3.4 Cellular Respiration and Photosynthesis

Two Pathways, One Energy Currency

Almost every Praxis Biology cluster on energy ties back to one of two pathways: cellular respiration (oxidizes glucose to capture energy) and photosynthesis (uses light to build glucose). Both ultimately rely on the same trick — a proton gradient across a membrane drives ATP synthase to make ATP. The chemistry mirrors itself in reverse:

• Respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP
• Photosynthesis: 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

Cellular Respiration

1. Glycolysis (Cytoplasm)

Glycolysis splits one glucose (6C) into two pyruvate (3C each). It costs 2 ATP up front (the investment phase) and produces 4 ATP (the payoff phase), for a net gain of 2 ATP by substrate-level phosphorylation. It also makes 2 NADH. Glycolysis is anaerobic and happens in the cytoplasm of every cell, prokaryotic and eukaryotic.

2. Pyruvate Oxidation (Mitochondrial Matrix)

Each pyruvate enters the mitochondrion and is converted by the pyruvate dehydrogenase complex into acetyl-CoA (2C). One CO2 is released and one NADH is generated per pyruvate, so two pyruvates yield 2 acetyl-CoA, 2 CO2, and 2 NADH.

3. Krebs (Citric Acid) Cycle (Matrix)

Each acetyl-CoA enters the Krebs cycle, which turns twice per glucose. Per turn, the cycle releases 2 CO2, generates 3 NADH, 1 FADH2, and 1 ATP (or GTP). So per glucose, the cycle yields 4 CO2, 6 NADH, 2 FADH2, and 2 ATP.

4. Electron Transport Chain and Oxidative Phosphorylation (Inner Mitochondrial Membrane)

The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2 deliver high-energy electrons to the ETC. As electrons hop down the chain, protons (H+) are pumped from the matrix into the intermembrane space, creating an electrochemical gradient.

At the end of the chain, electrons reduce O2 to H2O — which is why oxygen is the final electron acceptor and why respiration is called aerobic. Without oxygen the chain backs up and shuts down.

Protons flow back into the matrix through ATP synthase, which spins like a tiny turbine and phosphorylates ADP to ATP. This process — chemiosmosis — accounts for roughly 26–28 ATP per glucose, the bulk of cellular energy.

Total ATP Yield

Modern textbooks (and the Praxis) use 30–32 ATP because the energetic cost of importing NADH from the cytoplasm and of running ATP synthase is not perfectly 3 ATP per NADH.

Fermentation

When oxygen is unavailable, the ETC cannot turn over, NAD+ is not regenerated, and the Krebs cycle stalls. Cells fall back on fermentation — a way to recycle NAD+ so that glycolysis can keep producing the meager 2 ATP it generates on its own.

• Lactic acid fermentation — pyruvate is reduced to lactate (lactic acid), regenerating NAD+. Occurs in vertebrate muscle during intense exercise and in many bacteria (yogurt, cheese).
• Alcohol fermentation — pyruvate is decarboxylated to acetaldehyde, then reduced to ethanol. Occurs in yeast (bread, beer, wine) and produces CO2 as a byproduct.

In both pathways, the goal is not to make extra ATP; it is to regenerate NAD+ so glycolysis can continue.

Photosynthesis

Photosynthesis occurs in the chloroplast, an organelle with three membrane systems (outer, inner, and the internal thylakoid membranes stacked into grana). The fluid between thylakoids and the inner membrane is the stroma.

Light-Dependent Reactions (Thylakoid Membrane)

Light strikes photosystem II (PSII) first, exciting electrons that are passed down an ETC. Splitting (photolysis) of water releases electrons to refill PSII and also generates O2 as a byproduct — this is the oxygen we breathe. Proton pumping across the thylakoid membrane creates a gradient that ATP synthase uses to make ATP. Excited electrons from photosystem I (PSI) ultimately reduce NADP+ to NADPH.

Net outputs: ATP, NADPH, and O2 (released as waste).

Calvin Cycle (Stroma)

The Calvin cycle uses the ATP and NADPH from the light reactions to fix CO2 into organic carbon. Each turn fixes one CO2 onto a 5-carbon sugar called RuBP in a reaction catalyzed by rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase — the most abundant protein on Earth). Three turns are needed to net one G3P (a 3-carbon sugar); two G3P combine to form glucose.

The Calvin cycle is sometimes called the "dark reactions" because it does not require light directly, but in practice it runs during the day because it depends on ATP and NADPH that the light reactions make.

C3, C4, and CAM Plants

Rubisco occasionally grabs O2 instead of CO2, kicking off a wasteful side path called photorespiration. Hot, dry conditions worsen the problem because plants close their stomata to save water, dropping CO2 and raising O2 inside the leaf. Three strategies have evolved:

• C3 plants (rice, wheat) — fix CO2 directly into a 3-carbon compound. Vulnerable to photorespiration in hot weather.
• C4 plants (corn, sugarcane) — pre-fix CO2 into a 4-carbon compound (oxaloacetate) in mesophyll cells, then shuttle it into deeper bundle-sheath cells where rubisco operates. Spatially separating fixation keeps CO2 concentrated around rubisco.
• CAM plants (cacti, pineapple, succulents) — open stomata only at night to capture CO2 (stored as malate); during the day, stomata close to save water and stored CO2 feeds the Calvin cycle. Temporally separates fixation from the Calvin cycle.

The Praxis loves a stem that pairs an environment (hot, dry, arid) with the matching plant strategy.