Lebanese Baccalaureate - Life Sciences (LS) cheat sheet

Plasma membrane is a phospholipid bilayer with embedded proteins; hydrophobic tails face inward, hydrophilic heads face the aqueous compartments.

Passive transport (simple and facilitated diffusion) moves solute down its gradient without ATP; active transport moves solute against its gradient and requires ATP.

Na+/K+ ATPase pumps 3 Na+ out for every 2 K+ in, maintaining the resting membrane potential and osmotic balance.

Osmosis is water movement across a semipermeable membrane toward higher solute concentration; cells shrink in hypertonic and swell in hypotonic solutions.

Mitochondria: outer membrane, inner membrane folded into cristae, matrix containing Krebs cycle enzymes; site of oxidative phosphorylation.

Ribosomes on rough ER synthesize secreted and membrane proteins; free ribosomes synthesize cytosolic proteins.

Enzyme kinetics: Michaelis-Menten describes rate versus substrate concentration; Km is the substrate concentration at half-maximal velocity.

Cell cycle phases: G1, S (DNA replication), G2, M; checkpoints at G1/S and G2/M arrest the cycle if DNA is damaged.

DNA is antiparallel and complementary: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds); replication is semiconservative.

Transcription: RNA polymerase reads the template strand 3' to 5' and synthesizes mRNA 5' to 3'; uracil replaces thymine in RNA.

Translation: codon on mRNA pairs with anticodon on tRNA; start codon AUG codes methionine; stop codons UAA, UAG, UGA terminate translation.

Mendel's laws: segregation of alleles at meiosis I and independent assortment of unlinked genes.

Monohybrid cross Aa x Aa gives a 3:1 phenotypic ratio; dihybrid cross AaBb x AaBb gives 9:3:3:1 for unlinked genes.

A test cross with a homozygous recessive reveals the genotype of a dominant-phenotype individual.

Sex-linked recessive traits appear more often in males because they carry one X chromosome.

Mutations: substitution (missense, nonsense, silent), insertion or deletion causing frameshift; only germline mutations are inherited.

Negative feedback reverses a deviation from set point (e.g., blood glucose, body temperature); positive feedback amplifies it (e.g., oxytocin in labor).

Blood glucose regulation: insulin from beta cells lowers glucose; glucagon from alpha cells raises it by glycogenolysis and gluconeogenesis.

Nephron order: Bowman's capsule, proximal tubule, loop of Henle, distal tubule, collecting duct; most reabsorption occurs in the proximal tubule.

Gas exchange occurs by diffusion across alveolar walls; oxygen binds hemoglobin cooperatively, giving a sigmoid dissociation curve.

Cardiac cycle: systole is ventricular contraction (semilunar valves open), diastole is relaxation (AV valves open); normal rate 60-100 beats/min.

Thermoregulation: hypothalamic set point; sweating and vasodilation cool the body, shivering and vasoconstriction conserve heat.

Blood pH is buffered near 7.4; the bicarbonate buffer and respiratory rate adjust CO2 to correct acidosis or alkalosis.

Osmolarity is controlled by ADH from the posterior pituitary, which increases water reabsorption in the collecting duct.

Neuron parts: dendrites receive, soma integrates, axon conducts, axon terminals release neurotransmitter at synapses.

Resting potential is about -70 mV; depolarization to threshold opens voltage-gated Na+ channels, producing an action potential.

Action potential phases: depolarization (Na+ in), repolarization (K+ out), hyperpolarization; refractory period prevents backward conduction.

Myelin sheaths speed conduction by saltatory jumping between nodes of Ranvier.

Synaptic transmission is chemical: Ca2+ influx triggers vesicle fusion, neurotransmitter crosses the cleft, binds receptors on the postsynaptic membrane.

Reflex arc order: receptor, sensory neuron, integration center, motor neuron, effector.

Eye: cornea and lens focus light onto the retina; rods mediate dim-light vision, cones mediate color and sharp acuity; the fovea has the highest cone density.

