Bagrut Biology 5 Units cheat sheet

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; they have a nucleoid, 70S ribosomes, and often a peptidoglycan wall; eukaryotic cells have a nucleus, 80S ribosomes, and compartmentalized organelles.

Endosymbiotic theory: mitochondria and chloroplasts derive from engulfed bacteria; evidence = double membrane, own circular DNA, 70S ribosomes, binary fission, similar size.

Ribosome function: free ribosomes synthesize cytosolic proteins; rough ER-bound ribosomes synthesize proteins destined for secretion or membranes, then routed via ER → Golgi → vesicle → plasma membrane.

Lysosomes contain hydrolytic enzymes at acidic pH (~5); they degrade macromolecules and organelles (autophagy); peroxisomes instead carry out fatty acid β-oxidation and catalase-mediated H₂O₂ breakdown.

Fluid mosaic model: phospholipid bilayer with embedded integral proteins, peripheral proteins, cholesterol (modulates fluidity), and glycoproteins/glycolipids on the outer face for recognition.

Diffusion is passive down a concentration gradient; osmosis is water movement across a semipermeable membrane toward higher solute concentration; both require no ATP.

Facilitated diffusion uses channel or carrier proteins down a gradient (no ATP); active transport moves solutes against a gradient using ATP (e.g., Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in per ATP).

Surface area-to-volume ratio decreases as cell size increases; this limits cell size because exchange surfaces cannot keep pace with metabolic demand.

Water potential Ψ = Ψs (solute potential, negative) + Ψp (pressure potential); water moves from higher (less negative) Ψ to lower Ψ.

Condensation reactions join monomers with loss of water (glycosidic, peptide, ester bonds); hydrolysis breaks these bonds using water.

Carbohydrates: monosaccharides (glucose, C₆H₁₂O₆), disaccharides (sucrose = glucose + fructose; lactose = glucose + galactose), polysaccharides (starch, glycogen, cellulose); cellulose is β-1,4-linked and indigestible to humans.

Proteins: amino acids linked by peptide bonds; four levels of structure; primary sequence determines folding; denaturation by heat, extreme pH, or heavy metals disrupts hydrogen bonds and hydrophobic interactions, not peptide bonds.

Lipids: triglycerides = glycerol + 3 fatty acids (ester bonds); saturated fatty acids have no C=C and pack tightly; phospholipids are amphipathic and form bilayers.

Nucleic acids: nucleotides = phosphate + pentose + nitrogenous base; DNA has deoxyribose and thymine, RNA has ribose and uracil; A–T (2 H-bonds), G–C (3 H-bonds).

Enzymes lower activation energy; they are not consumed; activity is affected by temperature (denaturation above optimum), pH, substrate concentration (saturates at Vmax), and inhibitors.

Competitive inhibitors bind the active site and are overcome by higher substrate concentration; non-competitive inhibitors bind elsewhere and reduce Vmax; allosteric regulation changes enzyme conformation.

ATP is the universal energy currency: hydrolysis ATP → ADP + Pi releases energy; coupled to endergonic reactions; regeneration occurs in respiration.

Glycolysis (cytosol): glucose → 2 pyruvate, net 2 ATP and 2 NADH; aerobic respiration then proceeds to the Krebs cycle and oxidative phosphorylation in mitochondria, yielding ~30–32 ATP per glucose.

DNA replication is semi-conservative: helicase unwinds, DNA polymerase synthesizes 5'→3' on each template strand; leading strand is continuous, lagging strand forms Okazaki fragments joined by ligase.

Transcription: RNA polymerase makes mRNA from the template strand (5'→3'); in eukaryotes introns are spliced out and a cap and poly-A tail are added before export.

Translation: mRNA codons are read by tRNA anticodons at the ribosome; start codon AUG (methionine); stop codons UAA, UAG, UGA; genetic code is degenerate and nearly universal.

Mendel's law of segregation: the two alleles of a gene separate into different gametes; law of independent assortment applies to genes on different chromosomes.

Test cross: cross an unknown dominant phenotype with a homozygous recessive to determine genotype; dihybrid F₂ ratio 9:3:3:1 for unlinked genes.

Sex-linked inheritance: X-linked recessive traits appear mostly in males (hemizygous); an affected father cannot pass an X-linked allele to sons; carrier mothers pass to 50% of sons.

