Membrane structure

What is a detergent

small amphiphilic molecules of various structure 

• Hydrophobic ends of detergents bind to hydrophobic regions of membrane proteins and bring membrane proteins into solution 

  • Good for studying membrane protein characteristics and activities 

• Pulls membrane proteins into solution (disrupting bilayer) 

What is cholesterol

Serves to increase impermeability of the lipid bilayer and prevents hydrocarbon tails from crystallizing (is a sterol) 

• Found in the bilayer (lots in eukaryotes) 

  • Supportive region to stiffen fatty acid tails 

• Oriented with its hydroxyl polar head close to polar heads of phospholipids 

What is a ganglioside

complex glycolipids found mostly in nerve cells

-         Tay shaches disease

What is a glycolipid

sugar-containing lipid molecules found exclusively in noncytosolic side (exterior side of cell) of lipid bilayer 

• Sugars are added in lumen of Golgi apparatus (environment similar to extracellular) 

• Function = to help cell interact with its surroundings, protect from harsh surroundings, affect electrical field of membrane and ion concentration 

What are membrane anchors (example)

example =  Glycosylphosphatidylinositol anchor (GPI) = special membrane proteins that are actually extracellular made in the ER of the cell as single-pass initially 

• Transmembrane segment is cleaved and the GPI anchor is added 

• Anchor is only hold on this protein 

• Protein is delivered via transport vesicle to the cell membrane

GPI is the anchor

What is the lipid bilayer

Serves as relatively impermeable barrier to water soluble molecules 

• Thin film of lipid and protein molecules held together by non-covalent interactions 

• Not rigid because has 2 layers 

• Proteins embedded 

What is a lipid droplet

excess cellular lipids stored in organelles surrounded only by a phospholipid monolayer 

• Contain neutral lipids like triglycerides and cholesterol esters 

• Formed from ER membrane 

• Ex: adipocytes = just one big fat droplet 

What is a phospholipid

most abundant membrane lipid 

• Has 2 hydrophobic hydrocarbon tails (fatty acids) 

What is glycosylation

adding sugars to proteins 

• Most cell membrane proteins are glycosylated (contain sugars) 

• Sugars added in the ER and Golgi to proteins destined for some membrane  

• Sugars are always present on the noncytosolic side of membrane (extracellular side) 

What is corralling

helps to concentrate activated signaling complexes so to increase signal

What are the common phospholipids

Most common phospholipids in mammalian cell membranes: 3-carbon glycerol backbone 

1. Phosphatidylethanolamine = Found mainly in E. Coli & mitochondrion 

2. Phosphatidylserine = found mainly in myelin & RBC 

3. Phosphatidylcholine = found mainly in the mitochondrion 

Shingomyelin = most common sphingolipid 

• Found mainly in the liver 

*Not all cells have the same lipid composition (varies between cell type and tissue) 

Describe the lipid bilayer

basic structure for all cell membranes 

• Has embedded proteins (50% covered in proteins) 

• Lipid molecules are amphiphilic (hydrophilic &hydrophobic) 

• Contains cholesterol and glycolipids 

• Disordered lipid molecules  

  • Irregular and flexible spacing 

    • Kinks in fatty acid tails allow for more flexibility  

    • Phospholipids are NOT bound to their location 

How is the lipid bilayer fluid (un/saturated)

Fluidity = must be maintained for transport and enzymes to work 

• Lipid bilayer has bent tails (double bond) to make the chains harder to pack together and the bilayer more difficult to freeze thus maintaining functionality at lower temps 

  • Unsaturated: cis-double bonds = freeze resistant, functionality kept 

  • Saturated: no double bonds = freeze at low temps, no movement 

What is a lipid raft

patch in lipid bilayer made of specific lipids and proteins 

• segregate into specialized domains 

• Small region of a membrane enriched in sphingolipids and cholesterol 

How and why is the lipid bilayer asymmetrical

Asymmetry = crucial for converting extracellular signals into intracellular ones (and for marking dying cells) 

