Download File - Ms. Richards IB Biology HL

Topic 9 Plant Biology
Part One
9.1 Transport in the xylem of plants
Nature of science: Use models as representations of the real world—mechanisms
involved in water transport in the xylem can be investigated using apparatus and
materials that show similarities in structure to plant tissues. (1.10)
Understandings
• Transpiration is the inevitable consequence of gas exchange in the leaf
• Plant transport water from the roots to the leave to replace losses from
transpiration
• The cohesive property of water and the structure of the xylem vessels allow
transport under tension
• The adhesive property of water and evaporation generates tension forces in leaf
cell walls
• Active uptake of mineral ions in the roots causes absorption of water by osmosis
9.1 Transport in the xylem of plants
Applications and Skills
• Application: Adaptations of plants in deserts and in saline soils of water
conservation
• Application: Models of water transport in xylem using simple apparatus
including blotting or filter paper, porous pots, and capillary tubing
• Skill: Drawing the structure of primary xylem vessels in sections of stems
based on microscope images
• Skill: Measurement of transpiration rates using potometer
• Skill: Design of an experiment to test hypothesis about the effect of
temperature or humidity on transpiration rate
Retro Topic 2.2 Water
Understandings:
• Water molecules are polar and hydrogen bonds form between them
• Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and
solvent properties of water
• Substances can be hydrophilic or hydrophobic
Applications and skills:
• Application: Comparison of the thermal properties of water with those of
methane
• Application: Use of water as a coolant in sweat
• Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen
and sodium chloride in blood in relation to their solubility in water
9.2 Transport in the phloem of plants
Nature of science: Developments in scientific research follow improvements in
apparatus—experimental methods for measuring phloem transport rates using aphid
stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes
became available. (1.8)
Understandings
• Plants transport organic compound from sources to sinks
• 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 causes the contents of the phloem to flow toward sinks
9.2 Transport in the phloem of plants
Applications and skills
• Application: Structure-function relationships of phloem sieve tubes
• Skill: Identification of xylem and phloem in microscope images of
stem and root
• Skill: Analysis of data from experiments measuring phloem transport
rates using aphid stylets and radioactively-labeled carbon dioxide
Flash Cards
For Plants Part I do flash cards for
• 9.1 Transport in the xylem of plants
• 9.2 Transport in the phloem of plants
Angiosperms
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•
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•
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What is the difference between an angiosperm and a gymnosperm?
Angiosperms have flowers or fruit
Gymnosperms are the “naked” seeds, cone-bearers
There are two classifications of angiosperms
Commonly recognized and are named for the number of cotyledons, or
seed leaves, present on the embryo of the plant
1. One group are the monocots which include orchids, bamboos, palms,
lilies, and yuccas as well as the grasses such as wheat, corn, and rice
2. The other group is the dicots which include roses, beans, sunflowers,
and oaks
Monocots
1. One cotyledon
2. Veins, which carry vascular tissue,
usually parallel
3. Vascular bundles usually complexly
arranged spread
throughout/random
4. Fibrous adventitious root system.
(Unbranched roots grow from stems)
5. Floral organ parts usually in multiples
of 3 (stamens, petals)
6. Pollen grain with one opening
Monocots
Dicots
1. Two cotyledons
2. Veins usually netlike
3. Vascular bundles usually arranged in
ring
4. Taproot with lateral branches usually
present
5. Floral parts usually in multiples of
four or five
6. Pollen grains with 3 openings
Dicots
Flowering Plant
• Plants have three basic
organs:
1. Root
2. Stems
3. Leaves
• These organs are composed
of different tissues and
these tissues are teams of
different types of cells
Tissue Types
• Dermal tissue or epidermis- single layer of tightly packed cells that
covers and protects all young parts of the plant
• The epidermis of leaves and most stems secrete a waxy coating called
the cuticle that helps the aerial parts of the plant retain water
• Root hairs are epidermal extensions found near the tips of roots and are
important in the absorption of water and minerals
• Increase surface area
Tissue Types
• Vascular tissue: continuous throughout the plant, is involved in the
transport of materials between roots and shoots
• Two types of vascular tissue
1. Xylem which conveys water and dissolved minerals upward from
roots into the shoots
2. Phloem which transports food made in mature leaves to the roots
and to non-photosynthetic parts of the shoot system, such as
developing leaves and fruit. More generally, transport of sugars
from sugar source to sugar sink
Tissue Types
• Ground tissue: tissue that is neither dermal tissue nor vascular tissue
• In dicot stems, ground tissue is divided into pith, internal to the
vascular tissue, and cortex, external to the vascular tissue
• Among the functions of pith are photosynthesis, storage and support
Types of cells
• There are three types of cells we will discuss:
parenchyma, collenchyma, and sclerenchyma
• Parenchyma cells: have primary walls that
are relatively thin and flexible.
