Explain that the cell membrane is selectively permeable and describe passive transport (diffusion and osmosis) and active transport, including the role of concentration gradients (Virginia 2018 Biology SOL BIO.3.b).

What this topic is asking

Virginia Biology SOL standard BIO.3.b asks how the cell membrane supports cell function by controlling what moves in and out. The Biology EOC expects you to know the membrane is selectively permeable, to distinguish passive transport (diffusion and osmosis, no energy) from active transport (energy required), and to predict the direction substances move using a concentration gradient. Osmosis questions, often with a diagram of a cell in different solutions, are common, and they connect directly to your experimental-design skills.

The selectively permeable membrane

The membrane's structure suits its function. Phospholipids have a water-loving (polar) head and water-hating (nonpolar) tails, so they arrange into a double layer with tails inward, forming a barrier to water-soluble substances. Transport proteins in the membrane provide channels and pumps for substances that cannot cross the lipid layer on their own.

Passive transport: diffusion and osmosis

Diffusion explains how oxygen enters a cell and carbon dioxide leaves, each moving from where it is more concentrated to where it is less concentrated. Osmosis is the same idea applied to water. The trick to osmosis questions is to find which side has more dissolved solute: water moves toward that side, because that side has less free water.

Active transport

Active transport moves substances against their concentration gradient, from low to high concentration, which the laws of diffusion would not do on their own. Because it works against the gradient, it requires energy in the form of ATP, and it uses membrane proteins (pumps) to carry the substance across. Cells use active transport to take up nutrients that are already more concentrated inside, or to pump ions to maintain the right balance. The key contrast for the EOC: passive transport is down the gradient with no energy; active transport is up the gradient and needs energy.

Predicting which way water moves

To predict osmosis, compare the solute concentration inside and outside the cell:

• In a hypotonic surrounding (lower solute outside, such as pure water), water moves into the cell; an animal cell may swell or burst, but a plant cell becomes turgid because its cell wall resists.
• In a hypertonic surrounding (higher solute outside), water moves out of the cell; the cell shrinks (a plant cell becomes flaccid or plasmolyzed).
• In an isotonic surrounding (equal solute), there is no net movement of water.

Try this

Q1. State whether active transport or diffusion requires energy, and explain why. [2]

• Cue. Active transport requires energy (ATP) because it moves substances against their concentration gradient; diffusion needs no energy because it moves substances down the gradient.

Q2. A cell is placed in a solution and neither gains nor loses water. Describe the solution relative to the cell. [1]

• Cue. The solution is isotonic to the cell (equal solute concentration inside and out), so there is no net movement of water.