Transcript Transport in Angiospermatophytes

Transport in
Angiospermatophytes
Topic 9.2
Assessment Statements
9.2.1 Outline how the root system provides a large surface area for mineral ion and water
uptake by means of branching and root hairs.
9.2.2 List ways in which mineral ions in the soil move to the root.
9.2.3 Explain the process of mineral ion absorption from the soil into roots by active
transport.
9.2.4 State that terrestrial plants support themselves by means of thickened cellulose,
cell turgor and lignified xylem.
9.2.5 Define transpiration.
9.2.6 Explain how water is carried by the transpiration stream, including the structure of
xylem vessels, transpiration pull, cohesion, adhesion and evaporation.
9.2.7 State that guard cells can regulate transpiration by opening and closing stomata.
9.2.8 State that the plant hormone abscisic acid causes the closing of stomata.
9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate
of transpiration in a typical terrestrial plant.
9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.
9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino
acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds,
roots).
Roots and angiosperm transport
• Main function of roots is
to provide mineral ion
and water uptake for the
plant
• Efficiency due to
extensive branching
pattern and root hairs
• Root hairs increase
surface area by a factor
of 3
• Root cap protects apical
meristem during primary
growth
Root Zones
• Zone of cell division –
new undifferentiated
cells are forming, M
phase of the cell cycle
• Zone of elongation –
cells are enlarging in
size, corresponds to
G1 of the cell cycle
• Zone of maturation –
cells are becoming
functional to the plant
Water
• Epidermis → cortex
→ vascular cylinder
• Vascular cylinder
surrounded
endodermis and
pericycle
• Endodermis –
cylindrical layer of
cells that separates
the cortex from the
vascular tissue
epidermis
pericycle
cortex
xylem
phloem
endodermis
• Pericycle – layer of cells
that can become
meristematic; produces
the lateral or branching
roots of a plant
• Why must lateral or
branching roots
originate from this
region?
• To allow their connection
to the vascular cylinder
epidermis
pericycle
cortex
xylem
phloem
endodermis
Water enters a plant through root hairs by
osmosis
Symplastic route
• water moves from cell to
cell
Apoplastic route
• water moves through the
cell walls and the
extracellular spaces
Major processes that allow mineral ions to
pass from the soil to the root
• Diffusion of mineral ions and mass flow of water in
the soil carrying these ions
• Occurs when there is a higher concentration of
mineral outside the root than inside
• Minerals dissolved in and move via water
• Aid provided by fungal hyphae
• Filaments form a cover over the surface of young
roots
• Larger surface area is created for water and mineral
ion absorption
• Mutualisitic relationship referred to as mycorrhiza
• Active transport
• Energy must be expended when the ion the plant
needs cannot cross the lipid bilayer of the
membranes
• Ions must therefore pass through a transport protein
in the membrane
• Transport proteins are specific for certain ions
• They bind to the ion on one side of the membrane
and then release it on the other side
How the proton pump works
1. Proton pump uses energy from ATP to pump hydrogen
ions out of the cell
2. Result is higher hydrogen ion concentration outside the
cell than inside. This creates a negative charge inside
the cell.
3. This gradient results in the diffusion of hydrogen ions
back into the cell.
4. The voltage difference is called a membrane potential.
5. The hydrogen ion gradient and the membrane potential
represent forms of potential energy that can be used to
absorb mineral ions.
