Chloroplast

The chloroplasts ( from Ancient Greek χλωρός chloros "green" and ancient Greek πλαστός plastos "formed" ) are organelles of the cells of green algae and higher plants which carry out photosynthesis. For higher plants can from the photosynthetically active chloroplasts by differentiating chromoplasts leucoplasts ( amyloplasts, elaioplasts ), etioplasts and Gerontoplasten emerge (collectively, plastids ).

Structure of chloroplasts

In a photosynthetic active cell of a vascular plant are usually about 10 to 50 chloroplasts, having a diameter of about 4 to 8 microns. Many algae per cell, however, have only a single chloroplast which occupies a large part of the cell.

Similar to mitochondria possess chloroplasts both its own DNA and ribosomes and own two biomembranes as a shell. In its interior is as plasmatic phase of the stroma. The stroma in turn is traversed by thylakoid membranes - descendants of the inner membrane. With the exception of many of phototrophic protists are in the chloroplasts of higher phototrophs in several places flat, round protuberances of these membranes " money roll-like " superimposed. Such Thylakoidstapel called Granum ( pl. grana ). In the membranes of the thylakoids different pigments are incorporated, especially of the green pigment chlorophyll. Especially a lot of it is found in the membranes of the grana, which is why they appear intensely colored green. The pigments can absorb light of certain wavelengths, and the absorbed energy is used to produce ATP from ADP and phosphate (see phototrophy ). ATP serves as an energy carrier for the synthesis of glucose or starch from CO2 and water.

Establishment of a proton gradient

The biogenesis of these three membrane systems explains the fact that the proton gradient is established in chloroplasts the thylakoid membrane ( the thylakoid interior has an acidic environment on ), whereas mitochondria of the intermembrane space ( the area between the inner and outer membrane ) is loaded with protons. Similarly, the ATP synthase (also known as FoF1 - ATPase) in chloroplasts embedded in the thylakoid membrane enzyme ( CF1 - part extends into the stroma ) in mitochondria, a part of the inner membrane (F1 - facing part of the matrix ). In both systems, so ATP is released into the matrix / stroma. In contrast to ADP exchange it can pass into the cytosol of the cell.

Origin of chloroplasts - Endosymbiontentheorie

The structural design of a chloroplast resembles that of a cyanobacterium ( blue-green algae ). He has this as a ring -structured -stranded DNA genome and its own protein synthesis. The structure of this active ribosomes is the same as in cyanobacteria. Another argument is the original " proliferation " of chloroplasts without a structural coupling of cell division of the surrounding eukaryotic cell.

This has led to the endosymbiotic theory, which states that chloroplasts have evolved as endosymbiotic cyanobacteria and mitochondria as endosymbiotic bacteria. In the course of evolution and integration of the cyanobacterial precursor of the chloroplasts in the "host cell" led to various adjustments.

Among them is the adaptation of the chloroplast genome. The size of the genome decreased from approximately 3.5 million bases 120-160 thousand. In addition, a four-part structure of the genome began to emerge, which can be found in most photosynthetic organisms. Here are two opposing copies, which are interrupted by one-time items, the sizes of the three different DNA regions may vary depending on the species to a complete loss of the copied regions. The reduction of the genome was accompanied by the loss of genetic information and transfers to the nucleus. At the same time a complex machinery for the import of proteins from the cytosol into the chloroplasts developed. Thus one finds approximately 2000 proteins in the chloroplast, with only 100 genes are found in the genome. The genes encode products that can be divided roughly into two categories: maintenance of the genetic apparatus ( DNA polymerase, tRNAs and rRNAs ) and maintenance of photosynthetic capacity ( photosystem components and other proteins). It is not yet fully understood, such as the synchronization occurs in the expression between the nucleus and chloroplasts. This is necessary because plastid and nuclear- encoded products are assembled in all protein complexes in the chloroplast.

For a long time it was unknown how chloroplasts divide and change shape. Today we know that bacteria have a cytoskeleton, whose proteins are evolutionary precursors of the eukaryotic cytoskeleton. Experiments on the moss Physcomitrella patens (including with knockout mosses ) it is known that the FtsZ proteins, tubulin homologs, not only the division of chloroplasts cause, but can also form a complex network in chloroplasts. As these networks strongly reminiscent of the cytoskeleton, Reski 2000 coined the term " Plastoskelett " for this structure and postulated that it fills similarly complex functions in the plastids such as the cytoskeleton for the entire cell.