Cellular Respiration

Cellular Respiration — Overview

Part 1 of 7

Every living organism requires a continuous supply of energy to maintain order, grow, and reproduce. Cellular respiration is the set of metabolic reactions that convert the chemical energy stored in organic molecules — primarily glucose — into ATP (adenosine triphosphate), the universal energy currency of the cell.

The overall summary equation is:

$C_6H_{12}O_6 + 6\,O_2 \longrightarrow 6\,CO_2 + 6\,H_2O + \text{energy (ATP + heat)}$

This equation is essentially the reverse of photosynthesis, highlighting the complementary nature of the two processes in the biosphere.

The Four Stages at a Glance

Cellular respiration can be divided into four main stages, each occurring in a specific cellular location:

The total theoretical maximum yield is 30-38 ATP per glucose, depending on the shuttle system used to transport cytoplasmic NADH into the mitochondria.

Key Concept: The vast majority of ATP (~90%) is produced in the final stage — oxidative phosphorylation. The earlier stages primarily generate the electron carriers (NADH and FADH$_2$) that feed into the electron transport chain.

Checkpoint — The Big Picture

Oxidation-Reduction: The Engine of Energy Transfer

At its core, cellular respiration is a series of redox reactions. Understanding oxidation and reduction is essential for following how energy moves through the pathway.

OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons)

When glucose is oxidized, it does not lose all its electrons at once in a single explosive reaction. Instead, enzymes remove electrons gradually, passing them to the coenzyme NAD$^+$, which is reduced to NADH:

$\text{NAD}^+ + 2\text{H} \longrightarrow \text{NADH} + \text{H}^+$

Each NADH molecule carries a pair of high-energy electrons. These electrons travel through the electron transport chain in a controlled series of small energy releases, allowing the cell to capture energy as ATP rather than losing it all as heat.

Analogy: Think of a ball rolling down a staircase versus falling off a cliff. Both descend the same height, but the staircase releases energy in small, manageable steps — analogous to how the ETC harvests electron energy incrementally.

Checkpoint — Redox Chemistry

ATP: Structure and Function

ATP consists of three components: the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups. The bonds between the phosphate groups are often called "high-energy bonds" — not because the bonds themselves are unusually strong, but because hydrolysis of these bonds releases a large amount of free energy due to:

1. Electrostatic repulsion — the negatively charged phosphate groups repel each other
2. Resonance stabilization — the products (ADP + P$_i$) are more stable than ATP
3. Increased entropy — hydrolysis increases the number of molecules in solution

$\text{ATP} + \text{H}_2\text{O} \longrightarrow \text{ADP} + \text{P}_i + \text{energy} \quad (\Delta G = -30.5 \text{ kJ/mol})$

The cell maintains an ATP/ADP ratio far from equilibrium, ensuring that ATP hydrolysis always releases energy when and where it is needed.

Substrate-level phosphorylation vs. Oxidative phosphorylation:

• Substrate-level: An enzyme directly transfers a phosphate group from a substrate molecule to ADP. Occurs in glycolysis and the citric acid cycle.
• Oxidative: ATP synthase uses the proton-motive force (generated by the ETC) to phosphorylate ADP. Produces the vast majority of ATP.

Fill in the Key Terms

Match the Stage to Its Location

Exit Ticket — Part 1 Synthesis

Glycolysis — Splitting Glucose

Part 2 of 7

Glycolysis (from Greek glykys = sweet, lysis = splitting) is the first stage of cellular respiration and is one of the most ancient metabolic pathways — virtually all living organisms perform glycolysis, suggesting it evolved before the atmosphere contained significant oxygen.

