Cellular Respiration: Glycolysis and Krebs Cycle Study Notes

Cellular Respiration

Introduction to Cellular Energy

Cellular respiration is the process by which cells convert energy stored in organic molecules into usable chemical energy in the form of ATP. This process is essential for all living organisms and involves a series of metabolic pathways, including glycolysis and the Krebs cycle.

• Photosynthesis: Converts radiant energy from the sun into chemical energy stored in glucose (occurs in plants, algae, and some bacteria).

• Cellular Respiration: Releases energy from organic molecules (such as glucose) to produce ATP, the cell's energy currency.

• Energy Flow: Producers (plants) capture energy, which is transferred to consumers (animals) and decomposers (fungi, bacteria) through food chains.

Example: Ducks and ducklings obtain energy by consuming plants and other organisms, utilizing cellular respiration to convert food into ATP.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of redox (reduction-oxidation) reactions, where electrons are transferred from one molecule to another, gradually releasing energy.

• Oxidation: Loss of electrons or hydrogen atoms; results in decreased potential energy.

• Reduction: Gain of electrons or hydrogen atoms; results in increased potential energy.

• Redox Reaction Example: $AH + B \rightarrow A + BH$ (A is oxidized, B is reduced)

Key Point: The transfer of electrons from food molecules to electron carriers (such as NAD+ and FAD) is central to energy release in respiration.

Electron Carriers

NAD+ and FAD

Electron carriers are molecules that accept and transport electrons during cellular respiration. They play a crucial role in energy transfer.

• NAD+ (Nicotinamide Adenine Dinucleotide): Accepts electrons and hydrogen to become NADH.

• FAD (Flavin Adenine Dinucleotide): Accepts electrons and hydrogen to become FADH2.

Equations:

• $\text{NAD}^+ + 2H^+ + 2e^- \rightarrow \text{NADH} + H^+$

• $\text{FAD} + 2H^+ + 2e^- \rightarrow \text{FADH}_2$

Example: During glycolysis and the Krebs cycle, NAD+ and FAD are reduced as they accept electrons from metabolic intermediates.

ATP: The Energy Currency

ATP Synthesis and Hydrolysis

ATP (adenosine triphosphate) is the primary energy carrier in cells. It is produced by substrate-level phosphorylation and oxidative phosphorylation.

• ATP Hydrolysis: $\text{ATP} \rightarrow \text{ADP} + P_i$ ($\Delta G = -31$ kJ/mol)

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative Phosphorylation: ATP synthesis driven by the electron transport chain and ATP synthase.

Example: Glycolysis and the Krebs cycle generate ATP via substrate-level phosphorylation.

Glycolysis

Overview and Steps

Glycolysis is the first pathway in cellular respiration, occurring in the cytoplasm of almost every cell. It does not require oxygen and breaks down glucose into pyruvate.

• Location: Cytoplasm

• Oxygen Requirement: Not required (anaerobic)

• Reactants: Glucose (6C)

• Products: 2 Pyruvate (3C), 2 ATP (net), 2 NADH + 2H+

• Steps: 10 reactions, divided into energy investment and energy payoff phases

Key Points:

• ATP is consumed in steps 1 and 3.

• ATP is produced in steps 6 and 9.

• NAD+ is reduced to NADH during glycolysis.

Example: One glucose molecule yields two pyruvate, two ATP (net), and two NADH.

Pyruvate Oxidation

Conversion to Acetyl-CoA

After glycolysis, pyruvate is transported into the mitochondria and oxidized to acetyl-CoA, which enters the Krebs cycle.

• Location: Mitochondrial matrix

• Reaction: $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + H^+ + \text{CO}_2$

• Products: Acetyl-CoA, NADH, CO2

Example: Each pyruvate yields one acetyl-CoA, one NADH, and one CO2.

Fermentation

Anaerobic Pathways

When oxygen is absent, cells regenerate NAD+ through fermentation, allowing glycolysis to continue.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate (occurs in animal cells).

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2 (occurs in yeast).

• ATP Yield: 2 ATP per glucose (from glycolysis only)

Example: Muscle cells produce lactate during intense exercise when oxygen is limited.

Krebs Cycle (Citric Acid Cycle)

Overview and Steps

The Krebs cycle is a series of reactions in the mitochondrial matrix that oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

• Location: Mitochondrial matrix

• Reactants: Acetyl-CoA, NAD+, FAD, GDP (or ADP), Pi, H2O

• Products: CO2, NADH, FADH2, GTP (or ATP), CoA-SH

Equation:

• $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{GDP} + P_i + 2\text{H}_2O \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{GTP} + \text{CoA-SH} + 2H^+$

Key Steps:

• Acetyl-CoA combines with oxaloacetate to form citrate.

• Two CO2 molecules are released per cycle.

• Three NADH and one FADH2 are produced per cycle.

• One GTP (or ATP) is generated per cycle.

Example: For each glucose molecule, two turns of the Krebs cycle occur (one per acetyl-CoA).

Regulation of Cellular Respiration

Enzyme Regulation and Feedback

Cellular respiration is tightly regulated by feedback mechanisms that control enzyme activity based on energy needs.

• Phosphofructokinase: Key regulatory enzyme in glycolysis (step 3).

• Inhibition: High levels of ATP, citrate, NADH, or acetyl-CoA inhibit phosphofructokinase and other enzymes.

• Activation: High levels of ADP, AMP, NAD+, or CoA activate enzymes to increase energy production.

• Feedback Inhibition: Accumulation of products inhibits earlier steps, preventing excess ATP production.

Example: When ATP is abundant, phosphofructokinase is inhibited, slowing glycolysis.

Table: Summary of Key Pathways in Cellular Respiration

Pathway	Location	Reactants	Products	ATP Yield
Glycolysis	Cytoplasm	Glucose, NAD+, ADP, Pi	Pyruvate, NADH, ATP	2 (net)
Pyruvate Oxidation	Mitochondrial Matrix	Pyruvate, NAD+, CoA	Acetyl-CoA, NADH, CO2	0
Krebs Cycle	Mitochondrial Matrix	Acetyl-CoA, NAD+, FAD, GDP/ADP, Pi	CO2, NADH, FADH2, GTP/ATP	2 (per glucose)
Fermentation	Cytoplasm	Pyruvate, NADH	Lactate or Ethanol, NAD+	2 (from glycolysis)

Additional info:

• The images provided show molecular structures of enzymes involved in glycolysis and the Krebs cycle, as well as micrographs of intestinal cells with and without lactase, illustrating enzyme presence in lactose tolerance.

• Energy flow diagrams and photos of animals highlight the biological context of energy transfer in ecosystems.