Cell Structure, Function, and Interactions: Organelle Proportions, Endomembrane System, Cytoskeleton, and Cell Connections

Cell Structure and Function

Organelle Proportions

Different cell types in multicellular organisms have varying proportions of organelles, which are closely linked to their shape, size, and specialized functions.

• Organelle Proportions: The relative abundance of organelles (such as mitochondria, endoplasmic reticulum, etc.) differs among cell types depending on their roles.

• Functional Correlation: Changes in organelle proportions often reflect changes in cell activity or specialization.

• Examples:

  • Smooth Endoplasmic Reticulum (ER) is abundant in liver cells of alcoholics due to increased detoxification needs.

  • Muscle cells undergoing training show increased mitochondria for enhanced energy production.

Nucleus: Structure and Transport

The nucleus is a double-membraned organelle that houses genetic material and regulates molecular traffic through nuclear pores.

• Nuclear Envelope: Composed of two lipid bilayers, it separates nuclear contents from the cytoplasm.

• Nuclear Pore Complexes: Large protein assemblies (about 50 proteins) that control the movement of molecules in and out of the nucleus.

• Transport Mechanisms:

  • Small molecules (e.g., nucleotides) diffuse passively down their concentration gradient.

  • Larger molecules (e.g., proteins, RNA) require active transport, often mediated by nuclear localization signals (NLS), which act as "zip codes" for import.

  • Proteins with an NLS are recognized and imported into the nucleus.

• Import/Export:

  • Import: Activated nucleotides, enzymes for DNA/RNA synthesis, ribosomal proteins.

  • Export: mRNA, rRNA, and other RNAs (but not DNA).

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that manages protein synthesis, sorting, and secretion.

• Membrane Proteins: Fold based on their association with membranes.

• Ribosomes:

  • All ribosomes are structurally and functionally identical, whether free in the cytoplasm or bound to the ER.

  • Their location depends on the type of protein being synthesized.

• Protein Secretion:

  • ER supplies new membrane for the cell in addition to secreting proteins.

• Golgi Apparatus:

  • Sorts and further localizes proteins using specific signals (e.g., mannose-6-phosphate for lysosomal targeting).

• Exocytosis:

  • Vesicles merge with the plasma membrane, releasing contents outside the cell.

  • Transports membrane proteins and increases cell size.

• Endocytosis:

  • Plasma membrane pinches off to form vesicles, bringing external substances into the cell.

  • Decreases cell size.

  • Often involves receptor-mediated targeting; some viruses exploit this for cell entry (e.g., influenza virus).

Cytoskeleton

The cytoskeleton is a dynamic, complex network of protein fibers that provides structural support, maintains cell shape, and organizes internal contents.

• Functions:

  • Supports cell shape and structure.

  • Organizes organelles and internal transport.

  • Facilitates cell movement and changes in shape.

• Components:

  • Microtubules: Serve as tracks for vesicle transport; motor proteins use ATP to "walk" along these filaments.

  • Actin filaments and intermediate filaments: Provide additional support and flexibility.

• Dynamic Nature: Unlike bones, the cytoskeleton is constantly remodeled to adapt to cellular needs.

Cell Surface and Extracellular Matrix (ECM)

Cells have structural proteins inside and extracellular components outside the plasma membrane, which contribute to tissue integrity and communication.

• Extracellular Components:

  • Secreted by cells; more prominent in plants (cell wall) but also present in animals (ECM).

  • Composition and amount depend on tissue and cell type.

• Fiber Composites:

  • Most eukaryotic cells have an extracellular fiber composite.

  • Plant cell walls: Cellulose fibers resist tension; Pectin in ground substance resists compression.

  • Animal ECM: More protein, less carbohydrate; fibers are more bendable (e.g., elastin).

• Permeability:

  • Cell wall/ECM does not replace the plasma membrane and is not selectively permeable.

  • Spaces between fibers slow molecular movement.

Adjacent Cell Interactions

Multicellular organisms require coordination among cells, which is achieved through specialized cell junctions and communication pathways.

• Communication:

  • Cell membranes generally prevent free molecular traffic, but gaps such as plasmodesmata in plants allow signals to pass.

• Animal Cell Connections:

  • Held together by ECM, cell-cell adhesions, or both.

  • Wider variety of cell connections than plants.

• Types of Junctions:

  • Tight Junctions: Form watertight seals, preventing solution flow between cells (important in epithelial tissues).

  • Desmosomes: Provide strong adhesion using linking proteins called cadherins; more secure than tight junctions.

• Examples:

  • Blisters can form from desmosome loss.

  • Intercalated discs in heart muscle are specialized desmosomes for cell coordination.

Summary Table: Plant vs Animal Cell Surface Components

Feature	Plant Cells	Animal Cells
Main Extracellular Component	Cell Wall (Cellulose, Pectin)	Extracellular Matrix (Protein, Elastin)
Permeability	Not selectively permeable	Not selectively permeable
Protein Content	Low	High
Carbohydrate Content	High	Low
Flexibility	Rigid	Bendable

Key Terms

• Organelle: Specialized subunit within a cell with a specific function.

• Nuclear Localization Signal (NLS): Amino acid sequence that directs a protein to the nucleus.

• Endomembrane System: Group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Exocytosis: Process by which cells release substances to the extracellular environment via vesicle fusion with the plasma membrane.

• Endocytosis: Process by which cells internalize substances from the extracellular environment by engulfing them in vesicles.

• Cytoskeleton: Network of protein filaments that provides structural support and facilitates movement within the cell.

• Extracellular Matrix (ECM): Complex network of proteins and carbohydrates outside animal cells that provides structural and biochemical support.

• Desmosome: Cell junction that provides strong adhesion between cells, mediated by cadherin proteins.

• Tight Junction: Cell junction that forms a seal to prevent passage of materials between cells.

• Plasmodesmata: Channels between plant cells that allow for communication and transport of materials.

Additional info:

• Motor proteins such as kinesin and dynein use ATP to transport vesicles along microtubules.

• Cell signaling and adhesion are critical for tissue development and maintenance in multicellular organisms.