CELLULAR RESPIRATION All organisms require a constant expenditure of energy to maintain the living state - "LIFE". Where does the energy come from and how is it made available for life? With rare exception, ALL THE ENERGY FOR LIFE COMES FROM THE SUN. A very small amount of the total solar energy striking earth is captured by green plants and converted into chemical energy. This newly formed chemical energy is then available to sustain the various living organisms on earth. Solar Energy Cloroplasts (photosynthesis) CO 2 + H 2 O Organic + O 2 CO 2 + H 2 O Molecules Mitochondria Mitochondria (respiration) (respiration) ATP ATP (powers life) (powers life) HEAT HEAT [----------------GREEN PLANTS-------------------] [----------------------ANIMALS--------------------] The production of energy containing organic molecules by photosynthesis is a reduction of carbon as shown by the following equation: CO 2 + H 2 O + solar energy CH 2 O (organic molecule) + O 2 Likewise the energy contained in the organic molecules can be extracted for use by an oxidation of carbon as shown by the following equation: CH 2 O + O 2 CO 2 + H 2 O + ATP (usable energy)
It should be remembered that reduced carbon contains more energy than oxidized carbon, i.e. energy goes along with carbon reduction. [This also applies to other kinds of atoms but the carbon atom is the focus of living organisms.] HIGH ENERGY ORGANIC CARBON - FOOD (reduced state) (reduced carbon) PHOTOSYNTHESIS RESPIRATION (green plants) (green plants/animals) SUNLIGHT ATP (energy) (energy) LOW ENERGY CARBON DIOXIDE (oxidized state) (oxidized carbon) Have you thanked a green plant today? A Review of oxidation-reduction (redox) reactions - REDUCTION = The gaining of one or more electrons by a substance involved in a redox reaction. OXIDATION = The loss of one or more electrons by a stucstance involved in a redox reaction. Although there are oxidation-reduction reactions that involve unequal sharing of electrons rather than actual gain or loss of electrons, we will primarily deal with the gain/loss of electrons. Oxidation-reduction reactions are all coupled reactions, i.e. an oxidation reaction must be accompanied by a separate reduction reaction and visa versa. The above seems logical because for a substance to receive an electon (reduction), another substance needed to give up the electron (oxidation).
Lets look at some examples of redox reactions: NaCl (table salt) dissolved in water Na + (ox) + Cl - (red) Fe +2 ( ) + Cu +2 ( ) Fe +3 ( ) + Cu +1 ( ) Some redox reactions, that are important in living cells, involve the transfer of electrons along with one or more accompanying proton (H + ). Remember that a hydrogen atom can dissociate into a proton and an electron, which may then be used in different ways. H H + (proton) + e - (electron) Transfer of two electrons and one or more protons in a redox reaction - One-half of a redox reaction - NAD + (ox) + 2e - + 2H + NADH(red) + H + (transfer of 2e - and H + ) FAD ( ) + 2e - + 2H + FADH 2 ( ) (transfer of 2e - and 2H + ) A coupled redox reaction - NADH( ) + FAD( ) + H + NAD + ( ) + FADH 2 ( ) Cellular respiration - C 6 H 12 O 6 ( ) + O 2 ( ) 6CO 2 ( ) + 6H 2 0 ( ) The complete oxidation of one mole of glucose (C 6 H 12 O 6 ) yields (686 kcal), i.e. G = -686 kcal/mole. This energy can be released very rapidly during uncontrolled combustion (burning) in the form of heat. Heat energy is considered unusable for most living processes. In cells, the energy contained in organic compounds must be released in controlled, stepwise increments so that some of the released energy can be captured in other chemical forms (mostly as ATP) which can be used for maintaining life.
Cellular respiration consists of three distinctive processes: 1. Glycolysis 2. Kreb's cycle (aka "TCA cycle") 3. Electron transport chain (ETC) and oxidative phosphorylation 1. Glycolysis: Cellular location - cytosol (the semifluid portion of cytoplasm) Metabolic pathway (carbon) - Glucose (6C) 2 Pyruvate (3C) Net Production - 2 Pyruvate 2 NADH 2 ATP (by "substrate-level phosphorylation" of ADP to ATP) "Substrate-level phosphorylation" is the enzymatic transfer of a phosphate from an organic substrate to an ADP producing ATP - enzyme organic molecule - P + ADP organic molecule + ATP Only one step in the glycolytic pathway is a redox reaction (the one reducing NAD + to NADH). The majority of the reactions involve molecular rearrangements and phosphorylations / dephosphorylations (addition/removal of phosphate). GLYCOLYSIS DIAGRAM In the presence of oxygen, an intermediate step occurs which doesn't belong to glycolysis or Krebs cycle. This step oxidizes pyruvate and removes one of the carbons as carbon dioxide, producing acetate. Acetate, however, must be activated by the attachment of a coenzyme called "coenzyme A" which forms acetyl-coa. pyruvate (3C) acetate(2c) + CoA acetyl-coa CO 2 Cellular location - matrix of mitochondrion Metobolic pathway (carbon) - 2 Pyruvate + 2 CoA 2 Acetyl-CoA + 2 CO 2
Net Production - 2 CO 2 2 Acetyl-CoA 2 NADH 2. Krebs Cycle: Cellular location - matrix of mitochondrion Metabolic pathway (carbon) - 2 Acetyl-CoA (2C) 4 CO 2 + 2 CoA Net Production - 4 CO 2 6 NADH 2 FADH 2 2 ATP Cycle involves numerous redox reactions ( 4 out of 8 reactions!). One reaction involves production of ATP via substrate-level phosphorylation. The "intermediate step" (above) and Krebs cycle together cause the complete oxidation of energy rich organic carbon (e.g. glucose) to carbon dioxide. KREBS CYCLE DIAGRAM 3. Electron Transport Chain and Oxidative Phosphorylation: Cellular location - Inner mitochondrial membrane e - e - Metabolic pathway - NADH (FADH 2 ) Electron Transport Chain O 2 Proton Gradient (potential energy) Net Production - 34 ATP ATP Production What is an electron tranport chain (ETC)? Any ETC consists of a group of compounds that can undergo reversible redox reactions, i.e. each is capable of donating an electron to another compound and subsequently receiving an electron from a different compound. The various compounds, however, must be arranged in a particular sequence ("chain"), such that the electrons are passed energetically "downhill", i.e. to a lower electron energy state.
