ORT Biology cheat sheet

Prokaryotic cells lack membrane-bound organelles and a nucleus; their DNA is a single circular chromosome in the nucleoid region, and they may also carry plasmids.

The fluid mosaic model describes the plasma membrane as a phospholipid bilayer with embedded proteins, cholesterol, and glycolipids; hydrophobic tails face inward, hydrophilic heads face outward.

Enzyme activity is affected by temperature, pH, substrate concentration, and inhibitors; competitive inhibitors raise Km without changing Vmax, while non-competitive inhibitors lower Vmax without changing Km.

ATP is the universal energy currency: hydrolysis of ATP to ADP + Pi releases about 30.5 kJ/mol under standard conditions and drives endergonic reactions by coupling.

Glycolysis occurs in the cytoplasm and yields a net 2 ATP, 2 NADH, and 2 pyruvate per glucose; it does not require oxygen.

The Krebs cycle takes place in the mitochondrial matrix and yields 2 ATP, 6 NADH, 2 FADH2, and 4 CO2 per glucose (per two turns).

Oxidative phosphorylation occurs on the inner mitochondrial membrane; the electron transport chain pumps H+ to create a proton gradient, and ATP synthase uses that gradient to make about 26–28 ATP per glucose.

Osmosis is the diffusion of water across a selectively permeable membrane from low to high solute concentration; animal cells lyse in hypotonic solutions and shrink (crenate) in hypertonic solutions.

The cell cycle has interphase (G1, S, G2) and M phase; DNA replication occurs in S phase, and mitosis is divided into prophase, metaphase, anaphase, and telophase.

DNA is a double helix with antiparallel strands; adenine pairs with thymine (2 hydrogen bonds) and guanine pairs with cytosine (3 hydrogen bonds).

Semiconservative replication: helicase unwinds DNA, DNA polymerase synthesizes the leading strand continuously and the lagging strand in Okazaki fragments, and ligase joins the fragments.

Transcription produces mRNA from the template strand in the nucleus (in eukaryotes); RNA polymerase binds the promoter, and introns are spliced out before translation.

Translation occurs on ribosomes: mRNA codons are read in triplets, tRNA brings the matching amino acid, and the sequence always starts at the AUG start codon.

A monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio; a test cross uses a homozygous recessive partner to reveal an unknown genotype.

Dihybrid cross of two heterozygotes (AaBb × AaBb) gives a 9:3:3:1 phenotypic ratio when genes assort independently.

Sex-linked traits are carried on sex chromosomes; in humans, X-linked recessive disorders (e.g., haemophilia) affect males more often because males have only one X chromosome.

Mutations include substitutions (point mutations), insertions, and deletions; frameshift mutations alter the reading frame and usually have more severe effects than substitutions.

The lac operon in E. coli is an inducible operon: lactose binds the repressor, allowing RNA polymerase to transcribe the structural genes only when lactose is present and glucose is low.

Blood flows through the heart in this order: right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

The left ventricle has the thickest myocardium because it pumps blood at high pressure to the systemic circulation; the right ventricle pumps to the low-pressure pulmonary circuit.

The cardiac cycle: SA node initiates the impulse, AV node delays it, then the bundle of His and Purkinje fibres spread excitation; systole is contraction and diastole is relaxation.

Gas exchange occurs in the alveoli by diffusion; oxygen moves from alveolar air into the blood, and carbon dioxide moves from blood into the alveoli.

Nephron function: filtration in the glomerulus, reabsorption of glucose, amino acids, and most water in the proximal tubule, and secretion and concentration in the loop of Henle and collecting duct.

Neurons transmit impulses as electrical signals along the axon and as chemical signals across synapses via neurotransmitters; the resting potential is maintained by the Na+/K+ pump.

Homeostasis is maintained by negative feedback loops: a receptor detects a change, a control centre processes it, and an effector reverses the change (e.g., insulin lowers blood glucose).

The sliding filament model of muscle contraction: myosin heads bind actin, the power stroke shortens the sarcomere, and ATP and calcium ions are required.

A population is a group of individuals of one species in a given area; a community is all populations living together; an ecosystem includes the community plus the abiotic environment.

Population growth: exponential growth occurs with unlimited resources (J-shaped curve); logistic growth levels off at the carrying capacity K (S-shaped curve).

Energy transfer between trophic levels is about 10% efficient; this limits food chains to about 4–5 trophic levels and explains why top predators are few.

The carbon cycle involves photosynthesis (CO2 fixation), respiration, decomposition, combustion, and ocean absorption; the nitrogen cycle relies on nitrogen-fixing bacteria, nitrification, and denitrification.

Primary succession starts on bare rock with pioneer species such as lichens and mosses; secondary succession starts after a disturbance where soil already exists.

Symbiosis includes mutualism (both benefit), commensalism (one benefits, the other is unaffected), and parasitism (one benefits, the other is harmed).

Bioaccumulation is the buildup of a substance within an organism; biomagnification is the increasing concentration of a pollutant along a food chain, especially fat-soluble toxins such as DDT and mercury.

The greenhouse effect is the trapping of infrared radiation by gases such as CO2, methane, and water vapour; increased concentrations enhance global warming.

