DIFFERENCES BETWEEN PSI AND PSII
PSI (Photosystem I) and PSII (Photosystem II) are two integral components involved in the process of photosynthesis, but they have several key differences:
1. Location: PSI is located on the outer surface of the thylakoid membrane of the chloroplasts, while PSII is found in the inner surface of the thylakoid membrane. PSI is often referred to as "stromal lamellae," and PSII as "granal lamellae."
2. Photosynthetic Role: PSI primarily functions in cyclic electron flow and non-cyclic electron flow, which involves the transfer of electrons towards ferredoxin and NADP+ reduction. In contrast, PSII is responsible for the initial step of the light-dependent reactions, capturing light energy and performing oxygen-evolving photosynthesis.
3. Absorption Spectra: PSI absorbs light best at longer wavelengths (700nm) and reflects shorter wavelengths (680nm), while PSII absorbs light optimally at shorter wavelengths (680nm) and reflects longer wavelengths (700nm).
4. Pigment Composition: PSI mainly contains chlorophyll a, chlorophyll b, and a group of unique pigment molecules called P700, which give PSI its absorption peak at 700nm. Conversely, PSII contains chlorophyll a, chlorophyll b, and a different set of pigments called P680 that absorb light best at 680nm.
5. Oxygen Evolution: PSII is the only photosystem involved in oxygenic photosynthesis, responsible for the production of molecular oxygen (O2) through photolysis of water molecules. PSI does not participate directly in oxygen evolution.
6. Order in Electron Transport Chain: PSII functions as the initial electron donor in the light-dependent reactions, passing electrons to PSI through a series of electron carriers, including plastoquinone. PSI then takes the electrons to reduce NADP+ to NADPH.
In summary, PSI and PSII have distinct roles and functionalities within the photosynthetic process, differing in location, absorption spectra, pigment composition, oxygen evolution, and their respective positions in the electron transport chain.
PSI and PSII are two distinct photosystems found in the thylakoid membranes of chloroplasts. Here are some of the key differences between PSI and PSII:
1. Location: PSI, or Photosystem I, is located further downstream in the electron transport chain compared to PSII, or Photosystem II. PSII functions first in the light reactions of photosynthesis, followed by PSI.
2. Pigment Composition: PSI contains more chlorophyll a molecules with absorption maxima at longer wavelengths (around 700 nm), resulting in the ability to absorb light in the red portion of the spectrum. In contrast, PSII contains more chlorophyll a molecules with absorption maxima at shorter wavelengths (around 680 nm), allowing for the absorption of light in the blue portion of the spectrum.
3. Electron Source: PSII is responsible for the initial step in photosynthesis, where it receives electrons from water molecules that are split apart during the light-dependent reactions. PSI receives electrons from the electron carrier molecule, plastocyanin, which ultimately gets its electrons from PSII.
4. Production of ATP and NADPH: PSII generates ATP (adenosine triphosphate) through cyclic and non-cyclic photophosphorylation, and it also generates NADPH (nicotinamide adenine dinucleotide phosphate) as a reducing agent. PSI primarily generates ATP through cyclic photophosphorylation and provides additional reducing power to produce NADPH.
5. Role in the Z-Scheme: The Z-scheme describes the electron flow during photosynthesis. PSII operates at a lower energy level and receives electrons from water, while PSI operates at a higher energy level and receives electrons from PSII. This allows for the transfer of electrons, which generate chemical energy and eventually leads to the formation of ATP and NADPH.
In summary, PSI and PSII have different roles in the light reactions of photosynthesis and differ in terms of their pigment composition, electron source, and production of ATP and NADPH.