Transport of small molecules through the cell membrane can be:
1. Passive transport -no energy required
• Diffusion is the movement of molecules down the gradient (from higher to lower molecule concentration). It can be simple or facilitated with the help of membrane transport channel.
2. Active transport-energy required, against the gradient
• Primary active transport which directly uses ATP as energy source
• Secondary active transport-not direct use of ATP, involves more transport substances
On the other hand, large molecules use different mechanisms for cell membrane transport:
• Exocytosis-transport of the large molecule from the cell; molecules for the transport are packed inside the vesicles which then fuse with the plasma membrane and release their contents to the outside of the cell
• Endocytosis- transport of large molecule from outside the cell to the inside by engulfing it with the cell membrane.
Both of them are type of vesicle active transport.