Unit 2: Cells, Development, Biodiversity and Conservation

Cell theory states that all living organisms consist of one or more cells, cells are the basic functional units of life, and new cells arise from pre-existing cells.

All cells share a cell-surface membrane, cytoplasm, DNA and ribosomes. The rest of the ultrastructure differs: eukaryotes have membrane-bound organelles, while prokaryotes lack internal membrane compartments and have smaller ribosomes.

A bacterium and a plant cell look very different, but both contain DNA, ribosomes, cytoplasm and a boundary membrane. Those shared features support the common-cell principle; they do not imply identical cell functions.

Cell theory is a unifying model, not a claim that every cell has the same organelles. Viruses are not cells and therefore are not explained as living cellular organisms by this theory.

In a multicellular organism, specialised cells of the same type work together as a tissue; tissues combine into organs, and organs cooperate in organ systems.

Specialisation makes division of labour possible: a red blood cell transports oxygen, a muscle cell contracts, and an epithelial cell absorbs. Their organisation lets a larger structure perform a function no single cell performs as effectively.

Cardiac muscle, blood-vessel and connective tissues combine to form the heart. The heart then works with blood vessels as part of the circulatory system, so the labels describe nested levels rather than interchangeable names.

A tissue is more than a pile of similar cells: the cells must cooperate in a shared function. A single specialised cell is not itself an organ or an organ system.

Eukaryotic cells contain membrane-bound organelles whose structures support distinct functions. The nucleus stores chromatin, mitochondria make ATP in aerobic respiration, ribosomes translate mRNA, and the endoplasmic reticulum, Golgi, lysosomes and centrioles have different specialised roles.

The rough ER carries ribosomes and processes proteins; smooth ER lacks ribosomes and is involved in lipid and steroid production. The Golgi modifies and packages cargo, lysosomes digest material, and centrioles organise spindle fibres in animal-cell division.

A secreted protein follows a route from nucleus (transcription) to ribosome/RER (translation and folding), then in vesicles to the Golgi for modification and onward to the cell surface. A free ribosome instead makes a protein that remains in the cytoplasm.

Ribosomes are not membrane-bound, and plant and animal cells are both eukaryotic. Do not identify an organelle from a label alone; connect its visible structure to its function.

Ribosomes on rough endoplasmic reticulum make proteins that are destined for secretion, a cell membrane or another membrane-bound compartment. The RER folds and processes the new polypeptide in its lumen.

Transport vesicles carry the protein from the RER to the Golgi apparatus. The Golgi modifies and sorts it, then sends it in vesicles to its destination: outside the cell, into a lysosome or to another membrane system.

Insulin is a useful model of the pathway: a ribosome makes the polypeptide, the RER and Golgi process and package it, and a vesicle fuses with the cell-surface membrane to release it by exocytosis.

Free cytoplasmic ribosomes generally make proteins used inside the cytoplasm. The RER is not the same as the Golgi: one is the ribosome-studded production/folding site, the other the modification and sorting centre.

Prokaryotic cells are smaller cells without a nucleus or membrane-bound organelles. Their circular bacterial DNA lies in the cytoplasm, their ribosomes are 70S, and their cell wall contains murein (peptidoglycan).

Some prokaryotes also have plasmids, a capsule, flagella or pili. Plasmids are small DNA loops; a capsule can protect the cell; flagella support movement; pili can help attachment or DNA transfer. These structures are not present in every bacterium.

A plasmid can carry a resistance gene independently of the main circular chromosome. That explains why a bacterium may acquire a useful trait without changing the definition of its chromosome.

“Prokaryote” does not mean “no DNA” or “no membrane”: it has a cell-surface membrane and genetic material, but no membrane-bound nucleus. Size alone is not a structural comparison.

Electron microscopes resolve much smaller distances than light microscopes because electrons have a shorter wavelength. A TEM sends electrons through a thin specimen to show high-resolution internal structure; an SEM scans the surface to produce a three-dimensional view.

First decide whether the image is a 2D internal section or a 3D surface. Then use distinctive features—such as a nucleus, double-membraned mitochondrion with cristae, or stacked Golgi sacs—to propose an organelle, and check that the scale and context fit.

A high-resolution 2D image showing internal cristae is consistent with a TEM mitochondrion; a raised surface image is more consistent with an SEM. The image is a dead-specimen snapshot, not a movie of a living cell.

Magnification does not create information that resolution cannot support. Do not call every dark oval a nucleus or infer function from appearance without the structural evidence.

