What Is the Cell Membrane and How Does Transport Work?
The cell membrane is a selectively permeable barrier that controls what enters and leaves a cell. It uses passive and active transport mechanisms to move ions, nutrients and waste across the phospholipid bilayer.
The cell membrane is a phospholipid bilayer embedded with proteins that regulates the movement of substances via passive transport (diffusion, osmosis, facilitated diffusion) and active transport, which requires ATP.
- •No energy (ATP) required
- •Moves with the concentration gradient (high → low)
- •Includes simple diffusion, osmosis, facilitated diffusion
- •Example: O2 entering a cell
- •Requires ATP energy
- •Moves against the concentration gradient (low → high)
- •Uses carrier proteins like the Na+/K+ pump
- •Example: Sodium-potassium pump maintaining ion balance
Step-by-step worked examples
A red blood cell is placed in a hypotonic solution (0.1% salt) versus its normal isotonic plasma (0.9% salt). What happens?
Compare solute concentration inside vs outside the cell Outside solute (0.1%) is less than inside (~0.9%), so water moves INTO the cell by osmosis The cell swells and may burst (hemolysis)
The Na+/K+ pump moves 3 Na+ ions out and 2 K+ ions in for every 1 ATP hydrolyzed. How many ions cross per 5 ATP?
Per ATP: 3 Na+ out + 2 K+ in = 5 ions moved For 5 ATP: 5 × 5 = 25 ions total 15 Na+ out and 10 K+ in
A cell in a solution with 0.5 M glucose outside and 0.2 M glucose inside relies on facilitated diffusion via GLUT carriers. Which direction does glucose move?
Compare the gradient: outside (0.5 M) is greater than inside (0.2 M) Facilitated diffusion follows the gradient, no ATP needed Glucose moves INTO the cell through GLUT channel proteins
Flashcards
Quick quiz
Q1.Which transport type requires ATP?
Q2.A cell placed in a hypertonic solution will:
Q3.What is osmosis?
Q4.The sodium-potassium pump moves ions:
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Common mistakes
Diffusion and osmosis are the same thing. — Correct: Osmosis is diffusion of water specifically; diffusion applies to any solute.
All transport across the membrane needs energy. — Correct: Only active transport needs ATP; passive transport (diffusion, osmosis) does not.
A hypertonic solution makes a cell swell. — Correct: A hypertonic solution has more solute outside, so water leaves the cell and it shrinks.
Facilitated diffusion requires ATP because it uses proteins. — Correct: Facilitated diffusion is passive — it uses proteins as channels but no ATP is spent.
FAQ
What is the cell membrane?
A selectively permeable phospholipid bilayer that controls what enters and exits a cell, per the fluid mosaic model.
What is the rule for how transport direction is decided?
Passive transport always follows the concentration gradient (high to low); active transport moves against it and needs ATP.
What are examples of cell membrane transport?
Osmosis of water, diffusion of oxygen and CO2, facilitated diffusion of glucose, and active transport via the Na+/K+ pump.
How do you calculate how many ions the Na+/K+ pump moves?
Multiply the number of ATP molecules used by 5 (3 Na+ out + 2 K+ in per ATP) to get total ions transported.