Chloroplasts are found in plants and protista such as algae. Chloroplasts are not found in blue-green algae (cyanobacteria - kingdom Monera). However the cyanobacteria have their own membranes which function like the thylakoid membranes found in chloroplasts. The question of how plastids originated is being studied in a number of labs.
The chloroplast is a relatively large complex organelle with a double membrane. These membranes create two compartments the stroma, site of the light indpendent reacions of photosynthesis (Calvin cycle) and the inner thylakoid membrane with processes electrons used by the light reactions.
Chloroplasts have their own DNA (genes) and ribosomes. They are able to reproduce themselves and to carry out the synthesis of some (but not all) of their own proteins.
The chloroplast reproduces by simply pinching in two which looks like fission in prokaryotes.
Chloroplast means “green form”; green because the chloroplast contains the energy absorbing molecule chlorophyll.
Chloroplasts are essential in the process of photosynthesis in which carbon dioxide and water are changed into sugar and other important organic molecules. Oxygen is released as a by-product.
The cyanobacteria also contain energy gathering molecules related to the chlorophyll pigment found in eukaryotes.
Inside its double membrane, each chloroplast contains a clear, watery area called the stroma, which corresponds to the cytoplasm of a cell. It is in the stroma where the chloroplast makes sugar in a metabolism called the Calvin cycle.
This clear fluid area of the chloroplast is invaded by an extensive membrane system that forms saclike vesicles called thylakoids.
The cartoon of a thylakoid below gives a diagramatic rendering of the major molecular components necessary for the light reaction of photosynthesis. A 3D view of one of the molecules involved in the harvesting of light in photosystem II may be viewed at the Lund University, Sweden
When thylakoid membranes are folded into stacks they are referred to as grana, and where these membranes occur singly they are called stroma thylakoids.
The labeled illustration of a chloroplast found below may help you sort out the relationship between these terms.
Located in and on the thylakoid membranes are numerous granules called CF0CF1 complexes. The CF0 portion anchors the assemblage into the thylakoid membrane and forms a channel between the stroma and lumen, the two compartments of the chloroplast.
The CF1 portion contains the enzyme ATP synthase, and as you might expect is involved in the synthesis of ATP.
Besides the chloroplast, plants have other organelles called plastids which are related in origin but which have other functions.These include:
Visit the Virtual Cell to zoom into the thylakoid membranes and see how the light reaction works
Three structures mitochondria and chloroplasts have in common are:
In addition both mitochondria and chloroplasts reproduce independently of the "host" cell.
Mitochondria differ from chloroplasts because the final products of the mitochondrion are the reactants of the chloroplast. The use of sugar in aerobic respiration to produce energy by mitochondria is reversed in the chloroplast where energy is used to make sugar. Respiration is an exergonic reaction while photosynthesis is endergonic Many scientists propose that mitochondria and chloroplast are endosymbionts, that is these organelles are actually descendants of once-independent prokaryotic cells.
The similarities between both mitochondria and chloroplasts and the prokaryotes is powerful evidence for the serial endosymbiotic theory for the origin of these organelles.
Mitochondria are generally much smaller than chloroplasts, at least in cross section. However mitochondria may have a surprisingly convoluted structure.
In electron micrographs, mitochondria usually are oval, with the inner membrane exhibiting numerous folds called cristae.
The apparent purpose for this extensive folding is to increase the surface area where most of the biochemical work of the mitochondria is accomplished.
The following diagram illustrates relationships among the membranes and compartments found in mitochondria.
Electron microscope studies reveal that the cristae are covered with small, round bodies very similar to those in chloroplasts The spherical granules and their membrane bound tails are called F0F1 complexes which are similar to the CF0 CF1 complexes of chloroplasts. You can go to an animation of ATP synthase in action and visit a tutorial on metabolism.
Cells rich in mitochondria have either high metabolic rates, or a need for explosive energy output such as skeletal muscles.
For a complete discussion of mitochondrial structure and function visit the University of Texas Medical Branch site.
Click here to visit other sites describing
how
mitochondria work.
Modified July 9, 2005
