Final answer:
The missing word in the sentence from Biology Chapter 3 is 'systems', highlighting the conservation of energy principle, where energy cannot be created or destroyed but can be transferred or transformed.
Step-by-step explanation:
The phrase that completes the sentence 'Bio Ch 3 energy is conserved in energy _________' is 'systems'. The concept being referred to is the conservation of energy, which is a fundamental principle in both physics and biology known as the first law of thermodynamics.
This law states that energy cannot be created or destroyed, only transformed or transferred from one form to another. Within biological organisms, which are open systems, energy is exchanged with their surroundings as they carry out life processes such as photosynthesis and cellular respiration. The energy present before and after a transformation is always accounted for, even if it's in a different form, such as when chemical energy from food is converted into work, thermal energy, or stored as chemical energy in body fat.