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Membrane
Permeability

Explore how the phospholipid bilayer selectively controls what enters and exits cells: from simple diffusion to active transport.

AP Biology

What Is the Cell Membrane?

Every living cell is surrounded by a plasma membrane: a thin, flexible barrier composed primarily of a phospholipid bilayer studded with proteins. This membrane is not a rigid wall but rather a dynamic, fluid structure described by the fluid mosaic model.

The Phospholipid Bilayer

EXTRACELLULAR FLUID HYDROPHOBIC CORE INTRACELLULAR FLUID Head Tails

Why Selective Permeability Matters

Homeostasis
Maintains stable internal conditions by controlling ion and molecule concentrations.
Cell Signaling
Receptor proteins in the membrane detect hormones, neurotransmitters, and other signals.
Energy Production
Proton gradients across membranes drive ATP synthesis in mitochondria and chloroplasts.

Passive Transport: No Energy Required

Passive transport moves substances down their concentration gradient: from high concentration to low concentration. No cellular energy (ATP) is spent. The driving force is the natural tendency toward equilibrium.

Simple Diffusion
Small, nonpolar molecules (\( \text{O}_2 \), \( \text{CO}_2 \)) pass directly through the bilayer.
Facilitated Diffusion
Polar molecules and ions use channel or carrier proteins to cross.
Osmosis
Water moves through aquaporins or directly through the membrane toward higher solute concentration.

Simple Diffusion Across the Bilayer

PHOSPHOLIPID BILAYER HIGH CONCENTRATION LOW CONCENTRATION Molecules move NET downhill

Facilitated Diffusion: Protein Assistance

Channel
\(+\)
 Carrier Glc Glc EXTRACELLULAR INTRACELLULAR
Channel Proteins
Form hydrophilic tunnels for ions (\( \text{Na}^+ \), \( \text{K}^+ \), \( \text{Cl}^- \)). Some are gated (open/close in response to signals).
Carrier Proteins
Bind to specific molecules (like glucose) and change shape to shuttle them across. Slower than channels.

Osmosis: Water Follows Solute

Hypotonic Low solute Isotonic Equal solute Hypertonic High solute
\( \text{H}_2\text{O} \)
\( \text{H}_2\text{O} \)

A plant cell presses against a wall while an animal cell changes shape

Plant wall is thick and distinct from inner membrane; large central vacuole and peripheral cytoplasm visible. Animal cell has flexible boundary and no wall. Separate exact panel: Paired plant states retain the same wall while the inner membrane is pressed outward or retracted; animal membrane has no surrounding wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Wall support changes the response

Wall support changes the response. For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Active Transport: Against the Gradient

Active transport moves molecules against their concentration gradient: from low to high concentration. This requires cellular energy, usually in the form of ATP. The most famous example is the sodium-potassium pump.

\( \text{Na}^+/\text{K}^+ \)
Pump
\( \text{Na} \)
\( \text{Na} \)
\( \text{Na} \)
3 \( \text{Na}^+ \) OUT
\( \text{K} \)
\( \text{K} \)
2 \( \text{K}^+ \) IN
 ATP EXTRACELLULAR INTRACELLULAR

Passive vs Active Transport

Passive Transport Active Transport
Energy Required? No Yes (ATP)
Direction High → Low (down gradient) Low → High (against gradient)
Proteins Needed? Sometimes (facilitated) Always (pumps)
Examples \( \text{O}_2 \) diffusion, osmosis, GLUT1 \( \text{Na}^+/\text{K}^+ \) pump, \( \text{H}^+ \) pump
Reaches Equilibrium? Yes No (maintains gradient)

Bulk Transport: Endo- and Exocytosis

Large molecules and particles that cannot cross the membrane through channels or pumps are moved in vesicles: membrane-bound compartments that bud off from or fuse with the plasma membrane.

Endocytosis
Membrane folds inward to engulf material. Includes phagocytosis (solids), pinocytosis (fluids), and receptor-mediated endocytosis.
Exocytosis
Vesicles fuse with the membrane and release contents outside the cell. Used for secreting hormones, neurotransmitters, and waste.

Factors Affecting Permeability

1
Molecule size: Smaller molecules cross more easily. Large proteins cannot pass without vesicles.
2
Polarity and charge: Nonpolar molecules pass freely. Polar and charged molecules need protein assistance.
3
Temperature: Higher temperature increases membrane fluidity and diffusion rate.
4
Cholesterol content: Cholesterol buffers fluidity: prevents membranes from becoming too rigid (cold) or too fluid (warm).
5
Saturation of fatty acid tails: Unsaturated tails (kinked) increase fluidity; saturated tails pack tightly and reduce it.
A contextual liposome accompanies two strips of five two-tailed phospholipids. The low strip has two cis-kinked tails out of ten; the high strip has six out of ten. The companion has equal vesicles with ten gold dye markers each and the same exact tail counts.
Illustrative lipid-composition controls compare 2 / 10 tails cis-kinked (20%) with 6 / 10 tails cis-kinked (60%). Every strip has five two-tailed lipids. Vesicle size, trapped dye, temperature and time are matched; no measured leakage data are shown.
A contextual liposome accompanies two strips of five two-tailed phospholipids. The low strip has two cis-kinked tails out of ten; the high strip has six out of ten. The companion has equal vesicles with ten gold dye markers each and the same exact tail counts.
Illustrative lipid-composition controls compare 2 / 10 tails cis-kinked (20%) with 6 / 10 tails cis-kinked (60%). Every strip has five two-tailed lipids. Vesicle size, trapped dye, temperature and time are matched; no measured leakage data are shown.

