The key difference between photosystem 1 and photosystem 2 is that the photosystem 1 has a reaction centre composing of chlorophyll a molecule of P700 that absorbs light at a wavelength of 700 nm. On the other hand, the photosystem II has a reaction centre comprising chlorophyll a molecule of P680 that absorbs light at a wavelength of 680 nm.
Photosystems are a collection of chlorophyll molecules, accessory pigment molecules, proteins and small organic compounds. There are two main photosystems; photosystem I (PS I) and photosystem II (PS II), present in the thylakoid membranes of chloroplasts in plants. Both carry out the light reaction of photosynthesis. Accordingly, plants essentially need both these photosystems. It is because the stripping electrons from water require more energy than light-activated photosystem I can supply. Consequently, photosystem II can absorb shorter wavelength (higher energy) light and links in tandem to PS I, enabling non-cyclic electron flow.
CONTENTS
1. Overview and Key Difference
2. What is Photosystem 1
3. What is Photosystem 2
4. Similarities Between Photosystem 1 and Photosystem 2
5. Side by Side Comparison – Photosystem 1 vs Photosystem 2 in Tabular Form
6. Summary
What is Photosystem 1?
Photosystem I (PS I) is one of the two photosystems that involve in the light reaction of photosynthesis in plants and algae. Photosystem I discovered before the photosystem II. In contrast to PS II, PS I contains more chlorophyll a than the chlorophyll b. Also, PS I is present on the outer surface of the thylakoid membranes and can be easily visualized than the PS II. Furthermore, PS I participates in the cyclic phosphorylation and produces NADPH.
Moreover, there are two main parts in a photosystem such as an antenna complex (light-harvesting complex of pigment molecules) and a reaction centre. There are about 200-300 pigment molecules in a light-harvesting complex. Different pigment molecules are found in the photosystem to collect light and transfer from one to another and finally hand over to a specialized chlorophyll a molecule of the reaction centre. Photosystem I has a reaction centre composed of a chlorophyll a molecule of P700. It is capable of absorbing the light at wavelength 700 nm.
Figure 01: Light Reaction of Photosynthesis
When the light-harvesting complex of PS I absorbs energy and hands over to its reaction centre, the chlorophyll a molecule in the reaction centre excites and releases high energy electrons. These high energy molecules go via electron carriers while releasing their energy. Finally, they come to the reaction centre of PS II. When electrons travel via the electron transport chain, it produces NADPH.
What is Photosystem 2?
Photosystem II or PS II is the second photosystem that involves light dependent photosynthesis. It contains a reaction centre composed of chlorophyll a molecule of P680. PS II absorbs light at a wavelength of 680 nm. Furthermore, it contains more chlorophyll b pigments than chlorophyll a. PS II is present in the inner surfaces of thylakoid membranes. PS II is important since photolysis of water occurs associating with it. Furthermore, photolysis produces molecular oxygen that we breathe. Hence, similar to PS I, PS II is also extremely important for all living organisms.
Pigment molecules absorb light energy and transfer to P 680 chlorophyll molecules in the reaction centre of PS II. Hence, when P680 receives energy, it gets excited and releases high energy molecules. Consequently, primary electron acceptor molecules pick these electrons and finally hand over to PS I via going through a series of carrier molecules like cytochrome.
Figure 02: Photosystem II
When electrons are transferred through electron carriers of low energy levels, some of the energy released is used in the synthesis of ATP from ADP through a process called photophosphorylation. At the same time, light energy splits water molecules through the photolysis. Photolysis produces 4 water molecules, 2 oxygen molecules, 4 protons, and 4 electrons. These produced electrons replace the electrons lost from chlorophyll a molecule of PS I. Eventually, molecular oxygen evolves as a byproduct of photolysis.
What are the Similarities Between Photosystem 1 and Photosystem 2?
- Both PS I and PS II participate in the light-dependent reactions of photosynthesis. They are equally important in photosynthesis.
- They have two major parts such as antenna complex and reaction centre.
- Furthermore, they contain photosynthetic pigments that can absorb different wavelengths of sunlight.
- Also, both are present on the thylakoid membranes of the granna of chloroplasts.
- Besides, reaction centre of each photosystem comprises a chlorophyll a molecule.
What is the Difference Between Photosystem 1 and Photosystem 2?
Photosystem I has a chlorophyll a molecule of P700 in its reaction centre while Photosystem II has a chlorophyll a molecule of P680 in its reaction centre. Thus, PS I absorbs light at a wavelength of 700 nm while PS II absorbs light at a wavelength of 680 nm. Therefore, we can consider this as the key difference between photosystem 1 and photosystem 2. Both photosystems participate in the light-dependent reaction of photosynthesis. However, PS I involves in cyclic phosphorylation while PS II involves in noncyclic phosphorylation. Thus, it is also a difference between photosystem 1 and photosystem 2.
Moreover, a further difference between photosystem 1 and photosystem 2 is that the PS I is rich in chlorophyll-a pigments while PS II is rich in chlorophyll b pigments. Also, one important difference between photosystem 1 and photosystem 2 is the photolysis process. Photolysis occurs in PS II while it does not occur in PS I. Similarly, molecule oxygen evolves from the PS II while it does not occur in PS I. Furthermore, photosystem I is present in the outer surface of thylakoid membranes while photosystem II is present in the inner surface of the thylakoid membranes. Therefore, this is also a significant difference between photosystem 1 and photosystem 2.
Below infographic on the difference between photosystem 1 and photosystem 2 provides more information on these differences.
Summary – Photosystem 1 vs Photosystem 2
Photosystem I and Photosystem II are two major photosystems that carry out light-dependent reactions of photosynthesis in plants. PS I involves in cyclic phosphorylation while PS II involves in noncyclic phosphorylation. The reaction centre of PS I contains chlorophyll a molecule of P700 while the reaction centre of PS II contains chlorophyll a molecule of P680. Accordingly, PS I absorbs light at a wavelength of 700 nm while PS II absorbs light at a wavelength of 680 nm. Photolysis of water and production of molecular oxygen occur associating the PS II while those two events do not occur in PS I. Thus, this is the summary of the difference between photosystem 1 and photosystem 2.