Photosynthesis is the process by which phototrophic organisms use energy from the sun to drive the reduction of carbon dioxide into energy rich compounds like carbohydrates. This occurs via chemical energy in the form of ATP and NADPH. Chemotrophic organisms, including ourselves, can then oxidise these reduced organic carbons for energy, or use them as building blocks. We then release the carbon back into the atmosphere, for example, in the form of CO2. Oxygenic phototrophs also release O2 as a by-product, thereby completing the flow of energy and carbon.
Photosynthesis may be broadly divided into two types of reactions: the light reactions and the dark reactions. In the former, energy from sunlight excites electrons to higher energy levels in pigment molecules (such as chlorophylls and carotenoids). The electrons then fall back to the ground level and releases energy while doing this. This energy is used to pump protons across a membrane. The proton gradient created across the membrane drives the phosphorylation of ATP. Excited electrons are also passed onto NADP+ to generate NADPH as reducing power. As the name indicates, the dark reactions do not require energy directly from the sun. Instead, the chemical energy produced during the light reactions is used to fix carbon dioxide into carbohydrate molecules via the Calvin Cycle.
Chloroplasts are the photosynthetic organelles in plants. They are encircled by outer and inner membranes that allow free diffusion of substrate; hence they play no role in photosynthesis per se. The site of action is the membrane system consisting of continuous flattened sacks comprised of grana- and stroma-thylakoids. This is suspended in the rich aqueous medium of the chloroplast, known as the stroma, which contains enzymes needed in the Calvin cycle.
The major pigment in green plant photosynthesis is chlorophyll. Its serves two functions: to absorb light, and to drive charge separation. The structure of chlorophyll is very similar to the cyclic tetrapyrrole of haem, except that it has a central magnesium ion rather than a ferrous ion, and a saturated bond in the ring. Chlorophyll also has a long phytol chain that allows anchorage to the thylakoid membrane. Here it can contribute to electron transduction through the photosystems and light-harvesting complexes.
Chlorophylls a and b differ only slightly in their side-chain substituents: chlorophyll a has a methyl group while chlorophyll b has an aldehyde group. Hence, they both absorb in the violet-blue region (with the absorption maxima of chlorophyll a at 430nm and chlorophyll b at 453nm) and in the orange-red region (maxima at 662nm and 642nm respectively). They reflect at the green region of the electron magnetic spectrum, thus, appear green.
However, there are many types of pigment molecules, all of which absorb light at their characteristic wavelengths. Additionally, their absorption may vary depending on the microenvironment the pigment finds itself. This broadens the spectra of solar light that may be used for photosynthesis. We will return to this point later when discussing adaptation to facilitate photosynthesis.
When the energy from the photon of light hits a chlorophyll molecule, it excites an electron that is in the ground state of the molecular orbital to an excited singlet state. This absorption of energy to jump from the ground to the excited states corresponds to a specific wavelength. This is why all photosynthetic pigments have their own absorption spectrum. The electron in the excited state then returns to the ground state and upon doing so it emits energy. This energy of the excited state is transferred to neighbouring molecules, and may be used to translocate protons across the membrane. The equation to the right is used to calculate the energy from the photon.
It is worth noting that although blue light has greater energy, it is not more efficient in photosynthesis. This is because the life time of the higher excited state is relatively shorter (10-11s) than the lower excited singlet state (10-8s). Therefore, when the electron is excited to the higher level, it rapidly returns to the more stable lower level; it is the decay from the lower excited state to the ground level that releases photosynthetically-useful energy.
The photosystem consists of the unit of pigment proteins arranged to absorb and relay light energy, and the two reaction centres which receive this funnelled energy. The array of chlorophyll proteins that funnel the energy are known as the antenna proteins. Note that these only transfer the energy of the excited state to the reaction centres. Conversely, the reaction centres, are two special types of chlorophyll molecules that release the excited electron itself to nearby oxidation-reduction molecules.
Although energy transduction through the outer antenna proteins occurs at random, once this energy reaches the inner antenna proteins, there is a tendency for the energy to remain within them. This is due to the gradual dissipation of energy in the form of heat as it passes through the antenna proteins. To compensate for this loss, the microenvironment of the inner antenna proteins decreases the energy of its excited state. This implies that additional energy would be required for the electron to be excited in the reverse direction (i.e. from the inner antenna proteins towards the outer antenna proteins), which is not physically feasible. There is a further reduction in the energy of the excited state in the reaction centres. Hence the energy is ‘trapped’ upon reaching the reaction centre chlorophyll molecules.
The special pair of chlorophyll molecules within the reaction centres are known as P700, associated with reaction centre I, and P680 associated with reaction centre II. These ‘special pairs’ exist in 3 states. Taking pigment 680 as an example:
P680 which is the ground state, P680* the lowest excited singlet state, and P680+ the oxidised state following transfer of 1 electron.
