During photosynthesis, plants absorb sunlight and use that light to produce sugar, or glucose, used as energy for the plant. The beginning stage, the light-dependent reactions, is the process by which plants capture the stored energy from sunlight. The latter stage, the light-independent reactions, is the process of taking the energy and converting carbon dioxide to glucose. Aerobic respiration is a metabolic process where glucose is converted into CO2 and H2O in the presence of oxygen to produce energy. Because these processes are so similar, there are many commonalities among aerobic respiration and the light stages of photosynthesis.
A Process in Reverse
Photosynthesis and aerobic respiration are essentially reverse processes of each other. Photosynthesis begins with CO2 and uses energy from light to reduce this CO2 to sugar. Respiration begins with sugar and then oxidizes that sugar into CO2, a process that releases energy which is then collected as the carrier molecule ATP. Whereas carbon dioxide and water create glucose and oxygen in the process of photosynthesis, glucose and oxygen create carbon dioxide and water through the aerobic respiration process. The elements and outcomes are essentially the same, producing ATP or NADPH to generate energy. Aerobic respiration goes through four major stages, instead of two stages, as with photosynthesis.
Organisms require energy to survive. Both aerobic respiration and photosynthesis are biological processes involving living organisms and requiring energy. The way each process creates energy is similar in that both of these processes rely on existing molecules in cells to carry out the energy conversion reactions. In the light stages of photosynthesis, light energy is converted to chemical energy which is then used to produce organic compounds, but in aerobic respiration, sunlight is not required to create energy. The process for respiration requires oxygen to produce energy, but when there is no sun to convert CO2, plants also use this process.
Synthesis of ATP
ATP is the universal energy carrier. Both plant and animal cells undergo an aerobic process to produce ATP, which is used to transport chemical energy within cells for metabolism. In photosynthesis, plants are able to produce their own food using energy, or ATP converted from light energy, while animal cells cannot make ATP by photosynthesis. In both of the stages of photosynthesis and in aerobic respiration, glucose is the energy used to create ATP. Once ATP is supplied, cells use this energy for bodily or plant processes. The main difference is that in photosynthesis, glucose is created from the conversion of light, while during respiration, glucose is created from the conversion of oxygen, or oxidation.
Aerobic respiration and the light stages of photosynthesis both occur in organelles where processes happen that have complex internal membrane systems essential to both processes. Photosynthesis occurs in chloroplasts, organelles containing chlorophyll that capture the energy from sunlight and then store that energy in ATP and NADPH. Chloroplasts are similar to mitochondria, where aerobic respiration occurs. Both organelles contain their own DNA, are made of unit membranes, and have an outer envelope of two membranes. Functionally, both organelles contain similar enzymes and coenzymes for electron transport and ATP exchange, though these enzymes are used in different ways.
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