Key facts:

• Location: Cytoplasm (cytosol)
• Does not require oxygen (anaerobic)
• Converts one 6-carbon glucose into two 3-carbon pyruvate molecules
• Net yield: 2 ATP and 2 NADH per glucose

The Two Phases of Glycolysis

Glycolysis consists of 10 enzyme-catalyzed reactions divided into two phases:

Phase 1: Energy Investment Phase (Steps 1-5)

In this phase, the cell spends 2 ATP to phosphorylate and rearrange glucose:

1. Hexokinase phosphorylates glucose to glucose-6-phosphate (costs 1 ATP)
2. Isomerase converts glucose-6-phosphate to fructose-6-phosphate
3. Phosphofructokinase (PFK) phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate (costs 1 ATP) — this is the committed step and primary regulatory point
4. Aldolase splits the 6-carbon sugar into two 3-carbon molecules (G3P and DHAP)
5. Isomerase converts DHAP to G3P — from here, every reaction occurs twice (once per G3P)

Phase 2: Energy Payoff Phase (Steps 6-10)

Each G3P molecule is oxidized and rearranged, producing ATP and NADH:

1. G3P is oxidized; NAD$^+$ is reduced to NADH (×2)
2. Substrate-level phosphorylation produces 1 ATP per G3P (×2) 8-9. Molecular rearrangements prepare the substrate for the final step
3. Pyruvate kinase catalyzes the final substrate-level phosphorylation: 1 ATP per G3P (×2)

Net accounting per glucose:

Checkpoint — Glycolysis Steps

Regulation of Glycolysis

The rate of glycolysis is tightly controlled to match the energy needs of the cell. The key regulatory enzyme is phosphofructokinase (PFK), which is allosterically regulated:

This is a classic example of feedback inhibition — the end product (ATP) inhibits an early enzyme in the pathway, preventing wasteful overproduction.

Hexokinase is also regulated: its product, glucose-6-phosphate, acts as a competitive inhibitor. When glycolysis slows (backing up G6P), hexokinase activity decreases, preventing unnecessary glucose phosphorylation.

AP Exam Tip: The AP Biology exam frequently asks about regulatory mechanisms. Remember that PFK is the primary control point of glycolysis, and that ATP is both a product of the pathway and an inhibitor of PFK — a direct feedback loop.

Checkpoint — Regulation

Key Terms — Glycolysis

Match the Glycolysis Concept

Exit Ticket — Glycolysis Mastery

Pyruvate Oxidation — The Bridge Step

Part 3 of 7

After glycolysis, each glucose molecule has been converted into two molecules of pyruvate in the cytoplasm. Before pyruvate can enter the citric acid cycle, it must be transported into the mitochondrial matrix and converted into acetyl-CoA — a process called pyruvate oxidation (also known as the "link reaction" or "transition step").

This is a brief but critical step that connects glycolysis to the rest of aerobic respiration.

Pyruvate Transport into the Mitochondria

Pyruvate is a small, charged molecule that cannot freely diffuse across the mitochondrial membranes. It enters the mitochondrial matrix through a specific pyruvate translocase (a transport protein) in the inner mitochondrial membrane.

This transport is an example of facilitated transport — pyruvate moves down its concentration gradient (higher in cytoplasm, lower in matrix) through a carrier protein. It is co-transported with a proton (H$^+$), making it a symport mechanism.

Important: This transport step requires an intact inner mitochondrial membrane. Any damage to the membrane or inhibition of the translocase blocks all downstream aerobic respiration.

The Pyruvate Dehydrogenase Complex

Once inside the matrix, pyruvate undergoes an oxidative decarboxylation catalyzed by the pyruvate dehydrogenase complex (PDC) — a massive multi-enzyme complex consisting of three enzymes and five coenzymes.

