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Lipids and Biological Membranes > Biological Membranes > Flashcards

Biological Membranes Flashcards

1
Q

All of the biological membranes, as also the PLASMA MEMBRANE are composed by what?

A

They are composed by lipids and proteins and carbohydrates.

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2
Q

What is the fundamental structure of the membrane?

A

It is the PHOPHOLIPID BILAYER, forming a stable barrier between two aqueous compartments.

  • In the case of the plasma membrane, these compartments are the inside and the outside of the cell.
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3
Q

What are the individual units of MICELLE ?

A

They are CONE-SHAPED, because the cross section of the polar head is larger than that of the hydrophobic chains (wedge-shaped).

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4
Q

What are the individual units of BILAYER?

A

The individual units have CYLINDRICAL structure, when the cross section of the polar head is approximately equal to that of the hydrophobic chain.

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5
Q

What was the most important discovery with use of osmium, as a “dye” for electron microscopy?

A

It was the patter on trilaminar which is called UNITARY MEMBRANE.

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6
Q

(THE LATEST AND THE MOST RELIABLE MODEL : MOSAIC MODEL)

They proved that the polar lipids form a bilayer in which proteins are either what?

A

They proved that the polar lipids form a bilayer in which proteins are either INCLUDED(integral proteins) or SUPPORTED above (peripheral proteins) the layer.

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7
Q

(THE LATEST AND THE MOST RELIABLE MODEL : MOSAIC MODEL)

By an operative point of view membrane proteins can be defined as what?

A
  1. PERIPHERAL PROTEIN
    : They were difined as proteins that are laid above the bilayer and that dissociate from the membrane following treatments with polar reagent (such as solutions of extreme pH or high salt concentration, that do not break the phospholipid bilayer)
  • Once dissociated from the membrane, peripheral proteins are soluble in aqueous buffers.
  1. INTEGRAL PROTEINS
    : They cross the membrane and are embedded in the bilayer; it is possible to visualized them as “icebergs” floating in a 2-dimensional lipid “sea” with a random distribution.
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8
Q

What is the name of this model that the components are free to move independently of each other?

A

This model is referred to as the ‘Fluid Mosaic Model’.

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9
Q

(Fluid Mosaic Model)

Phospholipids can drift where?

A

Phospholipids can drift laterally in the plane of the membrane (an average lipid molecule can diffuse the length of a large bacterial cells in about 1 second) = LATERAL MOVEMENT

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10
Q

(Fluid Mosaic Model)

Phospholipids can migrate from where?

A

It can migrate from the monolayer on one side to that on the other = FLIP-FLOP MOVEMENT (rarely)

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11
Q

Why membrane proteins and phospholipids are unable to move back and forth between the inner and outer surfaces of membrane at an aprreciable rate?

A

This occurs by a FLIP-FLOP DIFFUSION.

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12
Q

But how the PROTEINS and LIPIDS are able to diffuse laterally through the membrane?

A

It is because they are inserted into a fluid lipid bilayer.

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13
Q

Not all of the proteins are able to diffuse freely through the membrane.
In some cases, the mobility of membrane proteins is restricted by what?

A

The mobility of membrane proteins is restricted by their ASSOCIATION WITH THE CYTOSKELETON.

*In other cases, the mobility of membrane proteins may be restricted by THEIR ASSOCIATIONS WITH OTHER MEMBRANE PROTEINS, with PROTEINS on the surface of adjacent cells, or with the EXTRACELLULAR MATRIX.

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14
Q

Which factors that determine the fluidity of the bilayer?

A
  1. Temperature
    - Minor Temperature -> Less fluidity
  2. A length of the acyl chains
    - A greater length of the acyl chains -> A minor fluidity
  3. Cholesterol
    - Greater presence of cholesterol -> Decreased fluidity
  4. Amount of unsaturated fatty acid
    - Increased unsaturated fatty acid -> Greater fluidity
  5. Amount of PROTEINS
    - Greater amount of PROTEINS has a decreasing effect on the fluidity of cell membranes.
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15
Q

Why the fluidity is determined by the amount of unsaturation of fatty acids?

A

This is the result of an increase in the volume occupied by each single hydrophobic tail which is reflected on a lower degree of packaging of the phospholipids and in the reduction of van der Waals forces.

