Cellular Respiration & Fermentation: Catabolic Pathways and Energy Harvest

Cellular Respiration & Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that break down organic molecules to release energy, primarily in the form of ATP, which powers most cellular work. This process occurs in both plant and animal cells and involves the oxidation of glucose and other fuels.

• ATP (adenosine triphosphate) is the main energy currency of the cell.

• Energy enters ecosystems as light and exits as heat; chemical elements are recycled.

• Electron transfer from food molecules to other molecules is central to energy release in catabolic pathways.

Catabolic Pathways and ATP Production

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic respiration: Similar to aerobic, but uses compounds other than oxygen as final electron acceptors.

The overall equation for aerobic cellular respiration is:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP + heat)}$

• Carbohydrates, fats, and proteins can all serve as fuel, but glucose is the primary molecule traced in cellular respiration.

• Catabolic pathways are linked to cellular work through ATP production.

The Stages of Cellular Respiration

Cellular respiration consists of several stages, each with specific roles in energy extraction:

1. Glycolysis: Occurs in the cytosol; splits glucose into two pyruvate molecules.

2. Pyruvate Oxidation: Pyruvate is converted to acetyl CoA in the mitochondrion.

3. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating ATP, NADH, and FADH2.

4. Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce most ATP.

ATP: The Energy Currency

• Glucose is like a large-denomination bill; ATP is like smaller bills or coins, easily spent for cellular work.

• Cellular respiration converts the energy in glucose into many molecules of ATP.

Substrate-Level Phosphorylation

ATP can be generated by two mechanisms:

• Oxidative phosphorylation: Accounts for ~90% of ATP, powered by redox reactions in the electron transport chain.

• Substrate-level phosphorylation: Enzyme transfers a phosphate group directly from a substrate to ADP, forming ATP. Occurs in glycolysis and the citric acid cycle.

Redox Reactions in Cellular Respiration

Oxidation of Organic Fuel Molecules

• During cellular respiration, fuel molecules (e.g., glucose) are oxidized (lose electrons), and O2 is reduced (gains electrons).

• Organic molecules with many hydrogens are excellent sources of high-energy electrons.

• Energy is released as electrons are transferred to O atoms.

Example equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

Electron Carriers: NAD+ and FAD

• NAD+ (nicotinamide adenine dinucleotide): Electron carrier and oxidizing agent in cellular respiration.

• Enzymes called dehydrogenases remove hydrogen atoms from substrates, transferring electrons and protons to NAD+, forming NADH.

• NADH stores energy used to synthesize ATP.

Electron Transport Chain (ETC)

• Series of molecules (mostly proteins) embedded in the inner mitochondrial membrane (or plasma membrane in prokaryotes).

• Electrons from NADH and FADH2 are passed through the chain in a series of redox reactions, releasing energy in small steps.

• O2 is the final electron acceptor, forming H2O.

• The energy released is used to regenerate ATP.

Glycolysis

Steps of Glycolysis

Glycolysis is the first step in glucose catabolism, occurring in the cytosol and not requiring oxygen.

• Energy investment phase: 2 ATP are used to split glucose into two three-carbon sugars.

• Energy payoff phase: 4 ATP are produced, 2 NAD+ are reduced to NADH, and 2 pyruvate and 2 H2O are formed.

• Net gain: 2 ATP per glucose by substrate-level phosphorylation.

Fate of Pyruvate

• Most energy remains in pyruvate after glycolysis.

• In eukaryotes, if O2 is present, pyruvate enters mitochondria for further oxidation.

• In aerobic prokaryotes, this occurs in the cytosol.

Oxidation of Pyruvate to Acetyl CoA

• Pyruvate is converted to acetyl coenzyme A (acetyl CoA) before entering the citric acid cycle.

• Pyruvate dehydrogenase catalyzes:

  • Oxidation of pyruvate's carboxyl group (releasing CO2).

  • Reduction of NAD+ to NADH.

  • Combination of the remaining two-carbon fragment with coenzyme A.

The Citric Acid Cycle (Krebs Cycle)

Overview and Steps

• Completes the breakdown of glucose by oxidizing acetyl CoA.

• Each turn generates 1 ATP, 3 NADH, and 1 FADH2; 2 CO2 are released as waste.

• Since 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule.

• Eight steps, each catalyzed by a specific enzyme.

• Acetyl CoA combines with oxaloacetate to form citrate; subsequent steps regenerate oxaloacetate.

