Key points and terms to know about Glycolysis and Cellular Respiration! Do not memorize all of the metabolic intermediates and their chemical structures!! But do know the facts and concepts outlined here. Remember change in Free Energy (∆G)? The overall equation for the catabolism of glucose is: C6H12O6 + 602 → 6CO2 + 6H2O ; ∆G = -686 kcal 2878 kcal ⏐ 3564 kcal (consumed) (released) -686 kcal = a lot of free energy released (lost) – but some is stored (in ATP)! Cells have found a way to oxidize (see below) molecules and harvest the free energy loss by storing some of that energy in the form of ATP = Major cellular currency! (Note that these same basic metabolic processes break down and extract energy from other biomacromolecules, like proteins and lipids!) -------------- The key to Cellular Metabolism is the Oxidation-Reduction Reaction (Redox) Oxidation : loss of electrons Reduction : gain of electrons In redox, electrons move to lower potential energy states, and energy is released (since energy must be conserved—the first law of thermodynamics!). In a polysaccharide, for example, many of carbon’s electrons are at a high potential energy state, since they are shared in nonpolar covalent bonds between two atoms with similar electronegativity (H and C). This means that the carbon’s electrons are not held very close to either atom, and is also why gasoline, a hydrocarbon, is such a great fuel— there is a lot of energy stored in its many C-H bonds. If the polysaccharide (or hydrocarbon, etc.) undergoes a chemical reaction and the C ends up bonded to O, which is much more electronegative, a polar covalent bond is formed, and the carbon’s electrons are drawn very close to the oxygen. Electrons near a very electronegative atom are at a lower potential energy state than electrons near less electronegative atoms, so a lot of energy is released in this reaction. In the equation above, the carbons in sugar start out in a relatively reduced state, but they end up very oxidized (their electrons drawn strongly away—though not removed!-- by the very electronegative oxygen in CO2). You should note that carbon can occur in a range of relatively reduced to relatively oxidized forms: O -C- → -C-OH → -C=O → -C-OH → O=C=O Increasing Oxidation -------------- If the catabolic reaction to break down glucose happened all at once (as it is written on the previous page), all that energy would be released as heat. Not only would we be unable to harness that energy fast enough to do cellular work, but we would also burst into flames! Instead, cells use what I have called “the slow burn”–-a sequence of glycolysis and cellular respiration- -to extract the energy from macromolecules slowly, in a stepwise fashion, and store some of it in ATP for short-term future use. Note that this is also a relatively “cold burn”, thanks to enzymes lowering the activation energy, and the gradual, stepwise release of energy. What catabolism shares with fire is that reactants are relatively reduced, and products leave relatively oxidized, and energy is released (including some heat) as a result. Cellular catabolic metabolism proceeds through three main pathways in animals: glycolysis, the Krebs cycle, and oxidative phosphorylation (the last two together are referred to as cellular respiration). Glycolysis: To extract the energy from glucose (ONE 6C sugar), it is broken down into pyruvate (TWO 3C sugars) in cytoplasm. Consume (2) and make (4) ATP along the way (substrate level phosphorylation to make ATP); also make 2 NADH. Redox reaction (glucose oxidized, NAD+ reduced), Overall glycolysis reaction: 1 glucose + 2ADP+ 2Pi + 2NAD 2 pyruvates + 2 ATP + 2 NADH This is the only pathway of the three that does not require oxygen! If oxygen is not present, after glycolysis, the 2 pyruvates undergo fermentation (either lactic acid or alcoholic) to extract further energy. This process does not produce as many ATP as cellular respiration and requires constant regeneration of NADH to create any ATP at all. If oxygen is present, the 2 pyruvates next enter the cellular respiration pathway, which uses two pathways (Krebs cycle and electron transport chain) to convert 2 pyruvates into CO2 and H2O. Krebs Cycle (= citric acid cycle): To enter the Krebs cycle, 2 pyruvates first undergo a Transition Reaction: 2 pyruvates (3C) 2 acetyl CoA molecules (2C) + 2 NADH. Acetyl CoA is the input molecule into the cycle part of the Krebs cycle, and preserves the two “best” (highest-energy) carbons on a carrier (CoA). Attaching Coenzyme A is to sugars reduces the stability (increases the reactivity) of the bonds, encouraging further oxidation. In eukaryotes, both the NADH and the acetyl CoA now enter the mitochondria from the cytosol (bacteria do all of cellular respiration in the cytosol!). In mitochondrial matrix (= lumen = intracellular space): Acetyl CoA (2C) is joined with OAA (4C) to make citric acid or citrate (6C). The 4C sugar is regenerated at the end of the Krebs cycle. Overall Krebs cycle reaction (1 glucose makes 2 pyruvate, so 2 acetyl CoA’s enter the Krebs cycle): 2 acetyl group (2 C) + 2ADP + 2Pi + 6NAD+ + 2FAD 4CO2 + 2ATP + 6NADH + 2FADH2 Redox rxn (AcetylCoA oxidized, NAD+ and FAD reduced), substrate level phosphorylation to make 2ATP Oxidative phosphorylation – Passes e- & H+ from NADH & FADH2 (from glycolysis and the Krebs cycle) to the electron transport chain (a series of enzymes and electron carriers in the cristae of the mitochondria). These e- and H+ are eventually passed to O2 to make H2O (oxygen is the final electron acceptor of respiration—see our equation at the beginning of the handout!). Overall electron transport chain equation: ½ O2 + 2e- + 2 H+ H2O ATP synthase is an enzyme in the mitochondrial inner membrane that harnesses the energy of the H+ gradient that results from the electron transport chain to regenerate ATP (see chemiosmosis, below). Energy Totals: ATP NADH FADH2 Glycolysis: 2 2 Krebs cycle: 2 8* 2 Electron Transport: 32-34** Grand Total: 36-38 ATP *note: after the transport of pyruvate (3C) into the matrix of the mitochondrion, NAD+ is reduced to NADH, and pyruvate goes from 3C to 2C and is then modified with CoA (acetyl-CoA), which makes it a more reactive species. That is combined with 4C sugar to make 6C sugar! **Because the NADH from glycolysis can donate its electrons to either NAD+ or FAD in mitochondria. If it donates to NAD+, you get 34 ATP; if it donates to FAD, you get 32 ATP. Some terms: Kinase – protein that transfers phosphate (for example, from ATP or Pi to another molecule, such as a sugar or protein). Substrate-level phosphorylation - the reverse of the example above- a kinase transfers phosphate from substrate molecule to ADP, regenerating ATP. Oxidative phosphorylation – kinase transfers phosphate from cellular pool of free inorganic phosphate (Pi) to ADP, regenerating ATP. Electron Carriers: in cellular respiration, stepwise oxidation of substrates is carried out by molecules that accept electrons, primarily the Coenzymes NAD+ and FAD (which are reduced to NADH and FADH2). Chemiosmosis—Happens at the end of the electron transport chain, when electrons have been pumped into the mitochondrial intermembrane space (between inner and outer lipid bilayers). H+ ions move down the concentration gradient, in across the inner mitochondrial membrane, through ATP synthase, a channel-shaped enzyme. This movement releases energy which is harnessed to make ATP through oxidative phosphorylation. Catabolic pathways – you should be able to explain why is energy both lost and stored during the catabolism of glucose, and which change is larger.
Pages to are hidden for
"Key points and terms to know about Glycolysis and"Please download to view full document