Cellular energy · Topic 11 of 17

Photosynthesis

Tap any line to see why it matters

Biology · cell energy

Photosynthesis

In chloroplasts, light reactions make ATP and NADPH and release oxygen; the Calvin cycle uses them to build sugar from CO₂. It mirrors respiration.

01 · The equation6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

02 · Two stagesLight reactions → Calvin cycle

03 · The mirrorThe reverse of cellular respiration

What it is

Photosynthesis is respiration in reverse: each one's products feed the other must know

Chlorophyll absorbs blue and red, reflects green

Step by step

Light reactions (thylakoids) → ATP + NADPH; Calvin cycle (stroma) → sugar

Stage 1 · Light-dependent reactions

Step by step

Light excites chlorophyll's electrons in the thylakoid membrane

Water is split → O₂ released (the O₂ comes from water, not CO₂)

Electron flow makes ATP and NADPH to carry energy to the Calvin cycle

Stage 2 · Calvin cycle

Step by step

Carbon fixation: CO₂ is attached to an organic molecule in the stroma

ATP + NADPH turn fixed carbon into G3P sugar (2 G3P → glucose)

Parts & jobs

Happens in chloroplasts; chlorophyll captures the light must know

CO₂ in, O₂ out through stomata; photosynthesis in the mesophyll

Know by heart

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ must know

Don't confuse

Autotrophs make their own food; heterotrophs eat other organisms

Photosynthesis or cellular respiration?

Commonly confused

	Photosynthesis	Cellular respiration
Where	Chloroplasts	Cytoplasm and mitochondria
Takes in	CO₂ + water + light energy	Glucose + oxygen
Gives off	Glucose + oxygen	CO₂ + water + ATP
Energy	Stores light energy in sugar	Releases sugar's energy as ATP
Who does it	Plants, algae, cyanobacteria	Plants and animals alike

Misconceptions

Trap: the Calvin cycle doesn't need darkness; it runs on light-made ATP + NADPH

Trap: plants do cellular respiration too (they have mitochondria)

More detail

Show 6 more facts

RuBP is regenerated; ADP and NADP⁺ go back to be recharged

Thylakoid stacks = grana; surrounding fluid = stroma (≠ stoma)

Photosystem II acts before photosystem I (named by discovery order)

RuBisCO fixes 1 CO₂ per turn: 3 turns → 1 G3P; 6 turns → 1 glucose

Both processes make ATP with an electron transport chain

Atom check: 6 C, 12 H, 18 O on both sides

Pearl

Photosynthesis makes the sugar and oxygen that respiration uses, and respiration hands back the CO₂ and water.

Sources · OpenStax Biology 2e §8.1 Overview of Photosynthesis · OpenStax Concepts of Biology §5.2 The Light-Dependent Reactions of Photosynthesis · OpenStax Biology 2e §8.3 Using Light Energy to Make Organic Molecules (Calvin cycle; energy cycle) · OpenStax Concepts of Biology §4.1 Energy and Metabolism (photosynthesis and respiration equations) · OpenStax Concepts of Biology §3.3 Eukaryotic Cells (chloroplast structure; plant cells have mitochondria) · OpenStax Biology 2e §7.4 Oxidative Phosphorylation (chemiosmosis also powers the light reactions of photosynthesis) · Union Test Prep — HESI A2 Biology study guide, p. 1: cells, organelles, membrane, respiration, photosynthesis · TSTC Library — HESI A2 guide (Biology: cell parts and organelles; compare photosynthesis and respiration) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Practice this topic, then test the whole section.

Start HESI prep free

HESI and HESI A2 are registered trademarks of Elsevier. NurseSavvy is not affiliated with or endorsed by Elsevier.