Semi-autonomous Organelles: Mitochondria and Chloroplast - Practice Questions & MCQ

Semi-autonomous Organelles: Mitochondria (Sing. Mitochondrion)

• Mitochondria are the double membrane bound cell organelles. 
• These are present both in plant cell and animal cell. 
• These are commonly referred to as the powerhouse of the cell because they are responsible for the production of ATPs during aerobic respiration. 
• Adenosine Triphosphate (ATP) is the main energy-carrying molecule of a cell. ATP represents the short-term stored energy of the cell. 

Structure of the Mitochondria

1. Membranes

• 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 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.
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2. Matrix 

Matrix is the jelly-like substance that contains the essential proteins and enzymes for aerobic respiration. Circular mitochondrial DNA, RNAs, 70S ribosomes etc. are also localised in the matrix.

3. Mitochondrial DNA
Although the major load of mitochondrial proteins comes from the nuclear DNA, mitochondria contain their own circular DNA that codes for some of the essential proteins. Mitochondria can reproduce through binary fission independently. These features make them semi-autonomous organelles. 
 

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 serve as a storehouse of the 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.

Semi-autonomous Organelles: Chloroplast
Chloroplasts are double membrane bound organelles that are present in plant cells and algal cells. These are primarily concerned with the process of photosynthesis. 

Structure of the Chloroplast

1. Membranes
The chloroplast is surrounded by a double-layered membrane - an outer and an inner membrane.  Each of these membranes is a phospholipid bilayer. The outer membrane is more permeable than the inner membrane. The space between the two membranes is called in the intermembrane space.

2. Stroma/Matrix
It is the jelly-like fluid present interior to the inner membrane. It is made up of water and soluble proteins. It contains DNA, RNA, ribosomes, enzymes etc. 

3. Thylakoids
These are flattened membrane system present in the stroma. These are stacked to form granum (pl. grana) in higher plants. Grana are connected to each other through a stretch of thylakoid called stroma lamellae. These membranes possess photosynthetic pigments coupling factors that perform ATP synthesis.