Photo-Phosphorylation: Cyclic and Non-cyclic and Effects of Red Drop and Enhancement


The process of producing ATP from ADP and inorganic phosphate in the presence of light is known as Photophosphorylation.

Photophosphorylation can be divided into two categories:

1. Photophosphorylation that is not cyclic 

2. Photophosphorylation in a cyclic manner

Photophosphorylation: Cyclic and Non-cyclic

A phenomenon known as Non-cyclic Photophosphorylation happens when the two photosystems act in sequence; first PS II, then PS I.

1. PSII absorbs red light with a wavelength of 680 nm, which excites electrons, which are then accepted by an electron acceptor, which takes them to an electron transport system, which then transmits the electrons to PSI.

2. When a wavelength of 700 nm is received, electrons in PSI are simultaneously stimulated.

3. Electrons are transported from the electron acceptor to the NADP+ molecule.

4. Adding these electrons to NADP+ reduces it to NADPH+ H+. This method of ATP production is known as non-cyclic photophosphorylation because the electrons lost by PSII do not return to it. When just PS I is active, the electron is circulated inside the photosystem, and phosphorylation occurs as a result of the cyclic flow of electrons. Phosphorylation could take place in the stroma lamellae. PS II and the NADP reductase enzyme are absent from the stroma lamellae. The excited electron is returned to the PS I complex rather than passing on to NADP+. As a result, the cyclic flow solely results in the production of ATP and not NADPH+H+.

When only light with wavelengths greater than 680 nm is available for excitation, Cyclic Photophosphorylation occurs. Only PSI is operational in this system. As a result, electrons circulate throughout the photosystem, resulting in a cyclic flow.

1. Because stroma lamellae lack both PSII and NADP reductase enzymes, this scheme could be occurring there.

2. Only ATP is synthesized by this cyclic flux, not NADPH + H+.

The hypothesis of Chemiosmotic Exchange

The chemiosmotic hypothesis explains ATP synthesis in chloroplasts. ATP generation during photosynthesis is triggered by the formation of a proton gradient across a membrane, such as a thylakoid membrane. The creation of a proton gradient across the thylakoid membrane involves the phases listed below.

1. Protons are transferred across the membrane as electrons flow through photosystems. The principal electron acceptor is found on the membrane’s outer surface. It transfers its electrons to an H carrier rather than an electron carrier. As a result, while delivering an electron, it removes a proton from the stroma. The proton is released into the inner side or lumen side of the membrane when an electron is delivered to the electron carrier on the inner side of the membrane.

2. The NADP reductase enzyme is found on the membrane’s stroma side. Protons are required for the conversion of NADP+ to NADPH+H+. The stroma is likewise cleared of these protons.

3. As a result, the number of protons in the stroma decreases, and they accumulate in the lumen. As a result, a proton gradient develops across the thylakoid membrane. In addition, the pH in the lumen has dropped noticeably.

4. When this gradient breaks down, energy is released. The breakdown of this gradient is caused by the passage of protons over the membrane to the stroma. The F0 of the ATPase has a transmembrane channel that allows protons to pass through.

5. There are two sections to the ATPase enzyme. The F0 is a component that is embedded in the membrane. This creates a transmembrane channel that allows protons to diffuse more easily across the membrane. F1 refers to the remaining portion. It protrudes on the lumen side of the thylakoid membrane’s outer surface.

5. When the gradient breaks, enough energy is released to trigger a shift in the ATPase’s F1 particle, resulting in the production of many molecules of energy-dense ATP.

To summarise, chemiosmosis necessitates the presence of a membrane, a proton pump, a proton gradient, and an ATPase. To establish a gradient of protons within the thylakoid membrane, energy is used to pump protons across a membrane. The ATPase enzyme has a channel. Protons can diffuse back across the membrane through this channel. The energy released by protons is enough to activate the ATPase enzyme. The ATPase enzyme is responsible for the production of ATP. The biosynthetic reaction that takes place in the stroma uses NADPH and ATP. This process is responsible for CO2 fixation and sugar synthesis.

A) Spectra of Absorption

The absorption spectrum shows how a pigment absorbs light of different wavelengths. In a spectrometer, a pigmy solution. It tells you what wavelength pigment absorbs. The absorption spectrum is calculated by plotting the light absorbed by the pigment versus wavelength.

1. The violet and orange-red regions of the visible spectrum are where chlorophyll a and b absorb the most light. Only a small amount of green and light is absorbed. Green plants use blue and red light as a source of energy for photosynthesis. The absorption of chlorophyll peaks at around 680 and 700 nm.

2. Between 449 and 490 mu, carotenoids absorb radiant radiation. The absorption peaks of carotene are 449 and 478 nm.

3. The peak wavelengths of xanthophylls are 440 and 490 nm.

B) Spectra of Action

 The action spectrum shows the relative efficacy of different wavelengths of ht in photosynthesis. The pigments that are truly active in photosynthesis are confirmed by the action spectrum. Action spectrum also found the two photosystems. Spectrographs are used to measure the action ctra. A sample is placed in a tram with a greater area. It is illuminated by monochromatic light. The action of the substance is then measured using a spectrograph.

The absorption spectra and the action spectrum are compared

All pigments have an absorption spectrum (Chl a. b, carotene, and xanthophylls). However, only chlorophyll provides the action spectrum. The reaction center is formed by chlorophyll a. The absorption spectra and the action spectrum of chlorophyll are vastly different. The action spectrum’s peaks are higher and its dips are narrower than the absorption spectrum’s. It denotes that all other pigments are merely decorative. Their energy is transferred to chlorophyll a. As a result, it produces more photosynthesis than it absorbs light. As a result, its action spectrum appears to be more effective than its absorption spectrum.

Effects of Red Drop and Enhancement

Emerson and Lewis’ demonstration of the red drop proved perplexing. Because the quantum yield only measures the light that has been absorbed, this is not attributable to a decrease in light absorption. This means that light with wavelengths longer than 680 nm is inefficient compared to light with shorter wavelengths. After modifying their fluorescence rates to give equivalent rates of photosynthesis, Emerson and his colleagues evaluated photosynthesis using red and far-red light in subsequent tests. The quantum yield obtained by combining red and far-red light was substantially higher than the sum of the yields obtained by combining red and far-red light separately. Emerson enhancement effect, or Emerson effect, is the name given to this occurrence.

These perplexing red drop and amplification effects led to the conclusion that photosynthesis involves two separate reaction centers or photochemical activities. Red light ( 680 nm) drives one event, while far-red light (> 680 nm) drives the other. When both activities are driven simultaneously or in rapid succession, optimal photosynthesis occurs. These two photochemical reactions are now referred to as Photosystem II and Photosystem I, and they work in tandem to optimize photosynthesis. Photosystem II absorbs red light with a wavelength of 680 nm well but is poorly driven by far-red light. Photosystem I, on the other hand, prefers to absorb far-red light with wavelengths larger than 680 nm.

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