Plan and conduct an investigation to provide evidence that the cell membrane controls transport and helps maintain homeostasis (Louisiana Student Standards for Science, High School Biology, HS-LS1-3).

What this topic is asking

Louisiana's LS1 standards (HS-LS1-3) ask you to investigate how feedback and structures keep an organism stable, and the cell membrane is where that begins. For LEAP 2025 Biology you should know the membrane's structure, the difference between passive and active transport, and how to predict water movement by osmosis. These items are common and often pair with a model of a cell in a solution, so reading the direction of movement is the key skill, an analyzing-data and planning-an-investigation practice.

The membrane: structure and the fluid mosaic model

Each phospholipid has a water-loving (hydrophilic) phosphate head and two water-fearing (hydrophobic) fatty-acid tails. In water, the phospholipids arrange themselves into two layers with the tails facing inward, forming the bilayer. Embedded proteins act as channels and pumps, so the membrane is described as a fluid mosaic: a moving, mixed surface. Small nonpolar molecules (such as oxygen and carbon dioxide) slip straight through, while larger or charged particles need a protein 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 it requires no energy from the cell.

• Diffusion is the net movement of dissolved particles from high to low concentration until they are evenly spread. Oxygen diffusing into a cell is an example.
• 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 still passive, but the substance (such as glucose or an ion) moves down its gradient through a membrane protein because it cannot cross the lipid bilayer on its own.

Active transport: against the gradient

Because it costs energy, active transport is how cells accumulate substances they need in higher concentration than the surroundings provide. Cells doing a lot of active transport (such as kidney or root cells) tend to have many mitochondria to supply the ATP, linking this topic back to organelles.

Osmosis and the cell: three solutions

Predicting water movement is the single most tested skill here. Compare the solute concentration outside the cell with the concentration inside:

• Hypotonic solution (less solute outside, more water outside): water moves into the cell. An animal cell swells and may burst; a plant cell becomes firm (turgid) but its cell wall stops it bursting.
• Isotonic solution (equal solute on both sides): water moves in and out equally, so there is no net change and the cell stays the same.
• Hypertonic solution (more solute outside, less water outside): water moves out of the cell. An animal cell shrinks; a plant cell loses turgor and wilts (plasmolysis).

The rule to memorize: water follows solute. Water always moves toward the side with the higher solute concentration.

How transport maintains homeostasis

Tying the topic to homeostasis: by controlling exactly what crosses the membrane and in which direction, the cell keeps its internal water balance, nutrient levels, and waste removal stable despite changes outside. This cellular-level control is the foundation for the organism-level homeostasis covered in the body-systems standards.

Try this

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

• 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 cell is placed in an isotonic solution. State what happens to the cell and why. [2]

• Cue. No net change in size, because the solute concentration is equal inside and outside, so water moves in and out at equal rates.