Structure and function of mitochondria and chloroplasts. Endosymbiosis.
- 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 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.
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.
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.
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.
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.
Want to join the conversation?
- Would it be possible for a modern day cell or organism to capture one of these cells and start a new symbiotic relationship with the host cell?(35 votes)
- That is an intriguing question. The origin of mitochondria is thought to have been a very rare event. The mitochondria seem to have their origin from one particular family of bacteria (the alpha-proteobacterium ) which are related to some existing bacteria. There are people trying to build cells so I guess maybe someone could try to create a modern endosymbiosis event.
There are many theories as to how eukaryotes came about. If you want to know more; see this paper: Endosymbiotic theories for eukaryote origin http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4571569/(23 votes)
- 1) Are there any animals that have chloroplasts? Or are they only in plants?
2) If they are only in plants, would it be possible for an animal to have chloroplasts, chemically speaking?(20 votes)
- Well according to modern classification, the organisms which have chloroplasts in them and have an advanced nucleus are called plants. Those organisms which show both plant and animal features(like Euglena) are kept in Kingdom: Protista
As for your second question, as Biology is science of exceptions we cannot publish any definite law. So in future, due to evolution, there may be animals with chloroplasts.(31 votes)
- It would be so cool if we could recreate endosymbiosis and incorporate chloroplasts in human cells so we could photosynthesize(24 votes)
- i hate to ruin your guys fun but i think we would die, if you put chlorophil into our bodies.(3 votes)
- is peroxisomes needed in a cell(11 votes)
- Before the ancestor of mitochondria entered the eukaryotic cells, Peroxisomes were asource of power to the cell. They used to break down H2O2(Hydrogen Peroxides) and would form water and and energy. But now, there only function is detoxification as the energy produced by mitochondria is much more than the energy produced by the peroxisome. So, peroxisomes are sort of vestigial organelles which had a major function in the past but aren't so useful in the present age..(3 votes)
- Does this mean that when you eat vegetables, you are you eating light energy?(6 votes)
- Yes, pretty much. In fact you could say you're eating the energy produced in the fusion reactions occuring inside of the Sun's core.(13 votes)
- if red blood cells do not have mitochondria, how are they considered to be eukaryotic cells?(4 votes)
- If mitochondria and chloroplasts evolved from bacteria, what did the cells use for energy before? Isn't this a case of you can't have the one without the other? Wouldn't the cell/animal/plant die without the energy from the chloroplast or mitochondria?(4 votes)
- Before mitochondria and chloroplasts became part of cells they had their own ways of producing energy from metabolizing food around them. These older methods of producing energy were lost along the way because the mitochondria and chloroplasts were better at it and it was a waste of resources to have the two ways of producing energy so the cells that eliminated the older way were more efficient and survived better that the ones that didn't.(6 votes)
- Have they proven endosymbiosis to be the reason why mitochondria and chloroplasts contain their own DNA and ribisomes? I am very blown by the idea that the mitochondria in our bodies were once bacteria and, if they really are bacteria before,we now have a gazillion of them.(3 votes)
- Proof is an elusive concept in science, but this theory is now generally accepted as the best explanation for multiple observations.
Another piece of evidence supporting the theory that these (and possible some other organelles) arose through endosymbiosis (aka symbiogenesis) is that if you make an evolutionary tree of the DNA within either of these organelles and bacteria, you find free-living bacteria that are closely related.
The group of bacteria from which mitochondria seem to have evolved are the Alphaproteobacteria — many of them are intracellular parasites, which suggests how they originally ended up inside the first eukaryotic cell.
Chloroplasts appear most closely related to the cyanobacteria.
You can read more about this theory here:
You might also find these interesting:
- In the endosymbiotic theory, what was the factor that decided that the chloroplast wouldnt converge with the other cells present on earth, so that there would be two types of cells,
1. plant cells
2. animal cells(4 votes)
- It just happened this way. Some phytoplankton probably merged with chloroplasts prokaryotic cells and absorbed them and formed a symbiotic relationship with them and also with mitochondria (they have both organelles), and some zooplankton probably got the short stick and managed to form symbiotic relationship only with mitochondria prokaryotic cells and from them out ancient common ancestor was created...(5 votes)