Semi autonomous Organelles: Meaning, Examples, Structure

What are Semi-Autonomous Organelles?

Semi-autonomous organelles are membrane-bound structures within eukaryotic cells that contain their genetic material and are partially self-replicating. Though not fully autonomous compared to some other organelles like the ribosomes, they still get some functions from the cell, but with a touch of independence. Mitochondria and chloroplasts are two major examples of these kinds of organelles.

Mitochondria

Mitochondria are double membrane-bound cell organelles. These are present both in plant cells and in animal cells.

These are commonly referred to as the powerhouse of the cell because they are responsible for the production of ATP during aerobic respiration. Adenosine Triphosphate (ATP) is the main energy-carrying molecule in a cell. ATP represents the short-term stored energy of the cell.

Structure of Mitochondria

• Mitochondria are spherical or rod-shaped organelles that are surrounded by two membranes.

• The outer membrane is smooth and contains special proteins called porins that allow easy diffusion of the particle. It is completely permeable to nutrient molecules, ions, ATPs etc.

• The inner membrane has a complex structure. It shows infoldings called cristae (sing. crista) where the enzymes for the electron transport chain are present. It is highly selectively permeable.

• The space between the outer and inner membrane is called the peri-mitochondrial or inter-mitochondrial space.

Functions of Mitochondria

• The most important function of the mitochondria is to produce energy in the form of ATPs during aerobic respiration. This process is called oxidative phosphorylation.

• Mitochondria also serves as a storehouse of calcium ions.

• They play an important role in the process of programmed cell death by removing the unwanted and excess cells during the development of an organism.

Chloroplasts

Chloroplasts are found in plant cells and some protists. These are responsible for photosynthesis, converting sunlight into chemical energy (glucose). They also have their own DNA (cpDNA).

Structure of Chloroplasts

• Thylakoid membrane is the inner membrane system inside the chloroplast where light-dependent photosynthesis reactions take place.

• The thylakoids are stacked in structures called grana, thus increasing surface area to maximize the amount of light absorbed.

• Stroma is the fluid-filled matrix outside the thylakoid membranes. The stroma contains enzymes that catalyze the light-independent reactions of photosynthesis (the Calvin cycle).

• Chloroplasts are enveloped by a double membrane structure.

• The outer membrane serves as a barrier between the chloroplast and the cytoplasm of the cell, and the inner one controls the passage of molecules into and out of the chloroplast.

Functions of Chloroplasts

• Chloroplasts are primarily responsible for converting light energy into chemical energy in the form of glucose through the process of photosynthesis.

• Chloroplasts assimilate carbon dioxide from the atmosphere during the Calvin cycle, using the ATP and NADPH generated during the light-dependent reactions.

• This process leads to the synthesis of carbohydrates such as glucose, which serve as energy sources and building blocks for plant growth and development.

• During photosynthesis, chloroplasts release oxygen as a byproduct of splitting water molecules.

• This oxygen is crucial for aerobic respiration in plants and other organisms that depend on atmospheric oxygen for survival.

Importance in Evolutionary Context

Semiautonomous organelles have originated from free-living prokaryotes through endosymbiosis. They provide evidence linking eukaryotic and prokaryotic cells.

Endosymbiotic Theory

According to the Endosymbiotic Theory, mitochondria and chloroplasts represent ancient symbiotic relationships between early eukaryotic cells and engulfed eubacteria and cyanobacteria, respectively. Genetic evidence includes similarities of organelle DNA to that of bacteria. Independent replication and possession of bacterial-like DNA are further evidence for the evolutionary relationship of these organelles to free-living bacteria.

Medical and Biotechnological Significance

Semiautonomous organelles play a key role in medicine and biotechnology.

Mitochondrial DNA Diseases and Therapy

The diseases, resulting from mitochondrial DNA mutations, pose an immense challenge to the cells in energy production. Gene therapy and replacement of mitochondria are two excellent therapeutic possibilities for these disorders.

Chloroplast Engineering in Agriculture

Engineering chloroplasts, on the other hand, have extreme potential in agriculture and biotechnology. It involves modifying the chloroplast DNA to improve crop traits.

Semi-Autonomous Organelles NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure and Function of Mitochondria
• Structure and Function of Chloroplast
• Endosymbiotic Theory

Practice Questions for NEET

Q1. Mitochondria are called powerhouses of the cell. Which of the following observations supports this statement?

1. Mitochondria synthesize ATP

2. Mitochondria have a double membrane

3. The enzymes of the Krebs cycle and the cytochromes are found in mitochondria

4. Mitochondria are found in almost all plant and animal cells

Correct answer: 1) Mitochondria synthesize ATP

Explanation:

Adenosine triphosphate (ATP), a tiny molecule that stores chemical energy for the cell, is produced by mitochondria.
Food-derived nutrients are transformed by mitochondria into ATP, the main energy source for the majority of physiological and metabolic functions.
Oxidative phosphorylation is the mechanism by which mitochondria use the energy released from oxidizing food to produce ATP. Two units are involved in this process: an electrical engine unit that uses the energy to produce ATP and a combustion unit that burns nutrients.

Hence, the correct answer is option 1) Mitochondria synthesize ATP.

Q2. Which of the following is absent in prokaryotes?

1. Cell wall

2. Nucleoid

3. Plasmid

4. Mitochondria

Correct answer: 4) Mitochondria

Explanation:

Because prokaryotes don't have mitochondria, prokaryotes make their ATP in the mesosomes. Hence option 4 is the correct answer.

Explanation for the incorrect options :

Option 1 is incorrect because prokaryotic cell walls are composed of peptidoglycan, a polymer of modified sugars (N-acetylglucosamine and N-acetylmuramic acid) that has been crosslinked with short peptides.

Option 2 is incorrect because bacteria and other prokaryotic cells contain a free-floating, often circular chromosome that is not protected by a nuclear membrane. Instead, the DNA is only present in the nucleoid, a part of the cell.

Option 3 is incorrect because prokaryotes have a nucleoid in the centre of the cell that houses the DNA but is not protected by a nuclear membrane. In addition to chromosomal DNA, many prokaryotes also possess smaller, circular DNA molecules known as plasmids, which can provide genetic benefits in particular situations.

Q3. Why is mitochondria known as a semi-autonomous organelle?

1. They possess their own ribosomes, mRNA, tRNA, and DNA

2. They have the ability to reproduce autonomously and use ribosomes to produce their proteins

3. They possess their own DNA but lacks ribosomes, mRNA, and tRNA

4. Both a and b

Correct answer: 4) Both a and b

Explanation:

Mitochondria are known as semi-autonomous organelles because they possess characteristics of both autonomy and dependence on the host cell.

• They possess their ribosomes, mRNA, tRNA, and DNA. This statement is true. Mitochondria have their own set of ribosomes, transfer RNA (tRNA), messenger RNA (mRNA), and circular DNA molecules called mitochondrial DNA (mtDNA). These components allow mitochondria to carry out protein synthesis independently to some extent.

• They can reproduce autonomously and use ribosomes to produce their proteins. This statement is also true. Mitochondria can reproduce by a process called fission, which is independent of the cell's division. They have their internal machinery, including ribosomes, which enables them to produce some of their proteins independently.

Therefore, the correct answer is option 4) Both a and b.

Also Read:

• MCQs on Semi-autonomous Organelles

• Discovery of Cells

• Ribosomes

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