Some organisms capture light and other forms of energy, both inorganic and organic, to produce their own food in processes called photosynthesis and chemosynthesis. Plants, algae, some protists, and some bacteria carry out photosynthesis, while bacteria mostly perform chemosynthesis. Photosynthesis relies on pigments, where the most common is chlorophyll, which gives plants their green color. Chemosynthesis involves the conversion of various inorganic chemicals, such as sulfur, into organic compounds. Both processes begin with reactions that capture energy and convert it into chemical energy in the form of adenosine triphosphate (ATP). This energy is used to fix carbon dioxide from the atmosphere into sugar molecules. One of the ways in which this process is achieved is through the Calvin cycle. Water is oxidized during the light reaction stage of photosynthesis. 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. When light is not available, organisms called chemoautotrophs use chemicals such as hydrogen sulfide to generate the energy needed to make organic compounds for themselves.
At A Glance
- Using chlorophyll and other pigments, photosynthetic organisms capture light energy and then extract carbon from CO2, which they use to make sugars, releasing oxygen in the process as a by-product.
Chloroplasts are thought to have evolved from free-living photosynthetic cyanobacteria.
- Chlorophyll a, chlorophyll b, and carotenoids each absorb particular wavelengths of light, enabling plants to use a variety of wavelengths.
- The light reactions make up the first stage of photosynthesis, using light energy to convert ADP into ATP and to reduce NADP+ to NADPH, as well as produce oxygen.
- Carbon is captured and fixed into sugar during the Calvin cycle, also known as light-independent reactions.
- 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.
- Some autotrophs make energy from inorganic chemicals, such as ammonia and hydrogen sulfide, instead of using sunlight.