Cell Structure

Introduction to Cell Theory and Cellular Organization

• Hierarchy of Biological Organization:

  • Biological study progresses systematically along a hierarchy of complexity: Chemical level $\rightarrow$ Cellular level $\rightarrow$ Tissue level $\rightarrow$ Organ level $\rightarrow$ Organ system level.

• Etymology and Discovery of the Cell:

  • Early microscopes provided approximately $40\times$ magnification power ($40\,\text{power}$).

  • Historical observations of thinly sliced cork (plant material) revealed rigid, rectangular compartment arrays.

  • These rectangular shapes resembled the small living quarters (chambers) occupied by monks in a monastery.

  • Derived from the Latin word sela (or cella), meaning "chamber" or "storeroom."

  • The term was ideal because cell walls gave the appearance of a monk's chamber and stored internal material.

• Definition and Quantitative Parameters of Human Cells:

  • Cell Definition: The basic living structural and functional unit of all organisms.

  • Unicellular vs. Multicellular: Simple organisms consist of a single cell, whereas humans are complex multicellular organisms.

  • Total Cell Count: The human body contains approximately $1 \times 10^{12}$ cells ($1,000,000,000,000$ or one million million / trillion cells).

  • Cell Diversity: There are $200$ distinct types of cells in the human body, varying in size, shape, and specialized function.

  • Genetics: All somatic cells in an individual are genetically identical. Gametes (reproductive cells) are the sole exception and are non-identical. Variations in cell structure and function stem from differential gene expression.

  • Size Dimensions:

    • Smallest human cell: $2 \times 10^{-6}\,\text{m}$ ($2\,\mu\text{m}$ or two microns) in diameter.

    • Largest human cell: Greater than $1\,\text{m}$ ($> 1\,\text{m}$) in length (specifically, single motor neurons extending from the lower back down to the bottom of the foot).

• Core Tenets of Cell Theory:

  • The cell provides structural framework and functional capabilities that allow life to exist.

  • The activity or function of an entire organism is the combined result of the collective activities of its individual cells.

  • The activity of a cell depends directly on its specific subcellular composition.

  • Principle of Complementarity: The structure of a cell determines its function (what a cell contains determines what it can do).

  • Continuity of Life: Cells arise only from pre-existing cells ($\text{cells} \rightarrow \text{cells}$).

    • Somatic cellular duplication occurs via mitosis.

    • New individuals are created through the union of two specialized reproductive cells known as gametes.

The Plasma Membrane: Composition and Structural Model

• General Features and Boundaries:

  • The plasma membrane serves as an outer boundary separating the intracellular fluid (inside the cell) from the extracellular fluid (outside the cell).

  • It isolates the internal cellular environment while regulating all exchange of materials between the internal and external compartments.

  • Generalized Animal Cell: A conceptual composite model featuring all possible organelles and structures. No single real human cell possesses all organelles in these exact proportions (e.g., mitochondria counts vary dramatically between active muscle cells and inactive cells).

  • Organelle Definition: Specialized structures within a cell that carry out specific tasks, providing a distinct division of labor so multiple chemical processes can occur simultaneously.

  • Cytoplasm Definition: The cellular region consisting of the fluid cytosol along with all suspended organelles, inclusions, and structural elements.

• Chemical Structure of the Plasma Membrane:

  • Phospholipid Bilayer: Formed by a double layer of phospholipids. The hydrophobic fatty acid tails compress together into the center, while hydrophilic phosphate heads face the aqueous outer and inner surfaces.

  • Dimensions: Super thin boundary measuring between $7\,\text{nm}$ and $10\,\text{nm}$ ($7\text{ to }10\,\text{nm}$) in thickness. Because it is extremely thin and lipid-rich, it is labile and malleable; it can be pulled apart, reformed, and resealed easily.

  • Fluid Mosaic Model: The plasma membrane is dynamic and flexible rather than rigid. Proteins embedded within the lipid bilayer drift and shift laterally like icebergs floating in a lipid sea.

• Membrane Components:

  • Integral Proteins: Proteins that span the entire thickness of the lipid bilayer, exposing structural domains on both the extracellular and intracellular surfaces. These function primarily as transport proteins (channel proteins and carrier proteins).

