Photosynthesis

A great variety of living things on earth, including all photosynthetic microorganisms, synthesize their foods from simple molecules such as carbon dioxide and water. In microorganisms, photosynthesis occurs in unicellular algae and in photosynthesizing bacteria such as cyanobacteria and green and purple sulfur bacteria.

Photosynthesis is actually two processes. In the first process, energy-rich electrons flow through a series of coenzymes and other molecules, and this electron energy is trapped. During the trapping process, ATP molecules and molecules of nicotinamide adenine di-nucleotide phosphate hydrogen (NADPH) are formed, both rich in energy. These molecules are then used in the second half of the process, where carbon dioxide molecules are bound into carbohydrates to form organic substances such as glucose.

Photosynthesis occurs along the thylakoid membranes of eukaryotic organisms. The thylakoids are somewhat similar to the cristae of mitochondria. Sunlight is captured in the thylakoid by pigment molecules organized into photosystems. The coenzyme NADP functions in the system. The photosystem includes the pigment molecules, as well as proton pumps, and coenzymes and molecules of electron transport systems. In prokaryotic microorganisms, the chlorophyll molecules are dissolved in the cell's cytoplasm and are called bacteriochlorophylls.

The process of photosynthesis is conveniently divided into two parts: the energy-fixing reaction (also called the light reaction) and the carbon-fixing reaction (also called the light-independent reaction, or the dark reaction).

The energy-fixing reaction. The energy-fixing reaction of photosynthesis begins when light is absorbed in a photosystem. The energy activates electrons to jump out of chlorophyll molecules in the reaction center. These electrons pass through a series of cytochromes in the nearby electron transport system. Some of the energy of the electrons is lost as they move along the chain of acceptors, but a portion of the energy is used to pump protons across a membrane, setting up the potential for chemiosmosis.

After passing through the electron transport system, the energy-rich electrons enter another photosystem. Light now activates the electrons, and they receive a second boost out of the chlorophyll molecules. The electrons progress through a second electron transport system and enter a molecule of NADP. Since NADP has acquired two negatively charged electrons, it attracts two positively charged protons from a water molecule to balance the charges, and the molecule is reduced to NADPH. This molecule contains much energy.

Because electrons have flowed out of the chlorophyll molecules, the latter are left without a certain number of electrons. These electrons are replaced by electrons secured from water molecules. The third product of the disrupted water molecules is oxygen. Two oxygen atoms combine with one another to form molecular oxygen. This oxygen is given off by cyanobacteria as the waste product of photosynthesis. It is the oxygen that fills the atmosphere and is used by all oxygen-breathing organisms.

What has been described above are the noncyclic energy-fixing reactions. Certain microorganisms are also known to participate in cyclic energy-fixing reactions.Excited electrons leave the chlorophyll molecules, pass through coenzymes of the electron transport system, and then follow a special pathway back to the chlorophyll molecules. Each electron powers the proton pump and encourages the transport of a proton across the membrane. This process enriches the proton gradient and eventually leads to the generation of ATP.

ATP production in the energy-fixing reactions of photosynthesis occurs by the process of chemiosmosis. Essentially, it consists of a rush of proteins across a membrane (the microbial membrane, in this case) accompanied by the synthesis of ATP molecules. It has been calculated that the proton concentration on one side of the membrane is 10,000 times that on the opposite side of the membrane.

In photosynthesis, the protons pass back across the membranes through channels that lie alongside sites where enzymes are located. As the protons pass through the channels, the energy of the protons is released to form high-energy ATP bonds. ATP is formed in the energy-fixing reactions along with NADPH formed in the main reactions. Both ATP and NADPH provide the energy necessary for the synthesis of carbohydrates that occurs in the second major set of events in photosynthesis.

The carbon-fixing reaction. In the carbon-fixing reaction of photosynthesis, glucose and other carbohydrates are synthesized. This phase of photosynthesis occurs in the cytoplasm of the microbial cell.

In the carbon-fixing stage, an essential material, carbon dioxide, is obtained from the atmosphere. The carbon dioxide is attached to a five-carbon compound called ribulose biphosphate (RuBP) to form a six-carbon product. This product immediately breaks into two three-carbon molecules (Figure 1 ). 


Figure 1

The carbon‐fixing reaction of photosynthesis. The ATP and NADPH used in this process are synthesized in the energy‐fixing phase.

The three-carbon molecule is called phosphoglycerate (PGA). Each phosphoglycerate molecule is converted to phosphoglyceraldehyde (PGAL) using the ATP and NADPH synthesized in the energy-fixing reaction. The organic compounds that result have three carbon atoms. They interact with one another and eventually join to form a single molecule of six-carbon glucose. The process also generates more molecules of ribulose biphosphate to participate in further carbon-fixing reactions.

The carbon-fixing reaction is often referred to as the Calvin cycle, for Melvin Calvin, who performed much of the biochemical research. The product of the reaction is glucose, a carbohydrate containing the energy of sunlight, which began the reactions in the chlorophyll molecule. This energy has passed through ATP and NADPH and is now present in the high-energy glucose molecules. Photosynthesizing microorganisms use the glucose to obtain the energy for their activities. Nonphotosynthesizing organisms use this same glucose by consuming the carbohydrate.