Cell Structure and Function Comprehensive Study Guide

Course Milestones and Lab Schedule

• Assessment Dates and Milestones:

  • Sections 2.1–2.5 Quiz and Notebook Check: September 25 ($9/25$)

  • Sections 2.6–2.10 Quiz and Notebook Check: October 2 ($10/2$)

  • Unit 2 Comprehensive Quiz and Notebook Check: October 5 ($10/5$)

• Required Laboratory Investigations:

  • Microscopy Lab: Examination of cellular structures and magnification principles.

  • Osmosis Lab: Quantitative measurement of water movement across selectively permeable membranes.

  • Plastid Lab: Analysis of plant cell organelles including chloroplasts and chromoplasts.

Section 2.1: Cell Structure and Function

• Prokaryotic vs. Eukaryotic Cell Comparison:

  • Membrane-Bound Organelles: Eukaryotes possess membrane-bound organelles that compartmentalize cellular reactions. Prokaryotes lack membrane-bound organelles.

  • Cell Wall Presence and Composition: Present in prokaryotes (peptidoglycan in bacteria) and select eukaryotes (cellulose in plants, chitin in fungi). Absent in animal cells.

  • Genetic Material Organization: Prokaryotic DNA is single, circular, and localized in a non-membrane-enclosed region called the nucleoid, often accompanied by extra-chromosomal plasmids. Eukaryotic DNA is linear, wrapped around histone proteins, and sequestered inside a double-membraned nucleus.

  • Multicellularity: Prokaryotes are strictly unicellular (though they can form colonies). Eukaryotes demonstrate both unicellularity and complex multicellularity.

  • Shared Cellular Structures: Both prokaryotic and eukaryotic cells possess ribosomes, cytoplasm (cytosol), and a plasma (cell) membrane.

• Taxonomic Structural Variations of the Cell Wall:

  • Bacterial Cell Wall (Peptidoglycan):

  • Composed of a polymer mesh network consisting of repeating disaccharides cross-linked by short amino acid peptide chains.

  • Contains embedded wall-associated proteins, teichoic acid polymers, and lipoteichoic acids anchored to the underlying cytoplasmic membrane.

• Fungal Cell Wall (Chitin & Glucan Layering):

  • Outer surface features a matrix of mannoproteins.

  • Middle layer composed of branched $\beta\text{-1,6-glucan}$ and $\beta\text{-1,3-glucan}$ networks synthesized by membrane-bound $\beta\text{-1,3-glucan}$ synthase utilizing $\text{UDP-Glc}$.

  • Inner structural layer composed of rigid chitin polymers synthesized by chitin synthase utilizing $\text{UDP-GlcNAc}$ directly above the phospholipid bilayer.

• Plant Cell Wall (Cellulose Architecture):

  • Built from linear chains of glucose molecules linked via $\beta\text{-1,4-glycosidic}$ bonds.

  • Parallel glucose chains bundle via hydrogen bonding into structural cellulose microfibrils, forming a high-tensile meshwork that surrounds the plasma membrane.

• Plant Cell vs. Animal Cell Distinction:

  • Plant Cells: Contain a rigid cellulose cell wall, a large central vacuole that maintains turgor pressure, chloroplasts for photosynthesis, and lack centrioles.

  • Animal Cells: Lack cell walls, contain multiple smaller vacuoles for material storage, possess centrioles/centrosomes involved in spindle fiber formation, and lack chloroplasts.

Detailed Organelle Structure and Function

• Nucleus:

  • Function: Primary repository for storing, protecting, replicating, and transcribing genetic information (DNA).

  • Structural Components:

  • Nuclear Envelope: Double-membrane barrier separating nuclear contents from the cytoplasm.

  • Nuclear Pores: Protein complexes bridging the nuclear envelope, regulating macromolecular transport (mRNA, proteins, ribosomal subunits).

  • Nuclear Lamina: Dense protein meshwork lining the interior surface of the nuclear envelope that maintains nuclear shape.

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

  • Chromatin: Complex of linear DNA strands packaged around histone proteins, suspended within the aqueous nucleoplasm.

