ZIMA Lecture 1 (Membrane Structure/Function) Flashcards
Define cell compartmentalization
Cell membranes and membranes of organelles constitute ___________\_for diffusion, preventing the mixing of distinct enviornments.
Transmembrane proteins act as a _______ of external signal or as ion channels and pumps
Cell compartmentalization: distinct chemical enviornments in the cytoplasm and inside intracellular organells
Cell membranes and membranes of organelles constitute an effecient barrier for diffusion, preventing the mixing of distinct enviornments.
Transmembrane proteins act as a sensor of external signal or as ion channels and pumps
What are the three major classes of membrane lipids?
What is the most abundant type of phosholipid?
What are they composed of?
Three major classes of membrane lipids:
phospholipids, cholesterols, and glycolipids
Most abundant phospholipids are phosphoglycerides
Made up of: polar molecule (choline, serine, etc) - phosphate - glycerol - two fatty acid chains (one is nonsaturated)
Which one has a kink? Nonsaturated or saturated?
NONSATURATED has a cis double bond that creates a kink in the fatty acid chain
What is spingomyelin?
It is a phospholipid.
It forms from choline, phosphate group, sphingosine, and fatty acid chains (fully saturated)
NO CIS DOUBLE BOUNDS FOUND IN THESE FATTY ACIDS
Explain the makeup of cholesterol.
What is its function?
Cholesterol:
- Sterol
- Up to one cholesterol for every phospholipid
- It contains a ring structure with polar group and a short non polar hydrocarbon chain
- Cholesterol enhances the permeability barrier of the lipid bilayer
- “It plays important surface membrane functions, it makes the membrane less permeable to water”
What molecules are major components of eukaryotic bio membranes?
PHOSPHOLIPIDS (“phospholipid bilayer”)
_________ of the phospholipid molecules causes them to form bilayer spontaneously without the use of biochemical energy.
Self-aggregation is ________ (to hide fatty chain and expose hydrophilic heads to water).
As a result the membrane possess a _______.
Amphiphilic nature of the phospholipid moecules causes them to form bilayer spontaneously without use of biochemical energy.
Self aggregation is energetically favorable (to hide fatty chain and expose hydrophilic heads to water)
As a result a membrane possess a self healing property.
Which of the following is a significant factor that governs stability to biological membranes?
Intermolecular interaction between fatty acid chains
Explain how lipid bilayers are a 2D fluid
Phospholipids in a membrane can move in different directions: rapid radial rotation, rapid lateral diffusion, slow flip flop exchange.
Flip flop RARELY occurs in eukaryotes
Phospholipid translocators catalyze the rapid flip flop of phospholipids from one monolayer to another.
- What does fluidity depend on?
- What is the phase transition temp/freezing point?
- What kind of acid chains are harder to freeze?
- What’s the relationship between fluidity and transition temp?
- What does cholesterol do?
- Fluidity (increased membrane dynamics) depends on the membrane composition and temperature.
- Temperature at which a lipid bilayer changes from a liquid to a crystalline state is called a phase transistion temp (or freezing point)
- Shorter fatty acid chains or fatty acid with double bonds favor fluidity (harder to freeze)
- As fluidity increases, the transition temperature decreases.
- Cholesterol decreases the permeability of the lipid bilayer, but not the fluidity.
Rafts: higher density of ________ & ________ in the lipid membrane.
Rafts could help_______
Rafts: higher density of sphingolipids & cholesterol in the lipid membrane. However, membrane proteins prefer specific membrane patches with whom they can interact.
Rafts could help organize membrane proteins- concentrating them for specific function (e.g. caveolae in the cell signaling)
Monolayers in membranes are _______.
Because spontaneous phospholipid flip-flop is a rare event, monolayers in membranes are asymmetric.
Lipid asymmetry of membranes plays a role in __________.
Lipid assymetry of membranes plays a role in converting extracellular signal into an intracellular one.
What is the major player in cell signaling within the membrane?
What happens with apoptosis?
Phosphatidylinositol (located in the inner monolayer) plays an important role in cell signaling.
Apoptosis: (programmed cell death): PS translocates to the outer monolayer by unspecific translocases (“scramblase”) and by the lack of activity of PS-translocase. The presence of PS in the outer monolayers triggers the cell’s phagocytosis by macrophages.
What is a glycolipid?
Where are they located?
Glycolipid: sugar containing lipids; they are made from sugars - sphingosine- fatty acids (no phosphate group)
They are located in the extracellular side of the bilayer
What is the physiological significance of glycolipids?
Glycolipids protect the membrane (charged gangliosides enhance concentration of Ca 2+ near the surgace of the membrane, cell-to-cell adhesion, etc.
SCREEN PROTECTION BARRIER, improve barrier for mechanical stress/physical force
How does cholera toxin work?
Cholera toxin binds to and enters cells that have GM1 (type of ganglioside) in their membranes and then causes increase in cAMP in the small intestine that leads to efflux of Na+ and water (diarrhea)
Transmembrane proteins have both ______ and ______ regions, making them “_______”.
Transmembrane proteins have both hydrophobic and hydrophillic regions- amphiphilic.
Hydrophobic AA’s mostly interact with the core of the lipid bilayer. Hydrophillic AA’s (side chains containing polar oxygen and nitrogens) tend to locate on the outer and inner membrane/water interfaces.
How are cytosolic and extracellular domains seperated?
Cytosolic and extracellular domains are seperated by the membrane spanning domains, which are composed largely of amino acids with nonpolar side chains.
Moreover, membrane proteins oragnize themselves in lipid bilayers in such a way as to maximize H bond interactions between polar groups forming regular alpha-helices
How can you predict which part of a peptide chain extend across the lipid bilayer?
From the amino acid sequence, it is possible to predict which parts of the peptide chain extend across the lipid bilayer. Segments containing 20-30 amino acids with a high degree of hydrophobicity can span a membrane as an alpha helix, and they can be identified in a hydropathy plot.
What is an alternative way for transmembrane proteins to pass a membrane?
Where is this structure located?
What is preferred structure in eukaryotes?
Formation of a beta sheet or a beta barrel is an alternative way for transmembrane proteins to pass a membrane. Hydrophilic AA’s will be inside the barrel. The beta barrel transmembrane proteins are limited to mitochondria, chloroplasts, and bacteria.
Cell prefers to use alpha helix in eukaryotes, more dynamic.
Which phospholipid is involved in apoptosis?
Phosphatidylserine
What is an important difference between proteins in the two sides of the membrane?
Important difference between proteins on the two sides of the membrane results from the reducing enviornment of the cytosol. Thus, disulfide (S-S) bonds will be formed only on the extracellular surface of the cell.
Many membrane proteins are often _______ on their extracellular sides, and extensive ________ creates a ________.
What role does that structure play?
Many membrane proteins are often glycosylated on their extracellular sides, and extensive glycosylation creates a glycocalyx.
That glycocalyx plays a role to protect cells against mechanical and chemical damage.
Remember: Reductive inside cell (cytosol),
Oxidative outside cell
Some membrane proteins diffuse along the ______, others are anchored via ________.
Some mebrane proteins diffuse along the bilayer plane, others are anchored via cytoskeleton proteins.
Majority of the membrane proteins in eukaryotes are:
alpha helices or beta sheets?
Answer: majority of eukaryotic transmembrane proteins are ALPHA HELICES
What plays the most important role in cell signaling?
What plays a role in apoptosis?
Cell signaling: phosphatidylinositol
Apoptosis: phosphatidylserine
(remember the PS gets moved from inside the cell to outside by the scramblase/translocator)
