Structure of Mitochondria
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| Structure of mitochondria |
Mitochondria are double-membrane-bound organelles responsible for energy production in eukaryotic cells. Their structure is highly specialized to support the biochemical processes of cellular respiration and ATP synthesis.
1. Shape and Size
Mitochondria show variability in shape, appearing rod-shaped, spherical, or filamentous depending on the cell type and physiological condition. The average size of a mitochondrion ranges from 0.5 to 1.0 micrometers (μm) in diameter and 1–10 μm in length. The number of mitochondria per cell depends on the energy requirement of that cell: muscle cells and neurons have a large number of mitochondria to meet high energy demand, while fat cells have fewer.
Example: A liver cell may contain over 2000 mitochondria.
2. Membrane System
Each mitochondrion is surrounded by two distinct membranes — an outer membrane and an inner membrane, both separated by an intermembrane space.
a) Outer Membrane
The outer membrane of the mitochondrion is smooth and unwrinkled, forming a simple outer layer around the organelle. It contains special porin proteins that act like tiny channels, allowing small molecules and ions to pass freely in and out. In this way, the outer membrane serves as both a protective barrier and a gateway, controlling the exchange of materials between the cell’s cytoplasm and the space between the two mitochondrial membranes.
b) Inner Membrane
The inner membrane of the mitochondrion is deeply folded inward, forming structures called cristae that greatly increase the surface area for energy-producing reactions. It is packed with enzymes and protein complexes that drive oxidative phosphorylation and the electron transport chain (ETC) — the key steps in ATP generation. This membrane also contains the respiratory chain complexes (I–IV) and ATP synthase particles (F₀–F₁ complexes) responsible for making ATP. Unlike the outer membrane, it is highly selective and impermeable to most ions and molecules, maintaining a controlled internal environment for efficient biochemical processes.
3. Mitochondrial Matrix
The matrix is a dense, enzyme-rich fluid enclosed by the inner membrane. It contains enzymes of the Krebs cycle, mitochondrial DNA (mtDNA), 70S ribosomes, tRNAs, and mRNAs. It is the site for the Krebs cycle, β-oxidation of fatty acids, and amino acid metabolism.
4. Cristae
Cristae are infoldings of the inner membrane extending into the matrix. They increase surface area for oxidative phosphorylation and house ATP synthase complexes (F₀–F₁ particles). Function as sites of ATP generation through the electron transport chain.
5. Mitochondrial DNA and Ribosomes
(a) Mitochondrial DNA (mtDNA)
The mitochondrial DNA (mtDNA) is circular and double-stranded, much like the DNA found in bacteria, and it lacks histone proteins that are usually present in nuclear DNA. It contains genes that code for a few essential mitochondrial enzymes, as well as rRNAs and tRNAs needed for protein synthesis inside the organelle. This DNA enables the mitochondrion to produce some of its own proteins, allowing it to function and replicate somewhat independently within the cell.
(b) Ribosomes
The mitochondrial ribosomes are of the 70S type, similar to those found in bacteria (prokaryotes). They play a crucial role in synthesizing proteins needed for the mitochondrion’s enzymes and structural components. Working together with mitochondrial DNA (mtDNA), these ribosomes allow the mitochondrion to replicate, transcribe, and translate some of its own proteins. This remarkable ability makes mitochondria semi-autonomous organelles, capable of partially managing their own functions within the cell.
Summary Table
Component | Structure/Composition | Function |
Outer Membrane | Smooth, with porins | Controls molecular exchange |
Inner Membrane | Folded into cristae | Site for ETC and ATP synthesis |
Cristae | Infoldings of inner membrane | Increase surface area for respiration |
Matrix | Enzyme-rich, contains mtDNA & ribosomes | Krebs cycle and metabolic reactions |
mtDNA & Ribosomes | Circular DNA, 70S ribosomes | Protein synthesis, partial independence |
Functions of Mitochondria
1. ATP Production: Through aerobic respiration.
2. Regulation of Metabolism: Involved in the Krebs cycle and fatty acid oxidation.
3. Heat Generation: In brown fat tissues.
4. Apoptosis: Plays a role in programmed cell death.
5. Calcium Storage: Helps in maintaining calcium ion homeostasis.
Semi-Autonomous Nature of Mitochondria
Mitochondria are termed semi-autonomous because they possess:
I. Their own DNA, RNA, and ribosomes.
II. The ability to synthesize some of their own proteins and enzymes.
III. The capacity to replicate independently by binary fission.
However, they still depend on nuclear genes for many proteins, hence not fully autonomous.
