Endocytosis is the process by which a cell pulls material from outside its membrane into its interior, wrapping portions of its surface around whatever needs to come in and pinching off a small bubble, or vesicle, that carries the cargo inward. Cells use endocytosis to take in nutrients, remove receptors from their surface, destroy invading microbes, and regulate how they communicate with their surroundings.1PubMed. Role of phospholipids in endocytosis, phagocytosis, and macropinocytosis The term literally means “into the cell” (from the Greek endo-, within, and kytos, cell), and it encompasses several distinct pathways that differ in what gets internalized, how the membrane bends, and what molecular machinery drives the process.
How the Basic Process Works
Every form of endocytosis follows the same general sequence. A patch of the cell’s outer membrane curves inward around a target, whether that target is a single protein molecule, a virus particle, or an entire bacterium. The curved membrane eventually closes and separates from the rest of the surface, forming a vesicle that drifts into the cell’s interior. From there, the vesicle fuses with internal compartments where the cargo is sorted, recycled, or broken down.
This bending and pinching requires energy. Many endocytic pathways depend on ATP, the cell’s energy currency, to drive the protein machines that sculpt the membrane and sever the forming vesicle. One well-studied family of proteins, the dynamins, acts like a molecular scissors, wrapping around the neck of the budding vesicle and squeezing until it snaps free. A related protein called EHD1 uses ATP to build scaffolds on membrane tubes, thinning them until they break apart, a step that is critical for recycling internalized material back to the cell surface.2Nature Communications. ATP-dependent membrane remodeling links EHD1 functions to endocytic recycling
Clathrin-Mediated Endocytosis
The best-understood form of endocytosis relies on a protein called clathrin. Clathrin molecules assemble into a lattice-like coat on the inner surface of the plasma membrane, pulling it inward to form a pit roughly 100 to 200 nanometers across. Adaptor proteins bridge the clathrin coat to the membrane and to the specific cargo molecules that need to be internalized. AP2, for instance, recognizes particular short amino acid sequences on the tails of transmembrane cargo proteins and simultaneously grabs clathrin, coupling cargo selection to coat assembly.3PubMed Central. Conformational Regulation of AP1 and AP2 Clathrin Adaptor Complexes – Section: Molecular Composition of APs Once the coated pit deepens enough, dynamin pinches it off, and the newly formed vesicle sheds its clathrin coat so the coat proteins can be reused.4PubMed. Capturing the mechanics of clathrin-mediated endocytosis
Clathrin-mediated endocytosis is selective. The cell picks which receptors to pull in and when, which makes this pathway central to activities like absorbing cholesterol. Low-density lipoprotein (LDL) particles bind to LDL receptors on the cell surface, get swept into clathrin-coated pits, and are delivered to compartments called lysosomes where the cholesterol is freed for use in building membranes or making hormones.5PubMed. Receptor-mediated endocytosis and the cellular uptake of low density lipoprotein Mutations that disrupt this process cause familial hypercholesterolemia, a condition where cholesterol accumulates in the blood because cells cannot efficiently internalize it.
Phagocytosis
Phagocytosis, sometimes called “cell eating,” is the pathway immune cells use to engulf and destroy large particles like bacteria, dead cells, and debris. Macrophages, neutrophils, and dendritic cells extend arm-like projections of membrane around a target, forming a cup-shaped structure that eventually closes into a large vesicle called a phagosome.6PubMed. The cytoskeleton in phagocytosis and macropinocytosis The phagosome then fuses with lysosomes, exposing the engulfed material to digestive enzymes and an acidic environment that breaks it down.
The membrane deformations involved in phagocytosis are extreme compared with the tiny pits formed during clathrin-mediated endocytosis. To push the membrane outward and wrap it around a bacterium, the cell relies heavily on its actin cytoskeleton, the internal scaffold of protein filaments that gives cells their shape. Receptor signals at the cell surface trigger rapid polymerization of actin right beneath the membrane, generating the force needed to extend the phagocytic cup.7PubMed Central. Generation of membrane structures during phagocytosis and chemotaxis of macrophages: role and regulation of the actin cytoskeleton This makes phagocytosis a much more energy-intensive affair than most other endocytic routes.
