Advanced Biology: Principles and Mechanisms

HOMEOSTASIS, THERMOREGULATION, AND OSMOREGULATION

Homeostasis is the state of physiological stability maintained within an organism despite fluctuations in the external environment. It ensures that the internal conditions, such as temperature, water potential, and chemical concentrations, stay within narrow limits conducive to enzymatic activity and cellular function.

Basic Mechanism of Homeostasis

The homeostatic control system consists of three main components:

• Receptors: These are components that detect changes in the external or internal environment (stimuli). Examples include thermoreceptors in the skin and osmoreceptors in the hypothalamus.
• Control Centre: The brain and spinal cord act as the control center. They receive information from receptors, process it, compare it to the 'set point,' and send signals to effectors.
• Effectors: These are muscles or glands that carry out the response ordered by the control center to restore the balanced state.

Feedback Mechanisms

• Negative Feedback: This is the most common mechanism where the response of the effector reverses the change in the system. For example, if body temperature rises, the control center triggers cooling mechanisms (sweating, vasodilation) to bring it back down to $37\,^{\circ}C$.
• Positive Feedback: This mechanism amplifies the stimulus rather than reversing it. An example is childbirth, where the hormone oxytocin stimulates uterine contractions, which in turn trigger more oxytocin release until the baby is born.
• Unregulated Positive Feedback: This can be dangerous, such as a very high fever (above $42\,^{\circ}C$) where the heat produces more metabolic heat, leading to a fatal spiral.

Osmoregulation

Osmoregulation is the maintenance of water and salt balance in the body. Animals are classified based on how they manage this balance:

• Osmoconformers: These animals maintain an internal osmotic concentration equal to their surrounding environment (isotonic). Most marine invertebrates are osmoconformers.
• Osmoregulators: These animals actively control their internal osmotic concentration regardless of the external environment. They must use energy to discharge excess water in hypotonic environments or take in water in hypertonic environments.
• Freshwater Habitats: Animals face the problem of water entering the body by osmosis and loss of salts. They excrete large amounts of dilute urine and actively pump salts into their bodies.
• Marine Habitats: Animals face water loss and salt gain. Marine fish drink large amounts of seawater and excrete excess salts through specialized cells in their gills.
• Terrestrial Habitats: The primary problem is dehydration. Adaptations include water-resistant skin (cuticle/keratin), drinking water, and excreting concentrated urea or uric acid.
• Anhydrobiosis: The ability of some animals (e.g., tardigrades) to survive almost complete dehydration in a dormant state until water returns.

Nature of Excretory Products

The type of nitrogenous waste excreted depends on the animal's habitat and water availability:

1. Ammonia ($NH_3$): Highly toxic and requires large amounts of water for excretion ($500\,cm^3$ water per $1\,g$ of Nitrogen). Excreted by aquatic animals (ammonotelic).
2. Urea ($CO(NH_2)_2$): Less toxic than ammonia and requires less water ($50\,cm^3$ water per $1\,g$ of Nitrogen). Produced in the liver via the urea cycle and excreted by mammals and adult amphibians (ureotelic).
3. Uric Acid ($C_5H_4N_4O_3$): Relatively non-toxic and insoluble in water, requiring very little water ($1\,cm^3$ water per $1\,g$ of Nitrogen). Excreted as a paste or pellet by birds, reptiles, and insects (uricotelic).

Thermoregulation

Animals balance heat gain and loss to regulate body temperature:

• Ectotherms: Animals that rely on external heat sources to regulate body temperature (e.g., invertebrates, fish, amphibians, reptiles).
• Endotherms: Animals that generate heat internally through metabolism (e.g., birds, mammals).
• Poikilotherms: Animals whose body temperature fluctuates with the environment.
• Homeotherms: Animals that maintain a constant body temperature despite environmental changes.
• Human Responses to Overheating (Hyperthermia): Sweating, vasodilation (widening of blood vessels), and decreased metabolism.
• Human Responses to Cooling (Hypothermia): Shivering (thermogenesis), vasoconstriction (narrowing of blood vessels), and pili erection (goosebumps).

