The Plasma
Membrane
Explore the fluid mosaic model, selective permeability, and the transport mechanisms that keep every cell alive.
AP Biology
What Is the Plasma Membrane?
The plasma membrane (cell membrane) is a thin, flexible boundary that separates a cell's interior from its external environment. It is only about 7 to 8 nm thick, yet it performs critical functions: controlling what enters and exits the cell, facilitating cell signaling, and maintaining structural integrity.
The Phospholipid Bilayer
Fluid membrane architecture preserves leaflet asymmetry
Polar phospholipid heads face water on both sides of the membrane while hydrophobic tails face inward. Carbohydrate chains on glycoproteins extend from the extracellular face.Two water-facing leaflets
Polar phospholipid heads face water on both sides of the membrane while hydrophobic tails face inward. Carbohydrate chains on glycoproteins extend from the extracellular face.Key Components of the Membrane
Phospholipids
Form the bilayer scaffold. Their amphipathic nature creates a self-sealing barrier.
Proteins
Integral proteins span the membrane; peripheral proteins sit on the surface. Both enable transport and signaling.
Cholesterol
Wedges between phospholipid tails, maintaining fluidity across temperature changes.
Carbohydrates
Glycoproteins and glycolipids on the outer surface aid in cell recognition and immune function.
The Fluid Mosaic Model
Proposed by Singer and Nicolson (1972), this model describes the membrane as a two-dimensional fluid in which proteins float like icebergs in a sea of phospholipids. The term "fluid" refers to the lateral movement of lipids and proteins, while "mosaic" describes the diverse patchwork of embedded molecules.
1
Lipids move laterally within each leaflet approximately \(10^7\) times per second.
2
Flip-flop (transverse movement between leaflets) is rare without enzyme assistance from flippases.
3
Membrane proteins drift, rotate, or are anchored by the cytoskeleton, creating functional microdomains.
Selective Permeability: The Core Rule
Passive vs Active Transport
Passive Transport
Moves substances down their concentration gradient. No energy (ATP) required. Includes simple diffusion, facilitated diffusion, and osmosis.
Active Transport
Moves substances against their concentration gradient. Requires energy (ATP). Includes the \( \text{Na}^+/\text{K}^+ \) pump and bulk transport (endo/exocytosis).
Types of Passive Transport
Simple Diffusion
Small nonpolar molecules pass directly through the bilayer: \( \text{O}_2 \), \( \text{CO}_2 \), ethanol.
Facilitated Diffusion
Polar molecules and ions pass through channel or carrier proteins: glucose via \( \text{GLUT} \), ions via ion channels.
Osmosis
The diffusion of water across a selectively permeable membrane through aquaporins, from low solute to high solute concentration.
Osmosis and Tonicity
A plant cell presses against a wall while an animal cell changes shape
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
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: The \( \text{Na}^+/\text{K}^+ \) Pump
Bulk Transport: Endocytosis and Exocytosis
Endocytosis
The membrane folds inward to engulf large particles. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.
Exocytosis
Vesicles fuse with the membrane to release contents outside the cell. Used for secreting hormones, neurotransmitters, and waste removal.
How to Identify the Transport Type
1
Determine the direction. Is the substance moving from high to low concentration (down the gradient) or from low to high (against it)?
2
Check energy use. If energy (ATP) is required, it is active transport. If not, it is passive.
3
Identify the molecule. Small and nonpolar: simple diffusion. Polar or charged: facilitated diffusion or active transport. Very large: bulk transport.
4
Look for proteins. If a transport protein is involved and no ATP is used, it is facilitated diffusion. If ATP is used, it is primary active transport.
Example 1: Red Blood Cells in Solutions
A red blood cell (RBC) is placed in a 0.1% NaCl solution. Normal blood plasma is about 0.9% NaCl. What happens to the cell?
1
Compare concentrations: 0.1% solution has less solute than the cell interior (0.9%). The solution is hypotonic relative to the cell.
2
Predict water movement: Water moves by osmosis from the hypotonic solution (more water) into the cell (less water).
