As a result of the light-absorbing pigment molecules that are found in their leaves, plants and other photosynthetic creatures are masters at the process of gathering energy from the sun. However, what happens to the light energy that is soaked in by the substance? Even while we don’t see the leaves of plants shining like light bulbs, we do know that energy can’t suddenly vanish into thin air (thanks to the First Law of Thermodynamics).
It has been discovered that a portion of the light energy that is taken in by pigments in leaves is changed into a new kind of energy known as chemical energy. During the initial stage of photosynthesis, which consists of a sequence of chemical processes known as the light-dependent reactions, the energy from light is turned into the energy that may be used by the plant’s cells to create new chemical bonds.
In this piece, we are going to investigate the light-dependent processes that take place in plants during the process of photosynthesis. In this section, we will analyze how pigment molecules take in light energy, how reaction center pigments transfer excited electrons to an electron transport chain, and how the energetically “downhill” movement of electrons results in the production of ATP and NADPH.
The light-dependent reactions and the light-independent reactions, often known as the Calvin cycle, are the two steps that make up the process of photosynthesis. When there is sufficient light, the reactions that rely on light will take place. The general equation for photosynthesis demonstrates that it is a redox process; in order to create oxygen, carbon dioxide is reduced while water is oxidized.
Energy + 6CO2 + H2O → C6H12O6 + 6O2
In chloroplasts, the light-dependent processes take place in the thylakoid membranes, whereas the Calvin cycle takes place in the stroma of the chloroplasts. Two photosystems, known as PS I and PS II, are found embedded inside the membranes of thylakoids. These photosystems are complexes of pigments that are responsible for absorbing sun energy. The wavelengths of visible light that are violet, blue, and red are taken in by chlorophylls a and b, whereas green is reflected by these pigments.
Carotenoids are pigments that reflect light in the yellow-to-orange spectrum and absorb light in the violet-blue-green range. The main colours at different times of the year are determined by environmental variables such as the length of the day and the temperature. Even if the two photosystems are both active at the same time, it is much simpler to investigate them independently. To get started, let’s talk about photosystem II.
The process of photosynthesis is kicked off when a photon of light collides with the antenna pigments of PS II. In the noncyclic route, PS II is able to absorb photons at an energy level that is marginally greater than that of PS I. (It is important to keep in mind that light with shorter wavelengths carries greater energy.)
The energy that is taken in is transferred to the reaction center of the antenna pigment, which is made up of chlorophyll a, where it raises the energy level of the electrons that make up the chlorophyll a. Following the acceptance of the electrons by a primary electron acceptor protein, they are subsequently transferred to the electron transport chain, which is likewise encased inside the thylakoid membrane.
The amount of energy that is taken in by PS II is sufficient to oxidize (split) water, which results in the release of oxygen into the environment. The electrons that are released as a result of the oxidation of water replace the electrons that were enhanced by chlorophyll in the reaction center. During the process in which electrons from the reaction center chlorophyll travel via a sequence of electron carrier proteins, hydrogen ions (H+) are pushed over the membrane and into the interior of the thylakoid through the process of chemiosmosis.
If you think you’ve heard this before, you probably have. As part of our investigation into cellular respiration (which we covered in Cellular Respiration), we looked at chemiosmosis. Because of this activity, a very high concentration of H+ ions is created, and when these ions pass through ATP synthase, molecules of ATP are produced as a byproduct of this process. During the Calvin cycle, which is the second stage of photosynthesis, these molecules of ATP will be utilized to give free energy.
This energy will be used for the production of carbohydrates. PS II and PS I are linked together via the electron transport chain. In a manner similar to that which takes place in PS II, this second photosystem is responsible for the absorption of a second photon of light, which ultimately leads to the production of a molecule of NADPH from NADP+. Additionally, the chemical events that make up the Calvin cycle are powered by the energy that is carried in NADPH.
An very large quantity of electromagnetic radiation is produced by the sun (solar energy). Only a small percentage of this energy can be seen by humans; the portion that can be seen is thus referred to as “visible light.” Waves are one way to think about the transmission of solar energy from one place to another. By measuring a wave’s wavelength, or the distance that separates each successive point on a wave, scientists are able to calculate the amount of energy that the wave has. The height of a single wave is determined by taking its measurement from two positions in quick succession, such as from one peak to the next or from one trough to the next.
