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The biochemistry of photosynthesis

The basic chemical equation of photosynthesis is:

n CO₂ + 2n H₂O + ATP + NADPH → (CH₂O)_n + n O₂ + n H₂O

This very simple equation suggests a very simple mechanism. In fact the mechanism of photosynthesis is extremely complex and requires the interplay of many complex proteins and other small molecules. The light energy that is the driving force behind photosynthesis is captured by molecules called pigments. Chlorophyll is perhaps the most well known of these pigments, but there is a diverse array of pigment molecules which participate in photosynthesis including: pheophytin, β-carotene, phycocyanobilin and phycoerythrobilin amongst others. Photosynthetic bacteria have there own versions of these pigments such as bacteriochlorophyll and bacteriopheophytin. They are very similar to the plant pigments.

The process of photosynthesis has two distinct stages: the light reactions and the dark reactions. As the name implies the light reactions involve the capturing of light. In plants there are two proteins that capture light which are called photosystem I and photosystem II respectively. In bacteria only one protein captures light energy. These proteins are called reaction centres and are discussed in detail below. Many other proteins such as cytochrome bf, plastocyanin and ferredoxin participate in the light reactions. These proteins cannot capture light themselves are serve primarily as electron carriers.

If plants and photosynthetic bacteria relied on the reaction centres capturing light on their own a small proportion of the available light would be captured. A group of proteins called light harvesting complexes serve to capture more light than the reaction centre could on its own. Light harvesting complexes surround the reaction centres and direct light inward toward them. Light harvesting complexes are discussed in detail below.

The product of the light reactions is not carbohydrate but ATP and NADPH. These molecules store potential energy – the energy of the light captured during the light reactions. The dark reactions use the NADH and ATP to synthesise carbohydrates amongst other things. The two stages of the dark reactions, the Calvin cycle and the pentose phosphate pathway are discussed in details below.

The Calvin cycle creates hexose sugars such as fructose. It is powered by the ATP and NADPH created during the light reactions. The Calvin cycle and is cyclical and extremely complex, including at least 17 distinct chemical reactions. The starting product of the Calvin cycle is 3 carbon dioxide molecules, 6 NAPH molecules and 6 ATP molecules. The end product is fructose 6-phosphate which is often converted into glucose then into starch.

The pentose phosphate pathway creates NADPH and synthesis 5-carbon sugars such as ribose. The pentose phosphate pathway is closely linked with the process which extracts energy from glucose: glycolysis.

The light reactions

Trapping light energy is the key to photosynthesis. The first step is the absorption of light by a photoreceptor molecule. The most common photoreceptor in green plants is chlorophyll a. This molecule is structurally similar to heme but it has a reduced pyrrole ring, a long hydrophobic alcohol called phytol esterified to an acid side chain (this is visible in the image as a long chain of carbon atoms beneath the rings) and it has magnesium at its centre instead of iron. Chlorophyll is a very effective photoreceptor molecule as it contains an alternating network of double and single bonds. Chemical compounds that are arranged in this fashion are called polyenes.

Electrons which orbit the nucleus of an atom reside in distinct energy levels. When light is absorbed by a photoreceptor the electron is raised from its ground state to an exited state. When this happens the electron will usually fall back down to its ground state releasing the potential energy of the absorbed light as heat or another photon. However, if a suitable electron accepter is nearby the exited electron can move from the photoreceptor to the electron acceptor. This produces a potential difference of charge between the two molecules which can be used do further work. The electron carrier can either be integral with the reaction centre, or it could be pat of an external electron carrying protein.

Photosynthetic bacteria such as Rhodopsudomonas Vridis contain a single reaction centre. The reaction centre reduces a single molecule of quinine upon absorption of two photons of light. The array of photoreceptors and electron carriers is almost symmetric about the centre of the reaction centre.

In plants

Two proteins, photosystem I and photosystem II, collect light during photosynthesis in green plants. These two proteins absorb maximally at 700nm and 680nm respectively. These two photosystems are present in all oxygen-evolving organisms. Electrons flow first through photosytem II, then through cytochrome bf, a membrane-bound complex homologous to t Q-cytochrome c, which participates in oxidative phosphorylation, and finally through photosystem I. When the path of electrons is shown in a diagram with the three proteins next to one another it resembles a z on its side. It’s often referred to the Z-scheme for this reason.

The electrons which pass through the Z-scheme originate at a water molecule which resides at the bottom of photosystem II. Two molecules of H₂O are oxidized to one molecule of O₂ for every four photons absorbed by photosystem II. The electrons which pass through the Z-scheme end up reducing NADP⁺ to NADPH. The Z-scheme also creates an uneven distribution of protons across the thylakoids membrane.

In bacteria and algae

Algae is a range from multicellular forms like kelp to microscopic, single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Very much like plants, algae have chloroplasts and chlorophyll, but various accessory pigments are present in some algae such as phycoerythrin in red algae (rhodophytes), resulting in a wide variety of colours. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. For example, in coral reefs, there is a symbiotic relationship between zooxanthellae and the coral polyps.

Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles), instead, photosynthesis takes place directly within the cell. Cyanobacteria contain thylakoid membranes very similar to those in chloroplasts and are the only prokaryotes that perform oxygen-generating photosynthesis, in fact chloroplasts are now considered to have evolved from an endosymbiotic bacterium, which was also an ancestor of and later gave rise to cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen. Some bacteria such as Chromatium, oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as waste.

Photosynthesis is affected by its surroundings. The rate of photosynthesis is affected by carbon dioxide, light intensity and the temperature.