The light-dependent reactions
The light-dependent reactions (LDR) are a series of processes that take place in the thylakoid membrane of chloroplasts. These reactions convert solar energy into chemical energy that plants can use to create glucose from carbon dioxide and water. The LDR are powered by photons from the sun, which are absorbed by pigment molecules in the thylakoid membrane. The pigment molecules then transfer the energy to chlorophyll molecules, which use it to split water molecules and produce oxygen gas.
The energy of photons
In the light-dependent reactions, the energy of photons is used to split water molecules into oxygen and hydrogen. This process also produces ATP and NADPH.
The electron transport chain
The light-dependent reactions use light energy to make organic molecules from simple inorganic molecules. This process is used by plants and other photoautotrophs to convert solar energy into useful chemical energy.
The light-dependent reactions take place in the thylakoid membranes of chloroplasts. The thylakoids are interconnected and form stacks called grana. The light-dependent reactions occur on the surface of these thylakoid membranes.
In the light-dependent reactions, pigment molecules absorb photons and transfer their energy to reaction-center chlorophyll molecules. This energy is used to move electrons from water molecules to NADP+, producing NADPH and oxygen gas. The electrons then flow through an electron transport chain, producing a proton gradient that generates ATP from ADP by chemiosmosis.
The light-independent reactions
The light-independent reactions, also known as the dark reactions, of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes.
The Calvin cycle
The light-independent reactions, also known as the dark reactions or reductive pentose phosphate (RPP) pathway, use the electrons that chlorophyll loses to the electron transport chain to ultimately convert NADPH and ATP into glucose. The light-independent reactions take place in the stroma of the chloroplast and can be summarized by the following equation:
CO2 + H2O + energy from ATP and NADPH → glucose (C6H12O6)
This process is also known as carbon fixation. Six molecules of carbon dioxide (CO2) are combined with six molecules of water (H2O) to produce one molecule of glucose. This reaction requires the energy from both ATP and NADPH, which are generated by the light-dependent reactions. The Calvin cycle is named after Melvin Calvin, who was awarded a Nobel Prize in Chemistry in 1961 for his discovery of this process.
The role of chlorophyll
The energy for most living systems comes from the sun. Green plants are the only organisms that can capture the energy in sunlight and convert it to chemical energy in the form of carbohydrates. This process of converting sunlight to chemical energy is called photosynthesis. Photosynthesis occurs in the chloroplasts, organelles in the plant cell that contain chlorophyll. Chlorophyll gives plants their green color.
The structure of chlorophyll
Chlorophyll is a green pigment found in cyanobacteria and the chloroplasts of algae and plants. Its structure is similar to that of hemoglobin, the red pigment in blood cells that captures oxygen from the lungs and transports it to the body’s tissues. Like hemoglobin, chlorophyll contains a central metal ion, in this case magnesium. Chlorophyll absorbs light in the violet-blue and red portions of the visible spectrum and uses the energy of these photons to drive the synthesis of organic compounds from simple inorganic molecules.
The function of chlorophyll
Chlorophyll is essential for photosynthesis, which allows plants to absorb energy from light. Chlorophyll molecules are located in the chloroplasts of plant cells. These molecules absorb light energy and transfer it to other molecules in the cell, which convert the energy into a form that the plant can use to create glucose from carbon dioxide and water.
The electrons that chlorophyll loses to the electron transport system are replaced by electrons from water molecules. This process liberates oxygen gas, which is a waste product of photosynthesis.