The story begins with a single eukaryotic cell. The cytoplasm of the said cell contains a system of membranous organelles that are suspended in it. The organelles are termed as a system even though they have different structures and functions as they are essential to the working of the cell. They work in coordination to achieve that. Some of the essential cogs in the machine are, vacuoles, lysosomes, Golgi complex and the endoplasmic reticulum (ER).
Since the functioning of other organelles like mitochondria, chloroplasts etc, are not coordinated with the functioning of the above, they are not considered part of the system. Let us read about these in brief.
- Endoplasmic Reticulum – ER represents a network of tiny tubular structures scattered in the cytoplasm. It divides the cytoplasm into two distinct compartments – luminal (inside ER) and extraluminal (outside ER). ER is present in the cytoplasm in two forms – Rough ER (embedded with ribosomes) and Smooth ER (free from ribosomes). Rough ER helps in synthesis and secretion of proteins whereas Smooth ER synthesizes lipids.
- Golgi bodies – Named after Camilo Golgi, the scientist who discovered them, Golgi bodies are a membraneous disc like sacs or cisternae of varying diameter between 0.5 – 1.0 µm stacked parallel like a lump of pancakes. Interconnected cis and trans faces or forming and maturing faces of cisternae are concentrically arranged around the nucleus. It serves mainly as the packaging center for endomembrane secretions like proteins.
- Lysosomes – Infamous as the ‘suicidal bags of cell’, these are membrane-bound vesicular structures containing hydrolytic enzymes capable of digesting carbohydrates, lipids, proteins and even nucleic acids.
- Vacuoles – The voluminous vacuole is the membrane-bound space of the cytoplasm that occupies as much as up to 90% volume of the plant cell. It contains water, sap, excretory products and other materials which are useless for the cell. The outer membrane of vacuoles, known as tonoplast, facilitates transport of ions against the concentration gradient into the vacuole. Contractile vacuoles in amoeba facilitate excretion along with transporting engulfed food via food vacuoles in some other protists.
To learn further about the enigmatic endomembrane system, visit Byju’s.