Cellular Biology

How Photosynthesis Works



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Photosynthesis is the process by which plants convert carbon dioxide, water, and sunlight into sugar and oxygen. The formula for photosynthesis is

Carbon dioxide + water + light energy -> carbohydrate (glucose) + oxygen + water

Water appears on both sides of the equation because it is produced as well as consumed; but only half as much water exits the process as enters it.

Photosynthesis takes place in chloroplasts; these are found in all green parts of plants but the process is mostly carried out in the leaves. Carbon dioxode enters the leaves and oxygen exits through stomata, pores in the leaf surface. Inside the leaves are chloroplasts, containing chlorophyll, which gives plants their green colour; they occur mostly in mesophyll cells in the middle of the leaf. There are 30-40 chloroplasts per mesophyll cell; they contain thylakoid membranes folded into stacks called grana. The chlorophyll molecules are embedded in the surface of the thylakoid membrane. The two types of chlorophyll molecules - a and b - are structurally very similar, consisting of a porphyrin ring and hydrocarbon tail.

Photosynthesis occurs in two parts: the light reactions, and the Calvin cycle (dark reactions or light-independent reactions). The first can only take place during daylight hours, the second at any time. In the light reactions, light molecules (photons) entering the leaf are collected by the light-harvesting complex. Chlorophyll a and chlorophyll b absorb light at slightly different wavelengths, mostly in the blue and red areas of the spectrum. Photons absorbed by chlorophyll b are transferred to chlorophyll a. A photon striking chlorophyll can excite an electron - boost it to a higher energy level. The energy bounces from molecule to molecule until it reaches the chlorophyll a molecule at the reaction centre, whic includes a primary electron acceptor. This traps the high-energy electron produced by chlorophyll a before it can drop back to its ground state.

The reaction centres and their surrounding chlorophyll molecules form photosystems. There are two photosystems, I and II, which differ in their reaction centres. Their reaction centre chlorophylls are P700 and P680 respectively; the numbers designate their wavelength absorbtion peaks. The chlorophylls differ only in their associated proteins.

Light is absorbed by PSII and the electron captured by the primary electron acceptor. The chlorophyll then accepts an electron from water as a replacement. The water molecule donating the electron is split into 2 hydrogen ions and 1 oxygen
molecule, which combines with another to form oxygen gas. The excited electron is passed via the electron transport chain to PSI. The ETC consists of the electron carrier plastoquinone (Pq); a two-cytochrome complex; and a copper-containing protein, plastocyanin (Pc). The electron loses energy, which is captured by the thylakoid membrane to produce ATP by photophosphorylation. This will provide energy for the synthesis of sugars during the Calvin cycle. When the electron completes its fall, it is passed to P700. P700 receives excited electrons from surrounding chlorophylls and passes them to a second ETC, which transfers them to ferrodoxin (Fd), an iron-containing protein. NADP+ reductase shifts them to NADP+, producing NADPH, which operates in the Calvin cycle. This process is known as non-cyclic electron flow.

Electrons can also follow a path of cyclic electron flow, which involves PSI only. In this process, electrons return to P700 from Fd; there is no NADPH production, but it does make ATP. Cyclic electron flow is used because the Calvin cycle uses more
ATP than NADPH, and that ATP needs to be replaced.

During the Calvin cycle, carbon dioxide is used up and sugar is produced for use by the plant. The process occurs in three phases. The first phase is carbon fixation. Each carbon dioxide molecule is added to ribulose bisphosphate (RuBP, a 5-carbon
sugar). This addition is catalysed by rubisco (RuBP carboxylase), the earth's most abundant protein. The product is unstable and immediately splits into 2 molecules of 3-phosphoglycerate. The second phase is reduction. Each 3-phosphoglycerate receives an extra phosphate group from ATP to become 1,3-bisphosphoglycerate. Electrons from NADPH reduce the carboxyl group to a carbonyl group, turning each 1,3-bisphosphoglycerate into G3P. For every three molecules of carbon dioxide, 6 molecules of G3P are formed. One exits the cycle to the cell, where it is used as the starting point for various biosynthetic processes; the remainder enter phase three, the regeneration of RuBP. Three more molecules of ATP are used to turn the 5 molecules of G3P into three molecules of RuBP, which re-enter the cycle, which then begins again.

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