Photosynthesis is a process that captures energy from the sun to produce sugars. It occurs in both prokaryotes like cyanobacteria and eukaryotic autotrophs. In eukaryotic autotrophs, photosynthesis takes place in the chloroplast.
The chloroplast is a double-membraned organelle that is compartmentalized into thylakoids, which are stacked into structures called grana. The fluid region outside of the thylakoids is called the stroma. Compartmentalization of the chloroplast helps to increase surface area and to decrease competition between competing interactions.
This increases the efficiency of the reactions that need to take place. The two main processes that occur in photosynthesis are the light dependent reactions, which take place in the thylakoid, and the Calvin cycle, which takes place in the stroma. Let's first look at the light dependent reactions.
which specifically take place in the thylakoid membrane and in the thylakoid compartment. Embedded into the thylakoid membrane is a series of proteins, including photosystem II, a proton pump, and photosystem I. There are several other membrane proteins as well that are involved in electron transport.
Water is split in the light-dependent reactions. This results in H plus ions, or protons, electrons, and oxygen gas as a byproduct. The electrons enter photosystem II.
The photosystems contain chlorophyll, a pigment that is capable of absorbing light energy from the sun. Photosystem II absorbs light energy, which is used to boost electrons to a higher energy level. The electrons become excited.
The high energy electrons are then passed through an electron transport chain, or ETC, in a series of chemical reactions. The reactions that occur as the electrons are transferred release energy that is utilized by the proton pump. The proton pump uses the energy from electron transport to establish a proton gradient. Protons are actively transported from the stroma to the thylakoid compartment. There is now an electrochemical gradient, a difference in both proton concentration and charge across the thylakoid membrane.
The proton gradient is necessary to produce ATP energy, which we will talk about in a moment. First, we'll talk about what happens to the electrons in the ETC. The electrons, now with lower energy, enter photosystem I.
Photosystem I contains the pigment chlorophyll, which absorbs light energy and excites the electrons once again. What's the purpose of re-exciting the electrons if we've already pumped protons and established a concentration gradient? High energy electrons are going to be picked up by a molecule called NADP. NADP is the final electron acceptor.
When NADP picks up electrons, it becomes NADPH. NADPH carries these electrons to the Calvin cycle to power reactions. Simply put, NADPH is an electron carrier molecule, and high energy electrons are needed for certain reactions to take place. Now we need to address the purpose of the proton gradient and an enzyme called ATP synthase. ATP synthase is a membrane enzyme that creates ATP by joining inorganic phosphate with ADP.
The flow of protons down their concentration gradient, from high to low concentration, is called chemiosmosis. ATP synthase is powered by chemiosmosis. As protons flow through the enzyme, the energy is used to create ATP from ADP and inorganic phosphate. ADP stands for adenosine diphosphate, di meaning two phosphate groups.
With the addition of inorganic phosphate, we can make energy-rich ATP, adenosine triphosphate. The addition of a phosphate group is called phosphorylation. Because the original source of energy came from the Sun this process is referred to as photophosphate So, photophosphorylation by ATP synthase is powered by chemiosmosis, but the original source of energy came from light.
Both the NADPH and ATP produced in the light-dependent reactions go on to the Calvin cycle. The Calvin cycle takes place in the stroma. During the Calvin cycle, a complex series of enzyme-catalyzed reactions converts CO2 into organic carbohydrates that become sugars.
NADPH and ATP power the reactions of the Calvin cycle. The conversion of NADPH to NADP provides electrons to facilitate reactions. The conversion of ATP to ADP releases energy which can power metabolic processes. Let's summarize what we've learned.
The light dependent reactions occur in the thylakoids. Light energy absorbed by chlorophyll in photosystems is used to excite electrons that power a proton pump to establish a proton gradient. ATP synthase makes ATP from ADP and inorganic phosphate. Photophosphorylation is powered by chemiosmosis. Excited electrons are picked up by NADP, the final electron acceptor in the ETC, to become NADPH.
NADPH goes to the Calvin cycle along with ATP. The Calvin cycle occurs in the stroma. ATP and NADPH are used to power reactions that convert carbon dioxide into organic carbohydrates. ADP and NADP return to the light-dependent reactions.