BIO 111 - Cell Structure & Function Flashcards

Introduction to Human Anatomy and Physiology

Scientific Disciplines and Scope

• Anatomy: The scientific discipline that investigates the body's structure.
  • Gross (Macroscopic) Anatomy: Examination of structures visible without a microscope.
    • Regional Anatomy: Study of the body area by area (e.g., head, thorax).
    • Systemic Anatomy: Study of the body system by system (e.g., cardiovascular, nervous).
    • Surface Anatomy: Study of external features and their relation to deeper anatomical structures (e.g., anatomical imaging such as X-rays).
  • Microscopic Anatomy: Examination of structures using a microscope.
    • Cytology: Cellular anatomy and structural features of individual cells.
    • Histology: Scientific study of tissues and their surrounding extracellular matrix.
• Physiology: The scientific investigation of the processes or functions of living things.
  • Cell Physiology: Examines the processes occurring within individual cells.
  • Neurophysiology: Focuses on the functions and operational mechanisms of the nervous system.
  • Cardiovascular Physiology: Examines the operational mechanisms of the heart and blood vessels.
• Integrative Disciplines:
  • Pathology: The study of structural and functional changes caused by disease.
  • Applied Physiology (Exercise Physiology): The study of structural and functional changes brought about by exercise and physical work.

Biological Levels of Organization

• Chemical Level: Atoms (e.g., hydrogen atoms, oxygen atom) bond to form molecules with three-dimensional structures (e.g., water molecule).
• Cellular Level: A variety of organic and inorganic molecules combine to form the fluid and specialized organelles of a body cell (e.g., smooth muscle cell).
• Tissue Level: A community of similar cells working together forms a body tissue (e.g., smooth muscle tissue).
• Organ Level: Two or more different tissue types combine to perform specific functions, forming an organ (e.g., urinary bladder containing smooth muscle and epithelial lining).
• Organ System Level: Two or more organs work closely together to perform the functions of a body system (e.g., urinary tract system composed of kidneys, ureters, urinary bladder, and urethra).
• Organismal Level: Many organ systems work harmoniously together to perform all life processes in an independent organism.

Characteristics of Life

• Organization: Specific relationships and structural arrangements among parts of an organism, enabling distinct functional roles.
• Metabolism: The sum total of all chemical reactions occurring within the body.
• Responsiveness: The ability of an organism to sense changes in its internal or external environment and adjust to those changes.
• Growth: An increase in the overall size of an organism, individual cells, or the total number of cells.
• Development: The structural and functional changes an organism undergoes over time.
  • Differentiation: The process wherein generalized stem cells change into specialized cells with specific structures and functions.
• Reproduction: The formation of new cells for tissue growth and repair, or the production of new individual organisms.

Homeostasis and Feedback Mechanisms

• Homeostasis: The maintenance of a dynamic, stable internal environment within physiological limits. Coined by Walter Cannon (1871–1945).
• Set Point and Normal Range:
  • Physiological variables fluctuate around an ideal normal value known as the set point.
  • The normal range represents the allowable upper and lower limits of fluctuation for a variable.
  • Body Temperature: Set point is $37^\text{o}\text{C}$.
  • Blood Glucose Level: Normal resting range is $70{-}110\text{ mg}/100\text{ mL}$.
• Homeostatic Disruptions: Driven by external stimuli (e.g., intense heat, cold, oxygen deprivation) or internal stimuli (e.g., psychological stress, physical exertion). Uncorrected severe disruptions lead to disease or death.

Components of Feedback Systems

• Stimulus: A disruption or deviation away from the normal set point of a physiological variable.
• Receptor (Sensor): A sensory structure that monitors the value of a physiological variable and sends input signals to the control center.
• Control Center: The processing center (e.g., brain) that establishes the set point, analyzes incoming input, and signals an effector.
• Effector: An organ or tissue that receives output signals from the control center and produces a response to alter the physiological variable.
• Response: The action taken by the effector that modifies the status of the controlled variable.

