AP Biology 3.5 Cellular Respiration Study Notes - New Syllabus Effective 2025
AP Biology 3.5 Cellular Respiration Study Notes- New syllabus
AP Biology 3.5 Cellular Respiration Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the processes and structural features of mitochondria that allow organisms to use energy stored in biological macromolecules.
Key Concepts:
- Cellular Respiration
3.5.A – Cellular Respiration & Mitochondria
🧬 What is Cellular Respiration?
- It’s the process of extracting energy (ATP) from macromolecules, especially glucose.
- The energy stored in chemical bonds of carbs, fats, or proteins is converted into ATP (the cell’s usable energy).
🔋 Where It Happens: The Mitochondrion
- Known as the “powerhouse of the cell“
- Has double membranes and its own DNA.
- Divided into 4 key areas:
- Outer membrane
- Inner membrane
- Intermembrane space
- Matrix
🧱 Key Structural Features That Support Function
| 🔬 Feature | 💡 Function |
|---|---|
| Double Membrane | Separates different reaction areas & helps form proton gradients |
| Cristae (folds in inner membrane) | Increase surface area for ATP production |
| Matrix | Location of the Krebs Cycle (citric acid cycle) |
| Inner Membrane | Where the Electron Transport Chain (ETC) and ATP synthase are located |
⚙️ Key Processes of Cellular Respiration
- Glycolysis
- Occurs in cytoplasm
- Breaks down glucose → pyruvate
- Makes a small amount of ATP (2 ATP)
- Doesn’t need oxygen (anaerobic)
- Krebs Cycle (Citric Acid Cycle)
- Occurs in mitochondrial matrix
- Processes pyruvate to release CO₂
- Makes NADH, FADH₂, and a little ATP
- Electron Transport Chain (ETC)
- Occurs in inner mitochondrial membrane
- Uses electrons from NADH/FADH₂ to pump protons (H⁺)
- Creates a proton gradient
- Chemiosmosis & ATP Synthase
- Protons flow back into matrix through ATP synthase
- This flow powers the enzyme to make LOTS of ATP
- Called oxidative phosphorylation
⚡ Overall Energy Outcome:
Total ATP per glucose = ~30 – 32 ATP
Most of it is made during ETC + chemiosmosis
💭 Summary Thought:
“Mitochondria turn food into fuel, using oxygen to power the ATP factory inside every cell.”
3.5.A.1 – Cellular Respiration & Energy Use
🎯 Big Idea:
All living organisms use cellular respiration (or fermentation) to convert energy stored in food (macromolecules) into ATP the energy currency of life.
🧪 What is Cellular Respiration?
- A biochemical process that breaks down biological macromolecules (like glucose, lipids, or proteins)
- The energy released is used to synthesize ATP (Adenosine Triphosphate)
🔄 ATP: Why It’s Important
ATP = Adenosine Triphosphate
Powers cell functions like:
- Muscle contraction
- Active transport
- Protein synthesis
- Cell division
🧬 Two Major Pathways for ATP Production:
| Process | Oxygen Used? | ATP Yield | Occurs In |
|---|---|---|---|
| Cellular Respiration | ✅ Yes (aerobic) | High (~36–38 ATP) | All eukaryotes + many prokaryotes |
| Fermentation | ❌ No (anaerobic) | Low (2 ATP) | All organisms (especially in low-O₂ environments) |
🧠 Fact: Even bacteria & archaea (the simplest life forms) can do respiration or fermentation!
🧩 Macromolecules Used for Energy:
Carbohydrates (like glucose): primary fuel
Lipids: long-term energy storage
Proteins: used when carbs & fats are scarce
💡 Key Takeaway:
- Cellular respiration is a universal process that all life forms use to extract usable energy from food and convert it into ATP.
- Therefore, respiration is a catabolic process, which breaks large molecules into smaller ones, releasing energy to fuel cellular activities.
