CH 09: Cellular Respiration: Harvesting Chemical Energy

Electron Transport Chain

• These electrons will then release their energy in a controlled fashion by passing through a series of molecules called the electron transport chain (ECT). This prevents an explosive release of energy.
• Each molecule in the chain is slightly more electronegative, thus, the electrons move "down" the chain, releasing some energy each time.
• At the end of the chain, the electrons are accepted by the final acceptor, oxygen.
• Thus, the pattern for energy and electron flow in cellular respiration is:
  • Food --> NADH --> Electron Transport Chain --> Oxygen
• The energy released by this process is what is used to drive the production of ATP in the following fashion.

Aerobic Cellular Respiration

• Occurs in three distinct stages:
  1. Glycolysis
  2. Kreb's (Citric Acid) Cycle
  3. Electron Transport Chain and Oxidative Phosphorylation - the focal point that we will concentrate on that will result in the formation of ATP.
     • Phosphorylation refers to the formation of ATP by combining ADP + P. There are two types that occur in CR:
       • Substrate-level Phosphorylation -- Enzyme transfers a phosphate directly from a substrate to ADP. An enzyme whose active sites fit Phosphate and ADP to make ATP. Lower levels than the next type of phosphorylation.
       • Oxidative Phosphorylation -- Results from the release of energy from electron transport chain. Engery is used to drive the formation of ATP.
         • Process B produces about 90% of the ATP generated in CR.

Glycolysis

• In ALL cells, there is a CR process that occurs known as glycolysis ("process of breaking sugar").
• It occurs in all cells because it makes no difference whether O2 is present or not because O2 is not involved in the reaction.
• Also, it occurs in the cytoplasm, ie, no special organelles are needed.
• It consists of 9 enzyme mediated steps.
• A type of fermentation reaction that does not require oxygen.
• It results in the partial breakdown of sugar.
• It is an endergonic reaction, requiring an initial input of 2 ATP.
• During the series of reactions, one glucose molecule (C6H12O6) is oxidized, forming two 3 carbon molecules known as pyruvic acid or pyruvate (equivalent terms).
• In addition, a total of 4 ATP molecules are generated by substrate-level phosphorylation. Only molecule that is simple enough that will go through the entire process from beginning to end; other biomolecules have to be broken down before entering the CR process.
• Two ATP's were used to start reaction, so there is a net gain of 2 ATP.
• As a result of glycolysis, two electrons are transferred to temporary electron acceptors NAD+, forming two NADH.
• If oxygen is present, the energy stored in NADH can be transferred to the electron transport chain for ATP production via oxidative phosphorylation.
• Occurs in membranes of prokaryotes and within mitochondria in eukaryotes. Will come back to what happens if there is no oxygen present.

Mitochondrion

• The mitochondrion has an outer membrane and a highly folded inner membrane.
• The folds are known as cristae. The electron transport molecules, which are iron containing molecules known as cytochromes are embedded in the cristae. (Literally "cell color").
• Between the outer membranes and within the cristae are fluid filled regions. The inner region is called the matrix.
• The 2nd and 3rd stages of Aerobic CR take place on the cristae, across the cristae and in the matrix.

Glycolysis as it relates to the Mitochondria

• The two pyruvates from glycolysis diffuse into the mitochondria.
• Each loses a carbon in the form of CO2. The remaining two carbon structure is called an acetyl group. It bonds with an enzyme to form acetyl coenzyme A, a critical molecule involved in the next stage of CR. (Some books note this as a separate step).
• In addition, two more NADH's are formed.

Kreb's Cycle

• This cycle is a series of 8 enzyme mediated oxidation reactions which begin and end with the same substance (a chemical "cycle" of reactions).
• The first product formed is citric acid or citrate, thus the cycle is also known as the citric acid cycle.
• Each acetyl coenzyme A helps to convert a molecule of oxaloacetic acid (oxaloacetate) into citric acid (citrate).
• There are two "spins" of the cycle, one for each acetyl coenzyme A. Produces no more ATP than Glycolysis; it's actual process is to provide temporary electron acceptors (NADH's and FADH2's) stacked up for Oxidative Phosphorylation.
• A total of 6 NADH and 2 FADH2 (temporary electron acceptors) are produced, along with 4 more CO2 and 2 ATP.
• By the end of the citric acid cycle (starting with Glycolysis), from 1 glucose, there are 12 temporary electron acceptors, 6 molecules of CO2 and 4 ATP generated.
• All ATP created up to this point is from enzyme mediated phosphorylation.

Electron Transport in the Mitochondria

• Electron Transport -- The electrons held by NADH and FADH2 transfer to the cytochromes of the electron transport chain.
  • This occurs on the cristae of the mitochondria in eukaryotes.
  • The electrons are passed from one cytochrome to the next. As they do so, they lose a bit of their energy.
• Chemiosmosis -- The energy released by the electrons is used to drive a "pump" known as the proton pump.
  • In the proton pump, H+ ions are actively transported from the matrix across the cristae. A Pump pushing electrons, using energy, against their concentration gradient.

