Ribosomes, mitochondria and plastids are important organelles taking part in protein synthesis, respiration and photosynthesis.
Ribosomes are non-membrane-bound structures composed of ribosomal RNA and proteins are found in both prokaryotic and eukaryotic cells. They synthesise proteins by attaching to a single messenger RNA and translate mRNA into proteins. While eukaryotic ribosomes are 80S, prokaryotic ribosomes are 70S. Apart from the cytoplasm, ribosomes also occur on the rough endoplasmic reticulum, in chloroplasts as well as within the mitochondria.
Mitochondria are sites of aerobic respiration and produce cellular energy in the form of ATP, hence called the powerhouse of the cell. It is a double-membrane-bound structure consisting of an outer and an inner membrane. The membranes divide the mitochondrial lumen into outer compartment and the inner compartment called the matrix, which contains a single, circular DNA molecule, a few RNA molecules and 70S ribosomes. While the outer membrane forms a continuous boundary, the inner membrane folds towards the matrix to form cristae, which increase the surface area.
Plastids are double membrane organelles that contain pigments and impart specific colours to plants. Based on the pigments they possess, plastids can be classified as chloroplasts, chromoplasts and leucoplasts. Chloroplasts contain chlorophyll and carotenoid pigments that capture light energy needed for photosynthesis, and chlorophyll imparts green colour to the leaves.
Chloroplasts possess an outer and an inner membrane with the inner membrane being less permeable than the outer membrane. Stroma, the space enclosed by the inner membrane, contains double-stranded, circular DNA molecules, 70S ribosomes and enzymes required for the synthesis of carbohydrates and proteins.
Stroma also contains thylakoids ―flattened membranous sacs containing chlorophyll pigment in their lumen, the space enclosed by the thylakoid membranes. Around twenty to fifty thylakoids are stacked like piles of coins, forming grana or intergranal thylakoids. The thylakoids of different grana are connected to each other by flat membranous tubules called stroma lamellae.
The chloroplasts as well as mitochondria are known as semi-autonomous organelles as they possess their own DNA. Chromoplasts contain carotene and xanthophylls that give red, yellow and orange colour to plant parts. Leucoplasts, are colourless plastids that store nutrients such as carbohydrates and fats. Based on the type of nutrient stored, leucoplasts are classified as amyloplasts, elaioplasts and aleuroplasts. Amyloplasts store carbohydrates, elaioplasts store oils and fats and aleuroplasts store proteins.