Structure and function of mitochondria and chloroplasts. Endosymbiosis.

Key points:

  • Mitochondria are the "powerhouses" of the cell, breaking down fuel molecules and capturing energy in cellular respiration.
  • Chloroplasts are found in plants and algae. They're responsible for capturing light energy to make sugars in photosynthesis.
  • Mitochondria and chloroplasts likely began as bacteria that were engulfed by larger cells (the endosymbiont theory).


You may know that your body is made up of cells (trillions and trillions of them). You may also know that the reason you need to eat food—such as veggies—is so that you have the energy to do things like play sports, study, walk, and even breathe.
But what exactly happens in your body to turn the food energy stored in broccoli into a form that your body can use? And how does energy end up stored in the broccoli to begin with, anyway?
The answers to these questions have a lot to do with two important organelles: mitochondria and chloroplasts.
  • Chloroplasts are organelles found in the broccoli's cells, along with those of other plants and algae. They capture light energy and store it as fuel molecules in the plant's tissues.
  • Mitochondria are found inside of your cells, along with the cells of plants. They convert the energy stored in molecules from the broccoli (or other fuel molecules) into a form the cell can use.
Let's take a closer look at these two very important organelles.


Chloroplasts are found only in plants and photosynthetic algae. (Humans and other animals do not have chloroplasts.) The chloroplast's job is to carry out a process called photosynthesis.
In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. The energy contained in these sugars is harvested through a process called cellular respiration, which happens in the mitochondria of both plant and animal cells.
Chloroplasts are disc-shaped organelles found in the cytosol of a cell. They have outer and inner membranes with an intermembrane space between them. If you passed through the two layers of membrane and reached the space in the center, you’d find that it contained membrane discs known as thylakoids, arranged in interconnected stacks called grana (singular, granum).
The membrane of a thylakoid disc contains light-harvesting complexes that include chlorophyll, a pigment that gives plants their green color. Thylakoid discs are hollow, and the space inside a disc is called the thylakoid space or lumen, while the fluid-filled space surrounding the thylakoids is called the stroma.
You can learn more about chloroplasts, chlorophyll, and photosynthesis in the photosynthesis topic section.


Mitochondria (singular, mitochondrion) are often called the powerhouses or energy factories of the cell. Their job is to make a steady supply of adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. The process of making ATP using chemical energy from fuels such as sugars is called cellular respiration, and many of its steps happen inside the mitochondria.
The mitochondria are suspended in the jelly-like cytosol of the cell. They are oval-shaped and have two membranes: an outer one, surrounding the whole organelle, and an inner one, with many inward protrusions called cristae that increase surface area.
Cristae were once thought to be broad, wavy folds, but as Sal discusses in his mitochondria video, they're now understood to be more like long caverns.1^1 Here is a 3D reconstruction of a slice cut from a mitochondrion:
The space between the membranes is called the intermembrane space, and the compartment enclosed by the inner membrane is called the mitochondrial matrix. The matrix contains mitochondrial DNA and ribosomes. We'll talk shortly about why mitochondria (and chloroplasts) have their own DNA and ribosomes.
The multi-compartment structure of the mitochondrion may seem complicated to us. That's true, but it turns out to be very useful for cellular respiration, allowing reactions to be kept separate and different concentrations of molecules to be maintained in different "rooms."
Although mitochondria are found in most human cell types (as well as most cell types in other animals and plants), their numbers vary depending on the role of the cell and its energy needs. For instance, muscle cells typically have high energy needs and large numbers of mitochondria, while red blood cells, which are highly specialized for oxygen transport, have no mitochondria at all.3^3

Where did these organelles come from?

Both mitochondria and chloroplasts contain their own DNA and ribosomes. Why would these organelles need DNA and ribosomes, when there is DNA in the nucleus and ribosomes in the cytosol?
Strong evidence points to endosymbiosis as the answer to the puzzle. Symbiosis is a relationship in which organisms from two separate species live in a close, dependent relationship. Endosymbiosis (endo- = “within”) is a specific type of symbiosis where one organism lives inside the other.
Bacteria, mitochondria, and chloroplasts are similar in size. Bacteria also have DNA and ribosomes similar to those of mitochondria and chloroplasts.4^4 Based on this and other evidence, scientists think host cells and bacteria formed endosymbiotic relationships long ago, when individual host cells took in aerobic (oxygen-using) and photosynthetic bacteria but did not destroy them. Through millions of years of evolution, the aerobic bacteria became mitochondria and the photosynthetic bacteria became chloroplasts.
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