Why this unit matters
Life depends on capturing, converting, and using energy to power growth, maintenance, and response.
What you will learn
- Describe how enzymes lower activation energy and how environmental factors affect enzyme activity.
- Connect light reactions and the Calvin cycle to the conversion of light energy into chemical energy.
- Compare glycolysis, Krebs cycle, and oxidative phosphorylation in terms of inputs, outputs, and ATP yield patterns.
Understand the core ideas
Cellular energetics tracks how energy is transferred, transformed, and conserved in biological systems. Enzymes accelerate reactions by lowering activation energy, but they do not change overall free energy change of a pathway. Enzyme activity is shaped by temperature, pH, substrate concentration, and regulatory molecules. In AP Biology, pathway questions often test whether you can infer where control points exist from rate changes or accumulation of intermediates. ATP serves as a short term energy carrier that couples exergonic and endergonic processes through phosphoryl transfer.
Photosynthesis converts light energy into chemical energy. In light reactions, photons excite electrons in photosystems, electron transport creates a proton gradient, and ATP synthase uses that gradient to make ATP. NADP plus is reduced to NADPH, providing reducing power. In the Calvin cycle, ATP and NADPH help fix and reduce carbon dioxide into carbohydrate precursors. The important model is that matter and energy have distinct accounting: carbon atoms enter as carbon dioxide, while energy enters primarily as light and is transferred into chemical bonds.
Cellular respiration oxidizes reduced carbon compounds to capture energy in ATP. Glycolysis yields pyruvate and reduced electron carriers. Subsequent pathways complete oxidation to carbon dioxide and transfer high energy electrons to carriers such as NADH. In oxidative phosphorylation, electron transport and chemiosmosis drive most ATP production. Oxygen functions as the terminal electron acceptor in aerobic respiration. Data analysis problems may compare oxygen consumption, carbon dioxide production, ATP output, or inhibitor effects, so strong reasoning links each observed change to a specific stage.
Key terms
- activation energy
- The energy barrier reactants must overcome to reach a transition state and proceed to products.
- chemiosmosis
- ATP production driven by diffusion of protons down an electrochemical gradient through ATP synthase.
- NADH
- A reduced electron carrier that donates high energy electrons to the electron transport chain.
- Calvin cycle
- A cyclic set of reactions in the chloroplast stroma that uses ATP and NADPH to fix and reduce carbon dioxide.
Data reasoning: locating a respiration inhibitor
After adding inhibitor X to aerobic cells, oxygen consumption drops sharply, NADH concentration rises, and glycolysis rate increases slightly.
- Use lower oxygen consumption to infer reduced electron flow to oxygen in the electron transport chain.
- Use NADH accumulation to infer that NADH oxidation is impaired downstream of carrier generation.
- Use slight glycolysis increase as partial compensation for reduced ATP from oxidative phosphorylation.
- Conclude inhibitor X most likely affects electron transport rather than glycolysis directly.
A common misconception
Claim: Cells make ATP only in mitochondria.
Correction: ATP is produced in multiple contexts, including glycolysis in the cytosol and photophosphorylation in chloroplasts.
Lessons in this unit
- Enzymes and metabolic pathwaysUse enzyme and substrate interactions to explain reaction rates and pathway regulation.
- Photosynthesis mechanismsMap carbon and electron flow through major photosynthetic stages.
- Cellular respiration stagesTrack matter and energy through aerobic respiration and ATP generation.
- Energy coupling with ATPExplain how ATP hydrolysis drives endergonic cellular work.
Study task
Unit checkpoint
What is the main role of ATP in cells?
ATP serves as an immediate energy carrier that couples energy-releasing reactions to energy-requiring cellular processes.