• Ribosome:

  • Composition: Non-membrane-bound complexes composed of ribosomal RNA (rRNA) and proteins, divided into a large subunit and a small subunit.

  • Evolutionary Significance: Found universally across all forms of life (prokaryotes and eukaryotes), reflecting a common ancestral origin for cellular protein synthesis.

  • Functional Roles: Synthesize polypeptide chains by translating nucleotide sequences carried by messenger RNA (mRNA).

  • Free Ribosomes: Suspended in the cytosol; produce cytosolic proteins.

  • Bound Ribosomes: Attached to the cytosolic face of the endoplasmic reticulum; produce membrane-bound, organellar, or secreted proteins.

• Endomembrane System Overview:

  • Components: Consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, transport vesicles, and plasma membrane.

  • Cooperative Function: Interconnected network that collaborates to synthesize, modify, package, isolate, and transport proteins, lipids, and complex carbohydrates throughout the cell or to the extracellular environment.

• Endoplasmic Reticulum (ER):

  • General Function: Membrane network providing structural support, maintaining cell morphology, and establishing intracellular transport pathways.

  • Rough ER: Surface studded with bound ribosomes. Compartmentalizes the cell and serves as the primary site for folding and modifying newly synthesized membrane and secretory proteins.

  • Smooth ER: Lacks attached ribosomes. Responsible for lipid and phospholipid synthesis, steroid hormone production, carbohydrate metabolism, and enzymatic detoxification of metabolic waste and xenobiotics.

• Golgi Apparatus:

  • Structure: Stack of flattened, membrane-bound sacs named cisternae. Possesses distinct directional polarity: the cis face (receiving side facing the ER) and the trans face (shipping side facing the plasma membrane).

  • Functions:

  • Correctly folds, processes, and chemically modifies proteins (e.g., glycosylation, phosphorylation).

  • Sorts, tags, and packages molecular products into transport vesicles for targeted intracellular delivery or exocytosis.

  • Operates via cisternal maturation, where cisternae physically progress and mature from the cis to the trans direction.

• Lysosome:

  • Structure: Single-membrane-enclosed vesicles containing acidic hydrolytic enzymes (hydrolases).

  • Functions:

  • Digestion of intracellular macromolecules, damaged cellular components (autophagy), and engulfed foreign materials or food particles (phagocytosis).

  • Plays a crucial role in regulated cell death pathways (apoptosis) by releasing hydrolytic content into the cytoplasm upon target signals.

• Vacuole:

  • Structure: Large membrane-bound sacs derived from the endoplasmic reticulum and Golgi apparatus.

  • Functional Diversity:

  • Plant Central Vacuole: Large, dominant sac that stores water, ions, pigments, and metabolic wastes. Regulates cellular osmolarity and exerts turgor pressure outward against the cell wall to maintain structural rigidity.

  • Animal Cell Vacuoles: Smaller, more abundant sacs utilized for temporary storage of nutrients, lipids, and endocytosed materials.

• Mitochondria:

  • Structure: Double-membrane metabolic organelle.

  • Outer Membrane: Smooth and permeable to small molecules.

  • Inner Membrane: Highly invaginated into folds called cristae, which maximize surface area for ATP synthase complexes and electron transport chain proteins.

  • Compartments: Intermembrane space (between inner and outer membranes) and the innermost fluid matrix.

  • Autonomy: Contains independent circular mitochondrial DNA (mtDNA) and prokaryote-like ribosomes. Dynamically undergoes fusion and fission events.

  • Function: Primary site of aerobic cellular respiration, metabolic citric acid cycle reactions, and oxidative phosphorylation for ATP generation.

• Chloroplast:

  • Structure: Double-membrane organelle present in plants and photosynthetic algae.

  • Thylakoids: Flattened membranous sacs containing chlorophyll light-harvesting complexes. Organized into stacks called grana (singular: granum).

  • Stroma: Fluid-filled inner matrix surrounding thylakoids; site of light-independent Calvin cycle reactions.

  • Autonomy: Contains self-replicating circular chloroplast DNA and 70S-type ribosomes.

