Cellular Energetics — Free AP Biology Review Games.
This unit covers photosynthesis, cell respiration and ATP cycle — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers photosynthesis, cell respiration and ATP cycle — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Photosynthesis
Photosynthesis converts light energy into chemical energy stored as glucose, occurring in two stages: the light-dependent reactions (thylakoid membranes) and the Calvin cycle (stroma). Students must know the inputs, outputs, and location of each stage, as well as how changes in environmental conditions affect the rate of photosynthesis.
Key Points
- Light reactions split water (photolysis), produce ATP and NADPH, and release O2 as a byproduct; occurs in thylakoid membranes
- Calvin cycle uses ATP and NADPH to fix CO2 into G3P via RuBisCO; 3 CO2 → 1 net G3P; occurs in stroma
- The overall equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
- Limiting factors include light intensity, CO2 concentration, and temperature — each affects the rate of a specific stage
A plant is placed in a sealed chamber with water and exposed to increasing light intensity. At low light, the rate of O2 production is proportional to light intensity. At high light intensity, O2 production plateaus even as light increases. What is the most likely limiting factor at high light intensity?
At low light, photons are limiting, so more light directly increases the rate of the light reactions and O2 production. Once light is saturating, the Calvin cycle becomes the bottleneck because ATP and NADPH are produced faster than RuBisCO can fix CO2. The plateau indicates that CO2 concentration or enzyme availability (not light) is now the limiting factor.
2 Cellular Respiration
Cellular respiration breaks down glucose to produce ATP through three interconnected stages: glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane). Students must know the net ATP yield per stage, the role of electron carriers (NADH, FADH2), and how the process differs under aerobic vs. anaerobic conditions.
Key Points
- Glycolysis: 1 glucose → 2 pyruvate, net 2 ATP, 2 NADH; occurs in cytoplasm; does not require O2
- Krebs cycle: each acetyl-CoA yields 3 NADH, 1 FADH2, 1 ATP (GTP); run twice per glucose; occurs in mitochondrial matrix
- ETC and chemiosmosis: NADH and FADH2 donate electrons; proton gradient drives ATP synthase; O2 is the final electron acceptor; ~32-34 ATP produced
- Fermentation (anaerobic): regenerates NAD+ to keep glycolysis running; yields only 2 ATP net (lactic acid or ethanol + CO2)
A researcher adds a chemical that makes the inner mitochondrial membrane freely permeable to H+ ions. Predict the effect on ATP production and explain which stage(s) are affected.
The chemical eliminates the proton gradient across the inner mitochondrial membrane by allowing H+ to leak back without passing through ATP synthase. Without the gradient, chemiosmosis cannot drive ATP synthesis, so oxidative phosphorylation produces no ATP. Glycolysis and the Krebs cycle are unaffected and continue producing small amounts of ATP directly (substrate-level phosphorylation), but total ATP yield drops dramatically from ~36-38 to only ~4 ATP per glucose.
3 ATP Cycle
ATP (adenosine triphosphate) is the universal energy currency of the cell, coupling energy-releasing (catabolic) reactions to energy-requiring (anabolic) reactions through the continuous cycle of ATP hydrolysis and regeneration. Students must understand the structure of ATP, how hydrolysis releases free energy, and how that energy drives cellular work.
Key Points
- ATP hydrolysis: ATP + H2O → ADP + Pi + ~7.3 kcal/mol free energy; energy released drives endergonic reactions
- ATP is regenerated from ADP + Pi using energy from cellular respiration (and photosynthesis in plants)
- ATP is used for three types of cellular work: mechanical (muscle contraction), transport (active transport pumps), and chemical (biosynthesis)
- ATP is not stored in large quantities; cells continuously recycle ADP → ATP; a human cell may recycle its ATP hundreds of times per day
The sodium-potassium pump moves 3 Na+ out and 2 K+ into a cell per cycle, against their concentration gradients, consuming 1 ATP per cycle. If this pump is given an inhibitor that blocks ATP hydrolysis, predict two downstream cellular effects.
Without ATP hydrolysis, the pump cannot change conformation and transport ions, so the Na+/K+ gradient collapses — intracellular Na+ rises and K+ falls. First, the resting membrane potential depolarizes because the electrochemical gradient that sustains it is lost, impairing nerve and muscle signaling. Second, because many secondary active transporters rely on the Na+ gradient as a driving force, glucose uptake and other coupled transport processes also fail, starving the cell of substrates for ATP synthesis.
Questions, answered.
What is Cellular Energetics?
Cellular Energetics is Unit 3 of AP Biology, covering photosynthesis, cell respiration and ATP cycle.
How to study for AP Biology Unit 3?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.