PHOTOSYNTHESIS and
RESPIRATION
Trapping the sun's energy by making
glucose: photosynthesis
If we are going to
follow the flow of energy through living cells, we have to begin at the very beginning. The energy which drives the biosphere comes from
the sun, where extremely high temperatures (15,000,000 o K) in the interior
lead to the fusion of hydrogen atoms to
form the heavier element helium. This
process is accompanied by the release of energy in the form of electromagnetic radiation. During the process of photosynthesis, this
energy from the sun is used to build glucose molecules out of carbon dioxide from the
atmosphere.
Photosynthesis takes
place within the cells of autotrophic organisms (both prokaryotic and eukaryotic) in a
special organelle called the chloroplast.
There are two very different stages in the process of photosynthesis: the first requires light, and the second does not. The light-dependent stage involves special
light-absorbing chlorophyll molecules
arranged in clusters called photosystems.
These molecules absorb photons (little packets of energy) from the sun, and then use the
energy to boost electrons from chlorophyll up to a higher potential energy level,
where they are then "captured" by an “electron acceptor” molecule. After capturing these high-energy electrons,
the electron acceptor transfers them (in pairs) to the first molecule in a chain
of molecules called an electron transport chain. Each molecule in the chain accepts a pair of
electrons from the previous molecule and then donates them to the next molecule in the
chain in a series of oxidation-reduction reactions, until finally, the
electrons are donated to a second, slightly different photosystem.
What is the point of all this activity? Well,
with each transfer of electrons within the electron transport chain, energy is given off. Some of this energy is lost as unusable energy
(entropy), but some of the energy can be used to do work. This work involves transporting H+ ions
across a membrane in the chloroplast. Once
across, the H+ ions are trapped, since the membrane itself is not very
permeable to H+ ions. This process
leads to high concentrations of H+ ions on one side of the membrane, creating a
buildup of pressure (like water behind a dam).
The only way for the
H+ ions to diffuse back from the area of high concentration on one side of this
membrane to the area of low concentration on the other side of the membrane is through
special channels that are part of an enzyme called an ATPase.
The ATPase enzyme catalyzes the addition of a phosphate group to the
molecule ADP to form the molecule ATP , an endergonic reaction. The energy supplied by the pressure of the H+
ions pushing back through the ATPase channels is used by the enzyme to drive the
reaction, again, like water turning a water wheel.
This is only part of
what is happening during the light-dependent phase of photosynthesis, though. Meanwhile, the second photosystem is also
absorbing light energy, which boosts electrons to a higher potential energy
level, where they are captured by a different acceptor molecule. Pairs of electrons from this acceptor are then
donated to a molecule called NADP+,
forming the molecule NADPH (another
endergonic reaction).
So, at the
end of the light-dependent stage, energy from the sun has been stored in the molecules ATP
and NADPH. The best current estimate
is that it takes 4 photons to produce one molecule of ATP and one molecule of NADPH. There is only one more thing: if you’ve been keeping track, you’ve
noticed that the first photosystem ends up short a pair of electrons, which must be
replaced before the process of energy absorption can begin again. This problem is solved by the breaking
down of a molecule of water to form an oxygen atom (which combines with another oxygen
atom and then is given off by the plant) and 2 hydrogen atoms. An electron is then donated to the photosysem by
each of the hydrogen atoms, leaving them as hydrogen ions (H+). Thus, the photosystem is reset for another round
of energy transfer.
Now the light-independent stage can begin. In this stage, the energy now trapped in
the molecular bonds of ATP and NADPH is further repackaged and stored in the bonds of the
molecule glucose. This series of reactions begins with the addition
of a molecule of CO2 to a 5-carbon sugar molecule called RuBP. The
resulting 6-carbon molecule is then broken into two identical 3-carbon molecules. This series of endergonic steps rely on
the energy released by the breakdown of ATP to ADP and phosphate and the oxidation of NADH
to NAD in order to proceed. The
steps are repeated six times, resulting in the formation of twelve of these 3-carbon
molecules. Ten of these molecules are
rearranged into six RuBP molecules (to replace the ones that were used up) and the
remaining two are put together to build a molecule of glucose. A total of eighteen molecules of ATP and
twelve of NADH are required to supply the energy to form one molecule of glucose.
