Cell Biology & Genetics

Biology is the study of life, and cells are the fundamental building blocks of all living organisms. The HESI A2 Biology section tests your understanding of cellular structure, function, genetics, and basic biological processes.

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Cell Theory

The modern cell theory states:

1. All living organisms are composed of one or more cells
2. The cell is the basic structural and functional unit of life
3. All cells arise from pre-existing cells through cell division

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Prokaryotic vs. Eukaryotic Cells

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Eukaryotic Cell Organelles

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Cell Membrane & Transport

The cell membrane is a phospholipid bilayer with embedded proteins. It is selectively permeable, meaning it controls which substances pass through.

Passive Transport (No ATP Required)

Osmotic Solutions:

• Hypertonic: Higher solute outside the cell → water moves OUT → cell shrinks (crenation)
• Hypotonic: Lower solute outside the cell → water moves IN → cell swells (may lyse)
• Isotonic: Equal solute concentration → no net movement → cell stays normal

Active Transport (ATP Required)

• Sodium-potassium pump: Moves 3 Na+ out and 2 K+ in against their gradients
• Endocytosis: Cell engulfs materials (phagocytosis = solids, pinocytosis = liquids)
• Exocytosis: Cell expels materials (e.g., neurotransmitter release)

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DNA & RNA

Central Dogma of Molecular Biology: DNA → (transcription) → mRNA → (translation) → Protein

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Cell Division

Mitosis (Somatic Cell Division)

• Produces 2 identical diploid (2n) daughter cells
• Used for growth, repair, and maintenance
• Phases: Prophase → Metaphase → Anaphase → Telophase (PMAT)

Meiosis (Reproductive Cell Division)

• Produces 4 unique haploid (n) daughter cells (gametes)
• Used to create eggs and sperm
• Two rounds of division: Meiosis I (separation of homologous pairs) and Meiosis II (separation of sister chromatids)
• Crossing over during Prophase I creates genetic variation

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Basic Genetics

• Gene: A segment of DNA that codes for a specific protein or trait
• Allele: Different versions of the same gene (e.g., brown eye allele vs. blue eye allele)
• Genotype: The genetic makeup (e.g., Bb)
• Phenotype: The physical expression (e.g., brown eyes)
• Homozygous: Two identical alleles (BB or bb)
• Heterozygous: Two different alleles (Bb)
• Dominant (B): Expressed when one or two copies are present
• Recessive (b): Only expressed when two copies are present (bb)

Punnett Square Example: Bb x Bb cross

Results: 1 BB : 2 Bb : 1 bb → 75% dominant phenotype, 25% recessive phenotype

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Types of Inheritance Beyond Simple Dominance

Sex-Linked Traits:

• Males are XY; females are XX
• If a recessive allele is on the X chromosome, males (XY) express the trait with just one copy
• Females need two copies of the recessive allele to express the trait
• Example: A carrier mother (X^H X^h) x normal father (X^H Y) → 50% of sons will have hemophilia

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The Scientific Method

The HESI A2 may test your understanding of the scientific method:

1. Observation — Notice a phenomenon or problem
2. Question — Formulate a question about the observation
3. Hypothesis — Propose a testable explanation
4. Experiment — Design and conduct a controlled experiment
5. Data collection — Record observations and measurements
6. Analysis — Interpret the data using statistics
7. Conclusion — Accept or reject the hypothesis based on evidence
8. Communication — Publish results for peer review

Key experimental terms:

• Independent variable — what the researcher changes (the cause)
• Dependent variable — what the researcher measures (the effect)
• Control group — receives no treatment (baseline for comparison)
• Experimental group — receives the treatment
• Constants — factors kept the same across all groups

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Biological Classification (Taxonomy)

Living organisms are classified into a hierarchical system from broadest to most specific:

Remember: "Dear King Philip Came Over For Good Spaghetti"

Key terms:

• Binomial nomenclature — organisms are named using Genus species (e.g., Homo sapiens, Escherichia coli)
• Genus is always capitalized; species is lowercase
• Both are always italicized (or underlined when handwritten)

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Photosynthesis vs. Cellular Respiration

These two processes are complementary — the products of one are the reactants of the other:

Stages of cellular respiration:

1. Glycolysis (cytoplasm) — glucose → 2 pyruvate; yields 2 ATP
2. Krebs cycle (mitochondrial matrix) — pyruvate is oxidized; yields 2 ATP + electron carriers
3. Electron transport chain (inner mitochondrial membrane) — yields ~34 ATP

Total ATP yield per glucose molecule: ~36-38 ATP (aerobic) vs. 2 ATP (anaerobic/fermentation)