Cell Membrane
Structure and Function
Why is the plasma membrane
necessary for cell survival?
What characteristics should
this barrier have?
Fluid Mosaic Model:
Phospholipid Bilayer (grout)
Phospholipids
Two fatty acid chains (non-polar) tails
linked to a phosphate group (polar head)
Double bonds increase fluidity
Amphipathic molecule
Fluidity
Fluid because phospholipid molecules are
not bonded to each other
Fluid Mosaic Model:
Phospholipid Bilayer
Selectively Permeable
Hydrophilic molecules cannot pass easily
through
Some very small or uncharged, lipid
soluble molecules can pass through easily
Cholesterol (in animal cells)
Makes bilayer stronger, less flexible, and
less permeable
Fluid Mosaic Model:
Mosaic of Proteins (tiles)
Proteins
Embedded in (integral) or attached to
surface (peripheral)
Three functional categories
Transport proteins
Receptor proteins
Recognition proteins
Membrane Related Health Issues
Infectious disease
Drug addiction
Allergies
Appetite signaling
Nervous system disorders involving
neurotransmitters
Transport Across Membranes:
Passive Transport
Substances move “down”
concentration gradient
Difference in concentration
No energy required
Simple Diffusion
Rate of simple diffusion
Concentration gradient
Molecule size
Lipid solubility
Examples
Facilitated Diffusion
Membrane transport membranes
Channel proteins
Carrier proteins
Osmosis
Diffusion of water across cell membrane
Isotonic cell environment
H2O concentration same in cell as
around cell
No net movement of water
Hypertonic cell environment
[solute] outside > [solute] inside cell
Water will flow out of cell
Cells shrivel
Osmosis
Hypotonic cell environment
[solute] outside < [solute] inside cell
Water will flow into cell
Cells will swell
Diffusion and Osmosis in the Human Body
As a person becomes very dehydrated, the concentration of water in her
blood decreases. In which direction will water move across the plasma
membranes of her blood cells? What will happen to the volume of the cells
as a consequence? Why is proper hydration important for multicellular
organisms.
Oops! You were cleaning out the aquariums in the biology lab and you
accidentally placed the freshwater fish in the saltwater tank. What will
happen to the cells of the fish?
In some countries, human waste is used as a fertilizer. If produce is grown
in this type of fertilizer, it is possible that it will be contaminated with the
protozoan that causes amoebic dysentery. One method of removing the
protist without ruining the food is to place the produce in a 5% salt solution.
Explain how this would kill the protist. If you cleaned your lettuce this way,
what would happen to the lettuce cells? How could you return them to
normal?
Transport Across Membranes:
Active Transport with Proteins
Use membrane proteins
Span the phospholipid bilayer
Use energy in the form of ATP
Two binding sites
Binds substance to be transported
Binds ATP for energy
Example – Na-K pump
Membrane potential
Transport Across Membranes:
Endocytosis
Pinocytosis
“Cell drinking”
nutrients
Phagocytosis
“cell eating”
Macrophages and white blood
cells
Transport Across Membranes:
Endocytosis
Receptor-mediated endocytosis
Selective process – receptor
proteins
Internalized in coated pit
Hormone insulin
Disorder: familial
hyperchloesterolemia
Connections Between Cells
Desmosomes – anchoring junctions
Adjacent cells held together by proteins and
carbohydrates
Skin, heart muscle, and neck of the uterus
Tight junctions
Impermeable junctions
Epithelial cells lining the digestive tract
Connections Between Cells
Gap junctions
Cells connected by hollow cylinders
Transmembrane proteins
Allow communication
Heart and smooth muscle cells
Plasmodesmata
In plants