This is a chloroplast with a double surrounding membrane. The outer membrane is believed to be eukaryotic in origin. The inner membrane is, according to the endosymbiont theory, the cell membrane of an ancient prokaryote that became an endosymbiont.
The fluid inside this double-membrane organelle is called the stroma. The stroma is, according to the endosymbiont theory, the cytoplasm of the prokaryotic endosymbiont. As one would suspect it has a nucleoid region to house the circular, naked DNA. It also holds 70S (prokaryotic-type) ribosomes.
The stroma is also the site of the Calvin Cycle. The Calvin Cycle is the series of enzyme-catalyzed chemical reactions that produce carbohydrates and other compounds from carbon dioxide. The Calvin Cycle is universal to all photosynthetic life forms.
Floating in the stroma are tiny membrane sacs. These are called thylakoids. The sacs are stacked in groups. Each group is called a granum. There are many grana in each chloroplast. The thylakoid membranes are the site of the photosynthetic light reactions. The thylakoids have intrinsic and extrinsic proteins, some with special prosthetic groups, allowing for electrons to be moved from protein complex to protein complex. These proteins constitute an electron transport system sometimes known as the Z-scheme.
The prosthetic group for two critical membrane proteins (P680 and P700) is a chlorophyll a pigment molecule. These chlorophyll-binding proteins give the thylakoids an intense green color. The many thylakoids in a chloroplast give the chloroplast a green color. The many chloroplasts in a leaf mesophyll cell give that cell a green color. The many mesophyll cells in a leaf give the leaf a green color. The chlorophyll molecule absorbs light energy and an electron is boosted within the electron cloud in a resonating chemical structure surrounding a magnesium ion. This excited electron is removed by the surrounding electron transport proteins in the membrane. The movement of these electrons, and accompanying protons, results ultimately in the trapping of energy in a phosphate bond in ATP.
The thylakoid is thus the location for light absorption and ATP synthesis. The stroma uses the ATP to store the trapped energy in carbon-carbon bonds of carbohydrates. Some chloroplasts show developing starch grains. These represent complex polymers of carbohydrates for long-term storage.
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