Photosynthesis and cellular respiration are often taught as two separate topics, but they are really two halves of one story. Together they explain where the matter in living things comes from and how the energy that powers a cell gets from sunlight to the work a cell does. The big idea in Living Earth is that atoms are never created or destroyed in these processes. They are rearranged, and energy is transferred and transformed along the way.
Photosynthesis: light energy becomes stored chemical energy
Photosynthesis takes place in the chloroplasts of plants, algae and some bacteria. Using light energy, the cell builds glucose from carbon dioxide and water, and releases oxygen as a by-product:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
The light energy does not vanish. It ends up stored in the chemical bonds of glucose, which is why glucose (and the larger molecules built from it) can later be used as fuel.
Cellular respiration: stored energy becomes usable energy
Cellular respiration takes place in nearly all living cells, in plants as well as animals. In aerobic respiration, glucose reacts with oxygen. The energy released is used to make ATP, the molecule cells use to power their work, and some of it leaves as heat:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP and heat)
Compare the two equations. The reactants of one are the products of the other. That is the cycle: carbon, hydrogen and oxygen atoms move back and forth between the air, the water and living things, while energy flows in one direction, from light to chemical bonds to ATP to heat.
Worked example 1: are the atoms conserved?
Count the atoms in the photosynthesis equation.
- Left side: 6 CO₂ gives 6 carbon and 12 oxygen. 6 H₂O gives 12 hydrogen and 6 oxygen. Total: 6 C, 12 H, 18 O.
- Right side: C₆H₁₂O₆ gives 6 C, 12 H and 6 O. 6 O₂ gives 12 O. Total: 6 C, 12 H, 18 O.
Both sides match, so the equation is balanced. The same counts hold for respiration, just read in the other direction. No atom is lost, which is why this is a model of conservation of matter.
Worked example 2: does the mass add up?
Use molar masses in grams per mole: CO₂ is 44.0, H₂O is 18.0, C₆H₁₂O₆ is 180.2 and O₂ is 32.0.
- Reactants: 6 × 44.0 g + 6 × 18.0 g = 264.0 g + 108.0 g = 372.0 g.
- Products: 180.2 g + 6 × 32.0 g = 180.2 g + 192.0 g = 372.2 g.
The totals agree to within rounding. Notice what this means: a plant making one mole of glucose takes in 264 g of carbon dioxide and 108 g of water. Most of the dry mass of a tree is built from carbon dioxide taken from the air, and it is the water and carbon dioxide, not the soil, that supply the atoms in the glucose.
Worked example 3: follow one carbon atom
Picture a carbon atom in a molecule of CO₂ in the air.
- A leaf takes in the CO₂, and photosynthesis builds the carbon into glucose.
- The plant uses the glucose for energy or converts it into other molecules, such as cellulose in its cell walls.
- A rabbit eats the plant. The carbon is now part of the rabbit's food molecules.
- The rabbit's cells carry out respiration and release the carbon as CO₂ when it exhales.
The same atom is back in the air, ready to be taken up again. The atom was never used up. The energy, however, did not make a loop. Some of it was released as heat at each step, so living systems need a steady input of energy, ultimately from the sun.
Why this matters for the Earth system
Because photosynthesis removes carbon dioxide and respiration returns it, the two processes link living things to the atmosphere, the oceans and the rock record. Photosynthesis also produced the oxygen that aerobic respiration needs. Scientists model these exchanges as flows of matter between parts of a system, and the equations above are the bookkeeping for those flows.
Common mistakes
- Thinking plants only photosynthesize. Plant cells also carry out cellular respiration, day and night, to get usable energy from their glucose.
- Saying that photosynthesis makes energy. It transforms light energy into chemical energy. Energy is conserved, not created.
- Saying that respiration is breathing. Breathing moves gases in and out of the body. Respiration is the chemical process inside cells.
- Thinking the mass of a plant comes from soil. Most of it comes from carbon dioxide and water.
- Writing an unbalanced equation. If the atoms do not match on both sides, a coefficient is wrong. Never change a subscript to fix it.
Practice Living Earth questions with an explanation for every answer, or read the guide to California Living Earth practice.
Source: the California Next Generation Science Standards (CA NGSS), as set out in the California Science Framework, HS-LS1-5, HS-LS1-7, HS-LS2-5.