MCB unit 2

Cellular Respiration Overview

• Cellular respiration is a process that converts glucose into usable energy (ATP).

• It breaks down carbon into carbon dioxide and hydrogen into water as waste products.

• The ultimate goal is to generate ATP, the energy currency of cells.

• Carbon dioxide and water are the least reactive forms of carbon and hydrogen, respectively, meaning they can't be further broken down for energy.

Phases of Cellular Respiration

• Phase 1: Glycolysis

  • Glycolysis consists of 10 steps that take place in the cytoplasm of cells.

  • Begins with glucose (a six-carbon molecule) and ends with two three-carbon molecules called pyruvate.

• Phase 2: Pyruvate Oxidation and Krebs Cycle

  • Involves the oxidation of pyruvate to produce Acetyl-CoA, which then enters the Krebs cycle (Citric Acid Cycle).

  • Both processes are interconnected and necessary for continued energy extraction.

• Phase 3: Oxidative Phosphorylation

  • Involves the electron transport chain and chemiosmosis.

  • Produces the majority of ATP during cellular respiration.

Glycolysis Detailed Steps

• Glycolysis has two main phases: the investment phase and the payoff phase.

• Investment Phase:

  • Initial reactions consume ATP to phosphorylate glucose and create fructose-1,6-biphosphate.

  • This phase requires energy input and is endergonic.

• Payoff Phase:

  • Involves the conversion of glyceraldehyde-3-phosphate (G3P) to pyruvate.

  • Produces ATP and NADH as part of the energy release process.

  • Energy is harvested through substrate-level phosphorylation, directly making ATP.

Steps in Glycolysis

1. Glucose to Glucose-6-Phosphate:

   • Enzyme: Hexokinase.

   • ATP investment to phosphorylate glucose.

2. Isomerization to Fructose-6-Phosphate:

   • Enzyme: Phosphoglucose Isomerase.

3. Phosphorylation to Fructose-1,6-bisphosphate:

   • Enzyme: Phosphofructokinase. ATP invested.

4. Cleavage into Two Triose Phosphates:

   • Fructose-1,6-bisphosphate splits into two three-carbon molecules.

5. Oxidation to Produce NADH:

   • Involves converting G3P into 1,3-bisphosphoglycerate, reducing NAD+ to NADH.

6. Substrate-Level Phosphorylation:

   • Creation of ATP from ADP during conversion of intermediate molecules.

   • Two ATP molecules are directly synthesized by removing phosphate groups from intermediates.

Summary of Glycolysis Outcomes

• Net gain of 2 ATP molecules and 2 NADH molecules from one glucose molecule.

• ATP production is crucial as it meets the energy needs of the cell.

• NADH acts as a carrier for electrons, which later contributes to ATP formation in oxidative phosphorylation.

Importance of Glycolysis

• Glycolysis is a universal process present in all living organisms.

• Fundamental for cellular metabolism and energy production.

• Does not require oxygen, allowing anaerobic organisms to generate energy.

Cellular Respiration Overview

Cellular respiration is a complex biochemical process that converts glucose into usable energy in the form of adenosine triphosphate (ATP). It involves a series of metabolic pathways that break down glucose to produce energy, alongside key waste products: carbon dioxide and water. The ultimate goal of cellular respiration is to generate ATP, regarded as the energy currency of cells, necessary for various cellular functions and processes.

During cellular respiration, glucose (a six-carbon molecule) undergoes catabolic reactions, leading to the breakdown of carbon into carbon dioxide, which is exhaled as a waste product, and hydrogen into water. It is noteworthy that carbon dioxide and water represent the least reactive forms of carbon and hydrogen, respectively, meaning they cannot be further broken down for additional energy extraction.

Phases of Cellular Respiration

Phase 1: Glycolysis

Glycolysis is the first phase of cellular respiration and consists of a series of 10 enzymatic steps that take place in the cytoplasm of cells. This process begins with one molecule of glucose (six-carbon) and concludes with two molecules of pyruvate (three-carbon each). Substantial energy investment in the form of ATP occurs during the initial steps to facilitate glucose breakdown.

Phase 2: Pyruvate Oxidation and Krebs Cycle

Following glycolysis, pyruvate undergoes oxidation to generate Acetyl-CoA, a crucial molecule that enters the Krebs cycle, also known as the Citric Acid Cycle. Both processes are highly interconnected, allowing for continuous energy extraction through a series of redox reactions. The Krebs cycle plays a pivotal role in producing electron carriers (NADH and FADH2) that are vital for the next phase.

Phase 3: Oxidative Phosphorylation

Oxidative phosphorylation comprises two integral processes: the electron transport chain and chemiosmosis. In this phase, the majority of ATP generated during cellular respiration is produced as electrons are transferred through a series of protein complexes, leading to the pumping of protons across the inner mitochondrial membrane. This creates a proton gradient, which drives ATP synthesis through ATP synthase, emphasizing the efficiency of energy conversion in aerobic organisms.

Glycolysis Detailed Steps

Glycolysis has two primary phases: the investment phase and the payoff phase.

Investment Phase:

In this initial set of reactions, ATP is consumed to phosphorylate glucose, transforming it into fructose-1,6-bisphosphate. This phase is characterized by energy input, signifying an endergonic reaction pathway.

Payoff Phase:

The payoff phase entails the conversion of glyceraldehyde-3-phosphate (G3P) into pyruvate, yielding energy products such as ATP and NADH. Energy harvesting occurs through substrate-level phosphorylation, which enables the direct synthesis of ATP.

