Comprehensive Study Notes on Human Anatomy, Biochemistry, and Cytology

Fundamental Concepts of Human Organization & Homeostasis

Anatomy vs. Physiology:
- Anatomy is the scientific study of physical body structures, forms, and their relationships. It is subdivided into:
  - Gross Anatomy: Examination of structures visible to the naked eye.
  - Systemic Anatomy: Study of specific organ systems (e.g., digestive or cardiovascular system).
  - Relational (Topographical) Anatomy: Study of how anatomical structures relate to one another in spatial position (e.g., the superior vena cava situated superior to the heart).
  - Microscopic Anatomy: Study of structures requiring magnification, including cytology (cells) and histology (tissues).
- Physiology is the study of the biological, mechanical, and biochemical functions of living systems. It is subdivided into:
  - Pathophysiology: Study of functional changes associated with or resulting from disease or trauma.
  - Immunology: Functional operation of the body's immune defense systems.
  - Cardiovascular Physiology: Functional mechanics of the heart and blood vessels.
  - Specialized Branch Physiology: Includes renal, respiratory, and musculoskeletal physiology.
Levels of Structural Organization:
- Chemical Level: Includes atoms—the smallest units of matter such as Carbon ( $\text{C}$ ), Hydrogen ( $\text{H}$ ), Oxygen ( $\text{O}$ ), Nitrogen ( $\text{N}$ ), Phosphorus ( $\text{P}$ ), and Sulfur ( $\text{S}$ )—and molecules, which consist of two or more atoms bound chemically (e.g., deoxyribonucleic acid, $\text{DNA}$ ).
- Cellular Level: Molecules combine to form cells, the basic structural and functional living units of the human body (e.g., smooth muscle cells).
- Tissue Level: Groups of specialized cells and the extracellular material surrounding them work together to perform a specific collective function. The four primary tissue types are epithelial, connective, nervous, and muscular tissue.
- Organ Level: Structures composed of two or more different tissue types that have specific functions and recognizable shapes (e.g., the stomach, which contains an outer serous membrane, smooth muscle tissue layers, and an inner epithelial tissue lining).
- Organ-System Level: Consists of related organs sharing a common physiological function (e.g., the digestive system, comprising the esophagus, liver, stomach, pancreas, gallbladder, small intestine, and large intestine). The human body contains 11 organ systems: Integumentary, Skeletal, Muscular, Nervous, Endocrine, Cardiovascular, Lymphatic, Respiratory, Digestive, Urinary, and Reproductive.
- Organismal Level: The highest functional level, representing any living individual as a unified whole.
Essential Characteristics of Life:
- Metabolism: The sum of all chemical reactions occurring within the body to obtain energy and build cellular structures.
- Responsiveness: The body's ability to detect and respond to internal or external environmental changes.
- Movement: Motion of the whole body, individual organs, single cells, or structures inside cells.
- Growth: An increase in body size resulting from an increase in the number of cells, the size of existing cells, or the amount of extracellular material.
- Digestion: The breakdown of ingested food molecules into simpler sub-units that can be absorbed and utilized for energy.
- Reproduction: The formation of new cells for tissue growth, repair, or replacement, or the production of a new individual sexually or asexually.
- Excretion: The elimination of metabolic waste products from the body.
Survival Requirements for Human Life:
- Sufficient nutrients, adequate oxygen ( $\text{O}_2$ ), abundant water ( $\text{H}_2\text{O}$ ), a stable internal environment, and an appropriate external environment.
Body Fluids & Compartments:
- Intracellular Fluid (ICF) / Cytosol: Fluid enclosed strictly within the plasma membranes of cells.
- Extracellular Fluid (ECF): Fluid located outside body cells. Subdivided into:
  - Interstitial Fluid: ECF filling the narrow spaces between tissue cells.
  - Blood Plasma: ECF located within blood vessels.
  - Other ECF Compartments: Includes lymph, cerebrospinal fluid (CSF), and the aqueous humor of the eyes.
