BIO 150 Forensic Biology - Cellular Structure and Eukaryotic Origins
3.1 Cellular Structure
Definition: A cell is the smallest unit of life; most cells are too tiny to be seen with the naked eye, so microscopes are used for study.
Origins of microscopy:
Antony van Leeuwenhoek (Dutch shopkeeper) in the 1600s observed protista and sperm, calling them “animalcules.”
Robert Hooke, in a 1665 publication Micrographia, coined the term “cell” for box-like structures seen in cork tissue.
In the 1670s, van Leeuwenhoek observed bacteria and protozoa.
Advances leading to cell theory:
Later improvements in lenses, microscope construction, and staining revealed internal cell components.
By the late 1830s, Matthias Schleiden (botanist) and Theodor Schwann (zoologist) proposed the unified cell theory: all living things are composed of one or more cells; the cell is the basic unit of life; new cells arise from existing cells.
Rudolf Virchow contributed to these ideas.
Significance: Establishes cells as the fundamental unit of life and basis for biology.
3.2 Prokaryotic and Eukaryotic Cells
Two major cell types:
Prokaryotes: domains Bacteria and Archaea; pro– = before; –karyon– = nucleus.
Eukaryotes: animals, plants, fungi, protists; eu– = true.
Common features of all cells:
Plasma membrane, cytoplasm, DNA, and ribosomes.
Prokaryotic cells: key differences from eukaryotes
Simple, unicellular organisms; lack a nucleus and membrane-bound organelles.
DNA located in the nucleoid (a darkened region).
Cell wall typically made of peptidoglycan; many have a polysaccharide capsule.
External structures: flagella (locomotion), pili (conjugation for gene transfer), fimbriae (attachment).
Eukaryotic cells: key features
Membrane-bound nucleus and other membrane-bound organelles; compartmentalization.
Prokaryotic components (overview):
Plasma membrane, cytoplasm, ribosomes, DNA, and often a cell wall and capsule.
Nucleoid: the central region where prokaryotic DNA is located.
3.3 Eukaryotic Cells
Form follows function: the principle that structure reflects function, evident in organisms and at the cellular level (e.g., streamlined bodies in birds and fish).
Eukaryotic cell definition: a cell with a membrane-bound nucleus and other membrane-bound organelles (organelles = "little organs").
Last common ancestor and fossil evidence:
Earliest fossils: about 3.5\,\text{billion years ago} and are likely cyanobacteria-like prokaryotes.
Most living eukaryotes have cells measuring about 10\,\mu\text{m} \text{ or greater}; fossils resembling eukaryotes appear around 2.1\,\text{billion years ago}.
Characteristics of the last common eukaryotic ancestor (shared by major lineages):
Nuclear envelope with a nuclear pore complex (cells with nuclei; the defining feature of a eukaryote).
Mitochondria (except in some highly reduced forms in anaerobes).
Cytoskeleton (actin microfilaments, intermediate filaments, microtubules).
Flagella and/or cilia (in many but not all lineages).
Linear chromosomes wrapped with histone proteins.
Endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vesicles).
Endosymbiosis and the origin of eukaryotes:
Endosymbiotic theory: a host cell engulfed an alpha-proteobacterium that became mitochondria; later, a cyanobacterium evolved into plastids (chloroplasts) in plastid-bearing lineages.
Evidence for endosymbiosis: mitochondria and chloroplasts have their own circular DNA, ribosomes similar to prokaryotic ones, and double membranes.
Endosymbiosis and metabolism:
Prokaryotic metabolism provided core energy-harvesting capabilities; aerobic respiration in mitochondria evolved from symbiotic relationships with aerobic prokaryotes.
Metabolism and oxygen in early life:
Before oxygen, organisms relied on fermentation; some prokaryotes evolved photosynthesis (e.g., cyanobacteria used water as a hydrogen source and released O2 as a waste product).
By ~2\times 10^9\text{ years ago} oxygen levels rose in the atmosphere; modern oxygen levels (~O_2\text{ ~ present}) arose within the last 7\times 10^8\text{ years}.
Margulis and the development of endosymbiotic theory:
Lynn Margulis (1960s–1990s) proposed and substantiated endosymbiotic theory; initially met with skepticism, it is now widely accepted (though some details remain debated).
Mitochondria:
Major features: energy production via aerobic respiration; two membranes; cristae; matrix; own ribosomes and DNA; can divide by binary fission; many mitochondrial genes transferred to nucleus over time.
