Mitochondria and Other Organelles

Mitochondria

We often refer to mitochondria (singular = mitochondrion) as the “powerhouses” or “energy factories” of a cell. Basically, this is because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell.

Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. As a matter of fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds or cristae, which increase its surface area. We refer to the space between the two membranes as the intermembrane space. Similarly, we refer to the space inside the inner membrane as the mitochondrial matrix. ATP synthesis takes place on the inner membrane. Image Attribution: modification of work by Matthew Britton; scale-bar data from Matt Russell

Cristae and Mitochondrial Matrix

Mitochondria are oval-shaped, double membrane organelles (see image above) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds which we refer to as cristae. We refer to the area which the folds surround as the mitochondrial matrix. Generally, the cristae and the matrix have different roles in cellular respiration.

Peroxisomes

Peroxisomes are small, round organelles that carry out oxidation reactions which break down fatty acids and amino acids. Single membranes enclose peroxisomes. Furthermore, peroxisomes detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H₂O₂, which would be damaging to cells. However, when these reactions are confined to peroxisomes, enzymes safely break down the H₂O₂ into oxygen and water.) For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. The membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components.