Final answer:
A substance must be small, uncharged, and hydrophobic to pass through a cell membrane by simple diffusion. Such substances blend into the lipid bilayer easily, while polar substances and ions require facilitated transport.
Step-by-step explanation:
To pass through a cell membrane by simple diffusion, a substance must typically be small, uncharged, and hydrophobic. The cell membrane is composed of a lipid bilayer that easily accommodates substances that are hydrophobic or non-polar, such as oxygen (O2) and carbon dioxide (CO2). Hydrophobic molecules can blend into the lipid core of the membrane without requiring additional energy or assistance.
Polar substances and ions, however, face challenges when trying to cross the cell membrane due to the hydrophobic nature of the lipid bilayer. Charged particles like sodium, potassium, calcium, and chloride ions cannot simply diffuse through; they require special channels or transport proteins to assist their passage via facilitated transport, which still does not require cellular energy.
Diffusion is a passive transport process where substances move from an area of higher concentration to an area of lower concentration, essentially moving down their concentration gradient. The cell utilizes this process to transport materials such as O2 and CO2, which do not require energy to move across the membrane. Water, despite not being hydrophobic, can also move through the cell membrane via special channels called aquaporins.