Cell Biology Study Guide: Tissue Structure, Cell Junctions, and ECM

Q1. Compare the structural organization of epithelial tissue and connective tissue. What does it mean for an epithelial cell to be "polarized," and what are its two distinct functional domains?

Background

Topic: Tissue Structure and Cell Polarity

This question tests your understanding of the differences between epithelial and connective tissues, as well as the concept of cell polarity in epithelial cells.

Key Terms:

• Epithelial tissue: Sheets of cells covering surfaces or lining cavities.

• Connective tissue: Cells scattered in an extracellular matrix, providing support.

• Cell polarity: The presence of distinct structural and functional domains within a cell.

• Apical domain: The surface facing the lumen or external environment.

• Basolateral domain: The surface facing underlying tissue and neighboring cells.

Step-by-Step Guidance

1. Describe the arrangement of cells in epithelial tissue (closely packed, forming continuous sheets) versus connective tissue (cells dispersed within an abundant extracellular matrix).

2. Explain the main functions of each tissue type (epithelial: protection, absorption, secretion; connective: support, binding, transport).

3. Define what it means for an epithelial cell to be "polarized"—having distinct structural and functional regions.

4. Identify the two main domains of a polarized epithelial cell: apical and basolateral.

5. Discuss how these domains contribute to the cell's function (e.g., selective transport, signaling).

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Final Answer:

Epithelial tissue consists of tightly packed cells forming continuous sheets, with little extracellular matrix, while connective tissue has cells scattered within a large amount of extracellular matrix. Epithelial cells are "polarized," meaning they have two distinct domains: the apical domain (facing the lumen or external environment) and the basolateral domain (facing the underlying tissue and neighboring cells). This polarity allows for specialized functions such as absorption and secretion.

Q2. Compare adherens junctions and desmosomes in terms of (a) their shape/zone of attachment, (b) the transmembrane adhesion proteins they use, (c) their intracellular linker proteins, and (d) the specific cytoskeletal filaments they anchor to.

Background

Topic: Cell Junctions

This question tests your knowledge of the structural and functional differences between adherens junctions and desmosomes, two types of cell-cell junctions.

Key Terms:

• Adherens junctions: Cell-cell adhesion structures linked to actin filaments.

• Desmosomes: Cell-cell adhesion structures linked to intermediate filaments.

• Cadherins: Transmembrane proteins mediating adhesion.

• Linker proteins: Connect cadherins to cytoskeletal filaments.

Step-by-Step Guidance

1. Describe the shape/zone of attachment for each junction (adherens: belt-like; desmosomes: spot-like).

2. Identify the main transmembrane adhesion proteins (both use cadherins, but different types).

3. List the intracellular linker proteins for each junction.

4. Specify which cytoskeletal filaments each junction anchors to (actin for adherens, intermediate filaments for desmosomes).

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Final Answer:

Adherens junctions form belt-like zones around cells and use classical cadherins (e.g., E-cadherin) as adhesion proteins, linked to actin filaments via catenins and vinculin. Desmosomes are spot-like junctions using desmogleins and desmocollins (cadherin family) as adhesion proteins, linked to intermediate filaments via plakoglobin and desmoplakin. These differences allow each junction to provide distinct mechanical properties to tissues.

Q3. Explain how β-catenin, α-catenin, vinculin, and p120 catenin work together to regulate adherens junctions and respond to mechanical tension. Why is extracellular calcium (Ca2+) essential for cadherin function?

Background

Topic: Adherens Junction Regulation and Cadherin Function

This question focuses on the molecular regulation of adherens junctions and the role of calcium in cadherin-mediated adhesion.

Key Terms:

• β-catenin: Links cadherins to α-catenin.

• α-catenin: Connects to actin filaments and responds to tension.

• Vinculin: Reinforces actin linkage under tension.

• p120 catenin: Regulates cadherin stability and turnover.

• Ca2+: Required for cadherin adhesive function.

Step-by-Step Guidance

1. Describe how β-catenin binds to the cytoplasmic tail of cadherins and links to α-catenin.

2. Explain how α-catenin connects to actin filaments and senses mechanical tension.

3. Discuss the role of vinculin in strengthening the connection between α-catenin and actin under tension.

4. Describe how p120 catenin regulates cadherin stability at the membrane.

5. Explain why Ca2+ is necessary for cadherin function (structural stability and adhesion).

Try solving on your own before revealing the answer!

