Cell Structure and Function Complete Study Guide

Fundamentals of Cell Biology and Historical Foundations

• The cell is defined as the smallest structural and functional unit capable of performing all processes necessary for life.

• All living organisms are composed of one or more cells.

• Most cells are microscopic with diameters less than $1\,\text{mm}$, making them invisible to the unaided human eye; however, select single cells, such as frog eggs ($\sim 1\,\text{mm}$) or ostrich eggs, are large enough to be seen without magnification.

• Historical Milestones in Cell Discovery:

  • In the 17th century, Anton Van Leeuwenhoek invented the light microscope, observed living single-celled organisms for the first time, and termed them "animalcules".

• Robert Hooke confirmed Leeuwenhoek's observations and coined the term "cell" after observing the microscopic repeating structures in cork tissue.

• German scientists Theodor Schwann (zoologist), Matthias Schleiden (botanist), and Rudolph Virchow (pathologist) established that all plant and animal tissues are composed of cells and established that cells arise exclusively from pre-existing cells.

  • Tenets of Cell Theory:

• Classic Cell Theory:

  • All living organisms are composed of one or more cells.

  • Cells are the structural and functional basic units of life.

  • All cells arise from pre-existing cells through cellular division.

• Modern Cell Theory Additions:

  • All cells possess the same basic chemical composition and types of biomolecules.

  • Hereditary information (DNA) is passed from parent cell to daughter cell during cell division.

  • Metabolic energy flow (biochemical reactions) occurs internally within cells.

Biological Scale and Surface-Area-to-Volume Dynamics

• Scale of Microscopic and Biological Structures:

• Atoms: $\sim 0.1\,\text{nm}$.

• Small molecules and amino acids: $\sim 1\,\text{nm}$.

• Proteins and globular viruses: $10\,\text{nm}$ to $100\,\text{nm}$.

• Most bacteria and eukaryotic chloroplasts: $1\,\mu\text{m}$ to $10\,\mu\text{m}$.

• Eukaryotic plant and animal cells, human eggs: $10\,\mu\text{m}$ to $100\,\mu\text{m}$.

• Frog eggs: $\sim 1\,\text{mm}$.

• Macroscopic organisms (ants, mice, roses, humans, blue whales): $1\,\text{cm}$ to $100\,\text{m}+$

  • Limits of Resolution:

• Electron Microscope: Resolves structures from $0.1\,\text{nm}$ to $100\,\mu\text{m}$.

• Light Microscope: Resolves structures from $100\,\text{nm}$ to $1\,\text{mm}$.

• Unaided Human Eye: Resolves structures from $100\,\mu\text{m}$ ($0.1\,\text{mm}$) up to macroscopic scales.

  • Mathematical Principles of Surface-Area-to-Volume Ratio:

• Small cell size is optimal for multicellular organisms because it maximizes the exchange of nutrients (such as glucose) and waste products (such as $\text{CO}_2$) across the plasma membrane.

• As a cell grows larger, its internal volume increases at a cubic rate, whereas its surface area increases at a quadratic rate.

• Doubling a cell's linear dimension increases its surface area fourfold ($2^2 = 4$) and its volume eightfold ($2^3 = 8$).

• Geometric Comparison of Cube Volumes and Surface Area:

