Bulk Transport and Membrane Dynamics (Endocytosis, Exocytosis, and Membrane Synthesis)

QA flashcards covering bulk transport, endocytosis (phagocytosis, pinocytosis, receptor-mediated), exocytosis, and membrane synthesis/sidedness.

What is bulk transport across the plasma membrane?

Bulk transport moves large particles or large quantities by enclosing them in vesicles that bud from or fuse with the membrane.

Which processes constitute bulk transport?

Endocytosis and exocytosis.

What is endocytosis?

The cell takes in macromolecules and particulate matter by forming new vesicles from the plasma membrane.

During endocytosis, how is external material enclosed to form a vesicle?

The plasma membrane invaginates to form a pocket that deepens and pinches off as a vesicle containing exterior material.

Name the three types of endocytosis.

Phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Define phagocytosis.

Cellular eating; uptake of solid particles or whole organisms by engulfing with pseudopodia to form a phagosome.

Define pinocytosis.

Cellular drinking; nonspecific uptake of extracellular fluid and dissolved solutes via small vesicles.

What is receptor-mediated endocytosis?

Endocytosis initiated by binding of ligands to membrane receptors, forming coated pits and vesicles via clathrin.

What is the role of clathrin in receptor-mediated endocytosis?

Clathrin forms the coat of coated pits/vesicles with adaptor proteins, enabling vesicle formation and subsequent uncoating.

What is a coated pit?

Membrane region where receptors cluster and are coated by clathrin/adaptor proteins to form a coated vesicle.

What happens to internalized receptor-ligand complexes?

They move to early endosomes; ligands may go to late endosomes/lysosomes, and receptors are recycled to the membrane.

What is transcytosis?

Transport of ligands across the cell to the opposite membrane side for release.

What is exocytosis?

Export of bulky material by vesicles fusing with the plasma membrane and releasing contents outside the cell.

Give an example of exocytosis.

The pancreas secreting insulin into the bloodstream.

How does exocytosis affect the plasma membrane surface area?

Exocytosis helps maintain total plasma membrane surface area by replacing membrane patches.

What is meant by synthesis and sidedness of membranes?

Membranes have distinct inside and outside faces; lipid composition may differ; proteins have directional orientation (N-terminus outside, C-terminus inside).

Where are membrane proteins and lipids synthesized?

Synthesis occurs in the endoplasmic reticulum; proteins are further processed in the Golgi apparatus and transported by vesicles to the plasma membrane.

What happens in the Golgi apparatus regarding membrane components?

Glycoproteins and glycolipids undergo carbohydrate modification; lipids and proteins are prepared for transport to the plasma membrane.

What is the role of LDL in receptor-mediated endocytosis?

Low-density lipoprotein carries cholesterol and binds to LDL receptors on membranes to enable cholesterol uptake.

What is familial hypercholesterolemia?

An inherited disease with very high cholesterol due to defective or missing LDL receptor proteins.

What is a coated pit and what proteins are involved?

A region of the plasma membrane coated with clathrin and adaptor proteins, where receptor-bound ligands cluster before vesicle formation.

What happens during vesicle uncoating in receptor-mediated endocytosis?

The clathrin coat and adaptor proteins are removed; receptors are recycled back to the plasma membrane.

What is the fate of ligands after receptor-mediated endocytosis?

Ligands are released in early endosomes; some are transported to late endosomes/lysosomes for digestion, others may be transcytosed.

What is a phagosome?

A vesicle formed around ingested solid material; it fuses with a lysosome for digestion.

How do phagocytosis and pinocytosis differ in specificity?

Phagocytosis ingests solids (often by specialized cells); pinocytosis nonspecifically ingests extracellular fluid and solutes.

What is the purpose of secretory vesicles in membrane trafficking?

Secretory vesicles from the Golgi carry secretory proteins to the plasma membrane for exocytosis.

How are secretory proteins positioned on the cell membrane after secretion?

Carbohydrate portions of glycoproteins and glycolipids are placed on the extracellular face after vesicle fusion.