Size/number: mitochondria can be from 1 to more than 10\,\mu\text{m} in length; number varies by cell energy demand.
Plastids (including chloroplasts):
Chloroplasts in photosynthetic eukaryotes; chloroplasts also have their own DNA and ribosomes; thylakoids arranged into grana; stroma surrounds the grana.
Plastids appear to be derived from cyanobacteria; primary endosymbiosis involved two membranes, with additional evidence of secondary endosymbiosis in various algal lineages (three or more membranes in some cases).
Primary vs secondary endosymbiosis:
Primary endosymbiosis: cyanobacteria gave rise to chloroplasts in Archaeplastida; two membranes surround chloroplasts.
Secondary (and higher) endosymbiosis: engulfment of a photosynthetic eukaryote (green or red alga) by a non-photosynthetic eukaryote; these plastids can have three or more membranes and may retain remnants of the endosymbiont’s nucleus.
Archaeplastida and Paulinella examples:
Archaeplastida (e.g., glaucophytes) show a thin peptidoglycan layer between outer and inner plastid membranes in some lineages, indicating an early stage of plastid evolution.
Paulinella chromatophora provides a modern example of a more recent primary endosymbiosis event with a cyanobacterial endosymbiont.
Endosymbiosis and genetic integration:
Many endosymbiont genes have been transferred to the host nucleus; mitochondria and plastids still rely on host cell machinery for their expression and replication.
It remains an open question whether the first endosymbiosis included a host nucleus at that time.
Mitosis and sex in eukaryotes (brief):
Mitosis: nuclear division where replicated chromosomes are evenly separated via the cytoskeleton; universally present in eukaryotes.
Sex (meiosis and karyogamy): genetic recombination; meiosis yields haploid nuclei; karyogamy fuses haploid nuclei to form diploid zygote nucleus.
3.4 Cell Diversity
The human body contains many specialized cell types, each with a specific role:
Epithelial cells protect surfaces and line organs and cavities.
Bone cells provide support and protection.
Immune cells defend against pathogens.
Red blood cells transport oxygen via hemoglobin.
Common cellular characteristics across diverse cell types:
All cells share the basic cellular machinery and functions, adapted to their roles.
3.5 Organelles (Overview of the Eukaryotic Organelle Toolkit)
The plasma membrane:
Structure: phospholipid bilayer with embedded proteins; separates cell interior from environment.
Function: regulates selective transport; maintains internal conditions.
Microvilli: membrane folds that increase surface area for absorption (e.g., intestinal lining).
Celiac disease example: immune response damages microvilli, leading to malabsorption.
Cytoskeleton:
Types: microfilaments (actin), intermediate filaments, microtubules.
Functions:
Microfilaments: movement of cellular components; support for microvilli; muscle contraction.
Intermediate filaments: structural support and anchoring organelles.
Microtubules: structural support, organelle movement, chromosome separation during mitosis; form the core of flagella and cilia; arrange as 9+2 in cilia/flagella (nine double microtubules on outside, two in center).
Centrosome and centrioles:
Centrosome: microtubule-organizing center near the nucleus; contains a pair of centrioles (perpendicular arrangement).
Role in cell division: helps separate chromosomes; centrosome duplications precede cell division.
Note: some cells (e.g., plant cells) can divide without centrioles.
Flagella and Cilia:
Flagella: long, usually few per cell; used for locomotion (e.g., sperm, Euglena).
Cilia: numerous, shorter; move cells or move substances along the surface (e.g., paramecium; cilia in respiratory tract; Fallopian tube lining).
The endomembrane system:
A network of membranes and organelles that modify, package, and transport lipids and proteins.
Includes: nuclear envelope, lysosomes, vesicles, endoplasmic reticulum (ER), Golgi apparatus, and plasma membrane.
The Nucleus:
Generally the most prominent organelle; houses DNA as chromatin; directs ribosome and protein synthesis.
Nuclear envelope: double membrane; outer and inner membranes are phospholipid bilayers.
Nuclear pores regulate transport of ions, molecules, and RNA between nucleoplasm and cytoplasm.
Chromosomes: linear DNA molecules; in growth/maintenance, DNA is in chromatin form; in division, chromosomes condense and become visible.
Nucleolus: a dense region where ribosomal RNA (rRNA) is synthesized and ribosomal subunits assemble; subunits then pass through nuclear pores to cytoplasm.
The Endoplasmic Reticulum (ER):
Network of interconnected membranous tubules; lumen/cisternal space.
