Key Features of Living Things

Splitting and reforming water molecules
  =>  Electron transport chains
Cells and Membranes
DNA and RNA - reproduction and heredity

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:

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 non-cyclic 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 is:
>> Next: Cells and Membranes >>