Ear: sound waves vibrate the tympanic membrane, ossicles, and cochlear fluid, stimulating hair cells that transduce mechanical energy into nerve impulses.

Hormones act on target cells bearing specific receptors; steroid hormones enter the cell and act on genes, peptide hormones bind surface receptors and use second messengers.

Hypothalamus-pituitary axis: hypothalamic releasing hormones stimulate the anterior pituitary, whose tropic hormones control peripheral glands.

Negative feedback: rising thyroid hormones inhibit TRH and TSH release; cortisol inhibits CRH and ACTH.

Insulin lowers blood glucose; glucagon, cortisol, adrenaline, and growth hormone raise it.

Male reproduction: FSH stimulates spermatogenesis via Sertoli cells; LH stimulates testosterone secretion by Leydig cells.

Female cycle: FSH matures the follicle, a mid-cycle LH surge triggers ovulation, and the corpus luteum secretes progesterone to maintain the endometrium.

Menstrual phases: follicular, ovulation, luteal; falling progesterone and estrogen cause menstruation if no implantation occurs.

Pregnancy hormones: hCG maintains the corpus luteum early; progesterone and estrogen are later produced by the placenta.

Innate immunity is non-specific and immediate: physical barriers, phagocytes, complement, inflammation, and fever.

Adaptive immunity is specific and has memory: B lymphocytes produce antibodies (humoral), T lymphocytes act directly (cell-mediated).

Antibody structure: two heavy and two light chains, variable regions bind antigen, constant regions determine isotype (IgG, IgM, IgA, IgE, IgD).

Helper T cells (CD4) activate B cells and cytotoxic T cells; cytotoxic T cells (CD8) kill infected cells.

Primary response is slow with mainly IgM; secondary response is faster, stronger, and dominated by IgG due to memory cells.

Vaccination exposes the immune system to a harmless antigen, producing memory cells without disease.

HIV targets CD4 helper T cells, progressively destroying cell-mediated immunity.

Allergic reactions involve IgE bound to mast cells releasing histamine; autoimmune disease is immune attack on self-antigens.

Trophic levels: producers, primary consumers, secondary consumers, decomposers; only about 10% of energy transfers to the next level.

Food webs are more stable than simple chains because alternative pathways buffer species loss.

Population growth: exponential when resources are unlimited; logistic when carrying capacity K limits growth (S-shaped curve).

Density-dependent factors (competition, disease, predation) intensify with crowding; density-independent factors (fire, drought) act regardless of density.

Symbiosis types: mutualism (both benefit), commensalism (one benefits, one unaffected), parasitism (one benefits, one harmed).

Carbon cycle: photosynthesis fixes CO2, respiration and combustion release it; nitrogen fixation by bacteria converts N2 to usable ammonia.

Eutrophication: nutrient runoff causes algal blooms, then oxygen depletion and death of aquatic animals.

Biodiversity is highest in tropical regions and in habitats with high structural complexity and stable conditions.

Natural selection requires heritable variation, differential reproductive success, and environmental pressure.

Homologous structures share ancestry but may differ in function; analogous structures share function but not ancestry.

Genetic drift is random allele-frequency change, strongest in small populations; gene flow is migration of alleles between populations.

Speciation: geographic isolation produces allopatric speciation; reproductive isolation (prezygotic or postzygotic) defines separate species.

Hardy-Weinberg equilibrium holds with no mutation, no migration, random mating, large population, and no selection; p + q = 1 and p² + 2pq + q² = 1.

Binomial nomenclature: genus capitalized, species lowercase, both italicized (e.g., Homo sapiens).

Classification hierarchy: domain, kingdom, phylum, class, order, family, genus, species.

Fossils, comparative anatomy, embryology, and molecular sequences provide evidence for common descent.

Hypothesis must be testable and falsifiable; a prediction states the expected outcome under the hypothesis.

Independent variable is manipulated; dependent variable is measured; controlled variables are held constant.

Control group receives no treatment or a placebo; experimental group receives the treatment; only random assignment supports causal claims.

Validity requires measuring the intended variable; reliability requires repeatable results across trials.

