The Cell Membrane

Question 1

Functions of the PM
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•Serves as a ___ barrier
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•Serves as a ____ barrier - regulates ____ in and out of cell
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•Allows ____ ____by serving as a ____ for ___ ____ that bind signaling molecules, and by serving as the____ of ___ ____
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•Allows___ ____ through____ and ____ attached to the surface, and through specific ___ ____.
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•Provides___ ___ for ___ ___ or components of the ____ ___. This allows the cell to maintain its ____ and____
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•Helps ____ and ____ ____ with____ ____.
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•Provides a stable site for the ____g and____ of ____
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•Regulates the ___ of ____
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Answer 1

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•Serves as a protective barrier
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•Serves as a selective barrier - regulates transport in and out of cell
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•Allows signal transduction by serving as a platform for transmembrane receptors that bind signaling molecules, and by serving as the source of signaling molecules
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•Allows cell recognition through proteins and carbohydrates attached to the surface, and through specific lipid molecules.
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•Provides anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and migrate
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•Helps compartmentalize and form microdomains with specialized functions
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•Provides a stable site for the binding and catalysis of enzymes
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•Regulates the fusion of cells
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Question 2

A little bit of history

The “Fluid Mosaic Model” of PM

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•Proposed by ___ and ____ __ _____ ago
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•The plasma membrane consists of a____ of ____ and ____ molecules that can, for the most part, move ___ in the plane of the membrane

Answer 2

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•Proposed by Singer and Nicholson 4 decades ago
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•The plasma membrane consists of a mosaic of proteins and lipid molecules that can, for the most part, move laterally in the plane of the membrane

Question 3

Composition and structure of the PM
•According to the fluid mosaic model, biological membranes are ___ ___ _____ with “____” oriented ____ for which the lipids act as “___ ____”.
•The lipid bilayers form the basic ___ of the membranes and the proteins provide more ____.
•Lipids are amphipathic in that they have hydrophilic polar heads pointing out and the hydrophobic portion forming the core of the membrane.
•Proteins are____ in the bilayer. They may pass through the bilayer ( ____ proteins), or they may be attached at the _____________ face (____ proteins).
•Now we know that carbohydrates are also important components of biological membranes, found on the ____ side, attached to ____ and ___ (______)

Answer 3

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•According to the fluid mosaic model, biological membranes are 2D liquid matrices with “dissolved” oriented proteins for which the lipids act as “passive solvents”.
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•The lipid bilayers form the basic platform of the membranes and the proteins provide more structure.
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•Lipids are amphipathic in that they have hydrophilic polar heads pointing out and the hydrophobic portion forming the core of the membrane.
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•

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•Proteins are embedded in the bilayer. They may pass through the bilayer ( integral proteins), or they may be attached at the cytoplasmic or exterior face (peripheral proteins).
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•Now we know that carbohydrates are also important components of biological membranes, found on the extracellular side, attached to lipids and proteins (Glycocalyx)

Question 4

What more do we know now?
•Lipid diversity is ___ ___than what Singer and Nicolson imagined
•Lipids are ___ ______ distributed, neither____ nor in____
•Proteins are __ _____ dissolved, and not all proteins ___ ____
•Lipids are not just the “____” but they actively contribute to ___ ____
•Proteins and lipids on the membranes form ____ which have ___ ____
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Answer 4

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•Lipid diversity is much greater than what Singer and Nicolson imagined
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•Lipids are not homogeneously distributed, neither laterally nor in leaflets
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•Proteins are not homogeneously dissolved, and not all proteins diffuse freely
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•Lipids are not just the “platform” but they actively contribute to cell physiology
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•Proteins and lipids on the membranes form microdomains which have specialized functions
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Question 5

•Myelin, which i___ ___ ____, contains ___% protein and ___% lipid.
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•Mitochondrial inner membranes contain ___ protein and ____ lipid.
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•Plasma membrane contains nearly____ amounts of proteins and lipids.
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•Carbohydrates constitute about_____ of the plasma membrane.
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Composition of membranes

Answer 5

•Myelin, which insulates nerve fibers, contains only ~25% protein and ~75% lipid.
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•Mitochondrial inner membranes contain ~75% protein and only ~25% lipid.
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•Plasma membrane contains nearly equal amounts of proteins and lipids.
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•Carbohydrates constitute about ~2-10% of the plasma membrane.
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•

Question 6

Lipids in biological membranes

Lipid composition of different membranes ____ thruout the cell

Chol:pl on pm is ___.