  • Function: Converts solar light energy into chemical energy stored in carbohydrate bonds via photosynthesis.

• Endosymbiont Theory:

  • Theory Principles: Proposes that eukaryotic organelles evolved through an endosymbiotic relationship where an ancestral eukaryotic host cell engulfed free-living prokaryotic organisms.

  • Evolutionary Steps:

  1. An ancestral host cell engulfed an oxygen-utilizing, non-photosynthetic aerobic prokaryote, which survived and evolved over generations into modern mitochondria.

  2. A lineage of these early mitochondria-containing eukaryotes subsequently engulfed a photosynthetic prokaryote (cyanobacterium ancestor), which evolved into modern chloroplasts.

  • Supporting Evidence: Mitochondria and chloroplasts have double membranes, circular self-replicating DNA genomes, independent binary-fission division, and prokaryotic-like ribosomes.

• Peroxisome:

  • Structure: Single-membrane sphere often characterized by a dense, crystalline core of metabolic enzymes such as urate oxidase.

  • Functions:

  • Contains oxidative enzymes that strip hydrogen atoms from organic substrates and transfer them to molecular oxygen ($O_2$), producing hydrogen peroxide ($H_2O_2$) as a intermediate byproduct.

  • Utilizes catalase enzymes to immediately convert toxic hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen ($O_2$).

  • Performs fatty acid $\beta$-oxidation and toxic substance breakdown.

Section 2.2: Cell Size and Surface Area-to-Volume Ratio

• Surface Area-to-Volume Ratio ($SA/V$) Principles:

  • Cells remain microscopic because volume increases much faster than surface area as linear size scales up (V \n\n\propto r^3 vs. $SA \propto r^2$).

  • Mathematical Proof of Scaling:

  • Large Cube Model: Side length = $30\,\mu\text{m}$.

    • \n\text{Surface Area} = 6 \times (30\,\mu\text{m})^2 = 5{,}400\,\mu\text{m}^2\n

    • \n\text{Volume} = (30\,\mu\text{m})^3 = 27{,}000\,\mu\text{m}^3\n

    • \n\frac{SA}{V} = \frac{5400}{27000} = 0.2\,\mu\text{m}^{-1}\n

  • 27 Small Cubes Model: Side length = $10\,\mu\text{m}$ each (occupying the exact same total volume of $27{,}000\,\mu\text{m}^3$).

    • \n\text{Total Surface Area} = 27 \times [6 \times (10\,\mu\text{m})^2] = 16{,}200\,\mu\text{m}^2\n

    • \n\frac{SA}{V} = \frac{16200}{27000} = 0.6\,\mu\text{m}^{-1}\n

  • Conclusion: Dividing a large volume into smaller cellular units increases the total surface area threefold ($16{,}200\,\mu\text{m}^2$ vs $5{,}400\,\mu\text{m}^2$), drastically enhancing transport potential.

• Limiting Factors and Environmental Exchange:

  • The plasma membrane surface area constrains the rate at which nutrients can enter and metabolic wastes/heat can exit.

  • Cell volume dictates metabolic demand and resource consumption rates.

  • Cells with higher $SA/V$ ratios exchange substances with their surrounding fluid environment far more efficiently.

• Cell Geometry Efficiency Case Study:

  • Model Cell A (Rectangular Block Shape): Dimensions $4\,\mu\text{m} \times 6\,\mu\text{m} \times 25\,\mu\text{m}$.

  • \n\text{Volume}_A = 4 \times 6 \times 25 = 600\,\mu\text{m}^3\n

  • \n\text{Surface Area}_A = 2(4\times6 + 4\times25 + 6\times25) = 2(24 + 100 + 150) = 544\,\mu\text{m}^2\n

  • \n\left(\frac{SA}{V}\right)_A = \frac{544}{600} \approx 0.907\,\mu\text{m}^{-1}\n

  • Model Cell B (Cylindrical Shape): Height $h = 25\,\mu\text{m}$, Radius $r = 3\,\mu\text{m}$.

