Energy in the biological world is captured from sunlight by photosynthesis in plants and consumed by respiration in all organisms. Let us study these two major metabolic reactions together.
Basic Knowledge
Photosynthesis: A reaction in chloroplasts that uses light energy to synthesize organic matter (glucose) from carbon dioxide and water.
Reaction formula: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Photosynthesis converts light energy into chemical energy (the bond energy of organic substances) and is the energy source of nearly all organisms on Earth.
Respiration: A reaction in mitochondria that breaks down organic substances to produce ATP.
Reaction formula: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP)
Photosynthesis and respiration have reaction formulas that are exactly opposite in direction, but their actual pathways are completely different. Respiration proceeds in three stages: glycolysis→citric acid cycle→electron transport chain, producing a maximum of about 38 molecules of ATP from one molecule of glucose.
Photosynthesis (CO2+H2O+light→glucose+O2, occurs in chloroplasts)
Respiration (glucose+O2→CO2+H2O+ATP, occurs in mitochondria)
Chlorophyll (A photosynthetic pigment. Found in the thylakoids of chloroplasts)
Autotroph (An organism that makes its own organic matter. Plants・algae・and some bacteria)
Heterotroph (An organism that obtains organic matter from other organisms. Animals・fungi)
Fermentation (A pathway that breaks down organic matter without oxygen to obtain ATP. Alcoholic fermentation・lactic acid fermentation)
Glycolysis (A pathway that breaks down glucose into pyruvate in the cytosol)
In Depth
The essence of photosynthesis is the conversion of light energy into chemical energy. Chlorophyll absorbs photons and releases excited electrons, and ATP and NADPH are produced as those electrons flow through an electron transport chain (the light reactions). Using this energy, glucose is synthesized from CO2 (the Calvin cycle = dark reactions). The light reactions occur in the thylakoid membrane, and the dark reactions occur in the stroma.
The essence of respiration is the oxidation of organic matter (removal of electrons). When glucose (rich in C-H bonds and reduced) is ultimately broken down to CO2 and H2O (complete oxidation), electrons move from high energy to low energy, and ATP is synthesized using the resulting energy difference. The final electron acceptor is oxygen, which is why this is called “aerobic respiration.”
In environments without oxygen, fermentation occurs. Examples include alcoholic fermentation by yeast (glucose→ethanol+CO2) and lactic acid fermentation by lactic acid bacteria (glucose→lactic acid). Fermentation proceeds only through glycolysis and produces far less ATP (about 2 molecules) than respiration, but it allows organisms to live without oxygen. Lactic acid fermentation also temporarily occurs during intense exercise and was once considered one cause of muscle soreness (the mainstream view now attributes soreness to other factors rather than lactic acid itself).
- Photosynthesis = 6CO2+6H2O+light→C6H12O6+6O2
- Respiration = C6H12O6+6O2→6CO2+6H2O+ATP
- Photosynthesis occurs in chloroplasts, respiration in mitochondria
- Respiration = three stages: glycolysis・citric acid cycle・electron transport chain
- Respiration produces about 38 ATP from one glucose molecule
- Autotrophs (plants) and heterotrophs (animals)
- Fermentation = without oxygen, with low ATP yield (about 2)
Points to Note
① Photosynthesis (anabolism・energy absorption) and respiration (catabolism・energy release) have opposite reaction formulas, but their actual pathways are different. ② Plants also respire. During the day, photosynthesis exceeds respiration, but at night only respiration occurs. ③ Light reactions (thylakoids・require light) and dark reactions (Calvin cycle) (stroma・do not require light). The dark reactions simply can proceed in darkness; in reality, they also proceed in the light. ④ Fermentation is metabolism that does not use oxygen. Its ATP yield is low, but it is important in environments with no oxygen.
Practice
- Write the reaction formula for photosynthesis using an arrow (including substance names and numbers of molecules).
- State all three stages of respiration.
- What is the metabolic pathway that breaks down glucose without oxygen and produces ethanol?