Metabolic Pathways relating to energy: Photosynthesis and Cellular Respiration
The following is loaded with the terminology of metabolism and energetics. I have
underlined or italicized the key terms. The more you use them (read, write, listen, and
speak), the more familiar they will become to you. The concepts are difficult enough
without stumbling over the terminology. Read this as often as necessary until you can
read all the way through without stumbling.
Photosynthesis and Cellular Respiration are very significant metabolic pathways because
in these pathways we can see how energy enters the biosphere, how it is stored, how it is
transferred from one molecule to another, and how it is used. There are countless other
metabolic pathways, but these two are brought into focus because of their significance in
energy. Metabolic pathways are called "pathways" because the reactions involve
numerous steps, each a chemical reaction catalyzed by a specific enzyme. Each step
involves "intermediates" - partially-constructed molecules.
Anabolic pathways: When smaller molecules are built up into larger ones the metabolic
pathway is anabolic. Photosynthesis is an anabolic pathway, because cells are "building"
sugar molecules from carbon dioxide and water. Linking glucose molecules together to
make starch is another anabolic process. Anabolic pathways are endergonic. Ever heard
of anabolic steroids? What is being 'built up' with the help of anabolic steroids?
Catabolic pathways: When large molecules are broken down and their energy is
extracted, the pathway is catabolic. In aerobic cellular respiration, the energy contained
in the bonds of sugars (and amino acids and fatty acids) is extracted by oxidation… all
the way down to carbon dioxide. In digestion, polymers are hydrolyzed into their
monomers. Both are examples of catabolism at different levels, and both are exergonic.
The pathways of cellular respiration have the ultimate goal of extracting energy from
food and transferring it to ATP. It is the same basic process in all life forms from
bacteria to humans. Pretty much all of the enzymes involved are the same across all life,
and that means that the genes (DNA) coding for those enzymes are also the same. As we
will see, there is some wiggle room in the genes coding for the same protein/enzyme, so
by analyzing the sequence of A, T, C, and G in the DNA coding for any specific gene, we
can infer from accumulated changes (mutations) how distantly related any two organisms
are. This topic will have to wait.
Oxidation and Reduction: The energetics pathways involve many reactions that only
occur together (they are 'coupled'). Oxidation involves the loss of one or more electrons
and their energy. An oxidized molecule has less free energy than it had before it was
oxidized. The electrons lost by one molecule must be picked up by another molecule.
When a molecule receives electrons from another, it is reduced, and its free energy is
increased. So whenever a molecule is oxidized, some other molecule is reduced, and vice
versa. Because the two processes are inseparable, we often refer to such reactions as
REDOX (oxidation/reduction) reactions. You "saw" a redox reaction when you tested
reducing sugars with Benedict's reagent. A copper compound is reduced by a sugar
molecule (that gets partially oxidized). So in that reaction, a sugar loses an electron
(oxidized) and the copper compound in Benedict's reagent gains an electron (is reduced).
That's why the test only works for 'reducing' sugars. (Even though the sugar gets
oxidized, it was the source of the electrons that reduced the copper compound…. Thus
"reducing sugar")
There are molecules in both photosynthesis and cellular respiration that are so well-suited
to being reduced, oxidized, reduced, oxidized - in an endless cycle, that they are referred
to as electron carriers. NADH and FADH2 in cellular respiration, and NADPH in
photosynthesis are the reduced forms of the electron carriers. The oxidized forms are
NAD+, FAD++, and NADP+, respectively. All 3 are vitamins in the category of B
vitamins. In addition to being "electron carriers" and vitamins, these molecules are also
referred to as coenzymes, since they work with enzymes to catalyze reactions.
Interestingly, all of these electron carriers are like little tiny pieces of DNA or RNA (as is
ATP). NADH and NADPH are also chemically related to nicotine (the N stands for
nicotinamide). Like the relatedness of organisms that appear to be so different, these
related molecules that are found in every cell provide evidence for the common ancestry
of all life.
Phosphorylation: When a phosphate group (PO4) is bonded to a molecule, the bond is a
high-energy bond and the free energy of the recipient is increased. Most commonly,
"phosphorylation" refers to the bonding of the third phosphate group to ATP (ADP + P >
ATP). The bond between the second and third phosphate groups is the source of most
energy used by organisms to do work. Phosphorylation requires energy from food.
