3.1 Biology Fundamentals

Biology on the NEX

The Science section of the NLN NEX (Nursing Entrance Exam, the 2025 replacement for the older PAX) contains 60 questions in 60 minutes — about one minute per item. It is one of three timed sections, alongside Verbal Ability (58 questions) and Mathematics (45 questions), for 163 items total. Each section is separately timed at 60 minutes, and there are no physics questions on the NEX. Biology is the largest science subsection, roughly 20 items (~33%), so it deserves the most study time.

Cell Biology

The cell is the basic structural and functional unit of life. The first dividing line tested is prokaryotic versus eukaryotic.

Common trap: the NEX may ask which structure bacteria lack — the answer is a true nucleus, not DNA (bacteria do have DNA, just not enclosed in a nucleus).

Key Organelles

Membrane Transport

Whether a process needs ATP is the single most tested distinction. Movement down a concentration gradient (high to low) is passive and free; movement against the gradient costs energy.

• Passive (no ATP): simple diffusion (O2, CO2), osmosis (water across a membrane), facilitated diffusion (glucose via carrier proteins).
• Active (ATP required): active transport (the sodium-potassium pump, 3 Na+ out / 2 K+ in), endocytosis (cell engulfs material), exocytosis (cell expels material, e.g., neurotransmitter release).

Worked example: A red blood cell placed in distilled water swells and bursts. Distilled water is hypotonic (low solute), so water moves into the cell by osmosis — no ATP needed. In a hypertonic salt solution the same cell shrinks (crenates) as water leaves.

DNA and the Central Dogma

DNA (deoxyribonucleic acid) is a double helix held together by complementary base pairs: A-T and G-C. RNA substitutes uracil (U) for thymine, so in RNA adenine pairs with U. A gene is a DNA segment coding for one protein.

The flow DNA to RNA to protein is the Central Dogma.

Mitosis vs. Meiosis

Mitosis phases follow PMAT: Prophase (chromosomes condense), Metaphase (line up at the equator), Anaphase (sister chromatids separate), Telophase (nuclei reform, cytokinesis splits the cell).

Mendelian Genetics

Dominant alleles (capital letter, e.g., B) are expressed with even one copy; recessive alleles (lowercase, b) require two copies (homozygous bb) to appear. Genotype is the gene combination; phenotype is the visible trait.

For a Bb x Bb cross, the Punnett square gives BB : Bb : bb = 1 : 2 : 1 (genotype) and a 3 : 1 phenotype ratio (75% dominant, 25% recessive). Trap: "shows the recessive trait" means bb only = 25%, not 75%.

How to Read a Monohybrid Cross

To solve any single-trait Punnett square quickly, follow four steps. First, write each parent's two alleles. Second, place one parent's alleles across the top of a two-by-two grid and the other parent's alleles down the side. Third, fill each box by combining the row and column letter, writing the capital letter first. Fourth, count the offspring boxes to read the ratios.

A few crosses are worth memorizing so you never have to draw the grid under time pressure. A homozygous dominant parent crossed with a homozygous recessive parent (BB times bb) gives all heterozygous Bb offspring, so 100 percent show the dominant trait. A heterozygous parent crossed with a homozygous recessive parent (Bb times bb) gives a one-to-one ratio, so half show the dominant trait and half show the recessive trait. A heterozygous-by-heterozygous cross (Bb times Bb) gives the classic three-to-one phenotype split. These three patterns cover the large majority of NEX genetics items.

Ecology and Energy Flow

The NEX also samples basic ecology. Energy enters most ecosystems through producers (plants and algae) that capture sunlight, flows to primary consumers (herbivores), then to secondary and tertiary consumers (carnivores), and finally to decomposers (bacteria and fungi) that recycle nutrients back to the soil. Only about ten percent of the energy at one level passes to the next, which is why food chains rarely extend beyond four or five links.

Symbiotic relationships appear as well: mutualism benefits both partners, commensalism benefits one without harming the other, and parasitism benefits one at the host's expense. Knowing producer, consumer, and decomposer roles is usually enough to answer the ecology items correctly.