Biology · 11th Grade · The Molecular Basis of Life · Weeks 1-9

Cellular Respiration: Glycolysis and Krebs Cycle

Covers the breakdown of glucose into pyruvate, the subsequent conversion to acetyl-CoA, and the reactions of the Krebs cycle, generating ATP, NADH, and FADH2.

• Common Core State Standards
• HS-LS1-7

About This Topic

Cellular respiration is the process by which cells extract chemical energy from organic molecules and transfer it to ATP. In US 11th-grade biology at the HS-LS1-7 level, students trace the two initial stages: glycolysis and the Krebs cycle. Glycolysis occurs in the cytoplasm and converts one glucose molecule into two pyruvate molecules, generating a net yield of 2 ATP (by substrate-level phosphorylation) and 2 NADH, with no oxygen required. Under anaerobic conditions, pyruvate is reduced to lactate or ethanol to regenerate NAD+, allowing glycolysis to continue.

Under aerobic conditions, pyruvate is transported into the mitochondrial matrix and oxidatively decarboxylated by pyruvate dehydrogenase to form acetyl-CoA, releasing one CO2 and one NADH per pyruvate. Acetyl-CoA then enters the Krebs cycle, combining with oxaloacetate to form citrate. Each turn of the cycle releases two CO2, produces one ATP (via GTP), three NADH, and one FADH2. Since two acetyl-CoA molecules enter per glucose, the Krebs cycle runs twice per glucose, contributing 6 NADH, 2 FADH2, and 2 ATP before oxidative phosphorylation.

Active learning approaches that ask students to trace carbon atoms and electron carriers through both stages develop the causal reasoning needed to understand why NADH and FADH2 are the central products feeding into the electron transport chain.

Key Questions

1. Explain the initial steps of glucose breakdown and energy capture in glycolysis.
2. Analyze the role of the Krebs cycle in generating electron carriers for oxidative phosphorylation.
3. Differentiate between aerobic and anaerobic respiration pathways.

Learning Objectives

• Compare the net ATP and electron carrier yield from glycolysis and the Krebs cycle per molecule of glucose.
• Trace the path of carbon atoms from glucose through pyruvate to acetyl-CoA and into the Krebs cycle.
• Explain the role of NAD+ and FAD in accepting high-energy electrons during glycolysis and the Krebs cycle.
• Differentiate between the location and oxygen requirements of glycolysis versus the Krebs cycle.
• Analyze the significance of the Krebs cycle in producing reduced electron carriers for subsequent ATP synthesis.

Before You Start

Introduction to Metabolism

Why: Students need a basic understanding of how cells obtain and use energy before delving into specific metabolic pathways.

Structure and Function of Organelles

Why: Knowledge of the cytoplasm and mitochondrial matrix is essential for understanding where glycolysis and the Krebs cycle occur.

Basic Chemical Reactions

Why: Understanding concepts like oxidation, reduction, and the transfer of energy is foundational to grasping the biochemical steps involved.

Key Vocabulary

Glycolysis	The initial metabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate, occurring in the cytoplasm and yielding a small amount of ATP and NADH.
Pyruvate	A three-carbon molecule that is the end product of glycolysis, which can then be further processed in the Krebs cycle or through fermentation.
Acetyl-CoA	A molecule formed from the conversion of pyruvate, which enters the Krebs cycle to be further oxidized for energy production.
Krebs Cycle	A series of biochemical reactions in the mitochondrial matrix that oxidizes acetyl-CoA, generating ATP, NADH, and FADH2, and releasing carbon dioxide.
Electron Carriers	Molecules like NADH and FADH2 that accept high-energy electrons during cellular respiration, transporting them to the electron transport chain.

Watch Out for These Misconceptions

Common Misconception

Cellular respiration only happens in mitochondria.

What to Teach Instead

Glycolysis occurs in the cytoplasm and is the only stage available to prokaryotes, which lack mitochondria. Only the Krebs cycle and oxidative phosphorylation are mitochondria-specific in eukaryotes. Under anaerobic conditions, fermentation also occurs in the cytoplasm. Carbon-tracing diagrams that place glycolysis explicitly in the cytoplasm and the Krebs cycle in the mitochondrial matrix are effective at correcting this compartmentalization error.

Common Misconception

The Krebs cycle produces most of the cell's ATP directly.

What to Teach Instead

The Krebs cycle produces only 2 ATP per glucose directly via substrate-level phosphorylation. Its primary contribution is generating electron carriers, 6 NADH and 2 FADH2, that donate electrons to the electron transport chain, where most ATP is produced. ATP yield tables distinguishing direct substrate-level phosphorylation from electron-carrier-linked phosphorylation consistently correct students' overestimation of the Krebs cycle's direct ATP output.

Common Misconception

Fermentation is the same process as decomposition.

