The Plasma MembraneSection 7.2p.175-178

Maintaining a Balance• All living cells must maintain a balance

regardless of internal + external conditions• Survival depends on the cell’s ability to

maintain proper conditions within itself

Why cells must control materials• Cells need nutrients

to function– Glucose– Amino acids– Lipids

• Must enter through the plasma membrane– The flexible boundary

between the cell + its environment

Why cells must control materials• Substances such as wastes + products must

leave through the plasma membrane– Substances enter + leave to maintain homeostasis– Selectively permeable• Ability to allow some molecule to pass through while

keeping others out– Examples

» Water vs. Na+ ions

Selectively Permeable

Structure of the Plasma Membrane

• Recall – LIPIDS (Fatty Acids + glycerol)

• Phospholipid– Two fatty acids, glycerol, + a

phosphate group• Plasma membrane is

composed of a phospholipid bilayer

The Phospholipid bilayer• Fatty acids “tails” are hydrophobic

– Don’t want to interact with water• Phosphate group “head” is hydrophilic

– Wants to interact with water

• Creates a barrier that regulates the ease in which substances cross the plasma membrane

Fluid Mosaic Model• The proteins embedded in the bilayer create the

“mosaic”• The phospholipids move within the membrane

(like a fluid)

Other components of the P.M.• Cholesterol– Helps to stabilize the phospholipids by preventing

their FA’s from sticking together

Other components of the P.M.• Proteins

– Transport• Move substances through PM

– Markers• Identify chemical signals + each other

– Inner surface• Gives cell’s structure flexibility

A video clip….

• Plasma Membrane