Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

Primary Synthesis of Organic Substances. Photosynthesis
Chloroplasts

The most primitive Photosynthetic organisms are cyanobacteria. They appear to have been the first life forms capable of converting inorganic carbon from Earth's atmosphere into Organic compounds using Water and solar energy. Furthermore, this process released oxygen into the atmosphere, which paved the way for The Emergence of aerobic life.

Today, the primary photosynthetic organisms are plants, in which this process takes place within specialized Organelles called chloroplasts. Based on biochemical data, chloroplasts are believed to be the "descendants" of ancient cyanobacteria that entered Eukaryotic Cells and established a symbiotic relationship with them.

Chloroplasts are double-membrane, semi-autonomous Cell organelles. Ranging in size from about 3 to 10 µm (with an average of 5 µm), they are easily visible under a Light Microscope. In higher plants, chloroplasts are elongated and biconvex, whereas in Algae their shape can vary significantly: cup-shaped in Chlamydomonas, spiral in Spirogyra, and ring-shaped in Ulothrix.

Chloroplasts are enclosed by a double membrane (Fig. 8). Their interior is filled with the stroma, a semi-fluid matrix containing dissolved substances and various structures, including circular DNA molecules, RNA, Ribosomes, numerous Enzymes, starch grains, lipid droplets, and Proteins. The presence of DNA, RNA, and ribosomes indicates that chloroplasts are capable of autonomous Protein Synthesis AND independent division.

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Fig. 8. Structure of Chloroplasts. A — 3D diagram; B — planar structural diagram: 1 — outer membrane; 2 — inner membrane; 3 — stroma; 4 — grana; 5 — thylakoid; 6 — lamella; 7 — DNA; 8 — ribosomes

Embedded within the stroma are membrane-bound components known as thylakoids. Thylakoids are disk-shaped, membrane-enclosed sacs that form the Internal Structure of chloroplasts. Stacked together, they form grana (singular: granum), where each granum resembles a stack of coins. The grana are interconnected by single thylakoids called lamellae, which appear as flat plates. The light-dependent reactions take place on the thylakoid membranes, whereas the light-independent (dark) reactions of Photosynthesis occur in the stroma.

Chloroplasts owe their green color to the pigment chlorophyll, which is localized on their membrane structures. Photosynthesis is directly linked to this pigment. Chlorophyll is capable of absorbing light quanta, leading to the excitation of its electrons. The shorter the wavelength, the higher the light energy, and the greater the probability of electrons transitioning to an excited state.

Several types of chlorophyll are found in chloroplasts, with chlorophylls a and b being the most widespread (Fig. 9). Chlorophyll a is present in large quantities in all plants and plays a central role in photosynthesis. It has a yellow-green color and absorbs light most intensely in the red and violet-blue spectra. There are several forms of this pigment that differ in THE POSITION OF their maximum absorption peak in the red region: chlorophylls 670, 690, 700 nm, etc.

Fig. 9. Absorption Spectra and photosynthetic rates for Different types of chlorophyll

Chlorophyll b is blue-green in color, absorbing energy more intensely in the violet spectrum and significantly less in the red spectrum. It is also found in Higher Plants and green algae. Brown algae and certain unicellular algae contain chlorophyll c, which has a green color. Red algae possess yet another variety, chlorophyll d, which is also green. Blue-green Bacteria (cyanobacteria) contain a different variant of photosynthetic pigment that is pale blue in color.

In addition to chlorophyll, the chloroplasts of all photosynthetic plants contain an orange pigment called carotene, which also participates in photosynthesis and Functions as an accessory pigment.