MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 2a Plasma Membrane Fluid Mosaic

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This deck focuses on 2a Plasma Membrane Fluid Mosaic, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is an integral (transmembrane) membrane protein?

Answer: 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.

Flashcard 2: What are the orientations of phospholipid heads and tails in a plasma membrane bilayer?

Answer: 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.

Flashcard 3: Where are carbohydrate chains located on plasma membrane glycoproteins and glycolipids?

Answer: 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.

Flashcard 4: What is the main structural difference between saturated and unsaturated fatty acid tails in membranes?

Answer: 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.

Flashcard 5: What is the glycocalyx?

Answer: 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.

Flashcard 6: Which change to fatty acid tails decreases membrane fluidity at a given temperature?

Answer: Increase saturated fatty acid tails. Straight saturated chains pack more densely via van der Waals forces, reducing molecular movement and membrane fluidity.

Flashcard 7: What property of transmembrane segments allows them to reside in the bilayer core?

Answer: 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.

Flashcard 8: Which membrane proteins are most directly responsible for selective ion passage across membranes?

Answer: 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.

Flashcard 9: At low temperature, how does cholesterol affect membrane fluidity?

Answer: Increases fluidity by preventing tight packing. Cholesterol disrupts the ordered gel phase by inserting between tails, maintaining a more fluid state in cold conditions.

Flashcard 10: At high temperature, how does cholesterol affect membrane fluidity?

Answer: 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.

Flashcard 11: Which solute crosses a pure phospholipid bilayer most readily: b^1) bda$^+$, b^2) glucose, b^3) $O_2$, b^4) $H_2O$?

Answer: $O_2$. As a small nonpolar gas, $O_2$ diffuses easily through the hydrophobic bilayer core, unlike charged ions or polar molecules like glucose and water.

Flashcard 12: Identify the best description of the plasma membrane as a barrier: What property makes it selectively permeable?

Answer: 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.

Flashcard 13: What term describes the temperature at which a membrane shifts from gel-like to fluid-like?

Answer: 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.

Flashcard 14: What is the primary role of cholesterol in animal cell membranes?

Answer: 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.

Flashcard 15: What is a peripheral membrane protein?

Answer: 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.

Flashcard 16: What term describes the unequal distribution of lipids and proteins between the two leaflets?

Answer: Membrane asymmetry. Lipids and proteins are distributed differently in the inner and outer leaflets due to biosynthetic processes and functional requirements.

Flashcard 17: Which change to fatty acid tails increases membrane fluidity at a given temperature?

Answer: 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.

Flashcard 18: Which interactions most commonly anchor peripheral proteins to the membrane surface?

Answer: 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.

Flashcard 19: Which enzyme class catalyzes phospholipid flip-flop to help maintain membrane asymmetry?

Answer: Flippases, floppases, and scramblases. These ATP-dependent enzymes translocate phospholipids across leaflets to establish and preserve asymmetric lipid distributions.

Flashcard 20: What term describes movement of a phospholipid from one leaflet to the other?

Answer: Flip-flop (transverse diffusion). This rare movement requires crossing the hydrophobic core, which is energetically unfavorable without enzymatic assistance.

Flashcard 21: Which membrane component primarily forms the semipermeable barrier to polar solutes?

Answer: 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.

Flashcard 22: Which is faster in membranes: lateral diffusion or flip-flop of phospholipids?

Answer: 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.

Flashcard 23: What model describes membranes as a dynamic phospholipid bilayer with mobile proteins?

Answer: 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.

Flashcard 24: How does increasing fatty acid tail length affect membrane fluidity and permeability?

Answer: 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.

Flashcard 25: Which type of diffusion is typical for lipids and most membrane proteins within the bilayer plane?

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