Transcript Lecture #4 The Plasma Membrane

Unit: Cellular Structure & Function
All cells have a cell
membrane that
separates the cell
from its non-living
surroundings.
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 the cell maintain homeostasis by
regulating what enters & leaves 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
Take 2 minutes to summarize what you have
learned regarding the importance of
the cell membrane to living things.
Discuss with your shoulder partnerWhat do you understand well?
What is a little confusing?
Cell membranes have a flexible, fluid like structure.
 This fluidity is caused by phospholipids,
which form the foundation of the plasma
membrane.
Remember: Lipids and water REPEL
one another
Phosphate Head
Phosphate group head
 Polar
 hydrophilic
Fatty acid tails
Fatty acid tails (2)
 Nonpolar
 hydrophobic
 The nonpolar “tails” cluster
together trying to get as far
away from the water as they
can.
Outside the cell
 The hydrophilic “heads” are
attracted to the water
Inside the cell

The resulting formation is
referred to as a lipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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)
Take 2 minutes to summarize what you have learned
regarding the structure of the lipid bilayer and the
reason and impact of its design
Discuss with your shoulder partnerWhat do you understand well?
What is a little confusing?
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.”
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
Take 2 minutes to summarize what you have learned
regarding the fluid mosaic model including the types of
molecules that are found in the cell membrane and
how they work together to help the cell perform the
activities of life.
Discuss with your shoulder partner What do you understand well?
 What is a little confusing?