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This illustration shows a chloroplast, which has an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Inside the inner membrane are flat, pancake-like structures called thylakoids. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoids is called the thylakoid space.
The chloroplast has an outer membrane, an inner membrane, and membrane structures – thylakoids that are stacked into grana. We call the space inside the thylakoid membranes the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and sugar synthesis takes place in the fluid inside the inner membrane, which we call the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals. Plants (autotrophs) are able to make their own food, like sugars used in cellular respiration to provide ATP energy generated in the plant mitochondria. Animals (heterotrophs) must ingest their food.

– What is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms?

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs we call thylakoids. Each thylakoid stack is a granum (plural = grana). We call the fluid enclosed by the inner membrane that surrounds the grana the stroma.

– What is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship?

Chloroplasts are believed to have arisen after mitochondria, since all eukaryotes contain mitochondria, but not all have chloroplasts. This is called serial endosymbiosis—an early eukaryote engulfing the mitochondrion ancestor, and some descendants of it then engulfing the chloroplast ancestor, creating a cell with both chloroplasts and mitochondria.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: