Plasma Membrane

Pre-Assessment

1. Which of the following statements concerning membrane proteins is incorrect?

A. They can act as a channel, allowing the transport of ions across the membrane.

B. They often require ATP to actively transport materials across the membrane against a concentration gradient.

C. They may be receptor proteins that bind specific molecules from the surrounding solution, which triggers endocytosis (i.e., receptor-mediated endocytosis).

D. They are usually not particular about what types of chemicals they will allow to cross the membrane.

2. Which molecule works to keep the membrane at optimal fluidity?

3. Which molecule is both hydrophobic and hydrophilic and aggregates as a bilayer to form the "fabric" of the membrane?

4. Which molecule may function in facilitated diffusion? 5. Which molecule might serve as a binding site for a

hormone, thereby eliciting a response in the cell?

Phospholipid Bilayer• Composed of fatty acid

tails connected to glycerol heads

• non-polar (hydrophobic) fatty acid interior impedes passage of water-soluble substances (it is liquid like a soap bubble)

• Polar heads (hydrophilic) are water soluble

Components of a Plasma Membrane

• Phospholipids• Proteins– Integral proteins – completely penetrate the lipid bilayer;

they control the entry and removal of specific molecules– Peripheral proteins – bound to the exterior surface of

the membrane; they have various functions

Components of a Plasma Membrane

• Cholesterol– Inside near the fatty acid tails; helps to regulate

membrane fluidity and is important for membrane stability

Fluid Mosaic Model

The 6 Membrane Protein Functions

1. Hormone binding sites – these proteins have specific shapes exposed to the exterior that fit the shape of specific hormones

2. Enzymes – catalyze chemical reactions; may be on interior or exterior of the cell membrane; often grouped together for a chain reaction (called metabolic pathway)

3. Cell adhesion – proteins hook together to provide temporary or permanent connections; these connections referred to as junctions

4. Cell-to-cell communication – many include carbohydrates attached to protein molecules on outside of cell; they provide an ID tag letting other cells know what type of cell they are

5. Channels for passive transport – integral proteins that have a channel in them to allow substances to move through; passive means substances move through from high to low concentration

6. Pumps for active transport – integral proteins that shuttle a substance from one side to the other by changing their shape; this process requires energy in the form of ATP