Walking along a beach, you find a section of an adult gray whale's vertebral column with three vertebrae. You saw one of the vertebra in half, revealing a hard outer layer of forming the wall, and a network of with in the center.

Walking along a beach, you find a section of an adult gray whale's vertebral column with three vertebrae. You saw one of the vertebra in half, revealing a hard outer layer of compact bone forming the wall, and a network of spongy bone with trabeculae in the center.

Explanation:

The harder structures of a skeleton are made by the compact bone. Compact bones are dense and rigid. They are non-porous.Spongy bones are porous bones which are covered by the compact bones as they themselves are not hard. They are also known as cancellous bones. Trabeculae are thin plates which  give a spongy structure to the spongy bones. The trabeculae are present in the center of the spongy bones.

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.