Chapter 3: Cell Structure and Function

Overview of Cellular Organization and Structure

• Cells serve as the fundamental structural and functional units of all living organisms.

• Primary Cellular Components:

  • Plasma Membrane: A semi-permeable phospholipid bilayer that forms a dynamic outer boundary regulating molecular transport.

  • Cytoskeleton: An internal network of protein filaments that provides mechanical support, maintains cell geometry, and facilitates intracellular transport and cellular motility.

  • Cytoplasm: The fluid medium (cytosol) surrounding organelles between the plasma membrane and the nucleus.

  • Nucleus: The membrane-bound command center housing genomic DNA and controlling cellular activities.

  • Organelles: Specialized intracellular structures executing distinct metabolic, synthetic, and hydrolytic functions.

Plasma Membrane Structure and Dynamics

• The plasma membrane physically separates the intracellular environment (cytoplasm and organelles) from the extracellular environment.

• Phospholipid Bilayer Architecture:

  • Hydrophilic Heads: Polar phosphate heads oriented outward toward aqueous intracellular and extracellular fluids.

  • Hydrophobic Tails: Nonpolar fatty acid chains oriented inward away from water, creating a selective permeability barrier.

• Membrane Components & Glycocalyx:

  • Glycoproteins & Glycolipids: Carbohydrate chains attached to membrane proteins and lipids on the extracellular surface, crucial for cell recognition, signaling, and cell-to-cell adhesion.

  • Cholesterol: Interspersed within the hydrophobic core to stabilize membrane fluidity across fluctuating physiological temperatures.

Cytoplasm and Cytoskeletal Architecture

• Cytoplasm:

  • Occupies the space between the plasma membrane and the nucleus.

  • Contains cytosol, dissolved electrolytes, metabolic substrates, and suspended organelles.

• Cytoskeletal Elements:

  • Microtubules:

  • Hollow cylinders composed of tubulin subunits.

  • Maintain cell structural integrity and shape.

  • Function as molecular "train tracks" for motor proteins transporting organelles and vesicles.

  • Cilia and Flagella:

  • Specialized microtubular projections from the cell surface enabling extracellular fluid movement or cell motility.

  • Intermediate Filaments:

  • Fibrous protein assemblies providing mechanical strength.

  • Essential for cell-to-cell junctions, tissue structure stability, and mechanical stress resistance during tissue development.

Nuclear Structure and Genetic Regulation

• Genomic Organization:

  • Contains the entirety of the organism's genetic blueprint ($DNA$).

  • Every nucleated somatic cell within an individual contains the exact same genetic sequence.

  • Cell specialization and tissue identity depend on selective gene expression (turning specific genes on or off) governed by signal transduction pathways and messenger RNA ($mRNA$) activation.

• Nuclear Components:

  • Nuclear Envelope: A double-membrane barrier enclosing nuclear contents, continuous with the rough endoplasmic reticulum.

  • Nuclear Pores: Protein-lined channels that selectively regulate nuclear import and export of macromolecules, proteins, and RNA transcripts.

  • Chromatin: Uncoiled strands of $DNA$ complexed with histone proteins; condenses into visible chromosomes during nuclear division.

  • Nucleolus (plural: Nucleoli): Dense nuclear sub-region dedicated to ribosomal RNA ($rRNA$) synthesis and ribosome assembly.

• Epigenetic Reprogramming:

  • Extracellular stimuli (e.g., regular exercise) induce epigenetic modifications such as histone acetylation (e.g., increased $H3K27ac$ marks at enhancer regions).

  • These modifications alter chromatin structure to upregulate target gene expression, modulating systemic phenotypes including myokine release, platelet function, blood metabolite profiles, and cognitive performance.

Cytoplasmic Organelles and Protein Synthesis Pathways

• Ribosomes:

• Non-membrane-bound complexes of $rRNA$ and proteins.

• Consist of a large subunit and a small subunit that assemble around $mRNA$.

• Function as the sites of translation, reading $mRNA$ codons to synthesize polypeptide chains.

• Exist free in the cytosol or attached to the cytosolic surface of the rough endoplasmic reticulum.

  • Endoplasmic Reticulum (ER):

• Rough ER:

  • Studded with ribosomes on its cytosolic membrane surface; continuous with the outer nuclear envelope.

  • Folds and modifies newly synthesized polypeptide chains into functional mature proteins.

• Smooth ER:

  • Devoid of ribosomes; continuous with rough ER.

  • Synthesizes lipids, phospholipids, and steroid hormones.

  • Detoxifies metabolic byproducts and drugs.

  • Stores calcium ions ($Ca^{2+}$) required for cell signaling and muscle contraction.

  • Facilitates intracellular transport through cisternal spaces.

