OpenStem

@openstem · Joined Jul 2026

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What is a keystone species, and why is its removal so consequential?1 / 10

A species that exerts an ecological influence disproportionate to its abundance, such that its removal triggers a cascade of change throughout the community. Sea otters are a classic example: by preying on sea urchins, they keep urchin populations low enough for kelp forests to persist. Remove the otters, and urchins overgraze the kelp, collapsing the habitat that many other species depend on.

Biology

Biology · L4 · Community Interactions and Succession

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What does a biodiversity index measure, conceptually?1 / 9

A biodiversity index combines two properties of a community into a single value: species richness (the number of different species present) and species evenness (how evenly individuals are distributed among those species). A community with many species that are all similarly abundant scores higher in diversity than one with the same number of species but dominated by just one or two of them.

Biology

Biology · L4 · Ecosystem Ecology and Conservation Biology

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What does the Michaelis-Menten equation describe, and what is its general mathematical form?1 / 10

The Michaelis-Menten equation describes how the initial reaction rate (v) of an enzyme-catalyzed reaction depends on substrate concentration ([S]) at a fixed enzyme concentration. Its form is v = (Vmax[S]) / (Km + [S]), which produces a rectangular hyperbola when v is plotted against [S]: rate rises steeply at low substrate concentration and levels off as the enzyme's active sites become saturated.

Biology

Biology · L4 · Michaelis-Menten Enzyme Kinetics

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What are the four main stages of cellular respiration, and where does each occur in a eukaryotic cell?1 / 10

(1) Glycolysis occurs in the cytoplasm. (2) Pyruvate oxidation (the link reaction) occurs in the mitochondrial matrix. (3) The citric acid cycle occurs in the mitochondrial matrix. (4) The electron transport chain and chemiosmosis occur across the inner mitochondrial membrane, where ATP synthase is also embedded.

Biology

Biology · L4 · Cellular Respiration Overview

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Describe the basic structural shape of an antibody monomer.1 / 10

An antibody monomer is Y-shaped, built from four polypeptide chains held together by disulfide bonds: two identical heavy chains and two identical light chains. The two arms of the Y form the antigen-binding regions, and the stem is the constant region.

Biology

Biology · L4 · Antibody Structure and MHC

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What is the complement system, in broad terms?1 / 10

The complement system is a group of roughly 30 blood plasma proteins, produced mainly by the liver, that circulate in an inactive state and become sequentially activated in a cascade to help destroy pathogens and amplify both innate and adaptive immune responses.

Biology

Biology · L4 · Complement and Hypersensitivity

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What ionic gradients does the Na+/K+ pump establish, and at what cost?1 / 10

The Na+/K+-ATPase pumps 3 Na+ out of the cell and 2 K+ in for every ATP hydrolyzed, against both ions' concentration gradients. This keeps Na+ concentrated outside the cell and K+ concentrated inside, establishing the steep electrochemical gradients that the resting potential and every subsequent phase of the action potential depend on.

Biology

Biology · L4 · The Action Potential in Ion-Channel Detail

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What triggers synaptic vesicle fusion once an action potential reaches the axon terminal?1 / 10

Depolarization of the axon terminal opens voltage-gated Ca2+ channels, and Ca2+ flows in down its steep electrochemical gradient. The resulting local rise in intracellular Ca2+ is sensed by synaptotagmin on the vesicle membrane, which triggers the SNARE complex (synaptobrevin, syntaxin, SNAP-25) to complete membrane fusion between the vesicle and the presynaptic membrane.

Biology

Biology · L4 · Synaptic Transmission and Neurotransmitters

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What happens during fertilization at the molecular level?1 / 10

A sperm binds species-specific receptors on the egg's outer coat (the zona pellucida in mammals), triggering the acrosome reaction — release of hydrolytic enzymes that let the sperm penetrate to the egg plasma membrane. Sperm-egg membrane fusion triggers a rise in intracellular Ca2+ in the egg (the cortical reaction), which causes cortical granules to release their contents and modify the zona pellucida, blocking additional sperm from binding (the block to polyspermy). Fusion of the haploid sperm and egg nuclei restores the diploid chromosome number and activates the egg to begin development.

Biology

Biology · L4 · Early Embryonic Development

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What are homeotic genes, and what phenotype results when one is mutated?1 / 10

Homeotic genes specify the identity of body segments along the anterior-posterior axis — they tell a segment what structures to build, not merely whether to build them. A homeotic mutation causes one body part to develop with the identity of another; the classic example is the Drosophila Antennapedia mutant, in which legs grow from the head in place of antennae because the antennal segment adopts the identity program of a thoracic leg-bearing segment.

