Transcript Plant Physiology - University of Windsor

Plant Physiology
To understand how plants work, you need to combine
knowledge of plant structure with understanding of plant
physiology. This lecture covers some aspects of physiology,
emphasizing transport and photosynthesis.
Transport
You’ve already seen the structure of xylem and phloem. How
does transport in these systems work? How is a redwood tree
able to move water from the soil to leaves 100m above the
soil?
Successful movement is based on the chemical nature of
water. Water molecules are bonded to each other by hydrogen
bonds. That makes the water in roots, xylem, and leaves a
continuous network. How does water move?
There are 5 major forces that move water from place to place:
1. diffusion – the net flow of molecules from regions of
higher to regions of lower concentration. This is the major
force moving water in gaseous (vapor) phase.
2. osmosis – the diffusion of liquid water molecules from a
dilute solution (more water, less solute) across a selectively
permeable membrane into a more concentrated solution
(less water, more solute). Osmosis is important in moving
water from the solution bathing cells (the apoplast) into
the cytoplasm. This flow will continue until the hydrostatic
pressure (turgor pressure) inside the cell balances the
osmotic pressure.
In intercellular spaces
Within cellular cytoplasm
3. capillary forces – not only is water cohesive (tends to stick
together), it is also adhesive, sticking to hydrophilic
surfaces. That includes carbohydrates (cellulose) of the
xylem tubes’ walls. They are very narrow in bore, and
water is pulled to cover the surface of the inside of the
tube. The force pulling is capillary force. How large can it
be? 1,000 atmospheres, or 15,000 lbs. Eventually the force
of gravity balances the upward pull, in theory. That
balance is not reached in plants, and capillary force moves
water upward to replace evaporative loss.
4. hydrostatic (turgor) pressure – this has already come up.
5. gravity – has also already been mentioned.
How much pressure is involved?
To move water to the top of a 33m elm tree (species doesn’t
matter) requires a pressure of 6.7 atmospheres (for those into
proper SI units, this is equivalent to 0.67 megapascals).
To move water to the top of a 100m redwood requires a
pressure of 20 atmospheres or 2 MPa.
Ecologists can measure the force exerted in a plant stem
using a tool called a Schollander Bomb.
Since the water column is continuous from the roots to the
leaves, water loss from transpiration affects the entire column.
Water is ‘pulled’ upward from the roots from the cohesion of
water in the entire system. How much water is transpired (and
replaced by absorption from soil into roots)?
One corn plant in the central U.S. uses (transpires) 50 – 100
gallons of water (text: 196 l)
A tomato plant ~ 120 l
An apple tree ~ 8000 l
date palm (warmer, wetter, more tropical habitat) ~ 140,000 l
Plant biologists determine how water will flow by combining
these forces into a measure called water potential. Water
flows from a region of high water potential to one having
lower potential.
There is generally a higher potential in roots and shoots than
in leaves. Transpiration involves water evaporating from the
humid interior of leaves and diffusing through stomates. That
loss generates strong forces pulling water up through the
plant, first from stems into leaves, then upward through the
xylem, from roots into xylem, and from soil into root tissues.
Let’s begin at the leaves and (briefly) follow the process and
forces…
Air spaces within the leaves are generally in equilibrium with
the liquid water in cellulose fibrils of cell walls, i.e. at 100%
relative humidity. Air outside the leaves is almost always at a
lower relative humidity.
That difference drives diffusion, as long as there is an
available pathway. Given hydrophobic cuticle, the pathway is
through stomates when they are open.
How fast water diffuses out is in part determined by the
thickness of the boundary layer around the leaf, a region of
almost unstirred air. Thicker boundary layers slow diffusive
loss.
What can make for a thicker boundary layer? A dense layer of
trichomes does the job. So does putting stomatal openings
below the surface of the leaf, in stomatal crypts.
Here’s a digrammatic representation of a cross section of a
yucca leaf:
Loss of water from intercellular spaces within the leaf causes
water to evaporate from the surfaces of cellulose cell walls.
That produces capillary forces attracting water from adjacent
areas of the leaf.
Much of that water comes from inside plant cells, moving
across the plasma membrane (osmosis). Turgor pressure
decreases. Cell walls ‘relax’, and, if sufficient water is lost
without replacement, the leaf wilts.
If the plant is well watered, then replacement is available from
the xylem. This water flows out of tracheids through the pits in
their secondary cell walls and into fibrous cell walls of the
mesophyll cells.
What follows on the next slide is a diagrammatic
representation of these movements…
Water flowing out of a tracheid pulls on the rest of the water in
the tracheid and on the walls of the tracheid. That force is
transferred by hydrogen bonding of water through the system.
The walls of the tracheid are strong and rigid, so the force
effectively acts only on the water column, producing a
hydrostatic tension.
