AP Biology 3.3 Cellular Energy Study Notes - New Syllabus Effective 2025
AP Biology 3.3 Cellular Energy Study Notes- New syllabus
AP Biology 3.3 Cellular Energy Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the role of energy in living organisms.
Key Concepts:
- Cellular Energy
3.3.A – Role of Energy in Living Organisms
🧬 Why Energy Matters:
- All living things need energy to grow, reproduce, maintain homeostasis, and carry out vital cellular functions.
- Energy is the driving force behind every biological process.
⚡ Key Functions Powered by Energy:
- Active transport across cell membranes
- Building macromolecules (like proteins & nucleic acids)
- Breaking down nutrients for usable energy (catabolism)
- Muscle contractions in animals
- Repairing cells and making new ones
🔄 Energy Transformation:
Organisms convert chemical energy (like glucose) into usable cellular energy (ATP).
This occurs through key biological processes:
- 🧪 Cellular respiration (in animals and plants)
- 🌞 Photosynthesis (in plants and algae)
🧠 Big Idea:
Energy flows through ecosystems and powers life at the cellular level.
3.3.A.1 – All Living Systems Require Energy
🌱 Big Idea:
Every living organism -whether it’s a tiny bacterium or a human needs a constant supply of energy to stay alive and function.
⚙️ Why Energy Is Required:
- To grow and repair cells
- To maintain internal balance (homeostasis)
- To power chemical reactions (like building proteins)
- To transport molecules across membranes
- To send signals (like nerve impulses)
📥 Source of Energy:
- Autotrophs (like plants) → Capture light energy via photosynthesis
- Heterotrophs (like animals) → Get energy by consuming other organisms
⚡ Key Term:
ATP (Adenosine Triphosphate) – The main energy currency used by cells
✅ Reminder:
Without energy input, cells would shut down and life would stop.
3.3.A.2 – Energy, Order, and Life
⚖️ Life & Thermodynamics:
Life runs on energy without breaking the rules of physics – especially these two laws:
1st Law of Thermodynamics → Energy cannot be created or destroyed, only transformed.
2nd Law of Thermodynamics → Systems tend toward disorder (entropy) unless energy is constantly added.
🧠 What This Means for Life:
🔁 Energy In > Energy Out
- To stay alive and organized, cells must take in more energy than they lose.
- If energy input drops too low → order breaks down → cell dies
🔗 Coupled Reactions
- Cells pair up reactions:
- 🔽 Some release energy (e.g., breakdown of glucose)
- 🔼 Others require energy (e.g., building proteins)
- Energy from one helps drive the other ➝ super efficient!
⚠️ No Energy = No Life
- If energy flow or cellular order stops → systems collapse → death
✅ Quick Recap:
To stay alive, life must fight disorder using a steady flow of energy – without it, things fall apart.
3.3.A.3 – Why Energy Pathways Are Step-by-Step
🧬 Metabolic Pathways = Controlled Energy Flow
- Biological systems use sequential reactions (step-by-step) instead of one big energy burst.
- These steps are part of a metabolic pathway – a series of enzyme-catalyzed reactions.
🔁 How It Works:
Product of one step ➝ becomes the reactant (input) for the next.
This creates a flow of reactions, like a conveyor belt.
💡 Why Do It in Steps?
⚙️ Better Control
- Energy is released in small amounts, not all at once prevents damage
🔌 Efficient Energy Transfer
- Energy can be captured and stored (like in ATP) instead of wasted as heat
🛠️ Regulation is Easier
- Enzymes can speed up, slow down, or stop the pathway at any point
📌 Example:
In cellular respiration:
Glucose → Glycolysis → Krebs Cycle → ETC
Each step is linked, and energy is harvested gradually
✅ Summary:
Living systems break down reactions into ordered steps so energy can be transferred efficiently, safely, and in a controlled manner.
3.3.B – Conserved Energy Processes Show Common Ancestry
🔬 Big Idea:
All living organisms from bacteria to humans share common energy processes, which is strong evidence that we all evolved from a common ancestor.
🧪 Shared & Conserved = Evolutionary Clues
- Shared processes = Found in many organisms
- Conserved processes = Stayed mostly unchanged over millions of years
➡️ These are too specific and complex to have evolved independently in different lineages → must be inherited!
⚡ Key Conserved Energy Features in All Life:
| Feature | What It Is | Why It Matters |
|---|---|---|
| ATP | Universal energy currency | Used in every known organism |
| Glycolysis | First step of breaking down glucose | Happens in cytoplasm in almost all life |
| Electron Transport Chain (ETC) | Series of proteins that transfer electrons to make ATP | Found in both mitochondria (eukaryotes) & plasma membranes (prokaryotes) |
| Chemiosmosis | Movement of protons (H⁺) to make ATP | Conserved from bacteria to humans |
🧬 What This Proves:
- These energy-making methods are ancient and were likely present in the earliest cells.
- It’s highly unlikely these complex processes evolved separately in different species.
- This supports the theory of common ancestry – we all evolved from a shared ancestor with these traits.
✅ Quick Summary:
The fact that all living organisms use the same energy molecules (like ATP), the same pathways (like glycolysis), and the same structures (like ETC proteins) shows that we’re all connected by evolution.
3.3.B.1 – Core Metabolic Pathways Are Conserved Across Life
🌐 Big Idea:
Core energy pathways like glycolysis and oxidative phosphorylation are found in ALL domains of life:
- 🧫 Bacteria
- 🌋 Archaea
- 🌱 Eukarya
This means these processes are ancient, essential, and came from a common ancestor.
⚙️ Core Metabolic Pathways You Need to Know:
| 🧪 Pathway | 🔍 Description | 💡 Why It Matters |
|---|---|---|
| Glycolysis | Breaks glucose into pyruvate in the cytoplasm | Happens in almost all cells, with or without oxygen |
| Oxidative Phosphorylation | Uses electron transport chain & chemiosmosis tomake ATP | Occurs in mitochondria (eukaryotes) or membranes (prokaryotes) |
🧬 Why Is This Important?
- These pathways are conserved – barely changed through evolution
- Used by all types of cells – evidence of a universal ancestor
- Too complex to have evolved independently – must have been inherited
🧠 Memory Tip:
“GO = Glycolysis + Oxidative phosphorylation” = Life’s Go-To energy paths.
No matter if it’s a bacterium or a blue whale – they use these same core steps!
✅ Key Takeaway:
The fact that organisms from all domains use the same core metabolic pathways supports the evolutionary theory of common ancestry.