Week 8 - Endocytosis L13

Endocytosis

Process where cells take in substances from outside by engulfing them with their plasma membrane.

Phagocytosis

Type of endocytosis known as 'cell eating' where large particles like bacteria or dead cell debris are engulfed.

Pinocytosis

Type of endocytosis known as 'cell drinking' where the cell takes in extracellular fluid and dissolved molecules. - Immune Cells - Macrophages

Receptor-mediated endocytosis

Type of endocytosis where specific molecules bind to surface receptors before being engulfed - LDL Cholesterol uptake

Bulk-phase Endocytosis

Nonspecific uptake of extracellular fluids where any present molecules are taken up. - Pinocytosis/Phagocytosis

Steady State Composition

Balance between exocytosis and endocytosis to maintain the composition of the plasma membrane.

Reactive Oxygen Species (ROS)

Substances produced in phagocytes that kill microorganisms.

Mycobacterium tuberculosis

Bacterium that inhibits phagosome-lysosome fusion, allowing it to multiply within the cell.

Coxiella burnetii

Bacterium that can survive in the acidic environment of the lysosome.

Listeria monocytogenes

Bacterium that produces proteins to escape the lysosomal membrane into the cytosol.

1. Recognition / Trigger

• The process begins when something outside the cell (nutrient, signaling molecule, or microbe) interacts with the cell membrane.

• In receptor-mediated endocytosis, specific molecules bind to receptors like a lock and key.

• In phagocytosis, immune cells detect and bind to large particles such as bacteria.

• In pinocytosis, the cell non-selectively takes in fluid and dissolved substances.

2. Membrane Invagination

• The plasma membrane bends inward, starting to wrap around the target material.

• Structural proteins such as clathrin, caveolin, and actin help shape and support the forming pocket.

3. Vesicle Formation (Budding and Pinching Off)

• The invaginated pocket deepens until the edges fuse, enclosing the material inside.

• The dynamin protein acts like molecular scissors to pinch the vesicle off from the membrane.

• The vesicle is now completely inside the cytoplasm.

4. Vesicle Trafficking

• The new vesicle travels through the cytoplasm using the cytoskeleton as a transport network.

• Motor proteins such as dynein and kinesin move the vesicle to its destination (e.g., endosome or lysosome).

5. Processing and Sorting

• The vesicle fuses with an early endosome, which acts as a sorting station.

• Useful materials are recycled back to the plasma membrane or sent deeper into the cell.

• Harmful or unwanted substances are directed to lysosomes, where they are broken down by enzymes.

Ongoing sampling

Process by which most cells continuously take in fluids and dissolved molecules through pinocytosis.

Early Endosome

The vesicle often fuses with an early endosome, a sorting station where useful goodies are recycled to the membrane or sent deeper.

Late Endosome

Late endosomes are located nearer the nucleus and are involved in sorting materials.

H+-ATPase Proton Pump

The acidic interior of endosomes is due to the H+-ATPase proton pump in the endosomal membrane.

Low Density Lipoproteins (LDLs)

Cholesterol is produced in the liver and packaged into LDLs for efficient transport in blood plasma.

LDL Receptor Function

LDL receptors are transported to the plasma membrane, concentrated in coated pits, and ready to take up LDL in blood.

LDL Receptor Structure

LDL receptors are transmembrane glycoproteins with multiple domains, including a binding site for apo B100 and apo E.

ApoE Function

Acting as a ligand for lipoprotein receptors, such as the LDL receptor.

ApoE Role in Metabolism

ApoE is necessary for the final hepatic clearance of specific lipoproteins, including VLDL, and IDL.

apoE

A protein that circulates in plasma and associates with different lipoproteins, facilitating the breakdown and processing of VLDL into LDL.

LDL receptor

A receptor that primarily recognizes and takes up LDL from the bloodstream via the apoB-100 protein.

apoB-100

A protein that binds to LDL receptors for the uptake of LDL particles from the bloodstream.

LDL particle clearance

The process by which LDL is recognized and taken up from the bloodstream, influenced by apoE and apoB-100.

Cholesterol de-esterification

The process by which cholesterol is released from LDL for use by the cell.

HDL cholesterol

High-density lipoprotein cholesterol, associated with a decreased risk of heart disease.

CETP

Cholesterol ester transfer protein that can transfer cholesterol from HDL to LDL, lowering HDL cholesterol levels.

IDL

Intermediate-density lipoprotein, a type of lipoprotein associated with the transition from VLDL to LDL.

Endothelial injury

Caused by ROS and oxidised LDL, attracting white blood cells and macrophages.

Foam cells

Macrophages that ingest oxidised LDL and deposit it in their cytoplasm.

Atherosclerotic Plaque

Restricts blood flow, is prone to rupture, and can trigger blood clots and heart attacks.

Statins

Drugs such as Lovastatin and Pravastatin used to lower blood LDL and reduce heart attack frequency.

Turgor pressure

Pressure that helps with plant cell expansion during growth.

Biogenesis of vacuoles

Similar to lysosomes; involves synthesis in the ER and processing in the Golgi Complex.

Provacuole

Intermediate structure that matures to form the functional vacuole.