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Unit: Cellular Structure & Function
All cells have a cellmembrane thatseparates the cellfrom its non-livingsurroundings.
It is a thin barrier;approximately 8 nm thick!
Problems with the cell membrane are involved in many diseases
Type 2 Diabetes, organ transplant rejection, Cholera, Cystic fibrosis, cancer
The cell membrane has several roles to play in a living cell.
For example:
• It helps thethe cell maintain homeostasis by regulating what enters & leavesleaves the cell (controls traffic in and out)
• It receives and produces signals to and from other cells.
• It identifies the cell as belonging to a particular organism and tissue
• It maintains connections between cells in organs and tissue
This fluidity is caused by phospholipids, which form the foundation of the plasma membrane.
Remember: Lipids and water REPEL one another
Cell membranes have a flexible, fluid like structure.
Phosphate group head Polar hydrophilic
Fatty acid tails (2)Nonpolar hydrophobic
Phosphate Head
Fatty acid tails
The nonpolar “tails” cluster
together trying to get as far
away from the water as they
can.
The hydrophilic “heads” are
attracted to the water
The resulting formation is referred to as a lipid bilayer
Outside the cell
Inside the cell
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
The Lipid Bilayer is Semipermeable
The lipid bilayer is a barrier that can be described
as Selectively Permeable (A membrane that
allows only certain substances in the cells
environment to pass through it.)
Some things have no trouble crossing the
Membrane (Ex. small, nonpolar things like O2 &
CO2) ;others need help across (ex: glucose)
1) Proteins
Some proteins stick to the surface of the lipid
bilayer (peripheral proteins) and others penetrate
the hydrophobic core (integral proteins).
Amino acids, which make up proteins, may be
polar or nonpolar. How do integral proteins stay in
place?
Answer: The water found on either side of the membrane repels the non-polar part of the membrane protein. It is attracted to the interior of the lipid bilayer
The polar part of a membrane protein is attracted to the water on both sides of the membrane.
This duel attraction holds the protein in place within the lipid bilayer
They may be attached to
some proteins
(glycoproteins) or lipids
(glycolipids) at the
membranes surface.
Function- act as a
chemical identification
card which allows cells to
recognize & interact with
one another.
Because the proteins embedded in the lipid bilayer can
move around and “float” among the lipids, and because so
many different kinds of molecules make up the cell
membrane, scientists describe the cell membrane as a
“fluid mosaic.”
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids