Week 2: Lipid Bilayer and Membrane Proteins

Question 1

Describe plasma membrane

Answer 1

protein-studded, lipid bilayer. Protects cell from outside environment. Composed of membrane lipids, membrane proteins and carbs. Proteins are inserted - penetrated by selective channels and transporter proteins -permeability barrier to most water-soluble molecules- proteins that act as sensors/receptors - is also self-healing.

Question 2

What 3 things does the plasma membrane achieve?

Answer 2

Receives information, imports/exports small molecules, allows capacity for movement and expansion

Question 3

What are the 2 properties of lipids?

Answer 3

hydrophilic head and hydrophobic tail - amphipathic

Question 4

What is the most common phospholipid in membranes and what is it composed of?

Answer 4

Phosphatidylcholine. Choline - Phosphate- Glycerol - two hydrocarbon tails.

Question 5

How do hydrophobic molecules react in water? Amphipathic molecules?

Answer 5

Coalesce into fat droplets.

Lipid bilayer

Question 6

How do lipids in bilayer stick together?

Answer 6

non-covalent forces, amphipathic phospholipids, weak bonds (van der Waals)

Question 7

What do pure phospholipids form?

Answer 7

closed spherical vesicles - liposomes. Can be used by scientists to measure movements of lipid molecules in lipid bilayer.

Question 8

What are lipid nanoparticles?

Answer 8

like liposomes but with more complex structure - used to deliver mRNA Covid vaccines.

Question 9

What is meant by 2D nature of membrane?

Answer 9

Individual lipids flex their tails and rotate rapidly, move and change position relative to each other - movement from one half to another is rare. Without proteins, tumbling between layers occurs less than once per month for any lipid. Constantly exchange places within their monolayer. Lateral diffusion, flexion (of tails) and rotation, flip-flop (rarely occurs).

Question 10

What does fluidity of bilayer depend on? (hint: 2 things)

Answer 10

Phospholipid composition and nature of hydrocarbon tails.

Question 11

What 2 properties of hydrocarbon tails affect tight packing in bilayer?

Answer 11

Length and saturation.

Shorter chain = more fluidity. Unsaturated tails = more fluidity.

Question 12

How is membrane fluidity modulated in bacteria and yeast?

Answer 12

lengths and saturation of tails are constantly adjusted to maintain membrane with relatively constant fluidity.

Question 13

How is membrane fluidity modulated in animal cells?

Answer 13

cholesterol - constitutes 20% of lipids by weight. Its short and rigid steroid ring structure can fill spaces between neighboring phospholipids left by kinks. It stiffens bilayer - less flexibility and less permeability.

Question 14

Why is cell fluidity important?

Answer 14

Fluidity enables membrane proteins to diffuse rapidly and interact with each other - important for cell signaling. Allows membrane lipids and proteins to diffuse from insertion sites to other cell regions. Ensures even distribution during replication. Can allow membranes to fuse.

Question 15

How are phospholipids made and oriented in ER?

Answer 15

Newly made phospholipids are added to cytosolic side of ER membrane and then redistributed by transporters - scramblases - that transfer them from one half of lipid bilayer to the other

Question 16

How are phospholipids confined to a side in Golgi?

Answer 16

Golgi membrane has transporters - flippases - remove specific phospholipids from side of bilayer facing exterior and flip them into one facing cytosol. This asymmetry is preserved as membranes bud from organelle and fuse. Conservation of orientation also applies to proteins in the membrane.

Question 17

Which lipid has most lopsided distribution?

Answer 17

The most lopsided distribution is glycolipids - only in noncytosolic half. The sugar groups face exterior - forming coat of carbs that surrounds and protects. The glycolipids acquire their sugar groups in Golgi - enzymes are oriented so the sugars are added only to lipid molecules in noncytosolic half. Once a glycolipid is created in this way, it remains trapped in this monolayer.

Question 18

What is a lipid raft?

Answer 18

Outer leaflet may have microdomains with distinct lipid composition. Contain higher proportion of sphingolipids. In artificial bilayers they float within plasma membrane. House a concentration of signaling and receptor molecules.

