Prokaryotic Cell Structure and Function

Basic Structure of Prokaryotic Cells

• Cell Envelope: Consists of the plasma membrane (phospholipid bilayer with proteins), cell wall (peptidoglycan in bacteria; Gram-positive have thick layers, Gram-negative have thin layers plus an outer membrane), and sometimes a capsule (polysaccharide layer for protection).

• Cytoplasm: Gel-like matrix containing water, enzymes, ions, ribosomes (70S), nucleoid region (circular DNA), and plasmids (extra-chromosomal DNA).

• Appendages: Flagella (motility), pili/fimbriae (adhesion, genetic exchange via conjugation).

Function of Prokaryotic Cells

• Metabolism: Diverse; includes autotrophy (photosynthesis, chemosynthesis) and heterotrophy.

• Reproduction: Binary fission (asexual); genetic variation via horizontal gene transfer (transformation, transduction, conjugation).

• Adaptability: Survival in extreme environments, endospore formation.

• Pathogenicity: Some cause diseases (e.g., Escherichia coli, Mycobacterium tuberculosis).

Medical Relevance

• Basis for antibiotic development and infectious disease treatment.

Eukaryotic Cell Structure and Function

Basic Structure of Eukaryotic Cells

• Plasma Membrane: Phospholipid bilayer with proteins, cholesterol, carbohydrates; regulates exchange and signaling.

• Cytoplasm & Cytoskeleton: Semi-fluid matrix with organelles; cytoskeleton (microfilaments, intermediate filaments, microtubules) for structure and transport.

• Nucleus: Double-membraned, contains chromatin, nucleolus (rRNA synthesis).

• Membrane-Bound Organelles: Mitochondria (ATP production), ER (rough: protein synthesis; smooth: lipid synthesis), Golgi apparatus (modification/packaging), lysosomes (digestion), peroxisomes (detoxification), vacuoles (storage in plants).

• Ribosomes: 80S (60S + 40S subunits), protein synthesis.

• Centrosome, Cilia/Flagella: Microtubule organization, cell movement.

• Cell Wall: In plants (cellulose), fungi (chitin), some protists.

Function of Eukaryotic Cells

• Energy production (mitochondria, chloroplasts in plants), protein synthesis/transport, cell division (mitosis/meiosis), communication, specialization.

Medical Relevance

• Central to understanding human biology, disease mechanisms, and therapeutic development.

Cell Communication

Cell Junctions

• Tight Junctions: Seal cells together (e.g., epithelia).

• Desmosomes: Mechanical strength via intermediate filaments.

• Gap Junctions: Direct exchange of ions/molecules (connexins).

• Plasmodesmata: Plant cell channels for transport/signaling.

Local and Long-Distance Signaling

• Local: Paracrine (growth factors), synaptic (neurotransmitters).

• Long-Distance: Endocrine (hormones via bloodstream).

Chemical Messengers & Receptors

• Messengers: Hormones, neurotransmitters, cytokines, second messengers (cAMP).

• Receptors: Cell surface (GPCRs, RTKs, ion channels), intracellular (steroid hormone receptors).

Stages of Cell Signaling

• Reception: Ligand binds receptor.

• Transduction: Signal relay/amplification (cascades, second messengers).

• Response: Cellular action (gene expression, enzyme activation).

Medical Relevance

• Disrupted signaling underlies diseases (cancer, neurodegeneration, immune disorders).

Cell Cycle, Mitosis, and Meiosis

Cellular Organization of Genetic Material

• DNA organized as chromatin; condenses into chromosomes (sister chromatids joined at centromere) before division.

Phases of the Cell Cycle

• Interphase: G1 (growth), S (DNA replication), G2 (preparation).

• M Phase: Mitosis (nuclear division) + cytokinesis (cytoplasmic division).

Stages of Mitosis

• Prophase, prometaphase, metaphase, anaphase, telophase; followed by cytokinesis.

Stages of Meiosis

• Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate); produces four haploid gametes.

Comparison of Mitosis and Meiosis

Origins of Genetic Variation

• Crossing over, independent assortment, random fertilization.

Medical Relevance

• Cell cycle errors cause cancer, chromosomal disorders (e.g., Down syndrome).

Cellular Respiration and Fermentation

Catabolic Pathways and ATP Production

• Breakdown of glucose to generate ATP via substrate-level and oxidative phosphorylation.

• Complete aerobic respiration yields 32–36 ATP/glucose; fermentation yields 2 ATP/glucose.