Negative Feedback Systems

• Mechanism: Reverses or negates the initial stimulus to restore a variable to its set point.
• Thermoregulation Example:
  • Stimulus: Body temperature exceeds $37^\text{o}\text{C}$.
  • Receptors: Nerve cells in the skin and brain detect elevated temperature.
  • Control Center: Temperature regulatory center in the brain processes information.
  • Effectors: Sweat glands throughout the body secrete perspiration, dissipating heat and lowering body temperature back toward the set point.
• Blood Pressure Regulation Example:
  • Stimulus: Elevated blood pressure (force exerted by blood against vessel walls).
  • Receptors: Baroreceptors in the walls of major arteries detect stretch and send nerve impulses.
  • Control Center: Brain interprets inputs and signals the cardiovascular system.
  • Effectors: Heart rate decreases and peripheral arterioles dilate, causing blood pressure to decline to normal.

Positive Feedback Systems

• Mechanism: Amplifies or reinforces the original stimulus, driving a variable further away from its baseline until a definitive climax event stops the loop.
• Childbirth (Parturition) Example:
  • Stimulus: Head of the baby pushes against and stretches the uterine cervix.
  • Receptors: Stretch receptors in the cervical wall send nerve impulses to the brain.
  • Control Center: Hypothalamus/pituitary gland secretes oxytocin into the bloodstream.
  • Effector: Smooth muscle in the uterine wall contracts more forcefully.
  • Outcome: Increased contraction pushes the baby further, increasing cervical stretch, triggering additional oxytocin release. The cycle terminates only upon delivery of the baby.

Homeostatic Imbalances

• Disorder: Any general derangement or abnormality of biological structure or function.
• Disease: A specific illness characterized by a recognizable set of clinical signs (observable objective changes) and symptoms (subjective experiences).
  • Local Disease: Affects one specific area or restricted region of the body.
  • Systemic Disease: Affects either the entire body or several organ systems simultaneously.
• Aging: Characterized by a progressive decline in the efficiency of the body's homeostatic restoration processes.

Anatomical Terminology and Body Plan

Anatomical Position and Body Orientations

• Anatomical Position: Standard reference stance: body erect, facing forward, feet parallel and together, upper limbs at the sides with palms facing forward.
• Supine Position: Lying flat on the back, face upward.
• Prone Position: Lying flat on the stomach, face downward.

Directional Terms

• Right / Left: Toward the right or left side of the body.
• Superior (Cephalic): Above or closer to the head relative to another structure.
• Inferior (Caudal): Below or closer to the tail/feet relative to another structure.
• Anterior (Ventral): Toward the front or belly surface of the body.
• Posterior (Dorsal): Toward the back of the body.
• Medial: Closer to the vertical midline of the body.
• Lateral: Farther from the vertical midline of the body.
• Proximal: Closer to the point of attachment of a limb to the trunk.
• Distal: Farther from the point of attachment of a limb to the trunk.
• Superficial: Toward or on the surface of the body.
• Deep: Away from the surface; internal.

Anatomical Body Regions

• Anterior Regions:
  • Cephalic (Head): Frontal (forehead), Orbital (eye), Nasal (nose), Oral (mouth), Mental (chin), Otic (ear), Buccal (cheek).
  • Cervical: Neck.
  • Thoracic (Thorax): Pectoral (chest), Sternal (breastbone), Mammary (breast).
  • Abdominal: Abdomen, Umbilical (navel).
  • Pelvic: Pelvis, Inguinal (groin), Pubic (genitals).
  • Upper Limb: Clavicular (collar bone), Axillary (armpit), Brachial (arm), Antecubital (front of elbow), Antebrachial (forearm), Carpal (wrist), Manual (hand: Palmar [palm], Digital [fingers]).
  • Lower Limb: Coxal (hip), Femoral (thigh), Patellar (kneecap), Crural (leg), Pedal (foot: Talus [ankle], Dorsum [top of foot], Digital [toes]).
• Posterior Regions:
  • Cephalic: Cranial (skull), Occipital (base of skull), Nuchal (back of neck).
  • Trunk/Dorsal: Acromial (point of shoulder), Scapular (shoulder blade), Vertebral (spinal column), Lumbar (loin), Sacral (between hips), Gluteal (buttock), Perineal (perineum).
  • Upper Limb: Olecranon (point of elbow), Dorsum of hand.
  • Lower Limb: Popliteal (hollow behind knee), Sural (calf), Plantar (sole of foot), Calcaneal (heel).