Water may move among neighboring tracheids under this
pressure, and may move up through the column of tracheids.
However, tracheids are connected only by pits, which makes
this path high resistance. Vessels are uninterrupted, have larger
diameters, and are, therefore, a low resistance path.
Tracheids are too small for bubbles to form, blocking flow and
there are many of them. Blocking one tracheid has little
impact. Not so in vessels.
What causes bubbles? Very high tension and freezing mostly.
Angiosperms have xylem with both tracheids and vessel
elements. Conifers (dominant in boreal forests with cold
climates) have only tracheids. This may explain, in part, their
dominance there.
Finally, there is flow into the xylem in roots. The xylem pulls
water from the intercellular space (the apoplast) of the stele
(the core of the root). The water flowing from apoplast into
xylem is replaced by water flowing into the stele from root
cortex. In turn, that cortical water is replaced by water drawn
into the root from the soil.
The movement from cortex to stele involves both apoplast
and symplast (the interconnected cytoplasms of adjacent
cells). The pathways work in parallel as shown below:
There is a limit to water movement through the apoplast.
There is a layer of cells around the stele called endodermis,
and these cells have something called a Casparian strip on
their walls made of suberin (and sometimes lignin) that
prevents intercellular water movement into the stele. Water is
transported to the stele at this point by the symplastic path
only.
Now let’s move to transport of sugars in phloem. It is
commonly sucrose (common table sugar) that is exported from
leaves. Export is by means of the sieve tubes of phloem.
The rate of movement in sieve tubes is ~ 1-2 cm/min. This is
faster than diffusion or cell-to-cell transport, but slower than
the movement of water in vessel elements of xylem.
The current belief is that the sucrose is carried along with a
bulk flow of solution. The flow is directed by a gradient in
hydrostatic pressure, and is powered by an osmotic pump.
Sucrose is the solute that is osmotically active. It is pumped
from photosynthetically active cells into sieve tubes of small,
minor veins. Accumulation of sucrose in sieve tube cells pulls
water into the cells by osmosis. That increases hydrostatic
pressure at a source of sucrose. That initiates flow.
Flow directs the water and sucrose to areas where sucrose is in
low concentration (a sink). At the sink, sucrose is removed
from sieve tubes (and water, as well) by companion cells. That
decreases hydrostatic pressure in the sieve tube at the sink, so
that a difference in pressure is maintained, and flow continues
from the source to the sink.
The same tissue can be a sink at one time and a source at
another, e.g.
• a young, growing leaf starts out as a sink; once mature its
sucrose is exported and it is a source
• carrots are biennial plants – they complete their life cycle
over two years. In year 1 the root is a sink, a storage
organ for starch and sugar in its parenchyma cells. In
year 2, when the shoot starts to bolt and flower, the root
becomes a source.
This diagram does not incorporate the importance of
companion cells in sieve tube unloading.
Photosynthesis
Photosynthesis involves two sets of reactions: the light
reactions and ‘dark’ reactions (that are otherwise called the
Calvin cycle).
The Light Reactions
These reactions occur on the thylakoid membranes of
chloroplasts. There are two photosystems involved, named,
logically enough, Photosystem I and Photosystem II.
Each of these photosystems contains proteins complexed with
cholorophyll pigments, and photosystem II also contains
carotenoids.
The chlorophyll and carotenoids are organized into light
harvesting complexes. They trap photons.
The energy of the trapped photon excites a chlorophyll a
molecule and, through what is called resonance, that energy is
transferred to a Photosystem reaction centre.
Either directly (if the photon excited Photoystem I) or
indirectly via electron transport (if the photon excited
Photosystem II), light energy is converted into electron energy
that is used in electron transport to NADP to reduce it to
NADPH, splitting water and releasing an electron and oxygen.
Showing both photosystems I and II…in addition to reducing
NADP to NADPH (photosystem I), ATP is produced
‘directly’when light excites photosystem II.
Fd – ferredoxin
Pq – plastoquinone
Pc - plastocyanin
The diagram on the previous slide showed the excitation of
Photosystem II caused P680 to become oxidized (lose an
electron). That electron is replaced by one from water (as part
of splitting water into hydrogen and oxygen). That electron is
passed through a series of carriers, losing some energy in each
step. The labeled sequence of acceptors are Pq
(plastiquinone), a cytochrome complex and plastocyanin.
That energy is captured (partly) in the formation of an ATP
molecule from ADP (called photophosphorylation).
The electron is eventually passed to an oxidized P700 of
photosystem I. When P700 was itself excited by light, it was
oxidized, and passed an electron through a series of acceptors,
with the energy used to reduce NADP to NADPH.
This whole process is non-cyclic photophosphorylation.