Stages of Cellular Respiration

• Glycolysis: In cytoplasm; glucose → 2 pyruvate, 2 ATP, 2 NADH.

• Pyruvate Oxidation & Citric Acid Cycle: In mitochondria; acetyl-CoA enters cycle, producing 2 ATP, 6 NADH, 2 FADH2, CO2.

• Oxidative Phosphorylation: Electron transport chain (ETC) and chemiosmosis; NADH/FADH2 donate electrons, proton gradient drives ATP synthase, O2 is final electron acceptor.

Anaerobic Respiration and Fermentation

• Anaerobic Respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate).

• Fermentation: Regenerates NAD+; lactic acid (muscle), alcohol (yeast).

Medical Relevance

• Disorders affect energy metabolism; fermentation relevant in muscle fatigue, cancer.

Viruses

Structure of Viruses

• Genetic material (DNA or RNA), capsid (protein coat), optional envelope (lipid, with glycoproteins), surface proteins for host recognition.

Viral Replication Cycle

• Attachment → Penetration → Uncoating → Replication/Synthesis → Assembly → Release (lysis or budding).

• Some (e.g., retroviruses) integrate into host genome.

Viral Diseases

• Cause respiratory, chronic, neurological, skin, and oncogenic diseases; emerging viruses (Ebola, Zika, SARS).

• Prevention: Vaccines; Treatment: Antivirals.

Medical Relevance

• Major infectious agents; research advances gene therapy, vaccines.

Bacteria and Archaea

Cell Surface Structures

• Bacteria: Peptidoglycan cell wall (Gram-positive/negative), capsule/slime layer, fimbriae/pili.

• Archaea: Pseudopeptidoglycan/protein/polysaccharide walls; unique membrane lipids.

Motility

• Flagella (bacteria: flagellin; archaea: archaellin), taxis (chemotaxis, phototaxis), gliding/twitching.

Internal Organization

• Nucleoid (circular DNA), plasmids, 70S ribosomes, inclusion bodies, specialized membranes (e.g., thylakoids).

Reproduction and Adaptation

• Binary fission, rapid mutation, horizontal gene transfer (transformation, transduction, conjugation).

Medical Relevance

• Pathogenic/beneficial roles, antibiotic resistance, extremophile research.

Chromosomal and Molecular Basis of Inheritance

Chromosomal Basis of Sex

• Sex chromosomes (XX/XY in humans); SRY gene on Y triggers male development.

Inheritance of X- and Y-Linked Genes

• X-linked: More frequent in males (e.g., hemophilia); Y-linked: Passed father to son.

Alteration of Chromosome Number and Structure

• Aneuploidy (e.g., trisomy 21), polyploidy, deletions, duplications, inversions, translocations.

Human Disorders Due to Chromosomal Alteration

• Down syndrome, Turner syndrome, Klinefelter syndrome.

Inheritance of Organelle Genes

• Mitochondrial DNA (maternal inheritance), diseases (e.g., Leber’s hereditary optic neuropathy).

DNA as Genetic Material & Structure

• Experiments (Griffith, Avery, Hershey-Chase) established DNA as genetic material.

• Watson & Crick model: Double helix, antiparallel strands, complementary base pairing (A-T, G-C).

Chromosome Structure

• DNA + histones = nucleosomes; euchromatin (active), heterochromatin (inactive).

Medical Relevance

• Genetic counseling, diagnosis, therapy for inherited diseases.

Genetic Mechanisms

Flow of Genetic Information

• Central dogma: DNA → RNA → Protein.

Replication

• Semi-conservative; enzymes: helicase, DNA polymerase, primase, ligase.

Transcription

• RNA polymerase synthesizes RNA from DNA template (initiation, elongation, termination).

Translation

• Ribosomes assemble proteins from mRNA; tRNAs bring amino acids; starts at AUG, ends at stop codon.

Genetic Code

• Universal, redundant, non-overlapping; codons specify amino acids.

Types of Mutations

• Point (silent, missense, nonsense), insertions/deletions (frameshift), chromosomal mutations.

Regulation of Gene Expression

• Transcriptional (factors, enhancers), post-transcriptional (splicing), translational, epigenetic (methylation, acetylation).

Medical Relevance

• Basis for genetic disorders, cancer, gene therapy, personalized medicine.

Tissues and Body Membranes

Types of Tissues

• Epithelial: Covers surfaces, lines cavities, forms glands; functions: protection, absorption, secretion, sensation.

• Connective: Supports, binds, protects; types: loose, dense, bone, cartilage, blood.

