Chloroplast

From Encyclopedia Jr, free information reference for Kids

Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. Chloroplasts absorb sunlight and use it in conjunction with water and carbon dioxide gas to produce food for the plant. Chloroplasts capture light energy from the sun to produce the free energy stored in ATP and NADPH through a process called photosynthesis. It is derived from the Greek words chloros which means green and plast which means form ( in biological terms it can be more roughly translated as organelle or cell ).

Origins

Chloroplasts are one of the many unique cells in the body, and are generally considered to have originated as endosymbiotic cyanobacteria. In this respect they are similar to mitochondria, but are found only in plants and protista. Both organelles are surrounded by a double celled composite membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many infoldings, filling their inner spaces.

In green plants, chloroplasts are surrounded by two lipid-bilayer membranes. The inner membrane is now thought to correspond to the outer membrane of the ancestral cyanobacterium. The chloroplast genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities. Many of the missing genes are encoded in the nuclear genome of the host.

It is interesting to note that in some algae (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have arisen through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts have themselves been engulfed by still other eukaryotes, forming tertiary endosymbionts.

Structure

The inside of a chloroplast with the granum circled.

Chloroplasts are flat discs usually 2-10 micrometer in diameter and 1 micrometer thick. The chloroplast has a two membrane envelope termed the Inner & Outer membrane respectively. Between these two layers is the intermembrane space.

The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes, though most of their proteins are encoded by genes contained in the cell nucleus, with the protein products transported to the chloroplast.

Within the stroma are stacks of thylakoids, the sub-organelles where photosynthesis actually takes place. A stack of thylakoids is called a granum (plural: grana). A thylakoid looks like a flattened disk, and inside is an empty area called the thylakoid space or lumen. The photosynthesis reaction takes place on the membrane of the thylakoid, and, as is also the case with mitochondria, involves the coupling of cross-membrane fluxes with biosynthesis.

Embedded in the thylakoid membrane is a dish of chlorophyll molecules known as an antenna complex. This outer array helps to increase the surface area of light capture. The light photons are then funneled to the centre of this complex. Two chlorophyll molecules are then ionised, producing an excited electron which then passes onto the photochemical reaction centre.

See also

• Chloroplast membrane

• Inner membrane
• Outer membrane

• Calvin cycle

References

• This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain

External links

• Chloroplasts and Photosynthesis: The Role of Light from Kimball's Biology Pages
• Chloroplast, Botany
• Use of chloroplast DNA in studying plant phylogeny and evolution
• 3D structures of proteins associated with thylakoid membrane