OpenStem

@openstem · Joined Jul 2026

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What is adaptive radiation?1 / 10

The relatively rapid evolution of many new species from a single ancestral lineage, with each new species becoming adapted to a different ecological niche. It happens fastest when a lineage suddenly has access to a wide range of unused resources and habitats.

Biology

Biology · L3 · Adaptive Radiation After Mass Extinction

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What is the main structural difference between prokaryotic and eukaryotic cells?1 / 11

Prokaryotic cells (bacteria, archaea) lack a membrane-bound nucleus; their DNA is in the cytoplasm as a nucleoid region. Eukaryotic cells (animals, plants, fungi, protists) have a membrane-bound nucleus plus membrane-bound organelles (mitochondria, ER, Golgi, etc.).

Biology

Biology · L4 · Cell Structure & Organelles

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State Mendel's law of segregation.1 / 11

Each individual has two alleles for a given trait (one from each parent). During gamete formation, these alleles segregate so each gamete carries only one allele. An offspring inherits one allele from each parent — restoring the diploid condition.

Biology

Biology · L4 · Mendelian Genetics & Heredity

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Describe the structure of DNA (Watson-Crick model).1 / 11

DNA is a right-handed double helix: two antiparallel polynucleotide strands wound around each other. Each strand is a polymer of deoxyribonucleotides linked by 3′-5′ phosphodiester bonds. Complementary base pairs are joined by hydrogen bonds: A–T (2 H-bonds) and G–C (3 H-bonds). The sugar-phosphate backbones face outward; bases face inward.

Biology

Biology · L4 · Molecular Biology — DNA, RNA & Protein

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State Darwin's four postulates of natural selection.1 / 11

(1) Variation: individuals in a population vary in heritable traits. (2) Heritability: traits are passed from parents to offspring. (3) Overproduction: more offspring are born than can survive. (4) Differential survival and reproduction: individuals with advantageous traits survive and reproduce more, passing those traits to the next generation.

Biology

Biology · L4 · Evolution & Natural Selection

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What is the difference between a food chain and a food web?1 / 8

A food chain is a single linear sequence of who eats whom (producer → primary consumer → secondary consumer → tertiary consumer). A food web is a network of interconnected food chains representing the more realistic, complex feeding relationships in an ecosystem. Each step is a trophic level; only ~10% of energy transfers to the next level.

Biology

Biology · L4 · Ecology — Populations, Communities & Ecosystems

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What is sensory transduction, in general terms?1 / 10

Sensory transduction is the conversion of a physical or chemical stimulus (light, sound pressure, mechanical deformation, or a chemical ligand) into a change in membrane potential of a specialized receptor cell — a receptor potential. Nearly every modality funnels through the same underlying logic: a stimulus alters the open probability of an ion channel, either directly or via a second-messenger cascade, changing ionic current and thus membrane voltage, which then modulates neurotransmitter release or action potential firing.

Biology

Biology · L4 · Phototransduction and Transduction Principles

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How does a cochlear hair cell convert mechanical deflection of its stereocilia into an electrical signal?1 / 10

Stereocilia are arranged in rows of increasing height and connected between adjacent rows by fine protein filaments called tip links. Deflection of the stereocilia bundle toward the tallest row increases tension on the tip links, which mechanically pulls open mechanoelectrical transduction (MET) channels at their lower end. Because the surrounding endolymph is K+-rich and positively charged relative to the hair cell interior, opening MET channels drives an inward K+ current that depolarizes the cell within microseconds — among the fastest transduction mechanisms in biology.

Biology

Biology · L4 · Hearing, Chemoreception, and Mechanoreception

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What are the three major classes of cell-surface receptors?1 / 10

(1) G-protein-coupled receptors (GPCRs): seven-transmembrane proteins that activate heterotrimeric G proteins. (2) Receptor tyrosine kinases (RTKs): single-pass transmembrane proteins with intrinsic kinase activity, activated by dimerization. (3) Ligand-gated ion channels: bind a ligand and directly open a pore, producing a fast electrical/ionic response (e.g. nicotinic acetylcholine receptor).

Biology

Biology · L4 · Signal Transduction Pathways

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How do cyclins and CDKs together control progression through the cell cycle?1 / 10

Cyclin-dependent kinases (CDKs) are constitutively present but catalytically inactive alone. Specific cyclins accumulate and decline at defined points in the cycle, binding and activating their partner CDK. Each cyclin-CDK complex phosphorylates a distinct set of substrates to drive a particular transition (e.g. cyclin D-CDK4/6 and cyclin E-CDK2 drive G1/S; cyclin B-CDK1 drives G2/M), and cyclin degradation by the ubiquitin-proteasome system switches the kinase back off.

