Cell Membrane, Diffusion & Osmosis, and Active Transport

Cell Membrane

Forms a boundary between a cell and the outside environment and controls the passage of materials into and out of a cell. Consists of a double layer of phospholipids interspersed with a variety of other molecules.


Molecule composed of three basic parts; a charged phosphate group, glycerol, two fatty acid chains. Polar head, nonpolar tail.

Cholesterol in the membrane

Strengthens the membrane.

Proteins in the membrane

Help materials cross the membrane.

Carbohydrates in the membrane

When attached to proteins, they serve as identification tags and enable cells to distinguish one type of cell from another.

Fluid Mosaic Model

Describes the arrangement of the molecules that make up a cell membrane. The cell membrane is flexible, and their is a variety of molecules.

Selective Permeability

It allows some, but not all, materials to cross. Enables a cell to maintain homeostasis.


A protein that detects a signal molecule and performs an action is respnse. Binds to a ligand.

Passive Transport

The movement of molecules across a cell membrane without energy input from the cell.


The movement of molecules in a fluid or gas from a region of higher concentration to a region of lower concentration.


The number of molecules of a substance in a given volume, varies from one region to another.

Concentration Gradient

The difference in the concentration of a substance from one location to another. Molecules diffuse DOWN the concentration gradient. (Region of higher concentration to region of lower concentration)


Diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration until the concentration on both sides is equal.

Isotonic Solution

A solution has the same concentration of dissolved particles as the cell. Water molecules move into and out of the cell at an equal rate, so the cell's size remains constant.

Hypertonic Solution

A solution that has a higher concentration of dissolved particles than inside the cell. So, water concentration is higher inside the cell. Water flows out of the cell, causing it to shrivel and die.

Hypotonic Solution

A solution that has a lower concentration of dissolved particles than inside the cell. Water molecules are more concentrated outside the cell. Water diffuses into the cell, and if too much enters, the cell membrane could possibly expand until it bursts.

Facilitated Diffusion

The diffusion of molecules across a membrane through transport proteins. The molecules move down the concentration gradient. (no energy)

Active Transport

Drives molecules across a membrane from a region of lower concentration to a region of higher concentration. Uses transport proteins powered by chemical energy.


Stores and transports chemical energy within cells.


The process of taking liquids or fairly large molecules into a cell by engulfing them in a membrane. The cell membrane makes a pocket around a substance. The pocket then breaks off inside the cell and forms a vesicle, which is then broken down by lysosoma


A type of endocytosis in which the cell membrane engulfs large particles. " Cell eating


The opposite of endocytosis. It is the release of substances out of a cell by the fusion of a vesicle with the membrane. A vesicle forms around materials to be sent out of the cell, then moves toward the cell's surface where it fuses with the membrane and