Section 4 Cellular Transport- Study Guide

What Is Cellular Transport?

Cells don't exist in isolation. They constantly move materials in and out through their membrane. Cellular transport is the process that controls what enters and leaves a cell.

This is a membrane selectivity topic. The cell membrane decides what passes through based on size, charge, and concentration gradients. Master this and you master one of the most tested concepts in biology.

Passive Transport: No Energy Required

Passive transport moves substances down their concentration gradient. That means from high concentration to low concentration. Cells don't spend ATP doing this.

Simple Diffusion

Small, nonpolar molecules slip through the phospholipid bilayer directly. Oxygen, carbon dioxide, and nitrogen move this way.

The rate depends on:

• Concentration difference between inside and outside
• Temperature
• Size of the molecule
• Properties of the membrane itself

Osmosis: Diffusion of Water

Water moves across membranes too. Osmosis is specifically the diffusion of water from an area of high water potential to low water potential.

You need to know the three scenarios:

• Isotonic — equal solute concentration on both sides. No net water movement.
• Hypotonic — lower solute concentration outside the cell. Water moves in. Animal cells may burst; plant cells become turgid.
• Hypertonic — higher solute concentration outside the cell. Water moves out. Animal cells shrivel; plant cells undergo plasmolysis.

Facilitated Diffusion

Large or polar molecules can't diffuse through the phospholipid bilayer alone. They use channel proteins or carrier proteins embedded in the membrane.

Channel proteins form pores that specific substances pass through. Carrier proteins change shape to shuttle molecules across.

This is still passive transport — no ATP is used. The molecules still move down their concentration gradient.

Active Transport: Energy Required

Active transport moves substances against their concentration gradient. Low concentration to high concentration. This requires ATP energy and specific protein pumps.

The Sodium-Potassium Pump

This is the most important example. In nerve cells especially, this pump:

• Moves 3 sodium ions out of the cell
• Moves 2 potassium ions into the cell
• Uses 1 ATP molecule per cycle

The result is a higher concentration of Na+ outside and higher K+ inside. This electrical gradient is critical for nerve impulses.

Other Types of Active Transport

Proton pumps move H+ ions across membranes, creating electrochemical gradients used to power other transport processes.

Cotransport uses the energy from one substance moving down its gradient to drive another substance against its gradient. This is how plants absorb glucose from root hairs.

Vesicle-Mediated Transport

Exocytosis

Materials exit the cell using vesicles. The cell packages substances into a vesicle at the Golgi apparatus. The vesicle moves to the cell membrane, fuses with it, and releases its contents outside.

This is how cells secrete hormones, neurotransmitters, and digestive enzymes.

Endocytosis

Materials enter the cell by the cell membrane wrapping around them and pinching off to form a vesicle inside.

Three main types:

• Phagocytosis — "cell eating." Large solid particles like bacteria get engulfed. The vesicle is called a phagosome.
• Pinocytosis — "cell drinking." Fluids and dissolved substances are brought in. Nonspecific uptake.
• Receptor-mediated endocytosis — Specific molecules bind to receptors on the cell surface. This triggers vesicle formation. Cholesterol uptake works this way.

Exocytosis vs. Endocytosis Comparison

Transport Mechanisms Comparison

How to Actually Learn This

Reading isn't enough. Here's what actually works:

Draw the Diagrams

Sketch cells in isotonic, hypotonic, and hypertonic solutions. Label water movement direction. Show what happens to animal versus plant cells in each scenario. This is the fastest way to internalize osmosis.

Memorize the Pump

Write out the sodium-potassium pump cycle until you can recite it without notes. 3 Na+ out, 2 K+ in, 1 ATP used. Repeat until it's automatic.

Match Examples to Mechanisms

For each transport type, you need one concrete example you can recall instantly. Oxygen diffusing into lungs. Plant roots absorbing water. Insulin being released from a cell. If you can't name an example, you don't know the mechanism.

Practice Questions

Look for questions that:

• Show a beaker with different solute concentrations and ask which direction water flows
• Describe a substance and ask which transport type moves it
• Give a scenario and ask whether ATP is required

If you can answer those three question types correctly, you understand cellular transport.

Common Mistakes to Avoid

• Confusing diffusion with active transport. If no ATP is used, it's passive. That's the dividing line.
• Forgetting that plant cells behave differently. They have cell walls, so they don't burst in hypotonic solutions — they become turgid instead.
• Mixing up endocytosis types. Phagocytosis is for large solids. Pinocytosis is for fluids. Receptor-mediated is for specific molecules.
• Thinking osmosis ignores solute. Water moves toward higher solute concentration, not away from water. The solute concentration is what drives the movement.

What to Study Next

After cellular transport, you need the cell membrane structure to understand why these transport mechanisms work. Review the phospholipid bilayer, the fluid mosaic model, and how membrane proteins function. Those concepts explain why transport happens the way it does.