Correlation does not establish causation; confounding variables can produce spurious associations.

Graph choice: bar graph for categorical comparisons, line graph for continuous change over time, scatter plot for correlation between two variables.

Mean is the average, median is the middle value, mode is the most frequent; standard deviation measures spread around the mean.

A larger sample size reduces sampling error and increases confidence in conclusions.

Energy flows one way through ecosystems while matter cycles; decomposers return nutrients to the soil.

Pyramid of energy is always upright; pyramids of numbers can be inverted (e.g., many insects on one tree).

Primary succession starts on bare rock with pioneer species; secondary succession follows disturbance where soil remains.

Climax community is the relatively stable end stage of succession in a given climate.

Biodiversity loss reduces ecosystem resilience to disturbance and disease.

Conservation strategies include protected areas, habitat corridors, captive breeding, and reduction of pollution.

Sustainable resource use means harvest rate does not exceed regeneration rate.

Mutation is the ultimate source of new alleles; recombination and gene flow reshuffle them.

Directional selection shifts a trait in one direction; stabilizing selection favors the mean; disruptive selection favors extremes.

Antibiotic resistance is natural selection acting on bacteria: resistant strains survive and reproduce.

Adaptive radiation is rapid diversification of one lineage into many niches (e.g., Darwin's finches).

Coevolution is reciprocal evolutionary change between interacting species, such as predator and prey.

Vestigial structures are remnants of organs that had a function in ancestors.

Molecular evidence: the more similar the DNA or protein sequences, the more recent the common ancestor.

Mass extinction events remove many lineages and open ecological niches for survivors.

A good experiment isolates one independent variable; changing several at once confounds results.

Blinding prevents bias: single-blind hides treatment from subjects, double-blind hides it from subjects and researchers.

Percent change equals (new value minus old value) divided by old value, multiplied by 100.

A rate is change in the dependent variable divided by the time or distance over which it occurred.

If a graph's slope is constant, the relationship is linear; a changing slope indicates a non-linear relationship.

Error bars show variability or uncertainty; non-overlapping error bars between groups suggest a real difference.

Qualitative data describe categories or observations; quantitative data are numerical and can be averaged and graphed.

Repeating an experiment by other researchers is replication, and it strengthens confidence in a conclusion.

Prokaryotic cells lack a nucleus and membrane-bound organelles; eukaryotic cells have both.

Plant cells have a cell wall, chloroplasts, and a large central vacuole; animal cells do not.

Cellular respiration summary: glucose plus oxygen yields CO2, water, and ATP; glycolysis in cytoplasm, Krebs cycle and electron transport in mitochondria.

Photosynthesis: light reactions in thylakoids produce ATP and NADPH; Calvin cycle in the stroma fixes CO2 into sugar.

ATP is the universal energy currency; hydrolysis of its terminal phosphate releases usable energy for cellular work.

Diffusion rate increases with steeper concentration gradient, higher temperature, smaller molecule size, and shorter distance.

Endocytosis imports material by membrane infolding; exocytosis exports material by vesicle fusion with the membrane.

Apoptosis is programmed cell death; uncontrolled division produces tumors.

Digestion: amylase breaks starch, pepsin digests protein in acid stomach, lipase digests fats; villi increase absorptive surface area.

Small intestine absorbs nutrients; large intestine reabsorbs water and ions.

Respiration mechanics: diaphragm contracts and flattens, thoracic volume increases, pressure falls, air enters.

Tidal volume is air moved per normal breath; vital capacity is the maximum volume exhaled after full inspiration.

Heart conduction order: SA node, AV node, bundle of His, Purkinje fibers; the SA node sets the rhythm.

Arteries carry blood away from the heart under high pressure; veins carry blood back and contain valves.

Liver functions include detoxification, bile production, and storage of glycogen.

Homeostatic control involves a sensor, an integrating center, and an effector responding to the deviation.

Genotype is the allele combination; phenotype is the observable trait resulting from genotype and environment.

Homozygous means two identical alleles; heterozygous means two different alleles.

