Biology 102 - General Biology
Energy Metabolism, Acquiring and Releasing
Photosynthesis, Glycolysis, Krebs Cycle,
Electron Transport Phosphorylation
Metabolism is the sum total of chemical reactions occurring in cells.
It includes both anabolism, the synthesis of the biomolecules (e.g., protein
synthesis, DNA replication, glucose synthesis in plants) and catabolism,
the degradation of molecules usually for the production of energy (e.g.,
glycolysis, Krebs Cycle). Metabolism is carried out by specific enzymes
which catalyze each step of a long series of reactions. The steps in a
metabolic pathway may be linear such as those in glycolysis or cyclic
as in the Krebs Cycle or light independent reactions of photosynthesis.
Each of the thousands of enzymes required for the thousands of metabolic
pathways are coded for in our genes. Each enzyme is specific for its substrates
and each produces unique products. Some metabolic pathways occur in all
or most cells and some are specific to the cell type.
Enzymes can put molecules
together (synthesize) or break molecules apart (degrade)
The Role of Enzymes
The enzymes that carry out all metabolic reactions are protein catalysts.
They speed up the rates of reactions by lowering the energy of activation
of the substrate molecules which they convert to product molecules. Enzymes
are proteins and, therefore, are composed of specific sequences of amino
acids which, in turn, are coded for by genes. Proteins usually have a
globular shape and they have "pockets" into which their substrates fit.
In order to carry out their specific reaction, they change their shape
to put "stress" on the bonds they are to break or the pockets can bring
parts of the substrate molecules into the position to react with one another.
Because enzymes are proteins, they are affected by temperature and pH
(acidity). Both temperature and pH affect the structure of the protein
and, therefore, their function. High temperatures and extreme acid base
changes denature (inactivate) them, they lose their specific shape and
pockets and can no longer do their job. Most enzymes work best at neutral
pH such as that found in the cytosol. However, some proteins work best
in acid environments. Pepsin, a digestive enzyme, functions in the acid
environment of your stomach and the lysosomal enzymes work only within
the acid environment of the lysosome. The lysosomal enzymes are inactivated
if the lysosomal membrane is broken and the enzymes are released into
the cytosol. An example of the effect of temperature is the pigment pattern
of the Siamese cat and Himalayan rabbit. These animals have darker fur
on their extremities because the enzyme that makes the dark pigment works
only at the cooler temperatures found at the animals' extremities. The
enzyme is denatured (inactivated) at the higher temperatures found in
the other parts of the animals' bodies.
This is an enzyme called a dehydrogenase...it
removes H atoms from substrates
All the energy that is required for the synthesis and maintenance
of living cells ultimately comes from the sun. The producers (autotrophs)
are those organisms which can synthesize their own biomolecules and enough
for the consumers, too. The consumers (heterotrophs) such as us and other
animals, rely on the metabolic capabilities of the producers. Thus, the
photosynthetic organisms, mainly plants, supply all of the basic biomolecules.
The recyclers, the fungi and bacteria, break down (degrade) the biomolecules
of dead organisms to carbon dioxide, water, and ammonia,which are then
used by the producers to make more biomolecules.
All organisms require "food" for two reasons: to use as
subunits to build the biomolecules of the organism and to provide energy
(ATP) for the maintenance and activities of the organism. Energy in the
form of ATP is a requirement for all cellular activities. ATP is produced
in the light reactions of photosynthesis (in chloroplasts of photosynthetic
eukaryotic organisms), in glycolysis (in the cytosol of all organisms),
and in aerobic respiration which includes the Krebs Cycle (matrix of mitochondria
and cytoplasm of prokaryotic cells) and electron transport phosphorylation
(in the mitochondria of eukaryotic organisms and on the inner plasma membrane
of prokaryotic cells).
Photosynthesis can be summarized in the equation:
6H2O (water) + 6CO2(carbon dioxide)------->(using
-------> 6O2 (oxygen) + C6H12O6
The energy of the sun is captured by pigments found in photosynthetic
organisms and the energy is captured in such a way as to produce ATP and
reducing power in the form of electrons which are carried by the coenzyme,
NADPH (formed from the vitamin niacin). In the process, the photons split
water into free oxygen and the electrons are captured by the coenzyme,
NADPH. NADPH is then used in the light independent (dark) reactions of
photosynthesis to reduce (add H atoms to) carbon dioxide to form glucose
and later other middle sized biomolecules (also known as carbon fixation).
Absorption of visible light by photosynthetic pigments,
The porphyrin ring in chlorophylls with Mg in the center
(the same ring is in hemoglobin but with Fe in the center)
(see Lecture 1)
The light reactions of photosynthesis occur in the grana (thylakoid membranes)
of the chloroplasts of eukaryotic cells. Chlorophyll a captures the blue-violet
and red wavelengths of light and chlorophyll b, the blue and red-orange
and the accessory pigments capture other wavelengths and pass them to
chlorophyll a in the reaction centers. The pigments are parts of a photosystem.
