What is Membrane Transport?
Membrane transport is the movement of substances like ions, nutrients, and waste across a cell's plasma membrane. Because the membrane is selectively permeable, cells use passive transport (no energy) and active transport (using ATP) to control exactly what enters and leaves.
Membrane transport is how molecules cross the cell membrane, either passively down their concentration gradient (diffusion, facilitated diffusion, osmosis) or actively against their gradient using ATP energy (active transport, such as the sodium-potassium pump).
- •Moves substances down the concentration gradient
- •Requires no ATP energy
- •Includes simple diffusion, facilitated diffusion, and osmosis
- •Example: oxygen diffusing into a cell
- •Moves substances against the concentration gradient
- •Requires ATP energy
- •Includes the sodium-potassium pump and endocytosis/exocytosis
- •Example: nerve cells pumping Na⁺ out and K⁺ in
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Step-by-step worked examples
A cell membrane has a surface area of 20 cm², a concentration difference of 15 mmol/L across it, and a thickness of 2 µm. Find the relative diffusion rate.
Use Rate = (SA × ΔC) / d. Rate = (20 × 15) / 2 Rate = 300 / 2 = 150 relative units.
A red blood cell is placed in a hypertonic solution (higher solute concentration outside than inside). Predict what happens to the cell and name the process.
Water moves by osmosis, from an area of high water concentration to low water concentration. Inside the cell there is more water (less solute) than outside, so water flows out of the cell. The cell loses water and shrinks — this is called crenation.
A nerve cell uses the sodium-potassium pump to move 3 Na⁺ ions out and 2 K⁺ ions in per cycle, against their concentration gradients. Which type of transport is this, and what powers it?
Both ions move against their concentration gradients (Na⁺ out despite being more concentrated outside already growing, K⁺ in despite being more concentrated inside), so this is active transport. Moving substances against a gradient always requires energy. The energy is supplied by ATP hydrolysis, which is why it is called the sodium-potassium ATPase pump.
Flashcards
Quick quiz
Q1.Which of these transport types requires ATP energy?
Q2.A membrane has SA = 10 cm², ΔC = 20 mmol/L, and d = 2 µm. What is the relative diffusion rate?
Q3.What happens to a cell placed in a hypertonic solution?
Q4.Which best describes facilitated diffusion?
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Common mistakes
Thinking all membrane transport requires energy. — Correct: Only active transport requires ATP; passive transport (diffusion, osmosis, facilitated diffusion) does not.
Confusing diffusion with osmosis. — Correct: Diffusion applies to any particle; osmosis specifically refers to the diffusion of water across a membrane.
Believing facilitated diffusion needs ATP because it uses a protein channel. — Correct: Facilitated diffusion still moves substances down their gradient, so it remains passive and needs no ATP.
Assuming increasing membrane thickness speeds up diffusion. — Correct: Diffusion rate is inversely related to membrane thickness — a thicker membrane slows diffusion, as shown by dividing by d in the rate formula.
FAQ
What is membrane transport?
It is the movement of molecules and ions across a cell's plasma membrane, either passively (no energy) or actively (using ATP).
What is the formula for the rate of diffusion across a membrane?
Rate = (Surface area × Concentration difference) / Membrane thickness — diffusion speeds up with more surface area or a bigger concentration gradient, and slows with a thicker membrane.
What are examples of membrane transport?
Oxygen diffusing into cells, water moving by osmosis, glucose entering via facilitated diffusion, and the sodium-potassium pump are all classic examples.
How is diffusion rate calculated?
Multiply the membrane's surface area by the concentration difference across it, then divide by the membrane's thickness.