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Y1 Fundamentals of Life Science > L7: Biological Membranes > Flashcards

L7: Biological Membranes Flashcards

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1
Q

What is the main function of the plasma membrane?

A

Separates the intracellular and extracellular environments, to form a layer of protection against the exterior environment

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2
Q

What components make up the plasma membrane?

A

-Composed of lipids (mainly phospholipids ) and proteins
- There are two opposing sheets of lipids into which proteins are inserted
- Each lipid has a hydrophobic tail (interior of membrane) and a hydrophilic head (exterior of membrane) which defines the lipid bilayer structure

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3
Q

What are some other functions of the plasma membrane?

A
  • Separates the intracellular and extracellular environments
  • Involved in cell signalling by receiving information via receptors on the proteins in the plasma membrane
  • Flexible to give cells the capacity to move and expand
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4
Q

What is the fluid mosaic model?

A

The phospholipid bilayer with proteins integrated within the spaces between phospholipids

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5
Q

What are the functions of membrane proteins?

A

Cell signalling, communication and selectively permeable

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6
Q

What are the main phospholipids found in the plasma membrane?

A
  • Phosphatidylcholine
  • Sphingomyelin
  • Phosphatidylserine
  • Phosphatidylethanolamine
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7
Q

What other lipids are found in the plasma membrane?

A
  • Cholesterol
  • Glycolipids
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8
Q

What is the structure of a phospholipid?

A
  • Non-polar, hydrophobic tail (hydrocarbon)
  • Polar, hydrophilic head containing a glycerol and phosphate group
  • Head group attached to the phosphate to form a specific phospholipid e.g. choline for phosphatidylcholine
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9
Q

What term is used describe a phospholipid?

A

Amphipathic (having both hydrophobic and hydrophilic regions)

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10
Q

What section of the phospholipid interacts with what region?

A
  • The hydrophilic head interacts with the intracellular and extracellular environments of the cell
  • The hydrophobic tail is in the centre and forms a bilayer to do so
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11
Q

What is the difference between saturated and unsaturated fatty acid tails?

A
  • Saturated tails contain no double carbon bonds
  • Unsaturated tails contain double carbon bonds
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12
Q

What is the effect of a double bond in the fatty acid tail?

A
  • Creates a kink in the tail which affects the ability to pack into the plasma membrane
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13
Q

What makes up a molecule of glycerol?

A

3 carbons
1 hydroxyl group

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14
Q

How are the fatty acid chains attached to the glycerol?

A

2 hydroxyl groups attached to the fatty acid chain by ester linkage

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15
Q

How is phosphate attached to the glycerol?

A

Phosphodiester bond

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16
Q

What charge does phosphatidylcholine have?

A
  • Neutral charge due to one positive and negative charge
  • Still means that it is highly hydrophilic
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17
Q

How does sphingomyelin different to the other phospholipids?

A
  • Does not have a glycerol
  • Has sphingosine instead
  • Same in every other way
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18
Q

What charge does phosphatidylserine have?

A
  • Negative charge - has 2 negative and 1 positive
  • Overall negative charge
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19
Q

What is the composition of the phospholipid bilayer?

A
  • Inner and outer monolayers have different compositions
  • Predominantly phosphatidylcholine with sphingomyelin on the extracellular leaflet
    Predominantly phosphatidylserine with phosphatidylethanolamine on the intracellular leaflet
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20
Q

What is the function of phosphatidylserine?

A
  • Important for protein kinase C activity in cytosol, convert extracellular signals to intracellular signals
  • Translocates to outer membrane during apoptosis and signals neighbouring cells to phagocytose cell
21
Q

What structure forms from a single fatty acid chain/monolayer?

A

Lipid micelle

22
Q

Why do lipid bilayers spontaneously close to form a sealed compartment?

A

It is energetically unfavourable for a lipid bilayer to exist as planar with edges exposed to cytosol - npo hydrophobic edges exposed

23
Q

Why is it important that biological membranes are fluid structures?

A
  • Allow signalling lipid molecules and membrane proteins to diffuse in the lateral plane and interact with one another
  • Ensures that membranes are equally shared between daughter cells following cell division
  • Allows cells to change shape (cell motility)
  • Allows membranes to fuse with other membranes (e.g. exocytosis)
24
Q

How can the membrane change to maintain fluidity? Give an example

A
  • Colder environments bacteria and yeasts synthesis shorter fatty acid chains which have increased ‘unsaturation’ - allows the membrane to remain fluid at lower temperatures
  • Humans have a fixed temp. so this isn’t a large issue
25
Q

What is cholesterol?

A
  • Lipid found in the phospholipid bilayer
  • Has a steroid ring structure (rigid)
  • Has a hydroxyl polar head which orientates the molecule in the membrane
  • Single hydrocarbon tail
  • Intercalates between phospholipids
26
Q

What is the function of cholesterol?

A
  • Reduces permeability of the bilayer, so the bilayer prevents hydrophilic molecules passing through into the hydrophobic region ( to small molecules)
  • Improves the rigidity of the membrane - less deformable because the bilayer is more tightly packed
27
Q

What is inositol?

A
  • 6 carbon ring structure
  • Unbonded has 6 hydroxyl groups attached to each carbon atom
  • When on the plasma membrane normally phosphorylated on the 4 and 5 carbon position
28
Q

What are some examples of intracellular signal transduction lipids?

A
  • Phosphatidylinositol
  • Diacylglycerol
  • Ceramide
  • Sphingosine-1-phosphate
29
Q

What are the roles of intracellular signal transduction lipids?