(Form of chemiosmosis)
Support in terrestrial plants
Support provided by
• Thickened cellulose
• Occurs in cell walls
• Cell turgor pressure
• Pressure inside the cell that is exerted on the cell wall
by the plasma membrane due to water that has
entered the cell wall by osmosis
• Important in keeping plants upright
• If the soil around a plant dries or gets too salty, water
may no longer move into the plant, water content
decreases, and the plant wilts
• Lignified xylem
• Rings of highly branched polymer
Transpiration
• Loss of water vapour from leaves and other aerial parts
of the plant
• Water lost through openings called stomata which are
opened and closed due to guard cells
• Transpired water has to be replaced from the roots to the
upper parts of the plant by absorption
• Column of water provides minerals and the water
needed for photosynthesis
• Water lost cools sun-drenched leaves and stems
Factors affecting transpiration
Environmental factor
Effect
Light
Speeds up transpiration by warming the
leaf and opening stomata
Humidity
Decreasing humidity increases
transpiration because of the greater
difference in water concentration
Wind
Increases the rate of transpiration because
humid air near the stomata is carried away
Temperature
Increasing temperature cause greater
transpiration because more water
evaporates
Soil water
If the intake of water at the roots does not
keep up with transpiration, turgor loss
occurs and the stomata close – this
decreases transpiration
Carbon dioxide
High carbon dioxide levels in the air around
the plant usually cause the guard cells to
lose turgor and the stomata to close
• Increase light → increase photosynthesis → carbon
dioxide used up → increase pH of guard cells →
stimulates conversion of starch to glucose → influx of
water into guard cells by osmosis → raises turgor
pressure opening stomata
• So carbon dioxide increase in air, lowers pH, glucose is
converted to starch, water leaves guard cells, turgor
pressure is lowered, and stomata close
Xerophyte (plants adapted to arid climates)
Modifications to decrease transpirational
water loss
• Small, thick leaves reduce water loss by decreasing
surface area
• Reduced number of stomata decreases the openings
through which water loss may occur
• Thickened, waxy cuticle
• Hair-like cells on the leaf surface trap a layer of water
vapour, thus maintaining a higher humidity near the
stomata
• Shed their leaves in the driest months
• Water stored in fleshy stems
• Alternative photosynthetic processes
CAM photosynthesis
• Close stomata during the
day
• Incorporate carbon
dioxide during the night
C4 photosynthesis
• Have stomata open
during the day
• Take in carbon dioxide
more rapidly than nonspecialised plants
Stomata and guard cells
• Closing of stomata is only
possible on a short-term
basis
• Why?
• Carbon dioxide must
enter the mesophyll so
that photosynthesis can
occur
• Stomata open and close
due to changes in the
turgor pressure of the
guard cells that surround
them
• Gain and loss of water in the
guard cells is largely due to the
transport of potassium ions
• Light triggers the activity of
ATP-powered proton pumps in
the plasma membrane of the
guard cells
• Potassium moves into cell
• Higher solute, water follows by
osmosis
• When potassium leaves, so
does water
• The plant hormone abscisic
acid causes potassium ions to
rapidly diffuse out of the guard
cells
• The result is stomatal closure
• Hormone is produced in the
roots during times of water
deficiency
Transport of water and mineral transport by
xylem
• Complex tissue
composed of:
• Tracheids
• Vessel elements
• Tracheids
• Dead cells that taper
at the ends and
connect to one
another to form a
continuous column
• Ancient plants only
had tracheids
• Vessel elements
• Most important
• Dead cells that have thick,
lignified secondary walls
interrupted by areas of
primary wall
• Have pits or pores that
allow water to move
laterally
• Attached end to end
• Have perforations allowing
water to move up
• Most modern flowering
plants only have vessel
elements
How water is carried by the transpiration
stream
1. Water moves down concentration gradient from high
concentration of water vapor within leaf to the
atmosphere which has a lower water vapor
concentration.
2. Water lost by transpiration is replaced by water from the
vessels.
3. Vessel water column is maintained due to cohesion
(water molecules held together with strong hydrogen
bonds) and adhesion (water molecules held to sides of
the vessels with hydrogen bonds) counteracting gravity.
4. Water is pulled from the root cortex into xylem cells,
again due to cohesion and adhesion.
5. Water is pulled from the soil into the roots due to the
tension created by transpiration.
The movement of organic molecules in
plants
• Organic molecules move via the phloem in plants
• Made up of living cells
• Sieve tube members
• Companion cells
• Sieve tube members are connected to one another by
sieve plates to form sieve tubes
• Sieve plates have pores that allow the movement of
water and dissolved organic molecules throughout the
plant
• Companion cells are connected to their sieve tube
members by plasmodesmata
• Phloem cells transport their contents in various
directions
• Direction of movement is from a source to a sink
• Source: a plant organ that is a net producer of sugar
either by photosynthesis or by the hydrolysis of starch
• Primary source are leaves
• Sink: a plant organ that uses or stores sugar
• Ex. Roots, buds, stems, seeds and fruits
• Some structures may be both a source and a sink, such
as tubers
Translocation
• The movement of organic molecules in plants
• Organic molecules are dissolved in water and the
solution is known as phloem sap
• Mostly sugars (sucrose most common)
• Amino acids
• Plant hormones
• mRNA
Pressure-flow hypothesis
1. Loading of sugar into the sieve tube at the source
(accomplished by active transport). This reduces the
relative water concentration in the sieve tube members
causing osmosis from the surrounding cells.
2. The uptake of water causes a positive pressure in the
sieve tube that results in a flow of the phloem sap.
3. This pressure is diminished by the removal of the sugar
from the sieve tube at the sink. The sugars are
changed at the sink to starch. Starch is insoluble and
exerts no osmotic effect.
4. Xylem recycles the relatively pure water by carrying it
from the sink back to the source.