Source: Campbell Biology, Chapter 7 (Section 7.5)
Tags: bulk transport, exocytosis, endocytosis, phagocytosis, pinocytosis, receptor-mediated endocytosis, vesicle, LDL, cholesterol uptake, membrane remodelling
Difficulty: Intermediate | Prerequisites: Sections 7.1 to 7.4 (membrane structure, selective permeability, passive and active transport). Familiarity with vesicles and the endomembrane system from Chapter 6 is also helpful.
Sections 7.3 and 7.4 covered how small molecules and ions cross the membrane. But large molecules, such as proteins and polysaccharides, are too big for transport proteins. This section explains how cells move bulk cargo in and out using vesicles: exocytosis (export) and endocytosis (import). These processes also serve a second purpose: they continually remodel the plasma membrane itself.
Cells export large molecules by fusing vesicles with the plasma membrane (exocytosis) and import them by pinching inward to form new vesicles (endocytosis). There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Both processes require energy and also serve to add or remove membrane material.
Exocytosis
The process by which a cell secretes molecules by fusing transport vesicles with the plasma membrane. The vesicle contents spill outside, and the vesicle membrane becomes part of the plasma membrane.
Endocytosis
The process by which a cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. A small area of membrane sinks inward, then pinches off as a vesicle inside the cell.
Phagocytosis
A type of endocytosis in which a cell engulfs a large particle (e.g. a bacterium or food particle) by wrapping pseudopodia around it and packaging it in a food vacuole. Think of it as "cell eating."
Pinocytosis
A type of endocytosis in which the cell continually gulps small droplets of extracellular fluid into tiny vesicles. It is nonspecific: whatever is dissolved in the fluid comes along. Think of it as "cell drinking."
Receptor-mediated endocytosis
A specialised type of pinocytosis in which specific solutes bind to receptor proteins on the membrane surface. The receptors cluster in coated pits, which then pinch off as coated vesicles carrying the bound molecules. This allows the cell to acquire bulk quantities of specific substances even when those substances are not very concentrated in the extracellular fluid.
Low-density lipoprotein (LDL)
A complex of lipids and protein that carries cholesterol in the blood. LDLs bind to LDL receptors on cell surfaces and enter the cell by receptor-mediated endocytosis.
Coated pit
A region of the plasma membrane lined on the cytoplasmic side by a layer of coat protein, where receptor-mediated endocytosis takes place.
Pseudopodium (plural: pseudopodia)
A temporary cytoplasmic extension of a cell, used in phagocytosis to surround and engulf particles.
Large molecules such as proteins and polysaccharides are too big for transport proteins. They leave the cell via exocytosis:
A transport vesicle budded from the Golgi apparatus moves to the plasma membrane.
Specific proteins rearrange the lipid molecules of both bilayers so that the vesicle membrane and the plasma membrane fuse.
The vesicle contents spill outside the cell, and the vesicle membrane is incorporated into the plasma membrane.
Exocytosis is used by secretory cells: neurons releasing neurotransmitters, pancreatic cells secreting insulin, and goblet cells secreting mucus are all examples. The process requires energy.
Endocytosis is the reverse of exocytosis: the cell takes in material by forming new vesicles from the plasma membrane.
A small area of plasma membrane sinks inward to form a pocket.
The pocket pinches off, forming a vesicle containing material from outside the cell.
There are three main types:
Phagocytosis ("cell eating")
The cell extends pseudopodia around a large particle (bacterium, food).
The particle is packaged in a membranous sac called a food vacuole.
The food vacuole fuses with a lysosome, and hydrolytic enzymes digest the contents.
Used by amoebas for feeding and by immune cells (macrophages, neutrophils) for destroying pathogens.
Pinocytosis ("cell drinking")
The cell continually gulps small droplets of extracellular fluid into tiny vesicles.
It is nonspecific: any solutes dissolved in the fluid are taken in.
The plasma membrane infolds, and the resulting vesicles are lined on their cytoplasmic side by coat protein.
Receptor-mediated endocytosis
Specific solutes bind to receptor proteins embedded in the membrane.
Receptors with bound ligands cluster in coated pits (regions lined with coat protein on the cytoplasmic side).
The coated pit pinches off to form a coated vesicle carrying the specific molecules inside.
This allows the cell to concentrate and import specific substances that may be at low concentration in the extracellular fluid.
Cholesterol uptake: a key example
Human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis and steroid production. Cholesterol travels in the blood packaged as low-density lipoproteins (LDLs). LDLs bind to LDL receptors on the plasma membrane and enter the cell by endocytosis.
Membrane remodelling
Endocytosis and exocytosis together provide a mechanism for rejuvenating and remodelling the plasma membrane. Membrane added by exocytosis is balanced by membrane removed by endocytosis, keeping the cell surface area roughly constant.
