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Ribosomes and Mitochondria

Describes cellular organelles.

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Ribosomes and Mitochondria

Sperm cells and muscle cells need lots of energy. What do they have in common?

They have lots of mitochondria. Mitochondria are called the power plants of the cell, as these organelles are where most of the cell's energy is produced. Cells that need lots of energy have lots of mitochondria.

Other Organelles

In addition to the nucleus, eukaryotic cells have many other organelles, including ribosomes and mitochondria. Ribosomes are present in all cells.


Ribosomes are small organelles and are the site of protein synthesis (or assembly). They are made of ribosomal protein and ribosomal RNA. Each ribosome has two parts, a large and a small subunit, as shown in Figure below. The subunits are attached to one another. Ribosomes can be found alone or in groups within the cytoplasm. Some ribosomes are attached to the endoplasmic reticulum (ER) (as shown in Figure below), and others are attached to the nuclear envelope.

The two subunits that make up a ribosome, small organelles that are intercellular protein factories.


A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the "power plants" of the cell because they use energy from organic compounds to make ATP (adenosine triphosphate). ATP is the cell's energy source that is used for such things such as movement and cell division. Some ATP is made in the cytosol of the cell, but most of it is made inside mitochondria. The number of mitochondria in a cell depends on the cell’s energy needs. For example, active human muscle cells may have thousands of mitochondria, while less active red blood cells do not have any.

(a): Electron micrograph of a single mitochondrion, within which you can see many cristae. Mitochondria range from 1 to 10 μm in size. (b): This model of a mitochondrion shows the organized arrangement of the inner and outer membranes, the protein matrix, and the folded inner mitochondrial membranes.

As Figure above (a) and (b) show, a mitochondrion has two phospholipid membranes. The smooth outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae. The fluid-filled inside of the mitochondrion, called matrix, is where most of the cell’s ATP is made.

Although most of a cell's DNA is contained in the cell nucleus, mitochondria have their own DNA. Mitochondria are able to reproduce asexually, and scientists think that they are descended from prokaryotes. According to the endosymbiotic theory, mitochondria were once free-living prokaryotes that infected ancient eukaryotic cells. The invading prokaryotes were protected inside the eukaryotic host cell, and in turn the prokaryote supplied extra ATP to its host.


  • Ribosomes are small organelles and are the site of protein synthesis. Ribosomes are found in all cells.
  • Mitochondria are where energy from organic compounds is used to make ATP.


Use this resource to answer the questions that follow.

  1. What happens at the ribosome?
  2. Describe the structure of a ribosome.
  3. How many ribosomes could a cell have?
  4. Where in the cell are ribosomes located?
  5. Why is the mitochondrion referred to as the "power plant" of the cell?
  6. Describe the structure of a mitochondrion. What is the cristae?
  7. What is located in the mitochondrial matrix?
  8. How many mitochondria does a typical liver cell have?


1. What is the function of a ribosome?

2. Identify the reason why mitochondria are called "power plants" of the cell.

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