Structure of Mitochondria

Why study the structure of mitochondria ?

The many functions of mitochondria include production of adenosine triphosphate (ATP) via the TCA Cycle (which is also as the Krebs Cycle and the Citric Acid Cycle). That process is an important metabolic pathway that is an essential part of carbohydrate and energy metabolism.
See also what is metabolism.

Read more about the functions of mitochondria.

Mitochondria are present in all eukaryotic cells - including all animal cells and plant cells. However the number of mitochondria in each cell varies considerably, from just one mitochondrion up to 10,000 mitochondria in some specialized types of cells. A "typical" number of mitochondria per cell is around 200 - but of course many diagrams of cells only show a small number to represent the presence of at least one mitochondrion in the cell. Mitochondria have an ellipsoid, that is "oval-shaped" or "rounded rod-like" shape. Some books describe them as "cigar-shaped". Mitochondria are continually active. They move and can change their exact shape. The following diagram of a mitochondrion is a very simple two-dimensional representation of the main parts and contents of a mitochondrion - intended to support introductory courses e.g. A-Level Biology.

Diagram of the Structure of Mitochondria (or the structure of a mitochondrion)

General Description of the Structure of a Mitochondrion

List of parts of a mitochondrion:

  • Outer membrane of mitochondrion
  • Inner membrane of mitochondrion
    also called the
    "Innermitochondrial membrane"
  • Cristae
  • F1F0 ATPases (also called F1F0-ATPases)
  • Intermembrane space
  • Matrix of the mitochondrion
  • Mitochondrial DNA (circular DNA)
  • Matrix granules
  • Ribosomes

Mitochondria are organelles that have a double membrane structure. The outer membrane defines the external shape of the mitochondrion. The inner membrane has many folds called cristae. (Cristae is the plural word for more than one crista.) The volume between the inner and outer membranes is called the intermembrane space, which is sometimes written "inter membrane space". The volume enclosed by the inner membrane is called the "matrix" or, if written in full, the matrix of the mitochondrion.

The matrix of each mitochondrion contains the enzymes of the TCA cycle, which is also known more fully as the tricarboxylic acid cycle - and also as the citric acid cycle, the Krebs cycle, and the Szent-Györgyi–Krebs cycle. The matrix also contains other structures and molecules including ribosomes, matrix granules and mitochondrial DNA.


Table of notes about the parts and contents of a mitochondrion

 

1.

Outer Membrane

The outer membrane of mitochondria (OMM) is permeable to oxygen, pyruvate, ATP and other molecules.

Thickness: The thickness of the outer layer is approx 40Å,
i.e. 40 ångström = 40 x 10-10m = 4 nm = 0.000004 mm
If these units are unfamiliar see the page about scientific numbers.

2.

Inner Membrane

The inner membrane of a mitochondrion is also known as the innermitochondrial membrane (IMM). It has many invaginations called cristae (see 4., below). The sections of the inner membrane that form cristae are covered with many tiny "stalked particles" called inner membrane spheres whose head, i.e. the "sphere" part, is on the matrix side of the inner membrane - as opposed to the intermembrane space side of the inner membrane - further description continues in (4.) below.

Many of the chemical reactions that take place within mitochondria occur on the inner membrane. It contains the electron transport system and the ATPase complex:

  • Electron transport system - generates a proton gradient.
  • ATPase complex - uses proton gradient to produce adenosine triphosphate (ATP) from adenosine diphosphate (ADP).

Hence the inner mitochondrial membrane is the site of oxidative phosphorylation.

Thickness: The thickness of the inner layer is about the same as that of the outer layer, so approx 40Å, i.e. 40 ångström = 40 x 10-10m = 4 nm = 0.000004 mm.
(So, one could fit 250,000 x 40Å such membranes along a line 1mm long, or 2.5 million such membranes along a 1 cm line !)

3.

Intermembrane Space

The space between the inner and outer mitochondrial membranes is called the intermembrane space (O cavity). It has a high proton concentration. This is due to the electron transport system of the inner mitochondrial membrane.

Width / Thickness: The space between the inner and outer mitochondrial membranes is approx 70Å, i.e. 70 ångström.

4.

Cristae, plural
(The singular term is crista.)

Cristae are folds of the inner mitochondrial membrane. The quantity and shape of the cristae may vary. For example, although the shape of cristae within mitochondria is often either flat or tubular, in some cases (e.g. in certain nerve cells) cristae take the form of prisms while in other cases (e.g. in some photoreceptor cone cells) they have a whorl-like shape.

Stalked Particles called inner-membrane spheres:
As mentioned in (2.) above, cristae are covered with many tiny "stalked particles" called inner membrane spheres that are also known as simply "spheres" or "knobs".

  • Their "heads", i.e. the "sphere" part, are on the matrix side of the inner membrane - as opposed to on the intermembrane space side of the inner membrane.
  • Each stalked particle has
    • a "head" or "sphere" diameter of 8-9 nm (so, approximately 8.5 x 10-9m)
    • a stalk 30-35Å (approx 3.25 x 10-9m) wide and 45-50Å (approx 4.75 x 10-9m) long.
  • They contain a protein called F1 which is often written "F1", that is with the "1" written as a subscript and an F0 portion, as shown.

The inner-membrane spheres play an important role in the production of adenosine triphosphate (ATP) in mitochondria - see [later pages] for further details.

5.

Mitochondrial Matrix

 

a.

Enzymes

The mitochondrial matrix contains a highly-concentrated mixture of hundreds of enzymes. These include most of the enzymes that participate in the TCA Cycle - which is also known as the "Krebs Cycle", among other names. The TCA Cycle occurs in the mitochondrial matrix.
(The only enzyme involved in the TCA Cycle that is not free in the mitochondrial matrix is succinate dehydrogenase - which is located on the inner-surface of the innermitochondrial membrane.)

b.

Ribosomes

The ribosomes in mitochondria are of 70S type - as found in prokaryotic cells (bacteria), as opposed to the 80S type present in many plant and animal cells. They can synthesize proteins.

c.

Mitochondrial DNA

Mitochondria have their own genetic material and the facility to produce their own ribonucleic acids (RNAs) and proteins. All of the mitochondrial DNA is maternal. Mitochondrial DNA carries genes necessary for the synthesis of many, but not all, mitochondrial proteins.

How do we know this, bearing in mind that mitochondria are very small ?

Detailed knowledge of the structure of mitochondria has been made possible by electron microscopy. During the 1940s and 1950s several eminent biologists including cell biologist the late Prof. George Palade (originally from Romania) and physician and histologist the late Prof. Fritiof Sjöstrand (from Stockholm, Sweden) studied the fine structure of mitochondria using electron microscopes. Although these scientists didn't work together their published papers and contributions to academic conferences indicate that both made significant progress in detailing and explaining the structure of mitochondria. Further information is included in the textbook "Discovering Cell Mechanisms: The Creation of Modern Cell Biology" by William Bechtel which is available for delivery within the UK from amazon.co.uk and in North America from amazon.com.

See also a comparison between animal cells, plant cells and bacteria cells.

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