Chloroplasts and Other Plastids

Chloroplasts
Chloroplasts
Plastids are highly specialized, double membrane-bound organelles found within the cells of all plants and algae. A type of plastid called the chloroplast is the cellular location of the process of photosynthesis.

Plastids exhibit remarkable diversity with respect to their development, morphology, function, and physiological and genetic regulation. Chloroplasts, a type of plastid, are arguably largely responsible for the maintenance and perpetuation of most of the major life-forms on earth through photosynthesis.

The process of photosynthesis uses visible light as an energy source to power the conversion of atmospheric carbon dioxide into organic molecules that can be used by living organisms.

As a by-product of photosynthesis, oxygen is released into the atmosphere and is used by living organisms in the energy-obtaining process of cellular respiration. Other plastid types are specialized for synthesis and storage of pigments, starch, and other secondary metabolites.

Plastid Structure


The typical plastid from the cell of a flowering plant is surrounded by a double membrane system consisting of an inner and outermembrane, with an intermembrane space between the two.

In chloroplasts, the photosynthetic pigments that are responsible for absorbing sunlight are located in the thylakoid membrane system. This continuous internal membrane system is found throughout the chloroplast stroma, an internal fluid matrix analogous to the cellular cytosol.

Granal thylakoids are organized into stacks, and the stromal thylakoids are unstacked and exposed to the stromal matrix. The internal space within the thylakoid membrane system is called the lumen.

The pigments and proteins involved in the light reactions of photosynthesis, the processes whereby light energy is converted into chemical energy, are embedded in the thylakoid membrane system. The dark reactions, or Calvin cycle, which is the carbon fixation pathway that leads to the formation of simple carbohydrates, occurs in the stroma.

Small starch granules and oil bodies, termed plastoglobuli, are often found in chloroplasts. These serve as energy storage reserves for the plant cell. Plastids other than chloroplasts typically lack thykaloids.

Proplastids

Plastids
Plastids
The developmental precursor to all plastid types is the proplastid. Proplastids are relatively undifferentiated plastids typically found in young, undifferentiated meristematic cells and tissues.

Under the appropriate cellular and environmental conditions, proplastids can undergo development and differentiation to any of three main plastid types: chloroplasts, chromoplasts, or leucoplasts.

Chloroplasts

Chloroplasts typically contain one or more of the three types of plastid chlorophylls (chlorophyll a, b, or c) and, often, members of the two classes of photosynthetic accessory pigments: carotenoids and phycobilins.

The most obvious and essential physiological process unique to chloroplasts is photosynthesis. In the energy transduction reactions (the light reactions), radiant energy in the form of visible light (mostly of the violet, blue, and red wavelengths) is harnessed primarily by the green pigment chlorophyll.

The harnessed energy is then used to phosphorylate adenosine diphosphate (ADP) to produce adenosine triphosphate (ATP) in a process termed noncyclic photophosphorylation and reduce the electron carrier nicotinamide adenine dinucleotide phosphate (NADP) to NADPH. Oxygen is liberated through the light-dependent oxidative splitting of water.

In the carbon-fixation reactions (often called the dark reactions, although they can occur in the presence of light) the ATP is used as an energy source for the attachment of atmospheric carbon dioxide to the simple sugar ribulose 1,5-bisphosphate (RuBP). The NADPH is used to facilitate the reduction of RuBP through a series of simple sugars in a biochemical set of reactions known as the Calvin cycle.

One of the products of this cycle, glyceraldehydes-3-phosphate (G3P), is used by the chloroplast to make glucose and other carbohydrates. G3P is also needed to perpetuate the Calvin cycle, so only one of every three produced is used for carbohydrate synthesis.

Chloroplasts
Chloroplasts
Chloroplasts are also the site of synthesis for the three aromatic amino acids: phenylalanine, tyrosine, and tryptophan.

The precursor compound aspartate is imported into chloroplasts from the cell cytosol and is used for the synthesis of the amino acids lysine, threonine, and isoleucine. An intermediate in the synthesis of threonine, called homoserine 4-phosphate, is exportedfromthe chloroplast into the cytosol as a precursor formethionine.