ATP Formation

• The H+ ions are what remain of the H's carried by NADH and FADH2. They can diffuse back into the matrix, BUT ONLY AT SPECIFIC CARRIER PROTEINS!
• In this case, the carrier proteins are an enzyme called ATP Synthase (Used to synthesize ATP).
• This facilitated diffusion process is called chemiosmosis.
• The flow of H+ ions across ATP Synthase drives phosphorylation. Phosphate gets bonded to ADP, forming ATP.
• Hydrogen electrons have already been stripped off their electrons for ATP and are floating around as Hydrogen ions; half an oxygen molecule and all the excess Hydrogen ions floating around, form H2O (water).
• This oxidative phosphorylation typically results in the formation of ~32 ATP molecules per glucose.
• The overall process of Aerobic CR in eukaryotes produces about 38 ATP per glucose molecule. (Figure 9.16, p.169)
• Glucose is NOT the only molecule that can produce ATP's from ACR. Disaccharides, starches and all manner of biomolecules are more complex than glucose, but need to be broken down and need to enter at different locations. (Figure 9.19, p.172)
• Other carbohydrates, as well as lipids and proteins also can be oxidized to form ATP.
• Proteins, lipids, and other CHO's enter the reaction pathways at different locations than glucose.
• 700 Kilocalories for one molecule of glucose; only using 36 ATP from the glucose molecule. 2% of the available energy in a glucose molecule. Some of the 686 is lost energy; unrecoverable. Some is heat energy generated by the chemical reaction energy.

Fermentation

• Aerobic organisms may occasionally be deprived of oxygen for a period of time. Glycolysis is a fermentation step (breaking down glucose is fermentation, without oxygen being present). Two other pathways that will continue to breakdown glucose if oxygen is not present (the mitochondrion will be shut down or inactive in the cellular respiration process).. the processes are fermentation.
• Some facultative aerobes, such as yeasts (unicellular fungus), are able to continue glycolysis and remain functional for a period of time without oxygen. We are obligative aerobes (we must have oxygen);
• Obligate aerobes may also may also continue glycolysis for a brief period.
• Both do so by the process of fermentation.
• Reason for Fermentation -- when O₂ is present in the cell, the NADH formed during glycolysis will move into the mitochondria and release their hydrogens (H+ ions plus electrons) at cytochromes.
• Without O₂, the NADH's have no final electron acceptor to pick up the electrons, thus when all NAD's are full, glycolysis stops (no energy -- > cell/organism dies). If there was a way to dump electrons someplace else to free up the NADH's so they can continue to pick up electrons during glycolysis. Fermentation does not provide any additional energy to a cell; it just allows the cell to continue to produce ATP through glycolysis.
• Fermentation is a temporary anaerobic pathway in an aerobic cell/organism in which an organic molecule acts as a final electron acceptor during glycolysis.
• Because the NADH can "hand off" their electrons, glycolysis can move forward, producing 2 ATP's per one glucose.
• Fermentation does NOT produce additional ATP! It only allows glycolysis to continue.
• By acting as the final electron acceptors, the organic molecules are changed, producing fermentation products in addition to the products of glycolysis (2 ATP, pyruvate).
• There are two fermentation pathways:
  1. Lactic Acid (Lactate) -- Pyruvate itself acts as the electron acceptor. In doing so, it is converted to the molecule lactate.
     • Lactate Fermentation is a is a very short term reaction path, because lactate can only build up to a point. Lactic acid cannot be converted to anything else and it is toxic to cells; it will build up very quickly. If the cells are not replenished, they will begin to die. (figure 9.17b, p.171)
     • Muscle tissue can undergo lactate fermentation. It is the "burn" you feel when overexerting a muscle. At some point, the muscle will become totally exhausted and will no longer function.
     • O₂ must be quickly supplied to the muscle.
     • This is why you "pant" (hyperventilate) after strenuous exercise.
     • Lactate can be converted back to pyruvate with the addition of O₂.
     • Oxygen Debt -- The amount of O₂ that must be "repaid" to a cell to convert all lactate back to pyruvate. The amount of oxygen that has to be used to flushed the lactate back to pyruvate and can't be used for Cellular Respiration.
     • Once the O₂ debt is "paid", if O₂ is still present, Aerobic Cellular Respiration can resume.
  2. Alcohol -- In this case, each 3 carbon pyruvates lose one carbon as CO₂ gas.
     • The 2 carbon products (acetaldehyde) act as the final electron acceptors.
     • In doing so, they are converted to ethyl alchohol (ethanol). 2 Products CO₂ and Alchohol. In wines and beer, the sugars are produced by grapes, in beer by grains. (figure 9.17a, p.171)
     • This is the pathway yeasts use when O₂ is absent. Used to make beer, wine, "leavened" baked goods. They can perform alcohol fermentation for months; they are not significantly effected by the absence of oxygen.

Disclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.