AP Biology 2.5 Membrane Transport Study Notes - New Syllabus Effective 2025
AP Biology 2.5 Membrane Transport Study Notes- New syllabus
AP Biology 2.5 Membrane Transport Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
2.5.A: Describe the mechanisms that organisms use to maintain solute and water balance.
2.5.B: Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.
Key Concepts:
- Membrane Transport
2.5.A – Mechanisms to Maintain Solute & Water Balance
⚖️ Why Balance Matters
- Cells must keep a stable internal environment → not too salty, not too watery
- This balance of solutes (like ions, sugars) and water is called osmotic regulation
🚛 Key Mechanisms Used by Organisms
| Mechanism | What It Does 🧪 | Example 🌿 |
| Osmosis | Passive movement of water across a membrane | Water moves into plant root cells |
| Diffusion | Solutes spread from high to low concentration | CO₂ diffuses into leaf cells |
| Facilitated diffusion | Proteins help move polar molecules without energy | Glucose enters cells via carrier |
| Active transport | Uses ATP to move solutes against concentration gradient | Sodium-potassium pump in neurons |
| Aquaporins | Channel proteins that allow faster water movement | Kidney cells reabsorb water |
| Contractile vacuoles | Pump out excess water to prevent bursting | Found in freshwater protists |
✅ Summary
To maintain water and solute balance, organisms use passive (osmosis, diffusion) and active (pumps, vacuoles) transport methods. These help cells stay hydrated, nutrient-balanced, and functioning properly.
2.5.A.1 – Membrane Permeability & Concentration Gradients
🧬 What Is a Concentration Gradient?
- A difference in solute concentration across a membrane
- One side = high
- Other side = low
🚪 How the Membrane Helps
- The membrane is selectively permeable → only allows certain molecules to pass
- This means:
- Some solutes get trapped on one side
- Others move slowly or need help
- Result? ➡️ A gradient forms
🎯 Why It Matters
- Gradients are essential for:
- Diffusion (solutes moving high → low)
- Osmosis (water balancing concentration)
- Active transport (pumps maintain or build gradients)
- Cells use gradients to move nutrients, ions, and signals!
✅ Summary
Selective permeability lets cells build and maintain gradients, which are key for transport, signaling, and homeostasis.
2.5.A.2 – Passive Transport
🧬Definition:
- Movement of substances down their concentration gradient
- → From high → low concentration
- → No ATP or energy required
🚛 Types of Passive Transport
| Type | How It Works | Example 🧪 |
| Simple diffusion | Molecules move directly through membrane | O₂ and CO₂ in/out of cells |
| Facilitated diffusion | Uses channel or carrier proteins | Glucose entering cell |
| Osmosis | Water moves through membrane or aquaporins | Water into plant root cells |
✅ Summary
Passive transport moves molecules without energy, using just the natural concentration gradient. It’s essential for gas exchange, water balance, and nutrient flow.
2.5.A.3 – Active Transport
Moves molecules against the gradient using energy (usually ATP).
🔋 What Is Active Transport?
Definition:
- Movement of substances from low → high concentration
- Goes against the gradient
- Requires energy (typically ATP)
🧪 Why It’s Important
- Used when the cell needs to:
- Accumulate nutrients inside (even if there’s more already)
- Remove wastes or ions against their flow
- Maintain internal balance (like ion concentrations)
🚛 Examples
| Example | Description |
| Sodium-potassium pump | Pushes 3 Na⁺ out & 2 K⁺ in using ATP (nerve cells) |
| Proton pump (H⁺ pump) | Moves H⁺ across membranes to create gradients |
| Endocytosis / Exocytosis | Large molecules transported using vesicles |
✅ Summary
Active transport pushes molecules against their natural flow using energy, helping the cell stay balanced and functional in all conditions.
2.5.B – Transport of Large Molecules Across the Membrane
Organisms use special transport methods to move large molecules into or out of cells.
🧱 Why Special Transport Is Needed
Large molecules (like proteins, polysaccharides, waste):
Too big for channels or pumps
Cell uses bulk transport via vesicles
Requires energy (ATP) = active process
🚪 Main Bulk Transport Mechanisms
| Type | Direction | What It Does 🧪 | Example 🧍🌿 |
| Endocytosis | Into the cell | Cell engulfs large particles or fluids using membrane | White blood cell engulfing bacteria |
| 1. Phagocytosis | “Cell eating” – Engulfs solids | Amoeba taking in food | |
| 2. Pinocytosis | “Cell drinking” – Engulfs fluids | Absorbing extracellular fluid | |
| Exocytosis | Out of the cell | Releases materials via vesicles fusing with the membrane | Secreting hormones, enzymes |
✅ Summary
To move big molecules, cells use vesicle-based transport like endocytosis (in) and exocytosis (out), which require energy and help maintain internal conditions.
2.5.B.1 – Endocytosis & Exocytosis (Bulk Transport)
Cells use energy to move large substances in and out using vesicles.
⚡ Why Use Vesicles?
- Some molecules are too big for protein channels (e.g., proteins, wastes, bacteria)
- Cells use vesicle transport → needs ATP
🔄 Two Main Processes
| Process | Direction | How It Works 🧪 | Example 🚀 |
| Endocytosis | Into the cell | Plasma membrane folds inward, forms vesicle that traps material from outside | White blood cells engulf bacteria (phagocytosis) |
| Exocytosis | Out of the cell | Vesicle from inside the cell fuses with plasma membrane and releases contents outside | Pancreas releasing insulin hormone |
✅ Summary
Endocytosis = bringing large substances in by forming vesicles
Exocytosis = sending large substances out by fusing vesicles with membrane
Both processes require energy and help cells manage communication, defense, and waste.