Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis
Living organisms require complex carbon compounds to build the structure of their cells and carry out life processes. Some organisms are able to make all the carbon compounds they need only using light energy and simple inorganic substances such as carbon dioxide and water. They do this using the process of photosynthesis.
Photosynthesis is an energy conversion, as light energy is converted into chemical energy in carbon compounds produced are carbohydrates, proteins, lipids and nucleic acids. This transformation supplies most of the chemical energy for life processes in ecosystems.
Conversion of carbon dioxide to glucose in photosynthesis using hydrogen from water
Plants convert carbon dioxide and water into carbohydrates by photosynthesis. The equation is as follows:
carbon dioxide + water –> glucose + oxygen
Hydrogen is needed for the reduction reaction that converts carbon dioxide into glucose. This hydrogen come from photolysis (a reaction that splits molecules of water using light). Hydrogen is released from water as separated protons (hydrogen ions) and electrons.
2H2O –> 4e- + 4H+ + O2
Oxygen is a waste product of this reaction. it diffuses out of photosynthesizing cells.
Oxygen as a by-product of photosynthesis
Oxygen is a by-product of photosynthesis, usually a waste product. it comes from the splitting of water in photolysis. Prokaryotes were the first organisms to perform photosynthesis, starting about 3,500 million years ago.
Photolysis increases the concentration of oxygen inside chloroplasts. This causes oxygen to diffuse out of chloroplasts and then out of leaf cells to air spaces inside the leaf. The oxygen then diffuses through stomata to the air outside the leaf.
Separation and identification of photosynthetic pigments by chromatography
Chloroplasts contain several types of chlorophyll, along with other pigments called accessory pigments. These pigments absorb different ranges of wavelengths of light, so they look different colours to us. Pigments can be separated by chromatography.
Thin layer chromatography gives the best results. This is done with a plastic strip that has been coated with a thin layer of porous material. A spot containing pigments extracted from leaf tissue is placed near one end of the strip. A solvent is allowed to run up the strip, to separate the different types of pigment.
Pigments
The first stage in photosynthesis is the absorption of sunlight. This involves chemical substances called pigments. Pigments absorb light and so appear different colours to us: the colours we see depend on the wavelengths of light the pigment absorbs and transmits.
- White and transparent substances are not pigments. White substances reflect all wavelengths of visible light, while transparent substances allow all wavelengths to pass through.
- Pigments that absorb all wavelengths of light appear black. (these pigments transform the light energy into other forms of energy, mostly heat.)
- Other pigments absorb some wavelengths of visible light but not others. For example, the pigment in a gentian flower absorbs all colors except blue. The flower appears then blue to us because this part of the sunlight is reflected and can pass into our eye, detected in the retina.
photons
A photon is a particle of light. Photons are discrete quantities of energy. The energy of a photon is related to its wavelength: the longer the wavelength the less energy a photon holds. Photons are absorbed by pigment molecules if the energy they hold causes an electron in an atom of the pigment molecule to jump to a higher energy level (excitation). A specific amount of energy is required for this to happen and this energy is only supplied by certain wavelength of light.
Chlorophyll
Photosynthesis involves a range of pigments, but the main photosynthetic pigments are chlorophylls. All forms of chlorophyll appear a shade of green to us. This is because photons in the red and blue parts of the spectrum can excite an electron in chlorophyll, but wavelengths in the green parts of the spectrum between red and blue cannot. Therefore, most greenlight is reflected. This is why green is the dominant colour and ecosystems dominated by plants.
The wavelengths of light absorbed by a pigment are shown on graphs called absorption spectrum.
- The horizontal X axis shows the wavelength of light in nanometres. The scale extends from 400 to 700 nm, reflecting the range of wavelengths invisible light. It is helpful to show the colours as well.
- The X axis shows absorption which is usually in a percentage
Similarities and differences of absorption and action spectra
An absorption spectrum is a graph showing the percentage of light absorbed at each wavelength by a pigment or a group of pigments.
An action spectrum is a graph showing the rate of photosynthesis at each wavelength of light.
When plotting both action and absorption spectra the horizontal X axis should show the wavelength of light in nanometres between 400 and 700.
On an action spectrum the Y axis should be used for a measure of the relative amount of photosynthesis. This is often given as a percentage of the maximum rate with a scale from 0 to 100.
On an absorption spectrum, the Y axis should be for the absorption of light either with a percentage scale or with arbitrary units. The spectrum for more than one pigment can be shown on the same graph.