- One $4\text{-cm}$ cube:
  - Total Surface Area: $\text{height} \times \text{width} \times 6\,\text{sides} \times 1\,\text{cube} = 4\,\text{cm} \times 4\,\text{cm} \times 6 \times 1 = 96\,\text{cm}^2$
  - Total Volume: $\text{height} \times \text{width} \times \text{length} \times 1\,\text{cube} = 4\,\text{cm} \times 4\,\text{cm} \times 4\,\text{cm} \times 1 = 64\,\text{cm}^3$
  - Surface Area to Volume Ratio: $\frac{96}{64} = 1.5:1$
- Eight $2\text{-cm}$ cubes:
  - Total Surface Area: $2\,\text{cm} \times 2\,\text{cm} \times 6 \times 8 = 192\,\text{cm}^2$
  - Total Volume: $2\,\text{cm} \times 2\,\text{cm} \times 2\,\text{cm} \times 8 = 64\,\text{cm}^3$
  - Surface Area to Volume Ratio: $\frac{192}{64} = 3:1$
- Sixty-four $1\text{-cm}$ cubes:
  - Total Surface Area: $1\,\text{cm} \times 1\,\text{cm} \times 6 \times 64 = 384\,\text{cm}^2$
  - Total Volume: $1\,\text{cm} \times 1\,\text{cm} \times 1\,\text{cm} \times 64 = 64\,\text{cm}^3$
  - Surface Area to Volume Ratio: $\frac{384}{64} = 6:1$

• A single large cell with a side length of $30\,\mu\text{m}$ yields a total surface area of $5,400\,\mu\text{m}^2$, whereas dividing that same volume into 27 smaller cubes of $10\,\mu\text{m}$ side length increases the total available surface area to $16,200\,\mu\text{m}^2$.

Taxonomic Domains and Structural Comparison

• Primary Domains of Life:

  • Prokaryotes: Archaea and Bacteria.

  • Eukaryotes: Eukarya (Animals, Plants, Fungi, Protists).

• Comparative Analysis of Cellular Domains:

• Archaea:

  • Cell Wall: Usually present, lacks peptidoglycan.

  • Plasma Membrane: Present (contains unique branched ether-linked lipids that confer stability in extreme environments).

  • Nucleus: Absent.

  • Membrane-Bound Organelles: Absent.

  • Ribosomes: Present ($70\text{S}$ size).

• Bacteria:

  • Cell Wall: Usually present, composed of peptidoglycan.

  • Plasma Membrane: Present.

  • Nucleus: Absent.

  • Membrane-Bound Organelles: Absent (some exhibit specialized internal plasma membrane invaginations for metabolic reactions).

  • Ribosomes: Present ($70\text{S}$ size).

• Eukaryotes:

  • Cell Wall: Present in plants (cellulose), fungi (chitin), and algae; absent in animal cells.

  • Plasma Membrane: Present.

  • Nucleus: Present.

  • Membrane-Bound Organelles: Present.

  • Ribosomes: Present ($80\text{S}$ size in cytoplasm/RER; larger than prokaryotic ribosomes).

Prokaryotic Cell Anatomy and Reproduction

• Structural and Functional Characteristics:

  • Structurally simple lacking internal compartmentalization, yet metabolically diverse and adapted to nearly every terrestrial and aquatic habitat.

  • Reproduction occurs exclusively through asexual division.

• Morphological Classifications:

• Bacillus: Rod-shaped bacterial cells.

• Coccus: Spherical or round bacterial cells.

• Spirillum: Long rod-shaped cells twisted into rigid spirals.

• Spirochete: Long rod-shaped cells twisted into flexible spirals.

  • Internal Cytoplasmic Components:

• Cytoplasm: Semifluid gel-like matrix composed of water, inorganic salts, and dissolved organic molecules.

• Nucleoid: Irregularly shaped, non-membrane-bound region holding the single circular bacterial chromosome.

• Plasmids: Small, extrachromosomal circular rings of accessory DNA that replicate independently and often carry genes for antibiotic resistance.

• Ribosomes: Thousands of small particle complexes ($70\text{S}$) suspended throughout cytoplasm conducting protein synthesis.

• Thylakoids: Internalized flattened membrane disks containing light-harvesting photosynthetic pigments present in photosynthetic prokaryotes such as cyanobacteria.

  • Cell Envelope and External Appendages:

• Cell Envelope Layers (from outermost to innermost):

  1. Glycocalyx: Outer polysaccharide coating protecting against desiccation, host immune recognition, and environmental stress. When organized and tightly attached, it is termed a capsule; when loose and diffuse, it is termed a slime layer.

