Fixation of carbon dioxide

NADPH remains within the stroma of the chloroplast for use in the fixation of carbon dioxide (CO₂) during the Calvin cycle. In a complex cycle of chemical reactions, CO₂ is bound to a five-carbon ribulose biphosphate compound. The resulting six-carbon intermediate is then split into three-carbon phosphoglycerate. With energy supplied by the breakdown of NADPH and ATP, this compound is eventually formed into glyceraldehyde 3-phosphate, an important sugar intermediate of metabolism. One glyceraldehyde molecule is exported from the chloroplast, for further conversion in the cytoplasm, for every five that undergo an ATP-powered re-formation into the five-carbon ribulose biphosphate. In this way three molecules of CO₂ yield one molecule of glyceraldehyde 3-phosphate, while the entire fixation cycle hydrolyzes nine molecules of ATP and oxidizes six molecules of NADPH.

Evolutionary origins

The mitochondrion and chloroplast as independent entities

In addition to their remarkable metabolic capabilities, both mitochondria and chloroplasts synthesize on their own a number of proteins and lipids necessary for their structure and activity. Not only do they contain the machinery necessary for this, but they also possess the genetic material to direct it. DNA within these organelles has a circular structure reminiscent of prokaryotic, not eukaryotic, DNA. Also as in prokaryotes, the DNA is not associated with histones. Along with the DNA are protein-synthesizing ribosomes, of prokaryotic rather than eukaryotic size.

Only a small portion of the mitochondrion’s total number of proteins is synthesized within the organelle. Numerous proteins are encoded and made in the cytoplasm specifically for export into the mitochondrion. The mitochondrial DNA itself encodes only 13 different proteins. The proteins that contain subunits synthesized within the mitochondrion often also possess subunits synthesized in the cytoplasm. Mitochondrial and chloroplastic proteins synthesized in the cytoplasm have to enter the organelle by a complex process, crossing both the outer and the inner membranes. These proteins contain specific arrangements of amino acids known as leader sequences that are recognized by receptors on the outer membranes of the organelles. The proteins are then guided through membrane channels in an energy-requiring process.

The endosymbiont hypothesis

Mitochondria and chloroplasts are self-dividing; they contain their own DNA and protein-synthesizing machinery, similar to that of prokaryotes. Chloroplasts produce ATP and trap photons by mechanisms that are complex and yet similar to those of certain prokaryotes. These phenomena have led to the theory that the two organelles are direct descendants of prokaryotes that entered primitive nucleated cells. Among billions of such events, a few could have led to the development of stable, symbiotic associations between nucleated hosts and prokaryotic parasites. The hosts would provide the parasites with a stable osmotic environment and easy access to nutrients, and the parasites would repay the hosts by providing an oxidative ATP-producing system or a photosynthetic energy-producing reaction.

Wilfred D. Stein

The cytoskeleton

The cytoskeleton is the name given to the fibrous network formed by different types of long protein filaments present throughout the cytoplasm of eukaryotic cells (cells containing a nucleus). The filaments of the cytoskeleton create a scaffold, or framework, that organizes other cell constituents and maintains the shape of the cell. In addition, some filaments cause coherent movements, both of the cell itself and of its internal organelles. Prokaryotic (nonnucleated) cells, which are generally much smaller than eukaryotic cells, contain a unique related set of filaments but, with few exceptions, do not possess true cytoskeletons. Their shapes and the shapes of certain eukaryotes, primarily yeast and other fungi, are determined by the rigid cell wall on the outside of the cell.

Four major types of cytoskeletal filaments are commonly recognized: actin filaments, microtubules, intermediate filaments, and septins. Actin filaments and microtubules are dynamic structures that continuously assemble and disassemble in most cells. Intermediate filaments are stabler and seem to be involved mainly in reinforcing cell structures, especially the position of the nucleus and the junctions that connect cells. Septins are involved in cell division and have been implicated in other cell functions. A wide variety of accessory proteins works in concert with each type of filament, linking filaments to one another and to the cell membrane and helping to form the networks that endow the cytoskeleton with its unique functions. Many of these accessory proteins have been characterized, revealing a rich diversity in the structure and function of the cytoskeleton.