The Plasma Membrane AP Biology All cells are

The Plasma Membrane AP Biology

All cells are separated from their environments by a plasma membrane.

Cell Membranes are Selectively Permeable • Selective Permeability – the property of the cell membrane that allows some substances in and out of the cell while refusing to let others pass through. • Small and nonpolar molecules pass easily through the cell membrane • Polar and charged molecules do not pass through easily. (CD Activity 5 G p 1)

Membrane Phospholipids form a Bilayer • A phospholipid is a special fat molecule with a polar (hydrophilic) head and a nonpolar (hydrophobic) tail region. • In an aqueous (water-based) solution, phospholipids arrange themselves into rows with the heads facing the water and tails facing each other

THE FLUID MOSAIC MODEL • The phospholipid bilayer combined with membrane proteins, cholesterol (in animal cells), and carbohydrates makes up the complete cell membrane. • It is called “fluid” because the lipid molecules and proteins flow around each other, and a “mosaic” because of its different parts and functions. See CD Activity 5 E

Functions of Membrane Proteins • • • Help support the delicate membrane. Catalyze cellular rxns (enzymes). Provide identification tags for cell types. Form junctions between cells. Receptor Proteins – enable cell to cell communication (signal transduction) by receiving molecular messengers from other cells (much like enzymes receive substrates) (CD activity 5 F) • Transport Proteins - help move large or polar molecules across the nonpolar phospolipid bilayer. (CD activity 5 G p 2)

Membrane Cholesterol • A lipid that stabilizes the fluidity of animal cell membranes by resisting temperature changes.

Membrane Carbohydrate Chains • get attached to phospholipids or proteins on the outside of membranes, which results in glycolipids and glycoproteins. • Function as identification tags to different cell types can be told apart. This allows: – Embyonic cells to sort themselves out into different tissues and organs. – The immune system to recognize and reject foreign cells like bacteria.