  2. Cell Wall: Rigid layer composed of peptidoglycan (a network of disaccharides cross-linked by peptide chains) surrounding the plasma membrane to maintain shape and prevent osmotic lysis.

  3. Plasma Membrane: Phospholipid bilayer regulating selective permeability into and out of the cell cytoplasm.

• Surface Appendages:

- Bacterial Flagellum: Rotating protein filament motor that propels the bacterium through liquid media.
- Fimbriae: Short, hair-like protein bristles on the cell surface that facilitate adhesion to host tissues or environmental substrates.
- Conjugation Pili: Rigid, hollow tubular structures used to transfer plasmid DNA horizontally between bacterial cells.

• Asexual Binary Fission Mechanism:

1. The single circular chromosome replicates at the origin of replication.

2. The cytoplasmic membrane elongates, physically separating the two replicated DNA molecules toward opposite poles.

3. A septum cross-wall begins to form as the cell wall and plasma membrane invaginate inward.

4. The cross-wall forms completely across the cell equator.

5. Two genetically identical daughter cells split apart.

Eukaryotic Cell Organization and Organelles

• Eukaryotic Complexity:

  • Characterized by membrane-bound compartments called organelles suspended in cytosol.

  • Organelles function like specialized factory departments, allowing incompatible biochemical processes to occur concurrently.

• Comprehensive Anatomy of Eukaryotic Cells:

• Organelle Summary and Functional Reference:

• Cell Wall: Rigid outer structural layer present in plant cells (composed of primary and secondary cellulose microfibrils), fungal cells (composed of chitin), and algae. Provides mechanical support, protection, and prevents osmotic bursting. Absent in animal cells.

• Plasma Membrane: Dynamic phospholipid bilayer embedded with integral and peripheral proteins. Defines cell boundary and regulates selective transport.

• Nucleus: Double-membrane enclosed organelle ($\sim 5\,\mu\text{m}$ diameter) containing the cellular genome structured as linear chromatin fibers wrapped around histone proteins. Serves as the control center for genetic replication and gene expression.

• Nucleolus: Dense, non-membrane-bound region within the nucleus where ribosomal RNA (rRNA) is synthesized and assembled with proteins into ribosomal subunits.

• Nuclear Envelope and Pores: Double phospholipid membrane system perforated by protein complex nuclear pores that regulate transport of nuclear proteins inward and ribosomal subunits/mRNA outward.

• Ribosomes: Complexes of rRNA and proteins consisting of one large subunit and one small subunit ($80\text{S}$ in eukaryotic cytosol). Synthesize polypeptide chains using mRNA transcripts as templates.

• Endoplasmic Reticulum (ER):

- Rough ER (RER): Network of folded flattened membrane sacs studded with ribosomes on its cytosolic face. Processes, folds, and modifies nascent polypeptides, packaging them into transport vesicles destination-tagged for the Golgi apparatus or cell membrane.
- Smooth ER (SER): Tubular membranous network lacking ribosomes. Synthesizes lipids (phospholipids, cholesterol, steroid hormones), metabolizes carbohydrates, stores intracellular calcium, and detoxifies drugs/xenobiotics in hepatic tissue.

• Golgi Apparatus (Golgi Body):

- Stack of 3 to 20 flattened, curved membranous sacs called cisternae with distinct polarities: *cis* face (receiving face oriented toward the ER) and *trans* face (shipping face oriented toward the plasma membrane).
- Functions as the cellular shipping center; modifies proteins and lipids (glycosylation, phosphorylation), sorts, packages, and routes products into transport or secretory vesicles.

• Lysosomes:

- Membrane-bound acidic vesicles containing hydrolytic digestive enzymes produced by the RER and processed in the Golgi.
- Perform intracellular digestion, phagocytosis of foreign pathogens, and autophagy of damaged organelles ("cellular recycling centers"). Found exclusively in animal cells; equivalent hydrolytic degradation in plants occurs within the central vacuole.

