Chloroplast Fact Sheet: Definition, Structure, Genome, and Function

Chloroplast Definition

Chloroplast, found in plant cells and some protists such as algae and cyanobacteria, is a cell organelle known as a plastid. Chloroplasts are the food producers of the cell by converting light energy of the sun into sugar that can be used by the cell. This entire process is known as photosynthesis and it all depends on the high concentration of chlorophyll, the molecule that absorbs light energy and gives plants and algae a green color. Hence, the name chloroplast refers to chlorophyll-containing plastids. Like mitochondria, chloroplasts are considered to have evolved from the bacteria.

Structure of Chloroplasts

Chloroplasts are commonly found in guard cells located in plant leaves, roughly 1–2 μm thick and 5–7 μm in diameter. Chloroplasts are oval-shaped and have two membranes: an outer membrane and an inner membrane. Between the outer and inner membrane is the intermembrane space approximately 10-20 nm wide. The space within the inner membrane is the stroma, the dense fluid within the chloroplast. This is the site at where the conversion from carbon dioxide to carbohydrates takes place.

Other chloroplast structures include:

• Thylakoid System: internal membrane system consisting of flattened sac-like membrane structures called thylakoids where light energy is converted into chemical energy. Thylakoids contain the light-harvesting complex, including the electron transport chains used in photosynthesis and pigments like chlorophyll and carotenoids.
• Granum: densely layered stacks of thylakoids (10 to 20) that are the sites of conversion of light energy to chemical energy.
• Chlorophyll: a green photosynthetic pigment sitting on the surface of thylakoids that absorbs light energy.
• DNA Ring: the circular DNA that is distinct from the nuclear DNA.

Figure 1. Plant cell chloroplast structure.

Chloroplast Genome

Like mitochondria, chloroplasts contain DNA and reproduce independently from nuclear and mitochondrial DNA through a division process similar to bacterial binary fission. The chloroplast genome is typically circular (though linear DNA has also been documented) and is approximately 120–200 kb in length. The modern chloroplast genome has been much reduced in size over the course of evolution and increasing number of chloroplast genes have been transferred to the nuclear genome. As a result, the nuclear genome is necessary to encode proteins that are responsible for chloroplasts function.

Chloroplast genomes (cpDNA) are relatively conserved among land plants in terms of their size, structure, and gene content. A study revealed that 81% of genes are shared between land plants and the most ancient algae species (Mesostigma viride). Chloroplast genomes contain about 120 genes on average, including rRNA genes, tRNA genes, at least three subunits of prokaryotic RNA polymerases and some other protein-coding genes like thylakoid proteins and ribosomal proteins.

The chloroplast DNA sequencing is a high-throughput sequencing of plant chloroplast genomes using Illumina or PacBio platforms to perform an in-depth analysis of cpDNA. Comparative genomic analysis obtains information including species classification, phylogenetic evolution, geographic lineage inheritance, disease diagnosis and forensics.

Function of Chloroplasts

Chloroplasts are essential for the survival and growth of plants and photosynthetic protists. They are responsible to carry out photosynthesis, the process of conversion of light energy into sugar and other organic molecules that are used by plants or algae as food. They also produce amino acids and lipid components that are necessary for chloroplast membrane production.

Photosynthesis has two stages:

(i) The 1^st stage: the light-dependent reactions occur. Sunlight is captured through chlorophyll and carotenoids and converted into adenosine triphosphate (ATP, the molecular unit of currency of intracellular energy transfer) and nicotinamide adenine dinucleotide phosphate (NADPH), which carries electrons.

(ii) The 2^nd stage: the light-independent reactions, also known as the Calvin cycle, occur. The electrons carried by NADPH convert carbon dioxide to carbohydrate, a process known as CO2 fixation. Hence, carbohydrates and other organic molecules can be stored and used for energy.