Homeostasis & Regulatory Systems:
- Definition: Homeostasis is a state of dynamic equilibrium in the body's internal environment maintained by regulatory processes (e.g., blood glucose concentration is maintained strictly within a physiological range of $70 - 110\,\text{mg}/100\,\text{mL}$ ).
- Regulation: Controlled primarily by the nervous system and endocrine system operating independently or synergistically:
  - Nervous System: Detects physiological shifts and transmits rapid electrical nerve impulses to counteract changes.
  - Endocrine System: Secretes chemical messengers called hormones into the bloodstream for slower, longer-lasting adaptations.
Feedback System Mechanics:
- A feedback system continuously monitors, evaluates, and adjusts controlled physiological conditions. It consists of three structural components:
  1. Receptor: A sensory structure that monitors changes in a controlled condition and sends input (nerve impulses or chemical signals) to a control center.
  2. Control Center: Sets the acceptable physiological range/set-point, evaluates input received from receptors, and generates output commands when necessary.
  3. Effector: A body structure that receives output commands from the control center and produces a response that alters the controlled condition.
- Negative Feedback Systems:
  - Reverses or negates a change in a controlled condition to restore it to its normal set-point.
  - Example: An elevated blood pressure stimulus triggers baroreceptors (receptors) in blood vessel walls to send nerve impulses (input) to the brain (control center). The brain interprets the input and sends nerve impulses (output) to the heart (effector), causing a decrease in heart rate, which lowers blood pressure back to normal.
- Positive Feedback Systems:
  - Strengthens or reinforces a change in a controlled condition until an outside event interrupts the cycle.
  - Example: Uterine contractions force the baby's head into the cervix, increasing cervical stretching. Stretch-sensitive nerve cells (receptors) in the cervix send nerve impulses (input) to the brain (control center). The brain releases oxytocin (output) into the blood, causing uterine wall muscles (effectors) to contract more forcefully. This pushes the baby further, stretching the cervix even more. The birth of the baby halts cervical stretching, interrupting the positive feedback cycle.
Homeostatic Imbalances:
- Disorder: Any abnormality of bodily structure or function.
- Disease: A specific illness characterized by a recognizable set of signs and symptoms.
- Signs: Objective, measurable physiological changes observed or evaluated by a clinician (e.g., fever, elevated blood pressure, skin rash).
- Symptoms: Subjective changes experienced by a patient that cannot be directly measured or observed by a clinician (e.g., headache, nausea, anxiety).

Anatomical Language, Body Planes, Cavities, and Regions

Anatomical Position:
- A standardized stance used for precise anatomical reference. The subject stands erect facing the observer, head level, eyes facing directly forward, feet flat on the floor, upper limbs positioned at the sides, and palms turned forward (ventral).
Reclining Positions:
- Prone: The body lies face down.
- Supine: The body lies face up.
Directional Terms:
- Superior (Cranial): Toward the head or upper part of a structure (e.g., the eyes are superior to the mouth).
- Inferior (Caudal): Away from the head or toward the lower part of a structure (e.g., the stomach is inferior to the heart).
- Anterior (Ventral): Nearer to or at the front of the body (e.g., the sternum is anterior to the heart).
- Posterior (Dorsal): Nearer to or at the back of the body (e.g., the brain is posterior to the forehead).
- Medial: Nearer to the midline of the body (e.g., the heart is medial to the lungs).
- Lateral: Farther from the midline of the body (e.g., the thumb is on the lateral side of the hand).
- Proximal: Nearer to the attachment of a limb to the trunk or origin of a structure (e.g., the knee is proximal to the ankle).
- Distal: Farther from the attachment of a limb to the trunk (e.g., the wrist is distal to the elbow).
Planes and Sections:
- Sagittal Plane: A vertical plane dividing the body or an organ into right and left sides.
  - Midsagittal (Median) Plane: Passes directly through the midline, producing equal left and right halves.
  - Parasagittal Plane: Divides the body or an organ into unequal left and right sides.
