Determine the role of cellular transport (diffusion, osmosis, facilitated diffusion, and active transport) across the selectively permeable membrane in maintaining homeostasis (GSE SB1.d).

What this topic is asking

Standard SB1.d asks you to determine the role of cellular transport in maintaining homeostasis. For the Georgia Milestones Biology EOC you must distinguish passive transport (no energy) from active transport (uses energy), predict the direction of diffusion and osmosis, and explain how controlling what crosses the selectively permeable membrane keeps internal conditions steady. Osmosis problems (hypotonic, hypertonic, isotonic) are a recurring item.

The selectively permeable membrane

Small, nonpolar molecules (oxygen, carbon dioxide) cross the bilayer easily. Charged ions and large polar molecules (like glucose) need transport proteins to cross.

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.

• Diffusion. The net movement of particles from high concentration to low concentration until evenly spread. Example: oxygen diffusing into a cell.
• 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. Passive movement of a substance down its gradient through a protein channel or carrier, used by molecules that cannot cross the bilayer alone. It still needs no energy.

Active transport: energy required

Active transport moves substances against their gradient, from low concentration to high, which diffusion cannot do. It requires energy (ATP) and uses membrane transport proteins (pumps). The classic example is the sodium-potassium pump in nerve cells. The exam clue for active transport is movement against the gradient or a statement that the cell uses energy.

Osmosis and tonicity

Osmosis problems are a staple EOC item. Compare the solute concentration inside the cell to the solution outside:

• Hypotonic solution (less solute outside than inside): water moves into the cell, so the cell swells (and an animal cell may burst).
• Hypertonic solution (more solute outside than inside): water moves out of the cell, so the cell shrinks.
• Isotonic solution (equal solute): no net water movement, so the cell stays the same size.

The single rule that solves these: water moves toward the higher solute concentration.

How transport maintains homeostasis

The link to homeostasis is the point of the standard. By controlling transport across the membrane, the cell keeps internal conditions (water balance, ion concentrations, nutrient levels, waste removal) within the narrow range its enzymes and processes need. For example, nerve cells use active transport (the sodium-potassium pump) to maintain the ion gradients that let them fire; root cells use active transport to take up minerals even when they are scarcer in the soil than in the cell. Passive transport handles routine gas exchange and water balance with no energy cost. Together, passive and active transport let the cell hold a steady internal state against a changing environment.

Try this

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

• Cue. Diffusion needs 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 plant cell is placed in a hypertonic solution. State which way water moves and the effect on the cell. [2 points]

• Cue. Water moves out of the cell; the cell loses water and the vacuole shrinks (the cell becomes flaccid, and the membrane may pull away from the wall).