3. Photosystems

The nitty-gritty of carbon-fixing reactions

Both ATP and NADPH are important products of the light reactions and both of them play roles in the synthesis of carbohydrates from atmospheric carbon dioxide. Although the carbon-fixing reactions do not require daylight, they generally are conducted during daylight hours as there is some indication that some of the enzymes required for the processes in carbon-fixing may require some level of light. These reactions take place in the stroma of the chloroplast.

Three known mechanisms of converting carbon dioxide to sugar: (1) Calvin Cycle, (2) 4-Carbon pathway, and (3) CAM photosynthesis.

The Calvin Cycle or the 3-carbon pathway—The Calvin cycle is the most common of the three mechanisms and has four main results:

  1. With the assistance of the enzyme rubisco (RuBP carboxylase), six molecules of atmospheric carbon dioxide combine with six molecules of ribulose 1, 5-bisphosphate (RuBP)
  2. The result of the first step is six unstable 6-carbon complexes, which immediately split into two 3-carbon molecules of 3-phosphoglyceric acid or 3PGA. This is the first stable compound in photosynthesis.
  3. NADPH and ATP from the light-reactions, supply the energy required to convert the 3PGA to 12 molecules of glyceraldehydes 3-phosphate (GA3P), which is a 3-carbon sugar phosphate complex.
  4. Finally, of the 12 molecules formed; 10 are restructured into six 5-carbon molecules of RuBP—the sugar that the process started with.

The sugars produced can either add to an increase in the sugar content (carbohydrate content) of the plant or they can be used in pathways that lead to the production of lipids and amino acids.

4-Carbon Pathway—C4 plants are plants that use a 4-carbon molecule called oxaloacetic acid in place of the 3-carbon 3-phosphoglyceric acid in step two of the Calvin cycle. Oxaloacetic acid is produced from a 3-carbon compound PEP—phosphoenolpyruvate and carbon dioxide. This process is enzyme-mediated and occurs in the mesophyll cells of the leaf. Some species will convert the resulting oxaloacetic acid to aspartic, malic or other acids.

Note that the acids do not substitute for 3PGA. The 4-carbon acids migrate to the bundle sheaths surrounding the vascular bundles, where they are further converted to pyruvic acid and carbon dioxide. In returning to the mesophyll cells and interacting with ATP molecules, the pyruvic acid molecules are able to produce additional PEP. In the bundle sheath cells, the carbon dioxide formed converts into 3PGA and other molecules, by combining with RuBP. The other molecules formed are similar to the other ones formed in the Calvin cycle. The C4 cycle furnishes carbon dioxide to the Calvin cycle in a more roundabout way than the C3 pathway, but there is an advantage to this extra pathway. The extra pathway greatly reduces photorespiration in C4 plants, and this is a good thing because photorespiration is in direct competition with the Calvin cycle and even takes place in the light while the Calvin cycle is functioning. During photorespiration, RuBP reacts with oxygen to create carbon dioxide; in contrast, during photosynthesis RuBP and carbon dioxide are used to form carbohydrates. C4 plants are able to pick up carbon dioxide in very low concentrations via the mesophyll cells. The Calvin cycle occurs in the bundle sheath where carbon dioxide is readily available. Because of the location, the enzyme rubisco will be in a prime spot to catalyze the reaction between RuBP and carbon dioxide, rather than oxygen. As a result, C4 plants have photosynthetic rates that are two to three times higher than C3 plants. There are a few other characteristic features of C4 plants worth noting:

C4 plants have two types of chloroplasts and an alternate pathway for using carbon dioxide. C3 plants only have one type of chloroplast and one pathway. Chloroplasts with starch grains and are large with very little grana, and sometimes none, in the bundle sheath cells. In the mesophyll, the small, but numerous chloroplasts have no starch grains and contain highly developed grana.

PEP carboxylase is found in high concentration in the mesophyll cells which permits ready conversion of carbon dioxide to carbohydrate at lower concentrations than does rubisco (in bundle sheath cells) of the Calvin cycle.

The temperature ranges for C4 plants are much higher than C3 plants that enable C4 plants to live well in conditions that would likely kill a C3 plant.

CAM photosynthesis—Crassulacean acid metabolism is a modified photosynthetic system that is somewhat similar to C4 photosynthesis in that 4-carbon compounds are produced during the carbon-fixing reactions. CAM plants accumulate malic acid in their chlorenchyma tissues at night, which is converted back to carbon dioxide during the day. In the daytime, malic acid diffuses out of the vacuoles and is converted to carbon dioxide for use in the Calvin cycle. PEP carboxylase is responsible for converting the carbon dioxide plus PEP to malic acid at night. This modification allows for a greater amount of carbon dioxide to be converted to carbohydrate during the day than would be otherwise converted given the conditions CAM plants generally grow in. CAM plants generally close their stomata during the day in order to reduce water loss. There are more than 20 families that contain CAM plants, including cacti, stonecrops, orchids, bromeliads, and many succulents growing in regions of high light intensity. There are some succulents that do not have CAM photosynthetic capabilities, as well as non-succulents that do have the ability.