- Frontal (Coronal) Plane: Divides the body or an organ into anterior (front) and posterior (back) portions.
- Transverse (Horizontal / Cross-Sectional) Plane: Divides the body or an organ into superior (upper) and inferior (lower) portions.
- Oblique Plane: Passes through the body or an organ at an angle between a transverse plane and a sagittal or frontal plane.
Body Cavities & Serous Membranes:
- Dorsal Body Cavity: Located near the posterior surface of the body. Enclosed by protective tissue membranes called meninges. Subdivided into:
  - Cranial Cavity: Formed by cranial bones; houses the brain.
  - Vertebral (Spinal) Canal: Formed by the vertebral column; contains the spinal cord.
- Ventral Body Cavity: Located near the anterior surface of the body. Organs inside are called viscera. Divided by the dome-shaped diaphragm muscle into:
  - Thoracic Cavity: Superior cavity enclosed by ribs, sternum, vertebral column, and muscles. Contains:
    - Pleural Cavities: Two fluid-filled spaces, each surrounding one lung.
    - Mediastinum: Broad median partition extending from the sternum to the vertebral column between the lungs. Contains all thoracic viscera except the lungs themselves (heart, aorta, esophagus, trachea, thymus).
    - Pericardial Cavity: Fluid-filled space within the mediastinum surrounding the heart.
  - Abdominopelvic Cavity: Inferior cavity extending from the diaphragm to the groin. Subdivided into:
    - Abdominal Cavity: Superior portion containing the stomach, spleen, liver, gallbladder, pancreas, small intestine, and most of the large intestine.
    - Pelvic Cavity: Inferior portion containing the urinary bladder, internal reproductive organs, and portions of the large intestine.
- Serous Membranes: Double-layered, slippery membranes that line ventral body cavities and cover internal viscera:
  - Parietal Layer: Lines the inner walls of the body cavities.
  - Visceral Layer: Covers and adheres to the outer surface of the viscera within the cavities.
  - Serous Fluid: Thin lubricating fluid secreted between the parietal and visceral layers to reduce mechanical friction during movement.
  - Specific Serous Membranes:
    - Pleura: Visceral pleura covers lungs; parietal pleura lines the thoracic chest wall.
    - Pericardium: Visceral pericardium covers the heart surface; parietal pericardium lines the pericardial sac.
    - Peritoneum: Visceral peritoneum covers abdominal viscera; parietal peritoneum lines the abdominal wall.
Abdominopelvic Regions and Quadrants:
- Nine Regions: Established by drawing four imaginary lines (two vertical midclavicular lines, an upper horizontal transpyloric line, and a lower horizontal transtubercular line):
  1. Right Hypochondriac Region
  2. Epigastric Region
  3. Left Hypochondriac Region
  4. Right Lumbar Region
  5. Umbilical Region
  6. Left Lumbar Region
  7. Right Inguinal (Iliac) Region
  8. Hypogastric (Pubic) Region
  9. Left Inguinal (Iliac) Region
- Four Quadrants: Established by passing vertical and horizontal lines directly through the umbilicus (navel):
  - Right Upper Quadrant (RUQ)
  - Left Upper Quadrant (LUQ)
  - Right Lower Quadrant (RLQ)
  - Left Lower Quadrant (LLQ)

Chemical Level of Organization & Biochemistry

Subatomic Particles & Atomic Structure:
- An atom consists of three primary subatomic particles:
  - Protons ( $p^+$ ): Positively charged particles located within the central nucleus.
  - Neutrons ( $n^0$ ): Uncharged (neutral) particles located within the central nucleus.
  - Electrons ( $e^-$ ): Negatively charged particles orbiting the nucleus in electron shells/clouds.
- Atomic Models: The Planetary Model depicts electrons in distinct concentric orbital rings; the Orbital Model depicts electrons as a probability cloud surrounding the central nucleus.
