Final answer:
Things are brought to early endosomes through receptor-mediated endocytosis, where they are internalized by vesicles that fuse with early endosomes for sorting. The endosomes serve as sorting stations for recycling or directing contents to lysosomes for degradation. The process also involves the complex sorting and modification of proteins and lipids by the Golgi apparatus.
Step-by-step explanation:
Things are brought to early endosomes primarily through a process called receptor-mediated endocytosis. In this process, receptors on the cell surface have a high affinity for specific substances. Once these substances bind to the receptors, they cluster in areas called coated pits. These pits invaginate and form a vesicle, called a coated vesicle, which internalizes the substances into the cell. Following internalization, the vesicles lose their clathrin coat and fuse with early endosomes. The early endosome serves as a sorting station where receptor proteins are often recycled back to the cell membrane, while other contents are delivered to late endosomes and eventually to lysosomes for degradation.
Additionally, proteins and lipids processed by the Golgi apparatus are packaged into transport vesicles, which may also fuse with early endosomes for sorting. During their journey through the endomembrane system, proteins are modified and sorted before becoming functionally active. This complex sorting and modification process ensures that substances are correctly delivered to their cellular destinations or released outside the cell if they are part of a secretion pathway.