Question

Which structure enables bacteria to harvest light energy in near total darkness? Please help!

a. Chlorosomes

b. Vesicles

c. Chloroplasts

d. Thylakoids

e. Chromatophores

Answer 1

Ans) a. Chlorosomes is the correct answer.

Explaination of all the Options:

a). Chlorosomes: Although the process of photosynthesis is most commonly associated with plants and algae, much of our understanding of the molecular basis for light energy capture and photochemical energy transduction has come from studies of photosynthetic bacteria.

In the mud layers at the bottom of lakes and the sea live bacteria that are so extreme that they can convert sunlight into usable energy. They do it with the help of special antennae and a team of scientists have now mapped the structure of a part of these antennae. The antennae are like micro-sized solar panels that use the energy in sunlight to grow.

Bacteria have developed a very complicated antennae known as chlorosomes. These special antennae contain up to 200,000 sun-catching molecules, which make them far and away the largest antennae known in nature.“These antennae are exceptionally effective at catching and converting the sparse solar energy that reaches them,”

We know the structure of many other antennae in photosynthetic organisms, but these are much smaller and have a very different structure than the baseplates of chlorosomes.

Precise functions of the baseplate are, First, it helps to orientate the antennae in the right direction to make the most of the limited sunlight available to them. Second, it helps to process photons of light more effectively by collecting them and sending them on to be processed.

Each reaction center is associated with an antenna, which contains several light-harvesting complexes (LHCs), packed with chlorophyll a and, depending on the species, chlorophyll b and other pigments. LHCs promote photosynthesis by increasing absorption of 680-nm light and by extending the range of wavelengths of light that can be absorbed.

b). Vesicles: Photosynthetic vesicles found in some bacteria constitute one of the simplest light-harvesting systems in nature. The overall architecture of vesicles and the structural integration of vesicle function is poorly understood till now. Structural model for an entire vesicle is presented, which improves upon earlier models by taking into account the stoichiometry of core and antenna(Chlorosomes) complexes determined by the absorption spectrum of intact vesicles.

c). Chloroplastes : The photosynthetic process in plants has four stages, each occurring in a defined area of the chloroplast: (1) absorption of light, (2) electron transport leading to the reduction of NADP⁺ to NADPH, (3) generation of ATP, and (4) conversion of CO₂ into carbohydrates (carbon fixation). All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1 – 3 are catalyzed by proteins in the thylakoid membrane.

d). Thylakoid: Thylakoid membrane it is the site of photosynthesis. In each chloroplast, the thylakoid membrane constitute a single, interconnected sheet that forms numerous small flattened vesicles. The thylakoid membrane contains a number of integral membrane proteins to which are bound several important prosthetic groups and light-absorbing pigments, most notably chlorophyll. Carbohydrate synthesis occurs in the stroma, the soluble phase between the thylakoid membrane and the inner membrane. In photosynthetic bacteria extensive invaginations of the plasma membrane form a set of internal membranes, also termed thylakoid membranes, or simply thylakoids, where photosynthesis occurs.

e). Chromatophores: Chromatophores are specialized cells which can contain or produce pigment to give the colour to the specific part. These can also reflect light in a specific way. Chromatophores are also found in membranes of phototrophic bacteria. These are used mainly for photosynthesis. They contain bacteriochlorophyll pigments and carotenoids. In purple bacteria, such as Rhodospirillum rubrum, the light-harvesting proteins are intrinsic to the chromatophore membranes. In green sulfur bacteria, chromatophores are arranged in specialized antenna complexes called as chlorosomes. Photons can be absorbed by any of the pigment molecules in each LHC(Light Harvesting Complex).

About Light Harvesting Complex:

The absorbed energy is then rapidly transferred (in <10⁻⁹ seconds) to one of the two chlorophyll a molecules in the associated reaction center, where it promotes the primary photosynthetic charge separation. Within an LHC are several transmembrane proteins whose role is to maintain the pigment molecules in the precise orientation and position that are optimal for light absorption and energy transfer, thereby maximizing the very rapid and efficient process known as resonance transfer of energy from antenna pigments to reaction-center chlorophylls. some photosynthetic bacteria contain two types of LHCs: the larger type (LH1) is intimately associated with a reaction center; the smaller type (LH2) can transfer absorbed light energy to an LH1.The molecular structures of plant light-harvesting complexes are completely different from those in bacteria, even though both types contain carotenoids and chlorophylls in a clustered geometric arrangement within the membrane.

Chlorophyll & Carotenoid pigment- IH2 LHI IlHg (Light Harvest ] Reaction centre

fig: Schematic depiction of the cylindrical LHC(Light Harvesting Complex) and the reaction center as viewed from above the plane of the membrane. Each LH2 complex consists of nine subunits and a total of 27 chlorophyll and 9 carotenoid molecules. The arrows trace the probable path by which light energy absorbed by an LH2 complex is transferred to the similar but larger LH1 complex and then to its final destination, the special pair of chlorophyll a molecules in the reaction center.

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