• Vacuoles and Vesicles:

- Vesicles: Small membranous sacs used for intracellular transport and temporary storage.
- Central Vacuole: Prominent fluid-filled membrane sac occupying up to $90\%$ of mature plant cell volume. Stores water, inorganic ions, sugars, pigments, and secondary metabolites/toxins; exerts turgor pressure against the cell wall to maintain structural rigidity (loss of turgor pressure causes wilting). Absent in fungal cells.

• Peroxisomes:

- Membrane-enclosed metabolic vesicles containing oxidative enzymes that break down fatty acids and amino acids, producing toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) as a metabolic byproduct.
- Contain the enzyme catalase, which rapidly degrades hydrogen peroxide into water and oxygen gas: $2\text{H}_2\text{O}_2 \xrightarrow{\text{catalase}} 2\text{H}_2\text{O} + \text{O}_2$
- Abundant in liver cells for bile salt synthesis and detoxification, and in germinating plant seeds for seed fat conversion.

Biochemistry of Enzymes and Biological Catalysis

• Enzyme Function and Thermodynamics:

  • Enzymes are specialized protein catalysts (or RNA molecules known as ribozymes) that accelerate chemical reaction rates without being consumed or permanently altered in the process.

• Biological Significance: Essential for life processes; without catalysis, metabolic chemical reactions would occur too slowly to sustain cellular survival.

• Catalytic Mechanism:

  • Enzymes bind substrate molecules at a specifically shaped pocket called the active site to form an enzyme-substrate complex.

  • Lower the activation energy ($E_a$) required to transition reactants into products, speeding up reaction rates.

  • Highly specific: Catalyze distinct reactions based on active site stereospecificity.

  • Reusable: After products are released, the active site returns to its initial conformation ready to bind another substrate molecule.

• Denaturation: Extreme temperature shifts or $\text{pH}$ deviations alter non-covalent secondary and tertiary bonds, unravelling protein conformation and abolishing catalytic activity.

The Endomembrane System

• Structural Composition:

  • Integrated functional network consisting of the nuclear envelope, rough ER, smooth ER, Golgi apparatus, lysosomes, vesicles, and the plasma membrane.

• Functional Workflow:

1. Genetic instructions are transcribed from DNA into mRNA within the nucleus and exported via nuclear pores.

2. Ribosomes bound to the Rough ER translate mRNA into polypeptides, which fold inside the ER lumen.

3. Membrane transport vesicles bud off the RER carrying modified proteins to the cis face of the Golgi apparatus.

4. Smooth ER synthesizes lipids and packages them into transport vesicles routed to the Golgi apparatus.

5. Proteins and lipids transit through Golgi cisternae, undergoing chemical modifications and destination tagging.

6. Secretory vesicles bud off the trans face of the Golgi and migrate along cytoskeletal tracks to fuse with the plasma membrane, discharging contents extracellularly via exocytosis.

7. Specialized vesicles containing digestive hydrolytic enzymes bud from the Golgi to form active lysosomes.

Energy-Converting Organelles: Chloroplasts and Mitochondria

• Cellular Bioenergetics:

• Solar energy captured by chloroplasts synthesizes high-energy carbohydrates, releasing oxygen gas as a byproduct.

• Mitochondria break down carbohydrates via aerobic cellular respiration in the presence of oxygen to generate chemical energy in the form of adenosine triphosphate ($\text{ATP}$), releasing low-energy $\text{CO}_2$ and $\text{H}_2\text{O}$ back into the environment.

  • Chloroplast Structure and Photosynthesis:

• Site of photosynthesis in plant cells, algae, and photosynthetic protists; absent in animal and fungal cells.

• Bounded by a double membrane (outer and inner membranes).

• Stroma: Fluid-filled internal compartment containing metabolic enzymes, chloroplast DNA loops, and $70\text{S}$ ribosomes.