The reaction proceeds in three steps:

1. Decarboxylation: The carboxyl group of pyruvate is removed as CO$_2$ (the first CO$_2$ released in respiration)
2. Oxidation: The remaining 2-carbon fragment is oxidized, and NAD$^+$ is reduced to NADH
3. Attachment to Coenzyme A: The oxidized 2-carbon fragment (acetyl group) is attached to coenzyme A (CoA), forming acetyl-CoA

$\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \longrightarrow \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH}$

Per glucose (2 pyruvates):

• 2 CO$_2$ released
• 2 NADH produced
• 2 Acetyl-CoA formed
• 0 ATP produced directly

The CO$_2$ released here (and in the subsequent citric acid cycle) is ultimately exhaled by the organism. This is literally where the carbon in your food becomes the carbon dioxide in your breath.

Checkpoint — Pyruvate Oxidation

Regulation of the Pyruvate Dehydrogenase Complex

The PDC is tightly regulated because it commits carbon to the citric acid cycle (an irreversible reaction):

The PDC is also regulated by covalent modification: a kinase phosphorylates (inactivates) the complex when energy is abundant, while a phosphatase dephosphorylates (activates) it when energy is needed.

Clinical Connection: Pyruvate dehydrogenase deficiency is a genetic disorder that impairs the conversion of pyruvate to acetyl-CoA. Patients accumulate pyruvate and lactate, leading to lactic acidosis and neurological problems, because the brain relies heavily on aerobic glucose metabolism.

The Anaerobic Alternative: Fermentation

When oxygen is absent, the electron transport chain cannot operate, NADH cannot be reoxidized, and the citric acid cycle stalls. Without NAD$^+$ regeneration, glycolysis would also stop.

Fermentation solves this problem by regenerating NAD$^+$ without using the electron transport chain:

Lactic Acid Fermentation: $\text{Pyruvate} + \text{NADH} \longrightarrow \text{Lactate} + \text{NAD}^+$

• Occurs in animal muscle cells during intense exercise and in certain bacteria (e.g., Lactobacillus in yogurt production)
• Pyruvate is directly reduced to lactate

Alcoholic (Ethanol) Fermentation: $\text{Pyruvate} \longrightarrow \text{Acetaldehyde} + \text{CO}_2$ $\text{Acetaldehyde} + \text{NADH} \longrightarrow \text{Ethanol} + \text{NAD}^+$

• Occurs in yeast and some plant cells
• Pyruvate is first decarboxylated, then the acetaldehyde is reduced to ethanol

Key Point: Fermentation does NOT produce additional ATP beyond the 2 ATP from glycolysis. Its sole purpose is to regenerate NAD$^+$ so that glycolysis can continue.

Checkpoint — Fermentation

Key Terms — Pyruvate Oxidation and Fermentation

Exit Ticket — Bridge Step Synthesis

The Citric Acid Cycle (Krebs Cycle)

Part 4 of 7

The citric acid cycle — also called the Krebs cycle (after Hans Krebs, who elucidated it in 1937) or the tricarboxylic acid (TCA) cycle — is the central metabolic hub of the cell. It completes the oxidation of the carbon atoms originally present in glucose, producing CO$_2$, and generates most of the NADH and FADH$_2$ that will drive ATP production in the electron transport chain.

Location: Mitochondrial matrix

Overall function: Oxidize the acetyl group from acetyl-CoA, releasing 2 CO$_2$ and capturing energy as NADH, FADH$_2$, and GTP (equivalent to ATP).

The Eight Steps of the Citric Acid Cycle

The cycle begins when acetyl-CoA donates its 2-carbon acetyl group to the 4-carbon molecule oxaloacetate, forming the 6-carbon molecule citrate:

The regenerated oxaloacetate is ready to combine with another acetyl-CoA, continuing the cycle.

Per acetyl-CoA (one turn):

• 2 CO$_2$ released
• 3 NADH produced
• 1 FADH$_2$ produced
• 1 GTP (= ATP) produced

Per glucose (two turns):

• 4 CO$_2$, 6 NADH, 2 FADH$_2$, 2 ATP

Note: The 4 CO$_2$ from the citric acid cycle plus the 2 CO$_2$ from pyruvate oxidation = 6 CO$_2$ total, matching the 6 carbons in the original glucose.