3.5.A.2 – Aerobic Cellular Respiration in Eukaryotes
Aerobic cellular respiration is a step-by-step process in eukaryotic cells where enzymes help release energy from macromolecules (like glucose) in the presence of oxygen.
🧪 What Does “Aerobic” Mean?
- Aerobic = with oxygen
- Requires O₂ to fully break down glucose into CO₂ + H₂O
- Much more efficient than anaerobic respiration (without oxygen)
⚙️ Key Characteristics:
- Involves multiple enzyme-catalyzed reactions
- Stepwise breakdown allows controlled energy release
- Energy is captured in ATP molecules
- Occurs mainly inside the mitochondria (except glycolysis, which is in the cytoplasm)
🧬 Sequence of Reactions:
| 🔬 Step | 🧠 What Happens | ⚡ Energy Output |
|---|---|---|
| 1. Glycolysis | Glucose (6C) → 2 Pyruvate (3C) | 2 ATP + 2 NADH |
| 2. Pyruvate Oxidation | Pyruvate → Acetyl-CoA + CO₂ | NADH |
| 3. Krebs Cycle | Acetyl-CoA → CO₂ | 2 ATP + NADH + FADH₂ |
| 4. Electron Transport Chain (ETC) | Electrons → ATP via proton gradient | ~28 ATP |
⚡ Why So Many Steps?
- Each enzyme plays a role in breaking bonds and capturing energy
- Prevents energy from being lost as heat
- Allows cells to regulate and adapt energy production
📌 Important Terms:
- Enzyme-catalyzed = Reactions sped up by proteins (enzymes)
- Oxidation = Losing electrons (energy release)
- Reduction = Gaining electrons (energy capture)
- ATP = Energy currency of the cell
🧠 Final Thought:
Aerobic respiration is like a slow-burning fire enzymes carefully guide every step to extract maximum energy from food.
3.5.A.3 – The Electron Transport Chain (ETC) in Cellular Respiration
🎯 Big Idea:
The Electron Transport Chain (ETC) transfers electrons through a series of steps that create a proton gradient, which powers the production of ATP — the main energy molecule in cells.
⚙️ 1. What Is the ETC?
- Final step of aerobic cellular respiration
- Occurs in the inner mitochondrial membrane (in eukaryotes) or plasma membrane (in prokaryotes)
- Involves redox reactions (electron transfer)
🧪 2. How It Works
i. Electron Transfer:
- NADH and FADH₂ (from earlier steps) deliver high-energy electrons
- Electrons move through a chain of proteins to the final electron acceptor:
- Oxygen (O₂) in aerobic respiration
- Other molecules (e.g., nitrate, sulfate) in anaerobic respiration (prokaryotes)
- O₂ + electrons + protons → H₂O (in aerobic respiration)
ii. Proton Gradient Formation:
- As electrons move through the ETC, H⁺ ions are pumped across the membrane
- High H⁺ concentration in the intermembrane space
- Low H⁺ concentration in the mitochondrial matrix
- Cristae (inner membrane folds) increase surface area → more ATP can be made
- In prokaryotes, the gradient forms across the plasma membrane
iii. ATP Production via Chemiosmosis:
- H⁺ ions flow back through ATP synthase (a membrane protein)
- This flow spins ATP synthase → forms ATP from ADP + Pi
- This process is called Oxidative Phosphorylation (powered by oxygen)
iv. Heat Production:
- Sometimes ETC and ATP production are decoupled
- Electrons still move, but ATP is not made
- Instead, energy is released as heat → helps endothermic organisms (like mammals) regulate body temperature
🧠 Key Terms Recap:
| Term | Meaning |
|---|---|
| ETC | Series of proteins that transfer electrons |
| NADH/FADH₂ | Electron carriers |
| Chemiosmosis | Movement of H⁺ through ATP synthase |
| Oxidative Phosphorylation | Making ATP using energy from ETC and oxygen |
| Cristae | Folds of the inner mitochondrial membrane |
3.5.B – How Cells Obtain Energy from Biological Macromolecules
🎯 Big Idea:
Cells break down macromolecules like carbohydrates, fats, and proteins to release energy in the form of ATP, which powers all cell functions.
