Cellular Biology I: Exocytosis and Endocytosis
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Questions and Answers

What role does dynamin play in receptor-mediated endocytosis?

  • It destroys receptors after internalization.
  • It binds to macromolecules for uptake.
  • It serves as a docking agent for clathrin.
  • It induces the invagination of the membrane. (correct)

Which protein complex connects the membrane to clathrin in receptor-mediated endocytosis?

  • LDL receptors
  • Dynamin
  • Transmembrane receptor proteins
  • AP2 adaptor proteins (correct)

What percentage of the total plasma membrane area do clathrin-coated pits typically occupy?

  • 5%
  • 10%
  • 2% (correct)
  • 20%

What happens to receptors during receptor-mediated endocytosis when they are internalized?

<p>They may be destroyed leading to down-regulation. (C)</p> Signup and view all the answers

What is the main function of receptor-mediated endocytosis in cells?

<p>It facilitates the uptake of specific extracellular ligands. (D)</p> Signup and view all the answers

What initiates the expression of transmembrane receptor proteins for LDL uptake?

<p>The need for cholesterol in membrane synthesis. (D)</p> Signup and view all the answers

How does receptor-mediated endocytosis increase the efficiency of ligand uptake?

<p>Through a selective concentrating mechanism. (A)</p> Signup and view all the answers

Which of the following statements about LDL receptors is accurate?

<p>They bind to specific cholesterol-containing particles for uptake. (B)</p> Signup and view all the answers

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

<p>It forms the vesicle membrane for internalization. (B)</p> Signup and view all the answers

Which process involves the destruction of receptors upon internalization to reduce cellular sensitivity?

<p>Receptor down-regulation (D)</p> Signup and view all the answers

What is the primary function of exocytosis?

<p>Releasing vesicle contents outside the cell (B)</p> Signup and view all the answers

Which type of endocytosis involves the uptake of large particles?

<p>Phagocytosis (B)</p> Signup and view all the answers

What role do clathrin-coated pits play in receptor-mediated endocytosis?

<p>They accumulate receptors for selective uptake of ligands (D)</p> Signup and view all the answers

Which of the following statements about pinocytosis is true?

<p>It involves nonspecific uptake of fluids and small macromolecules (A)</p> Signup and view all the answers

What types of materials does endocytosis primarily allow cells to uptake?

<p>Large macromolecules and fluids (A)</p> Signup and view all the answers

Which of the following is NOT a type of endocytosis?

<p>Transport-mediated endocytosis (D)</p> Signup and view all the answers

Which cells are primarily responsible for phagocytosis?

<p>Macrophages and neutrophils (C)</p> Signup and view all the answers

What distinguishes receptor-mediated endocytosis from other forms of endocytosis?

<p>It binds specific ligands to receptors on the plasma membrane (D)</p> Signup and view all the answers

Which of the following materials are typically taken up during receptor-mediated endocytosis?

<p>Nutrients and hormones (A)</p> Signup and view all the answers

What is a key characteristic of endocytosis compared to exocytosis?

<p>Endocytosis involves bringing materials into the cell (C)</p> Signup and view all the answers

What happens when the intracellular cholesterol concentration increases?

<p>LDL receptor production is inhibited (B)</p> Signup and view all the answers

What is the role of transferrin in the body?

<p>To bind and transport iron in the plasma (D)</p> Signup and view all the answers

How does the affinity of transferrin binding change with pH levels?

<p>Affinity decreases as pH decreases (D)</p> Signup and view all the answers

What is the purpose of H+ ATPases in early endosomes?

<p>To lower the pH of the vesicle for iron release (B)</p> Signup and view all the answers

What can result from blocked cholesterol uptake in the body?

<p>Accumulation of cholesterol in the blood (B)</p> Signup and view all the answers

What do extracellular vesicles (EVs) primarily carry for intercellular communication?

<p>Membrane proteins, lipids, RNAs, and signaling molecules (B)</p> Signup and view all the answers

What happens to the transferrin receptor after iron is released in the early endosome?

<p>It is recycled back to the plasma membrane (C)</p> Signup and view all the answers

Which statement about LDL receptors is true?

<p>They are inhibited when intracellular cholesterol levels are high (A)</p> Signup and view all the answers

Which of the following describes the function of transferrin receptors?

<p>They transport transferrin with iron to early endosomes (A)</p> Signup and view all the answers

What is a consequence of a reduced pH in the transferrin-iron complex?

<p>Transferrin releases iron ions (A)</p> Signup and view all the answers

Flashcards

Exocytosis

The final stage of vesicle trafficking where the vesicle merges with the plasma membrane, releasing its contents outside the cell.

Endocytosis

A cellular process that allows cells to take in materials from the external environment, including large macromolecules and fluids.

Phagocytosis

A type of endocytosis where cells engulf large particles, like pathogens, damaged cells, or debris.

Pinocytosis

A type of endocytosis where cells take in fluid and smaller molecules, including nutrients, through small vesicles.

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Receptor-mediated endocytosis

A specialized form of endocytosis where specific molecules bind to receptors, triggering their uptake into the cell.

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Clathrin

A protein that forms a cage-like structure on the cytoplasmic side of the plasma membrane during receptor-mediated endocytosis.

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Coated pits

Specific regions on the plasma membrane enriched with clathrin and specific phosphoinositides, involved in receptor-mediated endocytosis.

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Phagocytic cells

Specialized cells like macrophages and neutrophils that engulf large particles through phagocytosis.