HUMAN URINARY SYSTEM

The urinary system is responsible for nitrogenous waste removal, osmoregulation, and pH/ion regulation.

Anatomy of the Kidney

• Renal Capsule: The fibrous outer covering.
• Renal Hilum: The indentation where the renal artery enters and the renal vein and ureter exit.
• Renal Cortex: The outer region containing the functional units (nephrons).
• Renal Medulla: The inner region containing renal pyramids.
• Renal Pelvis: The funnel-shaped cavity that collects urine and leads to the ureter.

Structure of the Nephron

The nephron is the functional unit of the kidney, consisting of:

1. Renal Corpuscle: Contains the Glomerulus (capillary network) and Bowman's Capsule (cup-shaped structure that captures filtrate).
2. Renal Tubule: Consists of the Proximal Convoluted Tubule (PCT), Loop of Henle (Descending and Ascending limbs), and Distal Convoluted Tubule (DCT).
3. Collecting Duct: Receives urine from multiple nephrons and carries it to the renal pelvis.

• Cortical Nephrons: Located mostly in the cortex; have short Loops of Henle.
• Juxtamedullary Nephrons: Located at the border of the cortex and medulla; have long Loops of Henle that extend deep into the medulla, crucial for concentrating urine.

Functioning of the Kidney

1. Glomerular Filtration (Ultrafiltration): High blood pressure in the glomerulus forces water and small solutes (glucose, salts, urea) into Bowman's capsule, forming Glomerular Filtrate.
2. Selective Reabsorption: Useful substances (glucose, amino acids, $H_{2}O$, ions) are reabsorbed from the tubule back into the peritubular capillaries. $99\%$ of the filtrate is reabsorbed.
3. Tubular Secretion: Specific ions ($H^{+}$, $K^{+}$) and drugs are actively moved from the blood into the renal tubule to maintain pH and blood composition.

Regulation of Urine Concentration

• Counter-current Mechanism: The long Loops of Henle and vasa recta create an osmotic gradient in the renal medulla, allowing for the reabsorption of water and production of concentrated urine.
• Antidiuretic Hormone (ADH): Released by the posterior pituitary when the body is dehydrated; increases the permeability of the collecting ducts to water.
• Aldosterone: Secreted by the adrenal cortex; promotes reabsorption of sodium ($Na^{+}$) and water, increasing blood pressure and volume.

Disorders of the Urinary Tract

• Urinary Tract Infections (UTI): Commonly caused by Escherichia coli.
• Kidney Stones: Formed from minerals like calcium oxalate (hyperoxaluria) or uric acid (hyperuricemia). Treatment includes Extracorporeal Shockwave Lithotripsy (ECSWL) or Percutaneous Nephrolithotripsy.
• Kidney Failure: Treated via Haemodialysis (using a machine/dialyzer), Peritoneal Dialysis (using the abdomen's lining), or Kidney Transplant.

HUMAN NERVOUS SYSTEM

Nervous coordination involves the rapid transmission of electrochemical signals to respond to stimuli.

Neurons: The Structural Units

• Cell Body (Soma): Contains the nucleus and Nissl's granules (rough ER).
• Dendrites: Short fibers that receive impulses toward the cell body.
• Axon: A long fiber that carries impulses away from the cell body toward a synapse.
• Myelin Sheath: An insulating layer formed by Schwann cells (PNS) or Oligodendrocytes (CNS) that speeds up impulse transmission.
• Nodes of Ranvier: Gaps in the myelin sheath where saltatory conduction occurs.

Types of Neurons:

• Sensory Neurons: Carry impulses from receptors to the CNS.
• Motor Neurons: Carry impulses from the CNS to effectors (muscles/glands).
• Inter-neurons: Located within the CNS; process information.

