Cells, biological regulation, immunity and biotechnology
cell-biology
Cell biology links organelle structure with function, membrane exchange and cell division. This chapter reviews shared foundations and interprets supplied water-potential data. It does not assert a target paper structure or marking scheme.
Organelles: structure fits function
Prokaryotic cells lack a membrane-bound nucleus. Bacteria commonly have a nucleoid, 70S ribosomes and a peptidoglycan wall, but wall-less bacteria exist; archaeal walls do not contain peptidoglycan. Plasmids and flagella occur in some cells, not all. Eukaryotes compartmentalise functions: nucleus with chromatin and nucleolus; mitochondria with inner membrane and matrix; chloroplasts with thylakoids and stroma in photosynthetic cells; rough and smooth ER; Golgi apparatus and ribosomes. A typical plant cell additionally has a cellulose wall and a large vacuole. A structure should be identified by its actual features rather than assuming every member has every listed organelle.
Membranes and transport
The fluid mosaic model describes a lipid bilayer with proteins and, in suitable cells, sterols, glycoproteins and glycolipids. Cholesterol is characteristic of many animal membranes; sterol composition differs among organisms. Simple diffusion moves suitable molecules down a chemical gradient. Osmosis is net water movement down water potential across a selectively permeable membrane. Facilitated diffusion uses proteins without direct ATP hydrolysis; carrier-mediated flux can saturate. Primary active transport can use ATP directly, while secondary transport uses an electrochemical gradient maintained by another energy-dependent process. The mammalian sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP in its ordinary cycle. Sodium-coupled glucose uptake illustrates secondary transport. Vesicular transport uses energy. Surface area, diffusion distance, gradients and permeability together influence exchange.
Mitosis and the cell cycle
Interphase: G1 growth, S DNA replication, G2 organelle replication. Prophase - chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase: spindle microtubules attach through kinetochores at centromeric regions and chromosomes align at the equator. Anaphase - centromeres divide and sister chromatids move to opposite poles. Telophase - nuclei reform and chromosomes decondense. Cytokinesis is by cleavage furrow in animals, cell plate in plants. Mitosis maintains genetic constancy for growth, repair and asexual reproduction.
Worked example
A plant cell has solute potential -1200 kPa and pressure potential +400 kPa, giving water potential -800 kPa. In an external solution at -900 kPa, initial net water movement is out because -800 is higher than -900. As water leaves, pressure and solute concentration can change. The initial comparison does not prove complete plasmolysis: a new equilibrium may retain positive pressure. Distinguish a direction predicted by initial potentials from a final cellular condition that requires further information.
Traps that cost marks
Common errors include adding potentials with inconsistent signs, describing osmosis without comparing water potentials, and confusing centromeres with centrioles. Mitosis normally preserves the starting chromosome complement; it does not automatically turn every cell into a haploid or diploid product. DNA replication precedes chromosome separation. Specify whether an active transport step uses ATP directly or a pre-existing electrochemical gradient.
genetics
Genetics traces the storage, replication and expression of information, chromosome behavior and inheritance probabilities. Use explicit organism, ploidy and dominance assumptions. Shared practice does not establish a current official module or paper format.
DNA structure and replication
Nucleotides: deoxyribose, phosphate, and a base - purines adenine and guanine, pyrimidines thymine and cytosine. Nucleotides link 3′–5′ by phosphodiester bonds into two antiparallel strands; A pairs with T by two hydrogen bonds, G with C by three. Replication is semi-conservative: helicase unwinds, each strand acts as a template, DNA polymerase adds nucleotides 5′→3′, the leading strand is continuous while the lagging strand forms Okazaki fragments joined by ligase. Meselson–Stahl's density-gradient experiment supports this.
Gene expression
The code is a triplet, degenerate, non-overlapping and (nearly) universal, with AUG the usual start and UAA/UAG/UGA stop codons in the standard code; alternative initiation and code exceptions exist. Transcription: RNA polymerase makes mRNA from the template strand; eukaryotic mRNA is spliced to remove introns. Translation: mRNA binds a ribosome, tRNA anticodons pair with codons, peptide bonds form between amino acids, and the polypeptide is released at a stop codon.
Meiosis and inheritance
Meiosis I separates homologous chromosomes and meiosis II separates sister chromatids. A standard diploid-cell model gives four haploid products, though not all become equivalent functional gametes. Crossing over and independent assortment can create variation. An Aa by Aa cross gives a 3:1 phenotype expectation only with complete dominance and suitable viability assumptions. Aa by aa gives a 1:1 expectation; AA by aa does not. Codominance and multiple alleles require different genotype-to-phenotype rules. In a conventional XY model, an X-linked locus can be hemizygous in males; sex-determination systems differ among organisms. A 9:3:3:1 expectation requires an AaBb by AaBb cross, independent assortment, complete dominance at both loci and the stated viability model. Nearby linked loci often yield excess parental combinations in an informative test cross.
Worked example
A supplied informative fly test cross gives 41 grey-normal, 39 ebony-vestigial, 10 grey-vestigial and 9 ebony-normal offspring. Under the stated parental-phase and viability assumptions, the parental total is 80 and recombinant total 19, suggesting linkage rather than a 1:1:1:1 model. The observed recombinant frequency is 19/99, about 19.2 percent. Treating this percentage as a map distance is a short-interval approximation; undetected multiple crossovers can make actual genetic distance larger. The observation estimates a frequency, not a universally exact physical distance or an assumption-free genetic map.
