ATP

ATP is “energy currency” of the cell because the PO4 groups carry negative charges. The repulsion between these negatively charged components is essentially tearing the molecule apart due to repulsion. Why rely on such an unstable molecule for such a critical function? Much like cutting the rope during a “tug of war” when both teams are exerting maximum effort, breaking the bonds between the last two PO4 groups requires a small energy input with a great energy output that can be coupled with an otherwise unfavorable reaction. ATPADP +Pi is highly exergonic.

Remember that several molecules store energy (lipids, carbohydrates, etc.) but this energy must be converted to ATP before it is available to be used by the cell. ATP is the primary “energy currency” of the cell for the short term. Sugars and lipids are more stable and are thus better for longer term storage as glycogen (carbohydrate in the liver), fat, etc.

ENERGY AND ELECTRONS

Electrons may be found at different distances from the nucleus in energy levels. Farther from nucleus = higher energy level = higher amount of energy. Energy is released as electrons move down an energy level (and vice versa) If electrons are transferred from one substance to another, energy is transferred as well. This is a REDOX reaction (remember OILRIG: Oxidation is losing Reduction is gaining).

**The main idea of cellular respiration is that energy found in the electrons from the food we eat can be transferred through a series of “step down” redox reactions to eventually be used to join ADP +Pi yielding ATP.

Cellular respiration Step 1: Glycolysis

Activation energy input. The cell uses 2 molecules of ATP as activation energy to rearrange the glucose molecule into another 6-carbon molecule called fructose diphosphate (aka fructose bisphosphate) which can be split into two 3-carbon molecules.

Splitting the fructose. The fructose bisphosphate can be split into two 3-carbon molecules of PGAL (G-3-P). Energy can be harvested easily from PGAL.

Harvesting the energy. The energy is captured: 2 molecules of ADP are used to create 2 molecules of ATP. This is referred to as substrate level phosphorylation.

2 more ADP and 2 NAD+ molecules are used to make 2 molecules of NADH and 2 additional molecules of ATP

2 pyruvate (pyruvic acid) molecules remain, and these pyruvate molecules contain most of the original energy that was present in the original glucose molecule. NOTE: The purpose of aerobic cellular respiration is to harvest as much of the energy in the two 3-carbon pyruvate molecules as possible.

Summary: Glyco- (sugar or glucose); -lysis (break down). Glycolysis does not require oxygen, it occurs in the cytoplasm of the cell, and it is the one metabolic pathway that is found in all living organisms. Four molecules of ATP are produced in glycolysis, but two ATP’s must be used in the activation energy input. The net yield is only 2 ATP’s. If oxygen is not present, anaerobic fermentation reactions allow glycolysis to continue to produce ATP by recycling NADH to NAD+ molecules.

Cellular Respiration Step 2: Oxidation of Pyruvate and the Citric Acid Cycle

Prior to entering the Krebs Cycle, each pyruvate molecule moves from the cytoplasm of the cell to the matrix of a mitochondrion where the pyruvate molecule loses two electrons and a hydrogen to NAD to form NADH (electron carrier). At this point one of the carbons has been depleted of any useful energy, and it is removed as CO2. The remaining two carbons form an energy-rich acetyl group. This acetyl group unites with a coenzyme called Coenzyme A (CoA) to form acetyl-CoA. It is actually acetyl-Co-A that enters the Krebs Cycle.

As each of the two acetyl-CoA molecules enters the Krebs Cycle, it is joined to a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule called citrate (citric acid).

The purpose of the Kreb’s Cycle is to remove electrons and hydrogen ions from the citrate, joining the electrons and hydrogen ions with NAD+ and FAD to form NADH and FADH2 (the molecules that carry the electrons to the electron transport chain).

Follow the electrons: NAD+ + 2e- + H+ NADH Note the cyclic nature: The oxaloacetate must be regenerated as the Krebs cycle proceeds.

The waste CO2 is carried via the bloodstream to the alveoli of the lungs to be exhaled.

Summary: The Krebs cycle takes place in the matrix of the mitochondria and produces 6NADH, 2FADH2, 2ATP, and 4CO2 are produced per glucose. Most of the energy is now contained in the electron carriers NADH and FADH2.

Cellular Respiration Step 3: Electron Transport Chain

This is where most of the ATP production occurs through a stepwise release of energy. The ETC is located on the highly convoluted inner membrane of the mitochondria known as the cristae. This folding as is often the case in biology increases surface area.

Oxidative Phosphorylation: The high-energy electrons from the carrier molecules NADH and FADH2 are passed to a series of membrane-bound protein carrier molecules (proton pumps), each transferring energy to pump H+ into the inner membrane space before passing the electron to the next carrier.

Proton Motive Force (PMF): This increased concentration of H+ ions creates a PMF that pushes the H+ ions through the ATP synthase causing it to rotate.

Chemiosmosis: The energy of this rotation is utilized to phosphorylate ADP resulting in 32 ATP molecules per glucose. The production of ATP via ATP synthase and H+ pumps is called chemiosmosis.

Oxygen the final electron acceptor: The spent electrons are removed by oxygen which combines with the excess hydrogen ions to form water.

Why it’s Called Cellular Respiration:

In cellular respiration, CO2 is produced from the pyruvate and subsequent compounds as the electrons and hydrogen ions are stripped away during the capture of energy. When the energy has been depleted from the carbon, the carbon is removed, along with its associated oxygens, and the CO2 diffuses to the outside of the cell where concentrations are lower. From there the carbon dioxide is moved through the tissue fluid and picked up by the blood to be released from the body at the lungs. Oxygen is an electron acceptor and picks up the energy-depleted electrons that have traveled down the electron transport systems. When electrons have no more energy to contribute to ATP production, they are combined with oxygen and the excess hydrogen ions to form water as a product of aerobic respiration.

Fermentation

Fermentation is an anaerobic respiration process that recycles NAD+ from the NADH that is produced in glycolysis. The conversion of NADH to NAD+ is critical, as it allows glycolysis to continue to change glucose to pyruvate (with a net production of 2 ATP molecules). Fermentation takes place in the cytosol.

Fermentation occurs in two common forms: Alcoholic Fermentation: Converts pyruvate to carbon dioxide and ethanol

o Pyruvate+NADHEthanol+NAD++CO2 o Utilizedinyeastandsomebacteria. o Usedinmakingbreadandwine.

Lactic Acid Fermentation: Converts pyruvate to lactic acid o Pyruvate+NADHLacticacid+NAD++CO2 o Utilizedinanimalcellsandsomebacteria. o Strenuousexerciseresultsinmusclesusinguptheoxygencreatinganoxygendebtandproducing

lactic acid. The resulting decrease in pH causes fatigue in muscles. Lactic acid is later converted back to pyruvate in the liver as the individual breathes heavily to repay the oxygen debt.