Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 7.5 Bulk Transport Across the Plasma Membrane Occurs by Exocytosis and Endocytosis
Cells don’t just move small molecules. For bulk cargo—like proteins, polysaccharides, or even entire cells—they use specialized forms of vesicle transport.
This is where exocytosis and endocytosis take the stage.
1. Exocytosis: Pushing Things Out
How it works:
A vesicle containing cargo (like proteins or hormones) buds off from the Golgi apparatus.
It travels along the cytoskeleton toward the plasma membrane.
The vesicle fuses with the membrane and releases its contents outside the cell.
Examples:
Insulin secretion from pancreatic cells
Neurotransmitter release at nerve synapses
2. Endocytosis: Bringing Things In
The cell membrane folds inward, forming a vesicle that captures material from outside.
Types of Endocytosis:
Phagocytosis ("cellular eating"):
The cell engulfs large particles (e.g., bacteria).
Forms a food vacuole that fuses with a lysosome for digestion.Pinocytosis ("cellular drinking"):
The cell non-selectively engulfs extracellular fluid and its contents into small vesicles.Receptor-Mediated Endocytosis:
The cell uses receptors to bind and internalize specific molecules (e.g., cholesterol in LDL form).
3. Recycling the Membrane
Although membrane is added during exocytosis and removed during endocytosis, the total membrane surface area remains balanced over time.
These processes also help in remodeling the membrane and regulating protein composition on the surface.
In a Nutshell
Exocytosis moves large molecules out of the cell via vesicle fusion.
Endocytosis moves substances into the cell by forming vesicles from the membrane.
Three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated.
Together, they manage bulk transport and help maintain membrane homeostasis.