LIFESCI 7A Lecture Notes - Lecture 4: Oxidative Phosphorylation, Citric Acid Cycle, Redox
WEEK 4
7.1: An Overview of Cellular Respiration
● Cellular respiration uses chemical energy stored in molecules such as carbohydrates and lipids to
produce ATP.
○ Cellular respiration is a series of catabolic reactions that converts the energy stored in food
molecules, such as glucose, into the energy stored in ATP, and produces carbon dioxide as a
waste or by-product
■ Aerobic
cellular respiration (uses oxygen)
■ Anaerobic
cellular respiration (doesn’t use
oxygen)
○ From one molecule of glucose comes 32
molecules of ATP
● ATP is generated by substrate-level phosphorylation and oxidative phosphorylation.
○ substrate-level phosphorylation A way of generating ATP in which a phosphate group is
transferred to ADP from an organic molecule, which acts as a phosphate donor or substrate.
○ oxidative phosphorylation A set of metabolic reactions that occurs by passing electrons along
an electron transport chain to the final electron acceptor, oxygen, pumping protons across a
membrane, and using the proton electrochemical gradient to drive synthesis of ATP.
■ electron carrier A molecule that carries electrons (and energy) from one set of
reactions to another.
■ electron transport chain The system that transfers electrons along a series of
membrane-associated proteins to a final electron acceptor, using the energy released as
electrons move down the chain to produce ATP.
● Redox reactions play a central role in cellular respiration.
○ oxidation–reduction reaction A reaction involving the loss and gain of electrons between
reactants. In biological systems these reactions are often used to store or release chemical
energy.
■ NAD+ and FAD accept electrons,
■ NAD+ + 2e– + H+ → NADH
■ FAD + 2e– + 2H+ → FADH2
■ NADH and FADH2 can donate electrons
■ NADH → NAD+ + 2e– + H+
■ FADH2 → FAD + 2e– + 2H+
● Notice how an electron often comes with the H+ proton
● Cellular respiration occurs in four stages
○ Step 1: glycolysis The breakdown of glucose to pyruvate; the first stage of cellular respiration.
○ Step 2: pyruvate is oxidized to another molecule called acetyl-coenzyme A (acetyl-CoA),
producing reduced electron carriers and releasing carbon dioxide.
○ Step 3: citric acid cycle (Krebs Cycle) acetyl-CoA is broken down and more carbon dioxide is
released.
■ energy is transferred to ATP and reduced electron carriers.
■ The amount of energy transferred to ATP and reduced electron carriers in this stage is
nearly twice that of stages 1 and 2 combined.
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○ Step 4: oxidative phosphorylation A set of metabolic reactions that occurs by passing
electrons along an electron transport chain to the final electron acceptor, oxygen, pumping
protons across a membrane, and using the proton electrochemical gradient to drive synthesis of
ATP.
7.2: Glycolysis
● Glycolysis is the partial breakdown of glucose.
○ One 6-carbon glucose becomes two 3-carbon pyruvates
■ Net production of 2 ATP (2 used up and 4 produced) and 2 NADH
○ Does not require oxygen as it has evolved very early (before oxygen!); very widespread
metabolic pathway among organisms
○ Occurs in the cytoplasm
1. Glucose is phosphorylated (this uses to ATPs), this makes the glucose less stable and keeps it
within the cell (not permeable!)
2. Cleavage stage, glucose split into two 3-carbon pyruvates
3. Payoff stage, 4 ATP and 2 NADH produced
● The intracellular level of ATP is a key regulator of cellular respiration.
○ Level of ATP in a cell is indicator of how much energy is available inside the cell
○ In reaction #3 of glycolysis, , fructose 6-phosphate is converted to fructose 1,6-bisphosphate,
and a molecule of ATP is consumed.
■ Endergonic, irreversible
■ Catalyzed by enzyme phosphofructokinase-1 (PFK-1): Allosteric enzyme;
● ADP and AMP are activators that bind to it, increasing the rate of glycolysis+ATP
production
● ATP and citrate (citric acid cycle intermeidiate) are inhibitors that bind to it and
slows down glycolysis+ATP production
7.3 Pyruvate Oxidation
● First step that takes place in mitochondria (in the mitochondrial matrix)
○ Glycolysis doesn’t generate a lot of energy; pyruvate still has a lot of potential energy
● The oxidation of pyruvate connects glycolysis to the citric acid cycle.
○ intermembrane space The space between the inner and outer mitochondrial membranes.
○ mitochondrial matrix The space enclosed by the inner membrane of the mitochondria.
○ Pyruvate is converted to one CO2 and one acetyl group (OXYGEN REQUIRED); gives off
electron to NAD+, producing NADH
■ The acetyl group is then attached to Coenzyme A (CoA)
● Since glycolysis produces two pyruvates, after pyruvate oxidation we have two NADHs, two CO2, and
two Acetyl-CoA from one molecule of glucose
7.4 The Citric Acid Cyle
● Takes place in mitocondrial matrix
● Acetyl-CoA is completely oxidized into CO2; chemical energy transfered to ADP and to NAD+ and
FADH to create NADH and FADH2, supplying electrons to the electron transport chain
● The citric acid cycle produces ATP and reduced electron carriers.
○ Acetyl group (2C)detaches from CoA and joins oxaloacetate (4C), forming citric acid (6C)
■ Citric acid loses two carbons as CO2, and at the end of the cycle, is just oxaloacetate
again (acetyl group completely oxidized)
○ Per acetyl-CoA, three NADH’s and one FADH2 are produced, as well as one ATP via substrate
level phosporylation of GTP
● In the citric acid cycle, two of acetyl-CoA from one glucose yields
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Document Summary
Cellular respiration uses chemical energy stored in molecules such as carbohydrates and lipids to produce atp. Cellular respiration is a series of catabolic reactions that converts the energy stored in food molecules, such as glucose, into the energy stored in atp, and produces carbon dioxide as a waste or by-product. Anaerobic cellular respiration (doesn"t use oxygen) one molecule of glucose comes 32. Atp is generated by substrate-level phosphorylation and oxidative phosphorylation. Substrate-level phosphorylation a way of generating atp in which a phosphate group is transferred to adp from an organic molecule, which acts as a phosphate donor or substrate. Electron carrier a molecule that carries electrons (and energy) from one set of reactions to another. Electron transport chain the system that transfers electrons along a series of membrane-associated proteins to a final electron acceptor, using the energy released as electrons move down the chain to produce atp. Redox reactions play a central role in cellular respiration.