There are many other kinds of electromagnetic radiation that are released by the sun and other stars, but visible light is just one of them. Within the realm of the electromagnetic spectrum, researchers categorize the many distinct forms of radiant energy that come from the sun. The electromagnetic spectrum may be thought of as a range that encompasses all of the conceivable radiation frequencies (). The quantity of energy that is carried by different wavelengths is what determines the difference between them.
Every kind of electromagnetic radiation has its own unique wavelength at which it travels. When the wavelength grows longer (or when it looks to be more spread out in the figure), the amount of energy that can be conveyed decreases. The greatest energy is carried by waves that are shallow and close together. This could make no sense at first, but if you picture a piece of moving a hefty rope, it might make more sense. It doesn’t take much effort for a person to move a rope when the waves are long and broad. A person would need to expend a substantial amount of additional energy in order to make a rope move in quick, compact waves.
The electromagnetic spectrum displays many different kinds of electromagnetic radiation coming from the sun, such as X-rays and ultraviolet (UV) rays. [Case in point:] [Case in point:] The fact that higher-energy waves may enter tissues and cause damage to cells and DNA explains why X-rays and UV rays can be hazardous to living beings. UV rays are also known to cause skin cancer.
When pigments absorb light, this releases light energy, which then kickstarts the process of photosynthesis. Organic pigments, whether they are found in the retina of a human or in the thylakoid membrane of chloroplasts, have a limited range of energy levels that they are able to absorb. An orbital electron cannot be raised to a populatable, excited (quantum) state at energy levels lower than those indicated by red light because this amount of energy is inadequate.
In the process known as bleaching, the molecules will be physically torn apart if the energy levels are greater than those seen in blue light. Because retinal pigments can only “see” (absorb) light with a wavelength between 700 and 400 nanometers, this range of light is referred to as visible light. Plant pigment molecules can only absorb light with a wavelength between 700 nm and 400 nm for the same reasons; plant physiologists refer to this range as the photosynthetically active radiation for plants.
The spectrum of hues that make up the visible light that humans perceive as white light is truly present. White light may be seen in its component colors when certain things, like a prism or a drop of water, break up the light into its component wavelengths. The section of the electromagnetic spectrum that corresponds to visible light is represented by a rainbow of colors, with violet and blue having shorter wavelengths and, thus, more energy. On the opposite side of the spectrum, toward red, the wavelengths are longer and contain less energy because they are further apart.
There are many distinct types of pigments, each of which is capable of absorbing just certain wavelengths (colors) of visible light. Pigments are able to appear in their appropriate colors because they reflect or transmit the wavelengths of light that they are unable to absorb.
There are two primary categories of photosynthetic pigments that are present in plants and algae. These categories are chlorophylls and carotenoids, and each category has a wide variety of pigment molecules. There are five primary types of chlorophyll, which are designated as a, b, c, and d, in addition to bacteriochlorophyll, which is a similar chemical found in prokaryotes. Both chlorophyll a and chlorophyll b may be found in the chloroplasts of higher plants. These two pigments will be the primary focus of the next conversation.
Carotenoids are a significantly bigger category than the other pigments, and they come in dozens of distinct forms. Carotenoids, which are present in fruit, are employed as ads to attract seed dispersers. Examples of these types of carotenoids include the red pigment found in tomatoes (lycopene), the yellow pigment found in maize seeds (zeaxanthin), and the orange pigment found in orange peel (beta-carotene). In the process of photosynthesis, carotenoids play the role of photosynthetic pigments.
These pigments are molecules that are particularly effective at getting rid of surplus energy. When a leaf is exposed to direct sunlight, the light-dependent processes inside the leaf are needed to process a massive quantity of energy. If this energy is not managed correctly, it is capable of causing considerable harm to the leaf. As a result, several carotenoids call the thylakoid membrane their home. These carotenoids are able to safely absorb extra energy and then release it as heat.
The exact arrangement of visible light wavelengths that a certain kind of pigment takes in is referred to as its absorption spectrum, and it may be used to identify the pigment. The absorption spectra for chlorophyll a, chlorophyll b, and a form of carotenoid pigment called beta-carotene are shown in the graph located here (which absorbs blue and green light). Take note of the fact that each color exhibits its own unique pattern of absorption by exhibiting its own unique collection of peaks and troughs.
Chlorophyll an is able to absorb longer wavelengths of light from the blue and red ends of the visible spectrum, but not green light. Chlorophyll gives off a green appearance because green may be reflected or transmitted. Carotenoids have the ability to absorb light with shorter wavelengths, such as blue, and reflect light with longer wavelengths, such as yellow, red, and orange.
