Plant Physiology

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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. Transpor
t Youve 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?
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• There are 5 major forces that move water from
  place to place
• 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.
• 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.

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In intercellular spaces
Within cellular cytoplasm
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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.
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How much pressure is involved? To move water to
the top of a 33m elm tree (species doesnt
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.
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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
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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. Lets begin at the leaves and (briefly)
follow the process and forces
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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.
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Heres a digrammatic representation of a cross
section of a yucca leaf
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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
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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.
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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.
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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
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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.
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Now lets 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.
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• 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.

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This diagram does not incorporate the importance
of companion cells in sieve tube unloading.
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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.
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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.
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Showing both photosystems I and IIin addition to
reducing NADP to NADPH (photosystem I), ATP is
produced directlywhen light excites photosystem
II. Fd ferredoxin Pq plastoquinone
Pc - plastocyanin
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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). T
he 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.
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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.
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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. Heres
one diagram of the process.
The molecules involved RuBP ribulose
biphosphate PGA phosphoglyceric acid PGAL
phosphoglyceraldehyde rubisco ribulose
biphosphate carboxylase
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• The steps of the Calvin cycle are
• 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.
• The 6-carbon molecule formed is unstable, and
  very rapidly splits into two 3-carbon molecules
  of phosphoglyceric acid (PGA).
• 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.

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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.
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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 phosphoenol-
pyruvate, 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.
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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.
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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.
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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.
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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.
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And here is a diagram of the electron transport
chain. These proteins are all located on the
internal membrane (the crista) of the
mitochondrion.
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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.