A light microscope focuses light through a thin specimen; staining increases contrast when transparent cell structures would otherwise blend together. Electron microscopes use electrons with a shorter wavelength and therefore resolve finer detail.

Prepare a thin, supported specimen, add a suitable stain and begin with the low-power objective. Stain choice depends on the material and question: methylene blue highlights animal nuclei, iodine reveals starch, and differential staining can separate tissue types.

A clear onion or animal-cell sample may show almost no boundaries before staining, but nuclei become distinct after a suitable dye. An electron micrograph can show organelles that a light microscope cannot resolve.

Magnification makes an image larger; resolution is the ability to distinguish nearby points. More magnification cannot recover detail that the microscope never resolved, and a stain is not a natural colour of every structure.

For the animal-cell practical, make a thin, well-prepared slide, observe it systematically with a light microscope, and record only structures that are actually visible. Use a calibrated eyepiece graticule when a scale measurement is required.

Start on low power to locate the specimen safely, then increase magnification for cellular detail. Calibrate the graticule against a stage micrometer at the objective being used; the calibration changes when magnification changes. Make a clear, proportional labelled drawing without shading or invented structures.

If 40 graticule divisions equal 10 μm, one division is 0.25 μm. An object spanning five divisions is therefore 1.25 μm, provided the same objective calibration is still in use.

Image size, actual size and magnification must use compatible units. A plan drawing shows tissue arrangement at low power; a cell drawing shows detail at higher power. Neither replaces the observation itself.

A locus is the position of a gene on a chromosome. Different alleles occupy the same locus, but genes at different loci may be linked if they lie on the same chromosome and tend to travel together through meiosis.

Linked genes do not assort independently in the simple way that genes on different chromosomes do. Crossing over can separate linked alleles, so offspring may show parental combinations more often than recombinant combinations.

If two genes are close on one autosome, gametes usually retain the parental allele combinations; a crossover between their loci creates a less frequent recombinant combination.

Linkage is about physical chromosome location, not about two traits looking similar. Sex-linked genes require the X/Y context, while autosomal linkage concerns non-sex chromosomes.

Meiosis reduces a diploid nucleus to four haploid nuclei through two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II, so chromosome number halves in the first division.

Variation arises because homologous pairs align independently at metaphase I and non-sister chromatids exchange corresponding DNA at crossing-over in prophase I. Each gamete therefore receives a different combination of parental alleles.

For a species with haploid number n, independent assortment alone permits 2ⁿ chromosome combinations. In humans that is 2²³ possible combinations before crossing-over and fertilisation add further variation.

Crossing-over swaps segments between non-sister chromatids of homologous chromosomes; it is not the same as sister-chromatid separation. Meiosis produces haploid gametes, not two identical diploid cells.

Mammalian gametes are haploid cells produced by meiosis. Their structures are specialised for meeting, penetrating and supporting the next stage of reproduction.

Sperm have a flagellum for movement, many mitochondria to supply ATP and an acrosome containing enzymes that digest the zona pellucida. An egg is much larger, contains nutrient reserves, and has a zona pellucida that can become a barrier after fertilisation.

The acrosome helps one sperm reach the egg membrane; once fertilisation begins, changes to the egg’s zona pellucida help prevent additional sperm nuclei entering.

The egg’s size is not simply “for movement” and the sperm’s acrosome is not a nucleus. A gamete is haploid; the diploid chromosome number is restored only when nuclei fuse.

Fertilisation is the fusion of sperm and egg nuclei to form a diploid zygote. It begins when a sperm reaches the egg in the oviduct and its acrosome releases enzymes that digest a path through the zona pellucida.

After the sperm crosses the egg membrane, cortical granules release their contents. The zona pellucida hardens, producing the cortical reaction that blocks other sperm. The sperm and egg nuclei then fuse, restoring the full chromosome number; the zygote divides by mitosis.

The acrosome reaction permits entry, whereas the cortical reaction prevents polyspermy. They occur at different stages and solve opposite problems: access first, exclusion second.

Fertilisation is nuclear fusion, not merely sperm contact with the egg. The first divisions of the zygote are mitotic, so they increase cell number without halving chromosome number.

In flowering plants, fertilisation follows pollination: a pollen grain germinates on the stigma and grows a pollen tube down the style to an ovule. Male nuclei travel through the tube and one fuses with the egg nucleus.

The tube grows towards the ovary, enters the ovule through the micropyle and releases the male nuclei. Fusion produces a diploid zygote; the sequence matters because pollen-tube growth delivers the nucleus to the egg rather than fertilising on the stigma.