Thus, there is a strong integration of function among the chloroplast, cytosol, and nucleus, in that the enzymes involved in these amino acid biosynthetic pathways are nuclear-encoded, their mRNAs are translated using cytosolic ribosomes, and most of the biosynthetic enzymes are imported into the chloroplast.

Fatty acid biosynthesis is another biochemical function that occurs in chloroplasts. Fatty acids, as lipid precursors, might be either incorporated directly into chloroplast lipids via a plastid-localized biochemical pathway or exported into the cytoplasm for conversion into endoplasmic reticulum lipids.

Lipids found in the inner plastid membrane are plastid-synthesized, whereas those of the outer plastid membrane are synthesized in the endoplasmic reticulum.

Other Plastids

Other plastid types include chromoplasts, which typically contain carotene or xanthophyll pigments and are responsible for the colors of many fruits, flowers, and roots. Under some conditions chromoplasts can differentiate into chloroplasts. Leucoplasts are colorless and lack complex inner membranes.

One type of leucoplast, the amyloplast, synthesizes and stores starch. Other leucoplasts synthesize a wide range of products, including oils and proteins. Proplastids that are arrested during their normal development into chloroplasts are termed etioplasts. These typically are formed when developing plant tissues are deprived of light.

Evolutionary History

Plastids possess a number of features that provide insights into their remarkable evolutionary history. The chloroplasts of eukaryotic cells photosynthesize in a manner similar to the more ancient prokaryotic cyanobacteria by using membrane-bound chlorophyll to capture radiant energy.

Some plastids even bear a strong morphological similarity to cyanobacteria, being similar in size and having similar internal structures. Plastids divide by binary fission in a manner similar to bacterial reproduction.

Plastids also have a certain degree of autonomy in terms of their genetic system. Typically, the majority of flowering plant plastids contain multiple copies (50-100) of a circular chromosome, ranging in size from 130 to 180 kilobase pairs (kb) in higher plants. Chromosome size in algae is much more variable, ranging all the way from 57 kb to 1,500 kb.

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The plastid chromosome contains genes for RNAs, such as rRNA(ribosomal RNA) and tRNA(transfer RNA), and structural genes that code for polypeptides involved in photosynthesis, transcription, protein synthesis, energy transduction, and several other functions. Many of the genes on the chloroplast chromosome are organized into clusters termed operons in a manner similar to that found in eubacteria.

The nucleotide sequences of many plastid genes, especially the ribosomal RNA genes, are highly similar to those in eubacteria, and the ribosomes found within plastids have a similar composition and size to eubacterial ribosomes.

The plastid-encoded genes are transcribed either by a nuclear-encoded or plastid-encoded RNA polymerase, and the resultant mRNAs are translated by plastid ribosomes found within the stroma. The majority of plastid biochemical processes rely on both nuclear-and plastid-encoded genes.

Some proteins, such as RuBP carboxylase/oxygenase (Rubisco), are composed of both nuclear- and plastid-encoded protein subunits, again demonstrating the remarkable coordination of biogenesis anddevelopment between organelle and cytosol.

This evidence lends strong support to the endosymbiotic theory of the origin of plastids. This theory, in essence, states that plastids were once free-living, autotrophic (having the ability to obtain carbon from carbon dioxide), prokaryotic cells that were engulfed through phagocytosis by an ancestral heterotrophic nucleated cell (a cell having a metabolism where carbon must be obtained from organic molecules) termed a protoeukaryote.

Typically this engulfment would result in the ingestion and subsequent destruction of the engulfed prokaryotic cell. However, in one—or perhaps several—independent incidents, a symbiotic relationship was gradually established between the engulfed photosynthetic bacterium and the protoeukaryote.

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The captured bacterium provided an internal source of food production for the heterotrophic eukaryotic cell through photosynthesis. The bacterium, in turn, was provided with protection and a stable external environment in the cytosol of the eukaryotic cell.

To coordinate further the physiological and genetic interactions between the two, massive transfer of genes took place over time from the genome of the photosynthetic bacterium to the nuclear genome of the protoeukaryote, leading to the genetic capture and control of the photosynthetic endosymbiont.

Recent investigations have shown that this gene transfer event is an ongoing process, with examples of transfer documented in recent evolutionary time for both plastids and mitochondria in several different evolutionary lineages of flowering plants.

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