Final answer:
The claim that a variety of substances can move freely within cells is false, as most molecules that pass through the cell membrane are regulated, and larger or charged molecules require assistance to cross it.
Step-by-step explanation:
The statement that a wide variety of substances can move freely and interact within cells is false. While it is true that small, non-charged molecules such as oxygen, carbon dioxide, and water can move freely through the cell membrane, most substances cannot. Larger or charged molecules require specific pathways for transport. The plasma proteins in blood, for example, are too large to cross the capillary cell membrane via simple diffusion. They typically move through processes like facilitated diffusion, which uses protein channels or carriers to help them cross the membrane, or active transport, which requires energy input.
The fluidity of the cell membrane does indeed allow greater flexibility and motion of the membrane components which are essential for the transport of substances. Growth and dynamic homeostasis within an organism are sustained by the constant movement of molecules across membranes. This movement includes processes like diffusion, facilitated transport, and active transport.
It's incorrect to say that diffusion always moves solutes in opposite directions or that facilitated transport opposes active transport. Each type of transport serves a specific role for different molecules under various conditions.