Chloroplast
Chloroplasts are plastid organelles found in the cells of plants and algae. They contain chlorophyll and carotenoid pigments, and an elaborate system of internal membranes called thylakoids. They are active in photosynthesis, the all-important process that converts light into chemical energy. In algae, chloroplasts exist in a wide variety of sizes and shapes. Depending on the algal species, they may be simple disks or elaborate ribbons twisted into a spiral. Plant chloroplasts are usually disk-shaped and measure between 4 and 6 micrometers in diameter. They are especially abundant in leaf mesophyll tissue where a single cell may contain as many as 50. Specialized chloroplasts lacking grana, but not chlorophyll, are found in the bundle-sheath cells of plants with the C4 or Hatch-Slack photosynthetic pathway for carbon fixation.
The internal structure of chloroplasts is very complex, with a structure that facilitates the capture of light photons and channels the energy of the captured photons into chemical compounds such as adenosine triphosphate (ATP). ATP is formed from adenosine diphosphate and phosphate in a process called photophosphorylation. The energy stored in ATP is used to synthesize sugars within the chloroplast. A plasma envelope or membrane surrounds the interior of the chloroplast. Within the structure are chlorophyll-rich areas called grana formed from stacks of disk-like membranous structures called thylakoids. The grana are embedded in the stroma or ground substance of the plastid and are connected by stroma thylakoids that run between them. The light-capturing chlorophyll and carotenoid pigments are embedded, and precisely oriented, within the thylakoid membranes.
When chloroplasts have been in bright light for a time, they often contain starch grains formed from sugars produced by the rapidly photosynthesizing organelle. These temporary storage products soon disappear when the plant is placed in the dark, and the insoluble starch is converted back to soluble sugar and distributed throughout the plant.
Chloroplasts contain DNA, RNA, and ribosomes and are able to synthesize a number of proteins and lipids needed for their structure and activity. The replication of chloroplasts requires the parent cell, and appears to involve a cooperation between chromosomal and chloroplast DNA. Chloroplast ribosomes are only about two-thirds as large as the ribosomes of plants and other eukaryotes. In this respect, they closely resemble the ribosomes of bacteria. They are also sensitive to bacteria-inhibiting antibiotics. Unlike eukaryote DNA but reminiscent of bacteria, chloroplast DNA generally has a circular rather than a linear structure, is not associated with histones, and is not bounded by a membrane. The similarities between chloroplasts and bacteria has led to speculation that chloroplasts originated as photosynthetic bacteria that found shelter in larger heterotrophic cells and provided the larger cells with a built-in energy source. These larger cells became the precursors of eukaryotic cells when they acquired mitochondria.
This is the complete article, containing 454 words
(approx. 2 pages at 300 words per page).
