Explain the structure of the cell membrane and how diffusion, osmosis, facilitated diffusion, and active transport move substances across it, including the role of the concentration gradient and ATP (MA STE HS-LS1-4 supporting).

What this topic is asking

The Massachusetts STE framework expects you to explain how cells maintain a stable internal environment, and transport across the membrane is the mechanism behind it. On the High School Biology MCAS, transport questions almost always come with a stimulus: a diagram of a membrane, a data table of concentrations, or a description of a cell placed in a solution. You are asked to predict the direction of movement, decide whether energy is needed, and explain the result. The deciding factor is always the concentration gradient and whether the substance moves with it or against it.

The structure of the membrane

The cell membrane is a phospholipid bilayer. Each phospholipid has a phosphate "head" that is attracted to water (hydrophilic) and two fatty-acid "tails" that repel water (hydrophobic). The molecules arrange themselves into two layers with the water-loving heads facing out toward the watery surroundings and the water-fearing tails tucked inside. This arrangement makes the membrane a natural barrier to water-soluble substances, which is the key to control.

Embedded in the bilayer are proteins that act as channels, carriers, receptors, and markers. This picture of a flexible bilayer with proteins drifting in it is called the fluid mosaic model. Because the membrane lets some substances through more easily than others, it is described as selectively permeable (sometimes called semipermeable).

Passive transport: no energy needed

• Simple diffusion. Small or nonpolar molecules (oxygen, carbon dioxide) pass straight through the bilayer from high to low concentration.
• Osmosis. The diffusion of water across a selectively permeable membrane, moving toward the more concentrated (lower water) solution.
• Facilitated diffusion. Larger or charged particles (glucose, ions) move down their gradient but need a channel or carrier protein to cross, since they cannot pass through the lipid interior. It is still passive: no ATP is used.

Diffusion continues until the concentrations are equal (equilibrium), after which particles keep moving but there is no net change.

Active transport: energy required

Active transport moves a substance against its concentration gradient, from low to high concentration. Because this is the opposite of the spontaneous direction, it requires energy (ATP) and a carrier protein that changes shape to pump the substance across. The classic example is the sodium-potassium pump, which keeps the inside of a nerve cell low in sodium and high in potassium even though both ions are being pumped against their gradients. The rule is simple: with the gradient is passive (free); against the gradient is active (costs ATP).

Predicting water movement with tonicity

• Hypotonic solution: water moves into the cell. An animal cell swells and may burst; a plant cell becomes firm (turgid) because its wall resists.
• Hypertonic solution: water moves out of the cell. An animal cell shrinks (crenates); a plant cell's membrane pulls away from the wall (plasmolysis).
• Isotonic solution: no net water movement; the cell stays the same.

Water always moves toward the more concentrated solution, so you can predict the direction by asking where the water is more crowded.

Try this

Q1. Define osmosis and state whether it uses energy. [2]

• Cue. Osmosis is the diffusion of water across a selectively permeable membrane toward the more concentrated solution; it uses no energy (it is passive).

Q2. Explain why active transport needs ATP but diffusion does not. [2]

• Cue. Active transport moves a substance against its gradient (low to high), which is not spontaneous and so needs energy; diffusion moves a substance down its gradient, which happens on its own.