Just to make it all more complicated, photosystem I can
function independent of photosystem II, in cyclic
photophosphorylation. This sequence of electron transfer
produces ATP directly, rather than forming NADPH.
The energy captured in ATP and NADPH is used to drive the
chemical reactions of the Calvin-Benson cycle.
Melvin Calvin, from the University of California, won a Nobel
Prize for the ‘discovery’ and description of the dark reactions
(meaning not light requiring) of photosynthesis.
Here’s one diagram of the process.
The molecules involved:
RuBP – ribulose biphosphate
PGA – phosphoglyceric acid
PGAL – phosphoglyceraldehyde
rubisco – ribulose biphosphate
carboxylase
The steps of the Calvin cycle are:
1. Fixation of CO2 by enzymatically adding a carbon to
ribulose 1,5 biphosphate. The enzyme is rubisco (ribulose
biphosphate carboxylase. Rubisco is a common protein in
photosynthetic plants, representing from 1/8 to 1/4 of total
leaf protein.
2. The 6-carbon molecule formed is unstable, and very rapidly
splits into two 3-carbon molecules of phosphoglyceric acid
(PGA).
3. PGA is modified enzymatically (with the energy input from
one NADPH and one ATP from the light reactions) into two
molecules of glyceraldehyde phosphate (PGAL). Most of
the PGAL (10 out of every 12) is used to regenerate RuBP.
That makes the series of reactions cyclic.
4. The other two PGAL are re-combined enzymatically to
form a 6-carbon sugar, fructose 1,6 biphosphate. That sugar
molecule is converted rapidly to glucose, which is, in turn,
converted into sucrose or starch.
The Calvin-Benson cycle is universal in photosynthetic plants.
However, as you already know, there are alternatives in carbon
fixation.
In C4 photosynthesis, the initial carbon fixation step uses PEP
carboxylase to attach a carbon from CO2 to phosphoenolpyruvate, a 3-carbon molecule, to form a 4-carbon molecule,
oxaloacetate. There are then a cycle of reactions during which
a CO2 is passed to the Calvin cycle. Note the location of these
steps within the leaf. This mode of carbon fixation is called the
Hatch-Slack pathway.
CAM carbon fixation occurs as in the Hatch-Slack pathway,
fixing carbon into 4-carbon acids. The difference is in timing
and location. CAM plants fix carbon at night in the same
mesophyll cells that undergo light reactions during day.
This is the C4 pathway.
Carbon fixation in the
mesophyll, but Calvin
cycle reactions in the
bundle sheath.
In CAM, both light and
dark reactions occur in
mesophyll, but carbon
fixation is limited to
nighttime hours.
Photorespiration
Some of the CO2 fixed in photosynthesis is lost to
photorespiration.
The actions of rubisco depend on the relative concentrations of
CO2 and O2 in the leaf. When CO2 is high, rubisco acts to
catalyze the addition of CO2 to RuBP. However, when O2 is
high and CO2 low, rubisco catalyzes the addition of O2 to
RuBP. Eventually, CO2 is formed, but without formation of
ATP or NADPH.
This occurs in C3 plants, but not in C4 plants, and, on a hot
day, may cost a C3 plant as much as 50% of fixed carbon, at a
high energy cost. On cooler days (or in cooler climates) when
photorespiration is low or unlikely to occur, C3 plants are more
efficient (expend less energy) to fix CO2.
Photorespiration is, therefore, a key factor in explaining the
distributions of C3 and C4 species.
Cellular Respiration
All living organisms use energy, and form ATP to use in the
many enzymatic reactions involved in molecular synthesis.
The basic steps are glycolysis and the reactions of the Krebs
cycle.
Glycolysis means the splitting (lysis) of sugars. The usual
steps are to split a glucose molecule into two three-carbon
glyceraldehyde phosphate, then convert those molecules into
pyruvate molecules. The net energy-yielding result is 2 ATP
and 2 NADH being formed.
The pyruvate is transferred into mitochondria, where the
reactions of the Krebs cycle occur. The details (which you
were probably forced to learn in high school biology) are not
critical. What is important is the energy result of the cycle of
reactions.
The two molecules of pyruvate that are produced from one
molecule of glucose yield 2 ATP, 8 NADH and 2 FADH2 in
being carried through the Krebs cycle.
These energy-rich molecules are passed to the electron
transport system of the mitochondria, where more ATP is
produced (24 ATP from 8 NADH). The process is called
oxidative phosphorylation.
The Krebs cycle is diagrammed on the next slide.
And here is a diagram of the electron transport chain. These
proteins are all located on the internal membrane (the crista)
of the mitochondrion.
There are many more important components of plant
physiology, some of which may arise later in the semester
(e.g. plant hormones). This has been a “bare bones” treatment
of a few basics.