Question 19

How much of membrane mass is proteins in animal cells?

Answer 19

50% of mass of plasma membranes - 50 x number of lipid molecules to proteins.

Question 20

What are the different types of membrane proteins?

Answer 20

Transporters and channels, anchors, receptors and enzymes

Question 21

What are the different ways proteins associate with bilayer?

Answer 21

Transmembrane (amphipathic), monolayer associated (amphipathic alpha helix exposed on protein surface), lipid-linked and protein attached.

Question 22

What is the difference between integral and peripheral membrane proteins?

Answer 22

Proteins that are directly attached can be removed only by disrupting the bilayer with detergents - are integral membrane proteins. The others are peripheral membrane proteins - can be removed by extraction procedures that interfere with protein-protein interactions. Others are lipid anchored.

Question 23

How do transmembrane proteins cross lipid bilayer?

Answer 23

Specialized membrane-spanning segments of polypeptide chain - mostly of amino acids with hydrophobic side chains - interact with hydrophobic tails of lipids. Peptide bonds make the polypeptide backbone hydrophilic - atoms that are a part of it are driven to form hydrogen bonds with each other - maximized if polypeptide chain forms a regular alpha helix - majority of membrane-spanning segments of polypeptide chains are alpha helices - the hydrophobic side chains are exposed on outside of helix where contact the tails and hydrophilic backbone forms hydrogen bonds with one another within helix.

Question 24

Describe the different transmembrane proteins by how many times they cross bilayer.

Answer 24

For many transmembrane proteins, the polypeptide chain crosses the membrane only once - many of these single-pass transmembrane proteins are receptors. Other transmembrane proteins function as channels allowing small, water-soluble molecules to cross - consist of series of alpha helices that cross bilayer many times - multi-pass transmembrane proteins - 1+ membrane-spanning regions are amphipathic - forms from alpha helices. These amino acids are arranged so hydrophobic side chains fall on one side of helix and hydrophilic on other. In hydrophobic environment of lipid bilayer, alpha helices of this type pack side by side in ring - hydrophobic exposed to lipid tails, and hydrophilic making lining.
Alpha helices are most common, some transmembrane proteins cross as beta sheet rolled into a cylinder - beta barrel/large aqueous pore. Amino acid side chains facing inside are hydrophilic and outside are hydrophobic - ie. porin proteins - water-filled pores in mitochondrial and bacterial outer membranes - allow passage of small nutrients and inorganic ions.

Question 25

How are plant, yeast and bacteria cells strengthened and supported?

Answer 25

Cell Wall

Question 26

How are animal cells strengthened and supported?

Answer 26

Animal cells are stabilized by meshwork of filamentous proteins - cell cortex - attached to underside.
Studied well in RBCs - main component is spectrin - long thin and flexible rod - forms lattice - connected through intracellular attachment proteins.
Other animal cells are especially rich in actin and myosin - can be used to change shape, move, take up materials from environment, restrain diffusion of proteins etc.

Question 27

What are membrane domains? Describe what creates them?

Answer 27

Cells confining particular proteins to localized areas in bilayer creating membrane domains. Plasma membrane proteins can be tethered to structures outside of cell, or inside - especially to cortex (attachments to cytoskeleton - A, attachments to extracellular matrix - B, cell junctions, C + D). Cells can create barriers that restrict particular membrane components to a domain. In gut cells asymmetric distributions are maintained by tight junctions - specialized junctional proteins form continuous belt around cell creating a seal between adjacent plasma membranes - membrane proteins cannot diffuse past junction.

Question 28

What is glycocalyx?

Answer 28

All of the carbs on glycoproteins, proteoglycans and glycolipids is located on outside of membrane - forms carbohydrate layer - glycocalyx - protects cell surface from damage and because they attract water, they give cell slimy surface - helping them be motile. Play role in cell-cell recognition and adhesion. Transmembrane proteins on neutrophils - lectins - bind to particular oligosaccharide side chains that allow them to migrate into infected tissue.