  • Golgi Apparatus:

• Stacked membrane-bound flattened sacs (cisternae) possessing a receiving cis-face and a shipping trans-face.

• Chemically modifies, sorts, packages, and routes proteins and lipids arriving from the ER.

• Packages secretory products into vesicles for exocytosis across the cell membrane or yields hydrolytic enzymes packaged into lysosomes.

  • Lysosomes:

• Acidic membrane-bound vesicles generated by the Golgi apparatus containing hydrolytic enzymes.

• Phagocytosis: Fuses with endocytic vacuoles to digest internalized pathogens or foreign debris, providing immune protection.

• Autodigestion (Autophagy): Degrades damaged organelles and non-functional cellular components, preventing cellular degeneration and abnormal growth.

  • Peroxisomes:

• Membrane-bound vesicles containing oxidases and catalase enzymes.

• Break down long-chain fatty acids and neutralize toxic metabolic byproducts (such as hydrogen peroxide).

• Present in exceptionally high concentrations in liver and kidney cells responsible for systemic detoxification.

• Impairment of hepatic peroxisomal and metabolic pathways contributes to liver injury and chronic conditions such as liver cirrhosis.

  • Mitochondria:

• Double-membrane organelles responsible for aerobic cellular respiration and adenosine triphosphate ($ATP$) generation.

• Structural Features:

  • Smooth outer membrane.

  • Highly folded inner membrane forming cristae to maximize surface area for electron transport chain proteins.

  • Intermembrane space located between outer and inner membranes.

  • Gel-like matrix enclosed by the inner membrane containing metabolic enzymes, mitochondrial $DNA$, and mitochondrial ribosomes.

• Cellular Respiration Reaction:     $\text{Carbohydrate} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{ATP}$     $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

  • As carbohydrates (glucose) are oxidatively broken down to carbon dioxide and water in the presence of oxygen ($O_2$), the released free energy phosphorylates adenosine diphosphate ($ADP$) and inorganic phosphate ($\text{P}_i$) into $ATP$.

  • Applied Biological Scenarios:

• Sprinter vs. Marathoner Mitochondria Density: Endurance athletes (marathoners) possess significantly greater mitochondrial density in their skeletal muscle fibers compared to sprinters. Marathoners rely on continuous aerobic respiration for sustained $ATP$ production, whereas sprinters rely primarily on rapid, anaerobic energy systems.

• Post-Strength Training Muscle Repair: Ribosomes and the rough endoplasmic reticulum are the primary organelles responsible for rebuilding skeletal muscle tissue after strength training by upregulating protein translation to repair micro-tears in muscle fibers.

Cellular Transport Mechanisms

• The semi-permeable plasma membrane selectively controls internal concentration gradients using passive and active transport mechanisms.

Passive Transport Mechanisms (No $ATP$ Required)

• Simple Diffusion:

• Unassisted movement of small, nonpolar solutes down their concentration gradient (from high solute concentration to low solute concentration) until equilibrium is reached.

• Systemic $O_2$ Cascade: Minute ventilation $\rightarrow$ Pulmonary $O_2$ diffusion $\rightarrow$ Cardiac output $\rightarrow$ Circulatory $O_2$ delivery $\rightarrow$ Muscle $O_2$ diffusion $\rightarrow$ Muscle $O_2$ utilization and Muscle $ATP$ turnover.

  • Osmosis:

• Passive diffusion of water molecules across a selectively permeable membrane from an area of low solute concentration (high water concentration) to high solute concentration (low water concentration).

• Tonicity Environments:

  • Isotonic (Healthy Normal): Solute concentration inside and outside the cell is equal; no net water movement; red blood cells maintain normal biconcave shape.

  • Hypotonic (Water Intoxication / Overhydration): Extracellular fluid has lower solute concentration than cytoplasm; water rushes into the cell, causing swelling and potential osmotic lysis.

  • Hypertonic (Dehydration): Extracellular fluid has higher solute concentration than cytoplasm; water rushes out of the cell, causing cell shrinkage and crenation.

  • Facilitated Diffusion:

• Passive transport of polar or charged molecules (such as glucose, amino acids, or ions like $\text{Na}^+$) down their concentration gradient.

• Utilizes transmembrane protein channels or carrier proteins without expending metabolic energy ($ATP$).

  • Filtration:

• Movement of water and dissolved solutes across a membrane driven by hydrostatic pressure gradients (from high pressure to low pressure).

• Essential in renal physiology: blood pressure forces fluid and small waste solutes out of kidney capillaries into renal tubules, producing urine while retaining blood cells and large proteins.

Active Transport Mechanisms ($ATP$ Required)

• Primary Active Transport (Pumps):

• Solute transport against normal electrochemical/concentration gradients (from low concentration to high concentration).

