
Glucose and
fructose made by
photosynthesis are
linked to make
sucrose.
Pressure-flow 2
• Active transport is
used to load
sucrose into phloem
tubes against a
diffusion gradient.
Pressure-flow 3
• The high
concentration of
sucrose in the sieve
tube cells of the
phloem causes
water to move in by
osmosis, which
raises pressure and
causes the sap to
move.
Pressure-flow 4
• A developing fruit is
one example of a
sink. Sucrose may
be actively
transported out of
phloem into the fruit
cells. In a root,
sucrose is
converted into
starch, which keeps
sugar moving in by
diffusion.
Pressure-flow 5
• As the sugar
concentration drops
in the sieve tube
cells, osmosis
moves water out of
the tube.
Pressure-flow 6
• As water moves out
by osmosis, the
pressure in the
sieve tube cells
drops. The pressure
difference along the
column of sieve
tube cells keeps the
sap flowing.
Pressure-flow: Review
Thinking question
• Sugars are often stored in roots,
sometimes as starch. Why can a root be
both a “source” and a “sink” when it
comes to sugar transport?