Abdominopelvic Subdivisions

• Nine Abdominopelvic Regions:
  • Superior Tier: Right hypochondriac region, Epigastric region, Left hypochondriac region.
  • Middle Tier: Right lumbar region, Umbilical region, Left lumbar region.
  • Inferior Tier: Right iliac region, Hypogastric region, Left iliac region.
• Four Abdominopelvic Quadrants:
  • Right Upper Quadrant (RUQ), Left Upper Quadrant (LUQ), Right Lower Quadrant (RLQ), Left Lower Quadrant (LLQ).

Body Planes and Sections

• Median (Midsagittal) Plane: Vertical plane passing directly through the midline, dividing the body into equal right and left halves.
• Sagittal Plane: Vertical plane parallel to the median plane, dividing the body into unequal right and left portions.
• Frontal (Coronal) Plane: Vertical plane dividing the body into anterior and posterior sections.
• Transverse (Cross) Plane: Horizontal plane dividing the body into superior and inferior sections.
• Oblique Plane: Cut passing through the body or an organ at an angle other than a right angle.
• Organ Cuts:
  • Longitudinal Section: Cut running along the long axis of an organ.
  • Transverse Section: Cut at a right angle to the long axis of an organ.
  • Oblique Section: Cut at any non-perpendicular angle to the long axis.

Trunk Cavities and Serous Membranes

• Diaphragm: Muscular partition separating the ventral cavity into the superior thoracic cavity and inferior abdominopelvic cavity.
• Mediastinum: Central tissue compartment dividing the thoracic cavity; contains the heart, thymus, esophagus, trachea, and major blood vessels (excluding the lungs).
• Abdominopelvic Cavity: Encircled by abdominal walls, pelvic bones, and muscles; divided into an upper abdominal cavity and lower pelvic cavity.
• Serous Membranes: Double-layered membranes producing lubricating serous fluid:
  • Parietal Layer: Lines the internal surface of cavity walls.
  • Visceral Layer: Covers the outer surface of internal organs (viscera).
  • Serous Cavity: Potential space filled with thin serous fluid located between parietal and visceral layers.
  • Pleura: Serous membranes lining the pleural cavities and covering the lungs.
  • Pericardium: Serous membrane lining the pericardial cavity (parietal pericardium) and covering the heart (visceral pericardium).
  • Peritoneum: Serous membranes lining the abdominopelvic cavity and covering its enclosed abdominal organs.

Chemical Basis of Life

Matter, Atoms, and Atomic Structure

• Matter: Anything that occupies space and has mass.
• Mass: The absolute quantity of matter contained within an object.
• Weight: The gravitational force exerted on an object of a given mass.
• Element: The simplest form of matter possessing unique chemical properties; composed entirely of a single kind of atom.
• Atom: The smallest unit of an element that retains the distinct chemical characteristics of that element.

Subatomic Particles and Atomic Parameters

• Protons: Subatomic particles located in the nucleus with a single positive electrical charge ($+1$).
• Neutrons: Subatomic particles located in the nucleus with no electrical charge ($0$).
• Electrons: Subatomic particles occupying electron shells around the nucleus with a single negative electrical charge ($-1$).
• Nucleus: Dense central core of an atom formed by protons and neutrons; contains nearly all atomic mass.
• Electron Shells: Principal energy levels representing orbitals occupied by electrons around the atomic nucleus.
• Atomic Number: Equals the total number of protons in an atom's nucleus (which equals the number of electrons in a neutral atom).
• Mass Number: The sum of the number of protons plus the number of neutrons in an atom's nucleus.