Incomplete dominance blends phenotypes; codominance expresses both alleles distinctly (e.g., AB blood type).

Multiple alleles: the ABO system has IA, IB, and i alleles; IA and IB are codominant, i is recessive.

Pedigree rules: a recessive trait can skip generations; a dominant trait appears in every affected generation.

Karyotype shows chromosome number and structure; human diploid number is 46 (23 pairs).

Nondisjunction produces gametes with an extra or missing chromosome, causing conditions such as trisomy 21.

Restriction enzymes cut DNA at specific sequences; gel electrophoresis separates fragments by size, smaller fragments moving farther.

The nervous system divides into central (brain, spinal cord) and peripheral (somatic and autonomic) divisions.

Sympathetic division prepares the body for action (fight or flight); parasympathetic division promotes rest and digestion.

Neurotransmitters are removed from the synapse by reuptake, enzymatic degradation, or diffusion.

Cerebrum handles conscious thought and voluntary movement; cerebellum coordinates balance and fine motor control; medulla controls breathing and heart rate.

Brain regions: frontal lobe for planning and movement, occipital lobe for vision, temporal lobe for hearing and memory.

Sensory receptors transduce stimuli into receptor potentials; adaptation is reduced response to a constant stimulus.

Taste and smell are chemoreceptors; touch receptors include mechanoreceptors for pressure and vibration.

Reflexes are involuntary and rapid because they bypass conscious brain processing.

Endocrine glands secrete hormones into the blood; exocrine glands secrete through ducts.

Pancreas is both endocrine (islets of Langerhans) and exocrine (digestive enzymes).

Adrenal cortex secretes cortisol and aldosterone; adrenal medulla secretes adrenaline.

Thyroid hormones regulate metabolic rate; iodine deficiency causes goiter and hypothyroidism.

Pituitary is the master gland: posterior stores ADH and oxytocin, anterior secretes FSH, LH, TSH, ACTH, GH, and prolactin.

Diabetes mellitus type 1 is insulin deficiency; type 2 is insulin resistance.

Fertilization occurs in the fallopian tube; the zygote implants in the uterine wall about a week later.

Contraceptive hormones prevent ovulation by suppressing FSH and LH through negative feedback.

Antigens are molecules that trigger an immune response; epitopes are the specific binding regions recognized.

Phagocytosis: macrophages and neutrophils engulf and digest pathogens, then present antigens to T cells.

Inflammation signs are redness, heat, swelling, and pain, caused by vasodilation and increased permeability.

Antibodies neutralize toxins and viruses, agglutinate pathogens, and mark them for phagocytosis.

Blood groups: A and B antigens on red cells; anti-A and anti-B antibodies in plasma; O negative is the universal donor.

Rh incompatibility can cause hemolytic disease of the newborn in a second pregnancy.

Immunosuppression increases susceptibility to opportunistic infections.

Memory B and T cells persist after infection and enable a rapid secondary response.

Hormone action on target cells depends on receptor presence, not on hormone concentration alone.

Blood pressure is cardiac output times peripheral resistance.

Kidney filtration occurs in the glomerulus; glucose is normally fully reabsorbed, so glucose in urine signals high blood sugar.

Sweat glands and skin blood flow are effectors in temperature regulation.

Respiration rate rises with exercise to remove excess CO2 and supply oxygen.

Insulin promotes cellular uptake of glucose and glycogen storage in liver and muscle.

Osmoreceptors in the hypothalamus detect blood osmolarity and trigger thirst and ADH release.

Homeostatic imbalance examples: dehydration, hypoglycemia, fever, and acidosis.

Biotic factors are living (predators, competitors, pathogens); abiotic factors are non-living (light, temperature, water, soil).

Habitat is where an organism lives; niche is its role and resource use within the community.

Carrying capacity is the maximum population an environment can sustain indefinitely.

Predator-prey cycles often lag: prey rise, predators rise, prey fall, predators fall.

Competitive exclusion: two species cannot occupy the same niche indefinitely; one outcompetes the other.