Traps that cost marks
Confusing the template strand with the coding strand. Saying DNA polymerase works 3′→5′. Giving genotype ratios where gamete or phenotype ratios are asked. Using a test cross with anything other than a homozygous recessive. Assuming independent assortment for linked genes. Forgetting that crossing over occurs in prophase I, not anaphase.
physiology
Physiology
Items test how organ systems maintain internal stability, how exchange surfaces are built, and how structure at each level serves function.
Homeostasis and control
Negative feedback reverses a deviation: a sensor detects change, an integrating center compares it with a set point, and an effector counters it (insulin lowers glucose; sweating lowers temperature). Positive feedback amplifies a change and must terminate (oxytocin in labor, clotting cascade). Levels of organization run from cells to tissues (epithelial, connective, muscle, nervous) to organs to systems.
Systems
- Circulation: mammals have a four-chambered heart and double circulation; arteries carry blood away from the heart, and pulmonary arteries carry deoxygenated blood.
- Gas exchange: alveoli and gill lamellae maximize surface area and minimize diffusion distance; countercurrent flow in fish gills maintains a gradient along the whole lamella.
- Digestion: enzymes act at specific pH (pepsin acid; pancreatic amylase and lipase alkaline); villi and microvilli increase absorptive area.
- Excretion: the nephron filters at the glomerulus, reabsorbs glucose, water, and ions, and secretes wastes; the loop of Henle builds a concentration gradient.
- Nervous and endocrine: fast neuronal action potentials involve sodium and potassium channel currents; the sodium-potassium pump maintains gradients over time rather than directly causing rapid repolarization; hormones act on target cells bearing receptors.
- Plants: xylem moves water and minerals upward under transpiration pull (cohesion–tension); phloem translocates sugars by pressure flow; guard cells open stomata when turgid.
Worked example
After a carbohydrate-rich meal, blood glucose rises. Pancreatic beta cells detect it and secrete insulin; insulin promotes glucose uptake in responsive muscle and adipose tissue, promotes glycogen storage and suppresses hepatic glucose production, so glucose falls toward the set point. When glucose drops below the set point, alpha cells release glucagon, triggering glycogen breakdown and gluconeogenesis in the liver. Both arms are negative feedback: the response opposes the original deviation. In diabetes mellitus, insulin secretion and/or action is impaired and hyperglycemia can result; this model is not a clinical diagnosis.
Traps
Labeling a loop positive because the outcome is "good"; forgetting that pulmonary arteries carry deoxygenated blood; confusing xylem and phloem direction; assuming diffusion suffices in large animals; mixing the roles of the loop of Henle and the collecting duct; assuming all hormones are lipid-soluble.
microbiology-immunology
Esta revisión relaciona estructura microbiana, defensa inmunitaria y métodos de análisis de ADN. Se interpretan modelos celulares y datos suministrados; no se ofrecen diagnósticos, dosis, decisiones de tratamiento ni normas sanitarias extranjeras.
Virus y bacterias
Los virus son entidades acelulares con genoma de ADN o ARN, cápside y, en algunos casos, envuelta. Los ciclos lítico y lisogénico describen determinados bacteriófagos, no todos los virus. Los retrovirus utilizan transcripción inversa y pueden integrar ADN viral. Muchas bacterias poseen pared de peptidoglicano, pero existen bacterias sin pared; plásmidos, flagelos y endosporas no son universales. La estructura de una bacteria concreta debe leerse en los datos. Conjugación, transformación y transducción son mecanismos distintos de transferencia genética horizontal.
Inmunidad
Innata: barreras, pH, complemento, fagocitos, células NK e inflamación; reconoce patrones mediante receptores y no utiliza la memoria clonal clásica de linfocitos B y T; la inmunidad entrenada evita reducirla a una ausencia absoluta de memoria. Adaptativa: linfocitos B → células plasmáticas → anticuerpos (IgG, IgM, IgA, IgE, IgD); los linfocitos T auxiliares CD4 reconocen antígeno en MHC II y activan a B y a CD8; los T citotóxicos CD8 destruyen células infectadas presentadas por MHC I. Selección clonal, memoria y respuesta secundaria más rápida y con mayor título, sobre todo de IgG.
Biotecnología
Las enzimas de restricción reconocen secuencias determinadas y pueden producir extremos cohesivos o romos; no todas las enzimas reconocen el mismo tipo de secuencia. Las ligasas unen enlaces del esqueleto del ADN. Los vectores permiten estudiar secuencias introducidas en sistemas adecuados. PCR, electroforesis y secuenciación responden preguntas diferentes: amplificación de una región, comparación de fragmentos y orden de bases. En una comparación convencional de ADN lineal en el mismo gel, los fragmentos pequeños suelen migrar más lejos; la conformación y el sistema importan. Una banda no demuestra por sí sola identidad de secuencia ni expresión funcional. La ingeniería genética requiere comprobar tanto el cambio previsto como su efecto en el modelo estudiado.
Ejemplo resuelto
Partimos de una sola molécula de ADN y aplicamos PCR. En el modelo ideal de duplicación perfecta, tras 5 ciclos el número sería 2^5 = 32 y tras 20 ciclos aproximadamente 1,05 millones. El razonamiento es multiplicativo porque cada ciclo duplica cada molécula, hasta que la fase exponencial se satura al agotarse cebadores o dNTP. Si la muestra inicial tuviera 10 moléculas, tras 5 ciclos habría 320 copias.
Errores que cuestan puntos
Afirmar que los antibióticos matan virus, olvidar que los virus solo se replican dentro de una célula, tratar IgM e IgG como un diagnóstico binario de fase: una respuesta secundaria sistémica suele mostrar aumento de anticuerpos de alta afinidad, pero el isotipo depende del contexto y creer que los linfocitos T producen anticuerpos.