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Ligands

Molecules that bind to specific receptors on the plasma membrane, triggering receptor-mediated endocytosis.

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Recycling of plasma membrane

The internalization of portions of the plasma membrane, which are then recycled or degraded.

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Pit invagination

The inward folding of the cell membrane caused by the binding of a ligand to a receptor protein on the cell surface.

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Receptor protein

A protein that sits on the cell surface and is responsible for binding to specific molecules outside the cell, triggering a cellular response.

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Dynamin

A protein that helps to form clathrin-coated pits, which are essential for receptor-mediated endocytosis.

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AP2 adaptor proteins

A type of protein that helps in the formation of clathrin-coated pits.

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Clathrin triskelion

A cage-like structure composed of proteins that assembles around the pit in receptor-mediated endocytosis.

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Clathrin-coated vesicles

Small, spherical sacs that are formed by the pinching off of the cell membrane during receptor-mediated endocytosis.

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Housekeeping function of receptor-mediated endocytosis

The process where cells take in nutrients, remove waste products, and respond to signals from their environment. This process relies on the formation of clathrin-coated pits and vesicles.

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Signaling function of receptor-mediated endocytosis

The process where cells take in specific molecules that trigger a change in their behavior. Upon internalization, the receptor is often destroyed, reducing the cell's sensitivity to further stimulation.

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Cholesterol Uptake

A process where cells take up cholesterol from the bloodstream using LDL receptors.

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Downregulation of LDL Receptors

When cholesterol levels inside a cell are high, the cell reduces the production of LDL receptors, slowing down cholesterol uptake.

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Cholesterol Accumulation and Atherosclerosis

If cholesterol uptake is blocked, it accumulates in the blood, increasing the risk of atherosclerotic plaques forming in blood vessels.

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Transferrin

A protein in the blood that binds and transports iron.

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Transferrin Binding Sites

Transferrin has two specific binding sites for iron.

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Transferrin Receptors

Receptors on the cell surface that bind transferrin and deliver iron into the cell through endocytosis.

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pH-Dependent Binding of Transferrin and Iron

The strength of the bond between transferrin and iron changes with the pH of the environment.

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Iron Release from Transferrin

Transferrin releases iron when it enters the cell's acidic environment.

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Recycling of Transferrin Receptors

After releasing iron, the transferrin receptor is recycled back to the cell surface.

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Extracellular Vesicles (EVs)

Small particles secreted by cells that carry various molecules, including proteins, lipids, and RNA, to other cells.

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Study Notes

Cellular Biology I: Endocytosis

  • Endocytosis allows cells to take up materials (large molecules) from the external surface.
  • Endocytosis recycles resident proteins from the plasma membrane into internal compartments where they can be recycled or degraded

Endocytosis: Types

  • Phagocytosis: Uptake of particles larger than 0.5 μm (e.g., pathogens, damaged cells, debris); specific cells like macrophages and neutrophils are involved.
  • Pinocytosis: Nonspecific uptake of extracellular macromolecules and fluids; internalizing portions of plasma membrane for recycling purposes.
  • Receptor-mediated endocytosis: Selective uptake of macromolecules (ligands) by binding to specific receptors on the plasma membrane (e.g., hormones, growth factors, blood-borne nutrients).

Receptor-mediated Endocytosis: Details

  • Receptors accumulate in domains of the plasma membrane—enriched in clathrin and specific phosphoinositides (coated pits).
  • The specific coating made of clathrin on the cytoplasmic surface leads to pit invagination upon ligand-receptor binding
  • Protein Dynamin is crucial during the invagination stage.
  • Clathrin-coated pits typically occupy about 2% of the plasma membrane area.
  • Receptor-mediated endocytosis provides a mechanism to increase the efficiency of ligand internalization by many hundred folds.

Receptor-mediated Endocytosis: Function

  • Housekeeping Receptors: Uptake of materials needed for cell life and homeostasis; Delivery of ligands to the cell and return of receptors to cell surface.
  • Signaling Receptors: Bind to extracellular ligands, inducing changes in the cell; Receptor is destroyed (down-regulation) upon internalization, reducing sensitivity to further ligand stimulation.

LDL Receptor (Cholesterol Uptake)

  • Cholesterol is transported in the blood in low-density lipoproteins (LDL).
  • When a cell needs cholesterol for membrane synthesis, it expresses LDL receptors and inserts them into the plasma membrane for uptake.
  • When intracellular cholesterol concentration increases, LDL receptor production ceases (down-regulation).

Transferrin and Transferrin Receptor

  • Transferrin is an iron-binding plasma glycoprotein with two high-affinity iron binding sites.
  • Cell surface transferrin receptors deliver bound iron to early endosomes via receptor-mediated endocytosis.
  • The affinity of binding decreases as the pH of the vesicle decreases (from 7.2 to 5.5).
  • Transferrin releases its iron ions, while the receptor is recycled back to the plasma membrane.

Extracellular Vesicles (EVs)

  • EVs (e.g., exosomes, ectosomes) are key players in intercellular communication; they carry membrane proteins, lipids, RNAs, and cytosolic proteins for recipient cells.
  • Recent genomic studies have shown significant differences in EV cargo composition based on donor cell types, metabolic cues, and disease states.

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Description

This quiz covers the essential processes of exocytosis and endocytosis in cellular biology. Learn about how cells secrete materials and uptake large molecules through these vital mechanisms. Test your understanding of phagocytosis and the different types of endocytosis.

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