Receptor-mediated endocytosis of LDL is directly relevant to cardiovascular health. When LDL receptors are defective (as in familial hypercholesterolaemia), cholesterol accumulates in the blood, increasing the risk of atherosclerosis. Phagocytosis by white blood cells is a cornerstone of the innate immune system. Neurotransmitter release via exocytosis is the basis of all chemical signalling between neurons.
Students sometimes think endocytosis and exocytosis are forms of passive transport. They are not. Both require energy.
Pinocytosis is often confused with receptor-mediated endocytosis. Pinocytosis is nonspecific (it takes in whatever is dissolved in the fluid). Receptor-mediated endocytosis is highly specific, targeting particular molecules via receptor binding.
Students sometimes forget that exocytosis adds membrane material to the cell surface and endocytosis removes it. The two processes work together to maintain membrane homeostasis.
⚠️ Know the three types of endocytosis and be ready to distinguish them: what triggers each, whether it is specific or nonspecific, and what structures are involved (pseudopodia, coated pits, etc.).
⚠️ The LDL/cholesterol example is a common exam question for receptor-mediated endocytosis. Be able to trace the pathway from LDL in the blood to cholesterol inside the cell.
⚠️ Understand that exocytosis and endocytosis are both energy-requiring processes that also serve to remodel the plasma membrane.
True or False: Exocytosis removes membrane material from the cell surface. (False, it adds membrane material. Endocytosis removes it.)
Fill in the blank: A cell that engulfs a bacterium using pseudopodia is performing ______. (Phagocytosis.)
True or False: Pinocytosis is specific to certain molecules. (False, it is nonspecific.)
Fill in the blank: Cholesterol enters human cells via ______ endocytosis, after LDL binds to ______ on the cell surface. (Receptor-mediated, LDL receptors.)
True or False: Bulk transport by vesicles is a form of passive transport. (False, it requires energy.)
Q: Compare and contrast phagocytosis and pinocytosis.
A: Both are forms of endocytosis. Phagocytosis engulfs large particles (bacteria, food) using pseudopodia, forming a food vacuole that fuses with a lysosome. Pinocytosis takes in small droplets of extracellular fluid by infolding the membrane into tiny vesicles. Phagocytosis is selective in what it engulfs (the cell extends toward the particle). Pinocytosis is nonspecific: whatever is dissolved in the fluid enters the vesicle.
Q: Describe the process of receptor-mediated endocytosis and explain why it is more efficient than pinocytosis for importing specific molecules.
A: In receptor-mediated endocytosis, specific molecules (ligands) bind to receptor proteins on the cell surface. The receptors cluster in coated pits, which pinch off as coated vesicles carrying the bound molecules inside. Because receptors concentrate the target molecule, the cell can import large amounts of a specific substance even when it is present at low concentrations outside. Pinocytosis, by contrast, takes in fluid indiscriminately and would require enormous volumes to collect the same amount of a scarce molecule.
Q: How do exocytosis and endocytosis contribute to membrane remodelling?
A: Exocytosis adds vesicle membrane to the plasma membrane (the vesicle membrane is incorporated when it fuses). Endocytosis removes patches of plasma membrane (they are internalised as new vesicles). Together, these processes allow the cell to adjust its surface area, replace old membrane components, and recycle receptors.
Q: Trace the pathway of cholesterol from the bloodstream into a human cell.
A: Cholesterol circulates in the blood packaged as LDL particles (complexes of lipids and protein). LDLs bind to LDL receptors on the plasma membrane. The receptor-LDL complexes cluster in coated pits. The coated pit pinches off to form a coated vesicle inside the cell. The cholesterol is then released and used for membrane synthesis or the production of other steroids. The emptied receptors are recycled back to the plasma membrane.
Bulk transport connects to Chapter 6 (endomembrane system): vesicles budding from the ER and Golgi are the same vesicles that fuse with the plasma membrane during exocytosis. Phagocytosis connects to Chapter 43 (immune system), where macrophages and neutrophils use it to destroy pathogens. Receptor-mediated endocytosis connects to Chapter 11 (cell signalling), because signal molecules often enter cells through receptor binding and internalisation.
Bulk transport, exocytosis, vesicle fusion, secretion, secretory cells, neurotransmitter release, endocytosis, phagocytosis, cell eating, pseudopodia, food vacuole, lysosome, pinocytosis, cell drinking, nonspecific uptake, receptor-mediated endocytosis, coated pit, coated vesicle, coat protein, clathrin, LDL, low-density lipoprotein, cholesterol uptake, LDL receptor, membrane remodelling, membrane recycling, familial hypercholesterolaemia