Explain the role of the cell membrane as a highly selective barrier through passive transport (diffusion and osmosis) and active transport (NGSSS SC.912.L.14.2; Reporting Category 1, Molecular and Cellular Biology).

What this topic is asking

Part of NGSSS benchmark SC.912.L.14.2 asks you to explain the cell membrane as a highly selective barrier, including passive transport (diffusion and osmosis) and active transport. For the Florida Biology 1 EOC you need to tell apart transport that needs no energy (down the gradient) from transport that uses energy (against the gradient), and you must be able to predict which way water moves when a cell sits in a solution. This is one of the most frequently tested Reporting Category 1 ideas.

The selectively permeable membrane

The phospholipids have water-attracting (hydrophilic) heads facing out and water-repelling (hydrophobic) tails facing in, forming a barrier that small or fat-soluble molecules (oxygen, carbon dioxide) cross easily, but large or charged particles (ions, glucose) usually cannot cross without help from transport proteins. The flexible bilayer with its embedded proteins is exactly the structure that makes selective transport possible: a structure-and-function example.

Passive transport: no energy needed

Passive transport moves substances down their concentration gradient, from higher to lower concentration, and needs no energy from the cell.

• Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration until they are evenly spread. Oxygen diffuses into a cell because it is more concentrated outside.
• Osmosis is the diffusion of water across a selectively permeable membrane, from where water is more concentrated (fewer solutes) to where it is less concentrated (more solutes).
• Facilitated diffusion is passive transport through a membrane protein. A particle that cannot cross the bilayer on its own (such as glucose) moves down its gradient through a channel or carrier protein, still without energy.

Active transport: energy required

The EOC clue for active transport is movement against the gradient, or a statement that the cell uses energy or ATP. Passive transport never uses energy; active transport always does.

Predicting osmosis (tonicity)

Comparing the solution outside a cell to the inside tells you which way water moves:

• Hypotonic solution: more water (fewer solutes) outside than inside. Water moves in, and the cell swells; an animal cell can burst, a plant cell becomes turgid.
• Hypertonic solution: less water (more solutes) outside than inside. Water moves out, and the cell shrinks (a plant cell plasmolyzes).
• Isotonic solution: equal solute concentration. No net water movement; the cell stays the same.

The trick is to follow the water, which always moves toward the higher solute concentration. A plant cell survives in pure water because its rigid cell wall resists the pressure, so it becomes turgid rather than bursting.

Try this

Q1. State the difference between passive and active transport in terms of energy and direction. [2]

• Cue. Passive transport uses no energy and moves substances down the gradient (high to low); active transport uses energy (ATP) and moves substances against the gradient (low to high).

Q2. A cell is placed in a hypertonic solution. State which way water moves and what happens to the cell. [2]

• Cue. A hypertonic solution has more solute (less water) outside, so water moves out of the cell and the cell shrinks.