3.3 Cellular Energy
- Syllabus
- 2025
- Topic
- 3.3
- Level
- —
Learning objectives
3.3A—Describe the role of energy in living organismsDescribe the role of energy in living organisms.• All living systems require an input of energy.• Life requires a highly ordered system and does not violate the first and second laws of thermodynamics.- i. Energy input must exceed energy loss to maintain order and to power cellular processes.- ii. Cellular processes that release energy may be coupled with cellular processes that require energy.- iii. Significant loss of order or energy flow results in death.- Exclusion: Students will need to understand the concept of energy, but the equation for Gibbs free energy is beyond the scope of the AP Exam.• Energy-related pathways in biological systems are sequential to allow for a more controlled transfer of energy. A product of a reaction in a metabolic pathway is typically the reactant for the subsequent step in the pathway.3.3B—Explain how shared, conserved, and fundamental processes and features support the concept of common ancestry for…Explain how shared, conserved, and fundamental processes and features support the concept of common ancestry for all organisms.• Core metabolic pathways (e.g., glycolysis, oxidative phosphorylation) are conserved across all currently recognized domains (Archaea, Bacteria, and Eukarya). 72 Cellular Energetics UNIT 3 c
Energy Flow Sustains Living Systems
Living systems require a continual input of energy to maintain their highly ordered organization and power cellular processes. As energy is transferred, some becomes unavailable for biological work, so maintaining life requires energy input to exceed energy loss.
Cells couple processes that release energy to processes that require it. For example, energy released during catabolic reactions can be transferred through ATP to drive cellular work. Coupling lets an energy-requiring process proceed without implying that the cell creates energy.
Energy-related pathways occur as controlled sequences: the product of one enzyme-controlled reaction becomes the reactant for the next. Releasing or transferring energy in multiple linked steps gives the cell more control than transferring it in one large step.
Biological order does not violate thermodynamic laws. Organisms are not closed systems: they take in energy and release energy to their surroundings. The Gibbs free-energy equation is not required for this AP objective.
Conserved Metabolism Reveals Common Ancestry
A biological process conserved across very different groups can support common ancestry because the shared process can be inherited from an ancestral population and retained through descendant lineages.
| Domain | Conserved core metabolic pathways |
|---|---|
| Archaea | Glycolysis and oxidative phosphorylation |
| Bacteria | Glycolysis and oxidative phosphorylation |
| Eukarya | Glycolysis and oxidative phosphorylation |
These pathways perform fundamental energy transformations and are present across all currently recognized domains. Their broad conservation is consistent with an early origin followed by inheritance as the domains diverged. The shared underlying pathway is therefore evidence of evolutionary continuity, not merely a list of similar names.
Common ancestry does not require every organism to perform metabolism identically. Lineages can modify regulation and pathway details while retaining a conserved core inherited from a common ancestor.