A
  • Minor proportion of the (phospho)lipid content of intracellular membranes. Derived from lipids residing in the plasma membrane
  • Rapidly generated / destroyed by enzymes in response to a specific signal
  • Spatially & temporally generated = Highly specific
    signal
  • Bind specifically to conserved regions found within many different proteins and once bound, induce conformational and/or localization and activity changes) within these proteins
30
Q

Describe how lipid bilayers are assembled

A
  • Assembled in the endoplasmic reticulum
  • Fatty acid binding protein inserts the fatty acid tail into the outer cytosolic leaflet
  • Addition of glycerol, phosphate and head group
  • Scramblase enzyme flips some of the newly synthesised phospholipids from the outer to inner leaflets of the ER membrane so the phospholipids are evenly distributed between the leaflets
  • Newly synthesised membrane is transported to the plasma membrane in vesicles
31
Q

What are glycolipids?

A
  • Formed from sphingosine with a sugar attached
  • Always face the exterior environment of the plasma membrane - found exclusively on the non-cytosolic monolayer of the membrane
  • Formed through glycosylation in the lumen of the Golgi
32
Q

What is the inside of the Golgi topologically equivalent to?

A

The extracellular environment

33
Q

Describe the process by which vesicles are transported between compartments

A
  • Vesicles will bud off from the ER and fuse with the cis side of the Golgi
  • Movement of vesicle through the Golgi and undergo modification through the addition of proteins and lipids
  • Modified membrane containing vesicles leave the Golgi via the trans side
  • Transport vesicles move to the plasma membrane and fuse there. Other vesicles or membranes can be for other compartments but will still move through the Golgi e.g. early endosome to a lysosome
34
Q

What types of vesicles are transported to the plasma membrane?

A
  • Constitutive export - always being made and exported
  • Vesicles which are stored and released upon a particular signal
35
Q

What is glycosylation?

A
  • Starts in the ER and continues throughout the Golgi
  • Sugar or carbohydrate molecule is covalently attached to a protein or lipid to form a glycolipid - the protein or lipid is modified in the Golgi to allow this
36
Q

Describe the process of exocytosis

A
  • Vesicles fuses at the plasma membrane
  • The enzyme flippase, using ATP, flips phospholipids from the inner cytosolic leaflet, which holds the modified material, to the outer extracellular leaflet to release the material outside of the cell
37
Q

What mass of the membrane is made up of proteins?

A

50%

38
Q

What are integral or transmembrane proteins? What specific structure is formed?

A
  • Proteins which go all the way through the phospholipid bilayer
  • Form an alpha helix inside the bilayer itself - the hydrophobic R groups sit on the outside of the helix as the surrounding environment is hydrophobic
39
Q

What structural forms of integral/transmembrane proteins are there?

A

1) Single pass 2) Multipass - these have the hydrophobic amino acids with side chain interacting with the lipid monolayer
3) Beta-barrel - less common and found in porin proteins to allow water across the membrane - beta sheets organised into a barrel - hydrophobic side chains in the lipid bilayer
4) Lipid- linked - can help anchor the proteins
5) Peripheral membrane proteins - associate with the membrane because they are binding other proteins that are binding to the membrane

40
Q

What are the different functions that membrane proteins can have?

A
  • Transport
  • Enzymes
  • Signal Transduction
  • Cell-cell recognition
  • Intercellular joining - join cells together
  • Attachment to the cytoskeleton and extracellular matrix
41
Q

Where are proteins synthesised?

A

Ribosomes in the cytoplasm

42
Q

What do soluble proteins for export have?

A

A 20 amino acid hydrophobic signal peptide at the N-terminus

43
Q

What recognises the signal peptide on the soluble proteins? What is the function?

A
  • The signal peptide is recognised by the translocator protein on the ER membrane
  • This translocator protein starts to guide the growing polypeptide chain through the membrane (translation occurs whilst threaded through)
  • Protein threaded through and the signal peptide is cleaved off, releasing the newly synthesised protein into the ER lumen
  • This is the process for the secretion of the protein completely out of the membrane
44
Q

Describe the process for the translocation of a protein which is integrated into the membrane?

A
  • Has a signal peptide and a stop-transfer sequence
  • Signal peptide directs the growing polypeptide chain to the translocator and the growing polypeptide chain is threaded through the membrane
  • When the translocator interacts with the stop-transfer sequence translocation ceases and the protein is released into the bilayer.
  • Signal peptide is cleaved
45
Q

What are the functions of glycoproteins?

A
  • Cell recognition
  • Inflammatory response
  • Protection
46
Q

What is the glycocalyx?

A
  • Carbohydrate-rich layer surrounding cells
  • Composed of glycolipids & glycoproteins, and traps lots of water molecules
47
Q

What are the functions of the glycocalyx?

A
  • Protects cells against chemical and mechanical damage - slimey property
  • Vascular system - stops erythrocytes sticking to the endothelial walls of the vessels
48
Q

Some organelles have double membranes. Give an example of this

A
  • The nuclear envelope consists of an inner and outer membrane
  • Continuous with the ER membrane.
  • The envelope is penetrated by nuclear pore complexes - allow bidirectional exchange between nucleoplasmic and cytoplasmic compartments.
  • 3000-4000 pore complexes per nucleus
  • The mitochondria is another organelle
49
Q

How do you identify how fluid a membrane is?

A
  • Proteins with fluorescent tags in the plasma membrane
  • Shine a laser beam on a selected region, which bleaches the area and turns off the fluorescent tags
  • Can watch to see how the area recovers - molecules move into the area - shows fluidity and lateral movement of structure
  • FRAP (Fluorescence Recovery After Photobleaching)

Y1 Fundamentals of Life Science (25 decks)

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