Energy

Daily Energy Requirements

Molecules differ in the amount of energy they contain per gram:

Chemical Energy

The direction and extent of a reaction is determined by its enthalpy (H) and entropy (S), which together define free energy (G). Reactions with a negative G have a net loss of energy and will proceed spontaneously, while positive G reactions require energy to occur.

Reactions that have a large positive G can be coupled to a second process with a large negative G, such as the hydrolysis of ATP.

Chemical energy is stored in the bonds of various molecules.

ATP

Adenosine triphosphate, or ATP, is one of the most ubiquitous forms of stored energy in the cell. Each of ATP's terminal phosphate groups has a large, negative G value of -7300 cal/mol, making them high energy phosphate bonds.

The ratio of ATP:ADP is usually close to 3 in a cell, meaning a steady supply of stored energy is usually available.

NAD+/NADH

Nicotinamide Adenine Dinucleotide, or NAD, is present in the cell at a ratio of NAD+:NADH at 600-1200. This ratio favors an oxidative role for NAD+.

When the energy-rich coenzyme NADH is produced, as occurs during glycolysis or the TCA cycle, it donates electrons to oxygen and the electron transport chain during the production of ATP.

The transport of electrons from NADH to oxygen in the inner mitochondrial membrane produces 52,000 cal. Each ATP requires 7,300 cal to be formed from ADP + Pi, and as each NADH can produce 3 ATP, 3 x 7,300 is 21,900 cal. The free energy not trapped in ATP is released as heat.

2 NADH molecules are produced during glycolysis.

An accumulation of NADH, which means the TCA cycle is unable to cope with energy demands, provides negative feedback for various steps of carbohydrate metabolism. Lactate dehydrogenase is activated by increased NADH, shunting pyruvate away from the TCA cycle and into lactate.

NADP+/NADPH

NADPH differs from NADH only by the presence of a phosphate on one of its two ribose units, but thise change allows it to interact with NADP+-specific enzymes.

NADPH is made primarily through the pentose phosphate pathway through the oxidation of glucose 6-phosphate. The ratio of NADP+:NADPH is 0.002-0.01. This ratio is maintained through the continual removal of products, including carbon dioxide, from the reaction and favors the use of NADPH in reductive biosynthetic reactions.

NADPH is particularly in the liver and lactating mammary glands, where it is used for fatty acid synthesis, in the adrenal cortex, where it is used for steroid hormone production, and in RBCs, where it is used to keep glutathione reduced.

NADPH is also used in the respiratory burst in phagocytic cells during pathogen destruction.

A lack of NADPH can cause serious disease, as seen in patients with G6PD deficiency.