  • \n\text{Volume}_B = \pi r^2 h = \pi (3)^2 (25) = 225\pi \approx 706.86\,\mu\text{m}^3\n

  • \n\text{Surface Area}_B = 2\pi r^2 + 2\pi r h = 2\pi(9) + 2\pi(3)(25) = 18\pi + 150\pi = 168\pi \approx 527.79\,\mu\text{m}^2\n

  • \n\left(\frac{SA}{V}\right)_B = \frac{168\pi}{225\pi} = \frac{56}{75} \approx 0.747\,\mu\text{m}^{-1}\n

  • Efficiency Analysis: Cell A is significantly more efficient at exchanging substances with its environment because its geometry yields a larger surface-area-to-volume ratio ($0.907\,\mu\text{m}^{-1}$ vs. $0.747\,\mu\text{m}^{-1}$).

Section 2.3: Plasma Membrane Dynamics and Structure

• Amphipathic Phospholipid Bilayer:

  • Phospholipids possess an amphipathic chemical nature with polar hydrophilic phosphate heads and nonpolar hydrophobic fatty acid hydrocarbon tails.

  • In aqueous solutions, phospholipids spontaneously self-assemble into a bilayer structure:

  • Hydrophilic Heads: Positioned outward, directly contacting aqueous intracellular cytoplasm and extracellular matrix water molecules.

  • Hydrophobic Tails: Directed inward towards each other, creating an internal nonpolar core isolated from water.

• Fluid Mosaic Model Architecture:

  • Describes the plasma membrane as a dynamic structure composed of a fluid phospholipid matrix embedded with proteins, steroids (cholesterol), glycolipids, and glycoproteins.

  • Membrane components laterally drift and shift continuously.

• Classification of Membrane Proteins:

  • Integral / Transmembrane Proteins:

  • Penetrate into or span completely across the nonpolar hydrophobic core of the bilayer.

  • Possess nonpolar hydrophobic middle domains composed of helices (rich in nonpolar amino acids) and hydrophilic polar end domains exposed to aqueous surroundings ($\text{N-terminus}$ and $\text{C-terminus}$ regions).

• Peripheral Proteins:

  • Polar proteins lacking hydrophobic anchor regions; loosely attached to the extracellular or cytoplasmic surface of integral proteins or phospholipid heads.

  • Six Core Functions of Membrane Proteins:

• Transport: Channel and carrier proteins provide pathways to move specific solutes across the hydrophobic core passively or actively.

• Enzymatic Activity: Membrane-bound active sites catalyze specific metabolic steps within metabolic pathways.

• Signal Transduction: Specific receptor sites bind extracellular ligand signaling molecules, inducing conformational changes that initiate intracellular signaling pathways.

• Cell-Cell Recognition: Oligosaccharide tags attached to glycoproteins act as molecular identification tags recognized by other cell receptors.

• Intercellular Joining: Membrane proteins form permanent or temporary intercellular junctions (e.g., tight junctions, desmosomes, gap junctions).

• Attachment to Cytoskeleton and Extracellular Matrix (ECM): Microfilaments or ECM elements anchor to membrane proteins to stabilize cell shape and fix protein positioning.

• Regulation of Membrane Fluidity:

  • Fatty Acid Composition:

  • Unsaturated Fatty Acid Tails: Contain double bonds creating structural kinks that prevent tight lipid packing, increasing membrane fluidity at lower temperatures.

  • Saturated Fatty Acid Tails: Straight hydrocarbon chains that pack tightly together, increasing membrane viscosity.

  • Cholesterol Fluidity Buffer in Animal Cells:

  • At Moderate/High Temperatures: Cholesterol restrains phospholipid movement, lowering membrane fluidity and maintaining structural integrity.

  • At Low Temperatures: Intercalates between phospholipid tails, breaking up tight lipid packing to prevent membrane solidification.

• Environmental Adaptation Case Study (Winter Wheat):

  • When exposed to cold temperatures, winter wheat seedlings increase the proportion of unsaturated fatty acids relative to saturated fatty acids within membrane lipids, maintaining necessary membrane fluidity and active transport capacity during freezing conditions.

