Biology study notes
Biology for the NMAT
Key points
- Mitochondria generate ATP via aerobic respiration; chloroplasts capture light energy via photosynthesis.
- The plasma membrane's phospholipid bilayer and embedded proteins control diffusion, osmosis, and active transport.
- Meiosis, independent assortment, and crossing over generate genetic variation; Mendelian ratios predict inheritance patterns.
- The central dogma (DNA to RNA to protein) and epigenetic modification together determine which genes a cell expresses.
- Negative feedback loops, such as thyroxine regulating TSH, keep physiological variables like hormone levels and blood pressure stable.
- Hemoglobin's oxygen affinity shifts with local pH and CO2 (the Bohr effect), delivering more oxygen to actively respiring tissue.
- Population growth is exponential when resources are unlimited and logistic as it approaches carrying capacity.
- Energy flows one-way through trophic levels, losing about 90% at each transfer, while nutrients like nitrogen and carbon cycle repeatedly.
- Natural selection, genetic drift, and gene flow all shift allele frequencies, but only natural selection consistently favors reproductive fitness.
- Homologous structures, the fossil record, and molecular-clock comparisons together provide converging evidence for common ancestry.
Cell Structure and Function
Every living organism is built from cells, the basic structural and functional unit of life described by cell theory: all organisms are composed of one or more cells, the cell is the basic unit of organization, and all cells arise from pre-existing cells. Eukaryotic cells subdivide their functions into membrane-bound organelles, allowing specialized reactions to occur in separate compartments. The nucleus stores and protects the genetic material and coordinates gene expression through its nucleolus and chromatin. Mitochondria, bounded by a folded inner membrane (cristae) that increases surface area, generate the bulk of a cell's ATP through aerobic respiration. Ribosomes, either free-floating or docked on the rough endoplasmic reticulum, translate messenger RNA into protein; the rough ER also modifies proteins destined for secretion, while the smooth ER synthesizes lipids and detoxifies compounds. The Golgi apparatus further modifies, sorts, and packages proteins into vesicles bound for secretion or other organelles. Lysosomes digest cellular debris and worn-out organelles, and the plasma membrane's phospholipid bilayer, studded with proteins, gives the cell selective permeability, regulating diffusion, osmosis, and active transport. Prokaryotic cells, by contrast, lack these membrane-bound organelles entirely.
Cellular Energetics: Photosynthesis and Respiration
Cells acquire and convert energy through two complementary processes. Photosynthesis, carried out in chloroplasts, captures light energy and stores it in chemical bonds. In the light-dependent reactions occurring across the thylakoid membrane, chlorophyll absorbs photons, water is split (photolysis) to release electrons, protons, and oxygen, and the resulting proton gradient drives ATP and NADPH synthesis. These products fuel the Calvin cycle in the stroma, where carbon dioxide is fixed into three-carbon sugars that ultimately form glucose. Cellular respiration reverses this energy flow: glycolysis in the cytoplasm splits glucose into pyruvate, yielding a small net ATP gain and NADH. Pyruvate is then oxidized and fed into the citric acid cycle within the mitochondrial matrix, releasing carbon dioxide and generating NADH and FADH2. These electron carriers donate electrons to the electron transport chain embedded in the inner mitochondrial membrane, and the resulting proton gradient powers ATP synthase to produce the majority of a cell's ATP, a process called oxidative phosphorylation. When oxygen is unavailable, fermentation regenerates NAD+ by converting pyruvate to lactate or ethanol, allowing glycolysis to continue at a reduced rate without oxidative phosphorylation.
Genetics and Molecular Biology
Heredity follows patterns first described by Gregor Mendel: traits are controlled by discrete alleles, dominant alleles mask recessive ones in heterozygotes, and alleles segregate into separate gametes during meiosis while genes on different chromosomes assort independently. Punnett squares and probability rules predict offspring ratios, and test crosses with homozygous recessive individuals reveal an organism's unknown genotype. At the molecular level, DNA's double helix pairs adenine with thymine and guanine with cytosine, and semi-conservative replication copies each strand as a template. The central dogma describes information flow from DNA to messenger RNA via transcription, and from mRNA to protein via translation at the ribosome, where codons are matched to anticodons on transfer RNA. Gene expression is tightly regulated by promoters, transcription factors, and operons such as the lac operon, which is switched on only when lactose is present and glucose is scarce. Mutations, including point substitutions, insertions, deletions, and frameshifts, introduce genetic variation, some of which is silent and some of which alters protein function. Beyond the DNA sequence itself, epigenetic modifications like DNA methylation can silence or activate genes differently across cell types without changing the underlying code.
