MCAS Biology cheat sheet

Macromolecules: carbohydrates (C, H, O in ~1:2:1 ratio, monomers are monosaccharides, function is quick energy and structure), lipids (mostly C and H, glycerol plus fatty acids, long-term energy storage and membrane construction), proteins (C, H, O, N, sometimes S; monomers are amino acids joined by peptide bonds; function is enzymes, structure, transport), nucleic acids (C, H, O, N, P; monomers are nucleotides; store and transmit genetic information).

Cell theory: all living things are made of one or more cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells. Prokaryotes lack a membrane-bound nucleus and organelles; eukaryotes have a nucleus and membrane-bound organelles.

Organelle functions: nucleus holds DNA and directs activity; ribosome builds protein; rough ER folds and transports protein; smooth ER makes lipids and detoxifies; Golgi modifies, sorts, and packages; mitochondrion performs cellular respiration to make ATP; chloroplast performs photosynthesis; lysosome digests; vacuole stores water and solutes; cell membrane is a phospholipid bilayer controlling what enters and exits.

Photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2, occurring in chloroplasts; light-dependent reactions in the thylakoid membranes split water (releasing O2) and make ATP and NADPH; the Calvin cycle in the stroma uses ATP and NADPH to fix CO2 into glucose.

Cellular respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP; glycolysis in the cytoplasm yields 2 ATP and pyruvate; aerobic respiration continues in mitochondria with the Krebs cycle and electron transport chain, releasing far more ATP; fermentation (anaerobic) regenerates NAD+ and yields little ATP, producing lactate in muscle or ethanol and CO2 in yeast.

Photosynthesis and respiration are complementary: the products of one are the reactants of the other; both involve energy conversion, but photosynthesis stores energy in glucose while respiration releases it to form ATP.

Cell transport: diffusion and osmosis are passive (high to low concentration, no energy); facilitated diffusion uses channel or carrier proteins; active transport moves substances against the gradient and requires ATP (for example, the sodium-potassium pump); endocytosis and exocytosis move material in vesicles.

Osmosis rules: water moves toward the side with higher solute concentration; in a hypertonic solution a cell loses water and shrinks (plant cells undergo plasmolysis); in a hypotonic solution a cell gains water (animal cells may lyse, plant cells become turgid); isotonic means no net movement.

Enzymes are proteins that catalyze reactions by lowering activation energy; they are specific to a substrate at the active site, are reusable, and are affected by temperature and pH; beyond an optimum they denature, changing shape so the substrate no longer fits.

DNA structure: a double helix of nucleotides, each with a phosphate, deoxyribose sugar, and one of four nitrogen bases (A, T, G, C); A pairs with T and G pairs with C by hydrogen bonds; the sequence of bases encodes genetic information.

DNA replication is semiconservative: the helix unzips and each strand serves as a template for a new complementary strand, so each new DNA molecule keeps one original strand.

Protein synthesis: transcription copies a gene into mRNA in the nucleus; mRNA moves to a ribosome where translation reads each codon (three bases) and tRNA delivers the matching amino acid; the sequence of codons determines the sequence of amino acids and therefore protein shape and function.

Mutations: a substitution changes one base and may alter one amino acid or have no effect; insertions or deletions shift the reading frame and usually change many amino acids; mutations in gametes can be inherited, while body-cell mutations are not passed to offspring.

Mendelian genetics: alleles are alternate forms of a gene; a dominant allele masks a recessive one in heterozygotes; genotypes are homozygous or heterozygous, while phenotype is the observable trait; a Punnett square predicts offspring ratios.

Monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio (dominant to recessive) and a 1:2:1 genotype ratio (AA:Aa:aa); a test cross with a homozygous recessive reveals an unknown dominant genotype.

Human genetics: sex-linked traits such as red-green color blindness and hemophilia are carried on the X chromosome, so males (XY) express them more often; blood type shows multiple alleles (IA, IB, i) and codominance; incomplete dominance produces an intermediate phenotype.

Meiosis produces four genetically unique haploid gametes from one diploid cell through two divisions; crossing over in prophase I and independent assortment in metaphase I create variation; fertilization restores the diploid number.

Mitosis produces two genetically identical diploid cells for growth and repair; stages are prophase (chromosomes condense), metaphase (chromosomes line up at the equator), anaphase (sister chromatids separate), and telophase (nuclei reform), followed by cytokinesis.

Evolution by natural selection requires heritable variation, competition for limited resources, and differential survival and reproduction; individuals with traits better suited to the environment leave more offspring, so those traits increase in the population over generations.

Evidence for evolution includes the fossil record (showing change over time and transitional forms), homologous structures (same origin, different function), vestigial structures, comparative embryology, and molecular similarities in DNA and protein sequences.

Natural selection acts on phenotypes, not genotypes directly, and acts on individuals while populations evolve; acquired characteristics are not inherited, and selection produces adaptation to a specific environment rather than progress toward perfection.

Species and speciation: a species is a group of organisms that can interbreed and produce fertile offspring; geographic isolation can lead to reproductive isolation and new species; extinction occurs when a species cannot adapt to changing conditions.

Classification: organisms are grouped by shared characteristics into domains (Bacteria, Archaea, Eukarya) and kingdoms; binomial nomenclature gives each species a two-part Latin name (genus plus species); dichotomous keys identify organisms by paired choices.

Ecology levels: organism, population, community, ecosystem, biome, biosphere; a habitat is where an organism lives, a niche is its role; biotic factors are living and abiotic factors are nonliving.

Energy flow: producers (autotrophs) capture sunlight; consumers (heterotrophs) eat other organisms; decomposers break down dead matter; only about 10 percent of energy passes to the next trophic level, so food chains rarely exceed four or five links and energy pyramids narrow upward.

Symbiosis: mutualism benefits both species, commensalism benefits one with no effect on the other, and parasitism benefits one at the expense of the host; predation and competition are other interactions that shape populations.

Population and cycles: carrying capacity is the maximum population an environment can sustain; limiting factors (food, space, disease, predators) slow growth; the carbon cycle moves carbon through photosynthesis, respiration, combustion, and decomposition, and nitrogen is fixed by bacteria before producers can use it.