Biology

Biology · L4 · Cell Cycle Checkpoints and Apoptosis

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What is a promoter, and how is its position defined relative to the transcription start site?1 / 10

A promoter is a DNA sequence upstream of a gene that RNA polymerase (directly, in bacteria, or via transcription factors, in eukaryotes) recognises and binds to initiate transcription. Positions are numbered relative to the transcription start site (+1): upstream sequences are negative (e.g. -10, -35), downstream sequences are positive. The promoter itself lies almost entirely upstream of +1.

Biology

Biology · L4 · Transcription in Depth

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Describe the overall structure of the ribosome, comparing prokaryotic and eukaryotic subunit sizes.1 / 10

A ribosome has a large and a small subunit, each built from ribosomal RNA (rRNA) plus proteins. Bacterial (70S) ribosomes: a small 30S subunit (16S rRNA) plus a large 50S subunit (23S + 5S rRNA). Eukaryotic (80S) ribosomes: a small 40S subunit (18S rRNA) plus a large 60S subunit (28S + 5.8S + 5S rRNA). The peptidyl transferase activity that forms peptide bonds resides in the large subunit's rRNA, making the ribosome a ribozyme.

Biology

Biology · L4 · Translation and Gene Regulation

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What is the polymerase chain reaction (PCR), and why is it useful?1 / 10

PCR is an in vitro technique that exponentially amplifies a specific, targeted region of DNA, generating millions of copies from a trace starting sample. It underlies DNA fingerprinting, cloning, diagnostic testing, and sequencing workflows because most downstream techniques require far more DNA than a single cell provides.

Biology

Biology · L4 · Biotechnology — Core Lab Techniques

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What is CRISPR-Cas9, and where did it come from biologically?1 / 9

CRISPR-Cas9 is a gene-editing system adapted from a natural bacterial and archaeal immune defence against viruses. Bacteria store short snippets of past viral DNA in their genome (the CRISPR array) and use them, together with the Cas9 nuclease, to recognise and destroy matching viral DNA on reinfection. Scientists repurposed this system to cut DNA at any chosen site in a genome.

Biology

Biology · L4 · Biotechnology — Modern Applications

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What is a gene pool, and what is an allele frequency?1 / 9

A gene pool is the sum of all alleles at all loci in every individual of a population. An allele frequency is the proportion of one specific allele among all alleles at that locus in the gene pool — a value between 0 and 1 (or 0% and 100%).

Biology

Biology · L4 · Hardy-Weinberg Model

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What is genetic drift, and why is it stronger in small populations?1 / 9

Genetic drift is random change in allele frequency from generation to generation due to chance sampling of which individuals happen to survive and reproduce. In small populations, chance sampling error has a proportionally larger effect on allele frequencies — a few random deaths can shift frequencies noticeably — whereas in large populations random fluctuations tend to average out.

Biology

Biology · L4 · Forces That Change Allele Frequencies

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What is stroke volume, and what is a typical resting value?1 / 10

Stroke volume (SV) is the volume of blood ejected by one ventricle in a single contraction, equal to end-diastolic volume minus end-systolic volume. A healthy resting adult has a stroke volume of roughly 70 mL per beat.

Biology

Biology · L4 · Cardiac Output and the Cardiac Cycle

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What are baroreceptors, and where are they located?1 / 10

Baroreceptors are stretch-sensitive nerve endings located mainly in the walls of the carotid sinuses and the aortic arch. They detect changes in blood pressure by sensing how much the vessel wall stretches.

Biology

Biology · L4 · Blood Pressure Regulation

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What does Boyle's law state, and how does it apply to breathing?1 / 10

Boyle's law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume: as volume increases, pressure decreases, and vice versa. During breathing, the respiratory muscles change the volume of the thoracic cavity, which changes the pressure inside the lungs relative to atmospheric pressure, and air flows from the region of higher pressure to the region of lower pressure.

Biology

Biology · L4 · Ventilation Mechanics and Lung Volumes

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What does the oxygen-hemoglobin dissociation curve describe?1 / 10

The oxygen-hemoglobin dissociation curve plots the percent saturation of hemoglobin with oxygen against the partial pressure of oxygen in the surrounding blood. Its S-shape (sigmoidal) reflects cooperative binding: hemoglobin binds oxygen more readily once one subunit has already bound it, so saturation rises steeply over the partial pressure range found in respiring tissues.