Darwin's theory of natural selection: variation exists within populations, more offspring are produced than can survive, individuals with favourable traits survive and reproduce, and advantageous alleles increase in frequency over generations.

Evidence for evolution includes fossil records, comparative anatomy (homologous structures), embryology, molecular biology (DNA and protein similarities), and observed examples such as antibiotic resistance in bacteria.

Homologous structures share a common ancestry but may differ in function (e.g., human arm, bat wing, whale flipper); analogous structures have similar function but different ancestry (e.g., bird wing and insect wing).

A species is a group of organisms that can interbreed to produce fertile offspring; reproductive isolation leads to speciation, which can be allopatric (geographic separation) or sympatric.

The Hardy–Weinberg principle states that allele frequencies remain constant in a large, randomly mating population with no mutation, migration, or selection; deviations indicate evolution is occurring.

The binomial naming system uses genus and species (e.g., Homo sapiens); classification hierarchy is domain, kingdom, phylum, class, order, family, genus, species.

The three domains of life are Bacteria, Archaea, and Eukarya; the five-kingdom system includes Monera, Protista, Fungi, Plantae, and Animalia.

Biodiversity has three levels: genetic diversity, species diversity, and ecosystem diversity; it is threatened by habitat loss, overexploitation, pollution, invasive species, and climate change.

Photosynthesis equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2; it occurs in chloroplasts, with light-dependent reactions in the thylakoid membranes and the Calvin cycle in the stroma.

Light-dependent reactions split water (photolysis) to release O2 and produce ATP and NADPH; the Calvin cycle uses ATP and NADPH to fix CO2 into glucose.

Chlorophyll absorbs mainly red and blue-violet light and reflects green; accessory pigments such as carotenoids widen the range of light absorbed.

Xylem transports water and dissolved minerals from roots to leaves; transpiration pull, cohesion, and adhesion drive the upward movement. Phloem transports sugars from sources to sinks.

Stomata are pores on leaves controlled by guard cells; they open when guard cells are turgid and close when flaccid, balancing CO2 uptake with water loss.

Plant hormones: auxin promotes cell elongation and phototropism; gibberellin promotes stem elongation and seed germination; abscisic acid closes stomata and promotes dormancy; ethylene ripens fruit.

Monocot leaves have parallel veins and scattered vascular bundles in the stem; dicot leaves have branched veins and vascular bundles arranged in a ring.

Mineral deficiencies: nitrogen deficiency causes yellowing of older leaves and poor growth; magnesium deficiency causes yellowing between veins because Mg is central to chlorophyll.

Asexual reproduction produces genetically identical offspring (clones) by mitosis; examples include binary fission, budding, fragmentation, and vegetative propagation.

Sexual reproduction involves meiosis and fertilisation, producing genetically varied offspring; meiosis halves the chromosome number from diploid (2n) to haploid (n).

Meiosis has two divisions: meiosis I separates homologous chromosomes (crossing over in prophase I), and meiosis II separates sister chromatids, producing four haploid cells.

In humans, spermatogenesis produces four sperm per meiosis; oogenesis produces one ovum and three polar bodies, and the egg is released at ovulation under FSH and LH control.

The menstrual cycle: FSH stimulates follicle development, oestrogen thickens the uterine lining, the LH surge triggers ovulation around day 14, and progesterone maintains the lining after ovulation.

Fertilisation normally occurs in the fallopian tube; the zygote undergoes cleavage and develops into a blastocyst, which implants in the endometrium about 6–7 days after fertilisation.

The placenta exchanges nutrients, gases, and wastes between mother and fetus and secretes hormones including hCG, progesterone, and oestrogen; the umbilical cord connects the fetus to the placenta.

Growth can be measured by height, mass, or cell number; plant growth occurs at meristems (apical and lateral), while animal growth is usually determinate and controlled by hormones such as growth hormone and thyroxine.

Genetic engineering steps: isolate the gene of interest, cut DNA with restriction enzymes, insert the gene into a vector (often a plasmid), transform a host cell, and select and culture the transformed cells.

Restriction enzymes cut DNA at specific recognition sequences (palindromic), often producing sticky ends; DNA ligase joins DNA fragments by forming phosphodiester bonds.

Gel electrophoresis separates DNA fragments by size using an electric field; smaller fragments move faster through the gel, and DNA is negatively charged so it moves toward the positive electrode.

PCR (polymerase chain reaction) amplifies DNA in vitro using denaturation (about 95°C), annealing of primers (about 55°C), and extension by Taq polymerase (about 72°C) over repeated cycles.

CRISPR-Cas9 is a gene-editing tool that uses a guide RNA to target a specific DNA sequence and the Cas9 nuclease to cut it, allowing genes to be knocked out or modified.

Monoclonal antibodies are produced by hybridoma cells formed by fusing a B lymphocyte with a myeloma cell; they are used in diagnosis, pregnancy tests, and targeted therapies.

In industrial fermentation, microorganisms such as yeast or bacteria are cultured in bioreactors to produce products including antibiotics, insulin, and ethanol; optimal temperature, pH, and oxygen supply are controlled.

Tissue culture and micropropagation involve growing plant cells or tissues on sterile nutrient media with hormones to produce many genetically identical plants quickly.