Cell Structure and Organization — Comprehensive Notes (Transcript-Based)

The Fundamental Units of Life

• Cells are the building blocks of all organisms. In single-celled organisms the cell is everything.

CELL THEORY

• Foundational principle of biology credited to Theodor Schwann, Matthias Schleiden, and Rudolf Virchow.

• Core statements:

  • Cells are the basic units of life.

  • All living organisms are composed of cells.

  • All cells come from preexisting cells.

BASIC CELL COMPOSITION

• Cells contain at least these core components:

  1. Proteins — perform most of the cell’s functions.

  2. Nucleic acids — store, transmit, and process information.

  3. Carbohydrates — provide chemical energy, carbon, support, and identity.

  4. Plasma membrane — serves as a selectively permeable barrier.

  5. Ribosomes — sites of protein synthesis.

  6. Chromosome(s) — nucleic acid–protein structures that transmit hereditary information.

  7. Cytoplasm — internal cellular fluid (cytosol).

TYPES OF CELLS AND DOMAINS

• Cells are divided into two fundamental types by morphology:

  • Eukaryotes — have a membrane-bound nucleus.

  • Prokaryotes — lack a membrane-bound nucleus.

• Based on phylogeny (evolutionary history), organisms are divided into three domains:

  1. Bacteria — prokaryotic.

  2. Archaea — prokaryotic.

  3. Eukarya — eukaryotic.

TREE OF LIFE (OVERVIEW)

• Major domains depicted: Bacteria, Archaea, Eukaryota (Eukarya).

• Note: Prokaryotes include diverse groups such as Spirochetes, Green bacteria, Proteobacteria, Methanosarcina, Halophiles, Methanobacterium, Cyanobacteria, etc.; Eukaryota include animals, plants, fungi, and protists (e.g., Entamoebae, slime molds, ciliates, flagellates, Trichomonads, Microsporidia, Diplomonads).

MICROSCOPE BASICS AND SIZE SCALES

• Life’s tiny scale makes microscopy essential:

  • Prokaryotic cells are typically 0.1–5.0 μm in diameter.

  • Eukaryotic cells typically range from about 5–100 μm in diameter.

• Size implications:

  • Smaller cells have a higher surface area-to-volume ratio, favoring exchange with the environment.

  • Prokaryotes are generally smaller due to lack of internal transport modifications found in eukaryotes.

MICROSCOPE TYPES: LM vs EM

• Light microscopy (LM): uses visible light.

• Electron microscopy (EM): uses beams of electrons; higher resolution.

• Two main EM types:

  • Scanning Electron Microscope (SEM): views surface/topography (3D exterior).

  • Transmission Electron Microscope (TEM): views ultrastructure inside cells.

MAGNIFICATION AND RESOLUTION

• Magnification (M): image size relative to actual size.

• Resolution (R): the ability to distinguish two points as separate.

• Key values:

  • LM resolving power ≈ $2.00 imes 10^{-7} ext{ m} = 200 ext{ nm}$ (about 200 nm).

  • EM resolving power ≈ $2.0 imes 10^{-9} ext{ m} = 2 ext{ nm}$ (about 2 nm).

• Live vs fixed specimens:

  • LM can image live or fixed specimens.

  • EM typically images fixed or dried specimens.

LM vs EM: Practical Differences

• LM:

  • Illuminating source: light.

  • Specimen prep: minutes to hours.

  • Live specimens: possible.

  • Magnification: ~500× to 1500×.

  • Image color: colored with stains or natural color.

  • Resolution: ~0.25–0.3 μm.

• EM:

  • Illuminating source: electrons.

  • Specimen prep: usually days (more involved).

  • Live specimens: not possible; images are black and white and often require heavy metal staining.

  • Magnification: ~100,000× to 300,000×.

  • Resolution: ~2 nm (much higher).

PROKARYOTIC CELL STRUCTURES (A PARTS LIST)

• Prokaryotic cells contain distinctive structures and functions:

  • At least one chromosome.

  • Numerous ribosomes for protein synthesis.

  • Phospholipids with bacterial vs archaeal differences:

  • Bacterial phospholipids: fatty acids bound to glycerol.

  • Archaeal phospholipids: branched isoprenoid chains bound to glycerol.

  • Cytoplasm: all contents inside the membrane.

PROKARYOTIC DNA AND GENOME ORGANIZATION

• The chromosome is organized into a nucleoid:

  • Most bacteria/archaea have a single, circular chromosome.

