Cell Membrane

Identify the substance moving, compare concentration inside and outside, decide the direction, then name diffusion, osmosis, or active transport.

Draw a membrane with arrows showing diffusion, osmosis, and active transport in different directions.

Saying water always enters a cell is wrong because water movement depends on the relative concentration of solutions.

The cell membrane, also called plasma membrane, is a thin selectively permeable boundary that controls movement of substances into and out of the cell.

The cell membrane, also called plasma membrane, is a thin selectively permeable boundary that controls movement of substances into and out of the cell. Diffusion: higher concentration to lower concentration. Osmosis: water movement through a selectively permeable membrane. Active transport: movement using energy, often against the gradient. Raisins swell in water because water enters by osmosis through their outer membrane.

The membrane is mainly made of lipids and proteins. It allows some substances to pass more easily than others. Diffusion moves particles from higher to lower concentration, osmosis is diffusion of water through a selectively permeable membrane, and active transport uses energy to move substances against a concentration gradient. Diffusion: higher concentration to lower concentration. Osmosis: water movement through a selectively permeable membrane. Active transport: movement using energy, often against the gradient. If a cell is placed in very salty water, water moves out of the cell by osmosis because the outside solution is more concentrated. Questions may ask what happens to cells in hypotonic, isotonic, or hypertonic solutions, or why the membrane is selectively permeable. Saying water always enters a cell is wrong because water movement depends on the relative concentration of solutions.