Cellular Respiration and Fermentation: Study Notes

Chapter 9: Cellular Respiration and Fermentation

Overview and Learning Objectives

This chapter explores how cells obtain energy from organic molecules, focusing on the processes of cellular respiration and fermentation. Students should understand the importance of ATP, the mechanisms of redox reactions, and the stages of glucose oxidation.

• Explain why ATP is crucial for life

• Describe oxidation-reduction reactions, and define oxidizing and reducing agents

• List and describe the stages of glucose oxidation and ATP yield at each step

• Understand glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation

• Describe the three fates of pyruvate and the mechanisms of lactic acid and alcohol fermentation

Importance of ATP

ATP: The "Energy Currency" of Cells

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It stores and releases energy for cellular processes.

• ATP hydrolysis: When ATP is converted to ADP and inorganic phosphate ($ATP \rightarrow ADP + P_i$), energy is released for cellular work.

• ATP synthesis: When ADP is converted to ATP ($ADP + P_i \rightarrow ATP$), energy is stored.

• Cells must constantly regenerate ATP from ADP and $P_i$ to maintain cellular functions.

ATP Cycle and Cellular Energy Flow

ATP is used to power intracellular reactions, and energy from food replenishes ATP levels through cellular respiration.

• No ATP production leads to cessation of intracellular reactions and cell death.

• Energy from food is converted to ATP, which is then used for cellular work.

Energy Flow in Ecosystems

Organisms access energy through photosynthesis and cellular respiration.

• Photosynthesis: Converts light energy to chemical energy in plants.

• Cellular respiration: Converts chemical energy in food to ATP in mitochondria.

• Some energy is lost as heat during these processes.

Pathways and Production of ATP

Types of Metabolic Pathways

Cells use different pathways to produce ATP, depending on oxygen availability.

• Fermentation: Partial breakdown of sugars without oxygen.

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

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

Cellular Respiration Equation

The overall reaction for aerobic respiration using glucose is:

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

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, ATP, and heat

Oxidation-Reduction (Redox) Reactions

Definition and Mechanism

Redox reactions involve the transfer of electrons between reactants.

• Oxidation: Loss of electrons from a molecule

• Reduction: Gain of electrons by a molecule

• Oxidizing agent: Accepts electrons and is reduced

• Reducing agent: Donates electrons and is oxidized

General Redox Reaction Example

$Xe^- + Y \rightarrow X + Ye^-$

• Xe^- becomes oxidized

• Y becomes reduced

Na/Cl Redox Reaction Example

$Na + Cl \rightarrow Na^+ + Cl^-$

• Sodium (Na) is oxidized (loses an electron)

• Chloride (Cl) is reduced (gains an electron)

• Na is the reducing agent; Cl is the oxidizing agent

Redox Reactions and Covalent Bonds

Not all redox reactions involve complete electron transfer; some involve changes in electron sharing within covalent bonds.

• Oxygen atoms are highly electronegative and attract electrons more strongly than other atoms.

• Partial gain of electrons by oxygen and loss by bonding partners constitutes a redox reaction.

Stages of Cellular Respiration

Overview of Glucose Oxidation

Cellular respiration consists of several stages that extract energy from glucose:

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

2. Pyruvate Oxidation: Converts pyruvate to acetyl CoA in mitochondria (eukaryotes).

3. Citric Acid Cycle (Krebs Cycle): Completes glucose breakdown to CO2; produces NADH and FADH2.

4. Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis to generate most ATP.

Glycolysis

Glycolysis is the first step in glucose metabolism, occurring in the cytoplasm and does not require oxygen.

• Glucose is split into two three-carbon pyruvate molecules.

• Net yield per glucose: 2 ATP (via substrate-level phosphorylation), 2 NADH, 2 pyruvate, and 2 H2O.

• Two phases: Energy investment (uses 2 ATP) and energy payoff (produces 4 ATP).

Pyruvate Oxidation

Pyruvate produced by glycolysis has three possible fates:

• Aerobic respiration: Pyruvate enters mitochondria and is converted to acetyl CoA.

• Lactic acid fermentation: Pyruvate is reduced to lactate.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2.

Conversion to Acetyl CoA

• Pyruvate's carboxyl group is removed (releasing CO2).

• Remaining two-carbon fragment is oxidized, reducing NAD+ to NADH.

• Fragment combines with coenzyme A to form acetyl CoA.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle occurs in the mitochondria (eukaryotes) or cytoplasm (prokaryotes).

• Each turn produces: 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (or GTP).

• For each glucose, the cycle runs twice (once per pyruvate).

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

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which most ATP is generated, powered by redox reactions.

• Includes two main processes: Electron Transport Chain (ETC) and Chemiosmosis.

• NADH and FADH2 donate electrons to the ETC, which passes them to oxygen, forming water.

• Energy released pumps H+ ions across the mitochondrial membrane, creating a proton gradient.

• ATP synthase uses the proton-motive force to synthesize ATP from ADP and $P_i$.

Electron Transport Chain

• Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• Electron carriers alternate between reduced and oxidized states as they accept and donate electrons.

• Energy is released in small steps, preventing loss as heat.

Chemiosmosis

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

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

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

ATP Yield from Cellular Respiration

Approximately 34% of the energy in glucose is transferred to ATP, producing about 32 ATP per glucose molecule. Most energy is lost as heat.

• Exact ATP yield varies due to differences in NADH/FADH2 conversion and use of proton-motive force for other cellular work.

Fermentation and the Fates of Pyruvate

Fermentation Pathways

When oxygen is absent, cells use fermentation to regenerate NAD+ and allow glycolysis to continue.

• Lactic acid fermentation: Pyruvate is reduced by NADH to lactate, regenerating NAD+.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2 in two steps, regenerating NAD+.

• Fermentation is used by yeast (alcohol fermentation) and some bacteria and fungi (lactic acid fermentation).

• Human muscle cells use lactic acid fermentation during strenuous activity when oxygen is limited.

Regulation and Versatility of Cellular Respiration

Regulation of Respiration

Cellular respiration is regulated by feedback inhibition.

• If ATP levels drop, respiration speeds up.

• If ATP is adequate, respiration slows down.

• Regulation is controlled by key enzymes in the metabolic pathways.

Versatility of Catabolism

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration.

• All major macromolecules can enter the respiration pathway at different points.

• Catabolic pathways are interconnected and regulated to meet cellular energy needs.

Summary Table: Stages of Cellular Respiration and ATP Yield

Stage	Main Location	ATP Yield (per glucose)	Key Products
Glycolysis	Cytosol	2 ATP	2 NADH, 2 pyruvate
Pyruvate Oxidation	Mitochondrial matrix	0 ATP	2 NADH, 2 acetyl CoA, 2 CO2
Citric Acid Cycle	Mitochondrial matrix	2 ATP	6 NADH, 2 FADH2, 4 CO2
Oxidative Phosphorylation	Inner mitochondrial membrane	~28 ATP	H2O

Additional info: Table values are approximate and may vary depending on cell type and conditions.