The compounds near the terminal end are ones that have a strong tendency to receive electrons while the compounds near the start are ones that energetically tend to donate electrons and remain oxidized. An experimentally determined value called a "redox potential" can tell us the likelihood of a compound to receive or donate an electron under certain conditions. Redox potentials and other types of experimental observations were used to discover the general sequence of carriers in the mitochondrial ETC. How does the energy released in a series of redox reactions (ETC) produce stored chemical energy in the form of ATP? Peter Mitchell (1961) published his research that attempted to answer the above question. Although researchers at first had difficulty accepting his hypothesis, he was later proven correct and received the Nobel Prize. Mitchell's hypothesis proposed that the ETC did NOT directly cause the synthesis of ATP but instead stored potential energy in the form of a proton gradient (electrochemical gradient) across the inner mitochondrial membrane. High [H + ] in intermembrane space and low [H + ] in matrix. Electron Transport Chain proton gradient (potential energy) The potential energy stored in the proton gradient was then used to "energize" the enzyme ATP synthase which directly produced ATP. proton gradient energized ATP synthase ATP Collectively then the hypothesis stated: ETC proton gradient energized ATP sythase ATP How does the ETC create a proton gradient? Mitchell proposed: 1. a "mix" of ETC carriers - some carry a pair of electrons and associated protons, e.g. NADH, FADH2 and ubiquinone (Q) some carry only one electron, e.g. cytochromes (Fe) and ironsulfur proteins (Fe) 2. the carriers are asymmetrically (unevenly) distributed within the inner membrane in such a way that they can move protons from the matrix into the intermembrane space
intermembrane space 2 H+ 2 H+ 2 cyt X (Fe) 2 cyt Y (Fe) inner mito. membr. Q QH 2 matrix NADH + H+ 2 H+ 3. the mitochondrial inner membrane is relatively impermeable to protons (needed to maintain a proton gradient) It has been experimentally shown that the ETC of the mitochondrion has three sites along the chain where protons are translocated (not "pumped" according to textbook). How does some of the potential energy stored in the proton gradient get converted to chemical energy in ATP? ATP synthase complex (F 1 particle) consists of three distinct protein complexes: 1. membrane spanning channel - proton channel; spans inner membrane 2. ATP synthases - protrudes into matrix; enzyme complex that synthesizes ATP 3. "stalk" - energy of proton movement through channel is transmitted to ATP synthase by this structure Protons traveling back to the matrix through the ATP complex appear to cause molecular movements in all three components (allosteric movements?) that activate enzyme activity. A Summarization of Useful Energy Extracted from Glucose by Cellular Respiration: Glycolysis - 2 NADH (cytosol) 6 ATP 2 ATP 2 ATP Intermediate Step - 2 NADH 6 ATP Krebs Cycle - 6 NADH 18 ATP 2 FADH 2 4 ATP 2 ATP 38 ATP (38 X 7.3 kcal/mol = 277.4 kcal/mol)
Cellular respiration is approximately 40% effecient in the extraction of total energy found in one mole of glucose. (VERY GOOD!!!!) We have seen that oxygen is required for cellular respiration. What happens when oxygen is absent, even for a relatively brief period? First of all, the intermediate step, Krebs cycle, electron transport chain and oxidative phosphorylation would stop. How? Glycolysis could still produce pyruvate and a small amount of ATP only if it had a way to reoxidize the NADH formed in glycolysis. Why would NADH need to be reoxidized to allow glycolysis to continue? There are several ways in which NADH can get reoxidized, depending upon the organism. These processes are called FERMENTATION and are essentially extensions of glycolysis. TWO ways of reoxidixing NADH for glycolysis: 1. Lactic Acid Fermentation - pyruvate lactate NADH NAD + 2. Alcohol Fermentation - pyruvate acetaldehyde ethanol CO 2 NADH NAD + Fermentation is very, very inefficient but does allow cells to survive varying periods without oxygen. Glycolysis is found throughout the prokaryotes and eukaryotes; as such it is a very old metabolic pathway dating back nearly 3.5 billion years. Glycolysis and the Krebs cycle form what is often referred to a "central metabolism" because it is at the center of numerous anabolic/catabolic pathways.