Concepts from previous chapters will be revisited as we look at the process of cellular respiration. Some of those concepts include:

Chemical bonds and electron transfers – this is how energy is moved from place to place in the cell.

Enzyme function – to catalyze each step in the respiration process

Cellular structures – specifically the mitochondria

Membrane structure and properties – many of the reactions we discuss occur at membrane surfaces.

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Cellular respiration is the process of using O₂ to break down glucose to obtain energy. When O₂ is readily available it is called an aerobic environment, and much energy in the form of ATP can be captured by the cell’s metabolism of glucose. When O₂ is not available to the cell, then the cell must convert to an anaerobic means of metabolism called fermentation. There are various types of fermentation but we will primarily focus on alcoholic fermentation and lactic acid fermentation.

Approximately 40% of the total energy carried in the chemical bonds of a glucose molecule can be captured in ATP; the rest is lost as heat. (Remember the 2^nd law of thermodynamics?) Nevertheless, living cells are more efficient at performing those energy conversions than many other means (e.g. a car engine which is ~25% efficient).

Electrons can move from one molecule to another and will move only when there is some amount of energy released because that makes the molecule more stable.

Three major stages of cellular respiration:

Glycolysis

Kreb’s cycle

Electron transport chain.

Glycolysis – a series of redox reactions resulting in the breakdown of one glucose into two pyruvic acid molecules. (NOTE: By the end of the chapter, you should be able to determine which ATPs are made by substrate level phosphorylation and which ones are made by chemiosmosis in glycolysis and Kreb’s cycle.) Glycolysis occurs in the cytoplasm of the cell.

Kreb’s Citric Acid Cycle – Before molecules of pyruvic acid can move into the Kreb’s cycle to be further oxidized, the pyruvic acid must first be converted (by reaction with Coenzyme A) into Acetyl CoA—this is the first molecule that enters the Kreb’s cycle. Throughout the series of reactions, CO2 is released and several molecules of NADH or FADH₂ are reduced. These play a critical role in the next stage of respiration, the electron transport chain. These chemical reactions occur in the matrix of the mitochondria.

Electron Transport Chain/Chemiosmosis – This is where the majority of ATP molecules are synthesized for cellular respiration. Importantly, the mitochondrial membrane is involved with this series of redox reactions. The reactions can be likened to a "bucket brigade" whereby the proteins in the membrane use the energy released from the electrons to push H+ ions from the matrix side of the membrane to the intermembrane space of the mitochondrion. The accumulation of H+ in the intermembrane space generates a concentration gradient. The gradient can only be relieved when H+ is allowed to cross back to the matrix side of the membrane through a protein channel, called ATP synthase. This protein is an enzyme which uses the potential energy of the H+ gradient to generate ATP.

Notice at the end of the "bucket brigade" of proteins that there must be a final electron acceptor. Oxygen serves as that electron acceptor under aerobic conditions. The combination of oxygen, electrons from the transport chain and H+ from the chemiosmosis produces H₂O. When oxygen is not available, the electron transport chain ‘backs up’ since there is no final electron acceptor. In these anaerobic conditions, the cell has to switch to some form of fermentation.

Alcoholic fermentation. Irreversible. Pyruvic acid from glycolysis is converted to ethanol.

Lactic acid fermentation. Reversible. Pyruvic acid from glycolysis is converted into lactic acid which can be re-converted into pyruvic acid once conditions become more aerobic.