Biology: Cells to AP Biology · Module 3 of 15
Cellular Energy: Respiration & Photosynthesis
How cells capture, store and release energy through respiration, photosynthesis and fermentation.
ATP: The Energy Currency
- ATP (adenosine triphosphate) stores usable chemical energy in the bonds between its phosphate groups.
- When a cell needs energy, it breaks the bond to the last phosphate, releasing energy and forming ADP; ATP is then regenerated by cellular respiration.
Why is ATP often called the "energy currency" of the cell?
Solution. Like currency exchanged for goods, ATP is made in one process (respiration) and "spent" to power nearly all cellular activities, from muscle contraction to protein synthesis.
Cellular Respiration
- Overall equation: glucose + oxygen → carbon dioxide + water + ATP.
- Three stages: glycolysis (cytoplasm), the Krebs cycle, and the electron transport chain (both in the mitochondria).
- Aerobic respiration (with oxygen) produces far more ATP than anaerobic processes.
Where in the cell does the majority of ATP production occur during aerobic respiration?
Solution. In the mitochondria, primarily during the electron transport chain stage.
Photosynthesis
- Overall equation: carbon dioxide + water + light energy → glucose + oxygen.
- Occurs in chloroplasts: the light reactions (thylakoid) capture light energy, and the Calvin cycle (stroma) builds sugar from CO2.
- Photosynthesis is essentially the reverse of cellular respiration in terms of reactants and products.
Where does the oxygen released during photosynthesis come from?
Solution. From the splitting of water molecules during the light reactions.
Fermentation
- Fermentation lets glycolysis continue without oxygen by regenerating NAD⁺, but produces far less ATP than aerobic respiration.
- Lactic acid fermentation occurs in human muscle cells during intense exercise; alcoholic fermentation occurs in yeast.
Why might your muscles feel a burning sensation during intense exercise?
Solution. When oxygen is limited, muscle cells shift toward lactic acid fermentation to keep glycolysis running. The burning sensation is now understood to come mainly from the buildup of H+ ions (acidosis) that accompanies this rapid anaerobic ATP production, not from lactate/lactic acid itself.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What are the reactants of cellular respiration?
2. Where does the Calvin cycle occur?