Non-Mendelian patterns: incomplete dominance (blended phenotype), codominance (both expressed, e.g., AB blood type), multiple alleles (ABO: I^A, I^B, i), polygenic traits, epistasis.

Restriction enzymes cut DNA at specific palindromic recognition sites producing sticky ends; DNA ligase joins fragments; gel electrophoresis separates fragments by size (smaller fragments migrate farther).

PCR amplifies DNA: denaturation (~95°C), primer annealing (~55°C), extension by Taq polymerase (~72°C); each cycle doubles DNA.

Pedigree decision rules: recessive traits often skip generations; dominant traits appear in every generation; X-linked recessive affects males disproportionately and is not transmitted father→son.

Darwin's mechanism: variation exists within populations, more offspring are produced than survive, differential reproductive success (natural selection) leads to adaptation over generations.

Sources of genetic variation: mutation (ultimate source), independent assortment, crossing over, random fertilization, gene flow.

Hardy-Weinberg equilibrium: p + q = 1 and p² + 2pq + q² = 1; p = dominant allele frequency, q = recessive; genotype frequencies p² (AA), 2pq (Aa), q² (aa).

Hardy-Weinberg assumptions: large population, random mating, no mutation, no migration, no selection; deviations indicate evolution is occurring.

Types of selection: directional (one extreme favored), stabilizing (intermediate favored), disruptive (both extremes favored).

Genetic drift: random allele frequency change, strongest in small populations; bottleneck and founder effects reduce genetic diversity.

Speciation requires reproductive isolation (geographic/allopatric or sympatric); prezygotic barriers include temporal, behavioral, and mechanical isolation; postzygotic barriers yield sterile or inviable hybrids.

Evidence for evolution: fossil record, homologous structures (common ancestry), analogous structures (convergent evolution), vestigial structures, molecular/DNA sequence similarity, biogeography.

Allele frequency change = (new allele count / total alleles in population); selection against recessive homozygotes removes q² each generation but recessive alleles persist in heterozygotes.

Homeostasis uses negative feedback: sensor detects deviation, control center (often hypothalamus) integrates, effector reverses the change; positive feedback amplifies (e.g., oxytocin in childbirth).

Blood glucose: insulin (β-cells, lowers glucose by promoting uptake and glycogen synthesis) and glucagon (α-cells, raises glucose by glycogenolysis) act antagonistically.

Nervous vs endocrine: neurons act fast and locally via neurotransmitters across synapses; hormones act slower and longer via the bloodstream on target cells with specific receptors.

Action potential: resting potential ≈ -70 mV (Na⁺/K⁺ pump); depolarization by Na⁺ influx, repolarization by K⁺ efflux; refractory period prevents backward conduction; myelination enables saltatory conduction.

Respiratory system: gas exchange by diffusion across alveolar walls; hemoglobin binds O₂ cooperatively (sigmoid dissociation curve); CO₂ transported mainly as bicarbonate; breathing controlled by medulla chemoreceptors sensing CO₂/pH.

Circulatory system: double circulation; arteries carry blood away from the heart (thick elastic walls), veins toward it (valves), capillaries allow exchange; cardiac cycle includes systole and diastole.

Kidney function: filtration in glomerulus, selective reabsorption in proximal tubule, loop of Henle creates osmotic gradient, ADH increases collecting-duct water permeability; aldosterone increases Na⁺ reabsorption.

Thermoregulation: hypothalamus set point; sweating and vasodilation cool; shivering and vasoconstriction warm; negative feedback restores core temperature.

Digestion: enzymes include amylase (starch → maltose), pepsin (protein, acidic stomach), lipase (lipids → fatty acids + glycerol); villi and microvilli increase absorptive surface area.

Human reproduction is sexual with internal fertilization; spermatogenesis produces four haploid sperm per meiosis; oogenesis produces one ovum and polar bodies.

Hormonal control: FSH stimulates follicle development, LH triggers ovulation and corpus luteum formation; estrogen and progesterone regulate the menstrual cycle and maintain the endometrium.

Meiosis reduces chromosome number from diploid (2n) to haploid (n); crossing over in prophase I and independent assortment in metaphase I generate variation; fertilization restores 2n.

Fertilization occurs in the fallopian tube; the zygote undergoes cleavage → morula → blastocyst → implantation; gastrulation forms the three germ layers (ectoderm, mesoderm, endoderm).

Placenta mediates maternal–fetal exchange of nutrients, gases, and wastes; maternal and fetal blood do not mix; hCG maintains the corpus luteum early in pregnancy.