• They have a large central vacuole, are the
least specialized cells
• They perform most of the metabolic
functions of the plant.
• For example, photosynthesis occurs within
chloroplasts of parenchyma cells in the leaf
• Examples: Palisade mesophyll and spongy
mesophyll
• Collenchyma cells- have thicker primary
walls than parenchyma cells
• Grouped in strands or cylinders,
collenchyma cells help support young parts
of the plant shoot
• i.e. “strings” of a celery stalk
• Functioning collenchyma cells are living
and flexible and elongate with the stems
and leaves they support
• Normally located under the epidermis
• Sclerenchyma cells- have thick secondary walls usually
strengthened by lignin, much more rigid then collenchyma cells
• Mature sclerenchyma cells cannot elongate and they occur in
regions of the plant that have stopped growing in length
• Many are dead at functional maturity
• The water-conducting vessel elements and tracheids in the
xylem are sclerenchyma cells called fibers and sclereids that
specialize entirely in support
• i.e. fibers: hemp fibers used for making rope and flax fibers for weaving
into linen
• i.e. sclereids (shorter): impart hardness to
nutshells and seed coats and the gritty
texture to pear fruits.
Structure of Leaves
• The function of leaves is to produce food for the plant by
photosynthesis
• The leaf is adapted by its structure to carry out photosynthesis
efficiently
• The main part of the leaf is the leaf blade or lamina. It has a large
surface area to absorb sunlight but is very thin (0.3mm)
• It is composed of 4 thin tissue layers with veins at intervals
Tissue layers
• Upper epidermis: continuous layer of cells covered by a thick cuticle.
• It prevents water loss from the upper surface even when heated by
sunlight
• Lower epidermis: is in a cooler position and has a thinner waxy
cuticle
• Both upper and lower epidermis: First line of defense against
physical damage and pathogenic organisms
• Barrier is interrupted only by stomata and guard cells
Tissue layers- Palisade mesophyll
• Palisade mesophylldensely packed cylindrical
cells with many chloroplasts
• Main photosynthetic tissue
and is positioned near the
upper surface where the
light intensity is highest
• Parenchyma cells
Tissue layers- Spongy Mesophyll
• Loosely packed rounded cells with few chloroplasts.
• Labyrinth of air spaces through which gases circulate up to the palisade
region. Air spaces are large near the stomata
• Provides the main gas exchange surface so must be near the stomata in
the lower epidermis. Photosynthesis depends on gas exchange over a
moist surface
• Spongy mesophyll cell walls provide this surface. Water often evaporates
from the surface and is lost in a process called transpiration
• Transpiration is the loss of water vapor from the leaves and stems of plants.
There are adaptations that minimize the amount of transpiration.