On intracellular membranes the ratio is ____

3 primary sites of lipid syn: ___ ___ ____

Answer 6

The lipid compositional data are expressed as a percentage of the total phospholipid (PL) in mammals (blue) and yeast (light blue). As a measure of sterol content, the molar ratio of cholesterol (CHOL; in mammals) and ergosterol (ERG; in yeast) to phospholipid is also included. The main panel shows the site of synthesis of the major phospholipids (blue) and lipids that are involved in signalling and organelle recognition pathways (red).

Lipid composition of different membranes varies thruout the cell

Chol:pl on pm is 1

On intracellular membranes the ratio is less than 1

3 primary sites of lipid syn: ER golgi and mitochondria

Question 7

Lipids in the PM

•Lipids constitute ~___% of the mass of most animal membranes
•There are ~ _______ lipid molecules per ____ area of lipid bilayer
•3 major lipid components
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Answer 7

•Lipids constitute ~50% of the mass of most animal membranes
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•There are ~ 5 x 106 lipid molecules per 1 μm2 area of lipid bilayer
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•3 major lipid components
–Phospholipids
–Glycosphingolipids
–Cholesterol
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Question 8

Phospholipids

•Phospholipds are the ___ ___ ___ in the PM
_____ and _____ are classified as phospholipds

Answer 8

• Phospholipds are the most abundant lipids in the PM
• Glycerophospholipids and sphingomyelins are classified as phospholipds

Question 9

Glycerophospholipids

•Have a ___head group and___ hydrophobic hydrocarbon tails.
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•The tails are usually ___ ___, and they can differ in ____ (______carbon atoms).
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•One tail usually has one or more ___-____ ____s (i.e., it is unsaturated), while the other tail ___ ___(i.e., it is saturated).

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•The simplest form is ________, which consists of a ____ backbone, ___ fatty acids and a ____ group – does not have an ___ group attached.
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•The alcohol group could be____ ____ ____ ____.

Answer 9

•Have a polar head group and two hydrophobic hydrocarbon tails.
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•The tails are usually fatty acids, and they can differ in length (14 and 24 carbon atoms).
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•One tail usually has one or more cis-double bonds (i.e., it is unsaturated), while the other tail does not (i.e., it is saturated).

•The simplest form is phosphatidicacid, which consists of a glycerol backbone, two fatty acids and a phosphate group – does not have an alcohol group attached.
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•The alcohol group could be serine (phosphatidylserine), ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine) or inositol (phosphatidylinositol).
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Question 10

Sphingolipids
•Like the phospholipids, are composed of a ___ ___and ____ non-polar tails
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•An ___ Carbon___ ___, _____, forms the backbone of these lipids rather than glycerol.

•Sphingomyelin is a phospholipid with a ____ backbone; contains a ____ residue, ____ and a _____ ____ side chain (___ linkage)
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•Glycosphingolipids have sphingosine backbone; ___ ____
_____–___ sugar residue
____–branch chain up to ___ sugar residues
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sphingosinge

fa

R group
H

phosphocholine

sugars

Answer 10

•Like the phospholipids, are composed of a polar head and two non-polar tails
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•An 18-carbon amino alcohol, sphingosine, forms the backbone of these lipids rather than glycerol.

•Sphingomyelin is a phospholipid with a sphingosine backbone; contains a choline residue, phosphate and a fatty acid side chain (amide linkage)
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•Glycosphingolipids have sphingosine backbone; contains sugars
•Cerebroside–single sugar residue
•Gangliosides–branch chain up to 7 sugar residues
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H: ceramide

phosphocholine: sphingomyelin
sugars: glycosphingolipid

Question 11

Cholesterol

Cholesterol is represented (A) by a formula, (B) by a schematic drawing, and (C) as a space-filling model

•Eukaryotic plasma membranes contain large amounts of cholesterol—up to one molecule for every phospholipid molecule.
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•They orient themselves in the bilayer with their ____ groups close to the ___ ____ groups of the phospholipid molecules.

Contain ___ ___ and ___

Answer 11

Cholesterol is represented (A) by a formula, (B) by a schematic drawing, and (C) as a space-filling model

•Eukaryotic plasma membranes contain large amounts of cholesterol—up to one molecule for every phospholipid molecule.
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•They orient themselves in the bilayer with their hydroxyl groups close to the polar head groups of the phospholipid molecules.