• Requires direct expenditure of metabolic energy ($ATP$ hydrolysis).

• Example: Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ ATPase), which pumps $\text{Na}^+$ ions out of the cell and $\text{K}^+$ ions into the cell to maintain resting membrane potential.

  • Endocytosis:

• Vesicular active transport process by which the cell engulfs extracellular material by invagination of the plasma membrane.

• Phagocytosis ("Cell Eating"): Plasma membrane extends around large particulate matter, microorganisms, or cell debris to form an intracellular vacuole/vesicle.

• Pinocytosis ("Cell Drinking"): Plasma membrane invaginates to internalize droplets of extracellular fluid containing dissolved solutes.

  • Exocytosis:

• Process where intracellular secretory vesicles fuse with the plasma membrane to discharge their contents into the extracellular space.

• Example: Regulated secretion of peptide hormones, such as insulin release from pancreatic beta cells into the bloodstream.

The Cell Cycle, DNA Replication, and Protein Expression

• The cell cycle represents the continuous series of events a cell undergoes leading to growth and division.

• Major Cell Life Decisions at the Restriction Checkpoint:

  • Proceed to cellular division.

  • Remain specialized in a non-dividing state ($G_0$ phase).

  • Undergo programmed cell death (Apoptosis).

Interphase

• Interphase accounts for the vast majority of cellular life and comprises three distinct functional sub-phases: $G_1$, $S$, and $G_2$.

• $G_1$ Phase (Growth 1):

  • Cell undergoes rapid physical growth, doubles organelle numbers, and synthesizes metabolic proteins.

  • Executes standard physiological cell functions.

  • Highly active in gene expression via transcription and translation.

• Transcription:

  • Occurs inside the nucleus.

  • Nuclear $DNA$ serves as a template to form pre-$mRNA$.

  • $mRNA$ undergoes processing (capping, polyadenylation, splicing) before exiting the nucleus through nuclear pores.

• Translation:

  • Occurs outside the nucleus in the cytoplasm.

  • $mRNA$ binds to the small and large ribosomal subunits.

  • Transfer RNA ($tRNA$) molecules carrying specific amino acids bind to $mRNA$ codons via complementary anticodon base pairing.

  • Ribosome catalyzes peptide bond formation, synthesizing a polypeptide chain one amino acid at a time.

  • Deacylated $tRNA$ departs to reload its specific amino acid.

  • $S$ Phase (Synthesis / $DNA$ Replication):

• Complete replication of the nuclear genome.

• Begins with single unreplicated double-helix strands (chromatids).

• Mechanism:

  • Parental $DNA$ double helix is unwound and unzipped by breaking hydrogen bonds between complementary base pairs.

  • DNA Polymerase aligns free nucleotides with template strands following complementary base pairing rules ($A$ pairs with $T$, $G$ pairs with $C$).

  • Semi-conservative replication yields two identical daughter double helices, each consisting of one original (parental) strand and one newly synthesized strand (sister chromatids joined at a centromere).

  • $G_2$ Phase (Growth 2):

• Final preparations for nuclear division.

• Synthesizes division-specific proteins and replicates centrioles/centrosomes.

Mitosis ($M$ Phase) and Cytokinesis

• Purpose and Importance:

  • Division of one parent cell into two genetically identical daughter cells.

  • Preserves exact chromosome numbers ($2n$) across somatic cell generations.

  • Provides foundation for tissue growth, repair, regeneration, and organismal maintenance.

• Sequential Stages of Mitosis:

  • Early Prophase / Prophase:

  • Centrosomes duplicate and migrate toward opposite cell poles as spindle fibers begin assembling.

  • Chromatin condenses into distinct visible duplicated chromosomes (sister chromatids).

  • Nucleolus disappears and the nuclear envelope fragments.

  • Early Metaphase / Metaphase:

  • Nuclear envelope is fully dissolved.

  • Centromeric (kinetochore) spindle fibers attach to the centromeres of sister chromatids.

  • Polar spindle fibers overlap at the cell center.

  • Chromosomes align along the equatorial metaphase plate.

  • Anaphase:

  • Sister chromatids separate at their centromeres, becoming individual daughter chromosomes.

  • Motor proteins pull daughter chromosomes along shortening spindle fibers toward opposite poles.

  • Ensures each pole receives an identical set and number of chromosomes.

  • Telophase:

  • Daughter chromosomes arrive at opposite poles and begin uncoiling into indistinct chromatin.

  • Nuclear envelopes and nucleoli reassemble in each developing daughter cell.

  • Mitotic spindle disassembles.

  • Cytokinesis:

  • Physical division of cytoplasm.

  • A cleavage furrow forms along the equatorial plane, pinching the cell into two distinct, genetically identical daughter cells.