Transpiration
• Transpiration is the loss of water vapor
from the leaves and stems of plants
• Stomata are openings on the underside of
the leaf that allow for gas exchange during
photosynthesis
• Stomata allow for oxygen and water vapor
to exit and carbon dioxide to enter
• Due to this exchange transpiration is the
inevitable consequence
• Plants try to minimize this through guard
cells that open and close the stomata
Stomata
Vascular Tissue- Phloem
• In phloem sucrose, other organic compounds, and some mineral ions
are transported through sieve tubes formed by chains of cells called
sieve-tube members which remain alive at functional maturity
• These sieve-tube members are connected to nucleated companion
cells which are connected to phloem via plasmodesmata
• All that remains of sieve tubes are plasma membranes
• Also have porous end walls
• We will continue discussing phloem later…
Vascular Tissue- Xylem
• So how do plants replenish this lost water?
• Plants transport water from the roots to the
leaves through xylem
• The structure of xylem vessels allows for the
efficient transport of water
• Xylem is composed of tracheids and vessel
elements
• These are elongated cells that are dead at
functional maturity
• So at maturity xylem is non-living
Primary Xylem
• Xylem vessels are long continuous
tubes
• Primary xylem is helical or ring-shaped
thickenings
• Thickenings made of the cellulose cell
wall are impregnated with lignin. This
makes them hard to resist inward
pressures.
• Pores in the outer cellulose cell wall
conduct water out of the xylem vessel
and into cell walls of adjacent cells
Transport of materials
Transport in plants occurs on three levels:
1. Uptake and loss of water and solutes by individual cells such as the
absorption of water and minerals from the soil by cells of a root
2. Short-distance transport of substances from cell to cell at the level
of tissues and organs, such as loading of sugar from
photosynthetic cells of a mature leaf into the sieve tubes of
phloem
3. Long-distance transport of sap within xylem and phloem at the
level of the whole plant
Root absorption of mineral ions from the soil:
Active Transport
• Active transport is the pumping of solutes across membranes against
their electrochemical gradients
• The cell must expend metabolic energy ATP to transport a solute
uphill.
• Active transport in root cells is involved in the absorption of
potassium, phosphate, nitrate, and other mineral ions from the soil
• The concentration of these ions in the soil is usually much lower than
inside root cells
• Proton pump is the major pump in plant cells
• Used to generate a hydrogen ion gradient
and membrane potential (voltage)
• Inside of the cell is negative while the outside
of the cell is positive. This is a form of stored
energy that can be harnessed to perform
cellular work
• Plants use this energy storage in the proton
gradient and membrane potential to drive
the transport of many different solutes
• This mechanism is called co-transport
because the transport protein couples the
downhill passage of one solute (H+ ions) to
the uphill passage of another (e.g. nitrite)
Substitute K+
or Na+ or Clor any
Ion or mineral
Transport of water
•
•
Differences in water potential drive water transport in plant cells:
The net uptake or loss of water by a cell occurs by osmosis, the
passive transport of water across a membrane
• There are 2 factors that influence the direction of water
movement in plant cells
1) Water will move from a hypotonic (lower solute concentration) to
a hypertonic (higher solute concentration) area.
2) The cell wall adds a second factor affecting osmosis: physical
pressure. Physical pressure causes water to move. If a solution is
separated from pure water by a selectively permeable membrane,
external pressure on the solution can counter its tendency to take
up water due to the presence of solutes
Transport of water
• When plant tissue is placed in pure water, the cell
begins to swell and push against the cell wall,
producing a turgor pressure
• The partially elastic wall pushes back against the
pressurized cell. When this wall pressure is great
enough to offset the tendency for water to enter
because of the solutes in the cell a dynamic
equilibrium will be reached and the cell will be
turgid
• Healthy plant cells are turgid most of the time.
