Two Stages of Photosynthesis


Background on Photosynthesis

  • Photosynthesis is a biological process by which energy contained within light is converted into chemical energy of bonds between atoms that power processes within cells. It emerged roughly 3.5 billion years ago in geological history, has evolved complex biochemical and biophysical mechanisms, and occurs today within a variety of single-celled organisms, as well as in plants. It is on account of photosynthesis that Earth's atmosphere and seas contain oxygen.

Photosynthesis: Two Stages

  • While the mechanisms of photosynthesis are complex, the overall reaction is as follows: carbon dioxide + sunlight + water --> glucose (sugar) + molecular oxygen (O2). This reaction takes place through several steps which occur during two stages, or phases: the light phase and the dark phase.

    In the light phase, energy from light, powers reactions that split water to release oxygen. In the process, high energy molecules, called ATP and a reducing agent and NADPH are formed. Actually, it is the the chemical bonds in these compounds that stores the energy. The dark phase is also known as the Calvin Cycle. In this phase, which uses the ATP and NADPH from the light phase, carbcarbon dioxide used to make the sugar, glucose. Oxygen is a byproduct of the reaction.

How it Works

  • A key component of the biological machinery that drives photosynthesis is the molecule chlorophyll. Chlorophyll is a large molecule with a special structure that enables it to capture light energy and convert it to high energy electrons, which are used during the reactions of the two phases, which ultimately produce the sugar, glucose.

    In photosynthetic bacteria, the reaction takes place in the cell membrane and within the cell, but outside of the nucleus. In plants and photosynthetic protozoans (protozoans are single-celled organisms belonging to the eukaryote domain, the same domain of life which includes plants, animals and fungus), photosynthesis takes place within chloroplasts. Chloroplasts are a type of organelle (membrane-bound compartments, adapted for specific functions, like little organs for cells).

    While chloroplasts exist today within other cells, such as plant cells, they actually have their own DNA and genes. Analysis of the sequence of these genes has revealed that, in fact, chloroplasts evolved from independently-living photosynthetic organisms related to a group of bacteria called cyanobacteria. A similar process occurred when the ancestors of mitochondria, the organelles within cells where oxidative respiration--the chemical opposite of photosynthesis-- takes place. According to the theory of endosymbiosis, a theory which was given a boost recently, as a result of a new study published in the journal Nature, both chloroplasts and mitochondria once lived as independent bacteria, but were engulfed within the ancestors of eukaryotes, leading ultimately to the emergence of plants and animals.


  • Photo Credit Photosynthesis Tutorial Northern Michigan University
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