Unit 4 is about the cell membrane, the thin layer of lipids and proteins that decides what gets into and out of every cell. By the end, you should be able to explain how the membrane's structure controls permeability and predict whether a substance moves by diffusion, osmosis, active transport, or bulk transport.
Read the overview first, then work through each topic note and try the checkpoint question before checking the answer.
Every cell has to keep its inside different from its outside, and the plasma membrane is what makes that possible. This unit starts with what the membrane is made of, then covers the two big categories of transport. Passive transport does not directly use ATP. Solutes move down their electrochemical gradients, which account for both concentration and charge. Active transport spends energy.
Honors Biology Unit 4 covers the cell membrane and how materials cross it. You learn the fluid mosaic model (a phospholipid bilayer with proteins, cholesterol, and carbohydrate chains that can drift sideways), then passive transport, which includes simple diffusion, facilitated diffusion through channel and carrier proteins, and osmosis, the diffusion of water. You use tonicity (hypertonic, hypotonic, isotonic) to predict whether a cell swells or shrinks. The unit ends with active transport, including the sodium-potassium pump and cotransport, and bulk transport by endocytosis and exocytosis.
The core of the bilayer is made of nonpolar fatty acid tails, so small nonpolar molecules like O₂ and CO₂ pass easily. The oily middle layer strongly limits the passage of ions and large polar molecules like glucose. Some small uncharged polar molecules can cross it slowly. That is why selective permeability comes from the lipids themselves, not just from proteins.
Random motion drives diffusion. Uncharged particles diffuse down their concentration gradients, while ions move down their electrochemical gradients, which account for concentration and charge. Proteins can help particles cross by facilitated diffusion. Osmosis is the diffusion of water across a selectively permeable membrane.
Pumps like the sodium-potassium pump use ATP to push ions against their gradients. Cotransporters then use those gradients to drag other molecules, such as glucose, into the cell. Large particles and big batches of molecules move inside vesicles by endocytosis and exocytosis, which also costs energy.
To predict how a substance crosses a membrane, ask two questions. First, can it pass through the lipid bilayer, or does it need a protein or a vesicle? For a solute crossing through the bilayer or a protein, ask whether it moves down its electrochemical gradient (passive) or against it (active). Vesicle transport requires energy and is not classified by that gradient.
The plasma membrane is a phospholipid bilayer.
Diffusion is the net movement of particles from high concentration to low concentration.
Active transport uses energy to move substances against their electrochemical gradients.
The plasma membrane is a phospholipid bilayer. Each phospholipid is amphipathic, meaning it has a polar, water-loving phosphate head and two nonpolar, water-avoiding fatty acid tails. In water, the tails cluster inward and the heads face the watery fluid on both sides, so the bilayer forms on its own without any energy input.
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes this bilayer as a moving sheet with many kinds of molecules in it. "Fluid" means phospholipids and many proteins drift sideways. "Mosaic" means the membrane is a patchwork of different molecules.
Integral proteins are embedded in the bilayer, and transmembrane proteins span it completely. These include channels, carriers, and receptors. Peripheral proteins sit loosely on the surface and can be removed without breaking the bilayer.
Cholesterol sits between phospholipids and acts as a fluidity buffer. At high temperatures it limits phospholipid movement, so the membrane doesn't get too loose. At low temperatures it limits tight packing of phospholipids, so the membrane stays more fluid. Carbohydrate chains attached to proteins (glycoproteins) and lipids (glycolipids) face outward and act as ID tags for cell recognition. Your A, B, and O blood types come from markers like these on red blood cells.
Selective permeability comes from this structure. Small nonpolar molecules cross easily, while ions and large polar molecules need protein help.
Why can oxygen cross the membrane directly, while a sodium ion (Na⁺) cannot? O₂ is small and nonpolar, so it dissolves through the hydrophobic tail region. Na⁺ is charged, so the nonpolar core repels it, and it needs a channel or carrier protein.
Diffusion is the net movement of particles from high concentration to low concentration. The difference in concentration is called the concentration gradient. Diffusion uses no ATP because random molecular motion drives it. At equilibrium, molecules keep moving, but there is no net change in either direction.
Simple diffusion goes straight through the bilayer and works for small nonpolar molecules like O₂ and CO₂. Facilitated diffusion uses proteins to help ions and polar molecules move down their gradients. Channel proteins form pores, such as K⁺ channels and aquaporins for water. Carrier proteins bind a molecule and change shape, as GLUT transporters do for glucose. Facilitated diffusion is still passive.
Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity predicts a cell's change in volume based on the concentration of solutes that cannot readily cross the membrane. In a hypertonic solution, water initially leaves the cell. In a hypotonic solution, water initially enters. In an isotonic solution, the cell has no net change in volume. Pressure can also affect water movement, especially in plant cells.
Worked example. Imagine a cell with 1% salt inside and no other solutes. Its membrane allows water through but not salt. In a 5% solution of the same salt, the solution is hypertonic, so water moves out and the cell shrinks. In a 0.2% solution of the same salt, water initially moves into the cell.