• NADH and FADH2 produced carry electrons to the electron transport chain.

Oxidative Phosphorylation

Electron Transport and Chemiosmosis

• Electron transport chain proteins are embedded in the inner mitochondrial membrane (cristae).

• Electrons are passed through a series of carriers, including cytochromes (proteins with heme groups).

• Electron carriers alternate between reduced and oxidized states.

• Energy released is used to pump H+ ions across the membrane, creating a proton gradient.

Chemiosmosis and ATP Synthase

• H+ ions flow back into the mitochondrial matrix through ATP synthase, driving ATP synthesis.

• This process is called chemiosmosis: using energy in a H+ gradient to drive cellular work.

• The H+ gradient is called the proton-motive force.

ATP Yield

• Most energy flows: glucose → NADH → electron transport chain → proton-motive force → ATP.

• About 34% of glucose energy is transferred to ATP (about 32 ATP per glucose); the rest is lost as heat.

• Exact ATP yield varies due to indirect coupling and use of the proton-motive force for other work.

Anaerobic Respiration & Fermentation

Anaerobic Respiration

• Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate SO42−).

• Produces by-products such as H2S instead of H2O.

Fermentation

• Glycolysis oxidizes glucose to pyruvate without O2 or an electron transport chain.

• NAD+ is regenerated by transferring electrons to pyruvate or its derivatives.

• Net yield: 2 ATP per glucose by substrate-level phosphorylation.

Alcohol Fermentation

• Pyruvate is converted to ethanol in two steps:

  1. CO2 is released from pyruvate.

  2. NADH reduces acetaldehyde to ethanol, regenerating NAD+.

• Used by yeast in brewing, winemaking, and baking.

Lactic Acid Fermentation

• Pyruvate is reduced directly by NADH to form lactate and NAD+.

• No CO2 is released.

• Used by fungi and bacteria to make cheese and yogurt.

Comparing Aerobic and Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration/Fermentation
Final Electron Acceptor	O2	Other molecules (e.g., sulfate, pyruvate)
ATP Yield	~32 ATP per glucose	2 ATP per glucose
Electron Transport Chain	Present	Absent in fermentation

Obligate vs Facultative Anaerobes

• Obligate anaerobes: Only survive by fermentation or anaerobic respiration; O2 is toxic.

• Facultative anaerobes: (e.g., yeast, many bacteria) can survive using either fermentation or cellular respiration.

• For facultative anaerobes, pyruvate is a metabolic branch point.

Versatility and Regulation of Catabolism

Versatility of Catabolism

• Glycolysis can metabolize various carbohydrates (starch, glycogen, disaccharides).

• Proteins are digested to amino acids, which are deaminated before entering glycolysis or the citric acid cycle.

• Fats are broken down by beta oxidation to yield acetyl CoA, NADH, and FADH2.

Feedback Regulation of Cellular Respiration

• Feedback inhibition is the main mechanism for metabolic control, preventing wasteful ATP production.

• If ATP is low, respiration speeds up; if ATP is abundant, respiration slows down.

• Enzyme activity is regulated at key points in the pathway (e.g., phosphofructokinase in glycolysis).

Redox Principles

Oxidation-Reduction (Redox) Reactions

• Redox reactions: Chemical reactions that transfer electrons between reactants.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Oxidizing agent accepts electrons; reducing agent donates electrons.

Example equations:

$\text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^-$ $\text{Xe}^- + \text{Y} \rightarrow \text{Xe} + \text{Y}^-$

Electron Sharing in Redox

• Oxygen is highly electronegative and attracts electrons strongly.

• Partial electron sharing can also result in redox changes, as in organic molecules.

Summary Table: Key Steps and Products of Cellular Respiration

Stage	Main Location	ATP Produced	NADH Produced	FADH2 Produced	CO2 Produced
Glycolysis	Cytosol	2	2	0	0
Pyruvate Oxidation	Mitochondrial Matrix	0	2	0	2
Citric Acid Cycle	Mitochondrial Matrix	2	6	2	4
Oxidative Phosphorylation	Inner Mitochondrial Membrane	~26-28	0	0	0

Example: Yeast performing alcohol fermentation produces ethanol and CO2, while muscle cells under anaerobic conditions produce lactate.

Additional info: The regulation of cellular respiration is crucial for maintaining energy homeostasis in cells. Phosphofructokinase is a key regulatory enzyme in glycolysis, inhibited by ATP and citrate, and stimulated by AMP.