Major Body Elements:
- The principal elements building human structures include Carbon ( $\text{C}$ ), Hydrogen ( $\text{H}$ ), Oxygen ( $\text{O}$ ), Nitrogen ( $\text{N}$ ), Phosphorus ( $\text{P}$ ), and Sulfur ( $\text{S}$ ).
- Essential biological ions and trace elements include Sodium ( $\text{Na}$ ), Magnesium ( $\text{Mg}$ ), Potassium ( $\text{K}$ ), Calcium ( $\text{Ca}$ ), Iron ( $\text{Fe}$ ), Chlorine ( $\text{Cl}$ ), and Iodine ( $\text{I}$ ).
Isotopes and Ions:
- Isotopes: Atoms of a single element that possess identical proton numbers but differ in neutron numbers, resulting in different atomic masses (e.g., Oxygen isotopes: $^{16}\text{O}, ^{17}\text{O}, ^{18}\text{O}$ ; Hydrogen isotopes: $^1\text{H}, ^2\text{H}, ^3\text{H}$ ; Carbon isotopes: $^{12}\text{C}, ^{14}\text{C}$ ).
- Ions: Charged particles formed when an atom loses or gains one or more valence electrons.
  - Cation: A positively charged ion formed when an atom loses electrons.
  - Anion: A negatively charged ion formed when an atom gains electrons.
  - Common Biological Cations: Hydrogen ion ( $\text{H}^+$ ), Sodium ion ( $\text{Na}^+$ ), Potassium ion ( $\text{K}^+$ ), Ammonium ion ( $\text{NH}_4^+$ ), Hydronium ion ( $\text{H}_3\text{O}^+$ ), Magnesium ion ( $\text{Mg}^{2+}$ ), Calcium ion ( $\text{Ca}^{2+}$ ), Iron (II) ion ( $\text{Fe}^{2+}$ ), Iron (III) ion ( $\text{Fe}^{3+}$ ).
  - Common Biological Anions: Fluoride ion ( $\text{F}^-$ ), Chloride ion ( $\text{Cl}^-$ ), Iodide ion ( $\text{I}^-$ ), Hydroxide ion ( $\text{OH}^-$ ), Nitrate ion ( $\text{NO}_3^-$ ), Bicarbonate ion ( $\text{HCO}_3^-$ ), Oxide ion ( $\text{O}^{2-}$ ), Sulfate ion ( $\text{SO}_4^{2-}$ ), Phosphate ion ( $\text{PO}_4^{3-}$ ).
Molecules, Compounds, & Valence Shell Rules:
- Molecule: Two or more identical or different atoms joined together by chemical bonds (e.g., $\text{H}_2, \text{O}_2, \text{CO}_2, \text{C}_6\text{H}_{12}\text{O}_6$ ).
- Compound: A substance containing molecules composed of two or more different elements (e.g., $\text{H}_2\text{O}, \text{NH}_3$ ).
- Valence Shell: The outermost electron shell of an atom. Chemical bonding likelihood depends on electron vacancies in this shell.
- Octet Rule: Atoms containing incompletely filled valence shells tend to react chemically to produce a stable arrangement of 8 valence electrons (or 2 electrons in the first shell).
- Shell Capacities: The 1st shell holds up to 2 $e^-$ ; the 2nd shell holds up to 8 $e^-$ ; the 3rd shell holds up to 8 reactive $e^-$ (e.g., Magnesium has 12 $p^+$ and 12 $e^-$ : 2 in the 1st shell, 8 in the 2nd, and 2 reactive valence $e^-$ in the 3rd).
Chemical Bonding Types:
- Ionic Bonds: Formed by the complete transfer of electrons from a cation to an anion. The resulting oppositely charged particles associate electrostatically (e.g., large aggregates of $\text{Na}^+$ and $\text{Cl}^-$ form $\text{NaCl}$ salt crystals). Dissociated ionic compounds in aqueous solution are called electrolytes.
- Covalent Bonds: Formed when atoms share one, two, or three pairs of valence electrons without actual electron loss or gain. Covalent bonds represent the strongest chemical bonds in the body.