Innate immunity: barriers (skin, mucus), inflammation, phagocytes, natural killer cells; non-specific and immediate, no immunological memory.

Adaptive immunity: B lymphocytes produce antibodies (humoral); T lymphocytes include helper T (activate B cells) and cytotoxic T (kill infected cells); memory cells give long-lasting secondary responses.

Antibodies are proteins with variable antigen-binding sites; antigen–antibody complexes mark pathogens for destruction; primary response is slower and weaker than secondary.

Vaccination exposes the immune system to antigens (weakened/killed pathogen or subunit) to generate memory cells without causing disease; herd immunity reduces transmission.

Population growth: exponential (J-shaped) when resources unlimited; logistic (S-shaped) with carrying capacity K; growth rate = (births + immigration) − (deaths + emigration).

Population density and dispersion patterns (clumped, uniform, random) reflect resource distribution and interactions; mark-recapture estimates N = (marked × total caught) / marked recaptured.

Species interactions: competition (−/−), predation and parasitism (+/−), mutualism (+/+), commensalism (+/0); competitive exclusion vs resource partitioning (niche differentiation).

Trophic levels: producers → primary consumers → secondary consumers → decomposers; only ~10% of energy transfers between levels; energy flows one way, nutrients cycle.

Food webs: keystone species disproportionately affect community structure; removal causes trophic cascade; invasive species disrupt native interactions.

Biogeochemical cycles: carbon (photosynthesis/respiration, combustion), nitrogen (fixation, nitrification, denitrification), water cycle; human activity alters these cycles.

Succession: primary succession begins on bare substrate (pioneer species like lichens), secondary after disturbance; ends in a climax community; biodiversity generally increases with succession.

Human impact: greenhouse gases (CO₂, CH₄) cause warming; eutrophication from nutrient runoff causes algal blooms and oxygen depletion; habitat fragmentation reduces biodiversity; conservation strategies include protected areas and corridors.

Controlled experiment components: independent variable (manipulated), dependent variable (measured), controlled variables held constant, control group for comparison.

Hypothesis must be testable and falsifiable; prediction states the expected outcome if the hypothesis is correct; theories are well-supported explanations, not guesses.

Validity requires controlling confounding variables; reliability increases with repeated trials and larger sample sizes; random sampling reduces bias.

Correlation does not imply causation; an observed association may result from a third variable.

Data types: continuous (measured, e.g., length) vs discrete/categorical (counts, categories); choose appropriate graph (bar for categories, line for trends over time, scatter for relationships).

Central tendency: mean (sensitive to outliers), median (robust), mode; spread measured by range, variance, standard deviation; standard error indicates precision of the mean.

Error sources: systematic (consistent bias, affects accuracy) vs random (scatter, affects precision); calibrate instruments and use blinds to reduce bias.

Statistical significance: p < 0.05 commonly indicates results unlikely under the null hypothesis; report n, means, and variability; error bars showing non-overlap suggest a real difference.

Ethical and safety considerations: informed consent, humane treatment of animals, proper disposal of chemicals, and reproducibility of methods.

Microbiology and biotechnology: bacteria as model organisms; transformation introduces plasmids; selectable markers (antibiotic resistance) identify transformed cells; recombinant insulin as a product example.

Genetic engineering workflow: identify gene → cut with restriction enzymes → insert into vector → transform host → select and screen → express product; CRISPR-Cas9 uses guide RNA to target specific sequences for editing.

Human genetics and genetic testing: karyotype detects chromosomal abnormalities; carrier screening and prenatal diagnosis; ethical issues include privacy, insurance, and informed consent.

Bioinformatics: sequence alignment, BLAST searches, phylogenetic trees built from molecular data; conserved sequences suggest functional importance.

Immunology applications: monoclonal antibodies produced by hybridoma cells; ELISA uses antibodies to detect antigens; blood typing based on ABO and Rh antigens.

Neurobiology: neurons transmit electrical signals; neurotransmitters cross synapses; drugs and toxins can act as agonists or antagonists at receptors.

Environmental biotechnology: bioremediation uses microorganisms to degrade pollutants; biofuels and wastewater treatment rely on microbial metabolism.

Experimental analysis in electives: interpret graphs of enzyme kinetics, population data, or genetic crosses; identify variables, controls, and sources of error; draw conclusions supported by data.