21.4: Chloroplasts

Figure \(\PageIndex{1}\): Structure of the chloroplast. (Image by MolecularExpressions at Florida State University; reproduced with permission.)

Chloroplasts are organelles within the cells of algae and plants that are the sites of the photosynthesis reaction. There can be between one and ~100 chloroplasts per plant cell. They are less than 10 \(\mu\)m in diameter and ~2 \(\mu\)m thick. Within each chloroplast are multiple stacks of poker-chip-shaped discs called thylakoids. These structures contain chlorophyll, the enzyme which utilizes incoming wavelengths of light in the blue and red portions of the visible spectrum to catalyze the reduction of atmospheric carbon dioxide (\(\ce{CO2}\)) with water (\(\ce{H2O}\)), generating glucose (\(\ce{C6H_{12}O6}\)), along with free oxygen (\(\ce{O2}\)) as a waste product. (Chlorophyll appears green because that is the portion of the spectrum not utilized for photosynthesis. Rather than being absorbed, it is reflected.)

The anatomy of chloroplasts is shared by free-living cyanobacteria. Like cyanobacteria, chloroplasts have their own DNA. They reproduce independently of the rest of their host cell. Unlike cyanobacteria, they have two membranes separating them from their host cell. They make the inner of the two membranes, while the outer membrane is manufactured by the host cell.

We estimate that the endosymbiosis between an early eukaryote (already sporting mitochondria) and a cyanobacterium (the proto-chloroplast) to have occurred around 1.25 Ga. This date is supported both by genetic comparisons and the fossil record. It probably happened more than once, something that is thought to be more likely on the extreme population sizes and ubiquity of bacteria.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What is the function of the chloroplast in the cells of algae and plants?

a. It is the site where the host cell stores (and copies) its genetic information.

b. It is the site of photosynthesis, the biochemical process where chlorophyll is used to catalyze a reaction between carbon dioxide and water, producing glucose using energy from sunlight.

c. It is the site of respiration, the the biochemical process where pyruvate and oxygen are utilized to make molecules of ATP, a molecule that serves as a major "currency" of energy within the cell.

Answer

b. It is the site of photosynthesis, the biochemical process where chlorophyll is used to catalyze a reaction between carbon dioxide and water, producing glucose using energy from sunlight.

Exercise \(\PageIndex{2}\)

Which group of microbes is thought to be most closely related to the proto-chloroplast species?

a. the "Asgard" superphylum of Archaea

b. Methanogenic Archaea

c. Alphaproteobacteria

d. Cyanobacteria

e. Betaproteobacteria

Answer

d. Cyanobacteria

Recent analogues

Figure \(\PageIndex{2}\): Two cyanobacterial endosymbionts within a single Paulinella chromatophora amoeba. (Photograph by Ferry J. Siemensma; reproduced here with permission.)

An illuminating analogue can be found in the amoeba Paulinella, which incorporated a cyanobacterium into its cells much more recently, sometime between 140 and 90 Ma. Based on genetic similarity, the internal symbiont was probably an ancestor to one of the extant marine cyanobacterial genera Prochlorococcus or Synechococcus. Though the symbiosis that led to proper chloroplasts is more obscured by the depths of geologic time, that the same process has been “done over” independently in other groups such as Paulinella suggests the validity of the idea of cyanobacterial endosymbiosis as a critical process in the evolution of photosynthetic eukaryotic life.

Another example is Azolla. Technically a fern, it looks nothing like most ferns, and instead floats on the surface of freshwater ponds all over the world. Azolla hosts the cyanobacterium Anabaena within its cells. The Anabaena has the ability to capture nitrogen (\(\ce{N2}\)) from the surroundings and “fix” it in a biologically useful form, ammonia (\(\ce{NH3}\)). This is beyond the abilities of a typical chloroplast, but being a plant the Azolla already has typical chloroplasts. The endosymbiosis of the Anabaena is an additional cyanobacterial acquisition, for the sake of the nitrogen it can sequester and share. This is a huge advantage to the Azolla host, which is extremely capable of growth as a result – it can double its biomass in two days. The relationship between Azolla and Anabaena is not haphazard or improvised: It’s codified enough that the host Azolla cells pass the endosymbiont Anabaena cells directly to their offspring through cell division and reproduction, an example of “vertical” transmission.

Take two

There can also be multiple rounds of endosymbiosis. Imagine, for instance, a eukaryotic cell containing a double-membraned chloroplast. (This is the situation we have already described.) What happens when that cell gets engulfed by another, bigger cell? This is secondary endosymbiosis, a fresh iteration wherein the eater gets eaten. The chloroplast will now be quadruple membraned, with both the “original” two membranes, plus a third membrane from its first host and a fourth membrane from its newest, outermost host.

Figure \(\PageIndex{3}\): When a cyanobacterium is engulfed by a larger eukaryotic cell, you get a chloroplast. When that cell gets engulfed by another, it’s a case of secondary endosymbiosis.

When this process is recent, there may be a residual nucleus-like body between the third and fourth membranes: this is the nucleus of the first host cell. In time, the second host cell may digest (or assimilate) that DNA, and perhaps one or more of the membranes surrounding the chloroplast.

There is even a well-documented case of tertiary endosymbiosis, in the dinoflagellates, where a cyanobacterium was engulfed by an ancestral eukaryotic host, making a unicellular alga, which was then swallowed by a protozoan, and then that was engulfed by a haptophyte alga! Geobiologist Andy Knoll refers to dinoflagellates as “biological Matryoshka dolls,” for good reason. A good first guess about how many episodes of endosymbiosis a particular organelle has undergone can be arrived at by counting the number of membranes and subtracting 1. Just to drive the point home, it’s worth noting that dinoflagellates are the so-called “zooxanthellae” that are endosymbionts in corals: Unless the coral undergoes “bleaching,” evicting the dinoflagellates into the surrounding water, that’s one more Matryoshka doll yet!

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

A secondary endosymbiont will initially have how many layers of membrane surrounding it?

a. Four

b. Two

c. Six

d. One

e. Five

f. Three

Answer

a. Four