• Thylakoids: System of interconnected flattened membranous sacs suspended within the stroma.

• Grana: Stacks of thylakoid disks containing chlorophyll and light-harvesting protein complexes that capture solar photons to synthesize glucose and oxygen.

  • Mitochondria Structure and Cellular Respiration:

• Universal powerhouse of eukaryotic cells; present in plants, animals, fungi, and protists.

• Enclosed by a double membrane system:

  • Outer Membrane: Smooth boundary membrane.

  • Inner Membrane: Deeply convoluted inward folds called cristae that maximize surface area for electron transport chain protein complexes.

• Matrix: Central fluid-filled compartment containing respiratory enzymes, mitochondrial circular DNA, and $70\text{S}$ ribosomes.

• Converts chemical energy locked within food molecules into usable cellular currency ($\text{ATP}$) through aerobic cellular respiration.

Evolution of Eukaryotes and Endosymbiotic Theory

• Evolutionary Steps of Eukaryotic Origins:

1. An ancestral prokaryotic cell undergoes invagination and infolding of its plasma membrane around the central nucleoid region, creating a double-membraned nucleus and establishing an internal endomembrane system.

2. The early nucleated cell engulfs an aerobic heterotrophic bacterium via phagocytosis. Instead of digestion, a mutually beneficial symbiotic relationship develops: the engulfed bacterium becomes an obligate intracellular endosymbiont, evolving into the modern mitochondrion.

3. A lineage of these mitochondrion-bearing early eukaryotes subsequently engulfs a photosynthetic cyanobacterium, establishing a secondary endosymbiotic relationship that evolves into modern chloroplasts.

• Evidence Supporting Endosymbiotic Theory (MAD DR Mnemonic):

• Membranes: Both mitochondria and chloroplasts possess double membranes. The outer membrane reflects host vesicle origin, while the inner membrane reflects the original prokaryotic plasma membrane.

• Antibiotics: Susceptible to specific antibacterial antibiotics (e.g., chloramphenicol) that inhibit bacterial protein synthesis, proving prokaryotic origins.

• Division: Organelle reproduction occurs independently of host nuclear division through binary fission-like splitting processes.

• DNA: Both organelles possess their own non-histone bound, naked circular DNA genomes identical in structural organization to prokaryotic chromosomes.

• Ribosomes: Contain internal ribosomes that are $70\text{S}$ in size and exhibit rRNA sequence homology identical to bacterial ribosomes, rather than eukaryotic cytosolic $80\text{S}$ ribosomes.

Cytoskeletal System and Cellular Dynamics

• Overview of Cytoskeleton:

• Dynamic internal network of protein fibers assembled and disassembled as needed by the cell.

• Functions: Maintains cell shape, anchors organelles, drives intracellular transport, and provides mechanical force for cellular motility and cell division.

  • Three Major Filament Systems:

1. Actin Filaments (Microfilaments):

- Structure: Two thin, flexible strands composed of globular actin monomers twisted into a helical polymer ($\sim 7\,\text{nm}$ diameter).
- Functions: Forms a dense cortical web beneath the plasma membrane providing structural integrity; forms microvilli projections in intestinal epithelial cells; forms pseudopodia for amoeboid locomotion; drives cytokinesis cleavage furrow formation in animal cell division.
- Motor Interaction: Interacts with the motor protein **myosin** using $\text{ATP}$ hydrolysis to drive muscle cell contraction and cytoplasmic streaming.

2. Intermediate Filaments:

- Structure: Intermediate in diameter ($\sim 8\text{--}11\,\text{nm}$); tough, rope-like fibrous polypeptide chains assembled into thick cables.
- Functions: Provides structural support to the nuclear envelope (nuclear lamina); anchors cell-to-cell junctions (desmosomes) maintaining tissue structural integrity across epithelial sheets; composes structural proteins like keratin in human hair, nails, and skin.