Nerve Impulse

• Resting Membrane Potential (RMP): The charge difference across a non-conducting neuron membrane (approximately $-70\,mV$), maintained by the sodium-potassium pump ($3Na^{+}$ out, $2K^{+}$ in).
• Action Potential: A reversal in charge. Includes Depolarization ($Na^{+}$ influx), Repolarization ($K^{+}$ efflux), and a Refractory Period where the neuron cannot fire again briefly.
• Saltatory Conduction: In myelinated axons, the impulse "jumps" from one Node of Ranvier to the next, reaching speeds up to $120\,m/s$.

The Synapse

The junction between two neurons or a neuron and an effector. Neurotransmitters (e.g., Acetylcholine, Dopamine, Serotonin) are released from synaptic vesicles into the synaptic cleft to transmit the signal.

• Excitatory Neurotransmitters: Promote action potentials (e.g., Glutamate).
• Inhibitory Neurotransmitters: Prevent action potentials (e.g., GABA, Glycine).

Organization of the CNS

• Brain: Protected by Meninges (Dura mater, Arachnoid, Pia mater).
  • Cerebrum: Largest part; responsible for thinking, memory, and voluntary movement. Divided into lobes: Frontal (motor/speech), Parietal (touch/taste), Temporal (hearing), Occipital (vision).
  • Thalamus: Sensory relay station.
  • Hypothalamus: Regulates homeostasis (hunger, thirst, temperature) and the pituitary gland.
  • Limbic System: Includes the Amygdala (emotion) and Hippocampus (memory).
  • Brainstem: Includes the Midbrain (reflexes), Pons (breathing), and Medulla Oblongata (autonomic functions like heart rate).
  • Cerebellum: Coordinates balance and fine motor skills.
• Spinal Cord: Link between the brain and the rest of the body; center for spinal reflexes.

Peripheral Nervous System (PNS)

Consists of $12$ pairs of cranial nerves and $31$ pairs of spinal nerves. Divided into:

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.
• Autonomic Nervous System: Controls involuntary functions.
  • Sympathetic: "Fight or flight" (increased heart rate, dilated pupils).
  • Parasympathetic: "Rest and digest" (decreased heart rate, stimulated digestion).

Receptors and Drugs

• Receptors: Taste (taste buds), Smell (Olfactory cilia), Pain (Nociceptors), Touch (Meissner's corpuscles, Pacinian corpuscles).
• Drugs: Psychoactive drugs affect the CNS. Addiction involves physiological dependence. Tolerance requires higher doses for the same effect. Heroin acts as a depressant; Caffeine and Nicotine act as stimulants.
• Disorders: Stroke (vascular), Meningitis (infection), Alzheimer's and Parkinson's (degenerative diseases).

HUMAN ENDOCRINE SYSTEM

Endocrine glands secrete Hormones directly into the bloodstream to regulate distant target organs.

Chemical Nature of Hormones

• Peptide/Protein: e.g., Insulin, Glucagon, ADH.
• Glycoprotein: e.g., TSH, FSH, LH.
• Amino Acid Derivatives: e.g., Thyroxine, Epinephrine.
• Steroids: Derived from cholesterol; e.g., Estrogen, Testosterone, Cortisol.

Major Glands and Hormones

• Hypothalamus: The link between the nervous and endocrine systems. Produces releasing and inhibiting hormones.
• Pituitary Gland (Master Gland):
  • Anterior Lobe: Growth Hormone (GH), TSH, ACTH, FSH, LH, Prolactin.
  • Posterior Lobe: Stores and releases ADH and Oxytocin (produced by the hypothalamus).
• Thyroid Gland: Produces Thyroxine ($T_{4}$) and Tri-iodothyronine ($T_{3}$) (regulate metabolism) and Calcitonin (lowers blood calcium).
• Parathyroid Glands: Produce Parathormone (PTH), which increases blood calcium.
• Pancreas (Islets of Langerhans):
  • Alpha cells: Glucagon (increases blood glucose via glycogenolysis).
  • Beta cells: Insulin (decreases blood glucose via glycogenesis).
• Adrenal Glands:
  • Medulla: Epinephrine and Norepinephrine.
  • Cortex: Cortisol (glucocorticoid) and Aldosterone (mineralocorticoid).
• Gonads: Ovaries (Estrogen, Progesterone) and Testes (Testosterone).