• Muscle: Movement; types: skeletal (voluntary), cardiac (heart), smooth (organs).

• Nervous: Communication; neurons (signal transmission), neuroglia (support).

Body Membranes

• Serous (closed cavities), mucous (open cavities), synovial (joints), cutaneous (skin).

Medical Relevance

• Disorders: cancer (epithelial), arthritis (connective), inflammation (membranes).

Senses

Hearing and Equilibrium

• Ear: Outer (collects sound), middle (ossicles amplify), inner (cochlea for hearing, vestibular system for balance).

Visual Perception

• Eye: Cornea/lens focus light, retina (rods/cones) detect, optic nerve transmits to brain.

Taste and Smell

• Taste: Taste buds (sweet, sour, salty, bitter, umami); Smell: Olfactory receptors in nasal cavity.

Types of Sensory Receptors

• Mechanoreceptors, thermoreceptors, nociceptors, photoreceptors, chemoreceptors.

Medical Relevance

• Disorders: hearing loss, blindness, anosmia, ageusia; prosthetics (cochlear, retinal implants).

Nervous System

Organization

• CNS (brain, spinal cord), PNS (nerves); sensory (input), motor (output), somatic (voluntary), autonomic (involuntary: sympathetic, parasympathetic).

Central Nervous System

• Brain (cerebrum, diencephalon, cerebellum, brainstem), spinal cord; protected by skull, vertebrae, CSF, meninges.

Peripheral Nervous System

• Motor (skeletal muscle), autonomic (organs/glands).

Glia

• Astrocytes (BBB), oligodendrocytes/Schwann cells (myelin), microglia (immune).

Blood-Brain Barrier

• Tight endothelial cells; regulates CNS environment.

Nervous System Disorders

• Neurodegeneration (Alzheimer’s, Parkinson’s), infections, trauma, autoimmune (MS), mental health.

Neurons, Synapses, and Signaling

Neuron Structure and Function

• Cell body, dendrites (input), axon (output, myelinated/unmyelinated), nodes of Ranvier (saltatory conduction).

Resting and Action Potential

• Resting: -70 mV, maintained by Na+/K+ pump.

• Action: Depolarization (Na+ in), repolarization (K+ out), refractory period.

Conduction of Action Potentials

• Continuous (unmyelinated), saltatory (myelinated).

Postsynaptic Potential

• EPSP (depolarizing), IPSP (hyperpolarizing); integration determines firing.

Neurotransmitters

• Excitatory (glutamate), inhibitory (GABA), modulatory (dopamine, serotonin); removed by breakdown, reuptake, diffusion.

Medical Relevance

• Imbalances cause neurological/psychiatric disorders; demyelination (MS).

Hormones and Endocrine System

Intercellular Communication

• Endocrine (hormones via blood), paracrine, autocrine, neuroendocrine.

Endocrine Tissues and Organs

• Hypothalamus, pituitary, thyroid, parathyroid, adrenal, pancreas, gonads, pineal.

Chemical Classes of Hormones

• Peptide (insulin), steroid (cortisol), amine (adrenaline).

Multiple Effects of Hormones

• Same hormone, different effects (e.g., epinephrine on heart, liver, lungs).

Simple Hormone Pathways & Feedback

• Stimulus → hormone → target → response; negative/positive feedback (e.g., insulin, oxytocin).

Medical Relevance

• Disorders: diabetes, thyroid disease, Addison’s; basis for hormone therapies.

Immune System: Innate and Adaptive Immunity

Antigen and Antibody

• Antigen: Triggers immune response; antibody: B cell protein binds antigen (IgG, IgA, IgM, IgE, IgD).

Humoral and Cell-Mediated Immune Response

• Humoral: B cells → plasma cells → antibodies (extracellular pathogens).

• Cell-mediated: T cells (cytotoxic, helper) target infected/cancer cells.

Inflammatory Response

• Histamine release, vasodilation, immune cell recruitment; symptoms: redness, swelling, heat, pain.

Immunological Memory

• Memory B/T cells enable faster secondary response; basis for vaccination.

Allergies and Immunization

• Allergies: IgE-mediated hypersensitivity; immunization: exposure to antigens for memory cell formation.

Medical Relevance

• Immunodeficiency, autoimmunity, hypersensitivity; vaccine development.

Cardiovascular System

Organization

• Double-loop: pulmonary (heart-lungs-heart), systemic (heart-body-heart).

Heart and Rhythmic Beat

• Four chambers; SA node (pacemaker), AV node, bundle of His, Purkinje fibers; systole/diastole; valves ensure one-way flow.

