What is the Cell Membrane?
The cell membrane (plasma membrane) is the selectively permeable barrier surrounding every cell, built from a phospholipid bilayer studded with proteins. It controls what enters and exits the cell, letting it maintain a stable internal environment.
The cell membrane is a phospholipid bilayer with embedded proteins that separates a cell's interior from its surroundings and selectively controls the movement of substances in and out.
- •No energy (ATP) required
- •Moves with the concentration gradient (high→low)
- •Includes diffusion, osmosis, facilitated diffusion
- •Example: O2 entering a cell
- •Requires ATP energy
- •Moves against the concentration gradient (low→high)
- •Uses carrier proteins (pumps)
- •Example: Na+/K+ pump
Step-by-step worked examples
A red blood cell is placed in distilled (pure) water. Predict and explain what happens.
Distilled water has a much lower solute concentration than the cell's cytoplasm. Water moves into the cell by osmosis, from low solute (outside) to high solute (inside) concentration. The cell swells and may burst (lyse) because water keeps entering to equalize concentration.
Oxygen must move from the lungs' air sacs into the blood. Which transport type is used and why?
Oxygen concentration is higher in the air sacs than in the blood. Oxygen moves down its concentration gradient, from high to low. No ATP is needed, so this is simple diffusion (passive transport).
A nerve cell must keep sodium ions low inside and potassium ions high inside, against their gradients. Which mechanism does this and what does it cost?
Both ions must move against their natural concentration gradient. This requires a carrier protein and energy: the Na+/K+ pump. Each pump cycle uses one ATP molecule — this is active transport.
Flashcards
Quick quiz
Q1.What type of molecule forms the main structure of the cell membrane?
Q2.Which transport process requires ATP?
Q3.A cell placed in a hypertonic (high-solute) solution will...
Q4.Which best describes facilitated diffusion?
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Common mistakes
Thinking diffusion requires energy. — Correct: Diffusion is passive — it uses the natural kinetic energy of particles, not ATP.
Believing osmosis is about solute movement. — Correct: Osmosis is specifically water movement across a membrane.
Assuming all proteins in the membrane transport molecules. — Correct: Some proteins are receptors, enzymes, or structural anchors, not transporters.
Confusing hypertonic and hypotonic effects on cells. — Correct: Hypertonic = water leaves cell (shrinks); hypotonic = water enters cell (swells).
FAQ
What is the cell membrane?
A phospholipid bilayer embedded with proteins, cholesterol, and carbohydrate chains.
What is cell membrane transport?
It's the collective process by which substances (ions, gases, molecules) cross the plasma membrane via diffusion, osmosis, facilitated diffusion, or active transport.
What are examples of cell membrane transport?
Oxygen diffusing into cells, water moving by osmosis, glucose entering via facilitated diffusion, and the Na+/K+ pump using active transport.
How is transport direction determined across the membrane?
Passive transport follows the concentration gradient (high to low); active transport moves against it and requires ATP.