HUMAN REPRODUCTIVE SYSTEMS

Male Reproductive System

• Testes: Located in the scrotum. Contain seminiferous tubules where Spermatogenesis occurs. Sertoli cells provide nourishment, and Leydig cells produce testosterone.
• Ducts: Rete testis → Epididymis (storage) → Vas deferens → Ejaculatory duct → Urethra.
• Glands: Seminal vesicles (fructose/mucus), Prostate gland (alkaline fluid), Bulbourethral glands (lubrication).
• Sperm Structure: Head (nucleus and Acrosome containing enzymes), Midpiece (mitochondria), Tail (flagellum).

Female Reproductive System

• Ovaries: Produce ova via Oogenesis. Ovulation releases a secondary oocyte.
• Fallopian Tubes (Oviducts): Site of fertilization.
• Uterus: Site of embryo implantation. Layers include perimetrium, myometrium, and endometrium.
• Menstrual Cycle:
  1. Menstrual Phase (Days 1–5): Shedding of the endometrium.
  2. Proliferative Phase (Days 6–14): Endometrium regrows under estrogen; ends with Ovulation triggered by an LH surge.
  3. Secretory Phase (Days 15–28): Corpus luteum secretes progesterone to maintain the endometrium for potential pregnancy.

INHERITANCE

Genetics is the study of heredity and variation.

Mendelian Genetics

• Law of Segregation: During gamete formation, the two alleles for a trait segregate so that each gamete receives only one allele.
• Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation (applies to genes on different chromosomes).
• Testcross: Breeding an organism with a dominant phenotype but unknown genotype with a homozygous recessive individual to determine the unknown genotype.

Deviations and Complex Inheritance

• Incomplete Dominance: The phenotype is an intermediate blend (e.g., pink 4-o'clock flowers from red and white parents).
• Co-dominance: Both alleles are expressed equally (e.g., MN blood group, AB blood type).
• Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood groups: $I^{A}$, $I^{B}$, $i$).
• Rh Factor: The presence ($Rh^{+}$) or absence ($Rh^{-}$) of the D-antigen. Erythroblastosis Foetalis occurs when an $Rh^{-}$ mother carries an $Rh^{+}$ fetus.
• Epistasis: One gene masks or interferes with the expression of another (e.g., Coat color in Labradors; Bombay phenotype).
• Polygenic Inheritance: Multiple genes contribute to a single trait, showing continuous variation (e.g., human skin color, wheat grain color).

Sex Linkage

• Sex Determination: Humans use the XX-XY system (Males are heterogametic XY).
• X-linked Traits: Traits found on the X chromosome. Recessive X-linked disorders (e.g., Haemophilia, Color-blindness) are more common in males because they have only one X.

CHROMOSOMES AND DNA

Chromosome Structure

• Chromosomes consist of chromatin ($40\%$ DNA, $60\%$ Histone protein).
• Nucleosomes: DNA wrapped around an octamer of histone proteins (the "beads on a string" model).
• Morphology: Includes the centromere, kinetochore, and telomeres. Classified as metacentric, sub-metacentric, acrocentric, or telocentric.

DNA: The Genetic Material

• Griffith’s Experiment: Demonstrated "transformation" in Streptococcus pneumoniae.
• Hershey-Chase Experiment: Confirmed DNA (not protein) enters bacteria using radioactive isotopes ($^{32}P$ and $^{35}S$).