Human Anatomy and Physiology
Human physiology is a study in coordinated homeostasis across organ systems. The nervous system processes information rapidly through neurons that communicate via synapses and neurotransmitters, with reflex arcs routed through the spinal cord for near-instant responses, while the endocrine system uses slower-acting hormones and negative feedback loops, such as thyroxine suppressing TSH release, to maintain stable internal conditions. The cardiovascular system circulates blood through a four-chambered heart, distributing oxygen bound to hemoglobin in red blood cells, whose affinity for oxygen shifts with local pH and CO2 levels (the Bohr effect) to release more oxygen where tissues need it most. The respiratory system exchanges gases across the thin walls of the alveoli, while chemoreceptors monitor blood CO2, O2, and pH to adjust ventilation. The digestive system mechanically and chemically breaks down food, from pepsin in the acidic stomach to pancreatic and intestinal enzymes in the small intestine, for absorption. The urinary system's nephrons filter blood at the glomerulus, then selectively reabsorb and secrete solutes along the tubule, with hormones like ADH and aldosterone fine-tuning water and electrolyte balance. The immune system defends the body through innate barriers and adaptive responses, in which B cells produce antibodies and T cells orchestrate cell-mediated immunity.
Ecology and Ecosystems
Ecology examines how organisms interact with each other and their environment across increasing scales of organization. At the population level, growth may be exponential when resources are unlimited or logistic as it levels off near carrying capacity, with life-history strategies ranging from r-selected species (many offspring, little care, Type III survivorship) to K-selected species (few offspring, high investment, Type I survivorship). Communities are shaped by competition, predation, and symbiotic relationships, including mutualism, commensalism, and parasitism, as well as by succession, which rebuilds ecosystems after disturbance. At the ecosystem level, energy flows one-way from producers through consumers, losing roughly 90% at each trophic transfer, while nutrients like nitrogen and carbon cycle repeatedly through biotic and abiotic reservoirs via processes such as nitrogen fixation and decomposition. Some pollutants defy this dilution, instead biomagnifying to dangerous concentrations in top predators.
Evolution and Diversity of Life
Evolution explains both the unity and diversity of life. Natural selection acts on phenotypic variation within a population: individuals whose traits improve survival and reproduction leave more descendants, gradually shifting allele frequencies over generations. Other forces, including genetic drift (pronounced in small populations, as in founder effects and bottlenecks) and gene flow between populations, also reshape allele frequencies, sometimes independent of fitness. Evidence for evolution converges from multiple lines: the fossil record documents transitional forms, homologous structures reveal shared ancestry despite different functions, molecular sequence comparisons enable molecular-clock estimates of divergence times, and biogeography explains species distributions. When populations become reproductively isolated, often through a geographic barrier as in allopatric speciation, they can diverge into separate species. The tree of life spans prokaryotes (bacteria and archaea, lacking a nucleus), eukaryotic protists, fungi (heterotrophic decomposers with chitin cell walls), and viruses, which are obligate intracellular parasites unable to replicate without hijacking a host cell's machinery.
Test-Taking Strategies for the Biology Subtest
Because the Biology subtest rewards conceptual understanding over rote memorization, focus on structure-function relationships and mechanisms rather than isolated vocabulary. When a stem describes an unfamiliar scenario, identify which core process is being tested (transport, gene expression, feedback regulation, energy flow) and reason from first principles rather than searching for a memorized match. For quantitative items, such as Hardy-Weinberg or dihybrid-cross probability questions, work the calculation explicitly rather than guessing from pattern recognition. Eliminate answer choices that describe a real process occurring in the wrong location or in the wrong direction, since these are common traps. Finally, budget time by answering confidently-known items first, and flag multi-concept integration questions for a second pass, since these usually take longer to reason through carefully.
References
- OpenStax Biology 2e — OpenStax
- OpenStax Anatomy and Physiology 2e — OpenStax
- Khan Academy: AP Biology — Khan Academy
- Khan Academy: Ecology — Khan Academy
- Khan Academy: Natural Selection — Khan Academy
- NCBI Bookshelf: Molecular Biology of the Cell — NCBI Bookshelf
- LibreTexts: Cell and Molecular Biology — LibreTexts
Ready to practice?
Put these biology concepts to the test with practice questions.