Cell Membrane Transport – AP Biology

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Cell Membrane Transport

AP Biology · Unit 2: Cell Structure & Function

Learning Objectives

Distinguish passive, facilitated, and active transport

Explain osmosis and predict water movement

Describe the role of ATP in active transport

Compare endocytosis and exocytosis

Transport Types

Simple Diffusion: small nonpolar molecules move freely through lipid bilayer

Facilitated Diffusion: ions/polar molecules use channel/carrier proteins, no ATP

Active Transport: against gradient, requires ATP (e.g., Na⁺/K⁺ pump)

Osmosis: water moves through aquaporins from low → high solute

Key Rule

Passive transport moves high → low concentration (down gradient). Active transport moves low → high concentration (against gradient, needs ATP).

TransportMembraneAP BiologyUnit 2

Simple Diffusion

Facilitated

Active Transport

Osmosis

Outside: High

Inside: Low

Net flux: → inward

Outside Concentration 80%

Inside Concentration 20%

ATP Available High

Simple Diffusion: Small nonpolar molecules (O₂, CO₂, ethanol) pass directly through the phospholipid bilayer. No energy or proteins needed. Always moves high→low concentration.

📝 Quick Quiz▼

1. Glucose enters cells via facilitated diffusion through GLUT proteins. This process requires:

ATP (active transport) A concentration gradient (passive) Both ATP and a protein channel Osmotic pressure

2. A red blood cell placed in a hypertonic solution will:

Lyse (burst) by gaining water Stay the same size Shrink (crenate) by losing water Gain solutes from outside

3. The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP. This is an example of:

Simple diffusion Facilitated diffusion Osmosis Primary active transport