Their turgor contributes to support in non-woody
parts of the plant
Transport of water
• As of 1990, scientists have suggested that even water transport is
mediated by selective channels. They have since found these
channels in both plant and animal cells
• The specific channels for passive traffic of water are transport
proteins called aquaporins
• They do not affect the gradient or the direction of water flow, but
rather the rate at which water diffuses down its gradient
• So water is transported by osmosis (passive transport)
• Nutrients (mineral ions) are actively co-transported from soil to root
Absorption of Water and Mineral by Roots
• Route: enter through epidermis of roots, cross the root cortex, pass
into the stele, flow up xylem vessels to the shoot system
• Root tips: epidermis is permeable to water. Much of the absorption
of water and minerals occurs near root tips
• Root hairs are extensions of epidermal cells, and account for much of
the surface area of roots
Lateral Transport
• Water and solutes move from
one location to another within
plant tissues and organs
• This occurs when water and
minerals are absorbed by a root
from the outer cells and moved
to the inner cells of the root
Endodermis
• In order for water and minerals to pass from the soil and the root
cortex to the rest of the plant, they must enter the xylem of the stele
• The endodermis surrounds the stele and functions as a last
checkpoint for the selective passage of minerals from the cortex into
the vascular tissue
• Minerals in the cytoplasm continue through the endodermal cells via
plasmodesmata and into the stele
• Minerals traveling via the cortex encounter a dead end that blocks
their passage into the stele
Casperian strip
• In the wall of each endodermal cell is the Casperian strip, a belt of
suberin, a waxy material that is impervious to water and dissolved
minerals
• Material must cross the plasma membrane of the endodermal cell
and enter the stele via the cytoplasm
• This assures that all solutes must pass through at least one cell
membrane before entering the xylem
• Water then passes into the tracheids and vessel elements of the
xylem using both diffusion and active transport
Apoplastic vs Symplastic
• Apoplastic route is through the
cell walls (and intercellular
spaces)
• Symplastic route is through the
cytoplasm (and plasmodesmata)
• Water is pulled to the xylem due
to a pulling force called the
transpiration pull
Summary of movement of water from roots
to xylem
• Roots and root hairs increase surface area to absorb water in soil
• Water is absorbed by osmosis
• Solute concentration is higher inside the root than in the soil so
water moves in by osmosis
• Ions move in by active transport
• Both apoplastic and symplastic transport across the root
• Apoplastic route is through the cell walls and intercellular spaces
• Symplastic route in through the cytoplasm and plasmodesmata
• Water moves from epidermis to cortex to endodermis
• Water moves through the endodermis and is blocked by
casparian strips
• Water moves into the xylem due to pulling force called
transpiration pull
• Cohesion of water molecules makes this possible
Long Distance Transport
• Bulk flow functions in long-distance transport
• Diffusion is efficient for transport over distances defined by cellular
dimensions but is too slow for long-distance transport e.g. from roots to
leaves
• Water and solutes move through xylem vessels and sieve tubes by bulk
flow  the movement of a fluid driven by pressure
• In xylem, it is actually tension (negative pressure) that drives longdistance transport.
• Transpiration and evaporation of water from a leaf, reduces pressure in
the leaf xylem. This creates a tension that pulls xylem sap upward from
the roots
• In phloem, for example, hydrostatic pressure is generated at one end of a
sieve tube, forcing sap to the opposite end of the tube
Transport of Xylem Sap
• Tracheids are spindle-shaped, elongated cells
with pits through which water flows from cell to
cell
• They are dead at functional maturity
• When the living interior of a tracheid or vessel
element disintegrates, the cell’s thickened cell
walls remain behind forming a nonliving conduit
through which water can flow
• Water moves from cell to cell without having to cross
thick secondary walls
• The secondary walls of xylem are strengthened with
lignin so tracheids function in support as well as
water transport
• Vessel elements are generally wider, shorter, thinner
walled, and less tapered than tracheids.
• They are aligned end to end forming long micropipes,
the xylem vessels
• The end walls of vessel elements are perforated,
enabling water to flow freely through xylem vessels
• Water streams from cell to cell through perforated
end walls and can also migrate laterally between
neighboring vessels through pits
Other features of xylem
• There are no plasma membranes in xylem vessels, so water can move
in and out freely.
• The lumen of the xylem vessel is filled with sap because the
cytoplasm and nuclei have disintegrated.
• There are pores in the outer cellulose cell wall to conduct water out
of the xylem vessel and into cell walls of adjacent leaf cells.