  • Peripheral Proteins: Proteins attached strictly to one surface (inner or outer) of the plasma membrane. They function primarily as mechanical supports (providing structural integrity to the fragile bilayer) or as enzymes.

  • G-Proteins: Specialized peripheral enzyme proteins that are tightly linked to integral proteins. When a ligand binds to the integral protein, it activates the adjacent G-protein to trigger intracellular biochemical cascades.

  • Steroids / Cholesterol: Contains four fused hydrocarbon rings (derived from cholesterol). Cholesterol is intercalated within the phospholipid bilayer to stabilize membrane fluidity and structural integrity.

  • Carbohydrates and Glycoproteins: Carbohydrate chains covalently attach to membrane surface proteins, forming glycoproteins that function in cell recognition and receptor activity.

Determinants of Membrane Permeability and Transport Mechanics

• Selective Permeability:

  • The plasma membrane allows certain molecules to pass freely while restricting or blocking others.

• Four Primary Determining Factors of Membrane Permeability:

  • Molecular Size: Smaller molecules pass through the membrane far more easily than larger ones. An upper size threshold exists above which molecules cannot cross without tearing or destroying the cell.

  • Solubility: Lipid-soluble (nonpolar / fat-soluble) substances dissolve directly into the hydrophobic core and pass through freely. Water-soluble (polar) substances cannot cross the lipid core directly.

  • Ionic Charge: Membrane proteins contain charged amino acid residues. Like charges repel incoming charged particles, whereas opposite charges attract them toward the membrane surface.

  • Carrier / Transport Molecules: The presence, speed, and specificity of integral transport proteins determine whether specific hydrophilic or large molecules can cross.

Passive Transport Mechanisms

• Fundamental Principles of Passive Transport:

  • Passive processes require zero cellular energy input ($\text{ATP}$).

  • Movement is driven entirely by kinetic energy and concentration gradients, moving molecules from an area of high concentration to an area of low concentration ("down" or "with" the concentration gradient).

• Simple Diffusion:

  • The unassisted, passive movement of solute particles (gases, liquids, or solids) from high concentration to low concentration until dynamic equilibrium is established.

  • At equilibrium, molecular movement does not stop; instead, for every molecule that moves across in one direction, another moves across in the opposite direction.

  • Factors Affecting Rate:

    • Molecular Size: Smaller molecules diffuse faster than larger ones.

    • Temperature: Higher temperatures increase molecular kinetic energy, accelerating the rate of diffusion.

  • Permeable Substances: Nonpolar, fat-soluble substances and lipids freely cross via simple diffusion.

• Facilitated Diffusion:

  • Passive movement of polar or charged solutes down their concentration gradient mediated by integral membrane proteins.

  • Channel Proteins: Transmembrane tunnels through which matching solutes pass freely.

  • Carrier Proteins: Transmembrane proteins specific to particular solutes (e.g., glucose, amino acids, specific ions). Binding of the solute induces a conformational shape change in the carrier protein, closing one side and opening the other to transfer the solute.

  • Factors Affecting Rate: Concentration gradient, total number of available carrier/channel proteins, and the protein's "pickup speed" (the rate at which the carrier undergoes its conformational shape change).

• Osmosis:

  • The movement of water across a selectively permeable membrane.

  • Because water itself cannot be "concentrated," osmosis is regulated by two main forces: solute concentration (dissolved particles) and hydrostatic/osmotic pressure.

  • Water moves passively from a region of lower solute concentration (higher relative water concentration) to a region of higher solute concentration (lower relative water concentration).

• Tonicity Definitions and Cellular Responses:

  • Tonicity: A comparison of the relative solute concentrations of two solutions separated by a membrane (e.g., intracellular fluid vs. extracellular fluid).

  • Hypertonic Solution: A solution with a higher solute concentration relative to the inside of a cell.

    • Cellular Effect: Water leaves the cell by osmosis, causing the cell to shrivel up—a process known as crenation.

  • Hypotonic Solution: A solution with a lower solute concentration relative to the inside of a cell.

    • Cellular Effect: Water enters the cell by osmosis, causing the cell to swell and burst—a process known as lysis (lytic activity).

  • Isotonic Solution: A solution with an equal solute concentration relative to the inside of a cell (e.g., Ringer's lactate).

