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2.8: Ribosomes and Mitochondria

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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

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.

Ribozymes are RNA molecules that catalyze chemical reactions, such as translation. Translation is the process of ordering the amino acids in the assembly of a protein, and translation will be discussed more in another concept. Briefly, the ribosomes interact with other RNA molecules to make chains of amino acids called polypeptide chains, due to the peptide bond that forms between individual amino acids. Polypeptide chains are built from the genetic instructions held within a messenger RNA (mRNA) molecule. Polypeptide chains that are made on the rough ER (discussed below) are inserted directly into the ER and then are transported to their various cellular destinations. Ribosomes on the rough ER usually produce proteins that are destined for the cell membrane.

Ribosomes are found in both eukaryotic and prokaryotic cells. Ribosomes are not surrounded by a membrane. The other organelles found in eukaryotic cells are surrounded by a membrane.

Mitochondria

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.

Summary

  • 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.

Practice

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?

Review

1. What is the function of a ribosome?

2. What is a significant difference between the structure of a ribosome and other organelles?

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

Vocabulary

ATP (adenosine triphosphate)

ATP (adenosine triphosphate)

Energy-carrying molecule that cells use to power their metabolic processes; energy-currency of the cell.
cristae

cristae

Inner membrane folds of the mitochondrion.
matrix

matrix

Fluid-filled inside of the mitochondrion.
mitochondria

mitochondria

(singular, mitochondrion): Organelle in eukaryotic cells that makes energy available to the cell in the form of ATP molecules.
ribosome

ribosome

Organelle inside all cells where proteins are made; site of protein synthesis.
ribozymes

ribozymes

RNA molecules that catalyze chemical reactions; RNA molecules with enzymatic activity.
translation

translation

Process in which genetic instructions in mRNA are read to synthesize a protein.

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Date Created:

Feb 24, 2012

Last Modified:

Nov 11, 2014
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