Chapter 4: Cells and Organelles – Structure, Function, and Invading Agents

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Organic building blocks such as amino acids, simple sugars, and nucleotide bases likely formed abiotically on early Earth.
These subunits polymerized into the precursors of modern proteins and nucleic acids, though the exact process is not fully understood.
RNA is believed to have preceded DNA as the first informational molecule due to its ability to catalyze its own replication (ribozymes).
Self-replicating RNA may have been trapped in liposomes (simple fatty acid vesicles), forming the first cell-like structures.

Example: Artificial liposomes can be created in the lab to model primitive cell membranes.

Cells are classified as eukaryotic (with a nucleus: animals, plants, fungi, protists, algae) or prokaryotic (without a nucleus: bacteria and archaea).
Modern molecular data (e.g., ribosomal RNA sequences) support three domains of life: eukaryotes, bacteria, and archaea.

All cells have a plasma membrane that defines their boundary.
Organelles are found only in eukaryotic cells, enabling compartmentalization of functions.
Bacterial and archaeal cells are generally smaller and structurally simpler than eukaryotic cells, lacking internal membrane-bound organelles.

Cell size is limited by the need for sufficient surface area to exchange materials and maintain adequate concentrations of necessary compounds.
Eukaryotic cells are larger than prokaryotic cells and compensate for lower surface area/volume ratios by compartmentalizing materials within organelles.

Cell Size (μm)	Surface Area (μm2)	Volume (μm3)	Surface Area/Volume Ratio
1	6	1	6.0
2	24	8	3.0
4	96	64	1.5

Additional info: As cell size increases, the surface area-to-volume ratio decreases, making transport and communication more challenging.

Eukaryotic cells have a nucleus (containing DNA), a variety of organelles, and a cytoskeleton.
Plant cells have a rigid cell wall; animal cells are surrounded by a flexible extracellular matrix of collagen and proteoglycans.

Contains chromosomes (DNA complexed with protein as chromatin).
Enclosed by a nuclear envelope (double membrane) with pores for regulated exchange of macromolecules.

Mitochondria (double-membraned) degrade food molecules to make ATP; contain their own ribosomes and circular DNA.
Chloroplasts (in plants/algae) trap solar energy to convert CO2 and water into sugar and oxygen; also have ribosomes and circular DNA.

Suggests mitochondria originated from aerobic bacteria and chloroplasts from cyanobacteria, based on similarities in DNA, ribosomes, and membranes.

Endoplasmic reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids.
Golgi apparatus: Modifies, sorts, and packages proteins/lipids for secretion or delivery to other organelles.
Lysosomes: Contain hydrolytic enzymes for cellular digestion.
These organelles are collectively called the endomembrane system.

Peroxisomes: Generate and degrade hydrogen peroxide; involved in fatty acid metabolism and detoxification.
Ribosomes: Sites of protein synthesis; found in all cells.

Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments; provides structural support, motility, and intracellular transport.
Plant, fungal, bacterial, and archaeal cells have a rigid cell wall; animal cells have an extracellular matrix for support.

Do not satisfy the criteria for living things; lack cellular structure and independent metabolism.
Act as infectious agents causing diseases in plants and animals; used as laboratory tools in genetics.

Viroids: Small, circular RNA molecules that infect plants.
Prions: Misfolded proteins that can induce other proteins to misfold, causing neurodegenerative diseases.

Example: Prions are responsible for diseases such as Creutzfeldt-Jakob disease in humans and mad cow disease in cattle.