• Makes charges between the two halves of the bilayer different 

• Asymmetrical monolayer 

Creates electrical chemical gradient 

What is a hydrophathy plot

Hydropathy plot = illustrates the number of amino acids of a transmembrane protein located within a cell membrane’s lipid bilayer  (~20-30) 

• How many membrane-spanning domains there are 

 

What are the 3 types of membrane proteins

Types:  

Transmembrane  

Cytosolic but associate with cytosol side of bilayer   

Cell surface but attached to lipid bilayer  

What is GPI (glycosylphosphatidylinositol anchor)

special membrane proteins that are actually extracellular made in the ER of the cell as single-pass initially 

• Transmembrane segment is cleaved and the GPI anchor is added 

• Anchor is only hold on this protein 

• Protein is delivered via transport vesicle to the cell membrane 

GPI is the anchor  

What are peripheral membrane proteins

bound via some noncovalent interaction with other membrane proteins 

What are transmembrane proteins

held in the hydrophobic core of the lipid bilayer and CANNOT be released easily 

• Function on BOTH sides of the bilayer/transport molecules across it 

• Membrane-spanning domains contact hydrophobic part of lipid bilayer (tails) 

  • Because the inside of the membrane bilayer is hydrophobic, the transmembrane domains are made of non-polar amino acids 

    • Since every peptide bond is polar and no water is present in the bilayer, the amino acids form hydrogen bonds with each other, so peptide chain curls into an alpha-helix as it crosses bilayer 

• types: single pass, multi pass, and beta barrels
  

What is a single pass protein

cross the lipid bilayer only once  (transmembrane)

What is a multi-pass protein

cross the lipid bilayer multiple times (“porin proteins”) 

• Can form beta sheets which form into a barrel which is a protein to satisfy hydrogen bonding requirements 

• More often occur in bacterial membranes and mitochondrial membranes 

• Most are constructed from alpha-helices that can slide, open and shut, transport or transduce (in eukaryotic cells) 

• Multiple transmembrane helices can arrange to create channels through the membrane (helices can make channels too) 

What are beta barrels

can form channels 

• Some have projecting amino acid loops into the center, filling the hole 

  • Can function as receptors or enzymes 

 

What is a carbohydrate coat

carbs extensively coat the surface of all eukaryotic cells covalently bound to membrane proteins and lipids 

• Lectins = carbohydrate-binding proteins 

  • Mediate variety of cell-cell adhesion processes and other recognition routes 

  • Used to study carb coating 

What is the eukaryotic cell cortical cytoskeleton

nucleated cells contain a more complex network that makes up the cortical region of the cytoplasm 

• Cortex = covering 

• Called the cortical cytoskeletal network = spectrin 

  • Cytoskeletal filaments can be attached to cytosolic membrane and form barriers to membrane protein movement 

How and why is the cytoskeleton flexible?

Flexible cytoskeleton = Maintaining shape 

• Tethering membrane proteins to other proteins inside the cell 

• EX: RBC = spectrin protein forms cell cytoskeleton, this interacts with membrane proteins to produce a flexible membrane 

  • Hereditary spherocytosis (spectrin mutations) produces RBCs that have unusual cell membrane properties... unflexible, destroyed quickly leads to anemia 

What is a intracellular domain

• Among the best-characterized plasma membrane proteoglycans are the syndecans, which have a membrane-spanning core protein whose intracellular domain interacts with the actin cytoskeleton and with signaling molecules in the cell cortex 

Special cell domains for proteins 

Membrane proteins are segregated into specific areas of cell surfaces 

• Proteins don’t swim through the lipids 

• Held in place by tight junctions, cytoskeletal components or protein-protein interactions 

• Keeps proteins and lipids in appropriate places for specific functionality 

Protein-protein interactions = keep proteins in separate domains 

• Self-aggregation 

• Tethering  

• Interaction with other cells 

What are extracellular domains

The extracellular domain is linked to multiple GAG chains (primarily heparan sulfate). 

What are some protein-protein interactions

Protein-protein interactions = keep proteins in separate domains 

• Self-aggregation 

• Tethering  

• Interaction with other cells