DNA Replication

• Semi-conservative Model: Each new DNA molecule consists of one old strand and one new strand. Proven by the Meselson-Stahl experiment using $^{15}N$ isotopes.
• Mechanism:
  • Helicase unwinds DNA.
  • Primase adds an RNA primer.
  • DNA Polymerase III adds nucleotides in the $5'$ to $3'$ direction.
  • Leading Strand: Synthesized continuously toward the replication fork.
  • Lagging Strand: Synthesized discontinuously as Okazaki fragments, joined by DNA Ligase.

Gene Expression

1. Transcription: DNA is copied into mRNA. Includes Initiation (Promoters like TATA box), Elongation, and Termination (GC hairpin).
2. Genetic Code: The set of 64 triplets (codons). It is universal, triplet, and degenerate (redundant).
3. Translation: mRNA is decoded into a polypeptide chain at the ribosome. Uses tRNA (anticodons) and ribosomes (A, P, E sites).

BIOTECHNOLOGY

Recombinant DNA Technology

Tools include:

• Gene of Interest: The DNA segment to be cloned.
• Restriction Endonucleases: "Molecular scissors" that cut DNA at specific palindromic sequences, creating "sticky ends."
• Vectors (Plasmids): Carriers used to transport the gene into a host cell.
• DNA Ligase: Joins the gene and vector.

Polymerase Chain Reaction (PCR)

Invented by Kary Mullis to amplify DNA in vitro. Steps:

1. Denaturation ($94\,^{\circ}C$): Strands separate.
2. Annealing ($54\,^{\circ}C$): Primers bind.
3. Extension ($72\,^{\circ}C$): Taq polymerase synthesizes new strands.

GMOs and Applications

• Transgenic Organisms: Bacterially produced insulin, herbicide-resistant crops (Bt cotton), and transgenic animals (goats producing medicine in milk).

IMMUNITY AND BIOSTATISTICS

Lines of Defence

1. First Line: Physical/chemical barriers like skin, mucus, and stomach acid.
2. Second Line: Non-specific internal responses. Includes Macrophages, Neutrophils, Interferons, Complement system, Inflammation, and Fever (Pyrexia).
3. Third Line: Specific adaptive immunity.
   • B-cells: Produce antibodies (Humoral immunity).
   • T-cells: Direct cell-to-cell attack (Cell-mediated immunity). Includes Helper T-cells and Cytotoxic T-cells.

Biostatistics and Pharmacology

• Statistical Measures: Mean (average), Median (middle), Mode (most frequent), Standard Deviation (variation from mean).
• Antibiotics:
  • Penicillin: Inhibits bacterial cell wall synthesis.
  • Tetracycline: Inhibits protein synthesis at the 30S ribosome.
  • Fluoroquinolones: Inhibit DNA gyrase.
  • Sulfonamides: Competitively inhibit folic acid synthesis.

EVOLUTION AND ECOLOGY

Evolution

• Darwinism: Evolution via Natural Selection (Overproduction, Variation, Struggle for Survival).
• Neo-Darwinism: Modern synthesis combining Darwin's theory with genetics (Hardy-Weinberg equilibrium).
• Endosymbiotic Theory: Suggests mitochondria and chloroplasts originated from free-living prokaryotes ingested by ancestral cells.

Ecology

• Levels: Species → Population → Community → Ecosystem → Biome → Biosphere.
• Biogeochemical Cycles: Water cycle and Nitrogen cycle (Fixation, Nitrification, Assimilation, Ammonification, Denitrification).
• Greenhouse Effect: Trapping of heat by gases like $CO_{2}$, $CH_{4}$, and $N_{2}O$, leading to Global Warming and climate change.
• Ecosystems of Pakistan: Temperate Deciduous Forest, Tropical Thorn Forest, Alpine Meadows, and Mangrove Forests.