Materials are able to move across a cell membrane through one of two methods: active transport or passive transport. what is the difference between active transport and passive transport?

Active transport uses energy (ATP) and passive transport does not. Active transport goes against the concentration gradient whilst passive transport does not.

Active transport is the movement of molecules across the cell membrane from an area of lower concentration to an area of higher concentration. This process requires chemical energy made by the cell. Passive transport is the movement of molecules across the cell membrane from an area of higher concentration to lower concentration. This DOES NOT require chemical energy made by the cell. Here's a tip that always helped me remember this. If you are doing something active (playing a sport, etc), you are using a lot of your energy. However, if you are doing something passive (sitting on the couch, etc), you are not using your energy. It's like that with the active and passive forms of transport as well. Make sense? :)

Gravitational force i believe