Repackaging the energy --the
mitochondria
Regardless of
whether cells can build their own glucose molecules through photosynthesis or whether they
need to acquire glucose through their diet, they need to be able to break the
glucose down again and release the energy stored within. In a sense, the cell
needs to "make change", to repackage the large amount of energy in the glucose
molecule in smaller units that are easier to "spend". During the process of respiration, the energy in glucose is repackaged
into the bonds of the molecule ATP. The
energy released in the exergonic reaction that occurs when a phosphate group is
transferred from ATP to other molecules can then be used directly by the cell.
In many cells,
respiration is associated with organelles called mitochondria,
and like photosynthesis, respiration occurs in stages. The first stage is called glycolysis, and takes place outside of
the mitochondria. During glycolysis, a
molecule of glucose (which has 6 carbons) is broken down through a series of
enzyme-catalyzed chemical reactions into two 3-carbon molecules called pyruvate. Some
of the first steps in this process are endergonic and must be coupled to the breakdown of
two molecules of ATP in order to proceed. Later
steps are exergonic, however, and are used to generate ATP, and also to reduce the
electron-carrying molecule NAD+ to NADH. The
net result is that for each molecule of glucose, two molecules of pyruvate, two molecules
of NADH, and two molecules of ATP are made.
The two molecules of
ATP that are produced during glycolysis can be used immediately by the cell, but the other
products of glycolysis must continue through into the next stage of respiration, the Krebs cycle, which takes
place inside the mitochondria. There is a
special carrier molecule which carries the pyruvate molecules into the mitochondria, but
the NADH molecules can't enter and instead transfer their electrons through the membrane
to another electron carrier, FADH2, which is found inside the mitochondria. In the Krebs cycle, a pyruvate molecule
combines with a molecule called CoA, losing
one of its carbons (in the form of CO2)
in the process. The new
molecule formed by this reaction then combines with another molecule called oxaloacetate, and the resulting molecule
undergoes a series of enzyme-catalyzed reactions during which the remaining two carbons of
the original pyruvate are also given off in the form of CO2 . Thus, the 3-carbon pyruvate molecule is totally
consumed. At the end of the series of
reactions, oxaloacetate and CoA are regenerated, and the process can begin again with
another molecule of pyruvate.
Most steps
in this cycle are exergonic, and the excess energy which is released is captured by
coupling the exergonic reactions to endergonic oxidation-reduction and phosphorylation
reactions. The bottom line is that
for each molecule of pyruvate that moves through the cycle, enough energy is released to
drive the formation of four molecules of NADH, one molecule of FADH2, and one
molecule of ATP. Thus, the two pyruvates
produced from a molecule of glucose during glycolysis can produce a total of eight
molecules of NADH, two molecules of FADH2, and two molecules of ATP.
Now, the NADH and
FADH2 molecules must undergo one more process.
The energy in these molecules, along with the energy from the NADH made during
glycolysis is repackaged into ATP during the third stage of respiration: oxidative
phosphorylation. Oxidative phosphorylation takes place
within mitochondria, organelles separated from the cytoplasm by a double membrane
system. The outer membrane is permeable to
small molecules (containing, as it does, the transmembrane protein porin) while the inner,
folded, membrane is impermeable to ions. There
are, however, specific transport proteins operating across the inner membrane.
During oxidative
phosphorylation, a series
of oxidation-reduction reactions take place, beginning with the oxidation of NADH
or FADH2 and reduction of either the NADH-Q reductase complex or ubiquinone
(coenzyme Q), respectively. Electrons are
then passed down an electron transport chain from NADH-Q reductase to
ubiquinone to ubiquitol-cytochrome c reductase to cytochrome c to cytochrome oxidase to
molecular oxygen (where they combine with the oxygen and a couple of H+ ions to form water. While most of these constituents are tightly
membrane-bound, cytochrome c is not. Cytochrome
c consists of a single 104 residue chain wrapped around a covalently attached heme group. This molecule is ubiquitous, and varies little
between different eukaryotic species.
