Movement through the Membrane Chapter 7-3

Cell Membrane Regulates what enters and leaves the cell (kind of like a security guard) Made of a lipid bilayer 2 layers of lipids Provides a tough, flexible barrier between the cell and its surroundings Proteins and carbohydrates can also be found in the cell membrane.

Cell Membrane One of the most important functions of the cell membrane is to regulate the movement of molecules from one side of the membrane to the other. The cell membrane is selectively permeable (it doesn’t let just everything through; it is selective).

Equilibrium Equilibrium—same concentration (amount per area) on both sides; equal Molecules are constantly moving across the membrane. In equilibrium, the molecules are moving at the same rate.

Isotonic Solution same strength same concentration as cell water molecules move in and out at same rate

Hypertonic Solution above strength higher concentration than cell water moves out of the cell

Hypotonic Solution below strength lower concentration than cell water moves into the cell

4 Types of Movement through the Membrane Diffusion Osmosis Passive Transport (also called facilitated diffusion) Active Transport

Diffusion Diffusion is the movement of molecules from an area of high concentration (crowded) to an area of low concentration (uncrowded) until equilibrium is reached Diffusion does NOT require the cell to use energy.

Diffusion

Osmosis Osmosis is the diffusion of water. Water moves from high concentration to low concentration. It does NOT require energy.

Osmotic Pressure Fresh water is ALWAYS hypotonic to a cell. Water will move into a cell in fresh water, causing it to swell and possibly causing it to burst. The cell wall prevents the cell from expanding too much.

Passive Transport Some molecules cannot cross the lipid bilayer of the cell membrane. Proteins in the cell membrane act as a channel (path). The proteins passively allow the molecules through the membrane. High to low concentration Does NOT require energy

Active Transport LOW (uncrowded) to HIGH (crowded) concentration!!!! Proteins act as pumps and actively force molecules through the membrane Active transport REQUIRES energy!!!