Describe the role of the cell membrane in maintaining homeostasis, including selective permeability and the movement of materials by diffusion, osmosis, and active transport (TEKS Biology, Reporting Category 1; structure and function; stability and change).

What this topic is asking

The Biology TEKS ask you to explain how the cell membrane maintains homeostasis through selective permeability and the movement of materials in and out of the cell. For STAAR Reporting Category 1 you need to tell apart passive transport (diffusion and osmosis, no energy) from active transport (uses energy), and to connect both to keeping conditions inside the cell steady. This is a structure and function and stability and change topic.

The selectively permeable membrane

Small or fat-soluble molecules (such as oxygen and carbon dioxide) cross easily; larger or charged particles (such as ions and glucose) usually need help from membrane proteins. The membrane's structure, a flexible bilayer studded with transport proteins, is exactly what makes selective transport possible: another structure and function example.

Passive transport: diffusion and osmosis

Passive transport moves substances down a concentration gradient (from high to low concentration) and needs no energy.

• Diffusion is the net movement of particles from a region of higher concentration to a region 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 to where it is less concentrated. If a cell sits in a solution with more water outside than inside, water enters and the cell swells; if there is less water outside, water leaves and the cell shrinks.

Active transport

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

How transport keeps the cell in homeostasis

The membrane's control of transport is the reason a cell can hold steady internal conditions even when its surroundings change. By letting nutrients in, pushing wastes out, and pumping ions to set the right balance, the membrane maintains homeostasis, a stable internal state. This links Reporting Category 1 to the wider idea of homeostasis and cellular regulation.

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. Explain how the cell membrane helps maintain homeostasis. [2]

• Cue. It is selectively permeable, controlling which substances enter and leave, so the cell can keep stable internal conditions despite changes outside.