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Relationship Between Chloroplasts & Photosynthesis

Photosynthesis is found in a wide variety of organisms, including plants, protists and bacteria. Even some animals and fungi-organisms not normally associated with photosynthesis have formed symbiotic relationships with photosynthetic organisms as in the case of corals and lichens. Eukaryotes such as plants and algae conduct photosynthesis within specialized cell organelles called chloroplasts. Absent in bacteria, these structures are unique to photosynthetic eukaryotes but there does seem to be a connection between chloroplasts and bacteria called the endosymbiont theory.

Photosynthesis
- Photosynthesis involves two sets of chemical reactions to convert sunlight into organic molecules. The first set of reactions, termed the light-dependent reactions, uses pigments such as chlorophyll to capture sunlight and create chemical energy through a series of redox reactions on a membrane. The second set of chemical reactions, termed the light-independent reactions, utilizes the chemical energy from the light-dependent reactions to drive the reactions that fix carbon dioxide into more complex molecules.
Chloroplasts and Light-Dependent Reactions
- Chloroplasts are specialized organelles within eukaryotes used for photosynthesis. The chloroplasts contain stacks of membranes called thylakoid membranes. Embedded on the surface of these membranes are photosystems which house the pigments that capture energy from sunlight. This energy is then transferred via an energized electron through a series of molecules also embedded on the membrane. This electron transport chain creates a proton gradient within the inner spaces of these membranes. The protons flow across the membrane through channels created by the enzyme ATP synthase which produces ATP from the flow of protons.
Light-Independent Reactions
- The light-independent reactions, often called the Calvin Cycle, occur within the spaces surrounding the chloroplast, termed the stroma. The Calvin Cycle fixes carbon dioxide to a five-carbon molecule (ribulose-1,5-bisphosphate) to yield two three-carbon molecules (3-phosphoglycerate). One of the three-carbon molecules may be shunted away to synthesize glucose. The remaining steps of the Calvin Cycle use the remaining 3-phosphoglycerate to regenerate the ribulose-1,5-bisphosphate needed at the beginning of the cycle. It is this regeneration of the ribulose-1,5-bisphosphate that requires the ATP generated in the light-dependent reactions. A second energy carrier molecule called NADPH is also required but this too is generated in the light-dependent reactions as the final step of the electron transport chain.
Endosymbiont Theory
- Complex organelles such as chloroplasts are notably absent in photosynthetic bacteria. However, chloroplasts share similarities with bacteria. Chloroplasts possess their own circular chromosome independent of the chromosomes found in the nucleus. Bacteria also possess a circular chromosome unlike the linear chromosomes found in eukaryotes. Chloroplasts also reproduce independently of the host cell. Chloroplasts possess double membranes as with ingested organsims -- a host membrane surrounding the organism's own plasma membrane. These and other features have led some biologists to propose an endosymbiont theory where chloroplasts and mitochondria are the result of a bacterial symbiont integrated into the cells of early eukaryotes.

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