• Cellulose rings with lignin make xylem hard so that they can resist
inward pressures.
Accent of Xylem Sap
• Depends mainly on transpiration and the physical properties of
water. Must rise against gravity.
• At night, when transpiration is very low or zero (stomata are closed,
temperatures are lower), root is still using energy to pump mineral
ions into xylem
• Water will flow in from root cortex, generating a positive pressure
that forces fluid up the xylem. This upward push of xylem sap is
called root pressure
• In most plants, root pressure is not the major mechanism driving the
ascent of xylem sap. At most, root pressure can force water upward
only a few meters
• For the most part, xylem sap is not pushed from below by root
pressure, but pulled upward by the leaves themselves
• Transpirational pull: water vapor diffuses from the moist air spaces
of the leaf to the drier air outside via stomata
• Evaporation from the water film coating the mesophyll cells
maintains the high humidity of the air spaces
• This loss of water causes the water film to form menisci (singular
meniscus) that become more and more concave as the rate of
transpiration increases
• The tension of water lining the air spaces of the leaf is the physical
basis of transpirational pull, which draws water out of the xylem
• The cohesion of water due to
hydrogen bonding makes it
possible to pull a column of sap
from above without the water
separating
• Also helping to fight gravity is the
strong adhesion of water
molecules (H bonds) to the
hydrophilic walls of the xylem
cells
Skill: Drawing the structure of primary xylem
vessels in sections of stems based on microscope
images
Skill: Drawing the structure of primary xylem
vessels in sections of stems based on microscope
images
• Plan diagram of a dicot plant
Application: Models of water transport in xylem using simple
apparatus including blotting or filter paper, porous pots, and
capillary tubing
• Models allow one factor or aspect to be studied independently of
other factors
• Water has adhesive properties. Glass capillary tubes can be used to
model adhesion between water and xylem vessel walls. Water
adheres to glass so rises up the capillary tube. A substance like
mercury does not adhere so does not rise.
Application: Models of water transport in xylem using simple
apparatus including blotting or filter paper, porous pots, and
capillary tubing
• Porous pot can be used to model flow in a xylem vessel due to
transpiration from the leaf
• Porous pot is similar to leaf cell walls as water adheres to it and there
are many narrow pores
• Water evaporates from the surface of the pot so more water is drawn
into the pot to replace losses
• Water rises up in the tube
Application: Models of water transport in xylem using simple
apparatus including blotting or filter paper, porous pots, and
capillary tubing
• Blotting paper can be used to model capillary attraction or adhesion
• Strip of blotting, filter, or chromatography paper is suspended by a
rubber stopper from the top of a test tube into a small amount of
water at the bottom of the test tube
• Paper is made of cellulose (like cell walls) so water rises up through it
against gravity in pores in the paper
Skill: Measurement of transpiration rates using
potometer
Potometer
Factors that effect transpiration
1. Light
• Light leads to photosynthesis, the need for gas exchange, and increases in
transpiration. Light stimulates stomatal opening
• Guard cells close the stomata in darkness
2. Temperature
• Transpiration assists the plant in evaporative cooling and prevents the leaf
from reaching temperatures that could denature various enzymes involved
in photosynthesis and other metabolic processes
• So increases in temperature lead to increases in transpiration
• Heat is needed for evaporation of water from the surface of spongy
mesophyll cells, so as temperature increases, evaporation increases, and
transpiration (water loss) increases.
temperature
evaporation
transpiration
Factors that effect transpiration
3.) Wind
• Wind leads to increase in evaporation
• Wind blows the saturated air away and leads to increases in the rate of transpiration – which
leads to an increased movement of water through the xylem and an increase in transpiration
• High wind velocities can cause stomata to close
• Still air, reduces the rate of transpiration.
