Final answer:
Water moves freely through cellular membranes to balance solute concentrations via osmosis, while solutes like Na+ and amino acids require special transport proteins for movement. This selective permeability allows for the regulation of the internal environment of the cell, maintaining isotonic conditions when possible.
Step-by-step explanation:
The reason solutes generally do not move in and out of the cell as freely as water is due to the semipermeable nature of the cell membrane. This membrane allows the free movement of water but controls the movement of solutes using specialized structures like carrier or channel proteins.
Water moves to maintain an equal concentration on both sides of the membrane through a process known as osmosis. Different solutes move across membranes based on their concentration gradients, and this can occur through simple diffusion or facilitated diffusion.
Oxygen (O2) and carbon dioxide (CO2) can diffuse directly through the lipid bilayer without the need for energy, a process called passive transport. Other substances, like sodium ions (Na+), require membrane proteins to facilitate their movement across the membrane.
Cells maintain homeostasis by controlling the movement of solutes. Substances that cannot diffuse freely through the lipid bilayer of the cell membrane do so with the help of transmembrane proteins, such as sodium channels for Na+ ions, or aquaporin proteins for water.
When the extracellular medium and the cytosol are isotonic, they have equal solute concentrations, and there is no net water movement. However, in a hypotonic environment (less solute concentration outside the cell), water enters the cell, potentially causing it to swell and burst.
Conversely, in a hypertonic environment (higher solute concentration outside the cell), water leaves the cell, which can cause the cell to shrivel.