Quiz · Hard
Light reactions, the Calvin cycle, glycolysis, the Krebs cycle, the electron transport chain and fermentation. Tests where each step happens and what goes in and out.
Where does glycolysis take place?
Mitochondrial matrix
Cytoplasm
Inner mitochondrial membrane
Stroma
Glycolysis happens in the cytosol and needs no oxygen, which is why every cell, including bacteria, can do it. Pyruvate then enters the mitochondria if oxygen is available.
What is the NET ATP yield of glycolysis per glucose?
4 ATP
2 ATP
36 ATP
0 ATP
Glycolysis makes 4 ATP but uses 2 ATP in the investment phase, for a net of 2. It also yields 2 NADH and 2 pyruvate. 4 is the gross yield, a classic trap.
Where does the Krebs (citric acid) cycle occur in eukaryotic cells?
Cytoplasm
Intermembrane space
Mitochondrial matrix
Outer mitochondrial membrane
The Krebs cycle enzymes are in the matrix. The electron transport chain sits in the inner membrane and pumps protons into the intermembrane space.
Which equation summarizes photosynthesis?
Photosynthesis uses carbon dioxide and water (plus light energy) to make glucose and release oxygen. The reverse equation is cellular respiration, and the last option is alcoholic fermentation.
The oxygen gas released during photosynthesis comes from:
Carbon dioxide
Glucose
Water
RuBP
Photosystem II splits water (photolysis) to replace its lost electrons, and the oxygen atoms pair up as O₂. Isotope-labeling experiments showed the carbon dioxide's oxygen ends up in sugar and water, not in the released O₂.
Where do the light-dependent reactions take place?
Stroma
Inner mitochondrial membrane
Cytoplasm
Thylakoid membranes
Photosystems, the electron transport chain and ATP synthase of photosynthesis sit in the thylakoid membranes. The Calvin cycle runs in the stroma, the fluid around the thylakoids.
Which products of the light reactions power the Calvin cycle?
ATP and NADPH
ATP and NADH
O₂ and glucose
CO₂ and water
The Calvin cycle uses ATP for energy and NADPH as the electron donor to reduce CO₂ to sugar. NADH is the electron carrier of cellular respiration, a common mix-up.
What does the enzyme rubisco do?
Splits water in photosystem II
Makes ATP from ADP using a proton gradient
Attaches CO₂ to RuBP in the first step of the Calvin cycle
Converts pyruvate to acetyl-CoA
Rubisco catalyzes carbon fixation: CO₂ + RuBP → two 3-carbon molecules (3-PGA). It is probably the most abundant protein on Earth. Making ATP is the job of ATP synthase.
How many turns of the Calvin cycle are needed to make one molecule of glucose?
1
3
12
6
Each turn fixes one CO₂. Glucose has six carbons, so six turns are needed (three turns make one G3P, and two G3P make one glucose). It costs 18 ATP and 12 NADPH.
CAM plants such as cacti and pineapples reduce water loss by:
Opening their stomata at night and storing CO₂ as organic acids
Closing their stomata permanently
Running the Calvin cycle only at night
Skipping the light reactions
CAM plants take in CO₂ at night when it is cool, store it as malic acid, then release it to the Calvin cycle during the day with stomata closed. The Calvin cycle still runs in daylight because it needs ATP and NADPH from the light reactions.
What is the final electron acceptor of the electron transport chain in aerobic respiration?
Oxygen, which is reduced to water
NAD+
Carbon dioxide
Pyruvate
At complex IV, electrons combine with O₂ and H+ to form water. Without oxygen the chain backs up. NAD+ accepts electrons earlier, in glycolysis and the Krebs cycle.
During aerobic respiration, where is CO₂ released?
Glycolysis
The electron transport chain
Pyruvate oxidation and the Krebs cycle
ATP synthase
One CO₂ leaves per pyruvate when it becomes acetyl-CoA, and two more leave per turn of the Krebs cycle: six per glucose in total. Glycolysis releases no CO₂.
What directly drives ATP synthase in the mitochondria?
Electrons passing through ATP synthase
Protons flowing back into the matrix down their gradient
Direct transfer of phosphate from glucose
Oxygen binding to ATP synthase
The ETC pumps H+ into the intermembrane space. H+ then flows back through ATP synthase, turning it like a turbine (chemiosmosis). Electrons never pass through ATP synthase.
Cyanide blocks complex IV (cytochrome c oxidase) of the electron transport chain. What is the immediate effect?
Glycolysis stops because there is no glucose
Cells switch to photosynthesis
ATP production increases to compensate
Electron flow stops and oxidative phosphorylation stops making ATP
With complex IV blocked, electrons cannot reach oxygen, the proton gradient collapses and ATP synthase stops. Cells fall back on glycolysis, which cannot meet the energy needs of the heart and brain.
What is the main purpose of fermentation?
Making large amounts of ATP
Regenerating NAD+ so glycolysis can continue
Producing oxygen for the cell
Storing glucose as glycogen
Without oxygen, NADH cannot unload at the ETC. Fermentation passes those electrons to pyruvate (making lactate or ethanol), freeing NAD+ so glycolysis keeps making its 2 ATP. Fermentation itself makes no extra ATP.
During intense exercise, human muscle cells short on oxygen convert pyruvate into:
Ethanol and CO₂
Lactate
Acetyl-CoA
Glucose
Human muscle does lactic acid fermentation. Yeast does alcoholic fermentation, producing ethanol and CO₂. Acetyl-CoA forms only when oxygen is available.
One turn of the Krebs cycle (one acetyl-CoA) produces:
3 NADH, 1 FADH₂, 1 ATP (or GTP) and 2 CO₂
2 NADH, 2 FADH₂, 2 ATP and 1 CO₂
1 NADH, 1 FADH₂, 3 ATP and 2 CO₂
3 NADPH, 1 ATP and 2 O₂
Each acetyl-CoA yields 3 NADH, 1 FADH₂, 1 ATP/GTP and 2 CO₂. One glucose makes two acetyl-CoA, so double these numbers per glucose.
About how many ATP does complete aerobic respiration of one glucose produce, using current estimates?
2
About 30–32
4
About 100
Modern estimates are about 30–32 ATP per glucose. Older textbooks say 36–38, which assumed whole-number ATP yields for each NADH and FADH₂. Either way, it is far more than the 2 ATP from glycolysis alone.
Which statement correctly compares photosynthesis and cellular respiration?
Plants do photosynthesis instead of cellular respiration
Photosynthesis happens in mitochondria and respiration in chloroplasts
Only respiration involves electron carriers
Both make ATP by chemiosmosis using an electron transport chain and ATP synthase
Both processes pump protons across a membrane and use ATP synthase. Plants do both processes: their mitochondria respire day and night. Photosynthesis uses the carrier NADP+/NADPH, so the last option is wrong too.