Steps in Glycolysis

1. Glucose to Glucose-6-Phosphate:

   • Enzyme: Hexokinase

   • Description: ATP is invested to phosphorylate glucose.

2. Isomerization to Fructose-6-Phosphate:

   • Enzyme: Phosphoglucose Isomerase

   • Description: Rearrangement of glucose-6-phosphate into fructose-6-phosphate.

3. Phosphorylation to Fructose-1,6-bisphosphate:

   • Enzyme: Phosphofructokinase (PFK)

   • Description: An additional ATP investment leads to the formation of fructose-1,6-bisphosphate, a key regulatory step in glycolysis.

4. Cleavage into Two Triose Phosphates:

   • Description: Fructose-1,6-bisphosphate splits into two three-carbon molecules, namely dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).

5. Oxidation to Produce NADH:

   • Description: G3P is oxidized to 1,3-bisphosphoglycerate while reducing NAD+ to NADH, which will later participate in oxidative phosphorylation.

6. Substrate-Level Phosphorylation:

   • Description: Two ATP molecules are generated during the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate by transferring phosphate groups from intermediate molecules.

Summary of Glycolysis Outcomes

The overall net gain from one glucose molecule during glycolysis is 2 ATP molecules and 2 NADH molecules, crucial for meeting the energy demands of the cell. The ATP produced is vital in fueling various cellular activities, while the NADH generated serves as an electron carrier that contributes to ATP formation during oxidative phosphorylation.

Importance of Glycolysis

Glycolysis is a universal metabolic pathway present in nearly all living organisms, highlighting its fundamental biological importance. This anaerobic process does not require oxygen, enabling organisms, including anaerobes, to efficiently generate energy even in low-oxygen environments. Moreover, glycolysis serves as a key regulatory point in cellular metabolism, orchestrating multiple pathways and metabolic functions critical for life.

Glycolysis Overview

• Glycolysis is the first step in the breakdown of glucose for energy production.

• Gross yield: 4 ATPs produced.

• Net yield: 2 ATPs after investment of 2 ATPs to start the process.

• Additional products include 2 NADHs and higher-rate molecules.

• Total output: 2 ATPs, 2 NADHs, plus other byproducts.

Energy Transfer in Glycolysis

• Recognizing energy transfer is crucial—building molecules vs. breaking them down.

• Importance of understanding reactions is more about recognition rather than rote memorization.

Clicker Questions & Discussion

• Engaged students in interactive clicker questions regarding glycolysis steps and energy changes.

• Endergonic vs. Exergonic Reactions:

  • Understanding if reactions are endergonic (energy absorbed) or exergonic (energy released) based on coupled reactions.

  • Nomenclature of reactions (A ➜ B) can be determined by examining energy inputs/outputs rather than knowing A and B specifically.

NAD+ and NADH Dynamics

• NAD+ acceptance leads to NADH formation ➜ identified as an endergonic reaction.

• Free Energy Flow: Always couple endergonic reactions with exergonic ones to maintain balance.

• Exergonic reactions provide the necessary energy to drive the endergonic processes.

Conversion of Molecules in Glycolysis

• Discussed how changing molecules (e.g., B ➜ C) relates to energy costs.

• Adding a phosphate (substrate-level phosphorylation) denotes an endergonic reaction requiring energy (comes from ATP).

Net Results of Glycolysis

• Key outputs: 2 ATPs, 2 NADHs, and carbon dioxide is produced elsewhere in cellular respiration, not directly in glycolysis.

• Emphasis on how carbon dioxide (CO2) is not produced during glycolysis itself.

Energy Extraction Problems

• Glycolysis doesn’t extract enough energy from pyruvate (low energy state).

• Need to consider:

  • How do we extract more energy from pyruvate?

  • How to convert NADH to ATP effectively?

  • Regeneration of NAD+ is vital for glycolysis continuation.

Terminal Electron Acceptors

• Aerobic Respiration: Utilizes oxygen as the terminal electron acceptor; follows glycolysis with oxidative respiration.

• Anaerobic Respiration: In non-oxygen environments, can utilize other electron acceptors (like nitrate or sulfate).

• Differentiated between aerobic and anaerobic respiration based on electron pathways.

Pyruvate Oxidation to Acetyl CoA

• Conversion of pyruvate (3 carbons) to Acetyl CoA (2 carbons) results in the release of CO2.

• Total carbon count is conserved through the process, with CO2 being the byproduct.

Krebs Cycle & ATP Production

• Krebs Cycle to maximize energy extraction from acetyl coenzyme A.

• Overview of substrate-level phosphorylation occurring in Krebs cycle producing GTP, easily converted to ATP.

• Importance of NADH and FADH2 in energy transport.

• Cellular respiration primarily occurs in mitochondria for eukaryotic cells.

Mitochondrial Structure and Function

• Inner & Outer Membrane:

  • Outer membrane contains porins, allowing small molecules to pass freely, maintaining equilibrium.

  • Inner membrane is highly folded (forming cristae) to increase surface area for ATP production via the electron transport chain.

  • Matrix: Location of Krebs cycle enzymes and where molecules like ATP are synthesized.

Summary of Key Points

• Glycolysis provides a foundational role in cellular respiration; understanding its mechanics is crucial for comprehending energy transfer in wider metabolic pathways.

• The ability to regenerate NAD+ is critical to sustain glycolysis and energy production.

• Different pathways for dealing with pyruvate (aerobic vs. anaerobic) show the adaptability of organisms based on environmental conditions.