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Transport
Mechanisms

Explore how molecules cross cell membranes through diffusion, facilitated transport, and active transport: the essential processes that keep every cell alive.

Biochemistry

Why Transport Matters

Cells are surrounded by a selectively permeable phospholipid bilayer. This membrane controls which substances enter and exit. Without transport mechanisms, cells could not obtain nutrients, remove waste, or maintain homeostasis.

The Cell Membrane Structure

Channel Carrier
\(\text{O}_2\)
\(\text{Na}^+\)
\(\text{Glucose}\)
 Extracellular Intracellular Phospholipid Bilayer

The Concentration Gradient

A concentration gradient exists whenever a substance is more concentrated in one region than another. Molecules naturally move from high to low concentration: this is the driving force behind passive transport.

HIGH Concentration Net flux LOW Concentration "Down the gradient" = passive, no energy required

Three Transport Types

Simple Diffusion
Small, nonpolar molecules pass directly through the lipid bilayer. No proteins needed.
Facilitated Transport
Large or polar molecules move through membrane proteins (channels or carriers). Still passive.
Active Transport
Molecules move against the gradient. Requires ATP energy and carrier proteins.

Simple Diffusion in Detail

In simple diffusion, molecules move down their concentration gradient directly through the phospholipid bilayer. The rate is described by Fick's First Law:

A larger concentration difference or a thinner membrane increases the rate of diffusion. Small nonpolar molecules like \(\text{O}_2\), \(\text{CO}_2\), and \(\text{N}_2\) are the best simple diffusers.

Facilitated Transport

Lipid Bilayer Channel
\(\text{Ion}\)
 Carrier
\(\text{Glucose}\)
 Extracellular (HIGH) Intracellular (LOW)
Channel Proteins
Form a water-filled pore for ions and small polar molecules. Very fast, highly selective.
Carrier Proteins
Bind the solute and change shape to shuttle it across. Slower but handles larger molecules like glucose.

Saturation Kinetics

Unlike simple diffusion, facilitated transport shows a saturation curve. Once all carrier/channel proteins are occupied, the rate plateaus at \(V_{\max}\):

[Substrate] Concentration Rate of Transport
\(V_{\max}\)
 Simple Facilitated 0

Active Transport: Against the Flow

Active transport moves solutes from low to high concentration: against the gradient. This requires cellular energy, typically ATP hydrolysis.

\(\text{Na}^+/\text{K}^+\) Pump
3 \(\text{Na}^+\) out
2 \(\text{K}^+\) in
\(\text{ATP}\)
\(\text{ADP} + \text{P}_i\)
 Extracellular Intracellular

Primary vs Secondary Active Transport

Primary
Directly uses ATP hydrolysis to power the pump. Example: \(\text{Na}^+/\text{K}^+\) ATPase uses 1 ATP per cycle.
Secondary
Uses the electrochemical gradient created by primary transport. One ion moves down its gradient, driving another against its gradient.

Passive vs Active: The Key Line

Feature Passive Active
Energy (ATP) Not required Required
Direction High → Low Low → High
Proteins Optional (channels/carriers) Always required (pumps)
Saturation Only facilitated Yes (finite pumps)
Examples \(\text{O}_2\), \(\text{CO}_2\), glucose (GLUT) \(\text{Na}^+/\text{K}^+\) pump, \(\text{H}^+\) pump

How to Identify the Transport Type

1
Check the gradient direction. Is the substance moving from high → low concentration, or low → high?
2
Look for energy input. Is ATP being consumed, or is the movement driven purely by thermal kinetic energy?
3
Identify the molecule. Small and nonpolar → likely simple diffusion. Large or charged → needs a protein.
4
Check for saturation. If transport rate plateaus at high concentration, a protein mediator is involved.

Example 1: Oxygen in the Lungs

Scenario: Oxygen molecules move from the alveoli (air sacs) in the lungs into the blood capillaries. The partial pressure of \(\text{O}_2\) in alveolar air is about 104 mmHg, and in venous blood it is about 40 mmHg.

Alveolus
\(p\text{O}_2 = 104\text{ mmHg}\)
 Diffusion Capillary
\(p\text{O}_2 = 40\text{ mmHg}\)

Example 2: Glucose Uptake

Scenario: After a meal, blood glucose is high. Glucose enters a muscle cell through GLUT4 transporter proteins. The glucose moves from high blood concentration to low intracellular concentration.

1
Gradient direction: High → Low (down the gradient)
2
Energy: No ATP is consumed
3
Molecule: Glucose is large and polar: it cannot cross the bilayer alone
4
Conclusion: Facilitated diffusion via a carrier protein (GLUT4)

Example 3: The Sodium-Potassium Pump

Scenario: In a neuron at rest, the \(\text{Na}^+/\text{K}^+\) ATPase pumps 3 \(\text{Na}^+\) out and 2 \(\text{K}^+\) in. Sodium is already more concentrated outside, and potassium is already more concentrated inside.

Common Mistakes

✕ Facilitated = Active
Facilitated transport uses proteins but is still passive. No ATP is consumed: the gradient drives it.
✕ Osmosis is not Diffusion
Osmosis is diffusion, specifically of water across a semipermeable membrane. It follows the same gradient rules.
✕ Diffusion Stops at Equilibrium
At equilibrium, molecules still move in both directions. The net movement is zero, but individual molecules keep moving.

Summary Comparison

Feature Simple Diffusion Facilitated Active
Energy None None ATP
Gradient High → Low High → Low Low → High
Proteins None Channel / Carrier Pump
Specificity Low High Very high
Saturable No Yes Yes
Examples \(\text{O}_2\), \(\text{CO}_2\), steroids Glucose, ions (channels) \(\text{Na}^+/\text{K}^+\) pump

Question 1 of 5

A small, nonpolar molecule moves from a region of high concentration to a region of low concentration without any protein assistance. What type of transport is this?

Question 2 of 5

The \(\text{Na}^+/\text{K}^+\) ATPase pumps 3 \(\text{Na}^+\) out and 2 \(\text{K}^+\) in per cycle. What directly provides the energy for this process?

Question 3 of 5

Glucose enters cells via GLUT transporters, moving down its concentration gradient. At very high glucose concentrations, the transport rate reaches a plateau. This saturation effect occurs because:

Question 4 of 5

In secondary active transport, one molecule moves against its gradient while another moves down its gradient. Which of the following best describes the energy source for this process?

Question 5 of 5

At dynamic equilibrium, the net movement of molecules across a membrane by diffusion is zero. Which statement about the molecules is correct?