Isotopes and Radioactivity

• Isotopes: Structural variations of an element that possess the same atomic number (same protons and electrons) but differ in mass number due to different neutron numbers.
• Hydrogen Isotopes:
  • Protium ($^1_1\text{H}$): 1 proton, 0 neutrons, 1 electron.
  • Deuterium ($^2_1\text{H}$): 1 proton, 1 neutron, 1 electron.
  • Tritium ($^3_1\text{H}$): 1 proton, 2 neutrons, 1 electron.
• Atomic Mass: The weighted average mass of all naturally occurring isotopes of an element.
• Radioactive Isotopes (Radioisotopes): Unstable isotopes whose nuclei decay over time, emitting ionizing radiation to reach stability.
  • Cobalt-60: Used in radiation therapy to slow cancer progression.
  • Iodine-131: Clinical agent used to treat hyperthyroidism.
  • Carbon-14: Applied in radiocarbon dating and clinical diagnostic breath tests for detecting Helicobacter pylori (H. pylori), the bacterial cause of peptic ulcers.

Chemical Bonding

• Molecules: Two or more atoms chemically combined to form an independent functional unit (e.g., hydrogen gas, $\text{H}_2$).
• Compounds: Molecules composed of two or more different types of chemically combined atoms (e.g., water, $\text{H}_2\text{O}$).
• Ionic Bonds: Formed by the complete transfer of valence electrons from one atom to another, creating charged ions.
  • Cation: Positively charged ion formed by losing electrons.
  • Anion: Negatively charged ion formed by gaining electrons.
  • Example: Sodium ($\text{Na}$) transfers its single valence electron to Chlorine ($\text{Cl}$), creating an attraction between $\text{Na}^+$ and $\text{Cl}^-$ to yield sodium chloride ($\text{NaCl}$).
• Covalent Bonds: Formed when atoms share pairs of valence electrons. Represents the strongest chemical bond type.
  • Nonpolar Covalent Bond: Equal sharing of electrons between atoms, resulting in an even distribution of electrical charge (e.g., Methane, $\text{CH}_4$).
  • Polar Covalent Bond: Unequal sharing of electrons due to electronegativity differences, causing a slight negative charge (\text{̠}^-) near the electron-attracting atom and a slight positive charge (\text{̠}^+) on the other side (e.g., Water, $\text{H}_2\text{O}$).
• Hydrogen Bonds: Weak electrostatic attractions occurring between a partially positive hydrogen atom in one polar molecule and a partially negative atom (such as oxygen or nitrogen) in another polar molecule. Holds multiple water molecules together.

Chemical Reactions and Energy

• Reactants: Starting substances entering into a chemical reaction.
• Products: Substances produced as a result of a chemical reaction.
• Metabolism: The sum total of all anabolic (synthetic) and catabolic (decomposition) chemical reactions in the body.

Types of Chemical Reactions

• Synthetic (Anabolic) Reactions: Two or more reactants combine to form a larger product ($A + B \rightarrow AB$). Chemical bonds are formed and energy is stored. Responsible for growth, maintenance, and repair.
  • Dehydration Synthesis: A synthetic reaction where water ($\text{H}_2\text{O}$) is removed as a product during molecule assembly (e.g., joining monosaccharides to form disaccharides or polysaccharides).
• Decomposition (Catabolic) Reactions: A larger reactant breaks down into smaller products ($AB \rightarrow A + B$). Chemical bonds are broken, releasing energy.
  • Hydrolysis: The predominant catabolic reaction in the human body, where water ($\text{H}_2\text{O}$) is split into two parts to cleave chemical bonds.
• Reversible Reactions: Reactions that can proceed in either direction (reactants to products or products to reactants).
  • Chemical Equilibrium: State reached when the rate of product formation equals the rate of reactant formation.
  • Bicarbonate Buffer Equation: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

Energy Concepts

• Potential Energy: Stored energy residing within chemical bonds that can perform work upon release.
• Kinetic Energy: Energy actively doing work and moving matter.
• Mechanical Energy: Energy resulting from the position or physical movement of objects.
• Chemical Energy: A form of potential energy stored within chemical bonds.
• Heat Energy: Energy flowing spontaneously between objects of differing temperatures.

