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As you learned from the SparkNotes on Cell Respiration, the process of cellular respiration starts with a molecule of glucose. Most organisms obtain these glucose molecules by consuming other organisms. However, some organisms, such as plants, algae, and some prokaryotes, can produce their own glucose. This process is called photosynthesis and uses light energy to create chemical energy (glucose), which can be stored for use in biological processes. Photosynthesis is critical to life on earth because it is the primary source of organic material for nearly all organisms and produces oxygen for our atmosphere.
Origins of Photosynthesis
Photosynthesis first evolved in prokaryotic organisms. Over long periods of time, photosynthesis and the resulting production of oxygen changed the Earth’s atmosphere to be more oxygenated, allowing for other forms of life to evolve. As discussed in the Cell Structure and Function SparkNotes, some of these prokaryotes were engulfed by host cells and eventually developed into chloroplasts. In eukaryotes, chloroplasts are the organelle where photosynthesis occurs. There are a variety of pigments that can be used to absorb light in photosynthesis, but the primary pigment is Chlorophyll a. Chlorophyll a absorbs primarily blue and red wavelengths while reflecting green wavelengths.
Reactions
During photosynthesis, light energy is used to combine water and carbon dioxide to produce glucose and oxygen. There are two main stages of this process: light-dependent reactions and light-independent reactions.
The light-dependent reactions, also known as photophosphorylation, occur in the thylakoid membranes of the chloroplasts. In this stage, light energy, water, NADP\(^+\), ADP, and inorganic phosphate are converted into oxygen, NADPH, and ATP. The general steps are as follows:
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Light energy is absorbed by chlorophyll which excites electrons.
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These electrons move through the electron transport chain (ETC). The ETC is a series of molecules embedded in the membrane. As electrons are transferred from one molecule to the next, they release energy which is used to pump protons across the membrane into the thylakoid space.
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This build-up of protons creates a proton gradient that drives ATP synthase to synthesize ATP from ADP and inorganic phosphate. This is similar to the chemiosmosis process that occurs in oxidative phosphorylation.
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Electrons are added to NADP\(^+\), the terminal electron acceptor in photosynthesis, to create NADPH.
These reactions occur in photosystems I and II. These photosystems work in tandem to support photophosphorylation. Steps 1 and 2 occur in photosystem II. At the end of photosystem II, the electrons enter photosystem I where they are re-excited by light and used to complete step 4.
The light-independent reactions, also called the Calvin Cycle, occur in the stroma of the chloroplast. In this stage, ATP and NAPDH from the light-independent reactions, along with CO\(_2\), are converted to glucose or other sugars. The Calvin Cycle involves a series of controlled reactions that use the energy stored in ATP and NADPH to convert carbon dioxide into carbohydrates.