Photosynthesis can only occur in wavelengths of light that are absorbed by chlorophyll or other photosynthetic pigments therefore the curves on action spectra will show that .
Carbon dioxide enrichment experiments
In highlight intensity and warm temperatures rate of photosynthesis are frequently limited by carbon dioxide concentration. This has been demonstrated in greenhouse experiments both temperature and light intensities are kept constant at optimal levels while carbon dioxide concentration is varied. Increasing carbon dioxide concentration above current atmospheric levels of about 400 PPM has been found to increase rates of photosynthesis and plant growth. When growing crops such as tomatoes it is now common practice to raise carbon dioxide levels.
The extra carbon dioxide can come from boilers that burn natural gas to produce heat or electricity for the greenhouse. More sustainably it can come from compost making where plant waste or animal manure is already decomposed by bacteria and fungi.
Photosystems as a raise of pigment molecules that can generate excited electrons
Photo systems are pigment protein complexes located in the keloid membranes of chloroplasts. In a typical photo system there are about 100 chlorophyll molecules and 30 accessory pigment molecules arranged in a precise molecular array. However, there is a lot of variation between the photo systems that have evolved in different organisms. Carotene and xanthophyll are examples of accessory pigments.
Each photosystem has a core complex, connected to light harvesting antenna complexes. Pigment molecules within antenna complexes absorbed light because it causes an electron in one atom of the pigment to become excited and jump to a higher energy level. A specific amount of energy is required for this to happen, this precise amount of energy is only supplied by certain wavelengths. The amount of energy decreases as the wavelength increases so for example photons of blue light have more energy than photons of red light.
The light energy that is absorbed by a pigment can be remitted as light when the electron drops back down to its original energy level. This is called fluorescence however something different happens in a light harvesting complex. When the electron in a pigment molecule drops back down to its original level, the energy admitted is absorbed by an electron in the adjacent pigment molecule causing it to become excited. This process is called excitation energy transfer and it is repeated across the light harvesting complex. In this way energy is transferred from pigment to pigment until it reaches the reaction centre in the core complex. This process happens very rapidly taking only a few femtosecond. For this energy transferred to happen the pigment molecules must be held in a precise array, in terms of both the distances between them and their relative orientations. This is achieved by the protein subunits in the light harvesting complex.
Light energy absorbed by any of the pigments in the light harvesting complex is funnel into the core complex. Eventually it reaches a special pair of chlorophyll molecules in the reaction centre. These molecules are able to donate pairs of excited electrons to electron acceptor. This completes the task of the photo system. Light energy has been absorbed, generating excited electrons. These electrons are then emitted from the Photos system, carrying the energy needed for later stages of photosynthesis. In low light intensity this process is very efficient however in highlight intensities other factors make harvesting less efficient and some of the light energy absorbed is remitted by fluorescence.
Different types of photosystems
Photosystem I is mostly located in thylakoid membranes between the Grana. These are called stroma lamellae. The primary electron donor of the reaction centre is P 700, containing a pair of chlorophyll molecules with peak light absorbance at 700 nm. Transfer of excited electrons from the primary electron donor happened to the enzyme NADP reductase, which uses the electrons to reduce NADP. Lastly, the source of replacement electrons are two electrons from plastocyanin.
Photosystem II is mostly located in thylakoid membranes in the actual grana, these being cylindrical stacks of thylakoids. The primary electron donor in the reaction centre is P6 180, containing a pair of chlorophyll molecules with peak light absorbance at 680 nm. The transfer of electrons from the primary electron donor happened to plastoquinone which transfers the electrons to a chain of electron carriers. The source of replacement electrons is the photolysis of water.
Advantages of the structured array of different types of pigment molecules in a photosystem
There are significant advantages in having pigment molecules arranged in the structured array of a photosystem:
- Photos of light are scattered. Even in high intensity light, one pigment molecule would only intercept a few photons per second. A photo system combines over 100 pigment molecules, increasing the number of protons absorbed per second x 2 orders of magnitude.
-Individual pigment molecules only absorbed light in a narrow range of wavelengths. The range varies between pigments for example chlorophyll is do not absorb green wavelengths but carotene does. A Photos system combines different types of pigments in one array, so a greater proportion of the energy in sunlight can be used.