Rough ER (RER): studded with ribosomes; synthesizes proteins; proteins enter the lumen for folding/modification; synthesizes phospholipids for membranes.
Smooth ER (SER): lacks ribosomes; synthesizes carbohydrates, lipids, phospholipids; detoxifies drugs/poisons; metabolizes alcohol; stores Ca2+.
The Golgi apparatus:
Receives vesicles from the ER; modifies cargo (e.g., adds short sugar chains); sorts and tags lipids/proteins for destinations.
Has a receiving face (near ER) and a releasing face (toward the plasma membrane).
Modified products are packaged into vesicles; some vesicles become secretory vesicles that fuse with the plasma membrane to release contents.
Higher secretory activity correlates with abundant Golgi (e.g., salivary gland enzymes; antibodies).
In plants, Golgi also synthesizes polysaccharides for the cell wall.
Lysosomes:
Digestive organelles with hydrolytic enzymes active at low pH; break down proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.
Key in immune defense: macrophages phagocytose pathogens; vesicles fuse with lysosomes to digest invaders.
Vesicles and Vacuoles:
Vesicles: membrane-bound sacs for storage/transport; can fuse with membranes or target contents to specific destinations.
Vacuoles: larger sacs; in plants often serve storage and turgor-related functions.
Ribosomes:
Sites of protein synthesis; free ribosomes appear in cytoplasm or attached to ER; composed of large and small subunits; abundant in many cell types (e.g., protein synthesis for hemoglobin in immature red blood cells).
Mitochondria:
Powerhouses of the cell; ATP production via cellular respiration.
Structure: oval-shaped, double-membrane; inner membrane contains cristae; matrix inside.
Contain their own ribosomes and DNA; mitochondria reproduce by division; can move within the cell along cytoskeleton.
Mitochondria are the site of aerobic respiration; many nuclear-encoded genes also contribute to mitochondrial function.
Peroxisomes:
Oxidize fatty acids and amino acids; detoxify poisons (e.g., liver detoxification of alcohol).
Contain enzymes that produce hydrogen peroxide (H2O2) as a byproduct, which is degraded by catalase into water and oxygen.
Chloroplasts and plastids:
Chloroplasts perform photosynthesis; contain chlorophyll and pigments; have their own DNA and ribosomes; outer and inner membranes.
Thylakoids are organized into stacks called grana; the fluid surrounding grana is the stroma.
Plastids are derived from cyanobacteria (endosymbiotic origin).
Primary plastids: two membranes; secondary plastids can have three or more membranes and may retain remnants of endosymbiotic nuclei.
Plant cell specifics:
Plant cells have a cell wall (main component cellulose) external to the plasma membrane;
Chloroplasts and plastids; plasmodesmata connect plant cells through cell walls.
Central vacuole in plants regulates water balance and turgor pressure; contributes to cell growth and storage of proteins in seeds.
Animal cell vs. plant cell differences:
Animals: have centrioles/centrosomes; lysosomes present.
Plants: have cell walls, chloroplasts, plasmodesmata, large central vacuole; generally lack centrioles.
Cell-to-cell communication and extracellular matrix (ECM):
ECM in animal tissues consists of glycoproteins (e.g., collagen) that help hold cells together and enable communication.
Blood clotting example: tissue factor binding in ECM initiates platelet adhesion and clot formation.
Intercellular junctions:
Plant cells: plasmodesmata
Animal cells: tight junctions, gap junctions, desmosomes for cell–cell communication and adhesion.
Plant Cell Structure (Overview and Key Components)
Plant cells add to the organelle toolkit with cell walls, plastids (including chloroplasts), plasmodesmata, and a large central vacuole.
Chloroplasts and mitochondria share features with their bacterial ancestors (DNA, ribosomes, similar size).
Thylakoid membranes within chloroplasts arrange light-harvesting complexes; chloroplast DNA resembles cyanobacterial DNA.
Central vacuole and turgor pressure contribute to plant cell rigidity and growth; vacuolar contents deter herbivory via bitter compounds.
Endnotes on Organelles and Cellular Architecture
The cytoplasm (cytosol) vs cytoplasm in eukaryotes:
In eukaryotes, cytoplasm is everything between the plasma membrane and the nuclear envelope, containing cytosol plus organelles suspended in it.
In prokaryotes, cytoplasm is everything inside the plasma membrane (no nucleus).
The cytoskeleton supports cell shape, organizes cellular components, facilitates movement, and provides structural integrity during division.