    • Cellular Effect: No net movement of water occurs; cell volume remains unchanged.

• Molarity vs. Osmolarity:

  • Molarity ($\text{M}$): Measures chemical compound concentration in moles per liter.

  • Osmolarity ($\text{osmol}$): Measures the total number of individual solute particles resulting from dissociation in solution per liter.

  • Example 1: A $1\,\text{M}$ Sodium Chloride ($\text{NaCl}$) solution dissociates into $1\,\text{M}\text{ Na}^+$ and $1\,\text{M}\text{ Cl}^-$ ions, yielding a $2\,\text{osmol}$ saline solution.

  • Example 2: A $1\,\text{M}$ Carbonic Acid ($\text{H}_2\text{CO}_3$) solution dissociates into $2\,\text{H}^+$ ions and $1\,\text{CO}_3^{2-}$ ion, yielding a $3\,\text{osmol}$ solution.

  • Physiological Application: The human body (e.g., kidney tubules) actively pumps ions across membranes to create localized solute gradients, thereby forcing osmotic water movement to precisely regulate urinary output.

• Filtration:

  • A passive transport process that forces fluid and dissolved solutes across a selectively permeable membrane using hydrostatic pressure or gravity (a pressure gradient rather than a concentration gradient).

  • Physiological Examples: Occurs in kidney capillary beds and systemic blood vessels where blood pressure drives movement out of capillaries.

  • Analogy: A barista using a coffee filter acts as a physical filtration membrane.

Active Transport Mechanisms and Vesicular Transport

• Fundamental Principles of Active Transport:

  • Active processes require cellular energy assistance in the form of Adenosine Triphosphate ($\text{ATP}$).

  • Moves substances "against" or "up" their concentration gradient (from an area of low concentration to an area of high concentration).

  • Approximately $40\%$ of all $\text{ATP}$ generated by the human body is consumed strictly by active transport mechanisms.

• Active Transport Pumps:

  • Requires integral carrier proteins and direct $\text{ATP}$ hydrolysis.

  • Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ Pump): Actively expels sodium ions ($\text{Na}^+$) out of the cell while pumping potassium ions ($\text{K}^+$) into the cell, maintaining non-equal ionic steady states required for physiological function.

• Vesicular Transport:

  • Active transport of large molecules, macromolecules, or bulk fluids packaged inside membrane-bound sacs called vesicles.

• Exocytosis:

  • Process of expelling large substances out of the cell.

  • Substances (e.g., hormones, neurotransmitters, mucus, cellular waste, exported proteins) are enclosed within a vesicle whose wall is a phospholipid bilayer.

  • The vesicle migrates to the plasma membrane, fuses with the phospholipid bilayer, and discharges its contents into the extracellular space.

• Endocytosis:

  • Process of bringing large substances into the cell by engulfing them with the plasma membrane, which pinches off to form an intracellular vesicle.

  • Phagocytosis ("Cell Eating"): Ingestion of solid particles into a cell. Executed primarily by specialized immune cells such as macrophages, which engulf bacterial pathogens and cellular debris to destroy them via enzymatic breakdown.

  • Pinocytosis ("Cell Drinking"): Non-specific engulfment of extracellular fluid droplets containing dissolved solutes.

  • Receptor-Mediated Endocytosis: Highly selective uptake mechanism where target extracellular molecules bind to specific protein receptors on the outer cell surface before vesicle formation.

    • Target Examples: Iron, insulin, specific enzymes.

    • Viral Exploitation: The influenza (flu) virus tricks cells by binding to surface receptors intended for essential molecules, triggering receptor-mediated endocytosis to invade the host cell.

    • Clathrin-Coated Vesicles: The inner cytoplasmic face of the plasma membrane beneath target receptors is lined with a structural protein called clathrin. Clathrin reinforces and stabilizes the membrane during engulfment to prevent membrane damage.

Specialized Plasma Membrane Modifications and Cellular Junctions

• Microvilli:

  • Microscopic, finger-like projections extending from the free (unattached) apical surface of cells.

  • Functions strictly to dramatically increase membrane surface area for absorption.

  • Found extensively on epithelial tissues lining the small intestine and kidney tubules.

• Cell Junctions:

  • Tight Junctions (Impermeable Junctions):

    • Integral proteins of adjacent plasma membranes directly interlock and fuse together.

