Recording-2025-03-12T15:04:33.971Z

Glycolysis Overview

• Glycolysis consists of two main phases:

  • Energy Investment Phase

  • Energy Payout Phase

Energy Investment Phase

• In this initial phase, 2 ATP molecules are invested.

• The purpose of the investment is to 'supercharge' glucose by adding phosphates, placing it in a higher energy state due to high-energy bonds.

• The glucose (6-carbon) is split into two 3-carbon compounds: G3P (Glyceraldehyde 3-phosphate).

• G3P is characterized as a high-energy 3-carbon compound due to their phosphate groups.

Energy Payout Phase

• Each G3P is converted to pyruvate.

• During this process, a net of 4 ATP molecules is produced.

• Additionally, 2 NADH molecules are generated.

NADH Characteristics

• NADH is a reduced form of NAD+, acting as a 'hidden' form of energy because it carries electrons.

• The electrons in NADH represent a source of energy that will be utilized later in cellular respiration processes.

ATP Yielding Process

• The ATPs from glycolysis are produced through substrate-level phosphorylation.

• This contrasts with oxidative phosphorylation, which produces a majority of the ATP in cellular respiration.

• The net yield from glycolysis is 2 ATP (4 produced - 2 invested).

Energy Availability in Glucose

• At the end of glycolysis, it’s noted that not most energy from glucose has been tapped into yet, as the pyruvate holds much of the original energy.

• Carbon count remains unchanged: 6 from glucose splits to two 3-carbon pyruvates.

Transition to Citric Acid Cycle

• Pyruvate enters the mitochondria for further processing if oxygen is present (aerobic respiration).

• In the mitochondria, each pyruvate is converted to acetyl CoA, releasing CO2 and producing NADH (2 NADHs total from both pyruvates).

Acetyl CoA Formation

• The conversion of pyruvate to Acetyl CoA includes oxidation, which reduces NAD+ to NADH.

• Acetyl CoA has a high-energy bond, ready for entering the citric acid cycle (Krebs cycle).

Citric Acid Cycle Overview

• The cyclization begins with Acetyl CoA combining with oxaloacetate (4-carbon) to form citrate (6-carbon).

• Through a series of transformations, citrate loses carbons as CO2 (resulting in 2 CO2s, turning into 4-carbon molecules).

• Key outputs per acetyl CoA are: 3 NADH, 1 FADH2, and 1 ATP through substrate-level phosphorylation.

Overall Outputs of Glucose Breakdown

• From one glucose molecule, the totals after glycolysis and the citric acid cycle are:

  • 6 CO2 (2 from pyruvate decarboxylation, 4 from the cycle)

  • 4 ATP (2 from glycolysis, 2 from citric acid cycle)

  • 10 NADH (2 from glycolysis, 2 from pyruvate, 6 from citric acid cycle)

  • 2 FADH2 (only from citric acid cycle).

Summary of Energy Storage

• Most of the energy from glucose is stored in the electron carrier molecules (NADH and FADH2), not in ATP or CO2.

• ATP yields per glucose molecule ultimately reach between 36 - 38 ATP during complete cellular respiration, with only 2 ATP having been produced at the end of glycolysis.

Final Notes

• Understanding the distinction between linear (glycolysis) and cyclical pathways (citric acid cycle) is key.

• Recognizing that NADH and FADH2 carry electrons crucial for energy extraction in subsequent stages of respiration is a cornerstone concept.