  • Large DNA molecule associated with proteins; information storage.

  • Proteins provide structural support to the DNA.

• Some prokaryotes carry plasmids: circular, extrachromosomal DNA that can carry accessory genes and be exchanged between cells.

CHROMOSOME AND PLASMIDS DIAGRAMS (KEY IDEAS)

• Chromosome is usually long and circular within the nucleoid region.

• DNA is often supercoiled to fit inside the small cell.

• Plasmids can be present in multiple copies and can be transferred between cells.

RIBOSOMES

• Prokaryotic ribosomes are 70S in size (composed of large and small subunits).

• Ribosomes are RNA-protein complexes that synthesize proteins.

• Prokaryotic ribosomes resemble each other in size/function across bacteria and archaea, though their RNA and protein components differ.

CYTOSKELETON IN PROKARYOTES

• Prokaryotes possess long, thin protein filaments forming a cytoskeleton.

• Roles include maintaining cell shape, organizing cell components, and crucially in bacteria for cell division.

INTERNAL MEMBRANE SYSTEMS AND ORGANELLES (PROKARYOTES)

• Some photosynthetic prokaryotes have internal photosynthetic membranes—infoldings of the plasma membrane that house pigments and enzymes for photosynthesis.

• Some species harbor internal compartments (organelles) for specialized tasks:

  • Store Ca2+ ions.

  • Magnetite crystals for magnetotaxis (orientation using Earth's magnetic field).

  • Concentrate enzymes for specific biosynthetic tasks.

THE PROKARYOTIC CELL WALL AND EXOSKELETON

• The cell wall forms a protective exoskeleton surrounding the plasma membrane.

• In bacteria, the main structural component is peptidoglycan (polysaccharide).

• Some bacteria have an outer membrane composed of glycolipids (outer membrane in Gram-negative bacteria).

EXTRACELLULAR APPENDAGES (MOVEMENT AND ATTACHMENT)

• Flagella: long filaments that rotate to propel the cell.

• Fimbriae: needlelike projections that promote attachment to surfaces or other cells.

SIZE AND WHY PROKARYOTES ARE SMALL

• Typical prokaryote size: 0.1–5.0 μm in diameter.

• Reason for small size: favorable surface area-to-volume ratio for material exchange with the environment; lack of internal transport adaptations found in eukaryotes.

EUKARYOTIC CELLS: OVERVIEW AND ORGANELLES

• Eukaryotic cells are generally larger than prokaryotic cells (5–100 μm) and contain compartmentalized organelles.

• Compartmentalization offers advantages: separation of incompatible reactions, increased efficiency, and larger overall cell size due to higher surface-area-to-volume management.

ANIMAL CELL: GENERAL STRUCTURE

• Common components in the generalized animal cell:

  • Plasma membrane

  • Extracellular matrix (ECM)

  • Nucleus and nuclear envelope with nuclear pores

  • Nucleolus and chromatin

  • Cytoplasm: cytosol plus organelles

  • Mitochondria

  • Peroxisomes

  • Endoplasmic reticulum (ER): rough (RER) and smooth (SER)

  • Golgi apparatus

  • Lysosomes

  • Cytoskeleton (microtubules, intermediate filaments, microfilaments)

  • Centrosome (microtubule-organizing center)

• The nucleus, nucleolus, and nuclear envelope constitute the nucleus; the nuclear lamina provides structural support.

PLANT CELL DIFFERENCES

• Plant cells share most organelles with animal cells but include:

  • Chloroplasts for photosynthesis.

  • A rigid cell wall outside the plasma membrane.

  • A large central vacuole (tonoplast surrounds it) that regulates water balance and contributes to turgor pressure for cell expansion.

• Plant cells also have chloroplasts with thylakoid membranes arranged in grana, stroma, and their own DNA/ribosomes.

PLASMA MEMBRANE

• Phospholipid bilayer with embedded proteins.

• Embedded components include:

  • Glycoproteins and glycolipids (carbohydrates attached to proteins or lipids).

  • Peripheral and integral membrane proteins.

  • Cholesterol (helps modulate membrane fluidity).

  • Cytoskeletal filaments associated with the cytoplasmic face.

CYTOPLASM

• Region between the plasma membrane and the nuclear envelope.

• Contains organelles suspended in cytosol; the cytosol is ~70–80% water but has a semi-solid consistency due to proteins.

