Lec 7 - Plasma Membrane Structure and Function

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Questions and Answers

What initiates the process of phagocytosis in macrophages?

The fusion of lysosomes with endocytic vesicles

Hydrogen peroxide production

The formation of phagosomes (correct)

The signaling of membrane proteins

What is the consequence of lysosomal enzymes failing to reach the lysosomes?

Improved cell signaling

Lysosomal storage diseases (correct)

Enhanced protein degradation

Increased energy production

Which signals are crucial for the transport of soluble proteins to peroxisomes?

Lysosomal receptor signals

M6P signals

Peroxisomal transport signal (PTS) (correct)

Basic amino acid clusters

What happens to proteins that lack M6P tagging due to mutations in specific genes?

They follow the default secretory pathway Signup and view all the answers

What is the primary function of peroxisomes in cellular processes?

Oxidation of fatty acids Signup and view all the answers

In relation to organelle transport, what is 'protein sorting'?

Transporting proteins to designated organelles via receptor recognition Signup and view all the answers

How do lysosomes degrade the contents of phagosomes?

By fusing with phagosomes to release hydrolytic enzymes Signup and view all the answers

Which characteristic best defines a phospholipid bilayer?

Formed by amphipathic molecules with hydrophobic tails Signup and view all the answers

What is the role of ubiquitination in the process of selective autophagy?

It flags defective macromolecules for degradation. Signup and view all the answers

What is formed when an autophagosome fuses with a lysosome?

A degradation complex for digestion. Signup and view all the answers

How are acid hydrolases targeted to lysosomes?

Through mannose phosphorylation (M6P). Signup and view all the answers

What is the function of mannose-6-phosphate receptors (MPRs)?

To bind M6P proteins and direct them to lysosomes. Signup and view all the answers

What characterizes constitutive secretion?

It is a continuous and unregulated process. Signup and view all the answers

Which feature distinguishes vacuolar targeting sequences from mannose-6-phosphate signals?

Vacuolar targeting sequences are recognized during the protein's synthesis. Signup and view all the answers

What initiates the process of regulated secretion?

The recognition of a specific stimulus. Signup and view all the answers

What role do acid hydrolases play in cellular digestion?

They help in the digestion of macromolecules. Signup and view all the answers

In the context of selective autophagy, what is a primary function of autophagosomes?

To encapsulate and degrade obsolete components. Signup and view all the answers

What is the significance of the signal patch in protein targeting?

It verifies protein quality for mannose phosphorylation. Signup and view all the answers

Which type of vesicle is primarily involved in the recycling of endocytic components back to the cell’s exterior?

Recycling endosome Signup and view all the answers

What process primarily controls the uptake of cholesterol in cells?

Receptor-mediated endocytosis Signup and view all the answers

What structure do caveolae utilize for their formation during pinocytosis?

Caveolin I and II Signup and view all the answers

Which characteristic best describes macropinocytosis?

Actin cytoskeleton rearrangement at the plasma membrane Signup and view all the answers

What happens to LDL receptors when cholesterol is needed in the cell?

They are increased on the plasma membrane Signup and view all the answers

What is the result of the endosome evolving into a lysosome during receptor-mediated endocytosis?

Lipoproteins are digested by hydrolases Signup and view all the answers

Which type of endocytosis is primarily conducted by specialized immune cells?

Phagocytosis Signup and view all the answers

What is the primary role of hydrolases in the endocytic pathway?

To digest lipoproteins in lysosomes Signup and view all the answers

In the context of endocytosis, what is the function of the early endosome?

To determine the fate of transported substances Signup and view all the answers

What is the primary distinction between pinocytosis and phagocytosis?

Only phagocytosis is used by immune cells Signup and view all the answers

Study Notes

Plasma Membrane Structure and Function

Secretion at the plasma membrane transports products made in the secretory pathway to the cell exterior. It also inserts membrane proteins.
Secretion pathways are the default pathway for secretory vesicles; no signal is required for some. Vesicle targeting to other locations needs signals.
Lysosomes are dynamic organelles consisting of hydrolytic enzymes (hydrolases) to break down waste. They prefer acidic conditions (pH 4.5-5).
Lysosomes contain different enzymes for specific components (e.g., proteases break down proteins into amino acids). They have proton pumps to maintain the acidic environment.
Late endosomes are a stage before lysosome maturation. They have proton pumps generating acidic conditions but the pH isn't low enough for lysosome function.
Lysosomes are dynamic, changing size and shape. Vesicles from the Golgi can fuse with lysosomes, enabling the breakdown of components. These vesicles carry lysosomal enzymes.
Four major pathways deliver materials to lysosomes: one internal and three external pathways.

