Cellular Respiration — Comprehensive Notes Summary & Study Notes

🔥 Overview

Cellular respiration is the process that breaks down glucose to produce usable chemical energy in the form of $ATP$. It occurs in the mitochondria of eukaryotic cells and powers virtually all cellular functions. The overall reaction can be summarized as:

$C_6H_{12}O_6 + 6\ O_2 → 6\ CO_2 + 6\ H_2O + ATP$

This process yields about $36$–$38$ $ATP$ per glucose under typical cellular conditions.

🌱 Connection to Photosynthesis

Photosynthesis and cellular respiration are complementary. What is produced in one reaction is used by the other: photosynthesis makes glucose and oxygen; respiration uses those to make CO_2, water, and ATP. This cyclical relationship is central to energy and carbon flow in ecosystems.

🧬 Mitochondria: The Powerhouse (Structure & Function)

• Outer membrane: protective barrier controlling entry and exit.
• Inner membrane (cristae): highly folded to increase surface area for the electron transport chain and ATP production.
• Intermembrane space: area where $H^+$ ions accumulate, creating a proton gradient used to make $ATP$.
• Matrix: fluid-filled interior containing enzymes for the Krebs cycle and mitochondrial DNA/ribosomes.

All eukaryotes have mitochondria; prokaryotes (bacteria and archaea) do not.

🧪 Stages of Cellular Respiration (Big Picture)

Cellular respiration consists of three main stages:

1. Glycolysis (cytoplasm, anaerobic)
2. Pyruvate oxidation + Krebs (Citric Acid) Cycle (mitochondrial matrix, aerobic)
3. Electron Transport Chain (ETC) and Chemiosmosis (inner membrane/cristae, aerobic)

Each stage contributes specific electron carriers and $ATP$ toward the grand total.

1️⃣ Glycolysis

Location: cytoplasm. Oxygen requirement: none (anaerobic).

Glycolysis breaks one glucose into two pyruvate molecules via three phases: energy investment, splitting, and energy harvest. The cell invests $2\ ATP$ and later produces $4\ ATP$, giving a net gain of $2\ ATP$ per glucose. Glycolysis also produces $2\ NADH$ which carry high-energy electrons to later stages.

Key outputs per glucose: $2\ ATP$, $2\ NADH$, 2 pyruvate.

🔁 Pyruvate Oxidation (Link Reaction)

Location: mitochondrial matrix (after pyruvate enters mitochondrion). Each pyruvate loses a carbon as $CO_2$, and the remaining 2-carbon unit binds to CoA to form Acetyl-CoA. This step produces $2\ NADH$ per glucose (one per pyruvate) and feeds the Krebs cycle.

2️⃣ Krebs (Citric Acid) Cycle

Location: mitochondrial matrix. Oxygen requirement: indirect (requires oxygen as final electron acceptor in ETC).

Each Acetyl-CoA (from pyruvate) enters the cycle and is combined with a 4-carbon molecule to form citrate, then is progressively oxidized, releasing $CO_2$ and transferring electrons to carrier molecules.

Per glucose (two turns of the Krebs cycle): $2\ ATP$ (substrate-level), $6\ NADH$, $2\ FADH_2$, and $4\ CO_2$ produced by the cycle itself. Combined with pyruvate oxidation, total carbon released per glucose is $6\ CO_2$.

3️⃣ Electron Transport Chain & Chemiosmosis

Location: inner mitochondrial membrane (cristae). Oxygen requirement: yes (aerobic).

Electrons from $NADH$ and $FADH_2$ are passed through protein complexes in the inner membrane. As electrons flow, energy is used to pump $H^+$ ions into the intermembrane space, creating an electrochemical gradient (proton motive force).

During chemiosmosis, $H^+$ ions flow back into the matrix through ATP synthase, driving phosphorylation of ADP to produce $ATP$. Finally, $H^+$, electrons, and $O_2$ combine to form $H_2O$; this is why oxygen is essential as the final electron acceptor.

The ETC and chemiosmosis account for the majority of ATP produced during respiration (estimates vary with conditions; see energy accounting).

💰 Energy Accounting (Per Glucose, typical values)

• Glycolysis: Net $2\ ATP$, $2\ NADH$
• Pyruvate oxidation + Krebs: $2\ ATP$ (Krebs substrate-level), $8\ NADH$ (includes pyruvate oxidation + Krebs), $2\ FADH_2$
• Electron transport & chemiosmosis: ~$28$–$34\ ATP$ (from oxidative phosphorylation using electrons from $NADH$ and $FADH_2$)

Grand total: ~$36$–$38\ ATP$ per glucose (actual yield varies by organism and mitochondrial efficiency).

⚖️ Aerobic vs Anaerobic Respiration

• Aerobic respiration (with $O_2$): yields ~$36$–$38\ ATP$ per glucose because the ETC/chemiosmosis operate.
• Anaerobic respiration / fermentation (without $O_2$): only glycolysis operates, producing $2\ ATP$ per glucose, and regenerates NAD^+ via fermentation products.

Oxygen unlocks the large energy yield stored in glucose.

📊 Applying to Data & Experiments (Carbon Capture Example)

When studying how trees respond to increased atmospheric $CO_2$, common hypotheses include:

• H1: More $CO_2$ → more photosynthesis → larger trees → more carbon capture.
• H2: More $CO_2$ → higher temperatures or water stress → smaller/stressed trees → less carbon capture.

Basic calculations and graphing tips:

• Change in carbon = carbon gained − carbon lost.
• Independent variable (e.g., census interval in years) goes on the X-axis; dependent variable (change in carbon) goes on the Y-axis.
• If changes can be negative, center the Y-axis so negative values are visible (e.g., include both positive and negative ranges).

🔎 Key Terms to Remember

• Glycolysis: cytoplasm, anaerobic, net $2\ ATP$, $2\ NADH$.
• Pyruvate oxidation: forms Acetyl-CoA, releases $CO_2$, makes $NADH$.
• Krebs cycle: matrix, produces $ATP$, $NADH$, $FADH_2$, releases $CO_2$.
• Electron transport chain: creates proton gradient, drives ATP synthase.
• Chemiosmosis: $H^+$ flow drives $ATP$ production; oxygen forms $H_2O$.

These concise points give a clear framework to understand how cells convert glucose into usable energy and how this links to larger ecological processes like carbon cycling.