Endomembraanijärjestelmän

Endomembrane system is a complex network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins. It is composed of various organelles, including the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the nuclear envelope. These membranes are interconnected either directly or indirectly through the transfer of vesicles, small sacs that bud off from one membrane and fuse with another.

The endoplasmic reticulum (ER) plays a crucial role in protein and lipid synthesis. The rough ER, studded with ribosomes, is involved in the synthesis of proteins destined for secretion or insertion into membranes. The smooth ER, lacking ribosomes, is responsible for lipid synthesis, detoxification, and calcium storage.

Proteins synthesized in the ER are then transported to the Golgi apparatus for further modification, sorting, and packaging. The Golgi further modifies proteins and lipids, adding carbohydrates or removing specific amino acids, and then sorts them into vesicles for delivery to their final destinations. These destinations can include lysosomes, the plasma membrane, or secretion outside the cell.

Lysosomes are membrane-bound organelles containing digestive enzymes that break down waste materials and cellular debris. Vacuoles are also membrane-bound sacs, but their functions vary depending on the cell type. In plant cells, a large central vacuole stores water, nutrients, and waste products, and also helps maintain turgor pressure.

The nuclear envelope, a double membrane surrounding the nucleus, is considered part of the endomembrane system as it is continuous with the ER. It regulates the passage of molecules between the nucleus and the cytoplasm. The coordinated activities of these organelles ensure the proper functioning and integrity of the cell.