Aserves as a wrapping around largely mobile joints and secretes synovial fluid that serves to lubricate joint movement.

Synovial membrane. The synovial membrane secretes synovial fluid into synovial cavity, which lubricates the joint movements by reducing friction between the ends of bones.

Your asshahs has a nice

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.