Section 2 The Plasma Membrane Chapter 7 Section 2 The Plasma Membrane

Plasma Membrane Remember from section 1 notes: All cells have a plasma membrane Both Eukaryotic & Prokaryotic

What is the plasma membrane? A thin, flexible boundary between ALL cells and their environment that controls what can and cannot ENTER and EXIT Maintains HOMEOSTASIS - the balance in the cell’s internal environment

Selective Permeability The property of the plasma membrane that ALLOWS/PERMITS some substances to pass through while PREVENTING the passage of others The control of how, when, and how much of these substances enter and leave a cell relies on the STRUCTURE of the plasma membrane

Selective Permeability The plasma membrane is composed of a PHOSPHOLIPID bilayer, in which TWO layers of phospholipids are arranged tail-to-tail (which allows it to exist in a watery environment) Each phospholipid consists of a “head” with TWO “tails” The “head” is a PHOSPHATE which is polar – and is therefore ATTRACTED to water The “tails” are FATTY ACID, which are nonpolar – and are therefore REPELLED by water

Selective Permeability The fatty acid “tails” form the INTERIOR of the plasma membrane The phosphate “heads” face the INSIDE and OUTSIDE of the cell

Selective Permeability This arrangement allows the polar heads to be closest to the WATERY environments found inside and outside the cell and the nonpolar “tails” to be AWAY from the water Therefore, water-soluble substances CANNOT move easily through the plasma membrane

Fluid Mosaic Model A pattern created on the surface of the plasma membrane by PROTEINS, CARBOHYDRATES, and CHOLESTEROL moving with and among the phospholipids