Adenosine Triphosphate (ATP)

• Function: The universal chemical energy currency of living cells, powering cellular work such as muscle contraction, nerve impulse transmission, and active biosynthesis.
• Structure: Consists of an adenosine molecule (adenine nitrogenous base bound to a ribose pentose sugar) attached to three inorganic phosphate groups.
• Energy Storage and Release: Energy is stored within high-energy phosphate bonds. Cleavage of the terminal phosphate bond releases free energy:

$\text{ATP} \rightleftharpoons \text{ADP} + \text{P}_i + \text{Energy}$

Reaction Dynamics and Catalysts

• Factors Influencing Reaction Rates:
  • Temperature: Increasing temperature increases kinetic energy, causing molecules to move faster and collide more frequently and forcefully.
  • Concentration: Higher reactant concentration increases collision frequency, accelerating reaction rates.
  • Catalysts: Substances that increase chemical reaction rates without being permanently consumed or altered.
• Enzymes: Biological protein catalysts that accelerate chemical reactions by lowering the activation energy (the minimum energy input required for reactants to initiate a chemical reaction).

Water, Acids, Bases, and pH

Functions of Water

• High Specific Heat: Requires a large input of heat energy to raise its temperature, helping stabilize internal body temperature.
• Protection: Acts as a fluid lubricant and shock-absorbing cushion around internal organs.
• Chemical Medium: Serves as the primary solvent wherein metabolic reactions occur, participating directly in dehydration synthesis and hydrolysis.
• Mixing Medium: Acts as a biological solvent facilitating substance transport.

Acids, Bases, Salts, and Buffers

• Acid: A proton ($\text{H}^+$) donor; any substance that releases hydrogen ions in solution.
• Base: A proton ($\text{H}^+$) acceptor; any substance that binds or accepts hydrogen ions.
• Salt: An ionic compound containing a cation other than $\text{H}^+$ and an anion other than $\text{OH}^-$ (e.g., $\text{NaCl}$).
• Buffer: A chemical system consisting of a weak acid and its conjugate base present in similar concentrations that resists drastic changes in pH.

The pH Scale

• Definition: Logarithmic measure of hydrogen ion concentration in a solution ($\text{pH} = -\text{log}[\text{H}^+]$).
• Neutral Solution: $\text{pH} = 7.0$ ($[\text{H}^+] = [\text{OH}^-] = 10^{-7}\text{ mol/dm}^3$; e.g., distilled water).
• Acidic Solution: $\text{pH} < 7.0$ ($[\text{H}^+] > [\text{OH}^-]$).
  • Hydrochloric acid ($\text{HCl}$): $\text{pH } 0$
  • Stomach acid: $\text{pH } 1$
  • Lemon juice: $\text{pH } 2$
  • Vinegar, cola, beer: $\text{pH } 3$
  • Tomatoes: $\text{pH } 4$
  • Black coffee: $\text{pH } 5$
  • Urine / Saliva: $\text{pH } 6.0{-}6.5$
• Alkaline (Basic) Solution: $\text{pH} > 7.0$ ($[\text{OH}^-] > [\text{H}^+]$).
  • Blood: $\text{pH } 7.4$
  • Seawater: $\text{pH } 8$
  • Baking soda: $\text{pH } 9$
  • Great Salt Lake: $\text{pH } 10$
  • Household ammonia: $\text{pH } 11$
  • Soda ash: $\text{pH } 12$
  • Oven cleaner: $\text{pH } 13$
  • Sodium hydroxide ($\text{NaOH}$): $\text{pH } 14$

Organic Macromolecules

Carbohydrates

• Composition: Composed of Carbon, Hydrogen, and Oxygen.
• Monosaccharides: Simple sugars.
  • Hexoses (6-Carbon Sugars): Glucose, Fructose, and Galactose. Function as primary energy substrates. Glucose and Fructose are structural isomers; Glucose and Galactose are stereoisomers.
  • Pentoses (5-Carbon Sugars): Ribose and Deoxyribose. Structural components of nucleic acids (RNA, DNA) and ATP.
• Disaccharides: Two monosaccharides joined via dehydration synthesis.
  • Sucrose: Glucose + Fructose.
  • Lactose: Glucose + Galactose.
  • Maltose: Glucose + Glucose.
• Polysaccharides: Long polymers of monosaccharides linked by glycosidic bonds.
  • Glycogen: Highly branched storage polysaccharide synthesized by animals, stored in liver and skeletal muscle.
  • Starch: Storage carbohydrate formed by plants; digested by humans for glucose.
  • Cellulose: Structural polysaccharide formed by plants; indigestible to humans, serving as dietary fiber (bulk).