A pollen grain can germinate only when the stigma is compatible. The tube then provides a continuous route from the pollen grain to the embryo sac, where nuclear fusion occurs.

Pollination is transfer of pollen; fertilisation is fusion of nuclei. Do not treat pollen landing on a stigma as completion of fertilisation.

The cell cycle is the regulated sequence from one cell division to the next: G1 growth and normal work, S-phase DNA replication, G2 preparation and checking, M-phase nuclear division, then cytokinesis.

After S phase each chromosome has two sister chromatids. Mitosis separates those chromatids into two genetically identical nuclei, and cytokinesis partitions the cytoplasm so each daughter cell receives one nucleus.

A root-tip cell can spend much of its cycle in interphase, then pass through prophase, metaphase, anaphase and telophase before the plant cell forms a new wall between daughters.

Interphase includes G1, S and G2; it is not a single resting stage. Cytokinesis follows nuclear division and is not one of the four named stages of mitosis.

Root-tip meristems contain actively dividing cells, so a stained squash can show the stages of mitosis under a light microscope. Acid softens the tissue, stain makes chromosomes visible, and gentle squashing spreads cells into a thin layer.

Cut a root tip, treat and rinse it, stain the tissue, squash it carefully on a slide and observe with suitable magnification. Identify condensed chromosomes and classify cells by the visible stage rather than by where you expect them to be.

A metaphase cell has chromosomes aligned at the equator; an anaphase cell has sister chromatids moving apart. Counting every stage with visible chromosomes allows a mitotic index to be calculated.

The preparation is a snapshot of a fixed sample: it cannot show a living cell progressing through time. Do not count an unclear nucleus as a stage or damage the specimen by pressing too hard.

Mitotic index measures the proportion of cells in a sample that are undergoing mitosis: number of cells with visible chromosomes divided by the total number of cells counted. Multiply by 100 for a percentage.

Count cells systematically, include prophase, metaphase, anaphase and telophase in the numerator, and include every counted cell in the denominator. Keep the unit as a proportion or state clearly that it has been converted to a percentage.

If 32 of 42 cells show visible chromosomes, the mitotic index is 32/42 = 0.76, or 76% when expressed as a percentage.

A high index means more cells were observed in mitosis in that sample; it does not automatically prove faster growth. Sampling region, preparation quality and counting decisions affect the estimate.

A stem cell can divide by mitosis and produce cells that remain stem cells or differentiate. Totipotent cells can form embryonic and extra-embryonic tissues; pluripotent embryonic cells can form embryo cell types; adult stem cells are usually more limited (multipotent).

The morula is an early embryo with highly potent cells; the blastocyst contains an inner cell mass used as an embryonic stem-cell source. Adult tissue stem cells can replace particular cell types, such as blood or intestinal epithelium.

A bone-marrow transplant uses adult stem cells to restore blood-cell production. Embryonic cells could offer a wider range of targets, but their use raises consent, embryo status, regulation and immune-compatibility questions.

“More potent” does not mean automatically safer or better. Medical decisions weigh cell source, differentiation control, rejection risk, evidence and ethical viewpoints rather than potency alone.

Cells in one organism usually contain the same genes, but they become specialised because different genes are switched on or off. Active genes are transcribed into mRNA, translated into proteins, and those proteins alter cell structure or processes.

A transcription factor can increase or reduce RNA-polymerase access to a gene. Changing which proteins are produced therefore changes a cell’s properties without changing its entire DNA sequence.

A muscle precursor and a nerve precursor can share the same genome yet express different sets of proteins, giving them different structures and jobs. Environmental signals can alter transcription-factor activity and gene expression.

Differentiation is not caused by cells losing all unused genes. Epigenetic marks can change how DNA is read without changing the base sequence; that differs from a mutation.

Eukaryotic pre-mRNA contains coding exons and non-coding introns. Post-transcriptional splicing removes introns and joins exons before the mature mRNA leaves the nucleus.

If the exons are joined in different combinations, alternative splicing produces different mature mRNAs from one gene. Translation of those mRNAs can therefore produce different polypeptides.

The same antibody gene can be spliced to include or omit an exon encoding a membrane-binding region. The resulting protein is either cell-bound or secreted, even though the original gene is the same.

Alternative splicing changes the RNA message after transcription; it does not alter the DNA base sequence. Do not confuse intron removal with DNA replication or mutation.