3. Photosystems


Earlier we mentioned in passing a coupling factor. The enzyme necessary for the mediation of the splitting of water molecules is on the inside of the thylakoid membrane. As a result of this, a proton gradient forms across the membrane and the movement of these protons is thought to be a source of energy for generating ATP. The motion is thought to be similar to molecular movement during osmosis and has hence been termed chemiosmosis. As the protons move across the membrane, they are assisted in crossing by protein channels called ATPase or coupling factor. Because of the proton movement, ADP and phosphate combine which produces ATP.

3. Photosystems


Pq, the acceptor molecule, releases the excited electron into the care of an electron transport system that is sort of like a downhill bucket brigade. The transport system moves electrons extracted from water temporarily to a high-energy storage molecule called nicotinamide adenine dinucleotide phosphate (NADP+). NADP+ is an electron acceptor for the photosystem. The transport chain is essentially iron-containing pigments, cytochromes, a copper-containing protein called plastocyanin and other electron transferring molecules. As electrons are passed through the chain and protons are being shuffled through a coupling factor, ATP molecules are assembled from ADP and phosphate in a process called photophosphorylation.

A similar series of events occurs in photosystem I. After a photon of light strikes a P700 molecule, the resulting excited electron is passed along to an iron-sulphur molecule Fe-S which in turn passes it to another acceptor molecule ferredoxin, (Fd). The ferredoxin molecule releases the electron to a carrier molecule called flavin adenine dinucleotide (FAD) and then eventually on to NADP+. A reduction occurs and NADP+ becomes NADPH. Electrons from photosystem II and the activities of the electron transport system replace any electrons removed from the P700 molecule. Because the electrons move in one direction, the movement of electrons from water to photosystem II to photosystem I to NADP+ are said to be part of noncyclic electron flow. Any ATP that is produced is designated noncyclic phosphorylation.

It should be noted that photosystem I can operate independently of photosystem II. When this occurs, the electrons boosted from P700 reaction-center molecules (photosystem I) are passed through an intermediary acceptor molecule called P430 and then on to the electron transport chain. This is rather than to the ferredoxin and NADP+. After being passed through the electron transport chain, the electron is dumped back into the reaction-center of photosystem I. This process demonstrates cyclic electron flow and any ATP generated by cyclic electron flow is termed cyclic phosphorylation. Note, that no water molecules are split and no NADPH or oxygen is produced.

3. Photosystems


A photon of light strikes the photosystem II reaction center, the P680 molecule to be exact near the inner surface of a thylakoid membrane. The received light energy excites an electron (boosts it to a higher energy level) which is an unstable reaction and thus most of the energy is lost to heat.

Up to four photons at a time can strike the P680 molecule, however, it can only accept one electron at a time. The molecule of pheophytin picks up the excited electron, which then crosses the thylakoid membrane and is passed along to another acceptor called plastoquinone or Pq near the outside surface of the thylakoid membrane. Protein Z extracts electrons from water and replaces the ones lost by the P680 molecule. Protein Z contains manganese which is required in order to split water molecules. Simultaneously, as two water molecules are split and a molecule of oxygen and four protons are produced. This enzyme-mediated water splitting process is called photolysis.

3. Photosystems


The two types of photosynthetic units in most chloroplasts are what constitute photosystem I and photosystem II.

Photosystem I contains photosynthetic units with 200 or more molecules of chlorophyll a, small amounts of chlorophyll b, protein saddled carotenoid pigment and a pair of specialized reaction-center molecules of chlorophyll called P700. All pigments in a photosystem are capable of absorbing photons, however, only the reaction-center molecules can really utilize the light energy. The other pigments aren’t worthless in the system, as they act sort of like an antenna in gathering and passing light energy along to the reaction-center. Iron-sulphur complexed proteins initially receive electrons from P700 and serve as primary electron acceptors for the unit.

Photosystem II contains chlorophyll a, protein saddled beta-carotene, a small amount of chlorophyll b and special pair of reaction-center molecules of chlorophyll a otherwise called, P680. The photosystem has a primary electron acceptor called pheophytin or Pheo.
For the record, the 680 and 700 in the names of the reaction-center molecules stand for the peaks in the absorption spectra of light waves of 680 nm and 700 nm.