Light dependent reactions in Photosynthesis

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Photosystems:
  
Photosystems are light harvesting complexes which directly convert energy from the photons of light to excited electrons. They are embedded in the thylakoid membranes and they consist of a reaction center (where chlorophyll a is present) and many antenna pigments that funnel light energy to the reaction center. There are two photosystems: Photosystem II (P680) and Photosystem I (P700). The two photosystems cooperate, and photosystem II is where the whole series light dependent reactions starts.

What happens when the electrons in the reaction center get excited?

One possibility: Noncyclic photophosphorylation 

We have to keep in mind that photosystem II and photosystem I cooperate. However, here I'm going to describe the process starting from photosystem II. So first the light energy is captured by photosystem II, and the antenna pigments in photosystem II pass it on to the reaction center containing chlorophyll a. Next, two electrons in the reaction center get excited by the light energy and thus they move to a higher energy level. They are captured by primary electron acceptor. But: two electrons have just been lost from the reaction center, and they need to be replaced somehow. The solution to this is photolysis. Photolysis means essentially splitting of a water molecule to two protons, two electrons and one oxygen atom. Two oxygen atoms combine to form the diatomic oxygen molecule, and the two electrons will are now replaced by the electrons from water. 

After the electrons have been accepted by an electron acceptor, they move through an electron transport chain, which consists of plastoquinone (Pq), a cytochrome complex and plastocyanin (Pc). Keep in mind that as the electrons are transferred from one electron carrier to the next (in other words: as one electron carrier is oxidized and the other gets reduced), protons from the chloroplast stroma move through the thylakoid membrane into the thylakoid space called lumen. These protons are going to be essential in ATP synthesis. The ATP synthase is also embedded in the thylakoid membrane, and the protons that have migrated to the lumen are goig to flow through the ATP synthase, providing energy for the phosphorlyation of ADP into ATP ( the reaction is ADP + Pi -> ATP). This is just like in the electron transport chain in animal cells. 

Now, at the end of the first electron transport chain, these excited electrons form photosystem II will replace the excited electrons from photosystem I. What happens in photosystem I is very similar to the scenario in photosystem II. The electrons in the reaction center get excited by the photons of light, but this time, they are replaced by electrons from photosystem II, the same ones that have moved down this first electron transport chain. 

The excited electrons from photosystem I also move down an electron transport chain. The difference here, though, is that at the end of this electron transport chain, NADPH is made. What happens is that NADP+ is reduced to NADPH because it accepts two protons and these two electrons that have travelled down this electron transport chain.

Another possibility: Cyclic flow

 When there is a shortage of ATP, but not NADPH, more ATP can be produced using the cyclic flow mechanism. The essence of the cyclic flow is recycling of electrons. This takes place when the two excited electrons in photosysthem I are are accepted by primary electron acceptor, but then they travel back to the cytochrome complex in the electron transport chain which they have already passed through (the first electron transport chain). This causes an increase in proton gradient in the thylakoid lumen, which increases the rate of the phosphorylation of ADP into ATP.

Photosynthesis (big picture)

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Photosynthesis is important for all of us. It is the mechanism the plants utilize to convert light energy from a light source (like our sun) to chemical bond energy and fix the  that enters the plant into organic compounds (glucose). This is what makes the plant alive, so that either humans can eat it and get the nutrients they need, or animals can eat the plant to obtain essential nutrients and later humans can eat the animal and get the nutrients from the animal. This is why photosynthesis is essential for life on Earth. But how do plants do it?

The process of photosynthesis takes place in the chloroplast, which is an organelle in the plant cell. Photosynthesis is a series of different reactions, which are subdivided into light dependent and light independent reactions. The chloroplast is enclosed by a double membrane, and it contains grana, which consist of layers of thylakoid membranes where the light dependent reactions occur, and stroma, where the light independent reactions occur. In order to convert this light energy into chemical bond energy that plants can use, we need some substances that can absorb this light energy first and foremost, and these substances are called pigments.

The chloroplast looks like this:

Pigments absorbing light energy:

Light energy is captured by the pigments in the plants. There are different kinds of pigments, all depending on what wavelenghts of visible light they absorb. In plants we have chlorophylls (absorb wavelengths of light in red, blue and violet range) and cartenoids (absorb wavelengths of light in blue, green and violet range). 

There are two types of chlorophyll: chlorophyll a and b. Chlorophyll b and cartenoids are called antenna pigments because they absorb the light energy from the sunrays striking them and transfer photons of light to chlorophyll a, which is directly involved in the transfer of electrons which happens in light independent reactions. The picture below shows what wavelengths different pigments absorb:

Light dependent reactions: Light energy is directly used to make ATP and NADPH

The energy from the sun that is transferred to chlorophyll a by antenna pigments is used to excite electrons in chlorophyll a-molecules to make these electrons available to be caught by primary electron acceptor and then transported through the electron transport chain.The flow of electrons in the electron transport chain provides energy to reduce ADP and NADP+ to ATP and NADPH. Double bonds in chlorophyll a molecules are important because they are source of the excited electrons. The picture below shows the structure of chlorophyll a:

Light independent reactions, or the Calvin cycle: ATP and NADPH from light dependent reactions are used to fix carbon in carbondioxide to glucose.

This is a cycle where carbondioxide is fixed to form a 3-carbon sugar, phosphoglyceraldehyde (also called PGAL or G3P), after 3 runs of the cycle, so the cycle needs to run three times to produce one molecule of glucose. The Calvin cycle occurs only in light, even though it does not directly use light energy. Instead, it uses ATP and NADPH from the light reactions and oxidizes them back to ADP and NADP+.