Inside the Cell: Structure and Function of Prokaryotic and Eukaryotic Cells

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Chapter 7: Inside the Cell

Overview

This chapter explores the internal structures of cells, emphasizing how life's properties emerge from the collaboration of these structures. The focus is on comparing prokaryotic and eukaryotic cells, understanding their organelles, and relating structure to function.

Cell Theory and Universal Features

Fundamental Components of All Cells

Proteins: Perform most cellular functions, including catalysis, structure, and signaling.
Nucleic acids: Store, transmit, and process genetic information (e.g., DNA and RNA).
Carbohydrates: Provide chemical energy, carbon, support, and cellular identity.
Plasma membrane: Serves as a selectively permeable barrier, controlling entry and exit of substances.

Classification of Cells

Morphological and Phylogenetic Distinctions

Prokaryotes: Lack a membrane-bound nucleus.
Eukaryotes: Possess a membrane-bound nucleus.

Based on phylogeny, life is divided into three domains:

Bacteria (prokaryotic)
Archaea (prokaryotic)
Eukarya (eukaryotic)

Comparison Table: Eukaryotic vs. Prokaryotic Cells

	Eukaryotic Cells	Prokaryotic Cells
Size	Generally much larger	Generally much smaller
Chromosome Location	Enclosed in a nucleus	Located in nucleoid region (no membrane)
Organelles	Many distinct organelles	Few distinct organelles

Prokaryotic Cell Structure

Key Features

Prokaryotic DNA: Usually a single, circular chromosome located in the nucleoid region; may also contain plasmids (small, circular DNA molecules).
Supercoiling: Prokaryotic DNA is highly supercoiled to fit within the cell.
Ribosomes: Macromolecular machines made of RNA and protein; site of protein synthesis.
Photosynthetic Membranes: Some prokaryotes have internal membranes that convert sunlight to chemical energy, derived from plasma membrane folds.
Organelles: Some bacteria possess membrane-bound compartments for specialized functions (e.g., storing calcium, magnetite crystals for navigation, organizing enzymes).
Cytoskeleton: Protein fibers that assist in cell division and maintain cell shape.
Plasma Membrane: Phospholipid bilayer with integral and peripheral proteins; defines the cytoplasm.
Cell Wall: Tough, fibrous layer (peptidoglycan) providing structural support; some species have an additional outer layer of glycolipids.
External Structures: Flagella (for movement) and fimbriae (for attachment).

Eukaryotic Cell Structure

Key Organelles and Functions

Nucleus: Double-membrane structure containing chromosomes; nucleolus synthesizes ribosomal RNA and assembles ribosome subunits.
Ribosomes: Not membrane-bound; free in cytosol (make cytosolic proteins) or attached to endoplasmic reticulum (make exported or membrane proteins).
Endoplasmic Reticulum (ER): Extension of nuclear envelope; two types:
- Rough ER: Studded with ribosomes; synthesizes, folds, and processes proteins.
- Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies molecules, and serves as a Ca2+ reservoir.
Golgi Apparatus: Stacked, flat membranous sacs (cisternae); processes, sorts, and ships proteins from ER. Has cis (nucleus-facing) and trans (plasma membrane-facing) sides.
Lysosomes: Contain hydrolytic enzymes for digestion and recycling; maintain acidic pH (~5) via proton pumps. Not found in plant cells.
Vacuoles: Large, membrane-bound structures in plants and fungi; store water, ions, proteins, pigments, or defensive compounds.
Peroxisomes: Carry out oxidation reactions; contain catalase to detoxify hydrogen peroxide.
Mitochondria: Double-membraned; site of ATP production; contain their own DNA and ribosomes.
Chloroplasts: Found in plants and algae; site of photosynthesis; contain their own DNA and ribosomes; have thylakoid membranes arranged in stacks (grana).
Cytoskeleton: Network of protein fibers providing structural support, movement, and organization.
Cell Wall: Present in fungi, algae, and plants; provides structural support via carbohydrate rods in a matrix of polysaccharides and proteins.

Benefits of Compartmentalization

Advantages of Organelles

Separation of incompatible chemical reactions.
Increased efficiency of chemical reactions by localizing substrates and enzymes.

Structure-Function Relationships

Specialization of Cell Components

Cells with abundant rough ER are specialized for protein synthesis (e.g., enzyme production).
Cells with abundant smooth ER are specialized for lipid synthesis (e.g., steroid hormone production).
Cells with many mitochondria are specialized for energy production (e.g., muscle cells).
Cells with many chloroplasts are specialized for photosynthesis (e.g., leaf cells).

Endomembrane System

Components and Functions

Includes ER, Golgi apparatus, and lysosomes.
Responsible for manufacturing, shipping, and recycling cellular products.
Proteins are transported in vesicles from ER to Golgi, then sorted and shipped to their destinations.
Proteins are tagged for correct delivery; vesicles have molecular markers for targeting.

Lysosomal Recycling Pathways

Three Main Pathways

Receptor-mediated endocytosis: Specific particles bind to membrane receptors, are internalized, and delivered to lysosomes for digestion.
Phagocytosis: Cell engulfs large particles or other cells, forming a phagosome that fuses with a lysosome.
Autophagy: Damaged organelles or cytoplasmic components are enclosed in a membrane, forming an autophagosome that fuses with a lysosome for degradation.

Cytoskeleton

Types and Functions

Actin filaments (microfilaments): Smallest; composed of actin; involved in cell shape, movement, and muscle contraction.
Intermediate filaments: Provide structural support; include keratins and nuclear lamins.
Microtubules: Largest; hollow tubes made of tubulin; serve as tracks for vesicle transport, separate chromosomes during cell division, and form cilia and flagella.

Motor Proteins

Kinesin: Moves vesicles along microtubules via ATP hydrolysis.
Myosin: Interacts with actin for muscle contraction and cell movement.

Cell Motility Structures

Flagella: Long, whip-like structures for movement; differ structurally between prokaryotes and eukaryotes.
Cilia: Short, hair-like projections for movement or fluid transport; found in some eukaryotic cells.

Endosymbiosis Theory

Origin of Mitochondria and Chloroplasts

Suggests these organelles originated as free-living bacteria engulfed by ancestral eukaryotic cells.
Evidence: Both contain their own DNA, ribosomes, and replicate independently of the cell cycle.

Summary Table: Major Eukaryotic Organelles and Functions

Organelle	Main Function
Nucleus	Information storage, ribosome assembly
Ribosomes	Protein synthesis
Rough ER	Protein folding and processing
Smooth ER	Lipid synthesis, detoxification
Golgi Apparatus	Protein modification, sorting, shipping
Lysosomes	Digestion and recycling
Vacuoles	Storage, structure, protection
Peroxisomes	Oxidation reactions, detoxification
Mitochondria	ATP production
Chloroplasts	Photosynthesis
Cytoskeleton	Structure, movement, organization
Cell Wall	Structural support

Additional info: The notes have been expanded to include definitions, examples, and context for each organelle and process, as well as tables for comparison and summary.