October 2026 HESI A2 Biology Study Guide

Cellular biology The cell as the basic unit of life. Prokaryotic cells have no nucleus and no membrane-bound organelles; eukaryotic cells have both. Plant cells add a cell wall, chloroplasts and a large central vacuole. Read the full guide Cell physiology How a cell keeps itself running: producing energy, maintaining its internal environment, responding to signals and regulating what crosses its membrane. Homeostasis is the idea underneath all of it. Read the full guide Cellular transport Movement across the membrane. Passive transport needs no energy and follows the concentration gradient: diffusion, osmosis and facilitated diffusion. Active transport needs ATP and moves substances against the gradient, as the sodium-potassium pump does. Read the full guide Molecular biology The molecules that carry and express information. DNA stores it, RNA carries it, and proteins do the work. Transcription copies DNA into RNA in the nucleus; translation builds a protein from that RNA at the ribosome. Read the full guide Organelles and cell structures Each organelle has one job. Mitochondria produce ATP, ribosomes build proteins, the endoplasmic reticulum transports them, the Golgi apparatus packages them, lysosomes break material down, and the nucleus holds the DNA. Read the full guide Cell division and genetics Mitosis produces two genetically identical cells for growth and repair. Meiosis produces four cells with half the chromosome number, for reproduction. The halving is why offspring have two sets of chromosomes rather than four. Read the full guide Genetics How traits pass between generations. A dominant allele is expressed whenever present; a recessive one only when both copies are recessive. Genotype is the pair of alleles carried; phenotype is what you can observe. Read the full guide Cellular respiration Converting glucose into usable energy. Glycolysis happens in the cytoplasm, the Krebs cycle and the electron transport chain in the mitochondria. Aerobic respiration requires oxygen and yields far more ATP than the anaerobic route. Read the full guide Photosynthesis Plants converting light energy into chemical energy in the chloroplasts. Carbon dioxide and water in, glucose and oxygen out. It is, in inputs and outputs, cellular respiration run backwards. Read the full guide Microbiology Bacteria, viruses, fungi and parasites, and how they differ. Bacteria are living cells and can be treated with antibiotics. Viruses are not cells, cannot reproduce alone, and do not respond to antibiotics. Read the full guide Embryology Development from a fertilised egg. The zygote divides, becomes a blastocyst, implants, and differentiates into three germ layers that give rise to every tissue in the body. Read the full guide Plant biology Plant structure and function: roots absorbing water and minerals, stems transporting them, leaves carrying out photosynthesis. Xylem moves water upward, phloem moves sugars in both directions. Read the full guide Microscopy Using a microscope and interpreting what it shows. Total magnification is the eyepiece multiplied by the objective lens, and resolution is what actually determines whether more magnification is useful. Read the full guide