A plant cell is placed in distilled water. Which way does water move, and why doesn't the cell burst? Distilled water is hypotonic, so water initially moves into the cell. The rigid cell wall pushes back, creating turgor pressure that keeps the cell firm instead of bursting.
| Solution outside the cell | Nonpenetrating solute concentration compared to cell | Initial net water movement | Animal cell result | Plant cell result |
|---|---|---|---|---|
| Hypertonic | More solute outside | Out of cell | Shrivels (crenation) | Plasmolysis, membrane pulls away from wall |
| Hypotonic | Less solute outside | Into cell | Swells, may burst (lysis) | Turgid, which is the normal healthy state |
| Isotonic | Equal solute | No net movement | Normal shape | Flaccid (limp) |
Active transport uses energy to move substances against their electrochemical gradients. For ions, these gradients reflect differences in both concentration and electrical charge. In primary active transport, a pump uses ATP directly. When ATP is broken down, the pump changes shape and carries its cargo across.
The sodium-potassium pump is the standard example. For every ATP used, it moves 3 Na⁺ out of the cell and 2 K⁺ in. This keeps Na⁺ high outside and K⁺ high inside, which neurons and muscle cells depend on. Because more positive charge leaves than enters, the pump also helps make the inside of the cell negative relative to the outside.
Secondary active transport, also called cotransport, uses no ATP directly. Instead, it uses a gradient that a pump already built. In your small intestine, Na⁺ flows back into cells down its gradient and pulls glucose in with it through the same carrier, even when glucose is already more concentrated inside.
Worked example. A cell uses 50 ATP on sodium-potassium pumps. Na⁺ moved out is ions. K⁺ moved in is ions. The net charge moved is positive charges leaving the cell. This contributes to a more negative interior, although other ion movements can offset the effect.
Bulk transport moves large particles or large amounts in vesicles. In endocytosis, the membrane folds inward. Phagocytosis takes in solid particles, as when a macrophage engulfs a bacterium. Pinocytosis takes in fluid. Receptor-mediated endocytosis takes in specific molecules that bind receptors, such as cholesterol carried on LDL. In exocytosis, vesicles fuse with the membrane to release contents, such as insulin or neurotransmitters.
A poison stops ATP production. If no other process maintains the Na⁺ gradient, why would sodium-glucose cotransport in intestinal cells eventually stop? The cotransporter depends on the Na⁺ gradient, and the sodium-potassium pump needs ATP to maintain it. As the gradient runs down, it can no longer drive glucose uptake.
| Process | Energy needed? | Direction | What helps it cross | Example |
|---|---|---|---|---|
| Simple diffusion | No | Down gradient | Nothing, goes through bilayer | O₂ entering a cell |
| Facilitated diffusion | No | Down gradient | Channel or carrier protein | Glucose through GLUT |
| Primary active transport | Yes, ATP directly | Against gradient | Pump protein | Sodium-potassium pump |
| Secondary active transport | Yes, uses an existing ion gradient | One solute against gradient | Cotransporter protein | Na⁺-glucose symport |
| Endocytosis or exocytosis | Yes | Into or out of cell in bulk | Vesicles | Macrophage eating bacteria |
| Term | Definition |
|---|---|
| Fluid mosaic model | The model of the membrane as a phospholipid bilayer with proteins, cholesterol, and carbohydrates that can move sideways within it. |
| Phospholipid bilayer | Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward. |
| Selective permeability | The property of a membrane that lets some substances cross easily while blocking or slowing others. |
| Diffusion | The net movement of particles from an area of higher concentration to an area of lower concentration. |
| Tonicity | A comparison based on solutes that cannot readily cross the cell membrane, used to predict the cell's change in volume. |
| Hypertonic | Describes a solution with a higher concentration of nonpenetrating solutes than the cell, causing the cell to lose water and shrink. |
| Hypotonic | Describes a solution with a lower concentration of nonpenetrating solutes than the cell, causing the cell to gain water and swell. |
| Active transport | Movement of a substance against its electrochemical gradient using energy, either directly from ATP or indirectly from another gradient. |
| Endocytosis | The process of bringing materials into a cell by folding the plasma membrane inward to form a vesicle. |
At equilibrium, molecules keep bouncing across the membrane in both directions at equal rates. What stops is net movement. If a question asks what happens at equilibrium, say "no net change," not "no movement."
In a simple osmosis problem with no pressure difference, water moves toward the side with a lower concentration of free water. To predict a cell's volume change, compare solutes that cannot readily cross the membrane. A hypertonic solution causes water to leave the cell.
Using a protein doesn't make a process active. Facilitated diffusion moves substances down their electrochemical gradients without directly using ATP, so it is passive. Carrier proteins can also take part in active transport, which uses energy to move a solute against its electrochemical gradient.
Cholesterol works as a buffer in both directions. It reduces fluidity at higher temperatures and limits stiffening at lower temperatures. If a question involves temperature, describe both effects.