Lipids

• Composition: Composed primarily of Carbon, Hydrogen, and Oxygen; insoluble in water (hydrophobic).
• Neutral Fats (Triglycerides): Composed of one glycerol backbone attached to three fatty acid chains via ester bonds formed by dehydration synthesis. Functions: energy storage, thermal insulation, structural cushioning.
• Phospholipids: Structural lipids composed of a glycerol backbone, two hydrophobic nonpolar fatty acid tails, and one polar hydrophilic phosphate-containing head group. Essential structural component of cellular membranes.
• Steroids: Lipids characterized by four fused carbon rings derived from cholesterol. Water-insoluble hydrophobic regulators.
  • Examples: Cholesterol (membrane component), Bile salts (glycocholate for lipid digestion), Estrogen (estradiol), Testosterone.

Proteins

• Composition: Composed of Carbon, Hydrogen, Oxygen, Nitrogen, and occasionally Iodine or Sulfur.
• Building Blocks: Amino acids linked covalently by peptide bonds formed via dehydration synthesis.
• Functions: Enzymatic catalysis, structural support, muscular contraction, metabolic regulation (e.g., insulin), physiological transport, and immunological defense.
• Structural Organization:
  • Primary Structure: Linear sequence of amino acids in a polypeptide chain.
  • Secondary Structure: Localized folding and bending stabilized by hydrogen bonds ($\text{α}$-helices and $\text{β}$-pleated sheets).
  • Tertiary Structure: Overall three-dimensional folding of a single polypeptide chain.
  • Quaternary Structure: Spatial association of two or more polypeptide subunits into a functional protein complex.

Nucleic Acids and Gene Expression

• Composition: Composed of Carbon, Hydrogen, Oxygen, Nitrogen, and Phosphorus.
• Nucleotide Structure: Each nucleotide monomer contains a pentose sugar, a phosphate group, and a nitrogenous base.
• Nitrogenous Bases:
  • Purines (Two Rings): Adenine (A), Guanine (G).
  • Pyrimidines (Single Ring): Cytosine (C), Thymine (T - DNA only), Uracil (U - RNA only).
• Deoxyribonucleic Acid (DNA): Double-stranded helical molecule containing deoxyribose sugar and bases A, T, C, G. Serves as genetic material.
• Ribonucleic Acid (RNA): Single-stranded nucleic acid containing ribose sugar and bases A, U, C, G. Responsible for carrying instructions from DNA to synthesize proteins.

DNA Replication

• Process: Occurs during the S phase of interphase. Unwinds double helix and creates mirror copies along old strands.
• Enzyme: DNA polymerase synthesizes new strands by adding complementary nucleotides in the $5'$ to $3'$ direction.
• Mechanism:
  • Initiates at specific origins of replication.
  • Leading Strand: Synthesized continuously toward the replication fork from a single RNA primer.
  • Lagging Strand: Synthesized discontinuously away from the replication fork in short segments called Okazaki fragments, each requiring an RNA primer.

Protein Synthesis: Transcription and Translation

• Transcription: Process occurring in the nucleus where a DNA gene sequence is transcribed into a messenger RNA (mRNA) transcript by RNA polymerase.
  • RNA polymerase binds to a promoter region, reads the DNA template strand, and synthesizes a complementary single-stranded RNA transcript until reaching a termination signal.
  • DNA base triplets are transcribed into mRNA codons (3-nucleotide sequences).
• Genetic Code:
  • 61 codons code for specific amino acids.
  • Start Codon: $\text{AUG}$ (codes for Methionine).
  • Stop Codons: $\text{UAA}$, $\text{UAG}$, $\text{UGA}$ (signal termination of polypeptide synthesis).
• Translation: Process occurring at the ribosome in the cytoplasm where mRNA codons are translated into an amino acid sequence.
  • Transfer RNA (tRNA) molecules carry specific amino acids and possess complementary anticodons that pair with mRNA codons.
  • Ribosome shifts mRNA through E, P, and A sites, forming peptide bonds between amino acids to build a polypeptide.