Macropinocytosis
Macropinocytosis is a less discriminating process. Instead of locking onto a specific receptor or wrapping around a defined target, the cell ruffles its membrane outward, then folds those ruffles back to trap a large gulp of surrounding fluid along with whatever molecules happen to be dissolved in it. The resulting vesicles, called macropinosomes, can be several micrometers in diameter, making them far larger than clathrin-coated vesicles.8PubMed Central. Macropinocytosis: mechanism and targeted therapy in cancers
Cancer cells have drawn particular attention in this context. Tumors with mutations in the RAS signaling pathway often ramp up macropinocytosis to feed themselves. By gulping large volumes of extracellular fluid rich in proteins, these cells can scavenge amino acids to fuel their rapid growth, a nutritional shortcut that healthy cells rarely need. Researchers are exploring whether blocking macropinocytosis could starve certain tumors while leaving normal tissue unaffected.8PubMed Central. Macropinocytosis: mechanism and targeted therapy in cancers
Caveolae and Their Dual Role
Caveolae are tiny flask-shaped indentations in the plasma membrane, stabilized by proteins called caveolins and cavins. They participate in a form of endocytosis distinct from the clathrin pathway, but they also serve a mechanical function that has nothing to do with cargo uptake. When a cell is stretched or swollen, caveolae flatten out, effectively donating extra membrane to absorb the tension. This flattening happens quickly and does not require ATP or actin, acting as a rapid-response buffer against mechanical stress.9PubMed Central. Cells respond to mechanical stress by rapid disassembly of caveolae Once the stress passes, the cell rebuilds its caveolae in an actin- and ATP-dependent process.
The dual role of caveolae matters in disease. Patients with certain forms of muscular dystrophy lack a functional caveola reservoir in their muscle cells, and their membranes are more prone to tearing under mechanical strain.9PubMed Central. Cells respond to mechanical stress by rapid disassembly of caveolae This connection between endocytic membrane structures and tissue integrity illustrates how endocytosis-related machinery can have functions well beyond simple cargo uptake.
Clathrin-Independent Routes
Not everything enters cells through clathrin or caveolae. A growing family of clathrin-independent pathways handles bulk uptake of nutrients and certain membrane-anchored cargo. One of the best characterized is the CLIC/GEEC pathway, which operates without clathrin or dynamin and provides a high-capacity, fast-acting route for internalizing material.10PubMed Central. The Role of the CLIC/GEEC Endocytic Pathway for Mechanophysical Transfection of DNA This pathway also helps maintain membrane tension by continuously cycling membrane inward.
Pathogens have learned to exploit these alternative routes. Adeno-associated virus 2 (AAV2), for example, enters cells primarily through the CLIC/GEEC pathway rather than through clathrin or caveolae. Its entry depends on cholesterol in the membrane and on signaling molecules like Cdc42, and once inside, the virus travels to the Golgi apparatus using the same route as the bacterial toxin cholera toxin B.11PubMed Central. Adeno-associated virus 2 infection requires endocytosis through the CLIC/GEEC pathway Bacterial toxins such as Shiga toxin and cholera toxin also hijack this pathway, clustering lipids on the cell surface to trigger their own uptake.12PubMed. Mechanisms of Carrier Formation during Clathrin-Independent Endocytosis
What Happens After Internalization
Endocytosis does not end when a vesicle pinches off the surface. The vesicle quickly fuses with a sorting station called the early endosome, and from there, cargo molecules face one of several fates.13PubMed Central. The early endosome: a busy sorting station for proteins at the crossroads Some are sent to lysosomes for degradation. Others are recycled directly back to the plasma membrane along a fast track. Still others travel through a slower recycling loop via specialized recycling endosomes located near the cell’s nucleus.14PubMed Central. The enigmatic endosome – sorting the ins and outs of endocytic trafficking A fourth option routes cargo to the Golgi apparatus, where it can be repackaged or modified.
