A major function of plasma membranes is to:

Question

BlackTom AI · Accepted Answer

Selective permeability is the defining functional property of the plasma membrane: its phospholipid bilayer with embedded proteins controls which ions and molecules enter and leave the cell.

1) Relevant knowledge points and concepts
- Plasma membrane composition: phospholipid bilayer (hydrophilic heads, hydrophobic tails), membrane proteins (channels, carriers, receptors), cholesterol (fluidity), and carbohydrate chains (recognition).
- Transport mechanisms: simple diffusion, facilitated diffusion (protein channels/carriers), active transport (pumps using ATP), and bulk transport (endocytosis/exocytosis).
- Distinction from other organelles: energy generation (mitochondria/chloroplasts), genetic information storage (nucleus/DNA), and storage of metabolites (vacuoles/glycogen/lipid droplets).

2) Step-by-step reasoning process
- The phospholipid bilayer creates a hydrophobic interior that blocks free passage of most polar or charged molecules.
- Membrane proteins provide selective routes: ion channels are selective for particular ions, carrier proteins bind specific solutes, and pumps (e.g., Na+/K+ ATPase) actively move substances against gradients.
- Receptor proteins permit regulated uptake or signaling that alters transport or endocytosis.
- Because of these features, the membrane can maintain internal concentrations different from the external environment, which is essential for homeostasis.

3) Calculations or intermediate steps
- No numerical calculation is required. Intermediate conceptual steps above show how structure (lipids + proteins) leads to selective transport functions.

4) Why the correct answer is correct
- Option b (“Regulate the passage of molecules into and out the cell”) directly describes selective permeability and the membrane’s transport roles (passive and active), matching molecular structure and known transport mechanisms.

5) Why the other options are incorrect
- a: Storing organic chemicals is a function of organelles or cytoplasmic inclusions, not the membrane itself.
- c: Generating ATP is primarily the role of mitochondria (and chloroplasts in plants), not the plasma membrane.
- d: Providing genetic information is the role of DNA in the nucleus (or nucleoid in prokaryotes), not the membrane.

6) Conclusion
- Therefore, the best choice is b. This matches the provided correct answer: b. Regulate the passage of molecules into and out the cell.

A major function of plasma membranes is to:单项选择题

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