  - Nonpolar Covalent Bonds: Shared electrons are distributed equally between atoms; charge is balanced symmetrically across the molecule (e.g., $\text{H}_2, \text{O}_2, \text{N}_2, \text{CH}_4, \text{CO}_2$ ).
  - Polar Covalent Bonds: Shared electrons are pulled unequally due to differences in electronegativity, creating a slight negative charge ( $\delta^-$ ) at one end and a slight positive charge ( $\delta^+$ ) at the other (e.g., $\text{H}_2\text{O}$ , where the electronegative oxygen atom pulls shared electrons away from hydrogen atoms).
- Hydrogen Bonds: Weak attractive forces between a partially electropositive hydrogen atom ( $\delta^+$ ) of one polar molecule and an electronegative atom ( $\delta^-$ ) of another molecule. These bonds create surface tension in water and act as intramolecular bonds holding large proteins and $\text{DNA}$ in precise three-dimensional shapes.
Chemical Reactions & Bioenergetics:
- Law of Conservation of Mass: The total mass of all reactants in a chemical reaction equals the total mass of the resulting products.
- Metabolism: The sum of all anabolic and catabolic chemical reactions in the body.
- Exergonic Reactions: Reactions in which the chemical bonds broken contain more potential energy than the bonds formed, resulting in a net release of energy (usually as heat during food catabolism).
- Endergonic Reactions: Reactions requiring a net energy input (usually driven by $\text{ATP}$ hydrolysis) to form new bonds (e.g., binding amino acids together to build proteins).
- Activation Energy: The collision energy required to break chemical bonds and initiate a reaction.
- Catalysts / Enzymes: Biological substances that speed up chemical reactions by significantly lowering the required activation energy without being altered themselves.
Classifying Chemical Reactions:
- Synthesis (Anabolic) Reactions: Two or more atoms or molecules combine to synthesize larger, complex molecules ( $A + B \rightarrow AB$ ); typically endergonic.
- Decomposition (Catabolic) Reactions: Large molecules are split into smaller atoms, ions, or molecules ( $AB \rightarrow A + B$ ); typically exergonic.
- Exchange (Displacement) Reactions: Consist of both decomposition and synthesis; bonds are both broken and formed ( $AB + C \rightarrow AC + B$ ; e.g., $\text{ATP} + \text{Glucose} \rightarrow \text{ADP} + \text{Glucose-phosphate}$ ).
Inorganic vs. Organic Compounds:
- Inorganic Compounds: Structurally simple molecules that usually lack carbon-carbon or carbon-hydrogen bonds (e.g., $\text{H}_2\text{O}, \text{NaCl}$ ; exceptions containing carbon include $\text{CO}_2$ and $\text{H}_2\text{CO}_3$ ).
- Organic Compounds: Always contain carbon and hydrogen, usually contain oxygen, and are held together almost entirely by covalent bonds (e.g., carbohydrates, lipids, proteins, nucleic acids).
- Functional Groups: Reactive atom clusters attached to carbon backbones including Hydroxyl ( $-\text{OH}$ ), Amino ( $-\text{NH}_2$ ), Carboxyl ( $-\text{COOH}$ ), and Phosphate ( $-\text{PO}_4^{2-}$ ) groups.
Properties & Roles of Water ( $\text{H}_2\text{O}$ ):
- Polar Solvent: Dissolves polar covalent and ionic substances (hydrophilic), while repelling nonpolar substances (hydrophobic).
- High Heat Capacity: Absorbs or releases large amounts of heat energy with minimal changes in its own temperature due to hydrogen bonding, maintaining body temperature homeostasis.
- High Heat of Vaporization: Requires significant heat energy to evaporate; thermal energy is carried away when sweat evaporates from skin surfaces.
- Lubricant & Cushion: Reduces physical friction between organs (found in mucus, synovial joint fluid, and serous fluids).
