Explain how the selectively permeable cell membrane uses passive and active transport to move substances and maintain homeostasis (Ohio's Learning Standards for Science, Biology, B.C.2).

What this topic is asking

Ohio standard B.C.2 says cells "carry on specific functions that sustain life," and one of the most basic is controlling what crosses the boundary. Ohio's Biology EOC turns this into items on the selectively permeable membrane and the kinds of transport that move substances across it. The crosscutting idea is stability and change: transport keeps the cell's internal conditions stable, which is homeostasis. Many items pair a diagram of a cell in solution with a question about which way water or particles move.

The membrane: structure fits function

The structure suits the function. Small, non-polar molecules such as oxygen and carbon dioxide slip straight through the oily middle, but large or charged particles (ions, glucose) need a protein channel or pump. This selective control is what lets the cell hold an internal environment different from its surroundings, which is the basis of homeostasis.

Passive transport: no energy needed

Passive transport moves substances down their concentration gradient (from where they are more concentrated to where they are less concentrated) and needs no energy. There are three kinds.

• Diffusion. The net movement of particles from high to low concentration until evenly spread. Oxygen diffusing into a cell is an example.
• Osmosis. The diffusion of water across a selectively permeable membrane, from where water is more concentrated (low solute) to where it is less concentrated (high solute).
• Facilitated diffusion. Diffusion that uses a protein channel to help a substance (such as glucose) cross. It is still passive, because it moves down the gradient and uses no ATP.

Active transport: energy needed

Active transport moves substances against the gradient (from low to high concentration), which the particles would not do on their own. It therefore requires energy (ATP) and membrane proteins that act as pumps. The root-cell example in the practice question, taking in ions that are already more concentrated inside, is the classic case. Whenever the EOC says a substance moves against the gradient, or that the cell spends energy to move it, the answer is active transport.

Osmosis and tonicity

Tonicity describes a solution by how its solute concentration compares with the inside of the cell. Water always moves by osmosis toward the side with more solute (less free water).

• Hypotonic solution (less solute outside than inside): water moves into the cell, so the cell swells. An animal cell may burst; a plant cell becomes firm (turgid) because its wall resists.
• Isotonic solution (equal solute): no net water movement, so the cell stays the same.
• Hypertonic solution (more solute outside than inside): water moves out of the cell, so the cell shrinks. An animal cell crenates; a plant cell loses turgor (plasmolysis).

Transport and homeostasis

The point Ohio's standards make is that all this transport serves homeostasis: keeping internal conditions stable. By choosing what to let in (oxygen, glucose, ions) and what to push out (carbon dioxide, waste), the membrane keeps the cytoplasm in the narrow range the cell's reactions need. This is the cell-level version of the whole-organism homeostasis you meet again in the body systems.

Try this

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

• Cue. Diffusion 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 outside, so water moves out of the cell by osmosis and the cell shrinks.