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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.
Phospholipids form a double layer surrounding a cell
• Made of a charged phosphate group, glycerol, and two
fatty acid chains
•
Head is polar and forms hydrogen bonds with water
(water-loving… “hydrophilic”)
•
Tail is non-polar (water-fearing… “hydrophobic”)
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane has two major functions.
cell membrane
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
cell membrane
inside cell
outside cell
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.
• There are other molecules embedded in the membrane.
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
Other molecules
•
•
•
Cholesterol strengthens membranes,
Proteins aid cell identification and movement of molecules
across membranes and cell signaling
Carbohydrates aid cell identification
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 made of a phospholipid bilayer.
cell membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane is made of a phospholipid bilayer.
• There are other molecules embedded in the membrane.
• The fluid mosaic model describes the membrane.
cell membrane
carbohydrate
chain
cholesterol
protein
protein channel
protein
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The fluid mosaic model describes the membrane.
•
The membrane is flexible like a fluid and has a variety of
molecules like the variety of tiles in a mosaic (picture)
cell membrane
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
The cell membrane is selectively permeable.
• Allows some molecules to cross
• Can also use terms such as semipermeable and
selectively permeable
• Enables a cell to maintain homeostasis
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane is selectively permeable.
Some molecules can cross the membrane
while others cannot.
3.3 Cell Membrane
Cell membranes are composed of two phospholipid
layers.
• The cell membrane is selectively permeable.
Some molecules can cross the membrane
while others cannot.
3.3 Cell Membrane
Chemical signals are transmitted across the cell
membrane.
• Receptors bind with ligands and change shape.
– Detect a signal molecule and perform an action in
response
• 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:
Located inside the cell;
binds to molecules that
cross the membrane
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.
– membrane receptor:
Located in the membrane;
Binds to molecules that
Cannot cross the
membrane