Numerous photosynthetic organisms have a variety of pigments. By using these pigments, the organism is able to absorb energy from a larger spectrum of wavelengths. There are many different kinds of photosynthetic organisms, and not all of them can fully absorb sunlight. Underwater, where the strength of the light and the quality of the light both fluctuate with depth, some creatures are able to flourish. The presence of light encourages the growth of other creatures. Because the higher trees in the rainforest block the majority of the sunlight and scatter what little is left, the vegetation that grows on the forest floor must be able to take in even the little amount of light that does penetrate.
In the process of researching a photosynthetic organism, researchers might generate absorption spectra in order to identify the many kinds of pigments that are present. A piece of apparatus known as a spectrophotometer is able to determine which particular wavelengths of light a particular material can absorb.
Spectrophotometers are used to measure the amount of light that is transmitted and then calculate the amount of light that is absorbed from that. Scientists are able to determine the wavelengths of light that an organism can absorb by first extracting pigments from the leaves of the organism and then inserting these samples into a spectrophotometer. Chromatography, in its many forms, is another technique that may be used in the process of identifying plant pigments. This technique separates pigments according to their relative affinities to solid and mobile phases.
The overarching purpose of reactions that are reliant on light is to transform the solar energy that is present into chemical energy in the form of NADPH and ATP. This chemical energy is what enables the processes to occur even in the absence of light and what drives the building of sugar molecules. The responses that are affected by light are shown here. Together, protein complexes and pigment molecules are responsible for the production of NADPH and ATP.
In a multiprotein complex known as a photosystem, the actual procedure that turns light energy into chemical energy takes place. There are two kinds of photosystems that are found embedded in the thylakoid membrane; these are photosystem II (PSII) and photosystem I (PSI). The two complexes are distinct from one another in terms of the substances that they oxidize (that is, the source of the electron supply with low energy) and the substances that they reduce (the place to which they deliver their energized electrons).
Both photosystems have the same fundamental structure, which consists of a reaction center surrounded by a number of antenna proteins to which chlorophyll molecules are attached and which are responsible for carrying out photochemistry. Light is captured by the light-harvesting complex, which then transfers that energy to the reaction center. The light-harvesting complex is made up of multiple antenna proteins and contains a mixture of 300–400 chlorophyll a and b molecules in addition to other pigments such as carotenoids.
Each photosystem is serviced by the light-harvesting complex. Any one of the chlorophylls may get excited when it takes in a single photon, a discrete amount of light, or a “packet” of light. This causes the molecule to enter an excited state. To summarize, the light energy has been absorbed by biological molecules at this point, but it has not yet been stored in any form that is helpful. After what seems like a millionth of a second, the energy finally makes its way to the reaction center after being passed on from one chlorophyll to the next. To this point, there has been no exchange of electrons between molecules; only energy has been passed between them.
In the core of the process is a pair of chlorophyll a molecules, each of which has a unique characteristic. The excitement of these two chlorophylls may cause them to undergo oxidation, which means that they are able to donate an electron as part of a process known as a photoact. At this point in the photosynthesis process, the light energy is being turned into an excited electron. This takes place at this particular phase in the reaction center. In each of the ensuing stages, the electron must be transferred onto the energy carrier NADPH in order for it to be transported to the Calvin cycle.
Once there, the electron must be deposited onto carbon in the form of a carbohydrate in order to be stored for the long term. The photosynthetic electron transport chain also comprises the cytochrome complex as one of its primary components. PSI and PSII are two important components of this chain. The cytochrome complex is an enzyme that is composed of two different protein complexes. It is responsible for moving electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc). This action makes it possible for protons to be moved across the thylakoid membrane as well as for electrons to be moved from PSII to PSI.
The high-energy electrons are sent one at a time from the reaction center of PSII, which is termed P680, to the main electron acceptor. These electrons are then transported to PSI through the electron transport chain, which goes from Pq via the cytochrome complex to plastocyanine. After each photoaction, the missing electron in P680 is “replaced” by removing a low-energy electron from water; this results in the splitting of water molecules and the re-reduction of PSII.
The dissociation of a single molecule of H2O results in the release of two electrons, two hydrogen atoms, and one oxygen atom. In order to produce one molecule of diatomic oxygen gas, two molecules must first be split apart. In order to maintain oxidative phosphorylation, the mitochondria in the leaf use around ten percent of the oxygen present. The remaining is lost to the atmosphere, where it is taken up by organisms that participate in aerobic respiration and consumed as fuel.
