Photosynthesis and cellular respiration are the two major energy processes that connect plants, animals, and most ecosystems. This cheat sheet helps students compare where each process happens, what molecules enter and leave, and how energy changes form. Understanding these processes is essential for explaining carbon cycling, oxygen production, food webs, and how cells make usable energy.
It also helps students interpret diagrams of chloroplasts, mitochondria, and membrane-based energy transfer.
Photosynthesis stores energy by using light to convert carbon dioxide and water into glucose and oxygen. Cellular respiration releases energy by breaking glucose down with oxygen to produce carbon dioxide, water, and ATP. The main formulas are 6CO₂ + 6H₂O + light energy -> C₆H₁₂O₆ + 6O₂ for photosynthesis and C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6H₂O + ATP for cellular respiration.
The two processes are complementary because the products of one process are often the reactants of the other.
Key Facts
- The overall photosynthesis equation is 6CO₂ + 6H₂O + light energy -> C₆H₁₂O₆ + 6O₂.
- The overall cellular respiration equation is C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6H₂O + ATP.
- Photosynthesis occurs in chloroplasts, with light reactions in the thylakoid membranes and the Calvin cycle in the stroma.
- Cellular respiration occurs mainly in mitochondria, with glycolysis in the cytoplasm, the Krebs cycle in the matrix, and the electron transport chain on the inner mitochondrial membrane.
- Light reactions split water, release O₂, and make ATP and NADPH for the Calvin cycle.
- The Calvin cycle uses CO₂, ATP, and NADPH to build sugar molecules such as glucose.
- Aerobic respiration usually produces about 30 to 32 ATP per glucose molecule in eukaryotic cells.
- Photosynthesis stores energy in glucose, while cellular respiration transfers energy from glucose into ATP that cells can use.
Vocabulary
- Photosynthesis
- Photosynthesis is the process by which plants, algae, and some bacteria use light energy to make glucose from carbon dioxide and water.
- Cellular respiration
- Cellular respiration is the process cells use to break down glucose and make ATP, usually using oxygen.
- ATP
- ATP is the main energy-carrying molecule that cells use to power chemical reactions and cellular work.
- Chloroplast
- A chloroplast is the organelle in plant and algal cells where photosynthesis occurs.
- Mitochondrion
- A mitochondrion is the organelle where most ATP is produced during aerobic cellular respiration.
- Electron transport chain
- The electron transport chain is a series of membrane proteins that move electrons and help produce ATP.
Common Mistakes to Avoid
- Saying photosynthesis makes energy is wrong because it converts light energy into chemical energy stored in glucose rather than creating energy from nothing.
- Forgetting to balance the equations is wrong because photosynthesis and respiration must follow conservation of matter, so the numbers of C, H, and O atoms must match on both sides.
- Mixing up chloroplasts and mitochondria is wrong because chloroplasts carry out photosynthesis, while mitochondria carry out most steps of aerobic respiration.
- Thinking plants only do photosynthesis is wrong because plants also perform cellular respiration to make ATP for their cells.
- Calling oxygen the energy source in respiration is wrong because glucose is the fuel, while oxygen is the final electron acceptor in aerobic respiration.
Practice Questions
- 1 Write and balance the overall equation for photosynthesis using CO₂, H₂O, light energy, C₆H₁₂O₆, and O₂.
- 2 If a plant produces 12 molecules of O₂ during photosynthesis, how many molecules of CO₂ were used?
- 3 If one glucose molecule produces about 30 ATP during aerobic respiration, about how many ATP could 5 glucose molecules produce?
- 4 Explain why photosynthesis and cellular respiration are often described as complementary processes.
Understanding Photosynthesis & Cellular Respiration
Energy moves through these pathways by electron transfer. Electrons carry energy when they are held in chemical bonds or passed between molecules. In photosynthesis, chlorophyll absorbs certain wavelengths of light, especially red and blue light.
This excites electrons inside the thylakoid membrane. A chain of carrier proteins uses their energy to move hydrogen ions into a small internal space. The ions then flow back through ATP synthase, a protein that works like a tiny rotating turbine.
This flow provides the energy to join phosphate to ADP and form ATP. NADPH acts as another short term energy carrier. It delivers high energy electrons for building carbohydrates in the Calvin cycle.
The Calvin cycle does not directly need sunlight, but it depends on materials supplied by the light reactions. Its key job is carbon fixation. An enzyme called rubisco attaches carbon dioxide to an existing carbon compound.
After several steps, the cycle produces a small sugar called G3P. Cells can combine G3P molecules into glucose, starch, cellulose, or other organic compounds. This explains why a plant uses much of the sugar it makes for growth rather than keeping it as free glucose.
Plants must balance carbon intake with water loss. Carbon dioxide enters mainly through stomata, tiny pores in leaves. When stomata close during hot or dry conditions, less water escapes, though less carbon dioxide enters as well.
Respiration is controlled breakdown rather than one sudden release of energy. During glycolysis, enzymes split glucose into smaller molecules and capture a little ATP. They transfer many electrons to NADH.
Later stages remove carbon atoms as carbon dioxide and load more electron carriers. At the inner mitochondrial membrane, NADH and another carrier called FADH2 donate electrons to an electron transport chain. Oxygen accepts electrons at the end of this chain.
Without oxygen, the chain stops because electrons have nowhere to go. The chain pumps hydrogen ions across the membrane, creating a concentration difference.
ATP synthase uses that difference in the same basic way as in chloroplasts. This shared mechanism is called chemiosmosis.
Cells need ATP because most energy requiring tasks happen in small, frequent steps. ATP can power muscle contraction, active transport across membranes, protein building, nerve signaling, and cell division. It is not a long term storage molecule.
A cell uses ATP quickly, then rebuilds it continually. When oxygen is limited, some cells can keep glycolysis running through fermentation. Human muscle cells may produce lactate during intense exercise.
Yeast produces alcohol and carbon dioxide. Fermentation makes far less ATP because it does not use the mitochondrial electron transport chain. When studying diagrams, track the membranes carefully.
Identify where hydrogen ions build up, where ATP synthase sits, and which molecules carry electrons. These details explain why location matters in both processes.