- Metabolic Reactant: Acts as a direct reactant in Hydrolysis (catabolic splitting of polymers by adding water) and a product in Dehydration Synthesis (anabolic linking of monomers by removing water).
Classes of Organic Biomolecules:
- Carbohydrates: Contain Carbon, Hydrogen, and Oxygen in a $1:2:1$ ratio [ $(\text{CH}_2\text{O})_n$ ]. Serve as cellular fuel sources (glucose) and structural units (ribose in $\text{RNA}$ ).
  - Monosaccharides: Simple sugars containing 3 to 7 Carbon atoms (Glucose, Fructose, Galactose, Ribose, Deoxyribose).
  - Disaccharides: Two monosaccharides linked by dehydration synthesis:
    - $\text{Sucrose} = \text{Glucose} + \text{Fructose}$
    - $\text{Maltose} = \text{Glucose} + \text{Glucose}$
    - $\text{Lactose} = \text{Glucose} + \text{Galactose}$
  - Polysaccharides: Large polymers containing hundreds of monosaccharides. The primary human storage polysaccharide is Glycogen (stored in liver and skeletal muscle tissue; hydrolyzed to release glucose when blood sugar drops).
- Lipids: Hydrophobic biomolecules containing Carbon, Hydrogen, and Oxygen (lacking a $2:1$ ratio of $\text{H}$ to $\text{O}$ ). Insoluble in polar water; bound to proteins (lipoproteins) for transport in blood.
  - Triglycerides: Neutral fats composed of a single 3-carbon glycerol molecule and 3 fatty acid chains bound by ester linkages. Functions include protection, insulation, and energy storage. Fatty acids may be Saturated (no double bonds) or Unsaturated/Monounsaturated (containing double bonds).
  - Phospholipids: Amphipathic molecules composed of a glycerol backbone, a polar phosphate head ( $\text{PO}_4^{3-}$ ) (hydrophilic), and 2 nonpolar fatty acid tails (hydrophobic). Form the fundamental lipid bilayer of cell membranes.
  - Steroids: Composed of four interlocking hydrocarbon rings. Cholesterol serves as a structural component of cell membranes and the precursor for bile salts, Vitamin D, adrenocortical hormones, and sex hormones.
  - Eicosanoids: Derived from a 20-carbon fatty acid called arachidonic acid. Includes Prostaglandins (modify responses to hormones, blood clotting, inflammation, immunity, stomach acid secretion, airway diameter, lipid breakdown, and smooth muscle contraction) and Leukotrienes (regulate allergic and inflammatory responses).
  - Other Lipids: Fat-soluble vitamins (A, D, E, K), Lipoproteins.
- Proteins: Macromolecules containing Carbon, Hydrogen, Oxygen, and Nitrogen ( $12 - 18\%$ of body mass).
  - Amino Acid Structure: Built around a central Carbon bound to an Amino group ( $-\text{NH}_3^+$ ), a Carboxyl group ( $-\text{COO}^-$ ), a Hydrogen atom, and a variable Side Chain ( $\text{R}\text{ group}$ ). There are 20 naturally occurring amino acids (classified as nonpolar, polar uncharged, or polar charged).
  - Peptide Bonds: Covalent bonds joining the amino group of one amino acid to the carboxyl group of another via dehydration synthesis. Chains of 3 or more amino acids form polypeptides.
  - Structural Levels:
    1. Primary: Unique linear sequence of amino acids in a polypeptide chain.
    2. Secondary: Localized twisting or folding into $\alpha$ -helices or $\beta$ -pleated sheets, stabilized by hydrogen bonds.
    3. Tertiary: Three-dimensional shape of a single polypeptide chain, maintained by hydrophobic interactions, ionic bonds, hydrogen bonds, and covalent disulfide bridges (between two cysteine sulfhydryl $-\text{SH}$ groups).
    4. Quaternary: Spatial arrangement formed by the combination of two or more individual polypeptide chains.
  - Denaturation: Unfolding of a protein's 3D shape caused by hostile environments (excess heat, acids, or salts), resulting in complete loss of functional capacity.