4.) Humidity
• Water diffuses out of the leaf when there is a concentration gradient between the air spaces
inside the leaf and the air outside
• The air spaces inside are always nearly saturated
• The lower the humidity outside the leaf, the steeper the gradient and therefore the faster the
rate of transpiration
• The higher the humidity outside, the less steep the gradient and transpiration rate is
decreased
5.) Number, size, and distribution of stomata
6.) Surface area of leaf
7.) Carbon dioxide levels in air
Skill: Design of an experiment to test hypothesis about the
effect of temperature or humidity on transpiration rate
• Keep everything else constant except the independent variable
• If examining temperature you can use 2 plants with 2 lights, one
putting out light and heat and the other one putting out just light at
the same wattage (compact fluorescent bulbs do not put out heat)
• If examining humidity you can also use 2 plants with and without use
of a mister and a plastic bag. Can monitor humidity using kestrel.
Control leaf exposed to air of known humidity. Experimental leaf
bagged with plastic bag, misted with 5 ml of water and sealed
• In both cases describe the potometer set up and/or the gas pressure
probe set up
Application: Adaptations of plants in deserts
and in saline soils of water conservation
• Xerophytes are plants adapted to arid climates. They have various
leaf modifications that reduce the rate of transpiration
• Cereus giganteus, the Saguaro or giant cactus that grows in deserts in
Mexico and Arizona
Adaptations of Xerophytes
1.
2.
3.
4.
5.
They have small, thick leaves, an adaptation that limits water loss by
reducing surface area relative to leaf volume
A thick cuticle gives some of these leaves a leathery consistency and
reduces water loss
Stomata are concentrated on the lower (shady) leaf surface, and they
are often located in depressions that shelter the pores from the dry
wind. These pits are surrounded by “hairs”.
During the driest months, some desert plants shed their leaves.
Other plants such as cacti, subsist on water the plant stores in its fleshy
stem during the rainy seasons (these modified stems are the
photosynthetic organs of cacti; the spines are modified leaves).
CAM Plants
• CAM stands for crassulacean acid metabolism. Succulents are CAM
plants. These plants have a metabolic adaptation that allow them to
incorporate carbon dioxide into organic acids during the night
• During the daytime, the organic acids are broken down to release
carbon dioxide in the same cells, and sugars are synthesized by the
conventional C3 pathway
• The leaf takes in its carbon dioxide at night, the stomata can close
during the day, when transpiration is most severe
Application: Adaptations of plants in deserts
and in saline soils of water conservation
• Halophytes are plants that live in saline soils
• They are adapted to grow in water with high salinity. They are a
promising biofuel because they do not compete with food crops for
resources
• Only 2% of all plant species
• Come in contact with saline water
through its roots or by salt spray,
such as in saline semi-deserts,
mangrove swamps, marshes,
and sloughs and seashores
Adaptations of halophytes
• Leaves are reduced to small scaly structures or spines
• They shed their leaves when water is scarce
• The stem becomes green and takes over roll of photosynthesis
• Water is stored in the leaves
• They have thick cuticle and multiple layered epidermis
• Stomata are sunken
• They have long roots that can grasp water
• They have structures for removing salt buildup
9.2 Transport in the phloem of
plants
9.2 Transport in the phloem of plants
• Nature of science: Developments in scientific research follow improvements in
apparatus—experimental methods for measuring phloem transport rates using aphid
stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes
became available. (1.8)
Understandings
• Plants transport organic compound from sources to sinks
• 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 causes the contents of the phloem to flow toward sinks
9.2 Transport in the phloem of plants
Applications and skills
• Application: Structure-function relationships of phloem sieve tubes
• Skill: Identification of xylem and phloem in microscope images of
stem and root
• Skill: Analysis of data from experiments measuring phloem transport
rates using aphid stylets and radioactively-labeled carbon dioxide
Application: Structure-function relationships of phloem
sieve tubes
• Phloem cells are sieve tube members which are arranged end to end to
form long sieve tubes
• Sieve tube members have no nuclei or ribosomes
• Between the cells are sieve-plates  porous cross-walls that allow the flow
of sap along the sieve tube
• Alongside each sieve-tube member is a nucleated companion cell
• This cell is a non-conducting cell which is connected to the sieve tube member by
numerous channels called the plasmodesmata
• Translocation is the transport of any biochemical in phloem whether
produced by the plant or not
• This includes plant hormones and small RNA molecules
Plasmodesmata
• Plasmodesmata are channels of cytoplasm that pass through cell
walls
Phloem Sieve Tube
Companion Cells
• Adjacent sieve-tube members
have no nucleus or ribosomes
• Companion cells help these
sieve-tube members
• In some plants, companion cells
in leaves also help load sugar
produced in the leaf into the
sieve-tube members
• The phloem then transports the
sugar to other parts of the plant
Direction of phloem sap
• Direction of phloem sap is variable but always moves from a sugar
source to a sugar sink
• Sugar source- sugar is produced by photosynthesis or through the
breakdown of starch. Mature leaves are major sugar source
• Sugar sink- an organ that is a net consumer or storer of sugar.
