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 (single circular chromosome), and plasmids (extra-chromosomal DNA).

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

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: Many 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, and carbohydrates; regulates exchange and signaling.

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

• Nucleus: Double-membraned, contains chromatin and 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), site of protein synthesis.

• Centrosome, Cilia, Flagella: Cell division and movement.

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

Function of Eukaryotic Cells

• Energy production (mitochondria, chloroplasts in plants), protein synthesis, cell division (mitosis/meiosis), communication, and 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: Provide mechanical strength.

• Gap Junctions: Allow direct exchange of ions/molecules (e.g., heart tissue).

• Plasmodesmata: Plant cell channels for communication.

Local and Long-Distance Signaling

• Local: Paracrine (nearby cells), synaptic (neurons).

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

Chemical Messengers and Receptors

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

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

Stages of Cell Signaling

• Reception: Ligand binds receptor.

• Transduction: Signal cascade (often involving second messengers).

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

Medical Relevance

• Disrupted signaling underlies diseases like cancer and neurodegeneration; many drugs target signaling pathways.

Cell Cycle, Mitosis, and Meiosis

Cellular Organization of Genetic Material

• DNA organized into chromosomes; chromatin condenses during division.

Phases of the Cell Cycle

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

• M Phase: Mitosis (nuclear division) and 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); results in four haploid gametes.

Comparison of Mitosis and Meiosis

Origins of Genetic Variation

• Crossing over, independent assortment, random fertilization.

Medical Relevance

• Cell cycle regulation is key in cancer; meiosis errors cause genetic disorders.

Cellular Respiration and Fermentation

Catabolic Pathways and ATP Production

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

• Cellular respiration yields up to 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 and chemiosmosis; produces ~28–34 ATP, water formed as 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 cells), alcohol (yeast).

Medical Relevance

• Defects cause metabolic diseases; fermentation relevant in muscle fatigue and cancer metabolism.

Viruses

Structure of Viruses

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

Viral Replication Cycle

• Attachment, penetration, uncoating, replication/synthesis, assembly, release (lysis or budding).

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

Viral Diseases

• Examples: Influenza, HIV/AIDS, hepatitis, COVID-19, HPV (oncogenic).

• Emerging viruses: Ebola, Zika, SARS.

• Prevention: Vaccines; Treatment: Antivirals.

Medical Relevance

• Major cause of infectious disease; advances in molecular biology and vaccine development.

Bacteria and Archaea

Cell Surface Structures

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

• Archaea: Pseudopeptidoglycan or 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 bacteria cause disease; antibiotic resistance is a major concern. Archaea are extremophiles with biotechnological potential.

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 common in males (e.g., hemophilia, color blindness).

• Y-linked: Passed father to son; few genes.

Alteration of Chromosome Number and Structure

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

Human Disorders Due to Chromosomal Alteration

• Down syndrome, Turner syndrome (XO), Klinefelter syndrome (XXY).

Inheritance of Organelle Genes

• Mitochondrial DNA (maternal inheritance); mutations cause metabolic diseases.

DNA as Genetic Material and Structure

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

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

Chromosome Structure

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

Medical Relevance

• Basis for genetic counseling, diagnosis, and therapy of 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 synthesize proteins from mRNA; tRNAs bring amino acids; start (AUG) and stop codons (UAA, UAG, UGA).

Genetic Code

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

Types of Mutations

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

Regulation of Gene Expression

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

Medical Relevance

• Foundation for understanding genetic diseases, cancer, gene therapy, and biotechnology.

Tissues and Body Membranes

Epithelial Tissue

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

Connective Tissue

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

Muscle Tissue

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

Nervous Tissue

• Neurons and glia; communication and regulation.

Body Membranes

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

Medical Relevance

• Basis for understanding organ function, pathology, and disease mechanisms.

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 light, 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; advances in sensory prosthetics.

Nervous System

Organization

• CNS (brain, spinal cord), PNS (nerves); 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

• Alzheimer’s, Parkinson’s, multiple sclerosis, infections, trauma, mental health disorders.

Neurons, Synapses, and Signaling

Neurons: Structure and Function

• Cell body, dendrites (input), axon (output); myelin sheath (faster 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) vs. saltatory (myelinated, nodes of Ranvier).

Postsynaptic Potential

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

Neurotransmitters

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

Medical Relevance

• Imbalances cause neurological/psychiatric disorders; demyelination causes multiple sclerosis.

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 and Feedback

• Stimulus → hormone → target → response; regulated by negative/positive feedback.

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: Produced by B cells, binds antigen.

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 response on re-exposure (basis for vaccination).

Allergies and Immunization

• Allergies: Hypersensitivity (IgE, histamine); Immunization: Induces memory without disease.

Medical Relevance

• Immunodeficiency, autoimmunity, hypersensitivity; vaccine development.

Cardiovascular System

Organization

• Pulmonary (heart-lungs-heart) and systemic (heart-body-heart) circulation.

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.

Blood Composition

• Plasma, erythrocytes (O2 transport), leukocytes (immunity), platelets (clotting).

Medical Relevance

• Diseases: Hypertension, anemia, arrhythmias, coronary artery 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: Diaphragm relaxes, volume decreases, pressure rises, air out.

Hemoglobin

• Binds O2 (cooperative binding), releases in tissues; also transports CO2 (as carbaminohemoglobin or bicarbonate).

Medical Relevance

• Asthma, COPD, pneumonia, anemia; O2 transport and gas exchange are critical for life.

Digestive System and Nutrition

Essential Nutrients

• Macronutrients (carbohydrates, 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 to anus), accessory organs (salivary glands, liver, pancreas, gallbladder).

Chemical Digestion

• Enzymatic breakdown: amylase (carbs), pepsin (proteins), lipase (fats); absorption in small intestine.

Dental Adaptation

• Herbivores (flat molars), carnivores (sharp 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, estrogen, progesterone), fallopian tubes, uterus, vagina.

Hormonal Control

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

Gametogenesis

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

Conception and Embryonic Development

• Fertilization (zygote), cleavage, blastocyst, implantation, gastrulation (germ layers), 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 also excrete wastes.

Kidney Structure

• Cortex (filtration), medulla (loops of Henle, collecting ducts), renal pelvis (urine collection).

Nephron Organization and Function

• Glomerulus (filtration), Bowman’s capsule, proximal tubule (reabsorption), loop of Henle (concentration gradient), distal tubule (ion/pH regulation), collecting duct (water reabsorption).

Kidney Function, Water Balance, Blood Pressure

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

Medical Relevance

• Kidney disease, hypertension, dehydration, diabetes insipidus.

Evolution

Evolution of Population

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

• Hardy-Weinberg equilibrium: (where and are allele frequencies).

Evolution of Human Beings

• From early primates to Australopithecus, Homo habilis, Homo erectus, Neanderthals, and Homo sapiens.

Origin and Evolution of Vertebrates

• Chordates → jawless fish → jawed fish → tetrapods → amniotes (reptiles, birds, mammals) → primates → humans.

Significance

• Explains diversity, adaptation, and relationships; informs medicine, ecology, conservation.

Genetics of Population

Genetic Drift and Effective Population Size

• Random allele frequency changes; more pronounced in small populations (effective population size, ).

Bottleneck Effect

• Sharp population reduction reduces genetic diversity (e.g., northern elephant seal).

Adaptation and Phenotypic Variance

• Adaptation via natural selection; phenotypic variance = genetic + environmental + interaction.

Hardy-Weinberg Equilibrium

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

• Equation:

Conclusion

• Population genetics explains genetic diversity, adaptation, and evolutionary change; essential for conservation and understanding genetic disorders.