- Energy is only transferred from one pigment molecule to another when the molecules are in close and precise orientation. Otherwise light energy is lost by fluorescence. The structured array also ensures that absorbed energy is funnel to the reaction centre of the Photosystem.
The pigment molecules in the structured array of a photo systems are interdependent. Individually they could not perform any part of photosynthesis, together they can harvest light energy very efficiently allowing for photosynthesis.
Generation of oxygen from the photolysis of water in photosystem II
Absorption of futons of light by Photosystem II causes a special chlorophyll called P680 in the reaction centre to become oxidised by emitting excited electrons. P680 is a powerful reducing agent, which is able to regain electrons from water. This happens in the oxygen evolving complex of photosystem two. The oxygen evolving complex contains a group of magnets, calcium and oxygen atoms and is in the core complex of the photo system. Next to the thylakoid space.
The oxygen evolving complex binds to water molecules and splits them to release four electrons and four protons. The remaining two oxygen bond together to produce a molecule of oxygen.
This splitting of water is called photolysis because it only happens in the light when P680 chlorophyll is oxidised, the formula is as follows:
2H2O —> O2 + 4H+ + 4e-
Photolysis happens in the oxygen evolving complex on the inner surface of the cycloid membranes. The electrons are transferred to the reaction centre, to replace those emitted by the P680 chlorophyll. The protons are released into the thylakoids space, contributing to a proton gradient across the thylakoid membrane. Oxygen molecules produced by photolysis are a waste product. They diffuse out from the thylakoids to the stroma of the chloroplasts. From there they diffuse through the cytoplasm of the cell and eventually out of the organism. In plants with leaves the oxygen diffusers through the stomata.
ATP production by chemiosmosis in thylakoids
Excited electrons generated by photosystem to are passed to plastoquinone, an electron carrier in the thyroid membrane. Plastoquinone except two electrons and also two protons from the stroma, becoming plastoquinol. This happens at binding sites in the reaction centre of photosystem 2.
Plastoquinol then moves through the thylakoid membrane to the cytochrome B6 F complex. It passes two electrons to the complex and releases two protons into the thyroid space, contributing to the proton gradient across the thyroid membrane. This converts plastoquinol back to plastoquinone , which can return to photosystem to to collect more excited protons and electrons.
The cytochrome B6 F complex contains electron transport chains, which transfer electrons from plastoquinol to another electron carrier called plastocyanin. While plastoquinone is hydrophobic and remains in the thylakoid membrane, plastocyanin is water soluble and is dissolved into the fluid space inside the thylakoid , where it is free to move. Plastocyanin picks up an electron from the B6F complex and transfers it to the reaction centre of photo system one.
The electrons reaching photosystem one carry less energy than they did when emitted by photosystem 2. Energy from the electrons has been used to pump protons from the stroma to the thylakoid space, generating a proton gradient. Photolysis also contributes to this gradient by releasing protons inside the thylakoid space. The concentration gradient of protons across the thylakoid membrane is a store of potential energy.
ATP synthase in the thylakoid membranes can generate ATP using the proton gradient. Protons travel across the membrane, down the concentration gradient, by passing through the enzyme ATP synthase. The energy released by the passage of protons is used to make ATP from ADP and an inorganic phosphate. This method of producing ATP is very similar to the process that occurs inside the mitochondria and is given the same name which is chemiosmosis.
The ATP produced by ATP synthase is released into the stroma here it provides the energy for the synthesis of sugars and other carbon compounds from carbon dioxide in later stages of photosynthesis.
Reduction of NADP by Photosystem one
Production of carbon compounds such as glucose by photosynthesis requires a supply of electrons. This is provided by NADP. NADP is identical to NAD which is used in cell respiration except that it has one extra phosphate group. Like NAD, NADP can exist in either a reduced or an oxidised state. It is converted to the reduced state by accepting two electrons.
Reduced an ADP is produced by the photosystem one. Energy from Photonz of light is absorbed by pigment molecules in the photo system and past to the reaction centre. Here it reaches a special pair of chlorophyll molecules called P 700 that act as a primary electron donor. An electron in one of these chlorophyll molecules is excited and then emitted from the reaction centre. It is passed via a short chain of electron carriers to the enzyme NADP reductase. This enzyme is positioned on the Stromer side of the thylakoid membrane where it can receive electrons from photosystem one. One ADP reductase has received too excited electrons, it can convert a molecule of NADP in the stroma to reduced NADP.