Contain polar OH, ring structure and hydrocarbon tail.

Question 12

Each lipid component is highly diverse

Question 13

Structures, distribution and characteristics of lipids determine the features of PM

____ ___

___

____(____ separation) ___ _ ___

____between___ and ___ ____

Answer 13

Bilayer structure

Fluidity

Heterogeneity (phase separation) within a leaflet

Asymmetry between inner and outer leaflets

Question 14

Bilayer structure of PM
•Two important features of phospholipids cause them to form bilayers in aqueous environments
1._____ – hydrophilic polar head groups and hydrophobic hydrocarbon tails. Therefore, phospholipids spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads to water
2_____

Answer 14

•Two important features of phospholipids cause them to form bilayers in aqueous environments

1. Amphipathic nature – hydrophilic polar head groups and hydrophobic hydrocarbon tails. Therefore, phospholipids spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads to water
2. Cylindrical structure

Question 15

The lipid bilayer is a two-dimensional fluid

•Phospholipid molecules readily exchange places with their neighbors ___ a ____~___ times per second) giving rise to rapid ___ ____
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•Phospholipid molecules rotate very rapidly about their ___ axis –____
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•The hydrocarbon chains of phospholipids are ____ – ___
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•Phospholipid molecules move ___ ___– ___ __ (____ diffusion). Aided by the proteins “___” or “____”
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Answer 15

•Phospholipid molecules readily exchange places with their neighbors within a monolayer (~107 times per second) giving rise to rapid lateral diffusion.
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•Phospholipid molecules rotate very rapidly about their long axis – rotation
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•The hydrocarbon chains of phospholipids are flexible – flexion
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•Phospholipid molecules move between leaflets – flip flop (traverse diffusion). Aided by the proteins “flippase” or “scramblase”
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Question 16

Fluidity of PM
•cis double bonds in fatty acid (unsaturated fatty acid) chains of phospholipids prevent the hydrophobic chains from___ ____ ___ together – therefore ____ membrane fluidity
•Cholesterol, depending on what phospholipd it interacts with, can ___ or ____ membrane fluidity
•Cholesterol makes saturated bilayers ___ fluid and unsaturated bilayers ___ fluid
Note – another factor that determines fluidity is____ – the warm body temperature maintains membranes in a fluid state

Saturated Lipids: _____ ___ ___
Saturated Lipids + chol: ____ phase (_____)

Unsat: ___ ____phase (____)

Answer 16

•cis double bonds in fatty acid (unsaturated fatty acid) chains of phospholipids prevent the hydrophobic chains from packing too closely together – therefore increase membrane fluidity
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•Cholesterol, depending on what phospholipd it interacts with, can increase or decrease membrane fluidity
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•Cholesterol makes saturated bilayers more fluid and unsaturated bilayers less fluid
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Note – another factor that determines fluidity is temperature – the warm body temperature maintains membranes in a fluid state

Saturated Lipids: solid crystaline Phase
+ chol: intermediate phase (liquid ordered or Lo)

Unsat: disordered fluid phase (Ld)

Question 17

Same features of the lipids also contribute to phase separation in PM– leads to formation of ______

Answer 17

Same features of the lipids also contribute to phase separation in PM– leads to formation of microdomains

Question 18

Asymmetry of lipids between inner and outer leaflets of PM
•The distribution of phospholipids in the plasma membrane bilayer is asymmetrical
•_______ and_____are found predominantly in the outer leaflet
•______ and ____ are found in the inner leaflet, with ___ being the most abundant
___ conatins a net ____ charge that makes the inner leaflet negatively charged
_____, which functions as a signaling ___ ____, is also found ___ in the ___leaflet
Note – in addition to lipids, ___ and ___ also contribute to asymmetry

Answer 18

•The distribution of phospholipids in the plasma membrane bilayer is asymmetrical
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•Phosphatidylcholine and Sphingomyelin are found predominantly in the outer leaflet
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•Phosphatidylethanolamine and phaphatidylserine are found in the inner leaflet, with PE being the most abundant
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•PS conatins a net negative charge that makes the inner leaflet negatively charged
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•Phosphatidylinositol, which functions as a signaling second messenger, is also found only in the inner leaflet
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Note – in addition to lipids, proteins and carbohydrates also contribute to asymmetry