    • Prevents molecules from passing through the extracellular space between neighboring cells.

    • Physiological Example: Found in the stomach lining to prevent highly concentrated gastric acid from leaking into adjacent tissues.

  • Desmosomes (Anchoring Junctions):

    • Internal protein plaques on adjacent cells linked together by interweaving protein filaments (acting like cellular Velcro).

    • Distributes mechanical tension and strain across an entire cellular sheet, preventing tissue tearing under high stress.

    • Physiological Example: Found in tissues subjected to high friction or pressure, such as the epidermis (skin), cardiac muscle wall, and arterial walls.

  • Gap Junctions (Communicating Junctions):

    • Hollow transmembrane channel proteins (connexons) of adjacent cells align and fuse to create continuous intercellular tunnels.

    • Allows ions, simple sugars, and small molecules to pass directly from the cytoplasm of one cell into the cytoplasm of another.

    • Physiological Example: Critical in electrically excitable tissues such as cardiac muscle and smooth muscle (e.g., driving wave-like peristaltic contractions in the digestive tract).

Membrane Potential and Electrochemical Gradients

• Membrane Potential:

  • The voltage or electrical charge difference across a plasma membrane caused by the spatial separation of oppositely charged ions.

  • Key involved ions include Sodium ($\text{Na}^+$), Potassium ($\text{K}^+$), and Chloride ($\text{Cl}^-$).

• Resting Membrane Potential:

  • The steady electrical state of a cell at rest, characterized by an unequal distribution of ions across the plasma membrane.

  • The interior face of the plasma membrane is electrically negative relative to the exterior face; the resting cell is electrically polarized.

• Electrochemical Gradient:

  • The combined influence of a concentration gradient and an electrical charge gradient across the membrane.

  • Controlled ion movements across the membrane alter the resting potential, driving nerve impulse conduction and muscle contraction.

Structure and Function of Subcellular Organelles

• Organelle Overview:

  • Derived from Latin organella, meaning "instrument" or "tool."

  • Specialized cellular structures with distinct morphology assigned specific roles in growth, repair, maintenance, and cellular control.

• Nucleus:

  • Functional Designation: The "Brains of the Cell."

  • Characteristics: The largest organelle. Most cells possess a single nucleus (mononucleated); some are multinucleated; mature red blood cells are anucleated ($0$ nuclei).

  • Genetic Material: Contains DNA (hereditary material; half inherited maternally, half paternally) that directs all protein synthesis and cellular activity. Exists as unwound chromatin in resting cells and condenses into chromosomes during cell division.

  • Nuclear Envelope: A double phospholipid bilayer membrane perforated by nuclear pores, which form where the inner and outer membranes fuse to regulate nuclear transport.

  • Nucleoplasm: The fluid gel-like substance enclosed within the nuclear envelope.

  • Nucleolus: Dense spherical body inside the nucleus responsible for synthesizing and assembling ribosomal subunits, which exit into the cytoplasm via nuclear pores.

• Mitochondria:

  • Functional Designation: The "Powerhouse of the Cell."

  • Function: Primary site of cellular respiration and $\text{ATP}$ synthesis.

  • Structure: Enclosed by a double phospholipid bilayer membrane.

  • Unique Features:

    • Self-Replication: Capable of replicating independently of cell division via binary division when cellular energy demands increase (e.g., during exercise training in muscle cells).

    • Mitochondrial DNA & RNA: Contains its own distinct circular DNA and RNA, which is inherited exclusively from the maternal lineage (mother).

• Ribosomes:

  • Functional Designation: The "Factory Floor."

  • Composition: Non-membrane-bound granules composed of ribosomal RNA ($\text{rRNA}$) and proteins.

  • Function: Site of cellular protein synthesis (translation).

  • Classes of Ribosomes:

    • Free Ribosomes: Float freely in the cytosol; synthesize soluble proteins intended for internal cellular use.

    • Attached Ribosomes: Bound to the membrane of the endoplasmic reticulum; synthesize proteins destined for insertion into membranes or for exocytosis/export.

• Endoplasmic Reticulum (ER):

  • Structure: Network of interconnected parallel membranes enclosing fluid-filled cavities (cisternae) continuous with the nuclear envelope.

