You must have heard of Popeye the sailor man and his amazing adventures?? Do you remember what gave power to Popeye?? Yes, it was spinach!! But, that was a cartoon character created to amuse us and to make us learn the benefits of spinach. Have you ever wondered what powers our cells to do all the work that keeps us alive? Where is all the energy for cellular activities created??

Yes, you are right ,it's the mitochondria. 

Mitochondria is like an electrical system of an apartment or a shopping mall which supplies electricity. It powers up the cell by supplying energy in the form of ATP (adenosine triphosphate) every now and then. Thus we call it the ‘powerhouse of a cell’.

Did you know that the mitochondria is also one of the organelles, other than the nucleus, that contains DNA and ribosomes?? Who knew, right??

Seems like the mitochondria is quite an interesting organelle which has a major role to play in the activities of the cell. Come, let us learn some more about it.

Table of Content:

• General Properties of Mitochondria
• Structural Organisation
• Semi-autonomous Nature of Mitochondria
• Functions of Mitochondria
• Practice problems of Mitochondria
• FAQs of Mitochondria

General Properties of Mitochondria

Mitochondria was discovered by Kolliker in 1880 and named by Benda in 1898. It is double membrane bound organelle that serves as the site for ATP synthesis in eukaryotic cells. Mitochondria is found in the cell cytoplasm of all the aerobic eukaryotes but absent in mature RBCs, anaerobic eukaryotes and all prokaryotes.

Cells that need more energy, such as the muscle cells, actively dividing meristematic cells in plants, etc., have more mitochondria. The giant hummingbird beats its wings 10-15 times per second. Thus, the muscle cells of hummingbirds have a high number of mitochondria present.

Structural Organisation of Mitochondria

The mitochondria is visible under light microscope but detailed study can be done under electron microscope only. It is a sausage-shaped or cylindrical structure which is 0.2-1.0 µm in diameter (average 0.5 µm) and 1.0-4.1 µm in length.

Mitochondria is bound by two membranes - an outer membrane and an inner membrane. The space between the two membranes is known as the intermembrane space or the perichondrial space. The space enclosed by the inner membrane is known as the matrix.

Outer Membrane

It is continuous and smooth and contains transport proteins. The membrane is selectively permeable and acts as a limiting boundary for the mitochondria. It also contains certain oxidising enzymes but the number of enzymes is much less compared to that of the inner membrane.

Inner Membrane

Inner membrane is also selectively permeable and regulates the passage of materials into and out of mitochondria. It is rich in a large number of enzymes, coupling factors, carrier proteins, electron carriers and channel proteins. It is folded to form finger-like projections called ‘cristae’ which increase the surface area of the membrane. Cristae contains numerous spherical or knob-like particles known as oxysomes or F0-F1 particles attached to the inner face of the inner mitochondrial membrane. 

Oxysomes

There may be around 104-105 oxysomes or F0-F1 particles per mitochondrion. The F1 subunit forms the head piece which has a diameter of about 8.5-10 nm. The F0 subunit forms the base piece which is about 11 nm long and 1.5 nm thick. A stalk of around 5 nm length joins the two subunits. 

The F0 subunit has a roter and a stator and remains embedded in the lipid bilayer of the inner membrane while the F1 subunit faces the inner matrix. The F0 subunit serves as a proton channel whereas the F1 subunit functions as ATP synthase enzyme that catalyses the oxidative phosphorylation of ADP and converts it into ATP.

Matrix

It is rich in enzymes involved in aerobic respiration. It has circular double stranded DNA called mitochondrial DNA (mtDNA). It also has 70S ribosomes (similar to prokaryotic ribosomes) and RNA molecules. Apart from this the mitochondrial matrix also contains proteins needed for the replication, transcription and translation of mitochondrial genes.

Semi-autonomous Nature of Mitochondria

Mitochondria are called ‘semi autonomous’ structures as they contain their own genetic material, that is mitochondrial DNA, which is similar to bacterial DNA in being circular, double stranded structures. Mitochondria also has its own RNA, ribosomes and proteins that allow it to independently replicate, transcribe and translate its DNA. During cell division, the mitochondria divides on its own by fission. 