How to Determine Transport Type

1
Identify the molecule: Is it small/nonpolar, polar, charged, or very large?
2
Check the direction: Is the molecule moving down its concentration gradient (high → low) or against it (low → high)?
3
Determine energy use: If down the gradient, it is passive. If against the gradient, it is active (needs ATP).
4
Identify the mechanism: Direct through bilayer = simple diffusion. Through a protein = facilitated diffusion or active pump. Via vesicle = endocytosis/exocytosis.

Example 1: Oxygen in the Lungs

In the alveoli of the lungs, oxygen concentration is higher in the air sacs than in the blood capillaries. How does \( \text{O}_2 \) enter the blood?

1
Molecule: \( \text{O}_2 \) is small and nonpolar.
2
Direction: High concentration (alveoli) → low concentration (blood) = down the gradient.
3
Energy: No ATP needed = passive.
4
Mechanism: Passes directly through the phospholipid bilayer = simple diffusion.

Example 2: Glucose Entering a Cell

After a meal, blood glucose is higher than intracellular glucose. Glucose enters cells via GLUT transporters. What type of transport is this?

1
Molecule: Glucose is polar and relatively large: it cannot cross the hydrophobic bilayer alone.
2
Direction: High (blood) → low (cell) = down the gradient.
3
Energy: No ATP = passive.
4
Mechanism: Requires a carrier protein (GLUT1) = facilitated diffusion.

Example 3: \( \text{Na}^+/\text{K}^+ \) Pump in Neurons

After a nerve impulse, the neuron must restore resting potential by pumping 3 \( \text{Na}^+ \) out and 2 \( \text{K}^+ \) in, both against their concentration gradients.

1
Molecules: \( \text{Na}^+ \) and \( \text{K}^+ \) are charged ions: they cannot cross the hydrophobic bilayer.
2
Direction: Both move against their gradient (low → high).
3
Energy: Requires 1 ATP per cycle = active transport.
4
Mechanism: A pump protein (\( \text{Na}^+/\text{K}^+ \) ATPase) changes shape using ATP hydrolysis.
A conceptual membrane has higher sodium outside and lower sodium in the cytosol. A violet pump moves three sodium out and two potassium in per ATP. A separate gold symporter brings one sodium and one glucose inward, using the gradient. Protein forms and their juxtaposition are schematic.
In this conceptual membrane, the Na+/K+ ATPase uses one ATP to move three Na+ out and two K+ in, maintaining a sodium electrochemical gradient. A separate symporter uses that gradient to bring Na+ and glucose inward; the 1:1 ratio is a selected teaching model and varies among transporters. The cutaways do not represent constitutively open pores: both transport cycles and the protein shapes are schematic. The juxtaposition does not assert a shared membrane face in a particular tissue.

Common Mistakes

✕ Osmosis = Solute Movement
Osmosis is the movement of water, not solute. Water moves toward higher solute concentration.
✕ Facilitated = Active
Facilitated diffusion uses proteins but no ATP. It is passive because molecules move down their gradient.
✕ All Proteins = Pumps
Channel proteins are passive (no energy). Only pump proteins use ATP for active transport.

Transport Summary

Type Energy? Protein? Example
Simple Diffusion No No \( \text{O}_2 \), \( \text{CO}_2 \), steroids
Facilitated Diffusion No Yes Glucose (GLUT), ions (channels)
Osmosis No Optional (aquaporins) \( \text{H}_2\text{O} \) across membrane
Active Transport Yes (ATP) Yes \( \text{Na}^+/\text{K}^+ \) pump, \( \text{H}^+ \) pump
Endocytosis/Exocytosis Yes (ATP) Yes (for formation) Phagocytosis, secretion

Question 1 of 5

A red blood cell is placed in a hypertonic solution. What will happen to the cell?

Question 2 of 5

Which molecule would most easily cross the phospholipid bilayer without any transport proteins?

Question 3 of 5

Facilitated diffusion differs from simple diffusion because facilitated diffusion:

Question 4 of 5

The \( \text{Na}^+/\text{K}^+ \) pump moves 3 \( \text{Na}^+ \) out and 2 \( \text{K}^+ \) in per ATP hydrolyzed. This transport is classified as:

Question 5 of 5

A cell needs to engulf a large bacterium. Which process will it use?