← Back to course

The Membrane & Cellular Transport

You’ll be able to

The fluid-mosaic model

The plasma membrane is a phospholipid bilayer: two layers of phospholipids with their polar (hydrophilic) heads facing the watery inside and outside, and their nonpolar (hydrophobic) tails tucked together in the middle. This arrangement makes the membrane selectively permeable. The fluid-mosaic model describes it as "fluid" because the phospholipids drift sideways, and a "mosaic" because it is dotted with embedded proteins, plus cholesterol (which buffers fluidity) and carbohydrate tags for cell recognition. Embedded transport proteins create pathways for substances that cannot cross the hydrophobic core.

Passive transport: going with the gradient

Passive transport requires no energy from the cell because substances move down their concentration gradient — from high to low concentration — driven by their own random motion. Simple diffusion lets small nonpolar molecules (O₂, CO₂) slip directly through the lipid bilayer. Facilitated diffusion handles molecules that cannot cross the hydrophobic core — ions and polar molecules like glucose — by passing them through channel or carrier proteins. It is still passive: the protein only provides a path, the gradient does the work.

Active transport: pumping uphill

Active transport moves substances against their gradient — from low to high concentration — so the cell must spend ATP. Carrier proteins called pumps do this work. The classic example is the sodium-potassium pump (Na⁺/K⁺ pump), which pushes 3 Na⁺ out and 2 K⁺ in against both gradients, powered by ATP; it maintains the ion imbalances nerve cells need to fire. For material too large for any protein, the cell uses bulk transport: endocytosis engulfs material by wrapping the membrane around it to form a vesicle, and exocytosis fuses a vesicle with the membrane to expel its contents. Both move large cargo and both require energy.

Transport at a glance
passive: high → low (no ATP) · active: low → high (ATP)
The direction relative to the concentration gradient tells you whether energy is needed. "Down" the gradient is free; "up" the gradient costs ATP.
Worked example

A muscle cell already holds far more K⁺ inside than the fluid around it, yet it continues to bring in still more K⁺. Is this passive or active transport, and what does the cell need to do it?

  1. 1.Compare concentrations: K⁺ is already higher inside than outside, so bringing in more K⁺ moves it from low (outside) to high (inside) — against the gradient.
  2. 2.Moving a substance against its concentration gradient cannot happen by diffusion; it requires energy input.
  3. 3.The cell uses a carrier protein pump (the Na⁺/K⁺ pump) that hydrolyzes ATP to force K⁺ inward against the gradient.
Answer: This is active transport: because K⁺ is moving from low to high concentration (up its gradient), the cell must spend ATP, using a pump such as the Na⁺/K⁺ pump.
Checkpoint

Glucose moves into a cell through a carrier protein, traveling from a region of higher glucose concentration outside to lower concentration inside, without the cell using ATP. What type of transport is this?

Watch out

Do not equate "uses a protein" with "active." Facilitated diffusion uses a protein but is still passive — no ATP — because it follows the gradient. The deciding question is always the direction relative to the gradient, not whether a protein is involved.

Checkpoint

A white blood cell engulfs a bacterium by extending its membrane around it and pinching off a vesicle. This process is best described as:

On the exam

For any transport question, run a two-step check: (1) Which way relative to the gradient? Down = passive/no ATP, up = active/ATP. (2) Does it need a protein? Small nonpolar = simple diffusion; ion or polar = protein-mediated. Those two answers name the mechanism.

Answer the 2 checkpoints as you read.

Sign in to save your progress