What to Teach Instead

Fermentation is a specific cellular anaerobic pathway in which organic molecules (pyruvate) are reduced using NADH to regenerate NAD+, allowing glycolysis to continue. Decomposition is the ecological breakdown of organic matter by decomposers in an ecosystem. While some decomposers use fermentation, the terms are not synonymous. Distinguishing them with concrete examples (yeast in bread dough vs. a compost pile) gives students two anchors for the distinction.

Active Learning Ideas

Carbon Tracing: Where Does Carbon From Glucose End Up?

Student pairs trace each carbon atom from glucose through glycolysis (to pyruvate), through the pyruvate dehydrogenase reaction (to acetyl-CoA and CO2), and through the Krebs cycle (to CO2). Using numbered carbon labels on a pathway diagram, they identify the step at which all 6 glucose carbons are released and determine where the oxygen in CO2 originates.

35 min·Pairs

Think-Pair-Share: What Does a Cell Do When Oxygen Runs Out?

Present the metabolic fork at pyruvate: aerobic path to mitochondria vs. anaerobic fermentation. Pairs reason through why NAD+ regeneration is critical for glycolysis to continue, explain the role of fermentation, and predict consequences for a muscle cell switching from aerobic to anaerobic during intense exercise. The class builds a shared diagram showing both pathways branching from pyruvate.

20 min·Pairs

Data Analysis: Comparing ATP Yields Across Respiration Stages

Pairs receive a table of ATP production per stage and must calculate the estimated total ATP yield when NADH and FADH2 enter the electron transport chain. They calculate the percent contribution of each stage and write a paragraph explaining why most ATP comes from oxidative phosphorylation rather than from the substrate-level phosphorylation in glycolysis and the Krebs cycle.

30 min·Pairs

Gallery Walk: Glycolysis as a Ten-Step Assembly Line

Post 10 stations around the room, each showing one step of glycolysis with the enzyme name, substrates, and products. Student groups rotate, adding to a cumulative flowchart tracking ATP invested, NADH gained, and carbon count. In the final discussion, the class identifies the investment phase (steps 1-5) and the payoff phase (steps 6-10), explaining why net yield is only 2 ATP despite producing 4.

40 min·Small Groups

Real-World Connections

• Biochemists studying metabolic disorders, such as diabetes or mitochondrial diseases, analyze the efficiency of glycolysis and the Krebs cycle in patient cells to understand energy production deficits.
• Food scientists use knowledge of fermentation, an anaerobic pathway that follows glycolysis when oxygen is absent, to produce products like yogurt, cheese, and bread.

Assessment Ideas

Quick Check

Provide students with a diagram showing glucose entering glycolysis and acetyl-CoA entering the Krebs cycle. Ask them to label the key outputs (ATP, NADH, FADH2, CO2) for each stage and indicate the cellular location.

Discussion Prompt

Pose the question: 'If a cell is deprived of oxygen, how does the fate of pyruvate differ from when oxygen is abundant, and what is the immediate consequence for ATP production?' Facilitate a discussion comparing anaerobic and aerobic fates.

Exit Ticket

Ask students to write down two differences between glycolysis and the Krebs cycle, focusing on location, inputs, and outputs. Then, have them write one sentence explaining why NADH and FADH2 are considered crucial products of these initial stages.

Frequently Asked Questions

What is the main purpose of glycolysis?

Glycolysis converts one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each), capturing a net gain of 2 ATP and 2 NADH. It is the universal first stage of cellular respiration, occurring in the cytoplasm of all living cells without requiring oxygen. In aerobic organisms, pyruvate feeds into the mitochondria for further oxidation; in anaerobic conditions, fermentation regenerates the NAD+ needed to keep glycolysis running.

What does the Krebs cycle produce?

Each turn of the Krebs cycle produces 3 NADH, 1 FADH2, 1 ATP (via GTP), and releases 2 CO2. Since two acetyl-CoA enter per glucose, the cycle runs twice per glucose, yielding 6 NADH, 2 FADH2, 2 ATP, and 4 CO2 total. The primary function is not direct ATP production but transferring chemical energy from carbon compounds to electron carriers that power the electron transport chain.

What is fermentation and when does it occur?

Fermentation is an anaerobic metabolic process in which pyruvate is reduced to regenerate NAD+ when oxygen is unavailable. In animals and some bacteria, lactic acid fermentation converts pyruvate to lactate (causing muscle soreness after intense exercise). In yeast, alcoholic fermentation converts pyruvate to ethanol and CO2. Fermentation allows glycolysis to continue generating ATP when the electron transport chain cannot operate.

How can active learning improve understanding of glycolysis and the Krebs cycle?

Tracing carbon atoms through both pathways on labeled diagrams transforms a confusing sequence of intermediates into a coherent story about carbon oxidation and energy capture. When students calculate the ATP yield at each stage and compare substrate-level phosphorylation to electron carrier production, they understand why the Krebs cycle matters even though it makes relatively little ATP directly. Gallery walks through glycolysis steps build familiarity with the investment-payoff structure before whole-class discussion cements it.