Chloroplasts Are the Site of Photosynthesis

Chloroplasts Are the Site of Photosynthesis

It is well known that photosynthetic organisms, such as green plants, convert carbon dioxide (CO₂) and water to carbohydrates such as glucose (written here as C₆H₁₂O₆) and molecular oxygen (O₂).

6CO₂ + 6H₂O - > C₆H₁₂O₆ + 6O₂

The equation actually represents two processes. One process, the oxidation of water to produce oxygen (the light reactions), requires light energy from the Sun. The light reactions of photosynthesis in prokaryotes and eukaryotes depend on solar energy, which is absorbed by chlorophyll to supply the energy needed in the light reactions. The light reactions also generate NADPH, which is the reducing agent needed in the dark reactions. The other process, the Þxation of CO₂ to give sugars (the dark reactions), does not use solar energy directly but rather uses it indirectly in the form of the ATP and NADPH produced in the course of the light reactions.

In prokaryotes such as cyanobacteria, photosynthesis takes place in granules bound to the plasma membrane. The site of photosynthesis in eukaryotes such as green plants and green algae is the chloroplast (Figure 22.1). Like the mitochondrion, the chloroplast has inner and outer membranes and an intermembrane space. In addition, within the chloroplast are bodies called grana, which consist of stacks of flattened membranes called thylakoid disks. The grana are connected by membranes called intergranal lamellae. The thylakoid disks are formed by the folding of a third membrane within the chloroplast. The folding of the thylakoid membrane creates two spaces in the chloroplast in addition to the intermembrane space. The stroma lies within the inner membrane and outside the thylakoid membrane. In addition to the stroma, there is a thylakoid space within the thylakoid disks themselves. The trapping of light and the produc-tion of oxygen take place in the thylakoid disks. The dark reactions (also called light-independent reactions), in which CO₂ is fixed to carbohydrates, take place in the stroma (Figure 22.2).

How does the structure of the chloroplast affect photosynthesis?

It is well established that the primary event in photosynthesis is the absorption of light by chlorophyll. The high energy states (excited states) of chlorophyll are useful in photosynthesis because the light energy can be passed along and converted to chemical energy in the light reaction. There are two principal types of chlorophyll, chlorophyll a and chlorophyll b. Eukaryotes such as green plants and green algae contain both chlorophyll a and chlorophyll b.

Prokaryotes such as cyanobacteria (formerly called blue-green algae) con-tain only chlorophyll a. Photosynthetic bacteria other than cyanobacteria have bacteriochlorophylls, with bacteriochlorophyll a being the most common. Organisms such as green and purple sulfur bacteria, which contain bacterio-chlorophylls, do not use water as the ultimate source of electrons for the redox reactions of photosynthesis, nor do they produce oxygen. Instead, they use other electron sources such as H₂S, which produces elemental sulfur instead of oxygen. Organisms that contain bacteriochlorophyll are anaerobic and have only one photosystem, whereas green plants have two different photosystems, as we shall see.

The structure of chlorophyll is similar to that of the heme group of myoglo-bin, hemoglobin, and the cytochromes in that it is based on the tetrapyrrole ring of porphyrins (Figure 22.3). The metal ion bound to the tetrapyrrole ring is magnesium, Mg(II), rather than the iron that occurs in heme. Another difference between chlorophyll and heme is the presence of a cyclopentanone ring fused to the tetrapyrrole ring. A long hydrophobic side chain, the phytol group, contains four isoprenoid units (five-carbon units that are basic building blocks in many lipids;) and binds to the thylakoid membrane by hydrophobic interactions. The phytol group is covalently bound to the rest of the chlorophyll molecule by an ester linkage between the alcohol group of the phytol and a propionic acid side chain on the porphyrin ring. The difference between chlorophyll a and chlorophyll b lies in the substitution of an aldehyde group for a methyl group on the porphyrin ring. The difference between bacteriochlorophyll a and chlorophyll a is that a double bond in the porphyrin ring of chlorophyll a is saturated in bacteriochlorophyll a. The lack of a conjugated system (alternating double and single bonds) in the porphyrin ring of bacteriochlorophylls causes a significant difference in the absorption of light by bacteriochlorophyll a compared with chlorophyll a and b.

The absorption spectra of chlorophyll a and chlorophyll b differ slightly (Figure 22.4). Both absorb light in the red and blue portions of the visible spec-trum (600 to 700 nm and 400 to 500 nm, respectively), and the presence of both types of chlorophyll guarantees that more wavelengths of the visible spectrum are absorbed than would be the case with either one individually. Recall that chlorophyll a is found in all photosynthetic organisms that produce oxygen. Chlorophyll b is found in eukaryotes such as green plants and green algae, but it occurs in smaller amounts than chlorophyll a. The presence of chlorophyll b, however, increases the portion of the visible spectrum that is absorbed and thus enhances the efficiency of photosynthesis in green plants compared with cya-nobacteria. In addition to chlorophyll, various accessory pigments absorb light and transfer energy to the chlorophylls (Figure 22.4b). Bacteriochlorophylls, the molecular form characteristic of photosynthetic organisms that do not pro-duce oxygen, absorb light at longer wavelengths. The wavelength of maximum absorption of bacteriochlorophyll a is 780 nm; other bacteriochlorophylls have absorption maxima at still longer wavelengths, such as 870 nm or 1050 nm. Light of wavelength longer than 800 nm is part of the infrared, rather than the visible, region of the spectrum. The wavelength of light absorbed plays a critical role in the light reaction of photosynthesis because the energy of light is inversely related to wavelength.

Most of the chlorophyll molecules in a chloroplast simply gather light (antennae chlorophylls). All chlorophylls are bound to proteins, either in antennae complexes or in one of two kinds of photosystems (membrane-bound protein complexes that carry out the light reactions). The light-harvesting molecules then pass their excitation energy along to a specialized pair of chlorophyll molecules at a reaction center characteristic of each photosystem (Figure 22.5). When the light energy reaches the reaction center, the chemical reactions of photosynthesis begin. The different environments of the antennae chlorophylls and the reaction-center chlorophylls give different properties to the two different kinds of molecules. In a typical chloroplast, several hundred light-harvesting antennae chlorophylls are present for each unique chlorophyll at a reaction center. The precise nature of reaction centers in both prokaryotes and eukaryotes is the subject of active research.

Summary

In eukaryotes, photosynthesis takes place in chloroplasts. The light reac-tions take place in the thylakoid membrane, a third membrane in chloro-plasts in addition to the inner and outer membrane.

The dark reactions of photosynthesis take place in the stroma, the space between the thylakoid membrane and the inner membrane of the chloroplast.

The absorption of light by chlorophyll supplies the energy required for the reactions of photosynthesis. Several different kinds of chlorophyll are known. All have a tetrapyrrole ring structure similar to that of the porphy-rins of heme, but they also have differences that affect the wavelength of light they absorb.

This property allows more wavelengths of sunlight to be absorbed than would be the case with a single kind of chlorophyll.