Functions of Mitochondria

The mitochondria serves as the site for the Krebs cycle and the electron transport chain during aerobic respiration and during the process helps to generate ATP from ADP due to the ATP synthase activity of the F0-F1 particles.

The mitochondria also helps in elongation of fatty acid fatty acid chains.

Practice Problems of Mitochondria

Q1. The mitochondria serves as the site for ATP production. Which of the following parts of the mitochondria is directly involved with this function?

• Outer membrane
• Perichondrial space
• Mitochondrial matrix
• Oxysomes

Solution: The inner membrane of the mitochondria is folded into cristae which contain numerous spherical or knob-like particles known as oxysomes or F0-F1 particles attached to their inner face. The F1 subunit of the oxysomes functions as ATP synthase enzyme that catalyses the oxidative phosphorylation of ADP and converts it into ATP. 

Thus, the correct option is d.

Q2. Which of the following is not a function of mitochondria?

• Fatty acid elongation
• Krebs cycle
• Electron transport chain and associated ATP production
• Glycolysis and associated ATP production

Solution: The mitochondria serves as the site for the Krebs cycle and the electron transport chain during aerobic respiration. It also helps to produce ATP from ADP using the energy generated due to movement of electrons across the electron transport chain. The mitochondria also helps in elongation of fatty acid fatty acid chains.

Glycolysis and associated production of ATPs occurs in the cytoplasm. Thus the correct option is d.

Q3. Mitochondria are considered to be a semi-autonomous organelle. Which of the following characteristics are responsible for its semi-autonomous nature?

1). Presence of circular double stranded DNA

2). Presence of 70S ribosome

3). Presence of respiratory enzymes

4). Ability to divide on its own.

• I and III
• I and II
• I, II and IV
• II and III

Solution: Mitochondria are called ‘semi autonomous’ organelles as they contain their own double stranded circular DNA, 70S ribosomes, RNAs and enzymes required to synthesise their own proteins. Another reason is that the mitochondria has the ability to divide on its own by fission during cellular division.

Thus, the correct option is c.

FAQs of Mitochondria

Ques.1- What are oxysomes?

Ans. The inner membrane of the mitochondria is folded into cristae which contain numerous spherical or knob-like particles known as oxysomes or F0-F1 particles attached to their inner face. There may be around 104-105 oxysomes or F0-F1 particles per mitochondrion. The F1 subunit of the oxysomes functions as ATP synthase enzyme that catalyses the oxidative phosphorylation of ADP and converts it into ATP. The F0 subunit serves as a proton channel .

Ques.2- What are cristae?

Ans. The inner membrane of the mitochondria is folded to form finger-like projections called ‘cristae’ which increase the surface area of the membrane.

Ques.3- What is the mitochondrial matrix composed of ?

Ans. Mitochondrial matrix is rich in enzymes involved in aerobic respiration. It has mitochondrial DNA (mtDNA), 70S ribosomes (similar to prokaryotic ribosomes) and RNA molecules. Apart from this the mitochondrial matrix also contains proteins needed for the replication, transcription and translation of mitochondrial genes.

Ques.4- Why is mitochondria called ‘powerhouse of the cell’?

Ans. Mitochondria powers up the cell by producing an energy rich compound known as adenosine triphosphate or ATP. ATP molecules have stored chemical energy which can be utilised by the cell for various cellular activities. Thus mitochondria is known as the ‘powerhouse of the cell’.

Ques.5- Where are mitochondria located?

Ans. Mitochondria is found in the cell cytoplasm of all the aerobic eukaryotes but absent in mature RBCs, anaerobic eukaryotes and all prokaryotes.

Related Topics to Cell Envelopes and Cell Membrane in Biology

• Differences between Prokaryotic and Eukaryotic Cell Difference
• Cell
• Eukaryotic cells 
• Cell Envelopes and Cell Membranes
• What is a Cell
• Animal Cell
• Plant Cell
• Prokaryotic Cells
• Types of Cells
• Endomembrane System

NCERT Class 11 Biology Chapters