Indicator species reveal environmental quality; lichens indicate clean air.

Renewable resources regenerate on human timescales; fossil fuels do not.

Habitat fragmentation reduces gene flow and increases extinction risk for isolated populations.

Species is a group whose members can interbreed and produce fertile offspring.

Phylogenetic trees show common ancestry; branch points represent divergence events.

Three domains of life: Bacteria, Archaea, Eukarya.

Viruses are not classified as living cells because they lack independent metabolism and reproduction.

Convergent evolution produces analogous traits in unrelated lineages facing similar environments.

Genetic variation within a population is measured by allele frequencies.

Bottleneck and founder effects reduce genetic diversity in a population.

Extinction occurs when a species cannot adapt to changing conditions or compete for resources.

Steps of inquiry: observation, question, hypothesis, prediction, experiment, analysis, conclusion.

A hypothesis supported by data is not proven; it remains provisional and may be revised.

Units must be stated with every measurement; SI base units include meter, kilogram, second, mole.

Precision refers to closeness of repeated measurements to each other; accuracy refers to closeness to the true value.

Percentage error equals the absolute difference between measured and accepted value divided by the accepted value, times 100.

A model is a simplified representation and has limits that must be acknowledged.

Tables should have labeled columns with units; graphs need labeled axes with units and a title.

A conclusion must be limited to what the data support and must state possible sources of error.

Cell theory: all organisms are made of cells, the cell is the basic unit of life, and cells arise from pre-existing cells.

Nucleus contains chromatin and the nucleolus; nuclear pores control exchange with the cytoplasm.

Lysosomes contain hydrolytic enzymes for intracellular digestion; peroxisomes break down fatty acids and hydrogen peroxide.

Cytoskeleton (microtubules, microfilaments) maintains shape and enables movement and organelle transport.

Golgi apparatus modifies, sorts, and packages proteins received from the ER.

Chloroplast structure: thylakoid stacks (grana) and stroma; chlorophyll absorbs mainly red and blue light.

Mitosis produces two genetically identical diploid cells; meiosis produces four genetically varied haploid cells.

Crossing over in prophase I of meiosis increases genetic variation.

DNA replication is semiconservative: each new double helix contains one original and one new strand.

Helicase unwinds DNA; DNA polymerase adds nucleotides only in the 5' to 3' direction.

Central dogma: DNA to RNA to protein; retroviruses reverse-transcribe RNA to DNA.

Genetic code is universal and degenerate: several codons can specify the same amino acid.

A gene is a DNA sequence coding for a protein or functional RNA.

Recombinant DNA technology joins DNA from different sources using restriction enzymes and ligase.

PCR amplifies DNA through repeated cycles of denaturation, annealing, and extension.

Transgenic organisms carry genes introduced from another species.

Neurons are excitable cells; glial cells support, insulate, and nourish neurons.

Threshold is the minimum stimulus needed to trigger an action potential; stronger stimuli increase frequency, not amplitude.

Saltatory conduction is faster than continuous conduction along unmyelinated axons.

Excitatory neurotransmitters depolarize the postsynaptic membrane; inhibitory ones hyperpolarize it.

The knee-jerk reflex involves a sensory neuron synapsing directly with a motor neuron in the spinal cord.

Retina layers: photoreceptors, bipolar cells, ganglion cells; the optic nerve carries signals to the brain.

Lens accommodation: ciliary muscles contract to thicken the lens for near vision.

Hearing range in humans is roughly 20 Hz to 20,000 Hz.

Hormone types: steroid (lipid-soluble), peptide and protein (water-soluble), and amine derivatives.

Steroid hormones act slowly and have long-lasting effects; peptide hormones act quickly via second messengers.

Feedback loops maintain hormone levels within narrow ranges; loss of feedback causes disorders such as hyperthyroidism.

Growth hormone promotes growth of bones and tissues; excess before puberty causes gigantism, after puberty acromegaly.

Insulin and glucagon are antagonistic hormones maintaining blood glucose near 1 g/L.

Oogenesis produces one ovum and polar bodies; spermatogenesis produces four sperm cells.