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16
Q

What is the factor which affect to the effect of cholesterol on the membrane fluidity?

A

It is the temperature.

  • At high temperatures, cholesterol interferes with the movement of the phospholipid fatty acid chains, making the outer part of the membrane less fluid and reducing its permeability to small molecules.
  • At low temperatures, however, cholesterol has the opposite effect.
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17
Q

By interfering with interactions between (what), cholesterol prevents membranes from freezing and maintains membrane fluidity?

A

Interactions between FATTY ACID CHAINS, cholesterol prevents membranes from freezing and maintains membrane fluidity.

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18
Q

The outer surface (extracellular side) consists predominantly of what?

A
  1. PHOSPHATIDYLCHOLINE
  2. SPHINGOMYELIN
  3. GLYCOSPHINGOLIPIDS
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19
Q

The inner surface (intracellular side) contains what?

A
  1. PHOSPHATIDYLSERINE
  2. PHOSPHATIDYLINOSITO
  3. PHOSPHATIDYLETHANOLAMINE
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20
Q

What are relatively minor membrane components, constituting only about 2% of the lipids of most plasma membranes?

A

GLYCOLIPIDS

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21
Q

What are present ONLY IN THE EXTRACELLULAR SIDE of the membrane and are associated with one another via hydrogen bonds?

A

GLYCOLIPIDS

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22
Q

The GANGLIOSIDES are the most frequent GLYCOLIPIDS, containing one or more which residues?

A

Containing one or more SIALIC ACID RESIDUES; they are most abundant in the membranes of nerve cells which constitute up to 10% of the total lipid mass.

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23
Q

Thanks to their localization (extracellular environment) GLYCOLIPIDS can intervene in what (5 things)?

A
  1. Binding function with the extracellular matrix
  2. Processes of cell recognition : GM1 ganglioside acts as receptor for the cholera toxin
  3. Electrical insulation in the myelin membrane
  4. Membrane protection from extreme conditions : Low pH , Degradative enzymes
  5. Electrical insulation in the myelin membrane
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24
Q

Many of the integral proteins are WHICH KINDS OF PROTEINS which completely span the membrane?

A

Many of the integral proteins are TRANSMEMBRANE PROTEINS which completely span the membrane.

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25
Q

The hydrophobic region of an WHICH PROTEIN consist of one or more stretches of nonpolar amino acids, usually coiled into alpha-helices.

A

An INTEGRAL PROTEIN.

  • The HYDROPHILIC PARTS are exposed to the aqueous solutions on either side of the membrane.
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26
Q

Membrane has distinct how?

A

Inside and outside faces.

  • This asymmetrical orientation begins during synthesis of new membrane in the endoplasmic reticulum.
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27
Q

The membrane-spanning portions of WHICH PROTEINS are usually alpha-helices of 20-25 hydrophobic amino acids?

A

TRANSMEMBRANE PROTEINS are usually alpha-helices of 20-25 hydrophobic amino acids, inserted into the membrane of the ENDOPLASMIC RETICULUM during synthesis of the polypeptide chain.

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28
Q

WHICH PROTEINS are then transported in membrane vesicles, starting from the endoplasmic reticulum, to WHERE, and AFTER to the PLASMA MEMBRANE.

A

TRANSMEMBRANE PROTEINS are then transported in membrane vesicles, starting from the ENDOPLASMIC RETICULUM, to the GOLGI APPARATUS, and AFTER to the PLASMA MEMBRANE.

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29
Q

Carbohydrate groups are added to where?

A

They are added to the POLYPEPTIDE CHAINS both in the ENDOPLASMIC RETICULUM and GOLGI APPARATUS, so that most of transmembrane proteins of the plasma membrane are GLYCOPROTEINS with their oligosaccharide chain exposed on the surface of the cell.

30
Q

(INTEGRAL PROTEINS)

Describe about Type I and II.

A

SINGLE TRANSMEMBRANE HELIX with different orientation of the AMINO or CARBOXYL ENDS.

31
Q

(INTEGRAL PROTEINS)

Describe about Type III.

A

There are many TRANSMEMBRANE HELICES, but all belong to the same polypeptide chain.

32
Q

(INTEGRAL PROTEINS)

Describe about Type IV.

