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 5G p1)

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 5E

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 5F) Transport Proteins - help move large or polar molecules across the nonpolar phospolipid bilayer. (CD activity 5G p2)

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.