The Cell Membrane
Ms. Napolitano & Mrs. HaasCP Biology
The Cell Membrane Thin, flexible barrier that surrounds
the cell
AKA plasma membrane
Selectively permeable – allows some substances to cross more easily than others
Usually ~8nm thick
The Phospholipid Bilayer
Fluid Mosaic Model Used to describe the structure of the membrane
Fluid structure of phospholipids with a mosaic of various proteins embedded in it
Cholesterol helps maintain fluidity Membrane becomes less fluid as temperature increases
by restricting movement Membrane becomes more fluid as temperature
decreases by maintaining space
Structure of the Membrane
Membrane Proteins Different types of cells contain different types of
membrane proteins
Integral proteins – embedded into the hydrophobic core of the lipid bilayer May or may not go all the way through the membrane
Peripheral proteins – on the bilayer surface
Integral Peripheral
Types of Membrane Proteins(1 of 2)
1. Transport Channel or carrier proteins (hydrophilic
tunnels) allow polar molecules and ions to pass through the hydrophobic layer
Proteins are specific for the substance they transport
2. Enzymatic3. Signal Transduction (chemical
messages) Receptor proteins transmit information from
outside of the cell to inside of the cell
Types of Membrane Proteins(2 of 2)
4. Recognition ID tags – glycolipids or glycoproteins
5. Intercellular Joining Join together adjacent cells
6. Support Attach to cytoskeleton or extracellular matrix
for stability
The phospholipid bilayer is __________ in the middle and __________ on the outsides.
1 2 3 4
25% 25%25%25%1. Hydrophobic, hydrophilic
2. Hydrophilic, hydrophobic
3. Hydrophobic, hydrophobic
4. Hydrophilic, hydrophilic
What is the function of cholesterol in the cell
membrane?
1 2 3 4
25% 25%25%25%1. Identification2. Transport3. Membrane
fluidity4. Recognition
The picture shows a peripheral protein.
1 2
50%50%1. True2. False
Which of the following is NOT a function of
membrane proteins?
1 2 3 4
25% 25%25%25%1. Signal transduction
2. Identification3. Transport4. Support
Passive Transport Passive Transport – the movement
across the cell membrane that does not require energy
Types: Diffusion Osmosis Facilitated Diffusion
Diffusion Diffusion – the movement of particles from an
area of high concentration to an area of low concentration until an equilibrium is reached Transports small, nonpolar molecules such as CO2
and O2
Concentration Gradient – the difference in the concentration of a substance across a space
Equilibrium – concentration of a substance is equal throughout a space
Osmosis Osmosis – diffusion of free water across a
selectively permeable membrane Water diffuses across the cell membrane
from the region of low solute concentration to that of a higher solute concentration until it reaches equilibrium
U-Tube (not YouTube!)
Which way will water flow?
U-Tube (not YouTube!)
This way!
Osmoregulation in Cells Without Cell Walls
Osmoregulation: control of water balance Tonicity: the ability of a solution to cause a cell to
gain/lose water Isotonic solution: no net movement of water across
the cell membrane Hypertonic solution: more free water inside the cell
(cells lose water & shrivel) Hypotonic solution: less free water inside the cell
(cells gain water & lyse) Some cells have a contractile vacuole to pump water
out of the cellhttp://www.youtube.com/watch?v=mTXRcbjuYGU
Osmoregulation of Cells With Cell Walls
Plant cells are healthiest in a hypotonic solution Osmotic pressure keeps cell walls turgid (very
firm)
Plant cells are flaccid (limp) in an isotonic solution
In a hypertonic solution, the cell membrane will shrink and pull away from the cell wall AKA plasmolysis (wilting)
http://www.youtube.com/watch?v=EsJ73x4ycp0
Facilitated Diffusion Facilitated Diffusion: passive transport aided by
proteins Molecules travel from high low concentration Transports small, polar molecules
Glucose Salts Ions Amino Acids Water (aquaporins)
Channel Proteins Hydrophilic passageways Some are always open for diffusion Rate of movement is determined by the
concentration gradient Some ion channels have gates that can be
opened by: Cell membrane stretching Change in electrical charge Binding of specific molecules
Carrier Proteins1. Specific substance binds to carrier protein
2. Protein changes shape & transports substances across the cell membrane
3. Molecule is released into the cell, and carrier protein returns to its original shape
http://www.youtube.com/watch?v=GFCcnxgXOhY
Active Transport Uses energy to move solutes AGAINST the
concentration gradient Carrier proteins act as “pumps” powered
by ATP Examples:
Sodium Potassium Pump Proton Pump Cotransport
Sodium Potassium Pump
Pumps 3 Na+ out of the cell and 2 K+ into the cell Actively transports both ions against their
concentration gradient Powered by ATP
http://www.youtube.com/watch?v=GTHWig1vOnY
Proton Pump Actively transport protons (H+) through the
internal membranes of mitochondria & chloroplasts
Cotransport (Coupled Transport)
Cotransport: The transport of one solute indirectly drives the transport of several other solutes (using ATP)
Example: As proton pump pumps H+ out, H+ ions diffuse back in pulling sucrose with it
Bulk Transport Substances that are too large to be
transported by carrier proteins Example: proteins leaving/entering the cell Uses vesicles Types:
Exocytosis Endocytosis
Exocytosis Exo = Export (recall exo means outside!!) Vesicles fuse with the cell membrane,
releasing the contents outside of the cell
Endocytosis Recall endo means inside!! Cell membrane engulfs particles Pinches off to form vesicles inside of the cell
http://www.youtube.com/watch?v=ygkvXT1BmaA
Top Related