Question 19

Which is most abundant pl in plasma membrane

Which has more unsat

Answer 19

PC

inner leaflet

Question 20

Asymmetry of PM is functionally important

• •Lipid head groups in the inner leaflet serve as _____ sites for ____
  
  • ___ ___ ___ binds to ________
• •Certain lipid head group can be ____ so that protein ___ ___ are created at a ___ ___and ___– important in ___ ____
  
  • Upon activation, _____ is ____ by a kinase and PIP ___ specific___ ___ ____ to the ___ ___of the PM
• •Lipids on ____ side of PM can be converted to ____ molecules
  
  • will learn more in detail in signaling lecture
• •Lipid asymmetry is important in distinguishing between ___and ___ cells
  
  • ____, which is normally confined to the____ leaflet of PM, rapidly ____ to the outer leaflet and serves as a signal to induce ___ to ____the dead cell and digest it
• •Protein asymmetry is important for their function
  
  • _____ vs ___
• •Carbohydrates are always found on the____leaflet
  
  • Important for ____ of cells from harsh conditions, cell____, cell ____

Answer 20

•Lipid head groups in the inner leaflet serve as docking sites for proteins
-Protein kinase C binds to phosphatidylserine
•Certain lipid head group can be modified so that protein binding sites are created at a particular time and place – important in cell signaling

• Upon activation, phosphatidylinositol is phosphorylated by a kinase and PIP

recruits specific intracellular signaling proteins to the cytosolic face of the

PM

•Lipids on cytosolic side of PM can be converted to signaling molecules

• will learn more in detail in signaling lecture

•Lipid asymmetry is important in distinguishing between live and dead cells

• Phosphatidylserine, which is normally confined to the inner leaflet of PM,

rapidly translocates to the outer leaflet and serves as a signal to induce

macrophages to phagocytose the dead cell and digest it

•Protein asymmetry is important for their function

• Cell surface receptors vs intracellular enzymes

•Carbohydrates are always found on the outer leaflet
-Important for protection of cells from harsh conditions, cell recognition, cell adhesion

Question 21

Proteins in biological membranes

•Broadly classified into

I.____membrane proteins – have __ or ___ segments ____ into the lipid bilayer

II____ membrane proteins - interact only with the ___ of the PM (E and F)

Answer 21

•Broadly classified into

I. Integral membrane proteins – have one or more segments embedded into the lipid bilayer (A-D)

II. Peripheral membrane proteins - interact only with the surface of the PM (E and F)

Question 22

Integral Membrane Proteins

•Two major types of integral membrane proteins
I.Proteins that___ ____the membrane (___ ___) – proteins that traverse the lipid bilayer

A. ___ ____ transmembrane protein (___________)

B.____ transmembrane protein (_________)

C.___ __ ___ ____

II. Proteins that ____ span the membrane (D), which are ____ to the ____ surface by an ____ ___ ____ that ____ into the ____ monolayer of the lipid bilayer

•Integral membrane proteins are _____ – their hydrophobic regions with non polar amino acids pass through the membrane and interact with the hydrophobic tails of lipid molecules; their hydrophilic regions with polar amino acids are exposed to aqueous environment (extracellular and cytosolic).

Answer 22

•Two major types of integral membrane proteins
I.Proteins that completely span the membrane (Transmembrane proteins) – proteins that traverse the lipid bilayer

A. Single pass transmembrane protein (a single α helix)

B. Multipass transmembrane protein (multiple α helices)

C. Rolled up beta sheet

II. Proteins that partially span the membrane (D), which are anchored to the cytosolic surface by an amphipathic α helix that partitions into the cytosolic monolayer of the lipid bilayer

•Integral membrane proteins are amphipathic – their hydrophobic regions with non polar amino acids pass through the membrane and interact with the hydrophobic tails of lipid molecules; their hydrophilic regions with polar amino acids are exposed to aqueous environment (extracellular and cytosolic).

Question 23

Peripheral Membrane Proteins

E. Anchored to one of the membrane leaflets by ___ ____ (___ ___, _____ group, ___ group) ____ attached to the ____ (where the___ part get’s ____ into the membrane)

or

F. _____ ______ to the membrane by interacting with ____ membrane proteins

•

Could be ____ or ____

In the case of proteins anchored by lipids

(i) The protein can be attached to the bilayer solely by a ___ ___ lipid chain—either a___ ___ chain or a ____group (occurs on the ____ side)

or

(ii) The protein can be attached via an ___ ____, to _________ in the _______monolayer.