Biology

Biology · L4 · Pattern Formation and Stem Cells

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What is a fixed action pattern (FAP), and what triggers it?1 / 10

A fixed action pattern is a stereotyped, species-typical behavioral sequence that, once initiated, runs to completion largely independent of further sensory feedback. It is released by a specific, often simple stimulus feature called a sign stimulus (or releaser) — for example, a red belly triggering aggressive display in male sticklebacks, or an egg-shaped object outside the nest triggering egg-retrieval in geese. FAPs are considered innate because they appear in full form even in individuals with no opportunity to learn them.

Biology

Biology · L4 · Innate Behavior and Learning

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What is the honeybee waggle dance, and what information does it encode?1 / 10

The waggle dance is a symbolic communication behavior performed by a returning forager on the vertical comb, in which the angle of the waggle run relative to vertical encodes the direction of a food source relative to the suns position, and the duration of the waggle run encodes distance. Dance vigor and repetition can also convey food quality, allowing the colony to recruit foragers efficiently to the best available resources without any individual needing to visit the site first.

Biology

Biology · L4 · Communication, Foraging, and Social Behavior

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Distinguish vicariance biogeography from dispersal biogeography as explanations for a disjunct species distribution.1 / 11

Vicariance explains a disjunct distribution (a group found on widely separated landmasses) as the result of a formerly continuous range being split by a later geological event, such as continental drift or the formation of a mountain range or seaway — the organisms did not need to cross the gap, because the gap did not exist when the ancestral population became distributed. Dispersal explains the same pattern by the ancestral organisms actively or passively crossing an already-existing barrier (rafting, wind, ocean currents, flight) to colonize a new area. Distinguishing the two often relies on comparing the timing of lineage divergence (from a molecular clock) against the known geological age of the barrier.

Biology

Biology · L4 · Patterns and Processes in Biogeography

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Distinguish first-, second-, and third-class lever systems in the musculoskeletal system, with an example of each.1 / 10

In a first-class lever, the fulcrum sits between the effort and the load (e.g. the joint between the skull and the first cervical vertebra, with neck muscles as effort balancing head weight as load). In a second-class lever, the load sits between the fulcrum and the effort (e.g. standing on tiptoes, where the ball of the foot is the fulcrum, body weight is the load in the middle, and calf muscle force pulling up on the heel is the effort) — this arrangement always produces a mechanical advantage greater than 1, trading speed/range of motion for force. In a third-class lever, the effort sits between the fulcrum and the load (e.g. the elbow joint during forearm flexion, with the biceps inserting close to the elbow fulcrum and the load held at the hand, far from the fulcrum) — the arrangement most common in vertebrate limbs, which sacrifices mechanical advantage (requiring more muscle force than the load itself) in exchange for greater speed and range of motion at the far end of the limb.

Biology

Biology · L4 · Muscle Mechanics and Locomotor Biomechanics

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Distinguish incomplete dominance from codominance, with an example of each.1 / 11

In incomplete dominance, the heterozygote shows an intermediate phenotype blending both alleles' effects (e.g. red-flowered × white-flowered snapdragons producing pink-flowered heterozygotes), because neither allele fully masks the other and the combined gene product level produces a phenotype between the two homozygous extremes. In codominance, the heterozygote shows BOTH parental phenotypes simultaneously and distinctly, not blended (e.g. the AB blood type, where both A and B antigens are fully expressed on the same red blood cells), because both alleles are independently, fully expressed rather than combining into an intermediate.

Biology

Biology · L4 · Mendelian and Non-Mendelian Inheritance Patterns

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Distinguish homologous structures from analogous structures, with an example of each.1 / 10

Homologous structures share a common evolutionary origin even if they now differ in function, reflecting descent from a shared ancestor — the pentadactyl limb bones of a human arm, whale flipper, and bat wing are homologous, built from the same ancestral bone set (humerus, radius, ulna, carpals) despite very different uses. Analogous structures perform a similar function but evolved independently from different ancestral origins (convergent evolution) — the wings of insects, birds, and bats are analogous in function but not built from homologous underlying structures.