The sorting decision is not random. Specific lipid molecules on the endosome’s membrane act as address labels, and each compartment along the pathway has its own characteristic lipid signature.15FEBS Letters. Spatial organization of phosphoinositide signaling In neurons, for example, one type of receptor stimulus causes internalized receptors to be routed to lysosomes and destroyed, while a different stimulus sends the same receptors through the recycling pathway so they can return to the surface and function again.16PubMed Central. Activity-dependent endocytic sorting of kainate receptors to recycling or degradation pathways This flexible sorting allows cells to fine-tune how sensitive they are to signals from their environment.
How Viruses Exploit Endocytic Entry
Many viruses depend on endocytosis to get inside host cells. Influenza is a classic case. The virus binds to receptors on the cell surface, triggers its own internalization through receptor-mediated endocytosis, and then uses the acidic environment inside the endosome to undergo a shape change that lets it fuse with the endosomal membrane and release its genetic material into the cell.17PubMed Central. Endocytosis of influenza viruses From the virus’s perspective, the endosome is not a trap but a delivery vehicle. The low pH that normally helps digest internalized cargo instead serves as the trigger for the virus to escape into the cytoplasm.
This dependence on endocytic entry is one reason researchers are interested in drugs that interfere with endosomal acidification. Blocking the drop in pH could, in principle, prevent the virus from completing its entry. The challenge is that healthy cells also rely on endosomal acidification for normal sorting and recycling, so the therapeutic window can be narrow.
Endocytosis and Alzheimer’s Disease
Disruptions in endocytic traffic are now recognized as one of the earliest detectable changes in the brains of people developing Alzheimer’s disease. In affected neurons, early endosomes become abnormally enlarged, and markers of both endocytic uptake and recycling are elevated, suggesting the system is working overtime but not efficiently clearing its cargo.18PubMed Central. Endocytic pathway abnormalities precede amyloid beta deposition in sporadic Alzheimer’s disease and Down syndrome These endosomal abnormalities appear before the hallmark amyloid plaques show up in the brain regions typically associated with Alzheimer’s, making endocytic dysfunction one of the earliest known intracellular changes in the sporadic form of the disease.
Because amyloid-beta peptides are generated in endosomes, a malfunctioning endocytic pathway could lead to greater production or poorer clearance of these toxic fragments.19PubMed Central. Endocytosis and Alzheimer’s disease Several Alzheimer’s risk genes identified through large genetic studies turn out to encode proteins involved in endosomal sorting and trafficking, reinforcing the idea that this pathway is not just a bystander but a contributor to disease progression.
Endocytosis as a Drug Delivery Strategy
The same pathways that viruses exploit are now being co-opted for medicine. Nanoparticle drug delivery systems are increasingly designed to enter cells through specific endocytic routes, the goal being to get therapeutic molecules past the plasma membrane and into the right intracellular compartment.20PubMed Central. Targeting receptor-mediated endocytotic pathways with nanoparticles: rationale and advances By decorating nanoparticles with ligands that bind particular cell-surface receptors, researchers can steer them toward clathrin-mediated uptake in tumor cells while sparing other tissues.
A persistent challenge is what happens after the nanoparticle gets inside. Many end up trapped in lysosomes, where the acidic, enzyme-rich environment degrades their cargo before it can reach its target. Designing particles that can escape from the endosome into the cytoplasm, without also causing membrane damage, remains an active area of work.21PubMed. Targeted endocytosis: Strategies in drug delivery systems for enhancing antitumor efficacy Some approaches use pH-sensitive coatings that swell and rupture the endosomal membrane only after the vesicle has acidified, releasing the drug into the cell interior.