However, in the light-dependent reactions of photosynthesis, ATP is phosphorylated by
the energy from light (photophosphorylation). This ATP is primarily to make sugar, and
then the plant can transport the sugar (not the ATP) to its roots, flowers, fruits, and other
non-photosynthetic organs. In cyanobacteria (the term applied to all of the blue-green
pigmented bacteria of which there are numerous species) the ATP from photosynthesis
can be used for cellular work (some ATP) or to make sugar (most).
Aerobic or anaerobic respiration:
Phosphorylation of ATP in cellular respiration can occur with (aerobic) or without
(anaerobic) oxygen. The yield of ATP is much greater in the aerobic pathways of
respiration. If we go back to the very origin of life on Earth, we are fairly certain that
there was little or no atmospheric or dissolved oxygen. Oxygen is simply too reactive to
exist for a significant amount of time. Today, all atmospheric oxygen is constantly being
regenerated by photosynthesis. So we can infer that phosphorylation of ATP was
primarily anaerobic (w/o oxygen) in the very earliest life forms. On today's earth, we
find microbes (bacteria and archaea) that live in oxygen-free environments, and that are
in fact killed by oxygen - so both anaerobic and aerobic respiration occur today.
At some point - evidence of photosynthetic bacteria dates back 3.8 billion years - the
oxygen-producing (photosynthetic) life forms began to proliferate. Oxygen released into
the water would react quickly. Anaerobes would be safe in their oxygen-free
environments, and the oxygen level was generally very low and localized. Some
organisms most likely evolved that could tolerate oxygen. And in fact, with oxygen the
efficiency of making ATP from food molecules goes up dramatically. (We know that a
single glucose molecule can phosphorylate 2 ATPs without oxygen, and 36 ATPs with
oxygen.) For these oxygen-using and oxygen-tolerant microbes, an 18-fold increase in
fuel efficiency would have created a significant advantage.
Aerobic respiration is such an advantage that all of the multicellular life on the planet
today is both oxygen-tolerant, and oxygen-dependent. As for the microbial world, we
see today a full spectrum of organisms with regard to their need for oxygen and their
ability to tolerate oxygen. Some microbes today are quickly killed by oxygen. These are
anaerobic organisms, and as you might imagine, they are found in anoxic (zero oxygen)
environments.
Oxygen has its drawbacks, even with us. You hear about foods rich in "antioxidants" and that's a good thing. Some vitamins have antioxidant properties. The reason we need
antioxidants is that free radicals of oxygen can disrupt DNA and lead to cancer. So even
though we must have oxygen to live, it is considered to be the #1 cause of cancer.
Let's get back to oxidative (aerobic) respiration. Oxygen exerts a very strong pull on
electrons, and it is the electrons moving through the electron transport system of cellular
respiration that allows us to phosphorylate enough ATP to keep us alive. The process of
producing ATP with oxygen is called oxidative phosphorylation. We can make a little
extra ATP without oxygen, and this can help when we are pushing ourselves to the limit.
The downside is that lactic acid is a byproduct of anaerobic respiration in muscles, and
can build up in muscle tissue resulting in soreness that can last for days.
In photosynthesis, electrons are driven off the chlorphyll molecules and pushed down an
electron transport system. Eventually, they are pulled off the end of the line by NADPH
(see above). The process of producing ATP in the light-dependent reactions of
photosynthesis is called photophosphorylation. This ATP, along with the reduced
NADPH is used to make sugar in the Calvin Cycle (light independent reactions).
Chemiosmosis: There is an electron transport system in both photosynthesis (the light
reactions) and aerobic cellular respiration (the electron transport system). They are
amazingly similar. Both involve the movement of electrons through molecules
embedded in cell membranes (thylakoid membranes in photosynthesis, and mitochondrial
membranes in oxidative respiration). In both, the moving electrons provide the energy to
pump protons (H+) across a membrane against their concentration gradient, and in both,
these protons rush through an enzyme complex called ATP synthase. The rush of protons
through ATP synthase provides the energy for phosphorylation of ATP. That's
chemiosmosis. "Chemiosmotic phosphorylation" is also called photophosphorylation (in
photosynthesis) and oxidative phosphorylation (in aerobic respiration).