Checkpoint — Citric Acid Cycle Steps

Regulation and Metabolic Hub

The citric acid cycle is regulated at three key enzymes:

1. Citrate synthase — inhibited by ATP, NADH, and citrate
2. Isocitrate dehydrogenase — stimulated by ADP; inhibited by ATP and NADH
3. $\alpha$-ketoglutarate dehydrogenase — inhibited by succinyl-CoA and NADH; activated by Ca$^{2+}$

The Citric Acid Cycle as a Metabolic Hub:

The cycle is not just for glucose catabolism. It intersects with many other metabolic pathways:

• Amino acid metabolism: Several amino acids can be converted to citric acid cycle intermediates (e.g., glutamate $\rightarrow$ $\alpha$-ketoglutarate)
• Fat metabolism: Fatty acids are broken down to acetyl-CoA via $\beta$-oxidation
• Gluconeogenesis: Oxaloacetate can be used to make new glucose
• Biosynthesis: Cycle intermediates serve as precursors for amino acids, fatty acids, and porphyrins

When intermediates are pulled out for biosynthesis, they must be replenished through anaplerotic reactions (e.g., pyruvate carboxylase converts pyruvate to oxaloacetate).

Checkpoint — Regulation and Integration

Key Terms — Citric Acid Cycle

Match the Cycle Component

Exit Ticket — Citric Acid Cycle Mastery

Oxidative Phosphorylation — The Main ATP Factory

Part 5 of 7

Oxidative phosphorylation is the culminating stage of aerobic respiration and produces the vast majority of ATP — approximately 30-34 ATP per glucose. It consists of two tightly coupled components:

1. The Electron Transport Chain (ETC): A series of protein complexes in the inner mitochondrial membrane that pass electrons from NADH and FADH$_2$ to O$_2$, pumping protons (H$^+$) into the intermembrane space
2. Chemiosmosis: The flow of protons back through ATP synthase, driving the phosphorylation of ADP to ATP

Location: Inner mitochondrial membrane and intermembrane space

The Electron Transport Chain

The ETC consists of four major protein complexes plus two mobile electron carriers:

The electron flow path:

$\text{NADH} \rightarrow \text{Complex I} \rightarrow \text{Q} \rightarrow \text{Complex III} \rightarrow \text{Cyt c} \rightarrow \text{Complex IV} \rightarrow \text{O}_2$

$\text{FADH}_2 \rightarrow \text{Complex II} \rightarrow \text{Q} \rightarrow \text{Complex III} \rightarrow \text{Cyt c} \rightarrow \text{Complex IV} \rightarrow \text{O}_2$

Why FADH$_2$ produces fewer ATP than NADH: FADH$_2$ enters at Complex II, which does NOT pump protons. NADH enters at Complex I, which does pump protons. Fewer protons pumped = smaller gradient = fewer ATP via chemiosmosis (1.5 ATP per FADH$_2$ vs. 2.5 ATP per NADH).

At Complex IV, molecular oxygen accepts the electrons and combines with H$^+$ to form water — this is why we breathe oxygen:

$\frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \longrightarrow \text{H}_2\text{O}$

Checkpoint — The Electron Transport Chain

Chemiosmosis and ATP Synthase

As electrons pass through Complexes I, III, and IV, the energy released is used to pump H$^+$ ions from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient (also called the proton-motive force):

• Chemical gradient: Higher H$^+$ concentration in the intermembrane space
• Electrical gradient: More positive charge in the intermembrane space

This gradient represents stored potential energy — like water behind a dam.

ATP synthase (sometimes called Complex V) is a remarkable molecular machine that harnesses this gradient. It has two main components:

• F$_0$ subunit: A channel embedded in the membrane through which H$^+$ ions flow back into the matrix
• F$_1$ subunit: The catalytic "head" that protrudes into the matrix and synthesizes ATP

As protons flow through F$_0$, the rotor spins (~100 revolutions per second), causing conformational changes in F$_1$ that catalyze:

$\text{ADP} + \text{P}_i \longrightarrow \text{ATP}$

Approximately 4 H$^+$ must pass through ATP synthase to produce 1 ATP.

