Download Photosynthesis and alternate pathways

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,
or to phosphorylate ADP to ATP.
Showing both photosystems I and II…in addition to reducing
NADP to NADPH (photosystem I) ATP is produced when
light excites photosystem II.
The equivalent diagram from the text looks slightly different,
but expresses exactly the same reaction system…
The diagram on the two previous slides 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 Pg
(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 capture of photon energy occurs on the thylakoid
membranes that form the characterstic grana stacks within the
chloroplasts. Once more, your text has an excellent figure…
The entire photochemical reaction occurs incredibly quickly.
From the initial capture of the photon to the formation of
NADPH and or ATP, the process is complete in a few
picoseconds (one picosecond = 10-12 seconds).
What is particularly important to us about photosynthesis is
that it is the process that produces/releases free O2 into the
atmosphere. There was no free oxygen formed before about
2-2.5 billion years ago, when photosynthesis first evolved in
some bacteria.
Even then, there was not enough formed to result in free
oxygen in the atmosphere. That only occurred about 1.5
billion years ago with an increase in the number of
photosynthetic organisms and a saturation of oxygen
absorbers like iron-containing (and magnesium- and other
metal ion containing) minerals.
The Dark Reactions
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 the most 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.
There are limitations to the amount of carbon fixed into
carbohydrates by photosynthesis and the Calvin-Benson
reactions:
1. The process may be limited by the amount of light
available, and in the basic C3 system by the balance between
photosynthesis and photorespiration. What is frequently
measured for plants is the light compensation point. That is
the light level at which photosynthesis and photorespiration
are in balance, and there is 0 net fixation of carbon.
In C3 plants photorespiration competes with photosynthesis
for ribulose biphosphate. In photosynthesis a carbon is
‘added’ and molecules of 3-phosphogycerate are formed. In
photorespiration the same enzyme (rubisco) binds oxygen to
ruBP.
It should be obvious that high O2 concentration in a leaf leads
to higher rates of photorespiration. It is not obvious, but true
that higher temperature also promotes photorespiration.
Photosynthesis releases free oxygen. Photorespiration,
logically enough, releases CO2 as its ‘waste’ product, but
without formation of ATP or NADPH.
Complete photorespiration involves not only the chloroplasts,
but also peroxisomes and mitochondria.
Photorespiration occurs in C3
plants, but not in C4 plants.
On a hot day, photorespiration 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.
Species have evolved physiological differences to parallel
their growth strategies. The light compensation point may
change seasonally (in part by changing amounts of
photosynthetic pigments and rubisco), and the maximum rate
of photosynthesis may also change. The text table presents
results of a study of forest understory herbs that grew (and
flowered) in spring (Allium tricoccum) when there was little
else in full leaf to shade the plant, in summer when light
competition is most severe (Viola pubescens), and a Saxifrage
that grows right through to autumn (Tiarella cordifolia).
Since light competition is so severe, it affects not just
interactions between species and individuals within a species,
but even the evolution of growth patterns within individuals.
Plants present their leaves so as to minimize the amount of
self-shading occurring among those leaves. How?
Among the taxonomic characteristics used to identify plants
are whether their leaves are opposite or adjacent, whether
leaves occur above one another or in whorls, whether the
leaves are entire or various more complicated shapes with
more-or-less finger-like projections. Some tropical plant
leaves even have holes in them. Each of these is a portion of
the adaptive scheme to maximize light capture efficiency and
minimize the cost of maintaining the light capture apparatus.
Leaves in whorls
Opposite leaves
Alternate leaves
Leaves may even have different shapes on the same plant
where they occur in open sun versus in the shade.
A related question is what part of sunlight does the
photosynthetic apparatus absorb? Not all light is
photosynthetically active radiation (or PAR). Leaves look
green because that part of the spectrum is not absorbed, but is
reflected, so that’s what we see.
2. There may be a limitation in the
amount of photo-synthetically active
radiation available. Only two forms of
chlorophyll are important in
photosynthesis – a (the photon capture
pigment for photosynthesis) and b, and
they have radically different absorption
spectra…
Latitude, and the thickness of
atmosphere that light has to penetrate
to reach the earth’s surface, also make
a difference in the relative intensities
of different parts of the light spectrum.
3. Under some circumstances there can also be a limit in the
amount or rate at which leaves can take in CO2. In part this is
due to the limits to diffusion rate of CO2 through the stomates
of the leaf.
One part of the limit is set by leaf conductance, which is the
rate of CO2 movement at a given difference between ambient
CO2 in the atmosphere around the leaf and intercellular
concentration of CO2 within the leaf.
Your text goes into further detail, subdividing the binding
process into compartments that include what goes on inside
the cells where photosynthesis occurs…
Another part of the limit is set by the binding coefficient for
CO2 by the rubisco enzyme, at least for plants with C3
photosynthesis.
There can even be significant variation in photosynthetic rate
across the area of a single leaf. This is logical based on
shading, leaf thickness, and spatial variation in water stress
leading to variation in stomatal opening…
Alternatives to C3 photosynthesis
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.
Carbon fixation only occurs in the mesophyll, using PEP
carboxylase, which has a far higher binding affinity for CO2.
The 4-C products of fixation are passed to the bundle sheath,
where the fixed carbon is moved into the Calvin-Benson
pathway.
C3 and C4 leaf structures are different, as well. C4 plants have
what is called the Krantz anatomy. Chloroplasts are more
densely packed in a ring of mesophyll cells directly
surrounding the bundle sheath, which is frequently protected
by high lignin and silica content in the C4 anatomy. Mesophyll
cells are more loosely packed and chloroplasts more
uniformly distributed in C3 plants.
Many other differences follow from the difference in carbon
fixation and anatomy.
The absence of photorespiration in C4 plants (rubisco is
mostly all in cells buried deep in the leaf surrounding the
bundle sheath) means that net photosynthesis will continue to
increase with leaf temperature to much higher temperatures in
C4s.
The concentration of CO2 in the bundle sheath of C4s is far
higher than in C3s.
Because PEP carboxylase’s binding affinity for CO2 is high,
C4 plants have the ability to continue to fix carbon for a while
even when their stomates are closed. That makes the water
use efficiency of C4s much higher than that for C3s.
That same affinity means that far less PEP carboxylase protein
is needed than the requirement for rubisco in C3s. That results
in a far greater nitrogen use efficiency, as well.
All these things, mostly resulting
from the limitations to using rubisco
to initially fix carbon, mean that C4
plants can continue to increase
photosynthesis as light intensity
increases to much higher light levels
than will saturate C3 plants.
What about CAM (Crassulacean Acid Metabolism) plants?
CAM uses the same pathways as C4, but with much different
localization and adaptations. CAM plants are mostly found in
hot environments and have thick, succulent leaves.
In CAM plants rubisco is distributed throughout the leaf
mesophyll, not just in the bundle sheath. Therefore, they can’t
afford to have their stomates open during the day. They fix
carbon at night with stomates open – it’s cooler and typically
more humid. They have to ‘store’ enough carbon overnight to
carry them through the day, when the light reactions occur.
Since stomates are then closed, the ‘release’ of carbon fixed by
PEP carboxylase overnight means that rubisco works in a high
CO2 – low O2 environment, minimizing any chance of
photorespiration.
So, the C4 system separates carbon fixation and rubisco
spatially (mesophyll versus bundle sheath), while CAM
separates PEP carboxylase and rubisco function temporally
(PEP carboxylase-driven carbon fixation at night versus
rubisco function with stomates closed during daylight).