NUCLEUS AND NUCLEUS-ASSOCIATED STRUCTURES

• Nucleus: large, membrane-bound compartment enclosed by a double membrane (nuclear envelope) with pores.

• Nuclear lamina: network of fibrous proteins providing structural support to the nucleus.

• Nucleoplasm: aqueous fluid inside the nucleus.

• Chromosomes: long DNA molecules packaged with proteins; exist as chromatin.

• Nucleolus: non-membrane-bound region where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled; prominent in non-dividing cells.

RIBOSOMES

• Complex molecular machines that synthesize proteins.

• Do not have a membrane.

• Composed of a large subunit and a small subunit; assembled from RNA and protein.

• Free ribosomes: synthesize proteins for the cytosol or import into nucleus.

• Bound ribosomes: attached to the rough ER; synthesize proteins destined for secretion, membranes, or organelles.

ENDOPLASMIC RETICULUM (ER)

• Extensive, membrane-enclosed factory continuous with the nuclear envelope.

• Two regions with distinct structure/function:

  • Rough ER (RER): studded with ribosomes; has cisternae (membranous sacs).

  • Synthesizes proteins that are shipped to other organelles, inserted into plasma membrane, or secreted.

  • As proteins are produced, they enter the ER lumen for folding and processing; glycoproteins and membrane transporters/pumps are common products.

  • Smooth ER (SER): lacks ribosomes; comprised of membranous tubules.

  • Enzymes catalyze lipid synthesis (needed by the organism) and lipid/ carbohydrate modifications.

  • Detoxification of some molecules.

  • Ca2+ storage reservoir.

GOLGI APPARATUS

• Receives products from rough ER on the cis side and ships them on the trans side.

• Functions: processes, sorts, and ships proteins (and lipids) synthesized in the ER.

• Structure: cisternae stacked into a polarized organelle with cis (near nucleus) and trans (toward plasma membrane) faces.

• Materials travel through cisternae via cisternal maturation, carrying and modifying cargo as they move.

LYSOSOMES

• Recycling centers found in animal cells; contain about 40 different enzymes.

• Enzymes are hydrolytic and function best at acidic pH (~5.0) due to proton pumps maintaining low internal pH.

• Digestive enzymes (acid hydrolases) degrade macromolecules.

• Part of the endomembrane system in which lysosomes, Golgi, and ER coordinate production, processing, and transport of macromolecules.

VACUOLES

• Organelles that vary in function; prominent in plants, fungi, and other eukaryotes.

• Roles:

  • Digest and recycle macromolecules (some share function with lysosomes).

  • Storage of water, ions, organic compounds; pigments in petals/fruits; noxious compounds to deter herbivory in leaves/stems.

  • In seeds, rich in storage proteins.

• Central vacuole (plant cells): occupies large volume; tonoplast surrounds it; helps regulate turgor and water balance; part of the plant endomembrane system.

PEROXISOMES

• Globular organelles found in all eukaryotic cells.

• Originate when ER-derived vesicles import peroxisome-specific enzymes.

• Functions:

  • Site of oxidation reactions; detoxification processes; reduction-oxidation (redox) chemistry.

  • Glyoxysomes (special plant peroxisomes) oxidize fats to form energy-storage compounds.

  • Hydrogen peroxide byproduct is detoxified by catalase.

MITOCHONDRIA

• Power-generating stations of the cell; site of cellular respiration and ATP production.

• Structure: two membranes (outer and inner), inner membrane folds into cristae; matrix is the interior fluid; intermembrane space between membranes.

• Mitochondria have their own DNA (mtDNA) and ribosomes; can grow and divide independently of the cell.

• Morphology is dynamic: fusion and fission yield networks or discrete organelles.

CHLOROPLASTS (PLANTS AND ALGAE)

• Sites of photosynthesis; contain three membranes (two envelope membranes and a third inner membrane associated with thylakoids).

• Thylakoids are arranged in stacks called grana; thylakoid lumen is the thylakoid space.

• Surrounding the thylakoids is the stroma, a fluid with enzymes for sugar production.

• Chloroplasts are double-membrane bound and contain their own DNA and ribosomes.

• Endosymbiosis theory: mitochondria and chloroplasts may have originated as free-living bacteria engulfed by ancestral eukaryotic cells.

CYTOSKELETON

• An extensive network of protein fibers that gives cells shape and structural stability, transports materials, and organizes organelles.