🍞 1. Main Energy Sources
- Carbohydrates → main & fastest source of energy (e.g., glucose)
- Lipids → store more energy per gram (used during long-term energy needs)
- Proteins → used as a backup energy source (mainly for growth & repair)
⚙️ 2. Energy Extraction Overview
| Process | Oxygen? | ATP Yield | Where It Happens |
|---|---|---|---|
| Aerobic Respiration | Yes | High (≈ 36–38 ATP/glucose) | Mitochondria |
| Anaerobic Respiration / Fermentation | No | Low (2 ATP/glucose) | Cytoplasm |
🧪 3. General Flow of Energy Extraction (Simplified):
- Glycolysis:
- Glucose → Pyruvate
- Small ATP yield
- Occurs in cytoplasm
- Anaerobic (no O₂ needed)
- Krebs Cycle (Citric Acid Cycle):
- Occurs in mitochondria (matrix)
- Breaks down pyruvate
- Produces NADH and FADH₂
- ETC & Chemiosmosis:
- Electrons power proton pumping
- ATP formed via ATP synthase
- O₂ is final electron acceptor → H₂O
🔁 4. Fermentation (When Oxygen Is Not Present)
- Less efficient backup process
- Produces only 2 ATP per glucose
- By-products include:
- Lactic acid (in animals)
- Alcohol + CO₂ (in yeast)
🔑 5. Why This Energy Matters
ATP powers essential cellular processes like:
- Muscle contractions
- Active transport (e.g., Na⁺/K⁺ pumps)
- Macromolecule synthesis
- Cell division and repair
📌 Summary Points:
- Energy comes from breaking down macromolecules
- ATP is the usable energy form for cells
- Aerobic respiration is efficient; fermentation is a backup
- Cells must constantly regenerate ATP to survive
3.5.B.1 – Glycolysis: Energy from Glucose
🧪 What is Glycolysis?
Glycolysis is the first step in breaking down glucose to get energy.
- Occurs in the cytoplasm of all cells.
- Does not require oxygen (anaerobic process).
⚙️ What Happens in Glycolysis?
Glucose (6-carbon) is broken down into 2 molecules of pyruvate (3-carbon).
A small amount of energy is released and stored as:
- ATP (energy currency of the cell)
- NADH (electron carrier, comes from NAD⁺)
📦 Key Products of Glycolysis:
| Product | Amount (per glucose) | Purpose |
|---|---|---|
| ATP | 2 net ATP | Immediate energy |
| NADH | 2 | For electron transport later |
| Pyruvate | 2 | Goes to Krebs cycle or fermentation |
🚫 No Oxygen Needed!
Glycolysis can happen even when oxygen isn’t present.
- If oxygen is available → aerobic respiration continues.
- If not → pyruvate is used in fermentation.
🧠 Why It’s Important:
- It’s the universal first step in cellular respiration.
- Found in all living organisms → supports idea of common ancestry.
- Fast way to make quick energy, especially in emergencies (like sprinting).
3.5.B.2 – Pyruvate Oxidation & Krebs Cycle (Citric Acid Cycle)
🚛 1. What Happens to Pyruvate?
- After glycolysis, pyruvate (3-carbon) is moved from the cytosol into the mitochondrion.
- It gets oxidized (broken down) before entering the next stage.
🔄 2. Pyruvate Oxidation:
- Each pyruvate loses 1 carbon as CO₂.
- The remaining 2-carbon fragment becomes acetyl-CoA.
- During this step:
- NAD⁺ → NADH (electron carrier made)
- CO₂ is released as a waste product.