•

Note – some proteins _____ly interact with the ___head groups

Answer 23

E. Anchored to one of the membrane leaflets by lipid moieties (fatty acid, prenyl group, palmitoyl group) covalently attached to the protein (where the lipid part get’s integrated into the membrane)

or

F. Bind noncovalently to the membrane by interacting with integral membrane proteins

•

Could be cytosolic or extracellular

In the case of proteins anchored by lipids

(i) The protein can be attached to the bilayer solely by a covalently attached lipid chain—either a fatty acid chain or a prenyl group (occurs on the cytosolic side)

or

(ii) The protein can be attached via an oligosaccharide linker, to phosphatidylinositol in the noncytosolic monolayer.

•Note – some proteins electrostatically interact with the lipid head groups

Question 24

For completion sake - The Glycocalyx of the PM

•Some of the ___ and ____ on the external surface of the membrane contain ___ __ _____(oligosaccharides) that extend into the _____environment
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•This_______ carbohydrate layer, called the glycocalyx, ____ the cell from ____ and___ the ___ of ___ ____.
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Answer 24

•Some of the proteins and lipids on the external surface of the membrane contain short chain carbohydrates (oligosaccharides) that extend into the aqueous environment
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•This hydrophilic carbohydrate layer, called the glycocalyx, protects the cell from digestion and restricts the uptake of hydrophobic compounds
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Question 25

Many membrane proteins diffuse in the plane of the membrane

•Membrane proteins do not______ across the lipid bilayer like the lipids, but they do____about an axis perpendicular to the plane of the bilayer (rotational diffusion). In addition, many membrane proteins are able to move ____ within a membrane leaflet (lateral diffusion).
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•Evidence for lateral diffusion of proteins came from _____________________ experiments
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•

(https://www.youtube.com/watch?v=FbCHqk37yQ4)

Answer 25

•Membrane proteins do not tumble (flip-flop) across the lipid bilayer like the lipids, but they do rotate about an axis perpendicular to the plane of the bilayer (rotational diffusion). In addition, many membrane proteins are able to move laterally within a membrane leaflet (lateral diffusion).
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•Evidence for lateral diffusion of proteins came from Fluorescence Recovery After Photo-bleaching (FRAP) experiments
•
•

(https://www.youtube.com/watch?v=FbCHqk37yQ4)

Question 26

Heterogeneity of protein distribution on PM

•The plasma membrane is ____ consisting of diverse ____
•
•This partitioning of membrane components, which compartmentalizes cellular processes, is regulated by ______ _____ and ____ interactions.
•

• A few known examples of membrane microdomains to date are
• Rafts - dynamic, submicroscopic domains enriched in ____, _____glycosylphosphatidylinositol-anchored (________) proteins, and proteins modified with ___ ___ ____ chains
• Clathrin-coated pits – required for ___ __ ____
• Caveolae –___-like domains that ____ a different set of ____ and are required for____
• Protein nanoclusters – formed by several proteins. Ex: ____

Answer 26

•The plasma membrane is heterogeneous, consisting of diverse microdomains.
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•This partitioning of membrane components, which compartmentalizes cellular processes, is regulated by lipid-lipid, protein-protein, and protein-lipid interactions.
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•
•

• A few known examples of membrane microdomains to date are
• Rafts - dynamic, submicroscopic domains enriched in sterols, sphingolipids, glycosylphosphatidylinositol-anchored (GPI-anchored) proteins, and proteins modified with saturated acyl chains
• Clathrin-coated pits – required for receptor-mediated endocytosis
• Caveolae – raft-like domains that recruit a different set of proteins and are required for endocytosis
• Protein nanoclusters – formed by several proteins. Ex: Ras

Question 27

Membrane microdomains are ___ and __ __ ___

Answer 27

CD59 – a marker protein for raft

CD81 – a marker protein for TEMs

When the two images are merged, the microdomains don’t overlap – shows that microdomains are distinct!

Question 28

Membrane microdomains are ____

Answer 28

• Electron microscope image of gold-labeled Ras molecules on 2-D plasma membrane sheets; bar =50 nm. Ras isoforms dynamically localize to distinct signaling nanoclusters
• Ras nanoclusters are dynamic – half life of ½ second

Question 29

Which best describes the structure of a PM

Answer 29

A. Proteins embedded within two layers of phospholipids.