• Major components include microfilaments (actin), intermediate filaments, and microtubules.

CELL WALL AND EXTRACELLULAR MATRIX (ECM)

• Eukaryotic cells often have a cell wall in plants, fungi, and some algae, outside the plasma membrane, providing rigid structural support.

• Animals lack a cell wall; instead, they rely on an extracellular matrix (ECM) composed of secreted proteins and polysaccharides for support and signaling.

COMPARISONS: PROKARYOTES VS EUKARYOTES IN PRACTICE

• Genomic organization:

  • Prokaryotes: nucleoid with single circular chromosome; may contain plasmids.

  • Eukaryotes: linear chromosomes within a nucleus; multiple chromosomes; more complex packaging with histones.

• Internal compartments:

  • Prokaryotes: few internal membranes; some possess internal photosynthetic membranes or organelles in certain species.

  • Eukaryotes: extensive compartmentalization (ER, Golgi, lysosomes, peroxisomes, mitochondria, chloroplasts in plants, etc.).

• Size and scale:

  • Prokaryotes: ~0.1–5.0 μm; SR>V favors rapid exchange with the environment.

  • Eukaryotes: ~5–100 μm; larger cells with more complex internal organization.

CONNECTIONS TO FOUNDATIONAL PRINCIPLES AND REAL-WORLD RELEVANCE

• Cell theory underpins modern biology, medicine, and biotechnology; understanding cell structure informs disease mechanisms, drug targeting, and tissue engineering.

• The endomembrane system demonstrates how cells compartmentalize functions to increase efficiency and control chemical environments.

• Mitochondria and chloroplasts exemplify endosymbiotic theory, highlighting evolutionary processes that led to eukaryotic complexity.

• The SA/V concept explains why cell size is constrained and how cells optimize transport and metabolic rates.

IMPORTANT NUMERICAL AND FORMAL REFERENCES (SUMMARY)

• Resolution (LM vs EM):

  • $ext{Resolution}_{LM} \,\approx\, 200\ \,\text{nm}$

  • $ext{Resolution}_{EM} \,\approx\, 2\ \,\text{nm}$

• Magnification ranges:

  • LM: $\text{Magnification}_{LM} \approx 5\times10^2 \text{ to } 1.5\times10^3$

  • EM: $\text{Magnification}_{EM} \approx 1\times10^5 \text{ to } 3\times10^5$

• Cell size ranges:

  • Prokaryotes: $0.1\ \mu\text{m} \le D \le 5.0\ \mu\text{m}$

  • Eukaryotes: $5\ \mu\text{m} \le D \le 100\ \mu\text{m}$

• SA:V considerations are summarized by the ratio $\frac{SA}{V}$; as cells grow, V increases faster than SA, reducing exchange efficiency.

ETHICAL, PHILOSOPHICAL, AND PRACTICAL IMPLICATIONS

• The endosymbiotic origin of mitochondria and chloroplasts informs our understanding of evolution and the interconnectedness of life.

• Advanced microscopy challenges and capabilities drive biomedical research (e.g., imaging organelle dynamics, drug localization).

• Recognition of cellular diversity (prokaryotes with internal membranes or organelles) broadens our view of what constitutes a "cell" and informs microbiology, ecology, and biotechnology.

SUMMARY: KEY TAKEAWAYS

• Cells are the fundamental units of life; all organisms are composed of cells, and cells arise from preexisting cells.

• Prokaryotic and eukaryotic cells differ in nucleus presence, internal organization, and size, but share core components (plasma membrane, cytoplasm, DNA, ribosomes).

• The prokaryotic cell features include nucleoid chromosome, plasmids, cell wall with peptidoglycan in many bacteria, ribosomes, cytoskeleton, and sometimes internal photosynthetic membranes and external structures (flagella, fimbriae).

• Eukaryotic cells feature extensive internal compartments (ER, Golgi, lysosomes, vacuoles, peroxisomes, mitochondria, and in plants, chloroplasts), a nucleus with nuclear envelope and pores, and a cytoskeleton for organization and transport.

• The endomembrane system coordinates synthesis, processing, and trafficking of proteins, lipids, and carbohydrates.

• Plant cells add a central vacuole, cell wall, and chloroplasts; animal cells rely on ECM and lack a cell wall.

• Microscopy (LM and EM) reveals DNA, organelles, and subcellular architecture at different scales and resolutions, with specific sample preparation and imaging trade-offs.