Question

Name two cell organelles that are double membrane bound. What are the characteristics of these two organelles? State their function and draw labelled diagrams of both.

Answer 1

Mitochondria and chloroplasts are the cell organelles that are double membrane bound.
Mitochondria:-
Characteristics:

1. Mitochondria are surrounded by two membranes designated as outer and inner membrane. The outer membrane is smooth and forms the continuous limiting boundary of the mitochondria whereas the inner membrane forms a number of infoldings called the cristae. Due to the presence of two membranes, the organelle is partitioned into two distinct chambers filled with aqueous fluid.
2. The matrix contains single circular dsDNA molecule, a few RNA molecules, 70S ribosomes and the components required for the synthesis of proteins. The matrix also contains enzymes for TCA (Tricarboxylic acid) cycle.
3. The two membranes have their own specific enzymes associated with the mitochondrial function. The mitochondria divide by fusion. The enzymes and electron carriers for formation of ATP are present only in the inner membrane.

Functions:

1. Mitochondria are called power houses of the cell because of the formation of ATP. ATP comes out of mitochondria and helps perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc.
2. Mitochondria provide important intermediates for the synthesis of several biochemicals like chlorophyll, cytochromes, pyrimidines, steroids, alkaloids, etc.
3. The matrix of the mitochondria has enzymes for the synthesis of fatty acids. Enzymes required for the elongation of fatty acids have been reported in the outer mitochondrial chamber.

Chloroplasts:-
Characteristics:

1. The inner membrane is relatively less permeable to substances than outer membrane and thus, has more proteins including carrier proteins. The space enclosed by the inner membrane of the chloroplast is called stroma. Stroma contains a large number of organised flattened membranous sacs called thylakoids.
2. The thylakoids of different grana are connected by flat membranous tubules called the stroma lamellae. The membrane of the thylakoids enclose a space called a lumen.
3. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes.

Functions:

1. Chloroplasts are the centres of photosynthesis or formation of organic compounds from inorganic raw materials.
2. Chloroplasts are able to trap sun energy and change it into chemical energy. The chemical energy is used by all living organisms to perform their life activities.
3. Chloroplasts liberate oxygen which is passed into the atmosphere. This keeps the balance of oxygen constant in the atmosphere, as oxygen is being consumed in respiration and combustion.