4.3 Membrane Structure
Membrane Function
Surrounds all cells
 Regulates cell’s internal environment

◦ Maintains HOMEOSTASIS
 Maintains the same conditions constant inside the
cell, regardless of external conditions
Membrane Function

Regulates what enters and exits the cells
◦ Selectively permeable
 Only allows specific materials to pass over the
membrane
 Examples: spaghetti strainer & mosquito netting
 Allows water & nutrients to enter the cell and
wastes to leave the cell
Membrane Structure:
1. Phospholipids

Phospholipid
◦ Lipid with…




2 fatty acids
1 glycerol
1 phosphate group
1 polar group
◦ Fatty acids are
hydrophobic
◦ The rest of the
phospholipid is
hydrophilic
Membrane Structure
1. Phospholipids

Form a bilayer
◦ Cell membrane has two layers of
phospholipids
◦ Polar “heads” attracted to water, carbs,
proteins outside the cell and in the cytoplasm
 Face outward
◦ Non-polar “tails” not attracted
to water
 Face inward
Membrane Structure
2. Cholesterol

Lipid
◦ Hydrophobic
 Fits in with fatty acid tails of phospholipids

Function:
◦ Keeps membrane from freezing or falling
apart
◦ Helps “stabilize” membrane
Membrane Structure
3. Membrane Proteins

Membrane proteins have:
◦ Hydrophobic regions & Hydrophilic regions
◦ These help membrane proteins STAY in
membrane
Membrane Structure
3. Membrane Proteins
Which color of the protein is hydrophilic?
 Hydrophobic?

Membrane Structure
3. Membrane Proteins

Function:
◦ Recognize material & transport into or out of
cell
Fluid Mosaic Model

Describes the cell membrane:
◦ Phospholipids are always moving like a fluid
membrane proteins make a pattern in the cell
membrane
REMEMBER THIS…?

What is concentration?
◦ The amount of particles in a particular area
Concentration gradient
Concentration gradient: unequal
distribution of particles (solute) in a
solution
 Particles will move until the
concentration is equal

◦ Dynamic equilibrium
Concentration Gradient

What will happen?
Concentration

Dynamic equilibrium!
Bozeman membrane

http://www.youtube.com/watch?v=y31DlJ
6uGgE