Electrons from photosystem one that are used to reduce an ADP are placed by electron carriers by plastocyanin. Photosystems one and two are therefore linked electrons excited in photo systems two are passed via plastoquinone and cytochrome b6f to plastocyanin which transfers them to photo system one.
The supply of NADP in a chloroplast sometimes runs out, because it has all been converted to reduced NADP. When this happens excited electrons photo system one are diverted to plastoquinone instead of being passed to an ADP. As the electrons flow back to the photo system via cytochrome B6F and plastocyanin , they cause proton pumping which allows ATP production by chemiosmosis. This process is cyclic photophosphorylation. It allows ATP to be produced when production of produced an ADP is impossible or unnecessary.
Thylakoids as systems for performing the light dependent reactions of photosynthesis
A thylakoid is a sack like vesicle that performs the light dependent reactions of photosynthesis. In these reactions light energy is absorbed and used to split water by photolysis, reduce NADP and produce ATP by chemiosmosis.
Cyanobacteria have thylakoids that are variable in shape and are attached to the plasma membrane. Eukaryotic algae and plants have two types of thylakoids inside their chloroplast.
— disc shaped thylakoids are arranged in stacks called Grana
— unstacked thylakoids known as stroma lamellae, form connections between thylakoids in Grana
A thylakoid is a system because it contains interacting components that individually would not be able to carry out their functions.
- thylakoid membrane separate the fluid inside the lumen of the thylakoid from a fluid in the surrounding stroma so a proton gradient can be maintained
-ATP synthase located in the thylakoid membrane uses the proton gradient to synthesise ATP on the Strotheide of thylakoid membranes
-The oxygen evolving complex of photosystem to split water in the lumen of the thylakoid by photolysis providing a supply of electrons
- Photos system two absorbs light and uses the energy from it to excite electrons which are passed to by plastoquinone
- plastoquinone and the cytochrome B6 F complex use energy carried by excited electrons to pump protons from the stroma to the lumen of the thylakoid
- plastocyanin transfers electrons from the cytochrome B6 F complex two photosystem one
- Photo system one absorbs light energy and uses energy from it to excite electrons. These electrons are used to reduce NADP on the stroma side of the thylakoid membranes
There is evidence that components are not evenly distributed between grana and stoma lamellea. Photosystem 2 and cytochrome B6 F complexes are mostly in the Grana. Photo system one and ATP synthases are mostly in the stroma lamellae, making synthesis of ATP and reduced NADP easier due to greater exposure to the stroma.
Carbon fixation by RuBisCO
Carbon dioxide is the carbon source for all organisms that carry out photosynthesis. It readily dissolves in water and passes across cell walls and membranes, so it diffuses from the atmosphere into Phosphosynthesizing cells. it also is able to diffuse out of cells and evaporate. Escape of carbon dioxide from photosynthesizing cells is prevented by carbon fixation.
In the carbon fixation reaction, CO2 is converted into a more complex carbon compound. This is arguably the most important chemical reaction in all living organisms. And plants and algae, it occurs in the stroma. The product of this carbon fixation reaction is a three carbon compound called glycerate 3-phosphate.
Carbon dioxide reacts with a five carbon compound called ribulose bisphosphate producing two molecules of glycerate 3-phosphate. The enzyme that catalyse this reaction is abbreviated to RuBisCO.
RuBisCO is surprisingly inefficient. Most enzymes convert thousands of molecules of substrate to product per second, however RuBisCO only fixes about three carbon dioxide molecules per 2nd to compensate for this they’re a very high concentrations of RuBisCO and the stroma. It is thought to be the most abundant enzyme.
Synthesis of Triose phosphate using reduced NADP and ATP
In sugars and other carbohydrates there are twice as many hydrogen atoms as oxygen atoms. RuBP is a five carbon sugar derivative. It is converted to glycerate 3-phosphate by the addition of carbon and oxygen, but not hydrogen as a result the amount of hydrogen relative to oxygen becomes less than 2 to one. Hydrogen has to be added to cell molecule by a reduction reaction to produce a carbohydrate. The carboxyl group in glycerate 3-phosphate is replaced by an aldehyde group.
This conversion involves both ATP and reduced NADP, produced by the light dependent reactions of photosynthesis. ATP provides the energy needed to perform the reduction and reduced NADP provides the electrons contained and hydrogen atoms. The product is a three carbon sugar derivative triose phosphate. Oxygen removed from the carboxyl group combined with hydrogen from reduced NADP used to produce water.
