Lecture 18 Flashcards
Membranes
- membranes are involved in receiving information information, export and import of molecules, motility
- they are selective barriers
- external and internal membranes (like ER, Golgi, nucleus)
membranes are _____
amphipathic –> one part hydrophilic, one part hydrophobic. Hydrophobic tails hide from water and hydrophilic heads are attracted
Hydrophilic vs hydrophobic
- hydrophilic molecules (acetone) form hydrogen bonds with water molecules
- hydrophobic molecules cannot. water forms a cage like structure around it (energetically unflavoured)
- if there are multiple hydrophobic molecules they will aggregate together to minimize contact with water
The lipid bilayer
- the lipids in the cell membrane are made out of phospholipids which have a phosphate-containing head and two hydrophobic tails
How are bilayers self-sealing
- bilayers spontaneously rearrange to eliminate free edges because this would expose hydrophobic tails to water
- this forms a liposome
In what ways can the lipid bilayer move?
- lipid bilayers have the ability to bend and adapt to changes in membrane structure
- lipids within the bilayer can move in lateral direction, flexion, rotation, and flip flop. NOT vertically
What is fluidity
- the ease with which the lipid molecules can move within the plane of the bilayer
What affects fluidity?
- length of carbon chain
- number of double bonds
Chain length
- shorter chain lengths: less tail-tail interaction, increasing fluidity
- longer chain lengths: decreasing fluidity because more tightly packed
Number of double bonds
- unsaturated chains form a kink and have double bonds which makes it more difficult to pack together, making the membrane more fluid
- saturated chains pack together very densely
saturated vs unsaturated chain
- saturated chain: the carbons are fully saturated with hydrogens
- unsaturated chain: the carbons are not fully saturated with hydrogens
Organisms like bacteria and yeast adapt to different
temperatures by adjusting the carbon chain length and
the unsaturation of double bond
To maintain membrane fluidity at colder temperatures
bacteria would:
Increase the number of double bonds in the carbon chains
Food for thought
- many fats produced by plants are unsaturated: more fluid, exists as liquids at room temperature, melting point is lower than room temperature
- fats produced are saturated: solid at room temperature, melting point is higher than room temperature
Cholesterol
- short, rigid lipid molecules that fill the space between neighbouring phospholipids caused by kinks in hydrophobic tail
- can stiffen membrane, making things more compact (decreasing fluidity)
- it will increase melting point
membrane assembly
- new phospholipids are manufactured on the cytosolic side of the endoplasmic reticulum
- unless they are transferred to the other leaflet of the bilayer by a protein, they will remain on that leaflet (monolayer)
What enzymes are used for the redistribution of phospholipids
- scramblases: randomly moves phospholipids from one monolayer to the other
- flippases: the Golgi membrane contains flippases which removes specific phospholipids from one side to the other
Redistribution of phospholipids causes…
- asymmetric distribution
Asymmetric distribution:
- phospholipids and glycolipids are distributed asymmetrically in the lipid bilayer of the eukaryotic plasma membrane
- for example: phophatidylserine is normally localized to the cytosolic face, but when facing the extracellular fluid it signals for apoptosis
membrane orientation
- phospholipids and membrane inserted proteins transported in the lumen will be exported to the plasma membrane and be in contact with the extracellular fluid
- cytosolic facing protein remain facing the cytosol when transported
- example: attached sugar to membrane protein is synthesized in the lumen of the Golgi
What are the membrane proteins
- transmembrane: extends through the bilayer
- monolayer associated a helix: amphipathic a helix is embedded in monolayer
- lipid linked: attached to the membrane through 1 or more covalently linked lipid groups
- protein attached: attached to the membrane indirectly through association with a membrane protein
Which membrane proteins are integral and peripheral
Integral: transmembrane, monolayer associated a helix, lipid linked
Peripheral: protein attached
detergents
- amphipathic molecules with only one hydrophobic tail
integral membrane proteins
- require detergents to remove from the membrane
peripheral membrane proteins
cambe removed by gentler procedures
Peripheral membrane proteins
- can be removed by gentler procedures
Why do detergents readily form micelles while phospholipids liposomes
detergents have a wedge shape and phospholipids are more cylindrical
How do polypeptide chains usually cross the lipid bilayer
- with an a-helix
single pass vs. multipass
- only crosses membrane once
- cross membrane multiple times (can create channels with hydrophilic interiors)
B-sheets cross the membrane as a
B-barrel
animal cells don’t have a cell wall like plant cells instead they have a ________
- cell cortex:
–> meshwork of fibrous proteins located on the cytosolic side of the membrane - involves restricting diffusion of protein within the membrane
- spectrin is a major component
cells can confine proteins to specific regions known as ______________
membrane domains
tight junctions
- prevent diffusion of proteins to other side of cell
- ensures unidirectional transport of macromolecules
Glycoproteins
- proteins with short chains of sugar
proteoglycans
- proteins with long chains of sugar
where are sugars located
sugars are located on the outside of the plasma membrane forming a carbohydrate layers known as glycocalyx
glycocalyx
- protects against mechanical damage, makes the cell slimy
- functions in cell-cell recognition and adhesion