In the light reactions, light photons from the sun, literally kick electrons
of the pigments up to a higher energy level. These higher energy electrons
are passed to chlorophyll a whose electrons are then passed along an electron
transport system (in the grana membranes) and the energy of the electrons
is captured in the form of ATP, and the electrons and H+ are
captured by NADPH. The electrons captured by NADP are replaced by the
splitting of water into free oxygen, hydrogen ions and electrons (photolysis).
All of the oxygen on this planet comes from the light reactions of photosynthesis.
When photosynthesis first appeared on the earth, its byproduct, oxygen,
presented a problem. Oxygen oxidizes other molecules which causes them
to break down. Soon some organisms "learned" how to utilize
this oxygen to their advantage by using it to completely break down their
"food" molecules to carbon dioxide and water and make many more
ATP molecules in the process!! Some organisms could not handle the oxygen
and dove underground....some still exist and they are the obligate anaerobes
who are killed by oxygen. Botulism is due to the action of such anaerobic
The light-independent reactions of photosynthesis (or dark reactions)
of the Calvin Cycle can occur in the absence of light. They utilize the
products of the light reactions, ATP and NADPH to reduce carbon dioxide
and to make glucose and the more complex middle sized biomolecules. The
enzymes that carry out these reactions are in the stroma (fluid) portion
of the chloroplast.
Gas exchange in complex plants occurs through the stomata (open pores)
on the underside of the leaves. Carbon dioxide enters and oxygen leaves
via these pores.
Glycolysis and the Krebs
cycle/Citric Acid Cycle are at the center of energy metabolism
Aerobic Respiration: C6H12O6
(glucose) + O2 ---------->
CO2(carbon dioxide) + H2O (water)
+ ATP (energy)
Glycolysis is the first part of aerobic respiration:
C6H12O6(glucose) --->pyruvate (or
lactate or ethanol and CO2)+ATP
While only some organisms carry out photosynthesis, all cells carry out
glycolysis. Glycolysis literally means the breakdown (lysis) of glucose
from glycogen. The series of reactions involved in glycolysis is carried
out by enzymes that are in the cytosol of all cells, both prokaryotic
and eukaryotic cells. Glucose, a six-carbon molecule, is broken down to
pyruvate, a three-carbon molecule. A net of 2 ATP molecules is produced
per glucose molecule that enters glycolysis. If the cell operates anaerobically,
the pyruvate is converted to lactate or ethanol but if the cell is aerobic
and oxygen is present, the pyruvate is converted to acetyl CoA. The pyruvate
(a 3 carbon molecule) produced from glycolysis is reduced by NADP and
after a CO2 is released, the two carbon molecule, acetyl CoA
is formed and enters the Krebs Cycle. While a net of only 2 ATP molecules
are produced by glycolysis, a total of 36 ATP molecules is produced when
glucose undergoes complete oxidation via the Citric Acid Cycle/Krebs Cycle
and is ultimately broken down completely to CO2 and H2O
in the mitochondria.
Aerobic Respiration: Krebs Cycle/Citric Acid Cycle and
oxidative phosphorylation via the mitochondrial electron transport system
pyruvate + O2-------------> CO2+
H2O + ATP
In eukaryotic cells the enzymes of the Krebs Cycle take pyruvate and
break it down to CO2 and water. The enzymes that carry out
the Krebs Cycle are in the matrix (fluid) portion of the mitochondria.
In prokaryotic cells, the Krebs Cycle enzymes are in the cytosol. Two
more ATPs are formed as pyruvate is oxidized to carbon dioxide. (The Krebs
Cycle, as the name implies is a cyclic series of reactions unlike those
of glycolysis which are linear.) In the oxidation of pyruvate through
the Krebs Cycle, NADH (a close coenzyme relative of NADPH) is also produced
as well as another related coenzyme, FADH. Both these coenzymes carry
hydrogen ions (H+) and electrons over to the inner membrane
of the mitochondria where electron transport phosphorylation occurs. As
the electrons are handed down a series of proteins (cytochromes which
contain iron, Fe++), ATP is produced and ultimately, at the
end of the line, the electrons are accepted by O2. So the oxygen
you breathe is the ultimate electron acceptor in these series of oxidations.
The oxygen becomes O- and combines with the H+ to
form H2O (metabolic water). In prokaryotic cells, the proteins
that perform electron transport phosphorylation are embedded in the inner
side of the plasma membrane. This is consistent with the idea that mitochondria
originated from aerobic bacteria that were endocytosed.
Mitochondrial Electron Transport System
This image depicts one
of the many OXPHOS systems embedded in the inner mitochondrial membrane.
These are the sites of ATP generation and the utilization of oxygen as
the ultimate electron acceptor.