A chloroplast is an organelle found in all plant and eukaryotic algae, but that isn’t how it started. As mentioned, somewhere between 1-2 billion years ago, a eukaryotic cell swallowed a photosynthetic bacterium. While this sort of event would normally mean that the smaller cell would be broken down through phagocytosis, in this case it did not.
Over the course of evolutionary history on this planet, there are many remarkable stories and leaps forward, some of which we are only now beginning to fully understand. The first spark of life remains the most mysterious of all, but since then, life has moved from the water to the land, has moved from unicellular to multicellular, and has even led to us – a sentient, self-aware species that is on the cusp of spreading into the stars.
One of the most interesting moments in evolution is often not discussed, and occurred between 1 and 2 billion years ago, leading to the very first chloroplast – a critical element of all plant life on the planet.
What is a Chloroplast?
A chloroplast is an organelle found in all plant and eukaryotic algae, but that isn’t how it started. As mentioned, somewhere between 1-2 billion years ago, a eukaryotic cell swallowed a photosynthetic bacterium. While this sort of event would normally mean that the smaller cell would be broken down through phagocytosis, in this case it did not. Furthermore, the cyanobacterium (a prokaryote) continued to photosynthesize and formed an endosymbiotic relationship with its host cell. In exchange for not destroying the bacterium, it began making food for the cell. This is done by capturing the energy from sunlight and then converting it into usable chemical energy to produce organic molecules, namely sugar.
Millions of years passed and the bacterium was passed down to each daughter cell through cell division; it also began to change, losing some of its initial genetic material and beginning to synthesize and bond with different proteins that changed its overall function. Gradually, the bacterium became assimilated and developed into an organelle of its own – the chloroplast. This “engulfing” theory is strongly supported by the structural similarities to cyanobacterium, and is largely accepted by the scientific community.
Despite its humble beginnings, the chloroplast eventually developed into a sophisticated and efficient organelle, and has a number of critical adaptations and structural components that allow it to serve such an important role in the cell. To begin with, chloroplasts tend to be concentrated in guard cells, which are located near the stomata and help to regulate liquid and gas exchange. Since chloroplasts require carbon dioxide for photosynthesis, their particular presence in these cells makes sense.
Similar to the nucleus, chloroplasts are surrounded by two lipid bilayers that form the membrane envelope; these membranes are selectively permeable and keep the stroma separated from the inner chloroplast space. Within the inner membrane of the chloroplast, there is a complex membrane system consisting of thylakoids, membranous sacs where photosynthesis occurs. The space inside these thylakoid sacs is called the lumen, and contains chlorophyll, the pigment in the chloroplast that absorbs energy from sunlight. These thylakoid sacs are stacked tightly in groups called grana, which is where the conversion of light energy to chemical energy occurs. Surrounding all of the thylakoid sacs, but held in by the inner membrane, is the stroma. This is similar to cytoplasm, but should not be ignored, as a number of important functions – such as the conversion of carbon dioxide into sugar – will occur.
The chloroplasts in plant cells are very similar to the mitochondria in other eukaryotic cells, including in animals. They are both closely linked to energy metabolism, have their own DNA, a tightly wound central space with perpetually dynamic activity, and they both likely evolved after being engulfed by a larger eukaryotic cells about a billion years ago!
Now found in every plant cell and a large number of photosynthetic protists, chloroplasts generate food for the cell, due to their photosynthetic ability. To properly appreciate the functional nature of a chloroplast, it is necessary to review the finer points of photosynthesis.
When sunlight energy strikes a plant, it is absorbed by the chlorophyll pigment located in the chloroplast. Photosynthesis is a two-stage process, consisting of the light reactions and dark reactions. The light reactions begin in the grana, explained above, where the energy is converted by chlorophyll a into ATP and NADPH. These are two forms of energetic “currency” for every cell, and also play a key role in cellular respiration (occurring in the mitochondria). The ATP and NADPH is generated by passing through 2 different photosystems, including an electron transport chain that creates a charge gradient within the chloroplast. This gradient enables the flow of hydrogen ions through a protein complex named ATP synthase, which produces ATP!
The ATP and NADPH generated during the light reactions will then be used in the dark reaction stage, which is often referred to as the Calvin Cycle. The ATP and NADPH, in combination with carbon dioxide, are transformed into sugar within the stroma of the chloroplast. Once those sugars are created within the chloroplast, they can be stored as starch, used in the production of cellulose, or utilized during cellular respiration.
What many people don’t realize is that the chloroplast has a number of other key functions besides generating food for the cell. The chloroplasts are also the site of amino acid synthesis, as well as other nitrogenous bases like purines and pyrimidines that the cell needs for DNA and RNA synthesis.
More Than What Meets the Eye
Chloroplasts are an interesting organelle because their numbers are dynamic, based on the needs of a plant. There can be anywhere from 1-100 chloroplasts in a single cell, and this number can fluctuate based on the type of cell, its location on the plant, and the amount of sunlight that is available for absorption and conversion by chlorophyll.
Without that fortuitous engulfing of a prokaryote cell a billion years ago, plant life would never have been able to develop, which would have prevented the oxygen levels in the atmosphere from changing. This, in turn, would have made the further evolution of eukaryotic cells impossible, meaning that humans might never have existed! So, while they may be small and seemingly insignificant organelles, completely unrelated to human beings, they are in fact a central figure in our survival!
- Georgia State University
- Florida State University
- Arizona State University
- Book: Molecular Biology of the Cell. 4th edition.