Electron transport chains - the building and storage of useful energy
Electron transport chains are key components of both respiration and photosynthesis that are
responsible for energy transfer and conversion
Electron transport chains in respiration
During various steps in glycolysis and the citric acid cycle, the oxidation of certain
intermediate precursor molecules causes the reduction of NAD+ to NADH + H+ and FAD to FADH2.
NADH and FADH2 then transfer protons and electrons to the electron transport chain to produce
additional ATPs from oxidative phosphorylation.
During the process of aerobic respiration, coupled oxidation-reduction reactions and electron
carriers are often part of an electron transport chain , a series of electron carriers that
eventually transfers electrons from NADH and FADH2 to oxygen.
The diffusible electron carriers NADH and FADH2 carry hydrogen atoms (protons and electrons)
from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle
to other electron carriers that are embedded in membranes.
These membrane-associated electron carriers include flavoproteins, iron-sulfur proteins,
quinones, and cytochromes.
The last electron carrier in the electron transport chain transfers the electrons to the
terminal electron acceptor, oxygen.
The chemiosmotic theory explains the functioning of electron transport chains.
According to this theory, the tranfer of electrons down an electron transport system through
a series of oxidation-reduction reactions releases energy. This energy allows certain
carriers in the chain to transport hydrogen ions (H+ or protons) across a membrane.
Depending on the type of cell, the electron transport chain may be found in the cytoplasmic
membrane or the inner membrane of mitochondria.
In prokaryotic cells, the protons are transported from the cytoplasm of the bacterium
across the cytoplasmic membrane to the periplasmic space located between the cytoplasmic
membrane and the cell wall.
In eukaryotic cells, protons are transported from the matrix of the mitochondria across
the inner mitochondrial membrane to the intermembrane space located between the inner
and outer mitochondrial membranes.
As the hydrogen ions accumulate on one side of a membrane, the concentration of hydrogen
ions creates an electrochemical gradient or potential difference (voltage) across the
membrane. (The fluid on the side of the membrane where the protons accumulate acquires
a positive charge; the fluid on the opposite side of the membrane is left with a negative
charge.)
The energized state of the membrane as a result of this charge separation is called
proton motive force or PMF.
This proton motive force provides the energy necessary for enzymes called ATP synthases,
also located in the membranes mentioned above, to catalyze the synthesis of ATP from ADP
and phosphate.
This generation of ATP occurs as the protons cross the membrane through
the ATP synthase complexes and re-enter either the bacterial cytoplasm or the matrix of
the mitochondria. (Proton motive force is also used to transport substances across membranes
during active transport and to rotate bacterial flagella.)
At the end of the electron transport chain involved in aerobic respiration, the last
electron carrier in the membrane transfers 2 electrons to half an oxygen molecule
(an oxygen atom) that simultaneously combines with 2 protons from the surrounding medium
to produce water as an end product.
Photosynthetic Electron Transport
The first phase of photosynthesis involves trapping light energy.When sunlight strikes a
plant leaf, the energyof certain wavelengths are absorbed by special photosynthetic pigments.
Each pigment has a characteristic color determined by the light it reflects.
Chlorophyll is the major photosynthetic pigment. Because it reflects green and yellow
light, chlorophyll appears green.
Chlorophyll can be found inside of chloroplasts. Chloroplasts have an inner and an outer
membrane. The inner membrane forms into stacks of disk like structures called thylakoids.
Each thylakoid contains from 200 to 400 molecules of chlorophyll.
Three events unfold during the light-dependent reactions:
First, pigments absorb light energy and give up electrons.
Second, electron and hydrogen transfers lead to ATP and NADPH formation.
Third, the pigments that gave up electrons in the first place get electron replacement.
Outside of the thylakoid membrane is a fluid called the stroma, this is the zone when
sucrose, starch, and other end products of photosynthesis are assembled. When light hits
a chlorophyll molecule, which is assembled upon photosystems, electrons absorb the energy.
There are two ways for the electrons to flow in light-dependent reactions. The most common is
non-cyclic photophosphorylation.
In non-cyclic photophosphorylation there is a one-way flow of electrons, ultimately
from water to NADP+. For every two electrons that enter this pathway, there is an energy
yield of of two ATP molecules and one NADPH molecule.
Plants can also employ cyclic photophosphorylation. For every two electrons that enter
cyclic photophosphorylation, one ATP molecule is synthesized by chemiosmosis. (similar to
respiration), NADPH is not produces and oxygen is not generated.
