Chapter 7: Inside the Cell – Cell Structure and Function

Chapter 7: Inside the Cell

Introduction: The Cell as the Fundamental Unit of Life

Cells are the basic structural and functional units of all living organisms. The properties of life emerge from the collaboration of internal structures within cells. Understanding cell structure is essential for grasping how life functions at the molecular level.

Cell Theory and Universal Cell Characteristics

Cell Theory

The Cell Theory, developed in the 1850s, states that all living things are composed of cells. This foundational concept underpins modern biology and chemistry.

• Cell Theory: All organisms are made of cells; cells are the smallest units of life; all cells arise from pre-existing cells.

Four Universal Characteristics of Cells

• Nucleic acids: Store and transmit genetic information (e.g., DNA and RNA).

• Proteins: Perform most cellular functions, including catalyzing reactions and providing structural support.

• Carbohydrates: Provide energy, structural support, and cellular identity.

• Plasma membrane: Selectively permeable barrier that separates the cell from its environment.

Bacterial and Archaeal Cell Structures and Their Functions

Types of Cells Based on Morphology

Cells are classified into two fundamental types based on their structure:

• Prokaryotic cells: Typically 1–10 μm in size; lack a membrane-bound nucleus.

• Eukaryotic cells: Typically 5–100 μm in size; possess a membrane-bound nucleus.

Three Domains of Life

Organisms are divided into three domains based on phylogeny (evolutionary history):

• Bacteria: Prokaryotic

• Archaea: Prokaryotic

• Eukarya: Includes protists, plants, animals, and fungi (eukaryotic)

Prokaryotic Cell Structure

General Features

• Domains: Bacteria and Archaea

• Prokaryotes are unicellular

Parts List for Prokaryotic Cells

• Chromosome: Most prokaryotes have a single, circular DNA molecule.

• Nucleoid: Region within the cell where the "supercoiled" DNA is located.

• Plasmid: Small, circular DNA molecules independent of the main chromosome; often carry genes for antibiotic resistance.

• Ribosomes (70S): Complexes of RNA and protein; site of protein synthesis; not surrounded by a membrane.

• Internal membrane systems: Found in photosynthetic prokaryotes; contain pigments and enzymes for photosynthesis.

• Membrane-bound organelles: Some bacteria have specialized compartments for storage or orientation (e.g., magnetite-containing organelles).

• Cytoskeleton: Protein fibers essential for cell division and maintaining cell shape.

• Plasma membrane: Selectively permeable boundary.

• Cell wall: Provides structural support; made of peptidoglycan in bacteria.

• Cytoplasm: Gel-like substance inside the cell.

• External structures: Flagella (movement) and fimbriae (attachment).

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Bacteria and Archaea (Prokaryotes)	Eukaryotes
Location of DNA	Nucleoid (no membrane)	Nucleus (membrane-bound)
Internal Membranes & Organelles	Few or none	Extensive; many organelles
Cytoskeleton	Present but simple	Complex and dynamic
Overall Size	1–10 μm	5–100 μm

Eukaryotic Cell Structures and Their Functions

General Characteristics

• Much larger than prokaryotic cells

• Extensive internal membranes

• Numerous membrane-bound organelles

• Dynamic cytoskeleton

Compartmentalization allows eukaryotic cells to separate incompatible chemical reactions and increase efficiency.

Parts List for Eukaryotic Cells

• Nucleus: Information center; enclosed by a double membrane (nuclear envelope) with nuclear pores; contains linear DNA (chromosomes) and nucleolus (ribosome production).

• Ribosomes (80S): Larger than prokaryotic ribosomes; free in cytoplasm or bound to membranes; site of protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; involved in protein synthesis and transport.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and calcium storage.

• Golgi Apparatus: Series of flattened sacs (cisternae); modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes; involved in breakdown and recycling of cellular materials.

• Vacuoles: Storage organelles; large central vacuole in plants for water and solute storage.

• Peroxisomes: Detoxification centers; contain enzymes for redox reactions (e.g., breakdown of hydrogen peroxide: $2H_2O_2 \rightarrow 2H_2O + O_2$).

• Mitochondria: Site of cellular respiration; produces ATP; contains circular DNA and 70S ribosomes; double membrane structure.

• Chloroplasts: Site of photosynthesis in plants and algae; contains circular DNA and 70S ribosomes; double membrane and internal thylakoid membranes.

• Cytoskeleton: Extensive network of protein fibers; maintains cell shape, enables movement, and organizes organelles.

• Cell Wall: Found in fungi, algae, and plants; rigid structure outside the plasma membrane composed of polysaccharides.

Endosymbiosis Theory

The Endosymbiosis Theory proposes that mitochondria and chloroplasts originated from prokaryotic cells engulfed by ancestral eukaryotes. Evidence includes their circular DNA, 70S ribosomes, and ability to divide independently by binary fission.

Cell Systems: Nuclear Transport, Endomembrane System, and Cytoskeleton

Nuclear Transport

• The nucleus is highly organized and contains genetic material (DNA).

• Molecules enter and exit the nucleus through nuclear pores.

• Proteins destined for the nucleus have a Nuclear Localization Signal (NLS), a specific amino acid sequence that directs their transport.

Endomembrane System

• Manufactures, ships, and recycles cellular "cargo" (proteins and lipids).

• Proteins are synthesized in the rough ER, processed in the Golgi apparatus, and transported to their destinations.

• Signal sequences ensure proteins reach the correct location.

• Glycosylation: Addition of oligosaccharides to proteins, forming glycoproteins; occurs in the ER and Golgi.

• Vesicle transport: Proteins are sorted and packaged into vesicles for delivery.

• Lysosomal recycling: Endocytosis brings materials into the cell; lysosomes digest and recycle cellular components (autophagy).

Dynamic Cytoskeleton

• Coordinates the physical relationship of organelles.

• Composed of three types of protein fibers:

  • Actin filaments: Smallest diameter; maintain cell shape, enable movement (cytokinesis, cell crawling).

  • Intermediate filaments: Provide structural support and anchor organelles.

  • Microtubules: Largest diameter; maintain cell shape, facilitate movement (flagella, cilia), chromosome separation, and vesicle transport.

• Motor proteins (e.g., kinesin) move vesicles along microtubule tracks.

• Flagella and cilia: Structures for cell movement; eukaryotic flagella/cilia have a 9+2 arrangement of microtubules.

Form and Function in Cells

Cell size, shape, and composition are correlated with their function. For example, muscle cells contain many mitochondria for energy, while adipose cells have large vacuoles for fat storage. Cells are dynamic, constantly undergoing metabolic and structural changes.

Example: Muscle vs. Adipose Cells

• Muscle cells: Rich in mitochondria for contraction and energy production.

• Adipose cells: Large vacuoles for storing lipids.

Additional info: These notes expand on the original content by providing definitions, examples, and context for key cell structures and processes, suitable for introductory chemistry and biology students.