Multiple Choice
Which structure is composed of two thin tissue layers that secrete fluids?
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6. The Membrane
Biological Membranes
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Plasma membranes are selectively permeable. This means that:
A
they allow all substances to pass through freely
B
they prevent all substances from entering or leaving the cell
C
they only allow water molecules to pass through
D
they allow some substances to pass through while blocking others
Verified step by step guidance1
Understand the concept of selective permeability: Selective permeability refers to the ability of the plasma membrane to regulate the movement of substances into and out of the cell. It allows certain molecules to pass through while restricting others.
Identify the structure of the plasma membrane: The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. The hydrophobic (water-repelling) interior of the bilayer plays a key role in determining which substances can pass through.
Determine the types of molecules that can pass freely: Small, nonpolar molecules (e.g., oxygen and carbon dioxide) can diffuse directly through the lipid bilayer due to their compatibility with the hydrophobic interior.
Understand the role of transport proteins: Larger or polar molecules (e.g., glucose, ions) require specific transport proteins to cross the membrane. These proteins facilitate movement via mechanisms such as facilitated diffusion or active transport.
Recognize the importance of selective permeability: This property is essential for maintaining homeostasis, as it allows the cell to control its internal environment by regulating the exchange of nutrients, waste products, and signaling molecules.
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