Energy Production

Oxidation-Reduction Reactions

Oxidation is the removal of one or more electrons from a substrate.  Protons (H⁺) are often removed with the electrons.

Reduction of a substrate refers to its gain of one or more electrons.

Each time a substance is oxidized, another is simultaneously reduced.

NAD⁺ is the oxidized form; NADH is the reduced form.

Glucose is a reduced molecule; energy is released during a cell’s oxidation of glucose.

The Generation of ATP

Energy released during certain metabolic reactions can be trapped to form ATP from ADP and phosphate.  Addition of a phosphate to a molecule is called phosphorylation.

During substrate-level phosphorylation, a high-energy phosphate from an intermediate in catabolism is added to ADP.

During oxidative phosphorylation, energy is released as electrons are passed to a series of electron acceptors (an electron transport chain) and finally to O₂ or another inorganic compound.

During photophosphorylation, energy from light is trapped by chlorophyll, and electrons are passed through a series of electron acceptors (an electron transport chain).  The electron transfer releases energy used for the synthesis of ATP.

Chemiosmosis

The energy from electron transfer through an electron transport chain drives proton pumps. 

The proton pumps move protons to one side of the membrane, increasing the concentration on one side and decreasing the concentration on the other side.

The protons diffuse down their concentration gradient through a membrane channel that has enzymatic (ATPase) activity. 

The energy of the protons moving through the channel drives phosphorylation of ADP to make ATP. 

Or, here is another way to say the same thing:

Generation of ATP

Organisms must regenerate ATP to continue their metabolic activites. There are three basic mechanisms an organism may use to generate ATP.

Mechanisms of ATP Generation

Substrate-level phosphorylation – an enzyme transfers a phosphate group from a donor molecule (the substrate) to ADP to form ATP.

Oxidative phosphorylation – electrons are transferred from organic compounds down an electron transport chain to a final electron acceptor; energy released during electron transfer is harnessed to make ATP by chemiosmosis.

Photophosphorylation – electrons are raised to a higher energy level by light, passed down an electron transport chain to a final electron acceptor, and energy released during electron transfer is harnessed to make ATP by chemiosmosis.

Two more things, both are a part of oxidative phosphorylation and photophosphorylation (call them mini-mechanisms if you want):

Electron transport chain - a series of molecules in a membrane that pick up electrons from carrier molecules like NADH and FADH₂, pass the electrons to sequentially to one another in a series of oxidation-reduction reactions, and use the energy released from the electrons to pump protons across the membrane in which they are embeded.

Chemiosmosis - the proton-motive force generated by the electron transport chain is harnessed by ATP synthase and used to make ATP from ADP and phosphate.

The processes that use the mechanisms are:

Cellular respiration – ATP is generated by oxidation of organic molecules, the passage of electrons down an electron transport chain, and chemiosmosis.  The final electron acceptor is almost always inorganic.

Aerobic – the final electron acceptor is O₂.

 

Anaerobic – the final electron acceptor is some inorganic molecule other than O₂.

Fermentation – ATP is generated by oxidation of organic molecules and the final electron acceptor is an organic molecule.  The electrons produced by oxidation are not used to make ATP.  ATP production is accomplished by substrate-level phosphorylation.

Photosynthesis – ATP is generated by photophosphorylation. 

Cyclic reactions produce ATP.  The electrons come from chlorophyll, pass down an electron transport chain, and return to chlorophyll. 

 

Non-cyclic reactions produce ATP and NADPH.  The electrons come from chlorophyll, travel down an electron transport chain, and are passed to NADP to form NADPH.  Chlorophyll is then reduced by H₂O or some other oxidizable compound like H₂S to replace the lost electrons. 

Here is an overview:

And here is the overview with a little more information (click here)

Metabolic Pathways of Energy Production

A series of enzymatically catalyzed chemical reactions called metabolic pathways store energy in and release energy from organic compounds.