What is the cell membrane made of?
Every cell needs a boundary that holds its contents in while still controlling what moves in and out. That boundary is the cell membrane, and it is built mainly from phospholipids, molecules with a phosphate head and two fatty acid tails. The head is hydrophilic (water-loving) and the tails are hydrophobic (water-fearing). Because one end is attracted to water and the other end avoids it, phospholipids are called amphipathic. In water, phospholipids automatically arrange themselves into a phospholipid bilayer, two layers with the hydrophilic heads facing outward toward the watery environment on both sides, and the hydrophobic tails tucked inward, away from water. This self-assembling structure is the foundation of every cell membrane.
Head loves water, tail avoids it
Picture the bilayer as two rows of molecules glued tail to tail. If the tails pointed outward they would touch water and the structure would fall apart. Tails always face each other on the inside, heads always face water, on the outside of the cell and on the inside facing the cytoplasm.
The fluid mosaic model
The fluid mosaic model describes the membrane as fluid and mosaic at the same time. Fluid means the phospholipids and proteins are not locked in place, they drift and slide within their layer. Mosaic means the membrane is a patchwork of different pieces, phospholipids, proteins, cholesterol, and carbohydrate chains, scattered throughout instead of arranged in one repeating pattern.
| Component | Location | Job |
|---|---|---|
| Phospholipid bilayer | forms the two layers | main barrier, blocks most water-soluble substances |
| Integral proteins | embedded through the membrane | act as channels or carriers, move substances across |
| Peripheral proteins | attached to one surface only | support structure, receive signals, help cells stick together |
| Cholesterol | wedged between phospholipid tails (animal cells) | keeps the membrane fluid, not too stiff or too runny |
| Glycoproteins / glycolipids | carbohydrate chains on the outer surface only | let cells recognize each other |
Cholesterol keeps the membrane steady
Cholesterol sits wedged between the phospholipid tails in animal cell membranes. At low temperature it keeps the membrane from becoming too stiff, and at high temperature it keeps the membrane from becoming too loose. It works as a buffer that steadies membrane fluidity.
Selective permeability: what crosses easily
Because the interior of the bilayer is hydrophobic, the membrane is selectively permeable, it lets some substances cross easily and blocks others. Small, nonpolar molecules slip directly through the lipid tails with no help. Large molecules, charged ions, and most polar molecules cannot pass through the hydrophobic interior on their own, they need a transport protein to cross.
| Crosses directly through the bilayer | Needs a transport protein |
|---|---|
| O₂ | Na⁺, K⁺, Ca²⁺ (ions) |
| CO₂ | glucose, amino acids |
| small nonpolar lipids | large or highly polar molecules |
| a small amount of H₂O (slow) | most H₂O (through aquaporins) |
Water needs help too
Water is polar, so only a small amount diffuses directly through the lipid bilayer. Most water crosses through channel proteins called aquaporins, which is why osmosis can happen quickly even though water is a polar molecule.
Passive transport: diffusion and facilitated diffusion
Passive transport moves substances across the membrane without using any cellular energy. It always moves substances down their concentration gradient, from an area of high concentration to an area of low concentration, the same direction a substance would move on its own. There are two kinds of passive transport. Simple diffusion happens when a small nonpolar molecule passes directly through the lipid bilayer, no protein required. Facilitated diffusion happens when a larger or charged molecule is helped across by a transport protein, either a channel protein (a pore that lets a specific ion or molecule pass) or a carrier protein (which changes shape to shuttle a molecule across). Example: Oxygen is more concentrated in the air in the lungs than in the blood, so O₂ diffuses into a red blood cell by simple diffusion. Glucose is often more concentrated outside a cell than inside, so a carrier protein moves it in by facilitated diffusion, still down its gradient, still with no ATP spent.
| Feature | Simple diffusion | Facilitated diffusion |
|---|---|---|
| Needs a protein? | No | Yes, a channel or carrier protein |
| Moves | small nonpolar molecules | ions, glucose, amino acids, and other large or polar molecules |
| Direction | down the concentration gradient | down the concentration gradient |
| Uses ATP? | No | No |
Passive does not mean random
Passive transport still only moves a substance in the direction its concentration gradient allows, downhill. A transport protein can speed up how fast that downhill movement happens, but it never uses ATP to push a substance uphill.
