Why are plants important?
Photosynthesis
Cellular Respiration
9.1 Transport in the xylem of plants
The Leaf has evolved to:
Basic leaf structure
Water, in the form of a gas, is
lost through the stomata.
Major structures of a generalized leaf
Leaf structure
•
•
•
•
•
Cuticle – protects against water loss
Epidermis – protection if no cuticle
Xylem – transports water
Phloem – carries products of photosynthesis
Palisade mesophyll – densely packed, large # of
chloroplasts
• Spongy mesophyll – loosely packed, provides gas
exchange surfaces
• Stomata- bottom surface, allow 02 & CO2
exchange
• Guard cells – open & close stomata
The dermal tissue system covers and
protects the plants surfaces
The epidermis covers the primary plant body
The periderm replaces the epidermis when
roots and stems increase in diameter and
become woody
Importance of tissue functions
• Palisade mesophyll – upper portion where
light will hit
• Veins – distributed throughout the leaf for
transport
• Spongy mesophyll – superior to stomata to
allow for continuous gas exchange
• Stomata – on underside so temperature is
lower & minimized water loss
Understandings
• Transpiration is the inevitable consequence of
gas exchange in the leaf.
Plant Water & Mineral Movement
• Transpired water must be replaced
• Steady stream of water provides minerals as
well as water
• Water loss cools sun-drenched leaves and stems
• Xylem supports the plant as well as conducting
water
The root system usually grows below ground
Plant organs are
composed of
three tissue
systems:
Vascular Tissue
Dermal Tissue
Ground Tissue
The vascular tissue distributes water and
solutes through the plant body
Understandings:
• The cohesive property of water & the structure
of the xylem vessels allow transport under
tension.
• The adhesive property of water & evaporation
generate tension forces in leaf cell walls.
Understandings:
• The uptake of minerals in the roots causes
absorption of water by osmosis.
Composed of
2 cell types
tapered
Pits allow
water to move
laterally
Attached end to
end to form
columns
Connect to one
another to form
columns
Virtual lab time
Stomata & Guard Cells
• Can only be closed a short time – why?
• Open & close because of turgor pressure
Why do guard cells gain & lose water?
• Due to transport of potassium ions
• Light from blue region of light spectrum
triggers the ATP powered proton pumps
• Potassium enters the guard cells
• Higher solute = inward movement of water
Plant hormone
• Why do potassium ions exit the cell?
• Abscisic acid causes exit of ions = closure of
stomata
• Abscisic acid is produced in the roots during
water deficiency
Other causes of closure
Carbon dioxide levels
Circadian rhythms
Understandings:
• Plants transport water from the roots to the
leaves to replace losses from transpiration.
The cohesion-tension theory of plant
fluid movement
Roots & fluid movement in plants
Cell Membrane & Transport
Root Anatomy
• Three Regions
– Meristematic – new
growth (M phase)
– Elongation –
enlarging (G1 phase)
– Maturation – cells
are a functional part
of the plant
Root Hairs & branching
• Root hairs & branching
greatly increase the
surface area which
water and dissolved
minerals can be
absorbed.
An adult rye plant
was found with 14
million branches
totaling 630
kilometers (380
miles)
Uptake of ions by the roots – HOW?
• Root interception
– Root grows & intercepts ions
– Example: Ca
• Simple diffusion
– Ions move down their concentration gradient
– No energy expense by plant
– Example: Zn, K, Fe, P
• Mass flow
– Bulk flow of water into the root “carries” ions to root
– Delivers N, Ca, Mg, S
• Active transport
– Ions move against their concentration gradient
– Requires a specific protein “pump” in the cell membrane
– Energy expense by plant
Hyphae?????
Fungal symbiotic
relationship =
greater surface
area (mutalistic)
Lab time
Microscope
plant lab
Adaptations for H2O conservation
Light
Humidity
Wind
Speeds up
transpiration by
warming the leaf
& opening stoma
Decreasing
humidity increases
transpiration due
to difference in
water
concentration
Increases the rate
of transpiration
because humidk
air near the
stomata is carried
away
Adaptations for H2O conservation
Temperature
Soil water
Carbon dioxide
Increasing
temperature causes
greater transpiration
because more water
evaporates
If intake of water at
root does not keep up
with transpiration,
turgor loss occurs &
the stomata close,
transpiration
decreases
High carbon dioxide
levels in the air
around the plant
usually cause the
guard cells to lose
turgor pressure & the
stomata close
Xerophytes
• Survive in
• Adapted for arid
climates
– Deserts
Halophytes
• Adapted to grow in
water with high levels
of salinity
• Studied for biofuels
This one is for
you, Alex!
Halophytes
• Store water = becoming succulent
• May secrete salt through salt glands =
mangrove
• Some can compartmentalize Na+ and Cl- in the
vacuoles of their cells
• Sunken stomata on thickened leaves reduce
water loss by creating a higher humidity near
the stomata
• Surface area of leaves is reduced
Lab time!
Measuring
transpiration
rates using
potometers
Lab Design
• Skill: Design an experiment to test hypothesis
about the effect of temperature or humidity
on transpiration rates. (think IA)
9.2 Transport in the phloem of plants
Understanding:
• Plants transport organic compounds from
sources to sinks
Lacks
nucleus &
cytoplasm
Have
nucleus &
cytoplasm
Sieve
plates
connect
sieve tubes
Moving from source to sink
• What’s a source?
– Producer of sugar (photosynthesis or hydrolysis of
starch)
• Leaves primary
• What’s a sink?
– Plant part that uses or stores sugar
• Roots, buds, stems, seeds, and fruit
Potatoes can be
both depending
on the time of
year
Phloem Sap
• Movement of organic molecules = translocation
• What makes up phloem sap?
– Sugar (sucrose)
– Amino acids
– Plant hormones
– Small RNA molecules
Understanding:
• Incompressibility of water allows transport
along hydrostatic pressure gradients
• Active transport is used to load organic
compounds into phloem sieve tubes at the
source
• High concentrations of solutes in the phloem
at the source lead to water uptake by osmosis
• Raised hydrostatic pressure caused the
contents of the phloem to flow toward sinks
The pressure-flow hypothesis
• Phloem sap can move 1
m/hr
• Measured by
– Aphid stylets
– Radioactively labeled
carbon dioxide
Nature of Science
Aphid stylets
• Aphids feed on by inserting
stylist “mouth” into a sieve
tube
• Pressure in sieve tube
forces the contents into the
stylet and insect’s gut
• Anaesthetize the insect and
cut off the gut
• Analyse the sap
Radioactive carbon dioxide
• Use radioactive form of
carbon so that the location
of carbon dioxide-fixing
reactions of photosynthesis
can be determined
• Autoradiography can then
be used to track the flow of
the carbohydrate, usually
sucrose, through the plant
Pressure-flow hypothesis
1. Load sugar into the sieve
tube at the source
reducing the relative water
concentration in the sieve
tube members causing
osmosis
2. The uptake of water
causes a positive pressure
(hydrostatic) in the sieve
tube which results in bulk
flow of the phloem sap
Pressure-flow hypothesis
3. Hydrostatic pressure is
reduced when sucrose
inters the sink (sugar
changed to starch
which is insoluble = no
osmosis)
4. Xylem recycles the
relatively pure water
by carrying it from the
sink back to the source
passive
Active
transport
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