As electrons pass down the electron transport chain, H+
ions are pumped from inside the mitochondria into the space in between the inner and outer
membranes. These ions cannot pass
back through the membrane, and instead must flow back through channels which are part of
enzymes called ATPases (just like in photosynthesis).
The force of the ions pushing back through the channels supplies the energy
necessary to make ATP. Each molecule
of FADH2 can produce two molecules of ATP, while each molecule of NADH can
produce three. Thus, the two NADH molecules
from glycolysis that handed their pair of electrons off to two FADH2 molecules,
along with the two FADH2 molecules from the Krebs cycle can produce a total of
eight molecules of ATP; and the eight NADH molecules from the Krebs cycle can produce
twenty-four molecules of ATP for a total of thirty-two molecules of ATP.
So what is the sum
total of all of this? The cell begins
with one molecule of glucose, and ends up with thirty-six molecules of ATP (two from
glycolysis, two from the Krebs cycle, and thirty-two from oxidative phosphorylation. This means that about 40 % of the energy that was
originally stored in the glucose molecule is now stored instead in ATP, and is available
for the cell to use.
Control of energy metabolism
During the processes of glycolysis and the Krebs cycle exergonic reactions
are used to drive the endergonic reactions that lead to the creation of the molecules ATP,
NADH, and FADH2. Then, during
oxidative phosphorylation, NADH and FADH2 are reoxidized and the energy
released in that process is used to drive the formation of more ATP. Let’s look at the control of these processes
in a little more detail.
First,
let’s look at the control of glycolysis. During
this process, one glucose molecule is converted into two pyruvate molecules. Other sugars, such as fructose and galactose, can
also enter this pathway (fructose through phosphorylation and conversion to
dihydroxyacetone phosphate and glyceraldehydes 3-phosphate; galactose through conversion
to glucose 1-phosphate). The keys to
control of the glycolytic pathway are the enzymes phosphofructokinase, hexokinase, and
pyruvate kinase.
Activity of
phosphofructokinase is inhibited by a high ratio of ATP/AMP and also by decreases in pH
and by citrate (an intermediate in the Krebs cycle).
Phosphofructokinase is, on the other hand, activated by fructose
2,6-bisphosphate (which is formed when fructose 6-phosphate and glucose are abundant). Hexokinase is inhibited by glucose 6-phosphate;
pyruvate kinase is inhibited by ATP and activated by fructose 1,6-bisphosphate.
Control of the citric acid or Krebs
cycle resides in three sites: the inhibition
of citrate synthase by ATP, stimulation of isocitrate dehydrogenase by ADP and inhibition
by NADH and ATP, and finally inhibition of a-ketoglutarate dehydrogenase by succinyl coA
and NADH.
Regulation of the rate of
oxidative phosphorylation is tied to levels of ADP. Rate of oxygen consumption is stimulated by
addition of ADP and slowed by ADP depletion. This
is because of tight coupling between the activity of the ATPase enzyme and electron
transport. Some organisms can uncouple these
two processes in order to produce heat through the use of thermogenin, an intrinsic
uncoupling protein.
Respiration without oxygen
Oxygen appears to
play a small part in respiration, serving only to accept the electrons coming off the
electron transport chain during oxidative phosphorylation.
That small role is critical, however, for without oxygen to accept those electrons,
the transport chain would become "backed up" with electrons and would cease to
function. Then, no H+ ions would
be transported, and no ATP would be made.
There are, though,
ways for cells to bypass this oxygen-requiring (or aerobic)
system. Some single-celled organisms as well
as some groups of cells in multi-cellular organisms can carry out a process called anaerobic respiration, or in other words,
respiration which does not require oxygen.
Anaerobic
respiration begins, as does aerobic respiration, with glycolysis. However, instead of entering into the Krebs
cycle, the pyruvate produced during anaerobic respiration undergoes a series of chemical
reactions known as fermentation. During
fermentation, pyruvate is converted by some types of cells (yeasts and bacteria, for
example) into ethyl alcohol, and by other types of cells (such as muscle cells) into
lactic acid. The net result of
fermentation is only two ATP molecules per molecule of glucose. Thus, anaerobic respiration is a much less
efficient process than aerobic respiration