🔁 3. Krebs Cycle (Citric Acid Cycle):
Acetyl-CoA enters the cycle in the mitochondrial matrix.
It’s fully broken down, releasing:
- More CO₂
- More NADH and FADH₂ (these go to ETC later)
- A small amount of ATP
💡 4. What’s Produced (per 1 glucose = 2 pyruvates)?
| Molecule | Made in Krebs Cycle |
|---|---|
| CO₂ | 4 |
| NADH | 6 |
| FADH₂ | 2 |
| ATP (direct) | 2 |
🔋 5. Why This Matters:
- NADH & FADH₂ store high-energy electrons → used in the electron transport chain.
- CO₂ is released as a waste gas.
- This stage prepares most of the energy output for aerobic respiration.
3.5.B.3 – Krebs Cycle (Citric Acid Cycle)
📍 Location: 🧬 Mitochondrial matrix
🔁 What Happens in the Krebs Cycle?
Carbon Atoms Removed as CO₂:
Carbon atoms are stripped from intermediates → CO₂ is released (waste gas).
ATP is Made:
A small amount of ATP is produced directly via substrate-level phosphorylation (ADP + Pi → ATP).
Electrons Are Captured:
- NAD⁺ → NADH
- FAD → FADH₂
These coenzymes grab high-energy electrons and take them to the electron transport chain (ETC).
📦 Main Products (per cycle):
| Output | Function |
|---|---|
| CO₂ | Released waste gas |
| ATP | Powers cell processes |
| NADH | Carries electrons to ETC |
| FADH₂ | Also carries electrons to ETC |
🔁 Remember: One glucose gives TWO turns of the cycle (one per pyruvate).
🧠 Why It Matters:
- The Krebs cycle completes the breakdown of glucose.
- It generates most of the cell’s high-energy electron carriers.
- It sets the stage for the massive ATP payoff in oxidative phosphorylation.
3.5.B.4 – Electron Transfer to the ETC (Electron Transport Chain)
📍 Location: 🧬 Inner mitochondrial membrane
🔋 Key Idea:
Electrons are extracted during glycolysis and the Krebs cycle, and then delivered to the ETC by coenzymes:
- NADH (from NAD⁺)
- FADH₂ (from FAD)
These carriers are like delivery trucks bringing high-energy electrons to the electron transport chain (ETC).
🛠️ Why This Is Important:
Electrons carried by NADH and FADH₂ are used in the ETC to:
- Create a proton gradient
- Drive ATP production (via oxidative phosphorylation)
3.5.B.5 – Proton Gradient in the ETC
🧬 Location: Inner mitochondrial membrane (in eukaryotes)
⚡ Key Concept:
As electrons move through the electron transport chain (ETC), they are passed from one molecule to another in a series of redox (oxidation-reduction) reactions.
🧪 What Happens?
These reactions pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
This creates a proton gradient:
- High H⁺ (low pH) in the intermembrane space
- Low H⁺ (high pH) in the matrix
This difference in proton concentration and charge is called an electrochemical gradient.
🎯 Why This Matters:
The proton gradient stores potential energy like a battery.
It’s later used to power ATP synthesis (via ATP synthase) – this process is called chemiosmosis.
🧠 In Short:
“Electrons move → protons pumped → gradient formed → ATP made ✅”
3.5.B.6 – Fermentation
🚫 When No Oxygen is Present:
- Fermentation is an anaerobic process (no oxygen required).
- It allows glycolysis to keep going by recycling NAD⁺.
🔁 What Does It Do?
Converts pyruvate (from glycolysis) into organic end products:
- Alcohol fermentation → produces ethanol + CO₂
- Lactic acid fermentation → produces lactic acid
🔋 Why It Matters:
- It regenerates NAD⁺ needed for glycolysis to continue making ATP.
- Provides short-term energy in low-oxygen environments (e.g. muscle cells during intense exercise).
🧠 Summary:
Fermentation = backup plan for making ATP without oxygen