3
Result: The RBC swells and may lyse (burst) because animal cells lack a cell wall to resist osmotic pressure.
Example 2: Glucose Uptake in Intestinal Cells
After a meal, glucose concentration in the intestinal lumen is higher than inside epithelial cells. But the cells also absorb glucose when concentrations are equal. How?
1
High lumen glucose: Glucose enters via facilitated diffusion through \( \text{GLUT2} \) transporters (passive, down the gradient).
2
Equal concentrations: The cell uses secondary active transport (\( \text{SGLT1} \) cotransporter), coupling glucose uptake with \( \text{Na}^+ \) moving down its gradient established by the \( \text{Na}^+/\text{K}^+ \) pump.
3
Overall: The \( \text{Na}^+/\text{K}^+ \) pump uses ATP to maintain the \( \text{Na}^+ \) gradient, which indirectly drives glucose absorption: secondary active transport.
Example 3: Signal Transduction at the Membrane
A water-soluble hormone like insulin cannot cross the hydrophobic bilayer. How does the cell respond?
1
Receptor binding: Insulin binds to a receptor protein (tyrosine kinase) on the extracellular surface of the membrane.
2
Signal relay: The receptor undergoes a conformational change, activating an intracellular signaling cascade (phosphorylation events).
3
Cellular response: \( \text{GLUT4} \) vesicles fuse with the membrane via exocytosis, increasing glucose uptake. The signal never had to enter the cell.
Common Mistakes
✕ Confusing tonicity direction
Students say "water moves to the hypotonic side." Water moves toward the hypertonic side (higher solute, lower free water).
✕ Calling osmosis "active"
Osmosis is a form of passive transport. It does not require ATP, even though aquaporin channels may be involved.
✕ Saying membranes are "static"
The fluid mosaic model shows that lipids and proteins are constantly moving laterally within the bilayer, not fixed in place.
Transport Comparison
| Feature | Passive | Active | Bulk |
|---|---|---|---|
| ATP needed? | No | Yes | Yes |
| Direction | High → Low | Low → High | Into/out of cell |
| Protein needed? | Sometimes | Always | Vesicles |
| Examples | \( \text{O}_2 \), osmosis, GLUT | \( \text{Na}^+/\text{K}^+ \) pump | Endo/exocytosis |
| Molecule size | Small to medium | Small ions | Large / macro |
Question 1 of 5
A plant cell is placed in a hypertonic solution. Which outcome is most likely?
In a hypertonic solution, water exits the cell by osmosis. In plant cells, the membrane shrinks away from the rigid cell wall: this is called plasmolysis. Lysis occurs in hypotonic solutions for animal cells.
Question 2 of 5
Which molecule can pass most easily through the phospholipid bilayer without assistance?
\( \text{O}_2 \) is a small, nonpolar molecule that readily dissolves in the hydrophobic interior of the bilayer. Glucose is polar and requires GLUT transporters; \( \text{Na}^+ \) is charged and needs ion channels; proteins are too large for simple diffusion.
Question 3 of 5
The \( \text{Na}^+/\text{K}^+ \) ATPase pumps \( 3~\text{Na}^+ \) out and \( 2~\text{K}^+ \) in per cycle. What is the net effect on the cell's interior charge?
Each cycle moves 3 positive charges out but only 2 positive charges in, producing a net loss of +1 charge from the interior. This makes the inside more negative, contributing to the resting membrane potential (approximately \( -70 \) mV in neurons).
Question 4 of 5
Cholesterol in the plasma membrane primarily functions to:
Cholesterol acts as a fluidity buffer. At high temperatures, it reduces membrane fluidity by restraining phospholipid movement. At low temperatures, it prevents tight packing and keeps the membrane flexible. It does not provide energy or function as a receptor.
Question 5 of 5
A white blood cell engulfs a bacterium by wrapping its membrane around it and forming a vesicle. This process is called:
Phagocytosis ("cell eating") is the endocytic process by which a cell engulfs large particles like bacteria. Pinocytosis is for fluids ("cell drinking"). Exocytosis releases material out of the cell. Facilitated diffusion involves no vesicle formation.