Implantation requires a receptive endometrium prepared by progesterone.

Puberty is triggered by increased GnRH, FSH, and LH secretion.

Lymphoid organs: bone marrow and thymus are primary; spleen and lymph nodes are secondary.

B cells mature in bone marrow; T cells mature in the thymus.

Antigen-presenting cells display antigens on MHC molecules to helper T cells.

Clonal selection: an antigen selects and stimulates the lymphocytes with matching receptors to multiply.

Passive immunity transfers antibodies (e.g., maternal antibodies through placenta or colostrum) and is temporary.

Active immunity follows infection or vaccination and generates memory cells.

Complement proteins form pores in bacterial membranes, causing lysis.

Fever is a defense response that can inhibit pathogen growth and speed immune reactions.

Blood composition: plasma, red cells (erythrocytes), white cells (leukocytes), and platelets.

Red blood cells lack a nucleus and carry hemoglobin for oxygen transport; lifespan is about 120 days.

Platelets and clotting factors form clots to stop bleeding; vitamin K is needed for clotting factors.

Muscle contraction: actin and myosin slide past each other when calcium and ATP are present.

Bone serves as calcium reserve; parathyroid hormone raises blood calcium, calcitonin lowers it.

Nerve-muscle junction releases acetylcholine to trigger muscle contraction.

Skin is the largest organ and a barrier against pathogens and water loss.

Exercise increases heart rate, breathing rate, and blood flow to muscles to meet higher oxygen demand.

Ecosystem includes the community of organisms plus the physical environment.

Biomes are large terrestrial regions defined by climate and vegetation, such as desert, tundra, and tropical rainforest.

The water cycle: evaporation, condensation, precipitation, runoff, and infiltration.

Human impacts include deforestation, pollution, overfishing, and greenhouse gas emissions.

Greenhouse gases such as CO2 and methane trap heat and contribute to global warming.

Recycling and renewable energy reduce resource depletion and pollution.

Invasive species can outcompete native species and disrupt food webs.

Ecological footprint measures the land and resources needed to support a population's consumption.

Lamarck proposed inheritance of acquired traits; Darwin proposed natural selection acting on heritable variation.

Variation arises from mutation and recombination, not from need or use.

Fitness is reproductive success relative to other individuals in the population.

Sexual selection favors traits that improve mating success, sometimes at a survival cost.

Geographic isolation plus time can produce reproductive isolation and new species.

The fossil record shows transitional forms and changing lineages over geological time.

Taxonomy groups organisms by shared characteristics and evolutionary relationships.

Biodiversity includes genetic, species, and ecosystem diversity.

A null hypothesis states no difference or no effect between groups; data may reject or fail to reject it.

Sample must be representative of the population; biased sampling invalidates conclusions.

Ethical considerations in biological experiments include informed consent and humane treatment of animals.

Interpreting graphs: read values from axes carefully, note units and scale, and identify trends and outliers.

A trend is a general direction in data; an outlier is a value far from the pattern and must be explained or excluded with justification.

Extrapolation beyond the data range is risky and should be labeled as uncertain.

Peer review and publication allow the scientific community to check methods and conclusions.

Scientific knowledge changes as new evidence accumulates; it is not fixed.

Enzymes lower activation energy and are not consumed; they are specific to substrate shape at the active site.

Enzyme activity is affected by temperature, pH, substrate concentration, and inhibitors; extreme heat denatures the protein.

Competitive inhibitors bind the active site and can be overcome by more substrate; non-competitive inhibitors bind elsewhere.

Cofactors (e.g., metal ions) and coenzymes (e.g., NAD+) help enzymes function.

ATP synthesis in mitochondria uses a proton gradient across the inner membrane (chemiosmosis).

Anaerobic respiration produces less ATP than aerobic respiration and yields lactate or ethanol.

Photosynthesis equation: 6CO2 + 6H2O + light yields C6H12O6 + 6O2.

Cell membranes are selectively permeable; small nonpolar molecules cross easily, ions need channels.