Autophagy

Autophagy is an internal delivery pathway where cells dispose of obsolete components (e.g., macromolecules).
Defective materials are often flagged (e.g., by ubiquitination).
Autophagosomes form from vesicles recognizing flagged signals and combining. This forms a double membrane around obsolete components. This temporary vesicle fuses with a lysosome to initiate digestion.
Selective autophagy vs non-selective: the latter uses cell starvation to increase autophagy engulfing cytosol.
Acid hydrolases are made, N-glycosylated, and folded in the ER. They move to Golgi via COP2 vesicles.
Mannose Phosphorylation: Certain mannose sugars of N-linked glycans are phosphorylated in the Golgi. This signals acid hydrolases to travel to lysosomes.
A signal patch (group of amino acids) determines if a protein can undergo mannose phosphorylation (receiving the M6P signal).
M6P proteins bind to receptors in the trans Golgi network for transport to lysosomes via clathrin vesicles.

Secretion

Constitutive secretion is the continuous creation and secretion of proteins in an unregulated manner. Most plasma membrane and extracellular components use this type of secretion.
Regulated secretion packages proteins into vesicles and releases them when a signal (e.g., variations in calcium ion concentration). This is how most neurotransmitters, hormones, and digestive enzymes are secreted.
Cells concentrate products into vesicles via similar products clustering. Clathrin vesicles pinch off, carrying contents, and undergo retrograde movement to the Golgi.

Endocytosis

Endocytosis forms vesicles at the plasma membrane.
The fate of substances from the extracellular matrix depends on whether they remain in the endosome (if so, it merges with a pre-lysosome, forming a lysosome and degrading its contents) or recycled (e.g., recycling endosome).
Pinocytosis(cell drinking) → Large amounts of substances around the plasma membrane are consumed by a cell. These can or cannot be clathrin-coated.
Caveolae are flask-like pinocytic vesicles which aren't coated by clathrin.
Macropinocytosis is an actin-based endocytic pathway that doesn't need clathrin. It uses actin polymerization to form ruffles/protrusions that engulf material.
Receptor-mediated endocytosis is how cells take up cholesterol (LDL). When needed, LDL receptors increase on the plasma membrane and LDL interacts to form clathrin-coated vesicles that transport the cholesterol to an early endosome.
Endocytosed receptors can be recycled (e.g., returned to plasma membrane) or degraded by lysosomes.

Phagocytosis

Phagocytosis is a type of endocytosis where specialized immune cells (e.g., macrophages) engulf materials in large vesicles (phagosomes). Enclosed material is then broken down in the lysosomes.

Protein Sorting and Peroxisomes

Protein sorting is the process of transporting proteins to the correct organelles. Proteins are sorted post-translation; not during translation.
Peroxisomes are single membrane organelles full of enzymes for crucial metabolic pathways (e.g, fatty acid oxidation) and other processes.
Precursors of peroxisomes come from the ER. Proteins enter or embed in peroxisomes via a signal.
Soluble and membrane proteins use different signals (PTS and mPTS) to find and enter peroxisomes. These processes require ATP.

Microscopy and Immunofluorescence

Fluorescent microscopy uses dyes (fluorophores) to highlight specific regions of a specimen.
Immunofluorescence uses fluorescent-tagged antibodies that bind proteins to signal the location of particular proteins in a specimen.

Antibody Production and Polyclonal Antiserum

Polyclonal antiserum is collected from the blood of an animal in which multiple antibody-producing cells have been stimulated to produce antibodies targeting various epitopes of an antigen.
Monoclonal antibodies are produced by cloning a single antibody-producing cell. This ensures all antibody molecules target the same epitope on antigens

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Description

This quiz explores the intricate details of the plasma membrane's structure and its functional roles in secretion and lysosome dynamics. It covers the pathways for secretory vesicles and the pivotal role of lysosomes in waste breakdown. Test your understanding of vesicle targeting, enzyme activity, and the characteristics of late endosomes.