Explain how the cell membrane controls the movement of materials by diffusion, osmosis and active transport, and relate membrane structure to selective permeability (NYSSLS LS1, structure and function; stability and change).

What this topic is asking

NYSSLS LS1 wants you to see the cell membrane as the boundary that keeps the inside of a cell different from the outside, and to explain how it controls transport. On the Life Science: Biology Regents this content very often appears as a cluster built on the Diffusion Through a Membrane investigation, so you should know both the science and the classic experiment.

Membrane structure and selective permeability

The lipid interior is nonpolar, so small nonpolar molecules (oxygen, carbon dioxide) and water cross fairly easily, while large molecules (such as glucose) and charged ions need transport proteins. This selectivity is why the membrane can keep the cell's contents different from the surrounding fluid.

Passive transport: diffusion and osmosis

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, that is, down the concentration gradient, until evenly spread. It happens because particles move randomly, and it requires no energy from the cell. Oxygen diffusing into a cell and carbon dioxide diffusing out are examples.

Osmosis is the diffusion of water across a selectively permeable membrane, from where water is more concentrated (a dilute, or hypotonic, solution) to where it is less concentrated (a concentrated, or hypertonic, solution). The direction of osmosis lets you predict whether a cell will gain water and swell, lose water and shrink, or stay the same in an isotonic solution.

Active transport

Sometimes a cell must move a substance against its concentration gradient, from low to high. This is active transport, and because it works against diffusion it requires energy (ATP) and a carrier protein that binds the substance and pumps it across. Examples include nerve cells pumping ions to reset for the next signal, and root cells taking up mineral ions from dilute soil water. Bulk transport (endocytosis and exocytosis) also moves large materials in vesicles and uses energy.

The Diffusion Through a Membrane lab

The classic New York investigation uses dialysis tubing as a model membrane. Starch and glucose solution go inside; the beaker holds water with iodine indicator (and sometimes glucose-test strips). Over time, small molecules (iodine, glucose, water) cross the tubing but the large starch molecules cannot. The iodine entering meets starch inside and turns blue-black, while the beaker water stays amber because no starch escaped. The lab is a direct demonstration of selective permeability based on molecule size, and clusters draw on it heavily.

Try this

Q1. Define osmosis. [2]

• Cue. The diffusion of water across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

Q2. Explain why active transport requires energy but diffusion does not. [2]

• Cue. Diffusion moves particles down their gradient (a spontaneous, passive process); active transport moves them against the gradient, which is not spontaneous, so the cell must supply energy (ATP).