Simplified Life Cycles

Known life comes in many forms and has diversified into a plethora of reproductive cycles.  Prokaryotes (bacteria other organisms lacking nuclei) represent the simplest of these, lacking any sexual or multicellular aspects, and relying solely on binary fission (simply duplicating all the cellular components and splitting the cell sown the middle).   The fungus wheat rust (Puccinia graminis); however, has two multicellular somatic (as in, not containing gametes) forms. One of these produces haploid (one set of chromosomes per nucleus, as opposed to diploidy, which involves two) monokaryotic (only one nucleus) spores; the other produces haploid dikaryotic spores, which have two nuclei, each with a different set of DNA.

To simplify things, three general models have been constructed that do a decent job of describing the vast majority of eukaryotic (having a nucleus) organisms. Each of the three, the so-called zygotic, gametic, and sporic cycles, describes its own set of organisms.

The gametic cycle contains only one multicellular stage, which happens to be diploid.  This diplodic mass eventually produces gametes (haploid cells which remain distinct individuals) through meiosis (nuclear division process that produces four haploid nuclei from one diploid nucleus).  The gametes generally pair up with other gametes of different mating types, and merge (in a process called fertilization). The product of fertilization is always a zygote (a single diploid cell), which in this cycle divides through mitosis (nuclear replication with near perfect preservation of genetic material) to produce the multicellular diploid body already mentioned.  Though you may never have though about animal reproduction in this way before, it perfectly, though overly simply, describes the process. Some protists and algae also have gametic cycles.

Zygotic life cycles have no diploid multicellular form; instead, meiosis occurs directly inside the zygote. Unlike the gametic cycle, the product of meiosis is not gametes, but haploid spores which grow into multicellular somatic bodies.  The multicellular form may be more of a loose association of communal cells, but either way, it produces gametes through mitosis. The gametes, of course, go on to form a zygote and complete the cycle.  Fungi and some algae fall into this category.

The sporic cycle has multicellular form in both haploid and diploid life stages. The zygote mitoticaly produces a diploid body (called a sporophyte) that produces spores through meiosis. Those spores grow into multicellular haploid bodies (called gametophytes) that mitoticly produce gametes. Fertilization brings the circle to a close.

Not to trip you up, but I realized as I was describing these cycles, that it is possible for some eukaryotic organisms to have no multicellular form.  All of those organisms would still fall within the range covered by these cycles, but a change of definition is needed.  The fact that a particular stage of life is multicellular is not important.  What is important is where in the cycle mitotic cell replication occurs.  Because multicellular bodies are usually built through mitosis, nothing really changes.  In the gametic cycle, mitosis occurs only in the diploid stage. In the Zygotic cycle, mitosis occurs only in the haploid stage. In the sporic cycle, mitosis occurs in both haploid and diploid stages.

Common Ancestry of Chloroplast and Mitochondria Revealed by Parallel Electron Transport Chains

Both the mitochondrion and the chloroplast have a double membrane system that isolates their energy conversion pathways from the reactions happening elsewhere in the cell.  Their similar anatomy allows for the function of similar chemical pathways, though there are different sources of energy, electrons, and hydrogen ions.

In both systems the space between the two membranes (the thylakoid lumen, in chloroplasts and the inter-membrane space, in mitochondria) has a high concentration of hydrogen ions relative to the fluid filling the inner membrane (the stroma and the matrix, respectively).  This hydrogen ion gradient is maintained by a series of chemical reactions catalyzed by a series of proteins specific to each organelle, though each protein complex parallels one in the other organelle, with a small exception.

The first of the protein clusters (Photosystem II, in chloroplasts and Complex I, in mitochondria) provides the electrons, hydrogen ions, and energy that fuel the electron transport chain.  Photosystem II uses energy from light to split oxygen from water, yielding two hydrogen ions and two electrons. The remaining energy from the light is stored in the electrons and shuttled down the chain.  The mitochondria; however, use the energy in NADH from the citric acid cycle to fuel the system. NADH is split into NAD+ and a hydrogen ion, while the two freed electrons are used by Complex I to shuttle one H+ across the membrane, against the potential.

From Photosystem II and Complex I, the electrons are passed to a Cytochrome b6/f complex or Complex III (Cytochrome b) depending on the organelle, via electron carriers.  In the chloroplasts this function is served by a host of lipid soluble molecules called plastoquinones, collectively called PQ for “pool of plastoquinones.”  Ubiquinone (also called coenzyme Q, or CoQ), a related chemical to plastoquinone, is the mitochondrial equivalent.

The cytochrome b6/f complex and complex III process the electron identically, passing them to plastocyanin and cytochrome c, pumping an H+ across the membrane along the way.  Plastocyanin and cytochrome c pass the electron to photosystem I of the chloroplasts and complex IV (cytochrome a/a3) of the mitochondria.

Photosystem I uses another boost from light to re-energize the electrons, then passes them down a chain of carriers, eventually reducing NADP+ to NADPH and relieving the transport chain of the electrons.  The energy stored in NADPH is later utilized in the Calvin cycle to produce more ATP.  Complex IV, on the other hand, uses the remaining energy to pump yet more hydrogen ions across the membrane, and then binds the electrons by triggering a reaction of hydrogen ions with dissolved oxygen.

ATP production works exactly the same in both systems, with the ATP synthase using the H+ potential to bind ADP and free phosphate.

There are certainly significant differences in the electron transport chains of mitochondria and chloroplasts, but all of those differences are dwarfed by the overall similarity of the two processes.

Genetic data shows fairly definitively that mitochondria where engulfed before chloroplast, of course assuming that both mitochondria and chloroplasts each had an ancestor that was free-living.  As an obvious statement, if chloroplasts were engulfed after the mitochondria, then the two were not engulfed as a singe organism that differentiated into the two organelles we have today.  Forced to accept that the organelles were engulfed at two different dates, and yet given the extreme similarity in electron transport chains, it seems not unreasonable to suspect that mitochondria and chloroplasts may share a common ancestor that had a comparable electron transport chain.