Cell Structure and Function

Overview and Parts of the Cell

• Cellular Diversity: Human body contains roughly $100\text{ trillion}$ cells categorized into approximately $200$ distinct cell types with varied shapes and sizes tailored to specific functions.
• Functions of the Cell: Basic structural and functional unit of life, provides protection and support, facilitates movement, permits intercellular communication, regulates cellular metabolism and energy release, passes on genetic inheritance.
• Three Principal Parts of a Cell:
  1. Plasma (Cell) Membrane: Flexible outer envelope surrounding cytoplasm.
  2. Cytoplasm: Cellular material between plasma membrane and nucleus.
     • Cytosol: Fluid portion containing water, ions, and dissolved solutes.
     • Organelles: Specialized cellular structures (excluding the nucleus).
  3. Nucleus: Membrane-bound organelle storing genetic material.

The Plasma Membrane

• Fluid Mosaic Model: Describes the plasma membrane as a sea of fluid lipids containing a mosaic of diverse proteins.
• Lipid Bilayer Composition: Two back-to-back layers made of three lipid types:
  • Phospholipids: Form a double row; amphipathic molecules with hydrophilic (water-attracting) polar heads facing outward and hydrophobic (water-repelling) nonpolar fatty acid tails facing inward.
  • Cholesterol: Molecules interspersed among phospholipids to stabilize membrane fluidity.
  • Glycolipids: Lipids with attached carbohydrate chains exposed to extracellular fluid.

Membrane Proteins

• Integral Proteins: Extend deep into or completely across the lipid bilayer among the hydrophobic fatty acid tails; many are transmembrane proteins.
• Peripheral Proteins: Attached loosely to the inner or outer membrane surfaces; can be removed without compromising membrane integrity.

• Functions of Membrane Proteins:
  • Ion Channels (Integral): Water-filled pores allowing specific ions to cross the membrane.
  • Transporters / Carriers (Integral): Bind specific substances, undergo conformational shape changes, and move them across the membrane.
  • Receptors (Integral): Cellular recognition sites that bind specific extracellular molecules (ligands; e.g., antidiuretic hormone) to alter cell function.
  • Enzymes (Integral and Peripheral): Catalyze chemical reactions at the inner or outer cell surface (e.g., lactase on intestinal epithelial cells).
  • Linkers (Integral and Peripheral): Anchor filaments inside and outside the plasma membrane to provide structural stability and shape, participate in cell movement, or link adjacent cells.
  • Cell Identity Markers (Glycoproteins): Distinguish self cells from foreign cells (e.g., Major Histocompatibility Complex [MHC] proteins).

Cytoplasmic and Organellar Structures

• Microvilli: Non-motile extensions of the plasma membrane containing actin filaments. Size is $1/10$ to $1/20$ that of cilia. Function to increase cell surface area for absorption.
• Ribosomes: Non-membrane-bound sites of protein synthesis. Composed of ribosomal RNA (rRNA) and ribosomal proteins. Assembled into small and large subunits within the nucleolus and exported through nuclear pores into the cytoplasm.
• Endoplasmic Reticulum (ER):
  • Rough ER: Outer surface studded with attached ribosomes; synthesizes and chemically modifies proteins destined for membranes, organelles, or secretion.
  • Smooth ER: Lacks ribosomes; synthesizes lipids, phospholipids, and steroids.
• Golgi Apparatus: Stack of flattened membranous sacs (cisternae). Modifies, sorts, packages, and distributes proteins and lipids from the ER into secretory vesicles for exocytosis or internal cellular use.
• Lysosomes: Membrane-bound digestive vesicles formed by the Golgi apparatus. Contain hydrolytic enzymes that break down endocytosed vesicles, worn-out organelles, and cellular debris.
• Peroxisomes: Small membrane-bound vesicles containing enzymes (oxidases) that break down fatty acids and amino acids; produce toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) as a byproduct, which is degraded by catalase.
• Proteasomes: Tiny cylindrical complexes containing protein-degrading enzymes that break down and recycle unneeded, damaged, or misfolded cellular proteins.
• Mitochondria: Double-membrane powerhouses responsible for cellular respiration and major ATP synthesis. Feature an outer membrane, inner membrane folded into cristae, and an intermembrane space. Contain independent mitochondrial DNA coding for mitochondrial proteins; increase in density in response to high metabolic demand.
• Cytoskeleton: Structural protein network maintaining cell shape, enabling motility, and aiding cell division:
  • Microtubules: Hollow cylinders ($25\text{ nm}$ diameter) composed of tubulin protein dimers.
  • Microfilaments: Thin strands ($7\text{ nm}$ diameter) composed of actin protein subunits.
  • Intermediate Filaments: Fibrous protein cables ($8{-}12\text{ nm}$ diameter; e.g., keratins) providing mechanical strength.

