Final answer:
Most molecules move across the placenta by diffusion, a passive transport mechanism where substances move from areas of higher to lower concentration without using energy. Facilitated diffusion is utilized for molecules that cannot directly diffuse through the membrane.
Step-by-step explanation:
The answer to how most things move across the placenta is c) By diffusion. This is a form of passive transport, where molecules move from a higher concentration to a lower concentration without the use of energy.
Diffusion is essential for the exchange of gases like oxygen and carbon dioxide between the mother and the fetus. Meanwhile, substances that are insoluble in lipids or too large to pass through the cell membranes by diffusion alone, such as glucose and certain amino acids, are transported via facilitated diffusion. This utilizes carrier proteins in the cell membrane to help molecules cross the membrane down their concentration gradient, still without using cellular energy. On the other hand, active transport is the process by which cells move molecules against their concentration gradient, which requires energy, typically in the form of ATP.