Organic Molecules:Entering and Leaving Fetus

The organic molecules enter and leaves the fetus through the cell membrane. Molecules both enter and leave the cell by crossing the plasma membrane.

• A few substances cross the phospholipid bilayer, but their molecules have to be small and uncharged: oxygen, carbon dioxide, and water are examples.
• Through diffusion, many substances enter and leave cells because their concentration on one side of the plasma membrane is uneven with the other side. Substances in solution tend to naturally spread until their concentration is uniform. 
• With active transport, it moves substance against their concentration gradient. This happens with energy from cell like hydrolysis of ATP.

This video will show you the process of endocytosis and exocytosis...

• Endocytosis is an energy-using process when cells absorb molecules by engulfing them.
• Exocytosis is the process of a biological cell releasing molecules into the extracellular fluid.
• Those are opposites

With the two process above, food can enter and leave the fetus.