PPT 14

BSC1010C: General Biology 1 - Cellular Respiration

Learning Objectives

• Discuss the importance of electrons in the transfer of energy in living systems.

• Explain how ATP is used by cells as an energy source.

• Describe the overall result in terms of molecules produced during the chemical breakdown of glucose by Glycolysis.

• Compare the output of glycolysis in terms of ATP and NADH molecules produced.

• Explain differences between a circular pathway (citric acid cycle) and a linear one (glycolysis).

• Describe how electrons move through the electron transport chain and explain what happens to their energy levels during this process.

• Explain how a proton (H+) gradient is established and maintained by the electron transport chain.

• Discuss the difference between anaerobic cellular respiration and fermentation.

• Describe the type of fermentation that readily occurs in animal cells and why it occurs.

• Discuss the ways in which carbohydrate metabolic pathways, glycolysis, and the citric acid cycle interrelate with protein and lipid metabolic pathways.

• Describe how feedback inhibition affects the production of an intermediate or product in a pathway.

• Identify the mechanism that controls the rate of the transport of electrons through the electron transport chain.

Glycolysis Overview

• Cells produce ATP from sugars and other high-potential energy compounds through a process called glucose catabolism or cellular respiration.

• The relationship between energy transfer and electron movement is crucial, as electrons possess high energy levels.

• Incremental electron transfers allow cells to harness food energy in the form of ATP.

Cellular Respiration Overview

• Life requires energy; ATP acts as the fuel for cellular work.

• Metabolic pathways extract energy from high-energy molecules, such as glucose, converting released energy to ATP via phosphorylation of ADP.

• Typical ATP reserves in cells sustain activity for approximately 30 seconds to a few minutes.

• Glucose is obtained through two main pathways:

  • Photosynthesis in plants.

  • Consumption of food in other organisms, with glucose being stored in the form of glycogen or starch.

Glucose Oxidation

• Complete oxidation of glucose to carbon dioxide occurs in cells through a series of controlled redox reactions:

  • Overall reaction of glucose oxidation: $C<em>6H</em>{12}O<em>6 + 6 O</em>2 <br>ightarrow 6 CO<em>2 + 6 H</em>2O + ext{~36 ATP}$.

• Energy is released primarily as heat and light.

• Fermentation, while also oxidizing glucose, produces largely reduced organic waste and is less energy-efficient than respiration.

Redox Reactions

• Defined as chemical reactions where electrons are transferred between molecules.

• Molecules serving as electron donors are reducing agents, while those accepting electrons are oxidizing agents.

• Reduced molecules gain electrons, whereas oxidized molecules lose electrons.

• Example of a redox reaction:
  $ext{AH (Reducing Agent) + X^+ (Oxidizing Agent)} <br>ightarrow ext{A^+ (oxidized) + XH (reduced)}$.

Electron Carriers

• Molecules such as Nicotinamide adenine dinucleotide (NAD) exhibit two states: oxidized (NAD+) and reduced (NADH), with NADH acting as an electron donor (carries 2 electrons and 1 H+)

• Electron carriers are vital in cellular respiration, facilitating the transfer of electrons to transport chains, where ATP is synthesized.

• Other electron carriers include FAD (oxidized) and FADH2 (reduced), NADP+ and NADPH.

ATP in Living Systems

• ATP hydrolysis is exergonic, splitting into ADP and inorganic phosphate (Pi), thus providing energy for endergonic reactions.

• **Processes: **

  • Dephosphorylation: Removal of a phosphate group from a molecule.

  • Phosphorylation: Addition of a phosphate group to a molecule, generally resulting in increased reactivity.

• ATP synthesis occurs through phosphorylation of ADP:

  • $(ADP + Pi <br>ightarrow ATP)$ via either substrate-level phosphorylation (coupled exergonic reactions) or chemiosmosis (involving ATP synthase).

Four Processes of Cellular Respiration

1. Glycolysis: Breakdown of a six-carbon glucose into two three-carbon pyruvate molecules.

2. Pyruvate Processing: Each pyruvate is oxidized to form acetyl CoA.

3. Citric Acid Cycle: Oxidation of each acetyl CoA to carbon dioxide (CO2).

4. Electron Transport and Oxidative Phosphorylation: Movement of electrons through a transport chain leading to ATP generation.

Glycolysis Details

• Input: 1 glucose, 2 NAD+, 2 ATP, 4 ADP.

• Output: 2 pyruvate, 2 NADH, and 4 ATP, resulting in a net gain of 2 ATP (as two ATP are consumed initially).

• Glycolysis involves:

  • 10 enzymatic reactions occurring in the cytosol with no requirement for oxygen.

  • An energy investment phase where ATP is used (steps 1-5) followed by an energy payoff phase (steps 6-10) generating ATP and NADH.

Specific Enzymatic Steps of Glycolysis

1. Hexokinase: Phosphorylates glucose using ATP.

2. Phosphoglucose Isomerase: Converts glucose-6-phosphate to fructose-6-phosphate.

3. Phosphofructokinase: Phosphorylates fructose-6-phosphate using ATP.

4. Aldolase: Cleaves fructose-1,6-bisphosphate into two 3-carbon sugars.

5. Triose Phosphate Isomerase: Converts dihydroxyacetone phosphate (DAP) to glyceraldehyde-3-phosphate (G3P).

6. Glyceraldehyde-3-Phosphate Dehydrogenase: Oxidizes G3P to produce NADH and attach a Pi to form 1,3-bisphosphoglycerate.

7. Phosphoglycerate Kinase: Transfers a phosphate from 1,3-bisphosphoglycerate to ADP to generate ATP and 3-phosphoglycerate.

8. Phosphoglycerate Mutase: Rearranges phosphate in 3-phosphoglycerate.

9. Enolase: Forms a double bond in phosphoenolpyruvate by removing a water molecule.

10. Pyruvate Kinase: Produces pyruvate and ATP by transferring a phosphate from phosphoenolpyruvate to ADP.

Pyruvate Processing to Acetyl CoA

• In eukaryotic cells, pyruvate produced in glycolysis moves into the mitochondria, while prokaryotic cells process pyruvate within the cytosol.

• Pyruvate undergoes a series of reactions by pyruvate dehydrogenase:

  • One carbon is converted to CO2, generating NADH.

  • The remaining two carbons are linked to coenzyme A, forming acetyl CoA.

Citric Acid Cycle Overview

• Acetyl CoA undergoes oxidation in the mitochondrial matrix, producing CO2, NADH, FADH2, and ATP.

• Overall, each glucose molecule results in 6 NADH, 2 FADH2, and 2 ATP.

• Cycle reactions regenerate oxaloacetate, driving the continual oxidation of acetyl CoA.

Summary of Outputs from Glucose Oxidation

• For each glucose molecule metabolized, outputs include:

  • 6 CO2, 10 NADH, 2 FADH2, and 4 ATP.

• Electron transfer occurs via the electron transport chain in the mitochondria, establishing a proton gradient leading to oxidative phosphorylation.