3.3 Cell Membrane
Bell Ringer
On page 79 Answer # 5
and # 6
3.3 Cell Membrane
KEY CONCEPT The cell membrane is a barrier that
separates a cell from the external environment.
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane has two major functions.
– forms a boundary between inside and outside of the
cell
– controls passage of materials
cell membrane
inside cell
outside cell
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane is made of a phospholipid bilayer.
• A phospholipid is composed of: a charged phosphate,
glycerol, & 2 fatty acid chains
cell membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• Glycerol + phosphate group = “head”
• Fatty acid = “tail”
• The head is has a charge just like water molecules.
– Hydrogen bonds form (attract water)
• The tails have no charge
– Repel water
cell membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• There are other molecules embedded in the membrane.
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• There are other molecules embedded in the membrane.
– Proteins- help materials cross & part of cytoskeleton
– Cholesterols- strengthen the cell
– Carbohydrates- are identification tags
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
According to the picture. Do you think
cholesterol is polar or nonpolar? Why?
cell membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The fluid mosaic model describes the membrane.
– Flexible
– Molecules can move
– Patterned like a mosaic
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane is selectively permeable.
(semipermeable)
Some molecules can cross the membrane
while others cannot.
3.3 Cell Membrane
Semipermeability enables a cell to maintain homestasis.
How a particular molecule crosses the membrane
depends on:
1. size of molecule
2. polarity
3. concentration difference from other side of cell
3.3 Cell Membrane
Small nonpolar molecules easily pass through
the cell membrane.
Small polar molecules used proteins.
Large molecules are moved in vesicles.
3.3 Cell Membrane
“Give an example of a semipermeable membrane with
which you are already familiar.”
3.3 Cell Membrane
Chemical signals are transmitted across the cell
membrane.
• Receptors bind with ligands and change shape.
• There are two types of receptors.
3.3 Cell Membrane
Chemical signals are transmitted across the cell
membrane.
• Receptors bind with ligands and change shape.
• There are two types of receptors.
– intracellular receptor
3.3 Cell Membrane
Chemical signals are transmitted across the cell
membrane.
• Receptors bind with ligands and change shape.
• There are two types of receptors.
– intracellular receptor
– membrane receptor