Biology

Biology · L4 · Homology, Analogy, and Body Plans

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How does the pectoral girdle differ in its connection to the axial skeleton between fish and tetrapods, and why does that matter functionally?1 / 10

In fish, the pectoral girdle is typically attached directly to the skull/opercular bones via bony connections, limiting independent head and forelimb movement and largely serving fin support for swimming. In tetrapods, the pectoral girdle is detached from the skull and instead suspended by muscles (and in some lineages a clavicle strut) from the axial skeleton, allowing independent movement of the head and forelimbs — essential for supporting body weight and generating propulsive force during walking, running, or flight.

Biology

Biology · L4 · Locomotor and Sensory Anatomy Across Vertebrates

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What is minimum viable population (MVP) size, and what does it attempt to quantify?1 / 10

Minimum viable population size is the smallest population size estimated to have a specified probability (e.g. 95%) of persisting for a specified time (e.g. 100 years) despite the combined effects of demographic stochasticity, environmental variation, genetic deterioration, and catastrophes. It is a probabilistic threshold, not a hard cutoff, generated using population viability analysis (PVA), and is used to set conservation targets such as the minimum area or number of individuals a reserve or captive-breeding program needs to support.

Biology

Biology · L4 · Extinction Risk and Population Viability

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Distinguish in situ from ex situ conservation, and give an example of each.1 / 10

In situ conservation protects species within their natural habitat, such as establishing a national park or marine protected area, preserving not just the target species but the full ecological community and evolutionary processes it participates in. Ex situ conservation protects species outside their natural habitat, such as captive breeding programs, zoos, seed banks, or cryopreserved genetic material, providing insurance against extinction in the wild and a source for reintroduction, but generally cannot replicate natural selection pressures or the full ecological interactions of the wild habitat, and can risk domestication-like selection or loss of natural behaviors over generations in captivity.

Biology

Biology · L4 · Conservation Strategies and Ecosystem Management

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Describe how parietal cells secrete hydrochloric acid into the stomach lumen, and what enzyme provides the substrate needed for this process.1 / 10

Parietal cells use carbonic anhydrase to combine CO2 and water into carbonic acid, which dissociates into H+ and bicarbonate; the H+ is actively pumped into the stomach lumen against a steep concentration gradient by an H+/K+-ATPase (the proton pump) in the apical membrane, while the bicarbonate is exchanged for chloride at the basolateral membrane (the chloride shift), with chloride then following H+ into the lumen through apical channels to combine and form hydrochloric acid. This mechanism can achieve a luminal pH as low as about 1-2, roughly a million-fold higher H+ concentration than in the blood.

Biology

Biology · L4 · Digestive Enzymes, Motility, and Regulation

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What types of DNA damage does base excision repair (BER) correct, and what is its overall mechanism?1 / 11

BER corrects small, non-helix-distorting base lesions such as deamination (e.g. cytosine to uracil), oxidation (e.g. 8-oxoguanine), and alkylation damage. A DNA glycosylase specific to the damaged base recognizes and flips it out of the double helix, cleaving the glycosidic bond to remove just the damaged base and leave an abasic (AP) site. AP endonuclease then nicks the DNA backbone at that site, DNA polymerase β fills the single-nucleotide gap using the intact complementary strand as a template, and DNA ligase seals the remaining nick.

Biology

Biology · L4 · DNA Damage and Repair Pathways

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What do the Michaelis-Menten parameters Km and Vmax each represent?1 / 10

Vmax is the maximum reaction rate the enzyme can achieve when substrate concentration is high enough to saturate every active site. Km is the substrate concentration at which the reaction rate equals half of Vmax, and it reflects the apparent affinity of the enzyme for its substrate under the given conditions: a LOW Km indicates the enzyme reaches half-maximal velocity at a low substrate concentration (high apparent affinity), while a HIGH Km indicates more substrate is needed to reach half-maximal velocity (lower apparent affinity).

Biology

Biology · L4 · Enzyme Kinetics and Allosteric Regulation

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Why is the oxygen-hemoglobin dissociation curve sigmoidal rather than hyperbolic?1 / 10

Hemoglobin is a tetramer of four subunits, each with its own oxygen-binding heme group, and oxygen binding at one subunit induces a conformational shift (transitioning the whole tetramer from a low-affinity tense (T) state toward a high-affinity relaxed (R) state) that increases the oxygen affinity of the remaining subunits — positive cooperativity. This produces the sigmoidal curve: hemoglobin is relatively reluctant to bind the first oxygen molecule at low pO2, but affinity increases sharply as more subunits become bound, allowing hemoglobin to load oxygen efficiently across the sharp pO2 range found in the lungs while still unloading substantial oxygen across the more modest pO2 drop found in respiring tissue.

