HESI A2 Biology cheat sheet

Cell theory: all living things are made of cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells by division.

Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles; they have a single circular chromosome, 70S ribosomes, and a cell wall. Bacteria are the only prokaryotic group tested.

Eukaryotes have a true nucleus bounded by a double membrane and membrane-bound organelles (mitochondria, ER, Golgi, lysosomes). Ribosomes are the only organelle shared with prokaryotes.

Organelle functions: mitochondria = ATP via aerobic respiration; ribosomes = protein synthesis; rough ER = protein processing and transport; smooth ER = lipid synthesis and detoxification; Golgi = modification, sorting, packaging; lysosome = intracellular digestion; chloroplast = photosynthesis.

Plasma membrane is a phospholipid bilayer with embedded proteins (fluid mosaic model). Small nonpolar molecules (O2, CO2) diffuse freely; ions and large polar molecules (glucose) need channels or carriers.

Osmosis moves water across a semipermeable membrane from low solute concentration to high solute concentration. Hypertonic solution = cell shrinks (crenation); hypotonic = cell swells or bursts (lysis); isotonic = no net change.

Mitosis produces two genetically identical diploid daughter cells for growth and repair; meiosis produces four genetically unique haploid gametes through two divisions with crossing over in prophase I.

DNA is a double helix of nucleotides (deoxyribose sugar, phosphate, nitrogenous base) with base pairing A-T and G-C. RNA is single-stranded, contains ribose and uracil instead of thymine.

Enzymes are proteins that lower activation energy and are not consumed in the reaction. They are specific to substrate, denature at high temperature or extreme pH, and follow lock-and-key or induced-fit models.

Punnett square rules: homozygous dominant x homozygous recessive gives 100% heterozygous offspring; heterozygous x heterozygous gives 3:1 phenotype and 1:2:1 genotype ratios.

Codominance = both alleles expressed equally (AB blood type); incomplete dominance = blended intermediate phenotype (red x white flowers give pink).

Genotype is the allele combination (AA, Aa, aa); phenotype is the observable trait. A recessive phenotype requires two recessive alleles.

Photosynthesis: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2, occurring in chloroplasts; light reactions in thylakoid membranes, Calvin cycle in the stroma.

Cellular respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP. Glycolysis in cytoplasm, Krebs cycle in mitochondrial matrix, electron transport chain on the inner mitochondrial membrane.

Aerobic respiration yields about 36-38 ATP per glucose; anaerobic fermentation yields 2 ATP and produces lactate (muscle) or ethanol and CO2 (yeast).

Classification order from broadest to most specific: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Binomial nomenclature italicizes genus and species (Homo sapiens).

Natural selection requires heritable variation, competition for limited resources, and differential reproductive success; it acts on phenotypes, changing allele frequencies over generations.

Homeostasis is maintained by negative feedback loops (a change triggers a response that reverses it), such as insulin lowering blood glucose; positive feedback amplifies a change (childbirth contractions).