An organism’s phenotype results from the interaction between its genotype and its environment. Epigenetic modifications can change which genes are expressed without changing the DNA sequence itself.

DNA methylation can reduce transcription, while histone acetylation loosens chromatin so transcription factors and RNA polymerase can access genes. These changes alter mRNA and protein production, and some can persist through cell division.

Genetically similar plants can differ in height when water or mineral supply differs. A lifestyle or environmental signal can also change gene expression through epigenetic tags, but that is not the same as creating a new allele.

An environmentally changed phenotype is not automatically inherited. Distinguish a reversible epigenetic state from a mutation in the DNA sequence, and state which evidence supports inheritance across cell division.

Some phenotypes depend on several genes or several alleles as well as the environment. Small additive effects from many loci produce a range of values rather than a few discrete categories.

A single-gene trait such as ABO blood group gives discontinuous categories. A polygenic trait such as height or mass is usually continuously distributed because different allele combinations add together and environmental conditions shift the result.

If two genes each contribute a small height effect, an individual carrying more contributing alleles can be taller than one carrying fewer; nutrition and water availability can still move either genotype away from its genetic potential.

Do not assume every gene contributes equally or that a continuous phenotype is purely genetic. “Polygenic” describes many genetic contributors; it does not remove environmental influence.

Plant cells share the core eukaryotic organelles but also contain structures specialised for a cell wall, photosynthesis, storage and communication between neighbouring cells.

The cellulose cell wall and middle lamella support and join cells. Plasmodesmata connect cytoplasm; pits are thin wall regions that aid transport. Chloroplasts contain thylakoid membranes and stroma for photosynthesis, amyloplasts store starch, and the vacuole is enclosed by a selective tonoplast and contains cell sap.

Water pressure against the vacuole makes a cell turgid, while cellulose and the middle lamella stop the surrounding wall from collapsing. A chloroplast is therefore not simply a “plant mitochondrion”: its compartments support photosynthesis.

Not every plant cell contains chloroplasts, and the tonoplast surrounds the vacuole rather than the whole cell. Compare the extra plant structures with the shared animal-cell organelles.

Identify a plant organelle in an electron micrograph by combining its shape, membranes and position with the surrounding cell context—not by relying on one dark spot.

A chloroplast has a double boundary and internal thylakoid stacks; a vacuole is a large space bounded by the tonoplast; a nucleus has an envelope and chromatin; an amyloplast contains starch granules. Use a TEM-style 2D internal view when the micrograph shows sections through organelles.

A plant cell containing a large vacuole, a cellulose wall and several chloroplasts is more likely to be a photosynthetic leaf cell than an animal cell. The combination is stronger evidence than any single feature.

Electron microscopy gives a dead specimen snapshot and the section plane can hide or distort structures. State what the image supports; do not infer an organelle that is not resolved.

Starch is a compact, insoluble storage polysaccharide of α-glucose. Cellulose is a structural polymer of β-glucose whose parallel chains form strong microfibrils through many hydrogen bonds.

Amylose is an unbranched helix with α-1,4 links; amylopectin has α-1,4 links plus α-1,6 branches, giving many terminal glucose units. Cellulose chains use β-1,4 links, so adjacent glucose units alternate orientation and align into fibres.

Starch granules can be stored without exerting a strong osmotic effect, then hydrolysed when glucose is needed. Cellulose microfibrils reinforce the wall so it withstands turgor pressure without bursting.

Starch and cellulose are both glucose polymers but their monomer configuration and bonding produce different functions. Hydrogen bonds between cellulose chains are not the same as the glycosidic bonds within each chain.

Plant fibres gain tensile strength from cellulose microfibrils arranged in a strong mesh and from secondary thickening of the cell wall, often with lignin.

Parallel or layered microfibrils distribute pulling forces, while a thick secondary wall resists stretching. These properties make sclerenchyma fibres and xylem useful as structural materials.

A rope made from plant fibres depends on the same principle: long, hollow fibres with thickened walls tolerate tension. The material can be exploited without confusing strength with flexibility or transport.

Sclerenchyma fibres support but do not conduct water; xylem also supports while conducting. A thick wall can strengthen a cell but can also prevent living contents and membrane transport.

Xylem vessels conduct water and mineral ions and support the stem; phloem translocates dissolved organic solutes; sclerenchyma fibres provide support without being a transport pathway.

Mature xylem vessels are dead, hollow and lignified, with pits for lateral water movement. Phloem contains living sieve-tube elements and companion cells, carries sap from sources to sinks in either direction, and lacks xylem’s lignified transport tube. Sclerenchyma fibres are dead, thick-walled support cells with end walls.

