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Facilitated
Diffusion

How cells move molecules across the membrane without spending energy: the role of transport proteins in passive transport.

AP Biology

What Is Diffusion?

Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration. This movement is driven by the kinetic energy of molecules and continues until equilibrium is reached.

The Membrane Barrier

Phospholipid Bilayer
\( \text{O}_2 \)
passes freely ✕ Glucose blocked Via protein facilitated Extracellular (HIGH) Intracellular (LOW)

Defining Facilitated Diffusion

Facilitated diffusion is the transport of molecules across a biological membrane down their concentration gradient through specific integral membrane proteins. It is a type of passive transport: no metabolic energy (ATP) is consumed.

Passive
No ATP required: molecules move down their gradient spontaneously
Protein-Assisted
Requires channel proteins or carrier proteins embedded in the membrane
Specific
Each protein transports only certain molecules: transport is selective

Two Types of Transport Proteins

+ + Channel Ions, water (aquaporins) G G Carrier Glucose, amino acids HIGH concentration HIGH concentration
Channel Proteins
Form a hydrophilic pore through the membrane. Molecules flow directly through. Example: aquaporins for water.
Carrier Proteins
Bind to a solute, undergo a conformational change, then release it on the other side. Example: GLUT1 for glucose.

The Concentration Gradient

Concentration Time Equilibrium Extracellular Conc. Intracellular Conc. Net flux

Saturation Kinetics

Rate of transport Solute concentration Simple diffusion Facilitated
\( V_{\max} \)

Facilitated vs. Simple Diffusion

Simple Diffusion Facilitated Diffusion
Protein needed? No Yes
ATP required? No No
Direction Down gradient Down gradient
Saturable? No Yes (\( V_{\max} \))
Specificity Low High
Examples \( \text{O}_2 \), \( \text{CO}_2 \), ethanol Glucose, ions, water

Passive vs. Active Transport

Facilitated Diffusion Active Transport
Energy (ATP) Not required Required
Direction Down gradient (high → low) Against gradient (low → high)
Protein needed? Yes Yes (pumps)
Saturable? Yes Yes
Example GLUT1 (glucose) \( \text{Na}^+/\text{K}^+ \) pump

How to Identify Facilitated Diffusion

1
Check the molecule: Is it large, polar, or charged? If yes, it likely cannot cross the bilayer alone.
2
Check for a protein: Is a channel or carrier protein involved? If yes, this is protein-mediated transport.
3
Check the energy source: Is ATP consumed? If no, the process is passive.
4
Check the direction: Does the solute move from high to low concentration? If yes: facilitated diffusion.

Example: Glucose Uptake

Cell Membrane GLUT1 Glc Glc Blood (5 mM) Cell (1 mM) Down gradient No ATP needed

Example: Aquaporins

AQP W W Selectivity filter: blocks ions, passes water Hypotonic side Hypertonic side Aquaporin

Example: Ion Channels in Neurons

In nerve cells, voltage-gated ion channels open in response to changes in membrane potential. When the membrane depolarizes, \( \text{Na}^+ \) channels open and sodium ions rush into the cell down their concentration gradient: this is facilitated diffusion.

\( \text{Na}^+ \) Influx
Extracellular \( \text{Na}^+ \) concentration is high (~145 mM). Intracellular is low (~12 mM). \( \text{Na}^+ \) flows inward.
\( \text{K}^+ \) Efflux
Intracellular \( \text{K}^+ \) is high (~140 mM). Extracellular is low (~4 mM). \( \text{K}^+ \) flows outward.

Factors Affecting Rate

Concentration Gradient
A steeper gradient means a faster rate of transport: more net movement per unit time.
Number of Proteins
More transport proteins increase \( V_{\max} \). At saturation, additional solute cannot speed up transport.
Temperature
Higher temperature increases molecular kinetic energy and protein flexibility, boosting transport rate.

Common Mistakes

✕ Thinking it uses ATP
Facilitated diffusion is passive. No energy is required. If ATP is involved, it is active transport.
✕ Confusing channels and carriers
Channels are open pores; carriers bind and change shape. Both facilitate diffusion, but they work differently.
✕ Saying it goes against the gradient
Facilitated diffusion always moves solutes down the gradient: from high to low concentration.

Transport Summary

Feature Simple Diffusion Facilitated Diffusion Active Transport
Protein? No Yes Yes (pump)
ATP? No No Yes
Direction High → Low High → Low Low → High
\( V_{\max} \)? No Yes Yes
Specificity Low High High

Question 1 of 5

Which of the following is not a characteristic of facilitated diffusion?

Question 2 of 5

A cell is placed in a solution where glucose concentration outside is 10 mM and inside is 2 mM. GLUT1 carriers are present. What will happen?

Question 3 of 5

What distinguishes a channel protein from a carrier protein in facilitated diffusion?

Question 4 of 5

A researcher observes that increasing solute concentration beyond a certain point no longer increases the rate of transport across a membrane. This observation is best explained by:

Question 5 of 5

Which of the following processes is an example of facilitated diffusion?