Cellular respiration vs photosynthesis, side by side

Cellular respiration and photosynthesis are close to mirror images. Photosynthesis uses light energy to build sugar from CO2 and water and releases O2. Respiration breaks sugar back down with O2 and releases CO2 and water, storing the energy in ATP. The overall equations run in opposite directions, but the machinery is not a simple reversal, and exam questions live in those differences.

The facts below follow OpenStax Biology 2e chapters 7 and 8.

The overall equations

• Photosynthesis: 6 CO2 + 6 H2O + light energy -> C6H12O6 + 6 O2
• Aerobic respiration: C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + energy (ATP and heat)

Plants do both. A leaf cell has chloroplasts and mitochondria, and it respires day and night. "Plants photosynthesize, animals respire" is a common wrong answer.

The comparison table

Where each stage happens

Respiration moves through the cell:

1. Glycolysis happens in the cytoplasm, in prokaryotes and eukaryotes alike, and doesn't need oxygen.
2. Pyruvate oxidation happens in the mitochondrial matrix.
3. The citric acid cycle also happens in the matrix.
4. Oxidative phosphorylation happens at the inner mitochondrial membrane.

Photosynthesis stays inside the chloroplast:

1. The light-dependent reactions happen in the thylakoid membranes.
2. The Calvin cycle happens in the stroma, the fluid around the thylakoid stacks (grana).

Watch the spelling trap: the stroma is inside the chloroplast; a stoma is a pore in the leaf surface where CO2 gets in.

Chemiosmosis happens in both

Both organelles make most of their ATP the same way: an electron transport chain pumps H+ across a membrane, and the H+ flows back through ATP synthase, which adds a phosphate to ADP. OpenStax says chemiosmosis makes about 90 percent of the ATP from glucose breakdown, and it's also how the light reactions make ATP.

The direction is what gets tested:

• Mitochondria pump H+ from the matrix into the intermembrane space. ATP synthase makes ATP on the matrix side.
• Chloroplasts pump H+ from the stroma into the thylakoid lumen. ATP synthase makes ATP on the stroma side, which is where the Calvin cycle needs it.

In both cases the compartment H+ is pumped into ends up with the lower pH.

Redox runs in opposite directions

In respiration, glucose is oxidized: it loses electrons to NAD+ and FAD, which become NADH and FADH2. Those carriers hand the electrons to the ETC, and oxygen accepts them at the end, picking up H+ to form water. OIL RIG (oxidation is loss, reduction is gain) keeps the direction straight.

In photosynthesis, light energizes electrons taken from water at photosystem II, which releases O2. The electrons pass through the chain to photosystem I, which uses a second photon to reduce NADP+ to NADPH. In the Calvin cycle, NADPH then reduces 3-PGA to G3P. Carbon goes from its most oxidized form (CO2) to a reduced sugar.

The numbers

Respiration, per glucose:

• Glycolysis: net 2 ATP (4 made, 2 used), 2 NADH, 2 pyruvate.
• Pyruvate oxidation: 2 NADH, 2 CO2, no ATP.
• Citric acid cycle (two turns): 2 ATP or GTP, 6 NADH, 2 FADH2, 4 CO2.
• Before the ETC that's 4 ATP, 10 NADH and 2 FADH2.

The total ATP is where textbooks disagree, and OpenStax gives no single number because the yield varies. Modern texts use about 2.5 ATP per NADH and 1.5 per FADH2:

• 10 x 2.5 + 2 x 1.5 + 4 = 32 ATP
• If the 2 NADH from glycolysis enter the mitochondrion by the glycerol phosphate shuttle at 1.5 each: 8 x 2.5 + 4 x 1.5 + 4 = 30 ATP

Older texts used 3 per NADH and 2 per FADH2, giving 10 x 3 + 2 x 2 + 4 = 38 (or 36 with the shuttle). Use the figure your course uses, and know that "about 30 to 32" is the modern one. Fermentation stops after glycolysis, so it gets 2 ATP per glucose. It regenerates NAD+ but makes no extra ATP.

Photosynthesis, Calvin cycle:

• Each turn fixes 1 CO2 and uses 3 ATP and 2 NADPH.
• Three turns make one G3P that can leave the cycle: 9 ATP and 6 NADPH.
• One glucose is built from two G3P, so it takes six turns: 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH.

The Calvin cycle uses more ATP than NADPH. Cyclic electron flow, which runs through photosystem I only and makes ATP without NADPH or O2, helps make up the difference.

How they connect

The products of one are the reactants of the other. O2 from photosynthesis is the final electron acceptor in respiration, and CO2 from respiration is the carbon source for the Calvin cycle. The key difference is the energy source. Photosynthesis runs on light. Respiration runs on the chemical energy photosynthesis stored.

Studying the pair

These two chapters are easiest to learn together. Make cards that force a comparison ("where is H+ pumped in a chloroplast?") rather than one-sided definitions, and quiz yourself instead of rereading; the testing effect explains why. For the broader plan, see how to study for a general biology exam, and what's in General Biology I shows where units 7 and 8 sit in the course. If you're taking chemistry at the same time, how to calculate pH for strong and weak acids covers the pH side of proton gradients.

Encodr's free General Biology I course gives cellular respiration and photosynthesis 11 chapters between them.