Inside the Cell: Structure and Function of Cellular Components

Chapter 7: Inside the Cell

Introduction

This chapter explores the internal organization of cells, focusing on the structures and functions of prokaryotic and eukaryotic cells. It highlights how cellular components collaborate to support life, and examines the mechanisms by which proteins and other molecules are transported and processed within the cell.

7.1 Bacterial and Archaeal Cell Structures and Their Functions

Overview of Prokaryotic Cells

• Prokaryotes include Bacteria and Archaea, which lack a membrane-bound nucleus.

• All cells contain proteins (cellular functions), nucleic acids (information storage and processing), carbohydrates (energy, support, identity), and a plasma membrane (selective barrier).

• Prokaryotic cells are more complex than previously thought, as revealed by transmission electron microscopy.

Key Structures of Prokaryotic Cells

• Chromosome: Usually a single, circular DNA molecule with associated proteins, located in the nucleoid.

• Plasmids: Small, circular DNA molecules that carry genes beneficial for survival.

• Ribosomes: Macromolecular machines for protein synthesis, composed of RNA and protein subunits.

• Cytoskeleton: Protein filaments that maintain cell shape and are essential for cell division.

• Internal Membrane Complexes: Found in photosynthetic species, these structures increase surface area for energy conversion.

• Organelles: Some bacteria have membrane-bound compartments for specialized functions (e.g., storing ions, orienting the cell, concentrating enzymes).

• Cell Wall: Provides structural support and shape; in bacteria, primarily composed of peptidoglycan.

• External Structures: Flagella (movement) and fimbriae (attachment to surfaces).

7.2 Eukaryotic Cell Structures and Their Functions

General Features of Eukaryotic Cells

• Eukaryotes include protists, fungi, plants, and animals; they possess a membrane-bound nucleus and are generally larger than prokaryotic cells.

• Contain numerous organelles that compartmentalize cellular functions, increasing efficiency and separating incompatible reactions.

Major Eukaryotic Cell Components

• Nucleus: Surrounded by a double-membrane nuclear envelope with pores; contains the nucleolus (site of ribosome assembly).

• Ribosomes: Sites of protein synthesis; can be free in the cytosol or bound to the endoplasmic reticulum (ER).

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Processes, sorts, and ships proteins from the ER; composed of stacked cisternae with distinct cis (receiving) and trans (shipping) faces.

• Lysosomes: Recycling centers containing acid hydrolases for macromolecule digestion; found mainly in animal cells.

• Vacuoles: Prominent in plant and fungal cells; function in storage, digestion, and defense.

• Peroxisomes: Centers for redox reactions; contain catalase to detoxify hydrogen peroxide.

• Mitochondria: Powerhouses of the cell, generating ATP via cellular respiration; contain their own DNA and ribosomes.

• Chloroplasts: Sites of photosynthesis in plants and algae; contain thylakoids, grana, and stroma, as well as their own DNA and ribosomes.

• Cytoskeleton: Network of protein fibers (actin filaments, intermediate filaments, microtubules) for structure, transport, and movement.

• Cell Wall: Provides structural support in fungi, algae, and plants; animals have an extracellular matrix instead.

7.3 Putting the Parts into a Whole

Structure-Function Relationships

• The structure and abundance of organelles vary according to cell type and function (e.g., fat cells store lipids, muscle cells are elongated for contraction).

• Cellular components are dynamic, with rapid turnover and movement of molecules and organelles.

7.4 Cell Systems I: Nuclear Transport

Nuclear Envelope and Transport

• The nuclear envelope separates the nucleus from the cytoplasm and contains nuclear pore complexes for selective transport.

• Large molecules require a nuclear localization signal (NLS) to enter the nucleus, functioning as a molecular "zip code." This ensures only specific proteins are imported.

7.5 Cell Systems II: The Endomembrane System Manufactures, Ships, and Recycles Cargo

Secretory Pathway and Protein Targeting

• Proteins destined for secretion or specific organelles are synthesized in the RER and transported through the Golgi apparatus via vesicles.

• The signal hypothesis explains how proteins are directed to the ER by a signal sequence recognized by the signal recognition particle (SRP).

• Proteins are modified (e.g., glycosylation) and sorted in the Golgi apparatus, then shipped to their final destinations using molecular tags and vesicle transport.

• Exocytosis is the process by which vesicles fuse with the plasma membrane to release contents outside the cell.

Lysosomal Recycling Pathways

• Cells recycle materials via lysosomes through three main pathways:

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptors.

  • Phagocytosis: Engulfment of large particles or cells.

  • Autophagy: Digestion of the cell's own damaged organelles or macromolecules.

7.6 Cell Systems III: The Dynamic Cytoskeleton

Types of Cytoskeletal Elements

• Actin Filaments (Microfilaments): Smallest, composed of actin; involved in cell shape, movement, and division.

• Intermediate Filaments: Provide structural support; include keratins and nuclear lamins.

• Microtubules: Largest, hollow tubes made of tubulin; serve as tracks for vesicle transport and are essential for chromosome separation during cell division.

Microtubule-Based Transport

• Vesicle transport along microtubules is powered by the motor protein kinesin, which uses ATP to "walk" along the microtubule track.

Flagella and Cilia

• Flagella and cilia are structures for cell movement; eukaryotic flagella move by undulation, while prokaryotic flagella rotate.

• Both structures are built on a "9 + 2" arrangement of microtubules (axoneme) and require the motor protein dynein for movement.

Table: Comparison of Prokaryotic and Eukaryotic Cell Structures

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present (membrane-bound)
Organelles	Few, not membrane-bound	Numerous, membrane-bound
Cell Size	Generally smaller	Generally larger
Cell Wall	Usually present (peptidoglycan in bacteria)	Present in plants, fungi, algae (cellulose, chitin); absent in animals
DNA Structure	Single, circular chromosome	Multiple, linear chromosomes
Examples	Bacteria, Archaea	Protists, fungi, plants, animals

Key Terms and Concepts

• Endosymbiosis Theory: Mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotes.

• Glycosylation: Addition of carbohydrate groups to proteins, forming glycoproteins.

• Exocytosis: Process by which vesicles release their contents outside the cell.

• Autophagy: Cellular process for degrading and recycling internal components.