Endosymbiotic Hypothesis
The Endosymbiotic Theory (proposed by Lynn Margulis, 1967) explains the origin of mitochondria.
Key Points:
a. Primitive aerobic bacteria were engulfed by ancestral anaerobic eukaryotic cells.
b. Instead of being digested, the bacteria formed a symbiotic relationship, supplying energy to the host.
c. Over time, these bacteria evolved into mitochondria, losing some genes to the host nucleus but retaining their own DNA and ribosomes.
Supporting Evidence:
Several lines of evidence support the endosymbiotic origin of mitochondria. These organelles contain circular DNA similar to that found in bacteria, suggesting a shared evolutionary ancestry. They also possess 70S ribosomes, the same type present in prokaryotes, which enables them to synthesize some of their own proteins. Mitochondria replicate by binary fission, a method typical of bacterial cell division, rather than through the mitotic process used by the host cell. Structurally, they are surrounded by a double membrane—the inner membrane is bacterial in origin, while the outer one is derived from the host cell during engulfment. Additionally, mitochondria exhibit antibiotic sensitivity, responding to drugs like chloramphenicol that specifically target bacterial ribosomes, further strengthening the evidence that they evolved from ancestral bacteria through symbiosis.
Conclusion
Mitochondria are vital organelles that not only generate energy but also reflect an ancient symbiotic event that shaped eukaryotic evolution. Their semi-autonomous nature highlights their unique evolutionary origin and functional independence within the cell.
FAQs on Mitochondria
Q: Why are mitochondria called the powerhouses of the cell?
A: Because they generate ATP through cellular respiration.
Q: What type of DNA is found in mitochondria?
A: Circular, double-stranded DNA similar to bacterial DNA.
Q: Who proposed the endosymbiotic hypothesis?
A: Lynn Margulis in 1967.
Q: How do mitochondria reproduce?
A: By binary fission, independent of the cell cycle.
Q: What type of ribosomes do mitochondria contain?
A: 70S ribosomes.
MCQs on Mitochondria
1. Mitochondria are known as:
a) Ribosomes b) Powerhouse of the cell ✅ c) Golgi bodies d) Lysosomes
2. The inner membrane of mitochondria is folded into:
a) Lamellae b) Thylakoids c) Cristae ✅ d) Cisternae
3. Which type of ribosomes are present in mitochondria? a) 80S b) 70S ✅ c) 60S d) 90S
4. Endosymbiotic hypothesis explains the origin of:
a) Ribosomes b) Nucleus c) Mitochondria and chloroplasts ✅ d) Golgi complex
5. Mitochondrial DNA is:
a) Linear b) Circular ✅ c) Segmented d) Double helix with histones
Worksheet: Mitochondria
A. Fill in the blanks
1. Mitochondria are known as the __________ of the cell.
2. The inner membrane of mitochondria is folded into __________.
3. Mitochondria possess __________ type of ribosomes.
4. The matrix contains enzymes for the __________ cycle.
5. The endosymbiotic hypothesis was proposed by __________.
B. Short Answer Questions
6. Describe the structure of mitochondria with a labeled diagram.
7. What is meant by the semi-autonomous nature of mitochondria?
8. Explain the endosymbiotic hypothesis with suitable evidence.
9. Mention any two functions of mitochondria.
10. How is mitochondrial DNA different from nuclear DNA?
References
1. Alberts, B. Molecular Biology of the Cell. 7th Edition, Garland Science, 2022.
2. Karp, G. Cell and Molecular Biology: Concepts and Experiments. Wiley, 2021.
3. Margulis, L. (1967). On the Origin of Mitosing Cells. Journal of Theoretical Biology.
4. Lodish, H. et al. Molecular Cell Biology. W.H. Freeman and Company, 2021.
5. www.zoologys.co.in – Educational resources in zoology and cell biology.
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