The endomembrane system includes: nuclear envelope, ER, Golgi, lysosomes, vesicles, and plasma membrane; these components work together to modify, package, and transport lipids and proteins.
The nuclear envelope and nuclear pores coordinate traffic between the nucleus and cytoplasm; the nucleolus assembles ribosomal subunits from rRNA and proteins.
The electron microscope reveals ribosomes as small dots; ribosomes exist as free-floating or attached to RER/ER membranes.
Key Terms Recap (with quick definitions)
Prokaryote: cell lacking a nucleus and membrane-bound organelles; DNA in nucleoid; includes Bacteria and Archaea.
Eukaryote: cell with a true nucleus and membrane-bound organelles; includes plants, animals, fungi, protists.
Nucleoid: region in prokaryotes where DNA is located.
Nucleus: membrane-bound organelle housing DNA and controlling gene expression.
Chromatin: DNA-protein complex; appears as thread-like material when not dividing.
Histones: proteins around which DNA wraps in eukaryotes to form chromosomes.
Endosymbiosis: one organism lives inside another; mitochondria and plastids originated via endosymbiosis.
Mitochondrion: organelle producing ATP via aerobic respiration; contains its own DNA and ribosomes.
Chloroplast: plastid that conducts photosynthesis; contains chlorophyll; has its own DNA and ribosomes.
Thylakoids/Grana: membrane-bound compartments in chloroplasts where light reactions occur; stacked in grana; stroma surrounds them.
Central vacuole: large plant cell organelle for storage and turgor.
Plasmodesmata: channels through plant cell walls enabling intercellular transport.
Extracellular matrix (ECM): network of glycoproteins and collagen that supports tissues and mediates signaling.
Intercellular junctions: plant plasmodesmata; animal tight junctions, gap junctions, desmosomes.
Cytoskeleton components: microfilaments (actin), intermediate filaments (keratin), microtubules (tubulin).
9+2 microtubule arrangement: the circumferential arrangement in cilia/flagella.
Equations and Quantitative Details (LaTeX)
Size comparisons:
Eukaryotic cells are typically 10\,\text{to}\,100\ \text{times larger} than prokaryotic cells.
Fossil/time scales:
First fossils: about 3.5\ \text{billion years ago}.
Appearance of most living eukaryotes: around 2.1\ \text{billion years ago}.
Rise of atmospheric oxygen to present levels occurred by about 0.7\ \text{billion years ago}.
Mitochondrial and plastid endosymbiosis:
Primary endosymbiosis: led to development of mitochondria (alpha-proteobacteria) and chloroplasts (cyanobacteria) with typically two membranes.
Some plastids show three or more membranes due to secondary endosymbiosis.
Chromosome counts (examples):
Humans: 46 chromosomes per cell.
Drosophila (fruit fly): 8 chromosomes.
Mitochondrial structure and scale:
Mitochondria lengths: 1 \leq \ell \leq 10\ \mu\text{m} (or larger) depending on energy demand.
Connections to broader concepts and real-world relevance
Form follows function in biology: structure of cells and organelles is shaped by their roles (e.g., microvilli for absorption; mitochondria for energy production; Golgi for secretion).
Endosymbiotic theory explains the origin of complex life and the integration of energy-harvesting organelles into host cells; this has implications for understanding disease, evolution, and bioenergetics.
The extracellular matrix and intercellular junctions underpin tissue integrity and communication, which is essential in developmental biology and pathology (e.g., cancer metastasis involves ECM remodeling).
Plant-specific features (cell wall, chloroplasts, central vacuole, plasmodesmata) reflect plant ecology and physiology (photosynthesis, water regulation, nutrient transport).
Ethical/philosophical angle: understanding the deep history of life at the cellular level informs debates on questions like the origins of life, the nature of life’s unity, and how best to interpret comparative biology data in medicine and conservation.
Quick study tips derived from the material
Link structure to function: examine why a feature exists (e.g., microvilli increase surface area for absorption).
Use endosymbiotic origin to explain why mitochondria and chloroplasts have their own DNA and ribosomes.
Distinguish prokaryotes from eukaryotes by nucleus presence and organelle complexity; always note the nucleoid vs nucleus distinction.
Remember key numerical anchors: oldest eukaryote fossils around 2.1\text{ bya}; mitochondria and chloroplasts are products of endosymbiosis; eukaryotic cells are generally 10\text{ to }100 times larger than prokaryotic cells.