As a result of transferring the excitation to P680, a weakly reducing agent (P680) is converted to a very strong reducing agent (P680*) so that it can give its electron to a nearby molecule, thereby driving an uphill flow of electron transfer reactions. Besides electron excitation in this special pair, the remaining electron transfer reactions are downhill. Energy from the exergonic flow of electrons is converted to a proton gradient, while the final electron reduces the cofactor NADP+.
The corresponding oxidised form of P680 (P680+) is an extremely strong oxidising agent. So much so that it can extract an electron from a water molecule, re-forming the ground state P680 molecule, and oxidising the water to an oxygen molecule in the process.
So we can imagine the flow of electrons beginning with P680+ in photosystem II drawing an electron from a water molecule. This electron is transferred to P680+ via manganese stabilising protein and the amino acid tyrosine on a subunit of photosystem II. This reduces P680+ to its ground state, where it can absorb the exciton channelled to it by the surrounding antenna proteins, thus forming the highly reducing P680* (with a reduction potential of approximately -0.8V). P680* is able to gives its electron to the neighbouring molecule pheophytin. Charge separation between the subsequent P680+ and pheophytin- produced prevents the electron in P680* returning to its ground state, where the energy would be dissipated by radiationless de-excitation (heat) or re-emission of the photon (fluorescence).
Instead, this electron is transferred to plastoquinone (pq). Plastoquinone is a small lipid soluble molecule, able to dissociate from the reaction centre and enter the lipid pool of the thylakoid membrane. Upon gaining two electrons form chlorophyll (one at a time) and 2H+ from the environment, the reduced form: plastoquinol migrates to and transfers its electrons to the cytochrome b6/f complex while releasing its protons into the lumen as it becomes re-oxidised to plastoquinone.
Next, the b6/f complex transfers the electrons to plastocyanin (pc), a peripheral membrane protein, which passes the electrons to photosystem I. Here, the oxidised P700+ once again is reduced to P700. The P700, when excited by a photon, lowers its reduction potential to about -1.3V. This allows transfer of electrons to ferredoxin (fd) via intermediary redox centres. The final transfer of electrons, from ferredoxin to NADP+, is catalysed by ferredoxin-NADP+ reductase.
The proton motive force created above is used to synthesise ATP from ADP and Pi in a process known as photophosphorylation. The enzyme catalysing this reaction is F0F1 complex in the thylakoid membrane. This enzyme is homologous to the ATP synthase in oxidative phosphorylation. However, unlike the mitochondrial membrane, the thylakoid membrane is permeable to small ions leading to loss of the electrical gradient i.e. the membrane potential. Therefore, the primary force driving photophosphorylation is the chemical gradient i.e. the difference in pH.
Although NADPH is needed for processes like carbon fixation, ATP is also required for housekeeping processes in addition to this. The relative production of ATP can be increased as per the need of the cell. This is done through cyclic electron transfer reactions involving photosystem I alone: the ferredoxin reduces the cytochrome b6/f complex which donates electrons to P700 via plastocyanin. Thus, protons are pumped across the membrane generating a proton motive force, but no NADP+ is reduced. This excess ΔpH drives bonus ATP synthesis.
Carbon assimilation occurs in the stroma where the enzymes, substrates, and co-enzymes such as ATP and NADPH synthesised by the light reactions above are found. This removes the need of transporting molecules to site of utilisation thus saving energy. This carbon-fixing reaction is termed the Calvin Cycle. The first step of this reaction attaches carbon dioxide to ribulose-1,5-bisphosphate at the carbonyl carbon. A transient intermediate is formed which rapidly breaks down to two 3-phosphoglycerate molecules. RUBISCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses this reaction. The second step of the reaction is the production of a high energy intermediate: glycerate-1,3-bisphosphate by the enzyme phosphoglycerokinase. In the following step, this intermediate is reduced by glyceraldehyde-3-phosphate dehydrogenase using the cofactor NADPH, forming glyceraldehyde-3-phosphate. These latter 2 steps are essentially the reverse of glycolysis but NADPH rather than NADH is used. One out of every six molecules of glyceraldehyde-3-phosphate formed enters sucrose biosynthetic reactions similar to gluconeogenesis, while the remaining 5 molecules enter a succession of reactions that regenerate ribulose-5-phosphate. Finally, phosphoribulokinase phosphorylates ribulose-5-phosphate to complete the cycle.
Only shorter wavelengths of light are able to penetrate the ocean. Because of this, photosynthetic organisms living deep under water would be compromised in their ability to photosynthesise due to the narrow spectrum of light reaching them. However, they overcome this problem by using alternative accessory pigments, such as phycobilins, that absorb at a different wavelength, namely the blue-green region of the electromagnetic spectrum. Phycoerythrin, from the phycobilin family of pigment proteins, in red algae enables their survival in deep sea waters. Alternatively, phycocyanin (another type of phycobilin) found in cyanobacteria allows absorption of the orange region penetrating the surface of shallow waters such as ponds habituated by these bacteria. Various accessory pigments, such as carotenoids, are also found in plants. These further expand the solar energy utilised in photosynthesis.
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