A

TRANSMEMBRANE DOMAINS are of different polypeptides that come together to form a channel through the bilayer.

33
Q

(INTEGRAL PROTEINS)

Describe about Type V.

A

Protein linked to lipids by covalent bonds

34
Q

(INTEGRAL PROTEINS)

Describe about Type VI.

A

Proteins with TRANSMEMBRANE HELICES and with LIPID ANCHORS.

35
Q

In EUKARYOTIC CELLS, how a large number of PROTEINS are linked to MEMBRANE?

A

They are COVALENTLY LINKED to MEMBRANE.

36
Q

What is the FIRST CLASS of the cell surface proteins?

A

The cell surface proteins of the FIRST CLASS are anchored to the EXTRACELLULAR SIDE by a GLYCOSYLATED PHOSPHOLIPID COMPLEX (GPI) (contains N-acetylglucosamine and inositol) which is a connecting bridge.

37
Q

At the FIRST CLASS belong SEVERAL PROTEINS INVOLVED in the MECHANISMS of what?

A
  • At this class belong SEVERAL PROTEINS INVOLVED IN THE MECHANISMS of CELL ADHESION (in particular, CADHERIN); other proteins are localized on the cell membrane of cardiomyocytes where they have a role in cell recognition and in promoting the processes of myocardial contraction.
38
Q

What are Cardiomyocytes?

A

Also known as myocardiocytes, cardiomyocytes are cells that make up the heart muscle/cardiac muscle.

39
Q

A SECOND CLASS of protein is anchored to where?

A

It is anchored to the cytoplasmic side of the membrane either via the MYRISTIC ACID (amide bond to the glycine residue present on the N-terminal of these proteins) or via the PALMIOTOYL ACID, (sulfamide bond with a cysteine residue present in the polypeptidic chain)

  • MYRISTIC ACID = SATURATED FATTY ACID WITH 14 CARBON ATOMS
40
Q

THIRD GROUP of PROTEINS are anchored to WHERE?

A

THIRD GROUP of PROTEINS are ANCHORED to the CYTOPLASMIC SIDE of the MEMBRANE by a FARNESYL RESIDUE (precursor of cholesterol) with thioester bond with a cysteine residue.

41
Q

(MEMBRANE PROTEINS)
What is the integral proteins (also glycoproteins) functioning as pores to allow the entry and the exit of certain substances in the cell?

A

CHANNELS

42
Q

(MEMBRANE PROTEINS)

What is the proteins allowing the selective passage of certain molecules or ions, through conformational changes?

A

CARRIERS

43
Q

What is the integral proteins that specifically recognize particular molecules (hormoes, neurotransmitters, nutrients, etc)?

A

RECEPTORS

44
Q

What is the peripheral and/or integral proteins that catalyze enzymatic reactions on the surface of the membrane?

A

ENZYMES

45
Q

What is the peripheral proteins, exposed on the cytoplasmic side; they serve to anchor the filaments of the cytoskeleton?

A

ANCHORS FOR CYTOSKELETON

46
Q

What is the glycoproteins (coordinates with some glycolipids) characteristic of each organism, which enable the identification of cells from other organisms?

A

MARKERS OF CELL IDENTITY

47
Q

(Functions of membrane proteins)

What is Anchoring?

A

Some membrane proteins, such as integrins, anchor the cell to the extracellular matrix, they also connect to microfilaments within the cell.

48
Q

(Functions of membrane proteins)

What is the Passive Transport?

A

Certain proteins form channels for selective passage of ions or molecules.

49
Q

(Functions of membrane proteins)

What is the Active Transport?

A

Some transport proteins pump solutes across the membrane, which requires a direct input of energy.

50
Q

(Functions of membrane proteins)

What is the Enzymatic Activity?

A

Many membrane-bound enzymes catalyze reactions that take place within or along the membrane surface.

51
Q

(Functions of membrane proteins)

What is the Signal Transduction?

A

Some receptors bind with signal molecules such as hormones and transmit information into the cell.

52
Q

(Functions of membrane proteins)

What is the Cell Recognition?

A

Some receptor proteins function as identification tags.

For example, bacterial cells have surface proteins, or antigens that human cells recognize as foreign.

53
Q

(Functions of membrane proteins)

What is the Intercellular Junction?