  - Enzymes: Protein catalysts. Structure: Apoenzyme (protein portion) + Cofactor (metal ion like $\text{Fe}^{2+}$ , $\text{Mg}^{2+}$ ) or Coenzyme (organic vitamin derivative) = Holoenzyme (functional enzyme). Substrates bind precisely at the Active Site.
- Nucleotides & Nucleic Acids:
  - Nucleotide Structure: Composed of a 5-Carbon pentose sugar (Ribose or Deoxyribose), a phosphate group ( $-\text{PO}_4^{2-}$ ), and a nitrogenous base (Adenine, Thymine, Cytosine, Guanine, or Uracil).
  - Adenosine Triphosphate (ATP): Consists of Adenine, Ribose, and 3 phosphate groups attached by high-energy phosphate bonds. Hydrolysis releases usable cellular energy: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$ .
  - DNA (Deoxyribonucleic Acid): Double-stranded helical molecule containing deoxyribose sugar and bases Adenine ( $\text{A}$ ), Thymine ( $\text{T}$ ), Cytosine ( $\text{C}$ ), and Guanine ( $\text{G}$ ). Base pairing rules: $\text{A}$ pairs with $\text{T}$ via 2 hydrogen bonds; $\text{C}$ pairs with $\text{G}$ via 3 hydrogen bonds. Encodes inherited genetic code.
  - RNA (Ribonucleic Acid): Single-stranded molecule containing ribose sugar and Uracil ( $\text{U}$ ) instead of Thymine. Three classes: Messenger RNA ( $\text{mRNA}$ ), Ribosomal RNA ( $\text{rRNA}$ ), Transfer RNA ( $\text{tRNA}$ ).

Cellular Structure, Membrane Transport, and Cytology

Cytological Foundations:
- Cell: The basic, living, structural, and functional unit of the human body.
- Cytology: The branch of science concerned with cell structure.
- Cell Physiology: The branch of science concerned with cell function.
Three Principal Parts of a Cell:
1. Plasma (Cell) Membrane: Selective outer boundary regulating movement into and out of the cell.
2. Cytoplasm: All cellular contents situated between the plasma membrane and nucleus. Composed of Cytosol and Organelles.
3. Nucleus: Large organelle housing $\text{DNA}$ and hereditary genes.
Plasma Membrane Architecture & Fluid Mosaic Model:
- Lipid Bilayer: Two back-to-back layers made of phospholipids. Phospholipid molecules are amphipathic: polar hydrophilic heads face outward toward aqueous ECF and ICF; nonpolar hydrophobic fatty acid tails face inward away from water.
- Fluid Mosaic Model: Structure composed of a sea of phospholipids containing embedded glycolipids, sterols (cholesterol), and integral/peripheral proteins that float dynamically across the layer.
Functional Roles of Membrane Proteins:
- Transport Proteins: Transmembrane proteins providing selective hydrophilic channels or hydrolyzing $\text{ATP}$ to pump solutes actively across membranes.
- Enzymatic Activity: Catalyze metabolic reactions on the inner or outer membrane face.
- Receptors for Signal Transduction: Binding sites with specific shapes matching extracellular chemical messengers (e.g., hormones) to initiate intracellular signal cascades.
- Intercellular Joining: Hook adjacent cells together via junctions; Cell Adhesion Molecules (CAMs) guide temporary cell binding and migration.
- Cell-Cell Recognition: Glycoproteins act as molecular identification tags recognized selectively by other cells.
- Attachment Points: Anchor internal cytoskeleton filaments and external extracellular matrix (ECM) proteins to stabilize cell shape.
Membrane Permeability & Gradients:
- Selective Permeability: Highly permeable to small nonpolar, uncharged molecules (e.g., steroids, $\text{O}_2, \text{CO}_2$ , small alcohols); impermeable to ions ( $\text{Na}^+, \text{K}^+$ ) and large uncharged polar molecules (glucose). Transmembrane channel and carrier proteins increase permeability for impermeable solutes.