Blood Vessels

• Arteries (away from heart), arterioles, capillaries (exchange), venules, veins (to heart); veins have valves.

Blood Pressure

• Systolic/diastolic; regulated by nervous system, hormones, kidneys; normal ~120/80 mmHg.

Blood Composition

• Plasma, RBCs (hemoglobin), WBCs (immunity), platelets (clotting).

Medical Relevance

• Diseases: hypertension, anemia, heart disease.

Respiratory System

Organization

• Upper (nasal cavity, pharynx, larynx), lower (trachea, bronchi, bronchioles, alveoli); diaphragm for ventilation.

Negative Pressure Breathing

• Inhalation: diaphragm contracts, thoracic volume increases, pressure drops, air in; exhalation: passive, air out.

Hemoglobin

• O2 transport (cooperative binding), CO2 transport (as bicarbonate, carbaminohemoglobin); Bohr effect (pH, CO2 influence O2 release).

Medical Relevance

• Respiratory diseases (asthma, COPD, pneumonia), anemia.

Digestive System and Nutrition

Essential Nutrients

• Macronutrients (carbs, proteins, fats), micronutrients (vitamins, minerals), water.

Dietary Deficiencies

• Protein-energy malnutrition, vitamin (C, D) and mineral (iron, iodine) deficiencies.

Organization of Digestive System

• Alimentary canal (mouth → anus), accessory organs (salivary glands, liver, gallbladder, pancreas).

Chemical Digestion

• Enzymatic breakdown: mouth (amylase), stomach (pepsin), small intestine (pancreatic enzymes, bile, brush-border enzymes).

Dental Adaptation

• Herbivores (flat molars), carnivores (canines), omnivores (mixed dentition).

Medical Relevance

• Malnutrition, obesity, GI disorders, metabolic diseases.

Human Reproduction and Development

Reproductive Anatomy

• Male: testes (sperm, testosterone), epididymis, vas deferens, accessory glands, penis.

• Female: ovaries (ova, hormones), fallopian tubes, uterus, vagina.

Hormonal Control

• GnRH (hypothalamus) → FSH/LH (pituitary) → gonadal hormones (testosterone, estrogen, progesterone).

Gametogenesis

• Spermatogenesis (4 sperm/germ cell), oogenesis (1 ovum/germ cell + polar bodies).

Conception and Development

• Fertilization (zygote), cleavage, blastocyst, implantation, gastrulation, organogenesis, placenta formation.

Birth

• Dilation, expulsion, placental stage; regulated by oxytocin.

Medical Relevance

• Fertility, pregnancy complications, developmental disorders.

Osmoregulation and Excretion

Excretory Organs

• Kidneys (urine formation), ureters, bladder, urethra; skin, lungs, liver (secondary roles).

Kidney Structure

• Cortex, medulla (pyramids), renal pelvis; blood supply via renal artery/vein.

Nephron Organization and Function

• Glomerulus (filtration), Bowman’s capsule, proximal/distal tubules, loop of Henle, collecting duct.

• Processes: filtration, reabsorption, secretion.

Kidney Function, Water Balance, Blood Pressure

• ADH (water reabsorption), RAAS (blood pressure), ANP (opposes RAAS).

Medical Relevance

• Kidney disease, hypertension, dehydration.

Evolution

Evolution of Population

• Genetic variation (mutation, recombination, gene flow), natural selection, genetic drift, sexual selection.

• Hardy-Weinberg equilibrium: (allele/genotype frequencies constant if no evolution).

Evolution of Human Beings

• From early primates → Australopithecus → Homo habilis → Homo erectus → Homo sapiens.

Origin and Evolution of Vertebrates

• Chordates → jawless fish → jawed vertebrates → tetrapods → amniotes → mammals/primates/humans.

Significance

• Explains diversity, adaptation, medical challenges (antibiotic resistance, emerging diseases).

Genetics of Population

Genetic Drift and Effective Population Size

• Random allele frequency changes; effective population size (Ne) is number contributing genes; small Ne increases drift.

Bottleneck Effect

• Population size reduction → loss of diversity (e.g., northern elephant seal).

Adaptation and Phenotypic Variance

• Adaptation: traits enhancing survival/reproduction; phenotypic variance = genetic + environmental + interaction.

Hardy-Weinberg Equilibrium

• Conditions: no mutation, random mating, no selection, large population, no migration.

• Equation:

Conclusion

• Population genetics explains diversity, adaptation, and informs conservation, epidemiology, and human genetics.