Lecture 6: Endocytosis, Phagocytosis, Pinocytosis, and Clathrin-Mediated Endocytosis

receptor down regulation (3)

- help turn off a signal Agonist-bound β-adrenergic receptors activate G proteins, which then stimulate adenylyl cyclase activity. A.Repeated or persistent stimulation of the receptor by agonist results in phosphorylation of amino acids at the C-terminus of the receptor by protein kinase A (PKA) and/or β-adrenergic receptor kinase (βARK). β-Arrestin then binds to the phosphorylated domain of the receptor and blocks Gs binding, thereby decreasing adenylyl cyclase (effector) activity. B. Binding of β-arrestin also leads to receptor sequestration into endosomal compartments, effectively neutralizing β-adrenergic receptor signaling activity. The receptor can then be recycled and reinserted into the plasma membrane. C. Prolonged receptor occupation by an agonist can lead to receptor down-regulation and eventual receptor degradation. Cells can also reduce the number of surface receptors by inhibiting the transcription or translation of the gene coding for the receptor (not shown).

exocytosis

- phago: At the end of the phagocytosis, exocytosis occurs in order to throw out the waste. - pino: No exocytosis occurs after pinocytosis

diameter

- phago: During phagocytosis, around 1-2 µm size particles are ingested. Vesicles formed during phagocytosis are comparatively large. - pino: During pinocytosis, around 0.1-0.2 µm size liquid droplets are ingested. Vesicles formed during pinocytosis are small.

examples

- phago: Engulfment of bacteria by white blood cells and engulfment of food particles by cells are examples for phagocytosis. - pino: Uptaking of enzymes and LDL from the extracellular fluid is an example for pinocytosis.

break down of particles

- phago: Ingested particles by phagocytosis are broken down into simple substances before absorption - pino: Ingested liquids by pinocytosis are readily absorbed.

method of protrusion

- phago: Pseudopodia are formed during phagocytosis. Vesicles are formed around the particle by evagination. - pino: mostly invagination during pinocytosis

definition

- phago: The ingestion of solid particle into the cell by phagocyte is referred to as phagocytosis. Called cell eating, mostly immune cells and single cellular organisms. - pino: The ingestion of liquid droplets into the cell by small vesicles is referred to as pinocytosis. Called cell drinking, present in every cell

function

- phago: generally used for the defensive purposes of the cell. Followed by exocytosis at the end. Or to take up food. - pino: used in order to intake important materials.

occurrence

- phago: occurs in tissue macrophages, neutrophils and monocytes, Kupffer cells in the liver. As well as protozoal cells. - pino: occurs in almost all the cells in the body of a multicellular organism.

function of receptor-mediated endocytosis

1.Take up nutrients, e.g. cholesterol, low density lipoprotein (LDL). 2.Recycling of synaptic vesicles. 3.Receptor Down-Regulation in the cell membrane 4.Transcytosis: antibody secretion in milk

recycling of synaptic vesicles (2)

A specialized kind of recycling from endosomes plays an important role in the transmission of nerve impulses across synapses

take up nutrients (1)

Cholesterol is transported through the bloodstream in the form of lipoprotein particles, the most common of which is called low-density lipoprotein, or LDL, that is bad cholesterol and atherogenic. Cells of these patients are unable to internalize LDL from extracellular fluids, resulting in the accumulation of high levels of cholesterol in the circulation.

endocytosis

Eukaryotic cells are able to take up macromolecules and particles from the surrounding medium by a process called endocytosisis. It is a fundamental process that enables cells to sample their environment, take up nutrients, eliminate pathogens, cancer, and apoptotic bodies, and regulate the expression of surface components.

transcytosis (4)

In polarized cells (e.g., epithelial cells), internalized receptors can also be transferred across the cell to the opposite domain of the plasma membrane provides a mechanism for the transfer of extracellular macromolecules across epithelial cell sheets

pinocytosis

is the ingestion of fluids along with the solutes into the cell by invagination. Almost all eukaryotic cells in the body of multicellular organisms perform pinocytosis. During pinocytosis, small uniform vesicles called pinosomes are formed. Some of the extracellular macromolecule like cholesterol are uptaken by receptor-mediated endocytosis. The process requires energy in the form of ATP.

phagocytosis

is the ingestion of large solid particles by the cell during endocytosis. Many protists and amoeba also uptake nutrients by phagocytosis. In multicellular organisms, particles like cell debris, aged cells, small mineral particles, dust, various colloids, and bacteria are phagocytized by phagocytes in the immune system, playing a vital role in the defense of the organism. The cells in the immune system like tissue macrophages, neutrophils and monocytes are called professional phagocytes.

Metchnikoff notabel achievement

the phagocyte is the first line of defense against acute infection in most animals, including humans.

clathrin-mediated endocytosis

•involves the formation of vesicles from clathrin-coated regions of the plasma membrane. • •The clathrin in the cytosol are then recycled back to the plasma membrane followed by movement of ingested materials from early endosome to the late endosome, finally fusing with lysosome to form the lysosome-endosome hybrid. • •The materials are then degraded by the low pH and enzyme-rich environment of the endo-lysosomal vesicle. •Involving Clathrin (pits), Dynamin (GTPase), Actin, AP-2,