= Chloroplast
{wiki}
A <chloroplast> is a photosynthetic <plastid> in <plant> and <algae> <biological cells>. The green-plant <chloroplast> modeled here has a <chloroplast envelope> surrounding <chloroplast stroma> and internal <thylakoids>. <Light-dependent reactions> occur at the <thylakoid membrane>; the <Calvin cycle> operates in the <chloroplast stroma>.<Biology 2e by Mary Ann Clark and others/8.1 Overview of Photosynthesis>[Biology 2e, 8.1, Basic Photosynthetic Structures and The Two Parts of Photosynthesis]{ref}
<Cyanobacteria> perform <oxygenic photosynthesis> without <chloroplasts>: the <prokaryotic> <biological cell> itself contains the photosynthetic machinery. Thus <chloroplast> identifies an <organelle>, while <oxygenic photosynthesis> identifies a process. <Endosymbiotic theory> explains their historical connection; <organelle protein import> explains how many nuclear-encoded <proteins> reach the modern <organelle>.
\Image[/chloroplast-envelope-thylakoid-boundary-map.png]
{title=<Chloroplast> compartments and boundary counting}
{description=Original simplified section and compartment-access model, not a micrograph or scale drawing. The enlargement separates <thylakoid lumen> from the <chloroplast stroma> between stacked sacs. The route counts crossings of membrane barriers, not molecular leaflets; actual permeability and transport mechanisms are omitted.}
The useful feature is selective separation: <enzymes> and <substrates> can share an <organelle> without sharing a compartment. For a constructed accessibility example, put a soluble <enzyme> in <chloroplast stroma> and its <substrate> in <thylakoid lumen>. Sharing the same <chloroplast> does not permit direct contact; contact requires a transport or boundary-disruption step. Placing the <substrate> between adjacent <thylakoids> instead puts it on the stromal side. The visual stack alone cannot establish chemical accessibility.
For the simplest boundary-counting model, represent surrounding <cytosol>, <chloroplast intermembrane space>, <chloroplast stroma> and <thylakoid lumen> by four <vertices> in a chain. Its three <edges> represent the outer envelope, inner envelope and <thylakoid membrane>, respectively. A route from <cytosol> to <chloroplast stroma> crosses two barriers; extending it into <thylakoid lumen> adds a third. This follows by listing the <edges> along the chain. Counting the two leaflets of each <lipid bilayer> separately would answer a different question. These counts describe this idealized geometry, not the number of <protein targeting> signals or the transport itinerary of every <protein>.
The <light-dependent reactions> article develops <ATP>, <NADPH> and <electrochemical gradient> coupling, and the <Calvin cycle> article develops <carbon fixation> bookkeeping. Their different locations make the <chloroplast> interesting as an organized chemical workspace. Light-independent <chemical reactions> still depend on supplied carriers and regulation; the name does not prescribe nighttime operation.
Back to article page