In a stem cross-section, xylem is toward the centre and phloem toward the outside, often associated with sclerenchyma. The position plus tissue structure helps identify function.

Phloem transport is not restricted to “upwards”, and sieve tubes are not simply miniature xylem vessels. Keep support, water transport and organic-solute translocation distinct.

To investigate plant tissues, cut a very thin transverse stem section, stain it, mount it without trapped bubbles and observe it under a light microscope. Use a plan diagram to record the tissue arrangement.

Identify xylem vessels, phloem sieve tubes and sclerenchyma fibres from their staining, wall structure and position. Draw the overall distribution at low power; draw recognisable cells at higher power, with labels connected directly to visible features.

Toluidine blue can make xylem and sclerenchyma blue-green while phloem appears pink-purple, helping distinguish the vascular bundle before you compare the inner xylem with outer phloem.

A plan diagram shows position and pattern, not individual cell detail. Air bubbles, a thick section or a misleading section plane can look like tissues, so only label what the image supports.

Plant fibres and starch are renewable feedstocks that can replace some oil-based plastics or fuels. Their sustainability advantage comes from renewability and biodegradability, not from the word “plant” alone.

Plant fibres can make ropes and fabrics; starch can make bioplastics or bioethanol. They may reduce fossil-fuel use and persistent plastic waste, but cultivation, processing, land use, strength and end-of-life conditions still affect the overall comparison.

A starch-based bioplastic may be renewable and biodegradable, while a plant-fibre rope may be weaker than an oil-based plastic rope. A fair judgement states which property and lifecycle stage is being compared.

Renewable does not mean impact-free or automatically sustainable. Include replacement rates, resource demands and performance rather than treating one environmental benefit as a complete verdict.

Plants absorb water and inorganic ions through roots and move them in xylem. Water supports photosynthesis, transport, turgor and temperature regulation; nitrate, magnesium and calcium ions support different molecules and structures.

Nitrate is needed for amino acids, proteins, DNA and chlorophyll. Magnesium is part of chlorophyll. Calcium contributes to cell-wall structure and normal growth. A shortage produces characteristic symptoms, but the symptom alone does not identify the cause without controlling other minerals.

A nutrient-broth experiment can compare seedlings receiving high, medium or low calcium while keeping light, age and other ions constant. Measure mass change and inspect symptoms rather than attributing every difference to water.

Water is a transport medium, not a replacement for mineral nutrients. A deficiency investigation needs matched seedlings, replication and controls so ion concentration is the tested variable.

Tensile strength is the maximum load a fibre carries before it breaks. To compare plant fibres, attach a fibre securely, add weights one at a time and record the breaking load.

Keep fibre length, width, moisture and loading method as constant as possible. Repeat with several fibres of the same type, calculate a mean and report variation; a single break is not a reliable material comparison.

If three equal-length fibres break at different loads, the mean breaking load estimates typical strength while the spread shows how consistent the material is. A rope’s performance depends on more than one strand.

Breaking load is not automatically tensile stress: stress also depends on cross-sectional area. Do not compare fibres of very different thicknesses as if load alone measured an intrinsic property.

Bacterial growth depends on resources and conditions that let enzymes and metabolism work: nutrients, a suitable temperature and pH, and enough oxygen for an aerobic species.

If temperature or pH moves too far from the organism’s suitable range, enzyme activity and cell processes fall. Nutrients supply materials for growth and respiration; oxygen matters when the bacteria respire aerobically.

An antimicrobial-plate investigation incubates bacteria on nutrient agar under controlled conditions. If the control grows but a plant extract leaves a clear zone, the zone can be attributed more confidently to antimicrobial action.

“No growth” can mean an antimicrobial effect, unsuitable culture conditions or contamination. Keep the control, incubation conditions and organism constant before drawing the conclusion.

An antimicrobial substance kills microorganisms or prevents their growth. Some plant compounds have antimicrobial properties and can be investigated as possible sources of treatments.

The useful claim is comparative: one extract produces less bacterial growth than a control under the same conditions. The size of a clear zone reflects inhibition only when diffusion, concentration and culture conditions are comparable.

Paper discs soaked in different plant extracts are placed on a bacterial lawn; a disc soaked only in ethanol is the control. A larger clear zone suggests stronger inhibition in that test, not a universal cure.

A plant being “natural” does not prove safety or effectiveness. Separate antimicrobial activity in an agar test from a clinically effective drug, and control contamination and solvent effects.