With cyclic photophosphorylation the pathway is cyclic because excited electrons that
originate from the pigment at the reaction center eventually return to it.
Energized electrons are transferred along an electron transport chain within the
thylakoid membrane.
As they are passed from one acceptor, the electron losses energy. An ATP synthetase enzyme
in the thylakoid membrane uses the energy of the proton gradient to manufacture ATP.
The cycle is completed when the spent electrons are returned to the pigment.
Cyclic photophosphorylation occurs in plant cells when there is too little NADP+ to accept
electrons from ferredoxin.
Inside the thylakoid are protons, but how do they get there?
During the start of the noncyclic pathway of photosynthesis, water molecules are split
into oxygen, protons and electrons (photolysis).
The oxygen is released as an end product and the electrons are sent through
transport systems. The hydrogen ions accumulate inside the thylakoid interior.
Hydrogen ions also accumulate in the compartment when electron transport systems are
operating. (This is true of both the cyclic and noncyclic pathway.)
When certain molecules of the transport system accept electrons, they also pick up
hydrogen ions from the stroma and release them inside the compartment.
The protons cannot diffuse out of the thylakoid membrane is impermeable except at
certain points bridged by an ATP synthetase enzyme.
This complex entends across the thylakoid membrane, projecting from the membrane surface
both inside and outside and forming channels through which protons can leak out of the
thylakoid. The formation of ATP and NADPH are important because they create energy and
nutrients for the plant.
Chloroplasts and Mitochondria
In plants the photosynthetic process occurs inside chloroplasts, which are small
organelles (5-10 microns across) found inside specialized cells.
The chloroplast consists of three membranes, an outer envelope membrane, an inner envelope
membrane, and an internal membrane system, known as the photosynthetic or thylakoid
membrane, which absorbs light, transfers electrons and protons, and produces ATP.
Mitochondria are organelles in animal and plant cells in which oxidative phosphorylation
takes place.
There are many mitochondria in animal tissues; for example, in heart and skeletal muscle,
which require large amounts of energy for mechanical work, in the pancreas, where there is
biosynthesis, and in the kidney, where the process of excretion begins.
Mitochondria have an outer membrane, which allows the passage of most small molecules and
ions, and a highly folded inner membrane (cristae), which does not even allow the passage
of small ions and so maintains a closed space within the cell.
The electron-transferring molecules of the respiratory chain and the enzymes responsible
for ATP synthesis are located in and on this inner membrane, while the space inside (matrix)
contains the enzymes of the TCA cycle.
The enzyme systems primarily responsible for the release and subsequent oxidation of
reducing equivalents are thus closely related so that the reduced coenzymes formed during
catabolism (NADH and FADH) are available as substrates for respiration.
Origin of Chloroplasts and Mitochondria
It has been proposed that Chloroplasts and Mitochondria were derived from primitive
prokaryotes) that were captured by other cells in a symbiotic relationship.
The mitochondria of eukaryotes are thought to have evolved from aerobic bacteria
(probably related to the rickettsias) living within their host cell.
The chloroplasts of eukaryotes are thought to have evolved from endosymbiotic
cyanobacteria (autotrophic prokaryotes).
The evidence for this origin:
# Both mitochondria and chloroplasts can arise only from preexisting mitochondria and
chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes
encode only some of the proteins of which they are made.
# Both mitochondria and chloroplasts have their own genome and it resembles that of
prokaryotes not that of the nuclear genome.
# Both genomes consist of a single circular molecule of DNA. There are no histones
associated with the DNA.
# Both mitochondria and chloroplasts have their own protein-synthesizing machinery,
and it resembles that of prokaryotes not that found in the cytoplasm of eukaryotes.
# Their ribosomal RNA (rRNA) and the structure of their ribosomes resemble those of
prokaryotes, not eukaryotes.
# The first amino acid of their transcripts is always fMet as it is in bacteria
(not methionine [Met] that is the first amino acid in eukaryotic proteins).
# A number of antibiotics (e.g., streptomycin) that act by blocking protein synthesis in
bacteria also block protein synthesis within mitochondria and chloroplasts.
They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.
# Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic
ribosomes do not - sensibly enough - have any effect on bacterial protein synthesis
nor on protein synthesis within mitochondria and chloroplasts.
# The antibiotic rifampicin, which inhibits the RNA polymerase of bacteria, also
inhibits the RNA polymerase within mitochondria.
It has no such effect on the RNA polymerase within the eukaryotic nucleus.