Growing roots, shoot tips, stems, and fruit are sugar sinks supplied by
phloem
Modified shoots
• A storage organ, such as a tuber (potato) or bulb (e.g. onion), may be
either a source or a sink depending on the season
• Other solutes, such as minerals, may be transported to sinks along
with sugar and later transported to developing fruit
Phloem Loading and Unloading
• Sieve tubes have no nuclei and ribosomes but have plasma membrane and
transport proteins
• Sugar must move into sieve tube members before it can be exported to sugar
sinks
Loading
1. Sugar can move through the cytoplasm (plasmodesmata) through diffusion
(Symplastic)
2. Sugar can move through a combination of the cytoplasm and cell wall
pathways (Apoplastic)
• Sucrose can be concentrated 2-3 times higher than concentrations in
mesophyll
• Phloem loading requires active transport
• Proton pumps do the work which enables the cells to accumulate sucrose
Phloem Loading- Short Distance Transport
Sieve Tube Pressure
• Concentration of free sugar in the
sink is lower than that in the sieve
tube
• As the result of this gradient, sugar
molecules diffuse from the phloem
into the sink tissues, and water
follows by osmosis
• Bulk flow or pressure flow is the
mechanism of translocation
Pressure Flow- Long Distance Transport
1. Step 1: Phloem loading results in a high solute concentration at the
source end of a sieve tube which lowers the water potential and
causes water to flow into the tube
2. Step 2: Hydrostatic pressure develops within the sieve tube and the
pressure is greatest at the source end of the tube
3. Step 3: At the sink end, pressure is relieved by the loss of water
which follows the exodus of sucrose
4. Step 4: Water is recycled back from sink to source by xylem vessels
Summary of Sugar Movement in Plants
• Cellular level- active transport across plasma membranes in phloem
cells
• Short distance level of lateral transport within organs- sucrose
migration from mesophyll to phloem via the cytoplasm through
plasmodesmata or through cell walls
• Long distance level of transport between organs- bulk flow/pressure
flow within sieve tubes
Skill: Identification of xylem and phloem in
microscope images of stem and root
• Stem
Root
Roots- X marks the spot!
Skill: Analysis of data from experiments measuring
phloem transport rates using aphid stylets and
radioactively-labeled carbon dioxide
• Phloem sap is an aqueous solution in which the prevalent solute is
sugar primarily sucrose
• It may be as high as 30% by weight
• Phloem sap may also contain minerals, amino acids, and hormones in
transit from one part of the plant to another
Skill: Aphids
• Using aphids is a method of obtaining
samples of phloem sap from single
sieve tubes
• Aphids have long piercing mouth parts
called stylets
• High pressure inside sieve tubes pushes
phloem sap out through the stylet and
into gut of aphid
• The aphid is cut off from the stylet
while it is still feeding
• The stylet is left and sap continues to
flow
• In 1940s began using 14CO2 in leaf for
photosynthesis  Labeled sucrose
produced