  • Rough Endoplasmic Reticulum (Rough ER):

    • Studded with attached ribosomes on its cytoplasmic surface.

    • Synthesizes all proteins secreted/exported from the cell.

    • Synthesizes integral membrane proteins and membrane phospholipids.

  • Smooth Endoplasmic Reticulum (Smooth ER):

    • Lacks attached ribosomes.

    • Synthesizes steroid-based hormones (e.g., sex hormones).

    • Acts as the primary site for cellular detoxification of drugs, alcohol, and foreign toxins.

    • Catalyzes the breakdown of stored glycogen into usable glucose.

    • Participates in lipid metabolism.

• Golgi Apparatus:

  • Functional Designation: The "UPS Store" of the cell.

  • Function: Modifies, concentrates, sorts, and packages proteins and lipids synthesized by the rough ER into membrane-bound vesicles for intracellular delivery or extracellular export.

• Lysosomes:

  • Function: Digestive vesicles formed by the Golgi apparatus containing hydrolytic enzymes.

  • Degrades cellular debris, worn-out organelles, endocytosed foreign material, and bacterial pathogens via lysis.

• Peroxisomes:

  • Function: Neutralize toxic free radicals (highly reactive, charged oxygen molecules released as metabolic byproducts during mitochondrial $\text{ATP}$ production).

  • Two-Step Enzymatic Neutralization Mechanism:

    1. Initial peroxisomal enzymes convert highly toxic free radicals into hydrogen peroxide ($\text{H}_2\text{O}_2$).

    2. Secondary peroxisomal enzymes (such as catalase) break down hazardous hydrogen peroxide ($\text{H}_2\text{O}_2$) into harmless water ($\text{H}_2\text{O}$).

Cytoplasm, Cytoskeleton, Cytoplasmic Inclusions, and Surface Structures

• Cytosol:

  • The viscous, semitransparent fluid portion of the cytoplasm consisting of water, dissolved proteins, salts, and nutrients.

• Cytoskeleton:

  • An elaborate network of protein rods running through the cytosol that gives the cell structural support, shape, and internal motility.

  • Microtubules:

    • Size: Largest cytoskeletal elements.

    • Structure: Hollow tubes composed of tubulin protein.

    • Function: Determines overall cell shape, anchors and distributes organelles, forms tracks for intracellular transport, and shifts chromosomes during nuclear division.

  • Microfilaments:

    • Size: Smallest cytoskeletal elements.

    • Structure: Thin strands composed of actin, concentrated heavily right beneath the plasma membrane.

    • Function: Strengthens the cell surface, drives cell shape alterations during exocytosis and endocytosis, and executes cytokinesis (splitting the cell during division).

  • Intermediate Filaments:

    • Size: Mid-sized cytoskeletal elements.

    • Structure: Tough, insoluble woven protein fibers.

    • Function: Provides high tensile strength to resist mechanical pulling forces exerted on the cell.

• Cytoplasmic Inclusions:

  • Inert, non-functional chemical substances stored directly within the cytosol (not enclosed by membranes).

  • Examples: Mucus droplets, stored lipid droplets in adipose cells, glycogen granules in liver/muscle cells, and melanin pigment granules in skin cells.

• Specialized Cellular Extensions and Centrosomes:

  • Cilia: Numerous short, hair-like projections extending from the free surface of specialized cells; beat in synchronized waves to move substances (such as mucus and trapped debris) across the cell surface (e.g., respiratory tract lining).

  • Flagella: A single, long, whip-like projection used exclusively for cellular locomotion. The human sperm cell is the only cell in the human body that possesses a flagellum.

    • Biological Note: Over $1,000,000$ sperm cells are required in an ejaculate to ensure successful fertilization of a single egg cell.

  • Centrosomes and Centrioles: Specialized cytoplasmic regions containing paired centrioles that organize the mitotic spindle apparatus and serve as microtubule anchoring sites during cell division.

Questions & Discussion

• Question: Regarding mitochondrial genetic material, is it only the DNA that comes strictly from the mother, or is the RNA maternal as well?

• Response: DNA directly produces RNA through transcription. Therefore, the RNA present inside the mitochondria is produced directly by the maternal mitochondrial DNA, meaning both mitochondrial DNA and mitochondrial RNA are inherited exclusively through the maternal lineage.