Endocytosis in Plants and Fungi
For a long time, biologists doubted that endocytosis could occur in plant cells at all. Plants have rigid cell walls and high internal turgor pressure, which pushes the plasma membrane outward against the wall, seemingly making it impossible to pull the membrane inward. That skepticism has been thoroughly overturned. Endocytosis is now recognized as essential for plant life, playing roles in hormone signaling, nutrient uptake, and defense against pathogens.22PubMed Central. Visualising endocytosis in plants: past, present, and future
Fungi face a similar mechanical problem. In fission yeast, researchers found that endocytosis is governed by a tug-of-war between inward forces generated by localized actin polymerization and the outward push of turgor pressure on the membrane. Lowering the effective turgor pressure (by adding a sugar solution to the surrounding medium) sped up the early steps of endocytosis and even rescued defects in mutant cells that normally struggle to pull their membranes inward.23PubMed Central. Role of turgor pressure in endocytosis in fission yeast Removing the cell wall entirely did not stop endocytosis, confirming that the wall is not mechanically required for the process and that the real obstacle is the pressure differential across the membrane.
How Researchers Watch Endocytosis in Real Time
Studying something that happens on a scale of nanometers and milliseconds is not easy. A wave of new fluorescent probes has made it possible to watch individual endocytic events in living cells rather than relying on fixed, snapshot-style images. One approach uses a small-molecule dye called ECGreen, which stays dim on the cell surface but lights up when it enters the acidic interior of an endocytic vesicle, giving researchers a real-time readout of internalization.24bioRxiv. A small-molecule fluorescent probe for live-cell imaging of endocytosis
A newer probe, called o-IP-IP, uses a dual-anchoring strategy that keeps it locked to the outer face of the plasma membrane for much longer than conventional dyes. Because it resists passive internalization, any fluorescence that appears inside the cell reflects genuine endocytic activity rather than dye leaking across the membrane on its own. This probe has been used to track endocytosis under osmotic stress and during changes in calcium concentration, giving researchers a cleaner picture of how cells adjust their uptake under different conditions.25PubMed. High-Fidelity Tracking of Endocytosis in Cancer Cells under Cellular Stress Using a Long-Term Anchoring Cell Membrane Fluorescent Probe In neuroscience, total internal reflection fluorescence microscopy has been adapted to image single endocytic events at synapses, capturing the moment individual synaptic vesicle proteins are pulled back into the presynaptic terminal after a round of neurotransmitter release.26PubMed Central. Live-cell imaging of endocytosed synaptophysin around individual hippocampal presynaptic active zones
Endocytosis and Exosome Biogenesis
Endocytosis is not just about bringing material in. The endosomal system also generates small vesicles that cells release outward, called exosomes. Exosomes originate inside endosomes when the endosomal membrane buds inward, creating tiny internal vesicles. If the endosome then fuses with the plasma membrane instead of a lysosome, those internal vesicles are released into the extracellular space as exosomes.27PubMed Central. The biology, function, and biomedical applications of exosomes The cargo packed into exosomes, including proteins, lipids, and RNA, reflects the sorting decisions made during endosomal trafficking. Exosomes can travel through blood and other body fluids to distant cells, making them a form of long-range intercellular communication that is rooted in the same endosomal machinery used for uptake and recycling.
An Ancient Cellular Innovation
Endocytosis is not a recent evolutionary invention layered onto already complex cells. Comparative studies across the full breadth of eukaryotic life, from animals and plants to single-celled protists, suggest that the last common ancestor of all eukaryotes already possessed a recognizable endocytic system. By examining which endocytic genes are shared across distantly related organisms, researchers have reconstructed a surprisingly sophisticated ancestral toolkit that included machinery for membrane bending, vesicle scission, and cargo sorting. The fact that organisms as different as amoebae and human neurons share core endocytic components speaks to how fundamental this process is to eukaryotic cell biology.