Plasma Membrane

PLASMA MEMBRANE

STRUCTURE

 Phospholipid bilayer

 Protein molecules embedded

partially or entirely

 Scattered throughout

proteins span the membrane

with hydrophilic (water loving)portions facing out and hydrophobic (water hating) portions facing in

 Fluid consistency (like olive

oil)

 Called the ‘fluid-mosiac

model’

 http://www.youtube.com/wa

ASYMMETRY AND

ARRANGEMENT:

 The upper and lower portions of the membrane are not

the same (identical)

 Same basic components, but arrangement may vary

between the two halves

 Carbohydrate chains of the glycolipids and glycoproteins

MEMBRANE FLUIDITY

 Molecules can be

‘exchanged’

horizontally, giving the membrane its fluid quality.

 Molecules cannot be

‘exchanged’ or move on a vertical plane. This maintains the integrity of the

THE LIPIDS

 Phospholipids arrange themselves into a

bilayer with their hydrophobic tails facing each other, and the hydrophilic heads facing both the intracellular and extracellular fluids of the cell.

 Glycolipids have a head which is a chain of

sugars forming a carbohydrate. (similar in structure otherwise to phospholipids)

 Cholesterol – found in animal cells and helps

CELL FINGERPRINTS

 Carbohydrate chains serve as ‘fingerprints’

for the cell

 Sequence and diversity of chains vary from

species to species making each cell unique

 Make cell recognition possible within the

body

 Foreign cells are destroyed by the immune

system

 Cells of different blood type, or foreign

TYPES OF PROTEINS

1. Channel protein

2. Carrier protein

3. Cell recognition protein

4. Receptor protein

CELL RECOGNITION PROTEIN

 Allows the body to recognize its own cells,

and foreign cells

 Makes transplants difficult, as well as blood

RECEPTOR PROTEIN

 Shaped so that certain molecules bind to

CHANNEL PROTEIN

 Allows particular molecules or ions to cross

CARRIER PROTEINS

 Combine with an ion or molecule before

transporting it

 Makes the membrane ‘differentially or

selectively permeable’ because it only allows certain molecules and ions to pass

 Are specific; only combine with specific

molecules

 Combines with molecule, changes shape and

TRANSPORT ACROSS A

MEMBRANE

 http://

www.wiley.com/college/pratt/0471393878/s tudent/animations/membrane_transport/ind ex.html

• Typically, a cell must transport various

materials back and forth across its membrane.

• These materials include water, waste,

FACILITATED TRANSPORT USING

CARRIER PROTEINS:

Large molecules such as glucose and amino acids down a concentration

gradient.

Eg. glucose carrier protein changes shape ‘back and forth’ approximately

100 times per second.

Glucose binds to protein, protein closes behind molecule and passes it

through to the cytoplasm

After glucose is released, carrier protein changes back to original shape

FACILITATED TRANSPORT

 This facilitated transport by the carrier

ACTIVE TRANSPORT

 Also uses a carrier protein (called a ‘pump’),

but this time, cellular energy as well

 Ions or molecules move across the membrane

and accumulate either inside or outside the cell

 These molecules often move in the opposite

direction of the concentration gradient

 Eg. sodium ions in urine cells, iodine in the

thyroid gland or the sodium-potassium pump

 40% of the cell’s energy is used in this

SODIUM POTASSIUM PUMP

the sodium and potassium bind, allowing it to combine

alternately with each ion

Change in shape is facilitated by

a phosphate group which is donated by ATP

Watch this animation:

ACTIVE VS. FACILITATED

 1) Compare/contrast active vs. facilitated

DIFFUSION AND OSMOSIS

 Diffusion – movement of molecules from high

to low concentration

 ‘Down the concentration gradient’ until

concentrations are equal on both sides of the cell membrane

 Osmosis is the diffusion of water across the

cell membrane (following the same principal)

 Eg. dye in water (solute – dye, solvent –

water)

VESICLE FORMATION AND THE TRANSPORT

OF MATERIALS WITHIN THE CELL:

Cells will manufacture molecules for secretion outside of the cell.

Some of these secretion molecules are complex combinations of proteins,

carbohydrates and lipids.

Watch this animation:

http://

www.sumanasinc.com/webcontent/animations/content/vesiclebudding.html

1. Protein is synthesised and present in the rER.

2. The protein is moved through the rER and modified.

3. A spherical vesicle is formed form the end of the rER with the protein inside. 4. The vesicle migrates to the golgi apparatus.

5. Vesicle and golgi membranes fuse. The protein is released into the lumen of

the golgi apparatus.

6. The golgi modifies the protein further by adding lipid or polysaccharides to

the protein.

7. A new vesicle is formed from golgi membrane which then breaks away. The

vesicles migrates to the plasma membrane.

8. The vesicle migrates to the plasma membrane fuses and secretes content its

ENDOCYTOSIS AND EXOCYTOSIS

 The fluidity of the membrane allows it to change

shape, break and reform during endocytosis and exocytosis.

 Watch this animation:

 http://highered.mcgraw-hill.com/olcweb/cgi/