Chloroplast

Chloroplast II

Bryum capillare leaf cells showing chloroplasts

Cyanidium O5A

Bornetia secundiflora herbarium item

AMP Capo Gallo 030 Peyssonnelia squamaria

Chloroplasts are organelles found in the cells of plants and algae. They are crucial for the process of photosynthesis, whereby sunlight is converted into chemical energy, specifically in the form of glucose, which serves as a vital energy source for the plant and, indirectly, for many other forms of life on Earth. Chloroplasts are characterized by their green color, which is due to the presence of chlorophyll, a pigment that plays a key role in absorbing light energy.

Structure

Chloroplasts are surrounded by a double membrane, consisting of an outer and an inner membrane. Between these membranes is an intermembrane space. Inside the inner membrane is the stroma, a fluid-filled space that contains enzymes, DNA, ribosomes, and other molecules necessary for protein synthesis and the replication of chloroplasts. Embedded within the stroma are thylakoids, flattened sacs where photosynthesis actually takes place. Thylakoids are often stacked in structures called grana (singular: granum). The space inside a thylakoid is known as the thylakoid lumen.

Function

The primary function of chloroplasts is to conduct photosynthesis. This process can be divided into two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). In the light-dependent reactions, which occur in the thylakoid membranes, sunlight is captured by chlorophyll and used to produce ATP and NADPH. These energy carriers are then used in the Calvin cycle, which takes place in the stroma, to convert carbon dioxide (CO2) from the air into glucose.

Chloroplasts also play roles in other plant cell functions, including fatty acid synthesis, amino acid synthesis, and the immune response.

Evolution

Chloroplasts are believed to have originated from a symbiotic relationship between a non-photosynthetic eukaryotic cell and a photosynthetic cyanobacteria. This theory is supported by the fact that chloroplasts, like mitochondria, contain their own DNA, which is circular and similar to bacterial DNA. This evolutionary process is known as endosymbiosis.

Genetics

Chloroplast DNA (cpDNA) is distinct from the plant's nuclear DNA and is inherited maternally in most plants. The chloroplast genome is relatively small and encodes some, but not all, of the proteins and RNA molecules required for chloroplast function. Many proteins found in chloroplasts are encoded by nuclear genes, translated in the cytosol, and then imported into the chloroplast.

Biotechnological Applications

Due to their unique properties, chloroplasts have been explored for various biotechnological applications, including the production of biopharmaceuticals, biofuels, and for use in phytoremediation. Genetic engineering of chloroplast genomes (plastome engineering) allows for the expression of foreign genes at high levels, which can be used for the production of vaccines, antibiotics, and other valuable compounds.