Cell Cycle, Mitosis, and Apoptosis

Interphase

• Definition: Non-dividing metabolic phase between cell divisions.
  • $\text{G}_1\text{ Phase (First Gap Phase):}$ Cytoplasmic growth and metabolic activity. Cells that permanently cease dividing enter the $\text{G}_0\text{ phase}$.
  • $\text{S Phase (Synthesis Phase):}$ Replication of nuclear DNA and centrosomes.
  • $\text{G}_2\text{ Phase (Second Gap Phase):}$ Final cytoplasmic growth and protein synthesis preparing for nuclear division.
• Chromosomes: Human somatic cells contain $46$ chromosomes ($23$ homologous pairs). During S phase, DNA content doubles. Replicated chromosomes consist of two identical sister chromatids joined at a central region called the centromere (containing protein kinetochores).

Mitotic Phase

• Prophase: Chromatin condenses into visible chromosomes; centrosomes move to opposite poles; mitotic spindle fibers emerge; nuclear envelope breaks down.
• Prometaphase: Chromosomes continue condensing; kinetochores appear at centromeres; spindle microtubules attach to kinetochores.
• Metaphase: Chromosomes align along the equatorial metaphase plate; each sister chromatid attaches to spindle fibers originating from opposite poles.
• Anaphase: Centromeres split; sister chromatids (now individual chromosomes) are pulled toward opposite poles; non-kinetochore spindle fibers elongate the cell.
• Telophase: Chromosomes reach opposite poles and decondense; nuclear envelope reforms around each chromosome set; mitotic spindle dissolves.
• Cytokinesis: Cytoplasmic division into two identical daughter cells. Forms a cleavage furrow in animal cells and a cell plate in plant cells.

Control of Cell Destiny

• Cellular Destinies: Remain alive and function without dividing, grow and divide, or undergo cell death.
• Maturation Promoting Factor (MPF): Protein complex that induces nuclear cell division.
• Apoptosis: Programmed, non-inflammatory cell suicide triggered by internal genetic programs or external signals.
• Necrosis: Uncontrolled pathological cell death resulting from tissue injury, disease, or severe hypoxia.
• Tumor-Suppressor Genes: Genes producing proteins that inhibit cell division, preventing uncontrolled cancerous growth.

Reproductive Cell Division: Meiosis

• Definition: Reproductive cell division occurring in gonads producing four haploid gametes containing $23$ chromosomes ($n$).
• Meiosis I (Reductional Division):
  • Prophase I: Synapsis of homologous chromosomes forms tetrads; crossing-over between non-sister chromatids occurs, leading to genetic recombination.
  • Metaphase I: Tetrads align along the metaphase plate.
  • Anaphase I: Homologous chromosome pairs separate and move to opposite poles (sister chromatids remain together).
  • Telophase I: Two haploid daughter cells form, each containing one set of replicated chromosomes.
• Meiosis II (Equational Division): Similar to mitosis (Prophase II, Metaphase II, Anaphase II [sister chromatids separate], Telophase II), yielding four genetically unique haploid daughter cells.

Histology: Human Tissues

Intercellular Junctions

• Tight Junctions: Watertight seals formed by fused strands of transmembrane proteins surrounding adjacent plasma membranes. Prevent fluid leakage between cells. Found lining the gastrointestinal tract and urinary bladder.
• Adherens Junctions: Contain plaque and transmembrane glycoproteins (cadherins) connected to microfilaments, forming continuous adhesion belts holding epithelial cells together.
• Desmosomes: Structural plaques attached to intermediate keratin filaments that cross the intercellular space, resisting cellular separation under mechanical stress. Provide support in cardiac muscle and epidermis.
• Hemidesmosomes: