Converting work to heat is an irreversible process, and the purpose of a heat engine is to reverse the effect partially. Heat engines aim to increase the efficiency of the reversal, that is, maximize the work retrieved from heat. If the efficiency of a heat engine were 100%, it would imply reversing the process completely without introducing any other effect. Thus, it would violate the second law of thermodynamics.
What could be the theoretical limit to the efficiency of a heat engine? The...

The citric acid cycle, also known as the Krebs cycle or TCA cycle, consists of several energy-generating reactions that yield one ATP molecule, three NADH molecules, one FADH2 molecule, and two CO2 molecules.

The citric acid cycle is termed an amphibolic pathway as it operates both anabolically and catabolically. The cyclic reactions balance the flux of the substrates to provide an optimal concentration of NADH and ATP to the cell.
Regulation of Citric Acid Cycle
The citric acid cycle is regulated in several ways, including feedback inhibition, regulation of enzyme activities, and associated anaplerotic or cataplerotic pathways.
The primary substrate of the TCA cycle—acetyl CoA—is...

In aerobic organisms, the citric acid cycle is the second stage of cellular respiration wherein molecules derived from the breakdown of carbohydrates, proteins, and fats are oxidized into carbon dioxide and energy. This process is also known as the tricarboxylic acid (TCA) cycle as the first product of the cycle, citric acid, contains three carboxyl groups in its structure. Alternatively, this cycle is also referred to as the Krebs cycle, in honor of its discoverer Sir Hans Krebs.
The citric...

The heart beats rhythmically in a sequence called the cardiac cycle—a rapid coordination of contraction (systole) and relaxation (diastole).
The Process
Electrical signals—sent from the sinoatrial (SA) node in the right atrial wall to the atrioventricular (AV) node between the right atrium and right ventricle—cause both atria to simultaneously contract. When the signal reaches the AV node, it pauses for approximately a tenth of a second, allowing the atria to contract and...

The electron transport chain or oxidative phosphorylation is an exothermic process in which free energy released during electron transfer reactions is coupled to ATP synthesis. This process is a significant source of energy in aerobic cells, and therefore inhibitors of the electron transport chain can be detrimental to the cell's metabolic processes.
Inhibitors of the electron transport chain
Rotenone, a widely used pesticide, prevents electron transfer from Fe-S cluster to ubiquinone or Q...