Chapter 9 | Cellular respiration and fermentation

Aerobic respiration

Consumes organic molecules and O₂ and yields ATP

Fermentation

Partial degradation of sugars that occurs without O₂. Does not yield additional energy

Redox reaction

Involves the transfer of electrons; atom that loses e^- is oxidized, atom that gains e^- is reduced

NAD+

A coenzyme that can accept electrons, becoming NADH. NADH temporarily stores electrons during cellular respiration

Step-wise release of energy

1. Glycolysis

2. Pyruvate oxidation and the citric acid cycle

3. Oxidative phosphorylation: electron transport and chemiosmosis

Glycolysis: Energy investment phase (1)

Spends 2 molecules of ATP, ends with 2 molecules of Glyceraldehyde 3-phosphate (G3P) + 2 molecules of ADP; G3P converts to 2 pyruvate

Glycolysis: Energy payoff phase (2)

Produces 4 molecules of ATP and 2 molecules of NADH, ends with 2 molecules of Pyruvate; net gain is 2 ATP (4 produced - 2 invested) and 2 NADH from G3P conversion

Substrate-level phosphorylation

The enzyme-catalyzed formation of ATP by direct transfer of a phosphate group to ADP from an intermediate substrate in catabolism

Glyceraldehyde 3-phosphate (G3P)

Is produced from the breakdown of glucose and serves as a crucial intermediate that can be further converted into pyruvate, leading to energy production

Pyruvate oxidation

1. Oxidizes pyruvate —> Acetyl CoA

2. Reduce 2NADH to NADH

3. 2CO₂ exit

4. Left with 2 carbons

Acetyl CoA / Acetyl coenzyme A

The entry compound for the citric acid cycle in cellular respiration, formed from a two-carbon fragment of pyruvate attached to a coenzyme

Citric Acid Cycle

A series of chemical reactions in the mitochondria that processes acetyl-CoA (derived from pyruvate) to produce energy; it generates high-energy molecules, including 3 NADH, 1 FADH₂, and 1 ATP for each turn, while releasing carbon dioxide as a waste product

Electron transport chain

Electrons (from NADH or FADH₂) move from an electron carrier with a lower affinity for electrons to an electron carrier down the chain with a greater affinity for electrons, releasing free energy

Complex I (Electron transport chain)

Accepts electrons from NADH and passes them to Coenzyme Q (ubiquinone)

Complex II (Electron transport chain)

Receives electrons from succinate and also passes them to Coenzyme Q

Complex III (Electron transport chain)

Transfers electrons from Coenzyme Q to cytochrome c

Complex IV (Electron transport chain)

Receives electrons from cytochrome c and uses them to reduce oxygen to water

Function of electron transport chain

NADH and FADH₂ donate electrons to the electron transport chain, regenerating NAD⁺ and FAD, which are crucial for sustaining glycolysis and the citric acid cycle

The electron transport chain creates a proton gradient across the inner mitochondrial membrane, with higher H⁺ concentration in the intermembrane space, storing energy that can be utilized to produce ATP

Chemiosmosis

An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP

Complexes I, III, and IV (Electron transport chain)

Act as proton pumps, using the energy from electrons to transport H+ ions from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient

Anaerobic respiration

A catabolic pathway in which inorganic molecules other than oxygen accept electrons at the “downhill” end of electron transport chains

Lactic acid fermentation

Glycolysis followed by the reduction of pyruvate to lactate, regenerating NAD⁺ with no release of carbon dioxide

Alcohol fermentation

Glycolysis followed by the reduction of pyruvate to ethyl alcohol, regenerating NAD+ and releasing carbon dioxide

How metabolic pathways are regulated

Phosphofructokinase; inhibited by ATP & citrate, stimulated by AMP

ATP synthase

An enzyme located in the inner mitochondrial membrane that synthesizes ATP from ADP and inorganic phosphate (P_i) using the energy from a proton gradient

ATP yield

30-32 ATP

Glycolysis

A series of reactions that ultimately spits glucose into pyruvate. Occurs in almost all living cells, serving as the starting point for fermentation, or cellular respiration