Osmosis: the diffusion of water
Osmosis is the diffusion of water across a selectively permeable membrane. Like any diffusion, water moves from an area of high water concentration to an area of low water concentration, until the concentration is equal on both sides. Water concentration is the flip side of solute concentration. A solution with more dissolved solute has less free water, and a solution with less dissolved solute has more free water. Water always moves toward the side with more solute, because that is the side with less water. Example: A cell is placed in a solution that has more dissolved sugar than the inside of the cell. The outside solution has less free water than the cell, so water diffuses out of the cell and into the solution, following its own concentration gradient from high water to low water.
Follow the water, not the solute
When comparing two solutions, ask which side has more free water. Water always diffuses toward the side with more solute, because that side has less water.
Tonicity and its effect on cells
Tonicity describes how a solution's solute concentration compares to the solute concentration inside a cell, and predicts which way water will move. An isotonic solution has the same solute concentration as the inside of the cell. Water still moves in both directions, but at equal rates, so there is no net change in cell volume. A hypotonic solution has a lower solute concentration than the inside of the cell, meaning more free water outside. Water moves into the cell by osmosis, and the cell swells. A hypertonic solution has a higher solute concentration than the inside of the cell, meaning less free water outside. Water moves out of the cell by osmosis, and the cell shrinks.
| Solution | Solute vs inside the cell | Water moves | Animal cell | Plant cell |
|---|---|---|---|---|
| Hypotonic | lower outside | into the cell | may swell and lyse (burst) | becomes turgid (firm, healthy, cell wall resists bursting) |
| Isotonic | equal | no net movement | normal shape | flaccid but not damaged |
| Hypertonic | higher outside | out of the cell | shrinks (crenation) | plasmolyzes (membrane pulls away from the cell wall) |
Name it from the solution's side
Tonicity is always named from the solution's solute level relative to the cell. A hypotonic solution is low in solute, so it pulls water into the cell. This trips students up because a low value sounds like it should mean less activity, but a hypotonic solution actually causes the most water to rush in.
Active transport: moving against the gradient
Active transport moves substances against their concentration gradient, from an area of low concentration to an area of high concentration. Because this goes against the natural direction of diffusion, active transport always requires energy in the form of ATP. Active transport uses carrier proteins called pumps. The best known example is the sodium-potassium pump (Na⁺/K⁺ pump), found in the membrane of nearly every animal cell. For every one ATP molecule it hydrolyzes, the pump moves 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, both against their gradients. This keeps sodium concentration high outside the cell and potassium concentration high inside the cell, a difference that nerve and muscle cells depend on.
Same protein family, different job
Both facilitated diffusion and active transport use carrier proteins that change shape to move a substance across the membrane. The difference is the direction and the energy cost, facilitated diffusion goes downhill and costs nothing, active transport goes uphill and costs ATP.
Putting it together: passive vs active transport
Every transport method sorts into one of two categories based on two questions, does it need ATP, and which direction does it move relative to the concentration gradient?
| Transport type | Direction | Needs ATP? | Examples |
|---|---|---|---|
| Simple diffusion | down the gradient | No | O₂, CO₂ |
| Facilitated diffusion | down the gradient | No | glucose via carrier protein, ions via channel protein |
| Osmosis | down the water gradient | No | water through aquaporins |
| Active transport | against the gradient | Yes | Na⁺/K⁺ pump |
One quick test
If a substance is moving toward where there is already more of it, and using ATP to get there, it is active transport. If it is moving toward where there is less of it and no ATP is spent, it is passive, whether that is simple diffusion, facilitated diffusion, or osmosis.
Try it yourself
potato-osmosis-lab.csvDownload potato-osmosis-lab.csv: https://restedteacher.com/practice-files/biology/potato-osmosis-lab.csv
Initial and final masses for 6 potato cores soaked for 24 hours in sucrose solutions of rising concentration.
Open it as a spreadsheet
Open the downloaded file in Google Sheets or Excel. For each core, percent mass change = (final mass − initial mass) ÷ initial mass × 100. A positive percent means the core gained mass (the surrounding solution was hypotonic to the potato, so water moved in). A negative percent means the core lost mass (the solution was hypertonic to the potato, so water moved out). A percent of exactly 0 means the solution was isotonic to the potato.
Using potato-osmosis-lab.csv:
1. Calculate the percent mass change for core 3 (0.4 M). Is the 0.4 M solution hypotonic, isotonic, or hypertonic to the potato?
2. Calculate the percent mass change for core 1 (0.0 M, plain water). Classify that solution's tonicity relative to the potato.
3. Which core(s) are in a solution that is hypertonic to the potato tissue?
4. As sucrose concentration rises from core 1 to core 6, does percent mass change increase or decrease?
5. Rank all six cores in order from the greatest mass gain to the greatest mass loss.