Plasma Membrane Structure, Fluid Mosaic Model (2A) - Bio & Biochem

What is an integral (transmembrane) membrane protein?

Protein embedded in bilayer, often spanning it. These proteins integrate deeply into the lipid bilayer through hydrophobic domains, often traversing it to connect intracellular and extracellular environments.
What are the orientations of phospholipid heads and tails in a plasma membrane bilayer?

Hydrophilic heads out; hydrophobic tails inward. Phospholipids self-assemble in aqueous environments to shield hydrophobic tails from water while exposing hydrophilic heads to the polar surroundings on both sides of the bilayer.
Where are carbohydrate chains located on plasma membrane glycoproteins and glycolipids?

On the extracellular (noncytosolic) surface. Carbohydrates are added in the ER and Golgi and oriented outward to participate in cell recognition and protection on the external face.
What is the main structural difference between saturated and unsaturated fatty acid tails in membranes?

Unsaturated tails have cis double bonds that kink. Cis double bonds in unsaturated tails introduce bends that prevent close packing, unlike the straight chains of saturated tails.
What is the glycocalyx?

Extracellular carbohydrate coat on the plasma membrane. The glycocalyx forms from oligosaccharides attached to membrane proteins and lipids, providing a protective layer and aiding in cell adhesion.
Which change to fatty acid tails decreases membrane fluidity at a given temperature?

Increase saturated fatty acid tails. Straight saturated chains pack more densely via van der Waals forces, reducing molecular movement and membrane fluidity.
What property of transmembrane segments allows them to reside in the bilayer core?

Hydrophobic amino acids (often an $b^1$ -helix). Nonpolar side chains in alpha-helical segments interact favorably with the hydrophobic bilayer interior, stabilizing the protein's position.
Which membrane proteins are most directly responsible for selective ion passage across membranes?

Ion channels and transporters (carriers/pumps). These proteins form selective pores or undergo conformational changes to facilitate controlled ion movement across the impermeable lipid bilayer.
At low temperature, how does cholesterol affect membrane fluidity?

Increases fluidity by preventing tight packing. Cholesterol disrupts the ordered gel phase by inserting between tails, maintaining a more fluid state in cold conditions.
At high temperature, how does cholesterol affect membrane fluidity?

Decreases fluidity by restraining phospholipid movement. Cholesterol fills spaces and interacts with tails to limit excessive motion, preventing the membrane from becoming too fluid in heat.
Which solute crosses a pure phospholipid bilayer most readily: $b^1$ ) $bd$ a $^+$ , $b^2$ ) glucose, $b^3$ ) $O_2$ , $b^4$ ) $H_2O$ ?

$O_2$ . As a small nonpolar gas, $O_2$ diffuses easily through the hydrophobic bilayer core, unlike charged ions or polar molecules like glucose and water.
Identify the best description of the plasma membrane as a barrier: What property makes it selectively permeable?

Hydrophobic core excludes most polar and charged solutes. The nonpolar interior of the phospholipid bilayer acts as a barrier, permitting nonpolar solutes while restricting polar and ionic ones.
What term describes the temperature at which a membrane shifts from gel-like to fluid-like?

Transition temperature (melting temperature). This temperature marks the point where thermal energy overcomes tail packing forces, transitioning the membrane from a rigid gel to a fluid state.
What is the primary role of cholesterol in animal cell membranes?

Buffers fluidity and decreases permeability to small polar solutes. Cholesterol intercalates between phospholipids to stabilize fluidity across temperature changes and tightens the bilayer to limit polar solute passage.
What is a peripheral membrane protein?

Protein loosely bound to membrane surface or other proteins. These proteins associate temporarily with the membrane via non-covalent interactions without embedding into the lipid core.
What term describes the unequal distribution of lipids and proteins between the two leaflets?

Membrane asymmetry. Lipids and proteins are distributed differently in the inner and outer leaflets due to biosynthetic processes and functional requirements.
Which change to fatty acid tails increases membrane fluidity at a given temperature?

Increase unsaturated (cis) fatty acid tails. Cis double bonds create kinks that disrupt tight packing of tails, allowing greater molecular motion and thus higher fluidity.
Which interactions most commonly anchor peripheral proteins to the membrane surface?

Electrostatic and hydrogen-bond interactions. Peripheral proteins bind via charge attractions and hydrogen bonds to polar head groups or integral proteins on the membrane surface.
Which enzyme class catalyzes phospholipid flip-flop to help maintain membrane asymmetry?

Flippases, floppases, and scramblases. These ATP-dependent enzymes translocate phospholipids across leaflets to establish and preserve asymmetric lipid distributions.
What term describes movement of a phospholipid from one leaflet to the other?

Flip-flop (transverse diffusion). This rare movement requires crossing the hydrophobic core, which is energetically unfavorable without enzymatic assistance.
Which membrane component primarily forms the semipermeable barrier to polar solutes?

Phospholipid bilayer hydrophobic core. The nonpolar interior of the bilayer repels polar and charged molecules, allowing selective passage primarily for nonpolar substances while blocking others.
Which is faster in membranes: lateral diffusion or flip-flop of phospholipids?

Lateral diffusion is much faster. Lateral diffusion occurs rapidly in the fluid plane, while flip-flop is slow due to the high energy barrier of the hydrophobic core.
What model describes membranes as a dynamic phospholipid bilayer with mobile proteins?

Fluid mosaic model. This model illustrates the plasma membrane as a flexible structure with phospholipids forming a fluid bilayer and proteins dispersed and mobile within it, reflecting its dynamic nature.
How does increasing fatty acid tail length affect membrane fluidity and permeability?

Decreases fluidity and decreases permeability. Longer tails enhance van der Waals interactions, leading to tighter packing that reduces both fluidity and the ability of solutes to permeate.
Which type of diffusion is typical for lipids and most membrane proteins within the bilayer plane?

Lateral diffusion. Lipids and proteins move freely sideways within the plane due to the fluid nature of the bilayer, allowing dynamic reorganization.

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