Biology

Biology · L4 · Gas Transport and Chemoreceptor Control

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What is the glomerular filtration rate (GFR), and what is a typical value in a healthy adult?1 / 10

GFR is the volume of filtrate formed by both kidneys per minute, typically ~125 mL/min (~180 L/day). Only ~1-1.5 L is excreted as urine each day because more than 99% of the filtrate is reabsorbed along the nephron.

Biology

Biology · L4 · Nephron Function In Depth

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What stimulates ADH (vasopressin) release, and where is it synthesized and released?1 / 9

ADH is synthesized in the hypothalamus and released from the posterior pituitary. Release is stimulated primarily by rising blood osmolality detected by hypothalamic osmoreceptors, and secondarily by large drops in blood volume or pressure detected by baroreceptors.

Biology

Biology · L4 · Hormonal Regulation Of Fluid Balance

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What are the two broad chemical classes of hormones, and what property determines which mechanism each uses?1 / 10

Lipid-soluble hormones (steroids, thyroid hormone) diffuse through the plasma membrane and bind intracellular receptors. Water-soluble hormones (peptides/proteins, catecholamines) cannot cross the lipid bilayer, so they bind membrane receptors and trigger intracellular second-messenger cascades. Solubility, set by chemical structure, dictates which mechanism a hormone must use.

Biology

Biology · L4 · Hormone Classes & Receptor Mechanisms

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Describe the general three-tier architecture shared by the HPA, HPT, and HPG axes.1 / 9

Each axis follows the same hierarchy: (1) the hypothalamus secretes a releasing hormone into the hypothalamic-hypophyseal portal system, (2) the anterior pituitary responds by secreting a tropic hormone into the systemic circulation, and (3) the tropic hormone stimulates a peripheral target gland to secrete its own hormone, which acts on body tissues and feeds back to shut off steps 1 and 2.

Biology

Biology · L4 · Hypothalamic-Pituitary Axes

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What is a motor unit, and what two components does it comprise?1 / 10

A motor unit is the functional unit of skeletal muscle control: one alpha motor neuron together with every muscle fiber it innervates via neuromuscular junctions. Because the neuron's action potential is all-or-none, stimulating it activates all of its fibers simultaneously and maximally.

Biology

Biology · L4 · Motor Units & Muscle Fiber Types

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What are osteoblasts, and what is their primary function in bone remodeling?1 / 10

Osteoblasts are bone-forming cells derived from mesenchymal stem cells. They synthesize and secrete osteoid (unmineralized collagen-rich matrix) and promote its mineralization with calcium and phosphate as hydroxyapatite. Osteoblasts that become trapped within the matrix they produce differentiate into osteocytes.

Biology

Biology · L4 · Bone Remodeling & Calcium Homeostasis

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What is horizontal gene transfer (HGT), and how does it differ from vertical inheritance?1 / 10

HGT is the movement of genetic material between organisms other than by parent-to-offspring (vertical) transmission. In bacteria it lets a cell acquire new traits — antibiotic resistance, toxin genes, novel metabolic pathways — in a single generation, without waiting for mutation and selection.

Biology

Biology · L4 · Horizontal Gene Transfer

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What are the four broad mechanistic categories of antibiotic resistance?1 / 10

(1) Enzymatic inactivation of the drug, (2) modification of the drug's target so it no longer binds, (3) reduced uptake or increased efflux of the drug, and (4) bypass — acquiring an alternative pathway or enzyme that performs the same function but is insensitive to the drug.

Biology

Biology · L4 · Antibiotic Resistance & Virulence

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What is auxin, and where is it produced in a shoot?1 / 10

Auxin (mainly indole-3-acetic acid, IAA) is a plant hormone synthesized primarily in the shoot apical meristem and young leaves. It is transported downward through the stem in a polar, cell-to-cell fashion via PIN efflux carrier proteins, and it drives cell elongation, apical dominance, and tropic responses.

Biology

Biology · L4 · Plant Hormones

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What is the cohesion-tension theory of xylem water transport?1 / 10

The cohesion-tension theory explains how water rises through xylem vessels to the tops of tall trees without any pump. Evaporation of water from leaf mesophyll cells into the substomatal air space creates negative pressure (tension) that pulls the entire water column upward. Hydrogen bonding between water molecules (cohesion) keeps this column unbroken, while adhesion of water to the hydrophilic walls of xylem vessels helps counteract gravity and resist column breakage.

Biology

Biology · L4 · Water Transport and Tropisms

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