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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The chloroplast is the organelle in which photosynthesis takes place. Inside the chloroplast there are disk like structures called thylakoids arranged in stacks called granum. The light dependent reaction takes place across the membranes of the thylakoids while the Calvin Cycle takes place in the stroma, which is the semi liquid substance inside the chloroplast.

One of the key differences between plant and animal cells is that plant cells as you see here, have a specialized organelle inside of them called a chloroplast that name means it's a [IB] which is a category of cell organelle that is green and that green color is key because it is what allows it to do photosynthesis, so the chloroplast is the organelle, and sometimes called of photosynthesis.

We zoom in closer on a chloroplast you'll see some basic structures that you'll find in every chloroplast and the first of which is its outer membrane. It actually has two membranes it has this outer membrane and then a nearly underneath it another membrane. This is evidence in part of how the chloroplast is developed through something called endosymbiosis probably about 500 million years ago give or take a few 100 million it used to be an independent organism but then it got eaten by a larger cell.

Once we're through with that outer envelope, it's sometimes called because it sounds cooler, you're inside the chloroplast and you'll see a bunch of structures floating around in a watery liquid. That watery liquid is called the stroma. That's very similar to the cytoplasm or cytosol of the larger outer cell that surround chloroplast.

Within the stroma you'll see these stacks of membrane discs. One individual stack is called a granum, collectively these stacks of discs are called grana. Now one disk within this granum is called a thylakoid, so the grana are made up of stacks of thylakoid membranes. The thylakoid membrane is the surface that has all the special molecules called chlorophyll and photosystems that allow for the absorption of light so that the plant organelle here, the chloroplast, can absorb that energy and then transfer it to enzymes that are floating around in the stroma those enzymes floating around the stroma then build glucose.

That process of absorbing light energy and then storing that energy in to glucose molecules is called photosynthesis thus the chloroplast is the organelle of photosynthesis.