Section 2.4: Selective Permeability

• Permeability Characteristics Across the Lipid Core:

  • Freely Permeable Molecules: Small nonpolar hydrophobic molecules ($N_2$, $O_2$, $CO_2$, steroids) dissolve directly in the nonpolar lipid interior and cross membranes unassisted down concentration gradients.

  • Impermeable / Transport-Dependent Substances: Hydrophilic substances, including charged ions ($Na^+$, $K^+$, $Cl^-$), large polar uncharged molecules (glucose), and charged organic compounds cannot penetrate the hydrophobic core and require specific embedded transport proteins.

  • Extremely Restricted Permeability: Small uncharged polar molecules ($H_2O$, $NH_3$) can diffuse across the bilayer simple diffusion pathways at very slow rates, requiring aquaporin channel proteins for rapid physiological transport.

• Structural Function of the Cell Wall:

  • Present in Bacteria, Archaea, Fungi, and Plants.

  • Serves as a physical barrier that defines cell boundaries, permits passive entry of small water/solute molecules, and counteracts internal osmotic pressure to prevent cell burst (osmotic lysis) in hypotonic environments.

Section 2.5 & 2.6: Membrane Transport Mechanisms

• Concentration Gradient Dynamics:

  • A concentration gradient represents a physical difference in solute solute density across a membrane.

  • Steep Gradient: Large difference in solute concentration between sides (e.g., 12 solute particles vs. 3 solute particles), resulting in rapid net diffusion rates.

  • Shallow Gradient: Minimal difference in concentration (e.g., 12 solute particles vs. 11 solute particles), resulting in slow net diffusion rates.

• Passive Transport Modes:

  • Net movement of solutes from regions of high concentration to regions of low concentration down a concentration gradient without direct input of metabolic energy (ATP).

  • Simple Diffusion: Direct, unassisted passage of small nonpolar molecules directly across the lipid bilayer.

  • Facilitated Diffusion: Passive passage of hydrophilic molecules or ions down concentration gradients utilizing specific transmembrane proteins:

  • Channel Proteins: Hydrophilic pore tunnels providing continuous aqueous passage for specific ions or water (aquaporins).

  • Carrier Proteins: Solute-binding transmembrane proteins that undergo conformational shape changes to flip target molecules across the membrane.

• Active Transport Mechanics:

  • Moves solutes against concentration gradients (from regions of low concentration to regions of high concentration).

  • Requires direct metabolic energy input (typically ATP hydrolysis or electrochemical potential energy).

  • Primary Active Transport: Direct utilization of ATP energy by protein pumps (e.g., $Na^+/K^+$ ATPase pump, Uniport pumps) to establish solute concentration gradients.

  • Secondary Active Transport (Cotransport):

  • Uses the potential energy stored in an electrochemical gradient established by primary active pumps to move a secondary solute against its concentration gradient.

  • Symport: Both the driving ion and target solute move in the exact same direction across the membrane.

  • Antiport: The driving ion and target solute move in opposite directions across the membrane.

• Bulk Transport Processes:

  • Requires direct ATP energy expenditure to move large macromolecules or massive quantities of materials into or out of cells in membrane-bound vesicles.

• Endocytosis Mechanisms:

  • The cell plasma membrane invaginates inward around target substances, pinching off internal vesicles from the external environment.

  • Phagocytosis ("Cellular Eating"): Pseudopods extend around large solid particles or whole cells, enclosing them within large food vacuoles for lysosomal fusion.

  • Pinocytosis ("Cellular Drinking"): Non-specific inward invagination of membrane capturing droplets of extracellular fluid into tiny vesicles.

  • Receptor-Mediated Endocytosis: Specialized inward uptake where specific extracellular target ligands bind to membrane-bound protein receptors, triggering assembly of clathrin-coated vesicles.

• Exocytosis Mechanics:

  • Intracellular secretory vesicles containing processed cellular products travel along microtubule tracks to the plasma membrane.

  • Vesicular membranes fuse with the inner plasma membrane face, expanding the total cell surface area and releasing internal luminal cargo into the extracellular space.