Membranous vs Nonmembranous Organelles
Prokaryotic cells do not possess membrane bound organelles whereas eukaryotic cells possess membrane bound organelles as well as non-membrane bound organelles.
Membrane Bound Organelles
Membrane bound organelles that are generally found in eukaryotic cells are rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, mitochondria, plastids and lysosomes. Endoplasmic reticulum consists of branching membranes. Some are connected to the plasma membrane, and some are connected to the nuclear membrane. Depending on the presence of ribosomes on the membrane of the tubules there are two types of ER. SER and RER. Tubules containing ribosomes on the surface are called rough endoplasmic reticulum, and that do not contain ribosomes on the surface are called smooth endoplasmic reticulum. Mitochondria are of different shapes. They may be spherical, oval, or rod shaped. They may be found anywhere in the cytoplasm. Mitochondrion is a double membrane organelle. That is, it has two membranes; a smooth outer membrane and an inner membrane. The inner membrane is folded in to the cytoplasm forming cristae. Cristae increase the surface area of the membrane. Cristae bear many oxysomes. Golgi apparatus is a single membrane bound organelle. Vesicles are separated from the cytoplasm by a unit membrane. Chororoplasts are double membrane organelles, in which, both the membranes are smooth. Cilia and flagella are also membranous structures. Structure of both cilia and flagella are similar. Flagella are long structures, and cilia are short structures. One cell normally contains one flagellum or 2 flagella, but a large number of cilia are present in a cell. Both cilia and flagella are surrounded by a unit membrane. They are made up of 2 central singlet microtubules and 9 pairs of peripheral microtubules. This is called 9 + 2 arrangement of microtubules. Prokaryotic cells also contain flagella. Flagella in prokaryotic cells do not have the 9+2 arrangement.
Non-membrane Bound Organelles
Organelles, which generally do not contain membranes, are ribosomes, cytoskeletal structures, centrioles, cilia, and flagella. Ribosomes can be found in both prokaryotic and eukaryotic cells. They are small granule like structures. They may be found anywhere in the cytoplasm. Ribosomes are of 2 types, 70s and 80s. 70s ribosomes are found in prokaryotic cells, and 80s ribosomes are found in eukaryotic cells. Cytoskeleton has two types of non-membranous components. Those are microfilaments and microtubules. All the three structures do not have a unit membrane. Microtubules are hollow and cylindrical structures. They are very fine unbranched structures. Microtubules are protein tubes made up of tubulin protein. Microfilaments are solid rod like structures. They are also unbranched structures. Microfilaments are protein fibres made up of actin protein. Centrioles are also not surrounded by a membrane. They are made up of triplets of microtubules, which are arranged around a cavity. There are no central microtubules. Therefore, it is called the 9 + 0 arrangement of microtubules. Only animal cells have centrioles. Plant cells do not have centrioles. Normally two centrioles are arranged perpendicular to each other. One such pair of centrioles is called a centrosome.
What is the difference between Membranous Organelles and Nonmembranous Organelles?
• Membranous organelles are surrounded by a unit membrane or a double membrane, and non-membranous organelles are not surrounded by any unit membrane.