Electrons lose their energy when they travel through the proteins that are located between the PSII and PSI levels. This energy is put to use in the process of transporting hydrogen atoms across the membrane, from the stromal side to the thylakoid lumen. These hydrogen atoms, in addition to the ones created by the process of splitting water, are accumulated in the thylakoid lumen, where they will be employed in a subsequent phase in the process of ATP synthesis. Because the electrons had lost energy before they arrived to PSI, it is necessary for PSI to re-energize them. As a result, another photon is taken in by the PSI antenna in order to do this task.
The PSI response center receives this energy, and it is sent to it (called P700). P700 undergoes oxidation, which results in the transfer of a high-energy electron to NADP+, which leads to the formation of NADPH. Therefore, PSII stores the energy required to generate proton gradients, which is necessary for the production of ATP, while PSI stores the energy required to convert NADP+ into NADPH. The two photosystems collaborate with one another, in part, to ensure that the amount of ATP and NADPH that are produced will be about equal to one another. There are further methods that can be used to fine-tune that ratio so that it precisely meets the chloroplast’s ever-evolving need for energy.
The accumulation of hydrogen ions within the thylakoid lumen, which occurs during cellular respiration, generates a concentration gradient in the same way as it does in the intermembrane space of the mitochondria. In the same way that electrons are transported through the electron transport chain during cellular respiration, ATP may be generated by harnessing the passive diffusion of hydrogen ions from high concentrations (in the thylakoid lumen) to low concentrations (in the stroma). The ions accumulate energy due to diffusion and the fact that they all have the same electrical charge, which causes them to be attracted to one another.
In order to free up this energy, hydrogen ions will burst through every opening they can find, much to how water would rush through a breach in a dam. This opening is a pathway via a particular protein channel known as the ATP synthase and it may be found in the thylakoid membrane. Because of the energy that is provided by the stream of hydrogen ions, ATP synthase is able to add a third phosphate group to ADP, which ultimately results in the formation of an ATP molecule (Figure 8.16). The process by which hydrogen ions travel from an area of high concentration to an area of low concentration inside ATP synthase is referred to as chemiosmosis. This is because the ions move from one region to another via a structure that is only partially permeable.
As the amount of carbon dioxide drops, the rate of photosynthesis will become more difficult to maintain.
The Calvin cycle is what ultimately makes use of the ATP and NADPH that are produced by the light reactions.
in the beginning
Both of these activities take place within organelles that have two layers of membranes surrounding them and require a variant of the cytochrome complex.
When photons hit photosystem (PS) II, the light energy is transferred from pigments to chlorophyll a molecules, which subsequently excite an electron. This electron is then transferred to the electron transport chain. The cytochrome complex is responsible for the transfer of electrons from PS-II to PS-I as well as the proton transport through the thylakoid membrane. The components of the reaction that is light dependent are what provide the energy for the Calvin cycle, which is what produces glucose.
because these mechanisms are less sophisticated in prokaryotes than in eukaryotes, indicating an earlier evolutionary start, and because anaerobes were better suited to early atmospheric conditions. anaerobic prokaryotes.
Redox processes are carried out by plastoquinone and plastocyanine, which make it possible for electrons to proceed farther down the electron transport chain and into Photosystem I.
The reduction of NADP+ to NADPH is caused by electrons that come from PS I.
P700 undergoes oxidation, which results in the transfer of a high-energy electron to NADP+, which leads to the formation of NADPH. Therefore, PSII stores the energy required to generate proton gradients, which is necessary for the production of ATP, while PSI stores the energy required to convert NADP+ into NADPH.
Since NADPH is produced on the stromal side of the thylakoid membrane, it is secreted into the stroma after its formation. The “normal” version of the light-dependent processes is referred to as non-cyclic photophosphorylation. This process involves the removal of electrons from water, which are then transferred to PSII and PSI before ultimately being converted into NADPH.
The reactions that are depending on light.
Light energy is converted into chemical energy via a series of processes that are reliant on light. In the light-dependent processes of photosynthesis, the primary objective is to harvest energy from the sun in order to catalyze the production of ATP and NADPH from the breakdown of water molecules. After that, these two molecules that can store energy are used in the processes that don’t need light.
After that, an electron with a high energy level is transferred from the reaction center chlorophyll to an acceptor molecule in the electron transport chain. Following this, electrons with a high energy level are transmitted across a chain of membrane carriers, which is connected to the production of ATP and NADPH.