Mitochondria and chloroplasts are the two organelles that are double-membranebound.

Characteristics of the mitochondria

Mitochondria are double- membrane-bound structures. The membrane of a mitochondrion is divided into the inner and outer membranes, distinctly divided into two aqueous compartments – outer and inner compartments. The outer membrane is very porous (containing the organelle), while the inner membrane is deeply-folded.

These folds are known as cristae. Cristae increase the surface area inside the cell. They are the sites for $ATP$ – generating chemical reactions. The membrane of a mitochondrion contains specific enzymes meant for specific mitochondrial functions. Hence, the mitochondria are the sites for aerobic respiration. They have their own $DNA$ and ribosomes. Thus, they are able to make their own proteins. This is why they are considered as semi-autonomous organelles

Characteristics of chloroplasts

Chloroplasts are double-membrane-bound structures.

They are divided into outer and inner membranes, further divided into two distinct regions:

$(i)$ Grana are stacks of flattened discs containing chlorophyll molecules. The flattened membranous sacs are called thylakoids. The thylakoids of adjacent grana are connected by membranous tubules called stroma lamellae.

$(ii)$ Stroma is a homogenous mixture in which grana are embedded. It contains several enzymes that are used for the synthesis of carbohydrates and proteins. It also contains its own $DNA$ and ribosomes.

Functions of the mitochondria:

$(i)$ They are the sites for cellular respiration.

$(ii)$ They provide energy in the form of ATP for all vital activities of living cells.

$(iii)$ They have their own $DNA$ and ribosomes. Hence, they are regarded as semiautonomous organelles.

$(iv)$ They have several enzymes, intermediately required for the synthesis of various chemicals such as fatty acids, steroids, and amino acids.

Functions of chloroplasts:

$(i)$ They trap solar energy and utilise it for manufacturing food for plants. Hence, they are involved in the process of photosynthesis.

$(ii)$ They contain the enzymes required for the synthesis of carbohydrates and proteins.