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

Two double membrane-bound organelles are the mitochondrion and the plastid (its green, photosynthetic form is the chloroplast).

Mitochondrion — bounded by a smooth outer membrane and an inner membrane thrown into folds called cristae, which project into the interior and increase its surface area. The matrix inside holds a single circular DNA molecule, a few RNA molecules and 70S ribosomes. Function: it is the site of aerobic respiration, generating cellular energy as ATP, so it is called the power house of the cell.

Chloroplast (plastid) — bounded by an outer membrane and a relatively less permeable inner membrane; the space enclosed by the inner membrane is the stroma. The stroma holds flattened membranous sacs called thylakoids, stacked like piles of coins into grana and interconnected by tubular stroma lamellae. The stroma also contains small double-stranded circular DNA and 70S ribosomes (smaller than the 80S cytoplasmic ones), and its thylakoids carry the chlorophyll pigments. Function: it traps light energy and is the site of photosynthesis. …

Among the organelles of a eukaryotic cell, two are enclosed by two membranes rather than one — the mitochondrion and the plastid, whose green, chlorophyll-bearing form is the chloroplast. The chapter itself draws this comparison directly, noting that plastids, like mitochondria, are double membrane bound. Each pairs a distinctive double-membrane structure with an essential energy-related function.

The mitochondrion.

• Characteristics: Each mitochondrion is a double membrane-bound structure, typically sausage-shaped or cylindrical. The outer membrane forms a continuous smooth boundary, while the inner membrane is thrown into numerous infoldings called cristae that project into the interior and greatly increase the internal surface area available for its enzymes. The two membranes divide the interior into an outer and an inner compartment; the inner one is filled with a dense matrix. The matrix holds a single circular DNA molecule, a few RNA molecules and 70S ribosomes, which is why a mitochondrion can even divide on its own by fission.
• Functions: The mitochondrion is the site of aerobic respiration. Through this it generates cellular energy in the form of ATP, which is why it is described as the power house of the cell.

The chloroplast (plastid).

• Characteristics: Plastids are found in all plant cells and in euglenoids; their green, photosynthetic form is the chloroplast. Like the mitochondrion, it is bounded by two membranes — an outer membrane and a relatively less permeable inner membrane. The space enclosed by the inner membrane is called the stroma. Within the stroma lie flattened, organised membranous sacs called thylakoids, arranged in stacks (like piles of coins) called grana, with flat membranous tubules called stroma lamellae connecting the thylakoids of different grana. The stroma also contains enzymes for carbohydrate and protein synthesis, small double-stranded circular DNA molecules, and ribosomes — the chloroplast's own ribosomes are smaller (70S) than the cytoplasmic (80S) ones. Chlorophyll pigments are held in the thylakoids.
• Functions: The chloroplast traps light energy in its chlorophyll and carotenoid pigments and is the site of photosynthesis — the grana carry out the light reactions and the stroma the dark reactions.

Method: Recognise the textbook's own explicit "double membrane bound" cue, then use a parallel four-part template per organelle

The phrase "double membrane bound" is not a random description here — it is exactly the wording the chapter uses when directly comparing two specific organelles to each other in a single sentence. When a question's wording echoes a specific comparison the textbook makes explicitly (rather than a general fact that happens to be true of several structures), that comparison is almost always the one being tested — so the first step is to recall where in the chapter "double membrane" is used as a direct point of comparison, rather than simply listing every organelle that technically has two membranes. …