Biology

Biology · L4 · Hemoglobin, Gas Transport, and Hemostasis

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What is the human microbiome, and how does it differ from the human microbiota?1 / 10

The human microbiota refers to the community of microorganisms (bacteria, archaea, fungi, viruses) living in and on the human body. The human microbiome refers more specifically to the collective genomes of that community — its total genetic and functional potential — which vastly exceeds the roughly 20,000 protein-coding genes in the human genome itself, since the gut microbiota alone is estimated to contribute hundreds of times more unique genes, encoding metabolic capabilities the human genome does not have.

Biology

Biology · L4 · Human Microbiome Composition and Function

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Summarize the overall products of the light-dependent reactions of photosynthesis that feed into the Calvin cycle.1 / 10

The light-dependent reactions, occurring across the thylakoid membrane, use light energy captured by photosystems II and I to split water (releasing O2), pump protons to build a proton-motive force that drives ATP synthase, and pass electrons through an electron transport chain ultimately reducing NADP+ to NADPH. These two products, ATP and NADPH, are the energy and reducing-power currency exported to the stroma to power carbon fixation in the Calvin cycle; the light reactions alone fix no carbon.

Biology

Biology · L4 · Photosynthesis: Light Reactions and Rubisco

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What is a monophyletic group (clade), and how does it differ from a paraphyletic or polyphyletic group?1 / 10

A monophyletic group (clade) consists of an ancestor and ALL of its descendants — a complete branch of the tree. A paraphyletic group includes an ancestor and SOME but not all of its descendants (e.g. traditional 'Reptilia' excluding birds, even though birds descend from the reptilian ancestor). A polyphyletic group includes members from separate branches that do not include their most recent common ancestor, usually because they were grouped by convergent similarity rather than shared ancestry (e.g. grouping 'warm-blooded animals' would lump birds and mammals despite their common ancestor being cold-blooded).

Biology

Biology · L4 · Reading and Building Phylogenetic Trees

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What is auxin (indole-3-acetic acid), where is it primarily synthesized, and how is it transported through the plant?1 / 10

Auxin is the primary hormone controlling directional growth, synthesized mainly in shoot apical meristems and young leaves. Unlike most hormones, which move passively through the phloem or diffusion, auxin undergoes polar transport: influx carriers (AUX1) and efflux carriers (PIN proteins), asymmetrically localized on one side of each cell, actively move auxin cell-to-cell in a consistent direction (typically basipetal, from shoot tip toward the root), and the polarized positioning of PIN proteins can itself be dynamically redirected in response to gravity or light, making auxin transport both directional and adjustable.

Biology

Biology · L4 · Plant Hormones

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What is the Cholodny-Went hypothesis, and how does it explain phototropism (bending toward light)?1 / 10

The Cholodny-Went hypothesis proposes that unequal light exposure causes auxin to be redistributed laterally within the stem tip, moving toward the shaded side, so the shaded side receives more auxin and elongates faster than the illuminated side, producing bending of the stem toward the light source. This lateral redistribution is achieved through light-induced relocalization of PIN auxin-efflux carriers to the shaded side of cells, directing auxin flow away from the illuminated side rather than requiring any change in total auxin synthesis.

Biology

Biology · L4 · Tropisms and Photoperiodism

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Describe the hypothalamic-pituitary-gonadal (HPG) axis and its major hormones.1 / 11

The hypothalamus secretes gonadotropin-releasing hormone (GnRH) in a pulsatile pattern, which stimulates the anterior pituitary to secrete the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These gonadotropins act on the gonads (ovaries or testes), stimulating gametogenesis and secretion of the sex steroids (estrogen and progesterone in females, testosterone in males), which in turn feed back on the hypothalamus and pituitary to regulate further GnRH/FSH/LH release, forming a classic multi-tiered endocrine axis common to both sexes despite very different downstream physiology.

Biology

Biology · L4 · Reproductive Endocrinology and Gametogenesis

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Define the biological species concept, and name one limitation of it.1 / 10

The biological species concept defines a species as a group of populations whose members can interbreed and produce fertile offspring in nature, and which are reproductively isolated from other such groups. It cannot be applied to asexual organisms (e.g. many bacteria), to fossils (where reproductive compatibility cannot be observed), or cleanly to organisms that hybridize extensively despite being otherwise distinct (e.g. many plants), which is why other species concepts (morphological, phylogenetic, ecological) are used alongside it.

Biology

Biology · L4 · Modes of Speciation

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