Photosynthesis & Cellular Respiration Notes

  • Mass in trees comes from CO_2 in the air, proven by Jan Baptiste van Helmont's experiment.

  • Joseph Priestly found plants restore air by releasing O2 using CO2. Basic photosynthesis equation: 6CO2 + 6H2O + light \rightarrow C6H{12}O6 + 6O2

  • ATP and NADPH are key energy carriers. Oxidation is electron loss, reduction is gain; in photosynthesis, H2O is oxidized, CO2 is reduced.

  • Glucose stores energy chemically. Chloroplast parts: outer/inner membrane, thylakoid membrane/lumen, stroma.

  • Chlorophyll, the main pigment, absorbs light energy in the thylakoid membrane. Carotenoids and xanthophylls help absorb additional light.

  • Chlorophyll absorbs blue and red, reflects green. Photosystems consist of antenna pigments, reaction center chlorophyll, and a primary electron acceptor.

  • ETC transfers electrons from Photosystem II to I, pumping H^+ ions, creating a proton gradient. ATP Synthase uses this gradient to make ATP from ADP + Pi.

  • Light-dependent reactions: electrons excited, water split, O_2 released, ATP and NADPH produced.

  • Calvin Cycle occurs in the stroma. CO_2 is fixed into an organic molecule using RUBISCO, the most important enzyme.

  • Calvin Cycle Steps: Fixation (CO_2 to RuBP via RUBISCO), Reduction (ATP/NADPH reduce 3-PGA to G3P), Regeneration (G3P regenerates RuBP).

  • Gluconeogenesis converts G3P into glucose. Photorespiration occurs when RUBISCO binds to O_2, avoided by C4 and CAM plants using PEP carboxylase.

  • C4 pathway: spatial separation in mesophyll/bundle sheath cells; CAM: stomata open at night, fix CO_2 and do Calvin Cycle during the day.

  • ETC (cellular respiration) yields the most ATP. Without oxygen, cells ferment (lactic acid or ethanol fermentation), producing 2 ATP per glucose.

  • Atmospheric CO_2 is monitored at Mauna Loa, currently around 425 ppm (as of 2025).

  • Seasonal CO2 oscillation: Spring/Summer (less CO2), Fall/Winter (more CO_2).

  • Humans disrupt the carbon cycle by burning fossil fuels, increasing atmospheric CO_2.

  • Glucose converts to ATP during respiration. Respiration occurs in all living cells (except dead cells and mature red blood cells); photosynthesis occurs in plant cells, some protists/bacteria.

  • Oxygen is the final electron acceptor in the ETC. Mitochondria are responsible for respiration; chloroplasts for photosynthesis.

  • C4 photosynthesis is more efficient than C3 in hot/dry/high-light environments. CAM plants (succulents) fix CO_2 at night, conduct the Calvin Cycle during the day.