3. Microtubules:

- Structure: Rigid, hollow cylinders ($\sim 25\,\text{nm}$ diameter) composed of globular $\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$ heterodimers arranged in 13 protofilaments.
- Controlled by the cellular Microtubule Organizing Center (MTOC), known as the **centrosome** in eukaryotic cells.
- Functions: Maintains cell shape; forms spindle fibers to pull replicated chromosomes apart during cell division; serves as structural tracks along which motor proteins carry transport vesicles and organelles.
- Motor Proteins:

  - **Kinesin**: Motor protein that uses $\text{ATP}$ to walk vesicles along microtubules toward the outer plasma membrane boundary (anterograde transport).
  - **Dynein**: Motor protein that walks vesicles along microtubules inward toward the nucleus (retrograde transport).

Centrioles, Cilia, and Flagella

• Centriole Structure and Centrosome Function:

• Located within the centrosome of animal cells (absent in plant cell MTOCs).

• Composed of short, perpendicular cylinders displaying a $9+0$ microtubule pattern: 9 outer microtubule triplets arranged in a ring surrounding an empty center.

• Organize the mitotic spindle apparatus necessary for chromosome separation during animal cell division.

  • Cilia and Flagella Structure and Mechanics:

• Membrane-enclosed cytoplasmic projections responsible for cellular movement or sweeping environmental fluids.

• Both display an internal axoneme featuring a $9+2$ microtubule pattern: 9 outer microtubule doublets encircling 2 central single microtubules, anchored to the cell by a basal body (which displays a $9+0$ triplet arrangement identical to centrioles).

• Comparison of Features:

  • Cilia: Short, hair-like, present in large quantities (thousands per cell); perform complex back-and-forth beating strokes.

  • Flagella: Long, whip-like, present in small numbers ($1$ to $8$ per cell); perform wave-like, circular, or propeller-like rotational motion.

• Functional Diversity and Specialized Biological Examples:

- Single-celled protists (e.g., *Paramecium*) use motile cilia for swimming.
- Ciliated tracheal epithelial cells lining human respiratory airways sweep inhaled dust and debris trapped in mucus upward away from lungs.
- Human spermatozoa utilize a long flagellum for propulsion toward egg fertilization.
- Non-motile primary cilia serve sensory roles, such as olfactory sensory neuron cilia sensing odorant molecules and rod photoreceptor connecting cilia transferring light signals.

Viral Exploitation of Eukaryotic Organelles

• Subversion of Host Machinery:

  • Viruses are non-living obligate intracellular parasites that must hijack host organelle systems to replicate.

• Mechanism of Enveloped Viral Replication (e.g., SARS-CoV-2 / COVID-19):

1. Viral spike proteins bind specific surface receptors (e.g., ACE2 receptors) on host plasma membranes, triggering viral entry via direct membrane fusion or receptor-mediated endocytosis.

2. The viral RNA genome is released into the cytoplasm and targets the Rough Endoplasmic Reticulum (RER), inducing double-membrane replication vesicles.

3. Viral RNA hijacks host cytosolic and RER-bound ribosomes to translate viral structural proteins and suppress host cell protein synthesis.

4. Spike proteins and structural capsid proteins assemble in the RER membrane and bud into vesicles routed to the Golgi apparatus.

5. Mature viral particles assemble within Golgi compartments and are packaged into secretory vesicles.

6. Secretory vesicles undergo exocytosis at the plasma membrane, releasing $\sim 1,000$ new virions per cell without immediate host cell lysis.

• Evolutionary Advantage of Non-Lytic Exocytosis:

  • Releasing virions via exocytosis keeps the host cell intact and viable longer, maximizing total viral particle output before cell death occurs.

• Lytic Viral Mechanism (e.g., Poliovirus):

  • Non-enveloped viruses replicate inside host cytoplasm until the host cell is completely filled, leading to osmotic lysis (cell bursting) and cell death (e.g., destruction of motor neurons in poliomyelitis).