Conversion of glycerate 3-phosphate to Triose phosphate happens in the stroma of the chloroplast. It is part of the light independent reactions of photosynthesis because light is not directly used. However, it can only continue for a short time in darkness as ATP and reduced NADP are required and they quickly run out.
Regeneration of RuBP in the Calvin cycle using ATP
The first carbohydrate produced by the light dependent reaction of photosynthesis is Triose phosphate. two Triose phosphate can be combined to form hexose phosphate. Hexose phosphate molecules can be combined by condensation reactions to form starch. When conditions in a leaf are suitable for photosynthesis start rapidly accumulates in chloroplasts.
All the Triose phosphate produced by photosynthesis was converted to hexose or starch, the supplies of RuBP in the chloroplast would soon be used up. This would cause carbon fixation to stop. Therefore, some trials phosphate has to be used to regenerate RUBP. This process is a conversion of a three carbon sugar into a five carbon sugar and it cannot be done in a single step. Instead, a series of reactions take place.
The light dependent reactions of photosynthesis form a cycle in which RUBP is both consumed and produced. This cycle was named the Calvin cycle.
For the Calvin cycle to continue indefinitely as much RUBP must be produced as consumed. When RUBP and CO2 are combined by RuBisCO only one of the six carbon atoms is newly fixed. For this reason only 1/6 the Triose phosphate molecules that are produced can be taken out of the Calvin cycle 5/6 of the Triose phosphate must be used to regenerate RUBP. FOR A NET GAIN OF ONE MOLECULE OF HEXOSE THE CALVIN CYCLE MUST HAPPEN SIX TIMES TO FIX SIX CARBON ATOMS.
Regeneration of RUBP requires the use of ATP. This is because triose phosphate is converted into ribulose phosphate and this must be converted to RUBP.
Synthesis of carbon compounds using the product of the Calvin cycle
Simple equations for photosynthesis usually show glucose as the end product. Plants require large quantities of glucose for cell respiration and for making cellulose. Six turns of the Calvin cycle are needed to produce one molecule of glucose, each term of the cycle contributes one of the fixed carbon atoms in glucose.
Glucose is usually converted to sucrose for transport from leaves to other parts of the plant at times glucose is produced more quickly than it can be transported. At these times it is converted to starch and stored temporarily inside chloroplasts. At night when photosynthesis has stopped. This starch is broken down and the carbohydrate is exported from the LEAF.
Chloroplast can also convert Triose phosphate from the Calvin cycle into fatty acids using enzymes of glycolysis pathway and link reaction to produce acetyl Coenzyme A and then linking together two carbon acetyl groups. Glycerol can also be made from triose phosphate and linked to fatty acids to produce triglycerides. Droplets of stored oil are often visible in chloroplasts.
Many other carbon compounds can be produced in photosynthesizing cells starting either with glycerate 3-phosphate or Triose phosphate from the Calvin cycle or with intermediate from pathways used for aerobic respiration. Mineral nutrients such as phosphate or sulphate are also needed to make compounds containing elements other than carbon hydrogen and oxygen. All amino acids are synthesis in photosynthesizing organisms using branching metabolic pathways? Nitrogen is supplied by nitrate or ammonius ions.
Interdependence of the light dependent and light independent reactions
The light dependent reactions in the thylakoid membranes are on the surface of them are as follows:
- Photolysis
- Light absorption by generation of electrons
- Transport of electrons by carriers
- ATP synthesis by chemiosmosis
- reduction of NADP
On the other hand, light independent reactions in the stroma are:
- Carbon fixation
- Synthesis of triose phosphate and other carbon compounds
- Generation of RuBP
Despite the name, light independent reactions can only continue for a few seconds in darkness. This is because they are dependent on substances produced by the light dependent reactions, which rapidly run out if they are not produced continuously. Similarly, light dependent reactions cannot continue indefinitely without substances produced by the light independent reactions. The two parts of photosynthesis are interdependent.
Light intensity effects which part of photosynthesis limits the overall rate at which carbon compounds are produced:
- In low light intensity the production of ATP and reduced NADP are restricted. Therefore, the conversion of glycerate 3-phosphate in the Calvin cycle is the rate limiting step
- And highlight intensity carbon fixation is usually the rate limiting stuff. Use of reduced NADP is restricted so supplies of NADP limit the light dependent reactions. Some photons of light absorbed by the photo systems are remitted as fluorescence.