Nobel Prize Connection: Peter Mitchell received the 1978 Nobel Prize for the chemiosmotic hypothesis — the idea that ATP synthesis is driven by an electrochemical proton gradient across a membrane. This was initially controversial but is now one of the most fundamental concepts in bioenergetics.

Checkpoint — Chemiosmosis

Key Terms — Oxidative Phosphorylation

Match the ETC Component

Exit Ticket — Oxidative Phosphorylation

Problem-Solving Workshop — Cellular Respiration

Part 6 of 7

This workshop applies concepts from Parts 1-5 to experimental scenarios and data-analysis problems commonly seen on the AP Biology exam. Work through each scenario carefully — these question types test higher-order thinking, not just memorization.

Scenario 1: Respirometer Experiment

A student uses a respirometer to measure the rate of cellular respiration in germinating vs. non-germinating pea seeds at two temperatures (10 °C and 25 °C). The respirometer measures O$_2$ consumption by tracking the movement of a fluid indicator in a sealed system. KOH is included to absorb any CO$_2$ produced, so the only gas change measured is O$_2$ uptake.

Key observations:

• Germinating seeds consume far more O$_2$ than non-germinating seeds
• Higher temperature increases O$_2$ consumption for both conditions

Scenario 1 Questions

Scenario 2: Metabolic Poisons

A researcher treats four groups of cells with different metabolic poisons and measures their effects:

Scenario 2 Questions

Scenario 3: ATP Accounting Challenge

Complete the ATP accounting table for the aerobic oxidation of one glucose molecule:

*Using 2.5 ATP per NADH and 1.5 ATP per FADH$_2$

**The cytoplasmic NADH from glycolysis may yield only 1.5 ATP each if transported via the glycerol-3-phosphate shuttle (instead of 2.5 via the malate-aspartate shuttle), reducing the total to 30 ATP.

Grand total: 30-32 ATP per glucose (4 by SLP + 26-28 by oxidative phosphorylation)

AP Exam Note: The AP exam uses the approximate value of 30-32 ATP per glucose. Older textbooks cite 36-38, but this figure has been revised downward based on more accurate measurements of the H+/ATP ratio and shuttle system costs.

ATP Accounting Questions

Apply Your Knowledge

Exit Ticket — Workshop Synthesis

AP Review — Cellular Respiration

Part 7 of 7

This final section presents AP-exam-style questions that integrate concepts from across all stages of cellular respiration. These questions emphasize experimental design, data interpretation, and conceptual connections — the skills most tested on the AP Biology exam.

Comprehensive Summary

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + \sim30\text{ADP} + \sim30\text{P}_i \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \sim30\text{ATP}$

Critical connections to remember:

1. Energy flows through carriers: Glucose $\rightarrow$ NADH/FADH$_2$ $\rightarrow$ proton gradient $\rightarrow$ ATP
2. Carbon tracking: 6C glucose $\rightarrow$ 2 × 3C pyruvate $\rightarrow$ 2 × 2C acetyl-CoA + 2 CO$_2$ $\rightarrow$ 4 CO$_2$ (from cycle) = 6 CO$_2$ total
3. Oxygen role: O$_2$ is ONLY used at Complex IV as the final electron acceptor — it does not participate in any earlier step
4. Water production: H$_2$O is formed at Complex IV when O$_2$ accepts electrons and combines with H$^+$
5. Coupling: The ETC does not make ATP directly — it builds the proton gradient that ATP synthase uses

AP-Style Questions — Set 1

AP-Style Questions — Set 2

Comprehensive Matching

Final Review — Key Numbers

Final Exit Ticket