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Some organisms produce their own food using light energy in a process called photosynthesis. Plants, algae, some protists, and some bacteria carry out photosynthesis. Photosynthesis relies on pigments, the most common of which is chlorophyll; chlorophyll gives plants their green color. Photosynthesis begins with reactions that capture energy from light and convert it into chemical energy in the form of ATP. This energy is used to fix carbon dioxide from the atmosphere into sugar molecules in a process called the Calvin cycle. Sometimes this cycle is interrupted by atmospheric oxygen, which is converted to carbon dioxide in a process known as photorespiration. To combat energy loss from photorespiration, some plants evolved alternative pathways to carbohydrate production. These plants are known as C4 or CAM plants, depending on the pathway they use.

At A Glance

  • Photosynthesis uses light energy to make sugars, which can be used later for fuel.
  • Photosynthesis takes place in chloroplasts. Chloroplasts are plastids in green plant cells that contain chlorophyll to aid photosynthesis.
  • Photosynthesis uses carbon dioxide, water, and light energy to make sugar.
  • The light reactions make up the first stage of photosynthesis. They use light energy to convert ADP to ATP, and to reduce NADP+ to NADPH.
  • The second stage of photosynthesis is the Calvin cycle, also known as the light-independent reactions.
  • Photorespiration involves the intake of O2 and release of CO2, consuming energy in the process. Adaptations to combat the energy loss of photorespiration include the C4 and CAM pathways.
  • In photorespiration, rubisco adds O2 to RuBP, consuming energy and generating CO2.
  • C4 plants fix carbon into four-carbon compounds before passing the carbon into the Calvin cycle.
  • CAM plants fix carbon into organic acids at night and carry out photosynthesis during the day using the carbon in these acids.