Endocytosis and Exocytosis

A bacterium is too big to squeeze through a channel or carrier protein, so a white blood cell handles it differently: it wraps its own membrane around the particle. In endocytosis, the plasma membrane folds inward, pinches off, and traps the external material inside a vesicle that buds into the cytoplasm. In phagocytosis specifically, the cell extends membrane around a large particle like a bacterium and engulfs it whole. This folding is not a side effect of some pump working, it is the cytoskeleton physically reshaping the membrane, and that reshaping costs ATP. That is why endocytosis counts as active transport even though no single protein carries a molecule across: the whole membrane is doing the work, not one transporter.

Exocytosis runs the same idea backward. A vesicle inside the cell, loaded with cargo like hormones or neurotransmitter, travels to the plasma membrane. The vesicle membrane fuses with the plasma membrane, opening up and dumping its contents into the extracellular space. Fusion adds the vesicle's membrane material directly into the plasma membrane, so the cell's surface area actually grows a little with each fusion event.

Neurons show why that growth matters. During a burst of signaling, many synaptic vesicles fuse with the presynaptic membrane at once, releasing neurotransmitter into the synapse but also adding a surge of extra membrane to the neuron's surface. Left unchecked, the membrane would keep expanding. So the neuron runs endocytosis right alongside exocytosis, pulling membrane back in and rebuilding vesicles, keeping surface area roughly constant while keeping vesicles in supply for the next signal.