Dr. digger, a paleontologist, finds a fossil that he describes in his journal. the creature was approximately 1 meter long, with its backbone fused into a large shell covering the body cavity. it had a beaklike mouth, without teeth, and all four limbs were in the form of flippers. based on anatomical similarities, to which modern animal is dr. digger’s creature most closely related?

The answer is a sea turtle.

The creature had a backbone, so it must be vertebrate. Among the vertebrates, there are no infinitive number of animals with shells. It could be only turtles. The characteristic such as a beaklike mouth without teeth supports a surmise. Limbs in the form of flippers suggest that animal must live in the water, and with such a dimensions it could live only in seas. All of these speak for the sea turtle fossil.

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.