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.

Most comparison questions come down to three differences. Learn those three rather than a long list.

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.

When a question asks what a cell "does" in a situation, the answer usually restores a balance rather than changes one.

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.

Osmosis is water moving. If the question is about water specifically, it is osmosis, not diffusion.

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.

DNA pairs A with T and C with G. RNA replaces thymine with uracil, so A pairs with U.

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.

Learn organelles as verbs rather than nouns. "Mitochondria make energy" sticks where "mitochondria are the powerhouse" does not.

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.

Mitosis makes two, meiosis makes four. The number in the answer usually tells you which process is meant.

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.

Draw the Punnett square rather than reasoning it out. It takes fifteen seconds and removes the most common error.

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.

Glucose and oxygen in; carbon dioxide, water and ATP out. Knowing the inputs and outputs answers most questions here.

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.

Compare the two equations side by side once. The symmetry makes both far easier to recall.

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.

That last distinction is both a favourite exam item and a real patient-safety issue. Do not lose it.

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.

Learn the three germ layers and one example organ from each. That is the level of detail assessed.

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.

Xylem carries water and phloem carries food. The two words appear in nearly every plant question on the exam.

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.

Always locate the specimen on the lowest power first. Questions about procedure test that order.