At a Glance

The Plasma Membrane Isolates the Cell While Allowing Communication with Its Surroundings
Membranes Are "Fluid Mosaics" in Which Proteins Move Within Layers of Lipids
The Phospholipid Bilayer Is the Fluid Portion of the Membrane
A Mosaic of Proteins Is Embedded in the Membrane

Molecules in Fluids Move in Response to Gradients
Movement Across Membranes Occurs by Both Passive and Active Transport
Passive Transport Includes Simple Diffusion, Facilitated Diffusion, and Osmosis
- Plasma Membranes Are Selectively Permeable to Diffusion of Molecules
- Some Molecules Move Across Membranes by Simple Diffusion
- Other Molecules Cross the Membrane by Facilitated Diffusion, with the Help of Membrane Transport Proteins
- Osmosis Is the Diffusion of Water Across Membranes
- Osmosis Across the Plasma Membrane Plays an Important Role in the Lives of Cells
Active Transport Uses Energy to Move Molecules Against Their Concentration Gradients
Cells Engulf Particles or Fluids by Endocytosis
- Pinocytosis Moves Liquids into the Cell
- Receptor-Mediated Endocytosis Moves Specific Molecules into the Cell
- Phagocytosis Moves Large Particles into the Cell
Exocytosis Moves Material Out of the Cell

Various Specialized Junctions Allow Cells to Connect and Communicate
- Desmosomes Attach Cells Together
- Tight Junctions Make the Cell Leakproof
- Gap Junctions and Plasmodesmata Allow Communication Between Cells
Some Cells Are Supported by Cell Walls

Chapter 4: Cell Membrane Structure and Function