A

Cell adhesion proteins attach membranes of adjacent cells.

54
Q

How the BACTERIORHODOPSIN consisted of?

A
  1. Seven HYDROPHOBIC alpha-helices conneted by beta-sheets, crossing for seven times the membrane.
  2. Each internal segment of alpha-helix is composed by 20 HYDROPHOBIC AMINO ACIDS; their hydrophobic interactions with fatty acids tails attach the protein to the membrane.
  3. The seven alpha-helices are grouped closely together and are perpendicular to the plane of the bilayer, forming a tight pathway for the TRANSMEMBRANE PASSAGE of PROTONS(H+).
55
Q

What is the things which is allow the passage of H2O across the membrane?

A

It is AQUAPORINS.

56
Q

Which type are the AQUAPORINS?

A

They are a complex of TYPE IV and consists of 4 INTEGRAL PROTEINS of TYPE III; each monomer has 6 TRANSMEMBRANE DOMAINS.

57
Q

By structural analysis, many PORINS don’t contain what?

A

Structural analysis has indicated that the many PORINS do not contain HYDROPHOBIC alpha-HELICAL REGION.

  • Instead, they cross the membrane as beta-barrels, in which beta-sheet fold up into a barrel-like structure ENCLOSING an AQUEOUS PORE.
58
Q

What is the PORE?

A

The term pore is used to describe the small openings in the skin in which oil and sweat reach the surface from their respective glands below.

59
Q

The molecular structure of AQUAPORINS is represented by the what?

A

HOURGLASS MODEL : 6 protein domains that cross the membrane, joined by loops of connection in part immersed in the cytoplasm.

  • SOME PORIN MONOMERS ASSOCIATE TO FORM STABLE TRIMERS, each contains three open channels through which polar molecules can diffuse across the membrane.
60
Q

What is the type of ERYTHROCYTE GLUCOSE TRANSPORTER (GLUT1)?

A

Complex of TYPE IV, with 5 integral monomers of type III, (with 12 hydrophobic elements) associated to form the inner channel presenting the polar or charged groups.

61
Q

The GLUT1 exists in how?

A

It exists in two conformations T1 and T2, and is arranged in a asymmetric mode in the erythrocyte membrane; constitutes a CONTROLLED PORE, since the GLUT1 transports internally Glucose without any carriage of small molecules or ions.

62
Q

Can the GLUCOSE TRANSPORT occur in both directions?

A

Yes, it can occur in both directions but is typically from the outside(plasma) inward to have a internal concentration.

63
Q

PAS staining is mainly used for staining structures containing what kind of stuffs?

A

Structures containing a high proportion of carbohydrate macromolecules (glycogen, glycoprotein, proteoglycans), typically found in e.g. connective tissues, mucus, the glycocalyx, and basal laminae.

64
Q

What are generally glycosylated?

A

The EXTRACELLULAR PORTIONS of PLASMA MEMBRANE PROTEINS.

65
Q

The Carbohydrate portions of glycolipids are exposed on where?

A

They are exposed on the outer face of the PLASMA MEMBRANE.

66
Q

The surface of the cell is covered by a CARBOHYDRATE COAT, known as (WHAT), formed by the oligosaccharides of glycolipids and transmembrane glycoporteins.

A

The GLYCOCALYX.

67
Q

What is the substance whose role is to protect the cell surface?

A

The GLYCOCALYX.

  • In addition, the oligosaccharides of the glycocalyx serve as markers for a variety of cell-cell interactions.
68
Q

What is the initial step in adhesion between leukocytes and endothelia cells?

A

It is mediated by a family of transmembrane proteins called SELECTINS, which recognize specific carbohydrates on the cell surface.

69
Q

What is the substance whose role is bind to specific oligosaccharides expressed on the surface of leukocytes?

A

Two members of the selectin family (E-selectin and P-selectin).

70
Q

A different selectin (L-selectin) is expressed by what and recognizes what?

A

It is expressed by LEUKOCYTES and recognizes an OLIGOSACCHARIDE on the surface of endothelial cells.

71
Q

What does the oligosaccharides provide which is exposed on the cell surface?

A

It provide a set of markers that identify the different cell types of multicellular organisms.

Lipids and Biological Membranes (2 decks)

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