Final Answer:

β-catenin binds to cadherins and links them to α-catenin, which connects to actin filaments. Under mechanical tension, α-catenin recruits vinculin to reinforce the actin linkage. p120 catenin stabilizes cadherins at the membrane and regulates their turnover. Extracellular Ca2+ is essential for cadherin function because it maintains the rigid structure of cadherin extracellular domains, enabling strong cell-cell adhesion.

Q4. Describe what happens at the molecular level during the Epithelial-Mesenchymal Transition (EMT) and explain how this process contributes to carcinoma metastasis.

Background

Topic: EMT and Cancer Metastasis

This question tests your understanding of the molecular changes during EMT and their relevance to cancer spread.

Key Terms:

• EMT: Epithelial-Mesenchymal Transition

• Cadherins: Cell-cell adhesion proteins lost during EMT

• Mesenchymal markers: Proteins gained during EMT (e.g., vimentin)

• Metastasis: Spread of cancer cells

Step-by-Step Guidance

1. Describe the loss of epithelial characteristics (e.g., E-cadherin expression, cell polarity).

2. Explain the gain of mesenchymal traits (motility, invasiveness, expression of vimentin).

3. Discuss the molecular signaling pathways involved (e.g., transcription factors like Snail, Twist).

4. Relate these changes to the ability of carcinoma cells to detach and migrate.

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Final Answer:

During EMT, epithelial cells lose cell-cell adhesion (downregulation of E-cadherin), cell polarity, and gain mesenchymal properties (upregulation of vimentin, increased motility). Transcription factors like Snail and Twist drive these changes. This enables carcinoma cells to detach from the primary tumor, invade surrounding tissues, and metastasize to distant sites.

Q5. Outline the three-step sequence by which circulating leukocytes exit the bloodstream at sites of inflammation, naming the key surface adhesion molecules involved at each stage (selectins, carbohydrates, integrins, and ICAM).

Background

Topic: Leukocyte Extravasation

This question tests your knowledge of the molecular mechanisms by which leukocytes exit blood vessels during inflammation.

Key Terms:

• Selectins: Mediate initial rolling adhesion.

• Carbohydrates: Ligands for selectins.

• Integrins: Mediate firm adhesion.

• ICAM: Intercellular Adhesion Molecule, ligand for integrins.

Step-by-Step Guidance

1. Describe the first step: rolling adhesion mediated by selectins binding to carbohydrate ligands on leukocytes.

2. Explain the second step: activation of integrins on leukocytes, leading to firm adhesion to ICAMs on endothelial cells.

3. Outline the third step: transmigration (diapedesis) of leukocytes through the endothelium.

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Final Answer:

The three steps are: (1) rolling adhesion via selectins binding to carbohydrate ligands, (2) firm adhesion via integrins binding to ICAMs, and (3) transmigration (diapedesis) through the endothelium. Each step involves specific adhesion molecules that facilitate leukocyte exit at sites of inflammation.

Q6. What are the two major functions of tight junctions in epithelial sheets? Explain the specific role of claudins in paracellular transport.

Background

Topic: Tight Junctions and Paracellular Transport

This question tests your understanding of tight junction functions and the role of claudins in regulating transport between cells.

Key Terms:

• Tight junctions: Seal spaces between epithelial cells.

• Paracellular transport: Movement of substances between cells.

• Claudins: Key proteins forming tight junction strands.

Step-by-Step Guidance

1. Identify the two main functions of tight junctions: barrier and fence.

2. Describe how tight junctions prevent free passage of molecules between cells (barrier function).

3. Explain how tight junctions maintain cell polarity by restricting movement of membrane proteins (fence function).

4. Discuss the role of claudins in forming selective channels for paracellular transport.

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Final Answer:

Tight junctions serve as a barrier to prevent paracellular movement of solutes and as a fence to maintain cell polarity. Claudins form the backbone of tight junction strands and determine the selectivity of paracellular transport, allowing specific ions or molecules to pass between cells.

Q7. Describe the structure of a gap junction channel, including the roles of connexins and connexons. What types of molecules can pass through these channels, and what is the invertebrate analog of connexin?

Background

Topic: Gap Junctions and Intercellular Communication

This question tests your knowledge of gap junction structure, function, and evolutionary analogs.

Key Terms:

• Connexins: Proteins forming gap junction channels.

• Connexons: Hexameric assemblies of connexins.

• Gap junctions: Channels for direct cell-cell communication.

• Innexins: Invertebrate analogs of connexins.

