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Stroma; thylakoi

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Grana (plural); Grannum (singular)

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Carbon dioxide

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Photosynthesis is the process used by all autotrophs, or producers, to generate energy without having to devour other organisms. Autotrophs play an essential role in supplying energy to the world’s organisms, as they convert the light from the sun (a form of energy) into chemical energy. This chemical energy is stored in carbohydrates for later use by the autotroph. When a heterotroph, or consumer, consumes an autotroph, it digests those carbohydrates and gain the energy stored within. Photosynthesis is one of the most important processes in nature, and it is performed by an organelle in the cells of photosynthetic organisms called chloroplast. 

chloroplastAs can be seen by comparing structure illustrations of chloroplasts and cyanobacteria, chloroplasts seem like nothing more than developed / modified versions of cyanobacteria, suited for function as an organelle of a plant or chromista cell instead of an independent single-celled organism. The predominant theory among scientists is that some phagocytic single cell eukaryotes must have swallowed cyanobacteria, and rather than breaking them down, gradually absorbed them and developed them as an organelle. Chloroplasts retain much from their single-celled ancestors like the stroma, which resembles cytosol in all cells, including cyanobacteria; double-membrane envelopes; nucleoids, complete with their own DNA and the supporting ribosomes; and thylakoids, substructures that serve as the site in chloroplasts and cyanobacteria of the conversion of light into chemical energy.

Chloroplasts are distributed throughout the cells of photosynthetic organisms. Different types of cells of photosynthetic organisms have different concentrations of chloroplasts, depending upon the type of cell and its function in the organism. For example, a leaf cell would have a far higher concentration of chloroplast than the rest of a plant since leaves are the parts of plants that absorb light to trigger photosynthesis, while the cells of other parts may not even contain chloroplasts.

Structure and Function

Chloroplasts are mostly ovoid in shape, and are enveloped by two membranes. The outer membrane is a semi-permeable envelope that encloses the organelle, protects its contents, and regulates entry to the chloroplast. The inner membrane further regulates entry and exit, and is also the site of lipid-like molecule synthesis. Some chloroplasts have an extension to the inner membrane called the peripheral reticulum. This plays host to vesicles that help transport materials between the chloroplast’s outer membrane and its inner contents, much like the vesicles in cell membranes. Most chloroplasts contain a thin intermembrane space. Within this space, some even have an extra peptidoglycan membrane resembling the cell wall of their cyanobacterial ancestors. Others host only empty space between these membranes.

Because of their evolutionary ancestry, chloroplasts are filled with a basic, aqueous fluid that resembles the cytosol of cyanobacteria. The stroma contains important proteins and the DNA-ribosome package required for transcription and translation. The nucleoids of chloroplasts don’t give the plant cell any special feature of inheritance, but do make the chloroplast more of an independent subunit that the plant cell relies upon to perform photosynthesis. As such, they are replicated along with chloroplasts. Attached to thylakoids throughout the stroma, plastoglobuli are important protein-lipid bubbles that consist of essential lipids like vitamin E, an antioxidant, and chlorophyll, the material that absorbs light and transports it into the thylakoid to start photosynthesis. The stroma also contains starches that are associated with the chloroplast’s synthesis and transport of sugars from photosynthesis. Among the most important proteins of the stroma (and most abundant in the world) is rubisco, the enzyme that speeds up the synthesis of sugar molecules from CO2 (carbon dioxide). The stroma is also the site of most of a plant’s amino acid synthesis and nitrogen compounds.

chlorophyll-structureThylakoids are sacks that host the process by which light is converted into chemical energy. They are bound by a membrane similar to that of the outer and inner membranes of the chloroplast. In most chloroplasts, thylakoids of a disk shape are arranged into a stack called a granum. These thylakoids are surrounded by stromal thylakoids that spiral around the granum and diverge on one end of the granum into multiple layers of thylakoids called lamellae. These grana of thylakoids are usually interconnected by their shared stromal thylakoids. Each granum often contains many thylakoids, depending upon how much light its particular type of plant cell receives.


From the attached plastoglobuli, thylakoids receive through their membranes the essential protein complexes containing carotenoids, which absorb and transport light, and chlorophyll, which converts that light into chemical energy. Light, which is a form of energy, excites electrons and strips them from substances like water, releasing hydrogen ions and oxygen gas that is eventually secreted as waste by the plant into the environment. The now excited hydrogen ions make their way into the thylakoid space (lumen), effectively making the contents more acidic. This triggers the annexation of these hydrogen ions to the adenosine diphosphate (ADP) molecules and nicotinamide adenine dinucleotide phosphate (NADP+) ions. The resulting adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) molecules are the energy currency of cells, and now store the original energy taken from the sun. The ATP and NADPH molecules are often found in the membranes of the stromal thylakoids. The rubisco enzyme in the stroma reacts with CO2 to produce the unstable ribulose bisphosphate (RuBP) molecules, which break down into 3-phosphoglyceric acid (3-PGA) molecules. The ATP and NADPH molecules react with 3-PGA molecules to form simple sugar molecules that can combine to form the larger carbohydrates used for long-term storage in various parts of the photosynthetic organism. Some of these conversions take place outside of the chloroplast.


There are different types of chlorophyll, each with its own unique molecular formula. All photosynthetic organisms' chloroplasts contain chlorophyll a. Plants, the most inclusive definition, have chloroplasts with chlorophyll a and b. Chromista chloroplasts have chlorophyll a, c1, and c2. Cyanobacteria have chlorophyll a, d, and f. The color of the chloroplast-rich parts of photosynthetic organisms is based on their types of chlorophyll and various concentrations of other pigment-type molecules, with plant leaves being green, chromista exhibiting a yellow-brownish color, and cyanobacteria a cyan color. With the few differences noted, photosynthesis is performed in essentially the same way across all photosynthetic species.


Photosynthesis is one of the most important processes in nature, and chloroplasts are the organelles in the cells of photosynthetic organisms that are responsible for carrying this process out. Owing to their evolutionary origin and consequent existing structure, chloroplasts function as independent subunits that perform photosynthesis for the autotroph so that it can generate and store energy that it needs for all of its activities. Photosynthetic organisms are an important food source for most lifeforms, and their waste products (such as oxygen) help make and maintain the atmosphere of the planet. All of this starts and is made possible first by the sun and then by the chloroplast’s ability to absorb, transport, and convert that light into chemical energy stored in as sugars.

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