Session 2.5 - Examplify Quiz - Session 2 Flashcards
Which of the following does not typically form part of the membrane bilayer? A. Lipid B. Carbohydrate C. Water D. Plasma E. Protein
D. Plasma
Plasma is a component of blood, not the membrane bilayer. All other substances are found in the membrane bilayer.
Describe the mechanism in which the membrane bilayer forms and is bonded together?
Membrane bilayers form SPONTANEOUSLY in water. They are bonded together by non-covalent bonds; namely van der Walls forces between the hydrophobic tails, and electrostatic/hydrogen bonds between the hydrophilic heads.
Which feature of the phospholipids in the membrane bilayer allows greater fluidity of the membrane? A. Hexagonal packing B. Saturated bonds C. Unsaturated bonds D. Cholesterol content E. Integral proteins
C. Unsaturated bonds
Unsaturated double bonds disrupt the hexagonal packing, therefore allowing more movement and fluidity. Although cholesterol does contribute to membrane fluidity, it is not a phospholipid, but a sterol that complexes between the phospholipid tails with its more hydrophilic A-ring associated with the hydrophilic heads. All the alternate answers reduce membrane fluidity.
In the Singer-Nicolson model of the plasma membrane (the fluid-mosaic membrane model), the membrane can be described as being both fluid and static at the same time. Why is this?
The term fluidity originates from the behaviour of phospholipids and other small lipids in the membrane where they can move laterally because they are not covalently linked to other molecules and so can move freely within the plane of the membrane leaflets. The static term originates from large conglomerates of membrane molecules that remain bound to each other (e.g. connexins, lipid rafts and adhesion molecules). The same membrane in a cell can contain both fluid and static elements: hence the term fluid-mosaic model.
Briefly explain why small polar molecules (e.g. [H2O or urea]) can diffuse through a lipid bilayer but even smaller ions (e.g. [H+ or F-]) cannot.
One of the important barriers to diffusion in lipid bilayers is the nonpolar (hydrophobic) layer created by the lipid tails in the middle layer of the bilayer. By definition hydrophobic and nonpolar molecules can diffuse across membranes relatively easily at a rate proportional to their size. Molecules with overt electrical charges complex water around them because they are hydrophilic and so become too large to penetrate this layer. Polar, uncharged molecules fall in the middle of the two extremes and are permeable at a rate proportional to both their size and their electric dipole (polarity).
Other than the fact that they are both attached to membranes, integral membrane proteins and lipid-anchored membrane proteins share an important property. What is it?
A. They are globular proteins B. They are multi-subunit proteins. C. They are held to the membrane by hydrophobic interactions D. Their peptide bonds are hydrophobic E. They have no disulphide bonds.
C. They are held to the membrane by hydrophobic interactions
A is incorrect because integral membrane proteins and lipid-anchored membrane proteins are not necessarily globular. Many other proteins consist of multi-subunits and this is not a special feature of the two protein types. Peptides bonds by definition are not hydrophobic and disulphide bonds are not necessary for all functioning proteins. Because the middle of the plasma membrane is hydrophobic, whilst the edges are hydrophilic, then for membrane proteins to stay in the membrane they need a hydrophobic amino acid-lipid interaction.
What structural feature distinguishes a multi-pass membrane protein from a multi-subunit membrane protein?
A multi-pass membrane protein consists of a single polypeptide chain that will penetrate the membrane at least twice with a ‘stitching’ pattern, whereas a multi-subunit membrane protein will by definition consist of more than one polypeptide chain, with each being called a subunit. In order for this to be considered a membrane protein, one of the subunits needs to be anchored in the membrane.
Which statement best explains why the a-helix is the most common secondary structure found in the membrane-spanning region of membrane-spanning proteins?
A. The α-helix is more hydrophobic than other secondary structures
B. Unlike other secondary structures, the α-helix is polar and so hydrophilic
C. The α-helix is most commonly modified by the addition of lipid tails that anchor it in the membrane
D. The number of amino acids needed to form the α-helix is lower than in other secondary structures
E. The position of side chains in the α-helix maximises the interaction between the polar polypeptide backbone and the hydrophobic acyl tails of membrane phospholipids.
E. The position of side chains in the α-helix maximises the interaction between the polar polypeptide backbone and the hydrophobic acyl tails of membrane phospholipids
The alpha helix ‘shape’ in itself is not hydrophobic, rather it enables the protein to separate the hydrophobic parts (acyl tails) from the hydrophilic parts (polypeptide backbone). This separation shields the hydrophilic part from the acyl middle layer of membrane and also creates a hydrophilic core through which polar substances can freely pass.
For each of the following components of the erythrocyte membrane, state whether they are 1) Integral or peripheral? and their 2) Main features 3) Function
Band 3 -
Integral / (A) / (B)
Ankyrin -
(C) / (D) / Links spectrin to band 3; restricts lateral mobility
Spectrin -
Peripheral / Long, rod-like protein. Forms heterotetramer (α2β2) / (E)
Glycophorin A -
(F) / Glycoprotein, hydrophilic / (G)
Band 4.1 -
(H) / Protein / (I)
Band 3
Integral
A) Glycoprotein, hydrophilic
B) Prevent flip-flop rotation
Ankyrin
C) Peripheral
D) Adapter protein
Links spectrin to band 3; restricts lateral mobility
Spectrin
Peripheral
Long, rod-like protein. Forms heterotetramer (α2β2)
E) Contributes to shape and flexibility of erythrocyte
Glycophorin A
F) Integral
Glycoprotein, hydrophilic
G) Prevent flip-flop rotation
Band 4.1
H) Peripheral
Protein
I) Links spectrin to glycophorin A; restricts lateral mobility
In a patient with sickle cell anaemia, the normal discoid shape of the red blood cell (erythrocyte) is disrupted and the normal biconcave disc loses its shape and becomes more like a crescent moon (or sickle scythe) shape. Through polymerization of the haem proteins at the membrane surface the plasma membranes become rigid by complexing with one of the structural proteins.
Which protein is most likely responsible for this effect?
Spectrin, because this structural protein provides the flexibility and thus the final structure of the erythrocyte.
Complete the flow diagram outlining the steps in membrane protein production:
Signal sequence recognised by (A) –>
Binding of (A) to the polypeptide chain prevents further protein synthesis –>
(A) is recognised by a receptor on the (B) –>
This releases (A) from the signal sequence, where the signal sequence then interacts with (C) –>
This is found within (D), allowing further protein synthesis through the pore –>
On membrane proteins, (E) forms the transmembranal region of the protein –>
Signal peptidases then cleave the signal sequence
A. SRP (signal recognition peptide) B. ER (endoplasmic reticulum) C. SSR (signal sequence receptor) D. A protein translocator complex (on ER membrane) E. Stop transfer signal