- Concentration Gradient: A difference in solute concentration between the cytosolic face and extracellular face of the membrane.
- Electrical Gradient: A difference in electrical charge across the membrane (inner surface is negative relative to outer surface). The combined influence is the Electrochemical Gradient.
Transport Mechanisms Across Plasma Membranes:
- Passive Transport (No cellular energy required; solutes move down gradients):
  1. Simple Diffusion: Passive mixing of particles occurring from high to low concentration due to intrinsic kinetic energy.
    - Rate Factors: Gradient steepness, temperature, particle size/mass, membrane surface area, diffusion distance.
    - Substances: Nonpolar, hydrophobic molecules ( $\text{O}_2, \text{CO}_2$ , lipids, small alcohols).
  2. Channel-Mediated Facilitated Diffusion: Passage of small inorganic hydrophilic ions ( $\text{Na}^+, \text{K}^+, \text{Cl}^-$ ) down gradients through selective transmembrane channel proteins (open or gated).
  3. Carrier-Mediated Facilitated Diffusion: Solute binds to a specific transporter protein on one side, triggering a conformational shape change that releases the solute on the opposite side (e.g., glucose, urea, fructose, galactose).
    - Transport Maximum ( $T_m$ ): The upper limit on diffusion rate occurring when all membrane transporters are completely occupied.
    - Glucose Transport Mechanism: Transporter (GluT) binds extracellular glucose $\rightarrow$ conformational shift moves glucose into cytoplasm $\rightarrow$ enzyme Kinase phosphorylates glucose into Glucose-6-Phosphate, keeping intracellular glucose concentration low and sustaining inward diffusion.
  4. Osmosis: Net diffusion of water molecules across a selectively permeable membrane toward the region of higher solute concentration.
    - Osmotic Pressure: Force proportional to the concentration of non-permeable solute particles in solution.
    - Tonicity Effects on Red Blood Cells (RBCs):
      - Isotonic Solution: RBCs maintain normal shape (solute concentration equal inside and outside).
      - Hypotonic Solution: Lower solute concentration outside $\rightarrow$ water rushes into cell $ ightarrow$ RBC undergoes Hemolysis (swells and bursts).
      - Hypertonic Solution: Higher solute concentration outside $\rightarrow$ water flows out of cell $ ightarrow$ RBC undergoes Crenation (shrinks).
- Active Transport (Requires energy; moves solutes against concentration gradients):
  1. Primary Active Transport: Energy derived directly from $\text{ATP}$ hydrolysis powers a transporter pump.
    - Sodium-Potassium Pump ( $\text{Na}^+/\text{K}^+$ ATPase): Consumes $40\%$ of cellular $\text{ATP}$ . Maintains low cytosolic $\text{Na}^+$ and high cytosolic $\text{K}^+$ by continually pumping $3\,\text{Na}^+$ out and $2\,\text{K}^+$ into the cell.
    - Digitalis Mechanism: Drug that slows $\text{Na}^+/\text{K}^+$ pumps, causing $\text{Na}^+$ accumulation inside cardiac cells. This slows secondary active $\text{Na}^+/\text{Ca}^{2+}$ exchange, allowing $\text{Ca}^{2+}$ to accumulate in cardiac muscle cells and strengthening heart contraction force.
  2. Secondary Active Transport: Energy stored in an ionic concentration gradient (established by primary active transport, like $\text{Na}^+$ or $\text{H}^+$ ) drives other substances against their gradients.
    - Antiporters: Move two substances in opposite directions (e.g., $\text{Na}^+/\text{Ca}^{2+}$ and $\text{Na}^+/\text{H}^+$ antiporters).
    - Symporters: Move two substances in the same direction (e.g., $\text{Na}^+/\text{Glucose}$ and $\text{Na}^+/\text{Amino acid}$ symporters).
- Transport in Vesicles:
  1. Endocytosis: Import of materials into vesicles budding inward from the plasma membrane.
    - Phagocytosis: