2.5 Membrane Transport

Membrane transport is how cells move materials across the plasma membrane to keep their internal conditions stable. Passive transport, including simple diffusion, facilitated diffusion, and osmosis, moves substances down a concentration gradient without energy. Active transport and bulk transport, including endocytosis and exocytosis, use ATP to move substances against gradients or move large amounts of material. For AP Biology, tie each transport type to gradient direction and energy use.

Why This Matters for the AP Biology Exam

This topic shows up across multiple-choice questions and evidence-based written responses because it connects membrane structure directly to function. You will be asked to predict which molecules can cross a membrane, decide whether a process needs energy, and explain how cells use these mechanisms to maintain homeostasis. Expect to interpret diagrams of membranes and transport proteins, analyze data about solute movement, and use the right vocabulary (gradient, passive, active, endocytosis, exocytosis) instead of analogies. Connecting transport mechanisms to energy use and gradient direction is exactly the kind of structure-to-function reasoning AP Biology rewards.

Key Takeaways

• Selective permeability comes from the hydrophobic membrane interior, which lets small nonpolar molecules cross freely but blocks ions and large polar molecules.
• Passive transport moves molecules down a concentration gradient with no metabolic energy input; active transport uses ATP and can move molecules against the gradient.
• Concentration gradients store potential energy and let cells keep internal conditions different from the outside.
• Endocytosis and exocytosis move large molecules or large amounts of material and both require energy.
• The direction of movement (high to low vs. low to high) is your fastest clue for whether a process is passive or active.
• Cells combine multiple transport mechanisms to maintain solute and water balance.

Selective Permeability and What Can Cross

The plasma membrane separates the inside of the cell from the external environment, and that separation lets a cell keep internal conditions different from its surroundings. This control comes from selective permeability, which results from the membrane's hydrophobic interior.

The hydrophobic fatty acid tails in the middle of the phospholipid bilayer repel charged and polar substances, making it hard for them to cross. Whether a molecule can pass depends on its size, charge, and polarity.

• Small nonpolar molecules (O₂, CO₂, N₂) pass directly through the bilayer without help.
• Small polar uncharged molecules (H₂O, NH₃) pass through in small amounts, but larger movements of water rely on transport proteins called aquaporins.
• Large polar molecules and ions (glucose, amino acids, Na⁺, K⁺) cannot cross on their own and need channel or transport proteins.

Quick Reference: How Molecules Cross the Membrane

Concentration Gradients

Selective permeability allows cells to build concentration gradients, which exist when a substance has different concentrations on either side of the membrane. These gradients matter because they:

• Drive passive transport
• Store potential energy the cell can use
• Let the cell maintain internal conditions different from the environment

Passive Transport

Passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy. Substances move down their gradient, similar to water flowing downhill.

1. Simple Diffusion

   • Molecules move directly through the phospholipid bilayer
   • No transport proteins needed
   • Works for small nonpolar molecules (O₂, CO₂, N₂)
   • Example: oxygen diffusing from your lungs into your bloodstream

2. Facilitated Diffusion

   • Still moves down the gradient (high to low)
   • Requires transport or channel proteins
   • Used for polar molecules and ions that cannot pass through the hydrophobic core
   • Example: glucose entering cells through transport proteins

3. Osmosis

   • Diffusion of water across a selectively permeable membrane
   • Water moves from higher free water concentration (lower solute concentration) to lower free water concentration (higher solute concentration)
   • Aquaporins can speed water movement, but osmosis is not limited to aquaporins
   • Example: water moving into or out of a cell depending on solute concentrations

Active Transport

Active transport requires the direct input of energy to move molecules, and in some cases it moves molecules from regions of low concentration to regions of high concentration (against the gradient). This energy usually comes from ATP. Picture pushing a boulder uphill: moving against the natural direction takes energy.

Key features of active transport:

• Uses membrane proteins, often called pumps
• Uses energy such as ATP
• Can create and maintain concentration gradients
• Important for processes like nerve function and nutrient absorption

Example: the sodium-potassium pump moves sodium ions out of the cell and potassium ions in, even though this works against their gradients.

Maintaining Solute and Water Balance

Organisms rely on these mechanisms to keep solutes and water in balance, which supports survival:

1. Selective permeability creates control. By regulating what crosses, cells maintain different concentrations inside versus outside.
2. Active transport maintains gradients. Cells use energy to pump substances against their gradients.
3. Passive transport allows equilibration. When appropriate, substances move down their gradients to balance conditions.
4. Coordinated transport. Cells often combine several mechanisms to maintain homeostasis.

Transport of Large Molecules

Molecules too big to fit through transport proteins move in bulk using vesicles. Both endocytosis and exocytosis require energy.

Exocytosis: Moving Material Out

• Internal vesicles fuse with the plasma membrane
• Contents are secreted to the outside
• Requires energy
• Used for secreting substances like hormones, neurotransmitters, and wastes

Endocytosis: Bringing Material In

• The plasma membrane folds inward, forming a vesicle around material
• The vesicle pinches off and enters the cell
• Requires energy

Three main types of endocytosis:

1. Phagocytosis ("cell eating")

   • Cell engulfs large particles or whole microorganisms
   • Forms food vacuoles
   • Common in white blood cells that take in bacteria

2. Pinocytosis ("cell drinking")

   • Cell takes in liquid droplets with dissolved substances
   • Non-specific: brings in whatever is in the liquid

3. Receptor-Mediated Endocytosis

   • Specific: only brings in molecules that bind to receptors
   • Receptors cluster in coated pits that form vesicles
   • Examples: cholesterol uptake, insulin uptake

How to Use This on the AP Biology Exam

Multiple Choice

• Use the direction of movement to classify a process fast. High to low with no energy is passive; low to high almost always means active transport.
• Match molecule type to mechanism. If a question shows a charged ion or large polar molecule crossing, expect a transport protein and not simple diffusion.
• Watch for energy cues. If a prompt mentions ATP, pumps, or moving against a gradient, the answer involves active transport, endocytosis, or exocytosis.

Written Responses

• Use precise terms (gradient, passive transport, active transport, endocytosis, exocytosis) rather than analogies. Analogy-only answers tend to miss the underlying concept.
• When you explain a transport process, state whether energy is required and which direction the substance moves relative to its gradient.
• Connect structure to function. Tie selective permeability back to the hydrophobic interior of the membrane when explaining why a molecule does or does not cross.

Data and Diagrams

• For membrane diagrams, identify whether a molecule is crossing through the bilayer or through a protein, and use that to name the mechanism.
• When analyzing solute data, link concentration differences to the predicted direction of movement.

Common Trap

• Forgetting that facilitated diffusion uses proteins but does not use energy. Proteins involved does not automatically mean active transport.

Common Misconceptions

• All transport across the membrane requires energy. Passive transport (simple diffusion, facilitated diffusion, osmosis) needs no metabolic energy because substances move down their gradient.
• Facilitated diffusion is a type of active transport because it uses proteins. It uses transport proteins but still moves substances down the gradient without ATP, so it is passive.
• Water cannot cross the membrane without aquaporins. Small amounts of water can pass directly through the bilayer; aquaporins speed up larger movements but are not strictly required for osmosis.
• Active transport always moves things from low to high concentration. It always requires energy, and it is often used to move against a gradient, but the defining feature is the direct input of energy.
• Endocytosis and exocytosis are passive because no pump is shown. Both move large amounts of material using vesicles and require energy.
• Concentration gradient means the substance is stuck. A gradient stores potential energy and is what drives passive movement when the membrane allows it.

Related AP Biology Guides

• Unit 2 Overview: Cell Structure and Function
• 2.1 Cell Structure and Function
• 2.2 Cell Size
• 2.4 Membrane Permeability
• 2.3 Plasma Membrane
• 2.6 Facilitated Diffusion