Step-by-Step Guidance

1. Describe how six connexin proteins assemble to form a connexon (hemichannel).

2. Explain how two connexons from adjacent cells align to form a gap junction channel.

3. List the types of molecules that can pass through gap junctions (ions, small metabolites).

4. Identify the invertebrate analog of connexin (innexin).

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Final Answer:

Gap junction channels are formed by two connexons (each made of six connexin proteins) from adjacent cells. These channels allow passage of ions and small molecules (up to ~1 kDa). Invertebrates use innexins as the analog to connexins for gap junction formation.

Q8. Identify the three main classes of molecules that make up the animal ECM. Contrast the structural properties and functions of collagen, elastin, and proteoglycans/GAGs.

Background

Topic: Extracellular Matrix (ECM) Composition

This question tests your knowledge of ECM components and their structural and functional differences.

Key Terms:

• Collagen: Provides tensile strength.

• Elastin: Provides elasticity.

• Proteoglycans/GAGs: Provide hydration and resistance to compression.

• ECM: Extracellular matrix.

Step-by-Step Guidance

1. List the three main classes of ECM molecules: collagens, elastins, proteoglycans/GAGs.

2. Describe the structural properties of collagen (triple helix, strong fibers).

3. Explain the properties of elastin (stretchable, cross-linked network).

4. Discuss the structure and function of proteoglycans/GAGs (hydration, gel-like matrix).

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Final Answer:

The three main classes are collagen (provides tensile strength), elastin (provides elasticity), and proteoglycans/GAGs (provide hydration and resistance to compression). Collagen forms strong, rigid fibers; elastin forms a flexible, stretchable network; proteoglycans/GAGs create a hydrated, gel-like matrix that resists compression.

Q9. How do fibronectin (via its RGD sequence) and laminin anchor cells to the ECM and basal lamina, respectively? Contrast how integrins connect to the cytoskeleton in focal adhesions versus hemidesmosomes.

Background

Topic: Cell-ECM Adhesion

This question tests your understanding of how cells anchor to the ECM and the differences in integrin-mediated connections.

Key Terms:

• Fibronectin: ECM protein with RGD sequence for integrin binding.

• Laminin: Basal lamina protein anchoring cells.

• Integrins: Transmembrane receptors connecting ECM to cytoskeleton.

• Focal adhesions: Link integrins to actin filaments.

• Hemidesmosomes: Link integrins to intermediate filaments.

Step-by-Step Guidance

1. Describe how fibronectin uses its RGD sequence to bind integrins and anchor cells to the ECM.

2. Explain how laminin anchors cells to the basal lamina via integrin binding.

3. Contrast the cytoskeletal connections: focal adhesions (actin) vs. hemidesmosomes (intermediate filaments).

Try solving on your own before revealing the answer!

Final Answer:

Fibronectin anchors cells to the ECM via its RGD sequence, which binds integrins. Laminin anchors cells to the basal lamina through integrin interactions. In focal adhesions, integrins connect to actin filaments, while in hemidesmosomes, integrins connect to intermediate filaments, providing different mechanical linkages.

Q10. List the three structural layers of a plant cell wall from outermost to innermost and describe their compositions. How do plasmodesmata (including the desmotubule and annulus) allow direct communication across rigid plant cell walls?

Background

Topic: Plant Cell Wall Structure and Intercellular Communication

This question tests your knowledge of plant cell wall layers and the function of plasmodesmata.

Key Terms:

• Primary cell wall: Outermost, flexible layer.

• Middle lamella: Pectin-rich layer between cells.

• Secondary cell wall: Innermost, rigid layer.

• Plasmodesmata: Channels for cell-cell communication.

• Desmotubule: ER-derived structure within plasmodesmata.

• Annulus: Cytoplasmic sleeve around desmotubule.

Step-by-Step Guidance

1. List the three layers from outermost to innermost: middle lamella, primary cell wall, secondary cell wall.

2. Describe the composition of each layer (pectin, cellulose, lignin).

3. Explain the structure of plasmodesmata, including the desmotubule and annulus.

4. Discuss how plasmodesmata allow direct communication between plant cells.

Try solving on your own before revealing the answer!

Final Answer:

The three layers are: middle lamella (pectin-rich), primary cell wall (cellulose and hemicellulose), and secondary cell wall (cellulose and lignin). Plasmodesmata are channels traversing these layers, containing a desmotubule (derived from ER) and an annulus (cytoplasmic sleeve), allowing direct exchange of molecules between plant cells despite the rigid cell wall.