To investigate plant antimicrobial properties, spread a known bacterial culture over nutrient agar, place equal paper discs containing plant extracts and a solvent control, then incubate safely and measure the clear zones.

Prepare extracts consistently, use sterile equipment, open plates briefly, tape and invert them, and incubate at a safe temperature. Measure diameters or areas, repeat at least three times and calculate a mean.

If a disc gives a 12 mm diameter zone, calculate its area using the radius, not the diameter. Compare that mean with the ethanol control before claiming an extract inhibits growth.

Aseptic technique protects the culture and the people handling it; it is not an optional decoration. A larger zone can also reflect faster diffusion, so concentration and disc size must be controlled.

Drug testing asks two different questions: is a treatment safe, and does it work better than a comparator? Modern development moves from laboratory and animal evidence to controlled clinical phases.

Phase 1 uses a small group to study dose and side effects; Phase 2 tests effectiveness in patients; Phase 3 compares the new drug with an existing treatment in a larger group. Placebos estimate expectation effects, and double blinding reduces observer and participant bias.

William Withering’s digitalis “soup” illustrates early dose-finding without modern controls. A current trial can compare a treatment with a placebo or existing drug while neither patient nor clinician knows the allocation.

A later phase is not automatically proof that every patient benefits. Sample size, comparator, blinding, side effects and outcome measures determine what the evidence supports.

Taxonomy organises organisms into nested groups using similarities and differences in phenotype and genotype. The species concept is the base: members can usually produce fertile offspring; higher ranks group progressively broader relationships.

Molecular evidence can overturn a classification based only on appearance. Comparing DNA, RNA or proteins supports molecular phylogeny: greater sequence similarity generally indicates a more recent common ancestor.

The three-domain system separates Archaea, Bacteria and Eukarya because molecular evidence showed that “prokaryote” was not one evolutionary group. A new grouping should be critically evaluated by the scientific community.

Classification is a model of relationship, not a fixed list of visual labels. A similar phenotype can evolve independently, and a binomial name is not evidence that two organisms are the same species.

Biodiversity includes genetic diversity within species, species diversity and the variety of habitats or ecosystems. It supports resilience, but human activities can reduce all three levels.

Habitat destruction and fragmentation remove living space and isolate populations; overexploitation and hunting remove organisms faster than they recover; intensive agriculture can reduce diversity through monoculture, pesticides and fertiliser loss; climate change shifts conditions.

A fragmented forest may lose species directly and also increase inbreeding in small isolated populations. A disease affecting a dominant species can then cause a larger ecosystem change when genetic and species diversity are low.

Species richness is only a count; species diversity also considers relative abundance. A single threat rarely explains every change, so distinguish the measured level of biodiversity from the proposed human cause.

Biodiversity is the variety of life at three linked levels: genetic diversity within species, species diversity within communities, and ecosystem or habitat diversity across a region. Endemic species occur naturally in only one area.

Greater variety can give ecosystems resilience because different genes, species and habitats provide alternative responses to disturbance. Low diversity can make a population or ecosystem more vulnerable to one pathogen or environmental change.

A coral reef may contain many habitats and niches, while an isolated island species may have high uniqueness but a small range and high extinction risk. “More species” alone does not describe all biodiversity.

Species richness counts species; species diversity also considers abundance. Genetic diversity is variation in alleles, not simply the number of visible body forms.

Genetic diversity can be estimated from the proportion of individuals that are heterozygous: heterozygosity index = number of heterozygotes ÷ total number of individuals.

Count the heterozygous individuals for the chosen gene or sample, divide by the full sample size and compare like with like. A larger value means more individuals carry two different alleles at that locus; it is not a complete census of every gene.

If 18 of 60 individuals are heterozygous, the index is 18/60 = 0.30. Comparing two populations is meaningful only when the locus, sample method and population context are comparable.

Heterozygosity is a proportion, not a percentage unless multiplied by 100. A single locus or small sample can miss genetic diversity elsewhere and can be biased by sampling.

The index of diversity compares communities using both the total number of organisms and how evenly individuals are distributed among species: D = N(N−1) / Σn(n−1). N is the total count and n is the count for one species.

Add all individuals to find N, calculate N(N−1), calculate n(n−1) for every species and sum those terms, then divide. A larger D indicates greater diversity under this formula.

For N = 202 and Σn(n−1) = 6884, D = 202×201 ÷ 6884 ≈ 5.90. Two habitats with the same richness can still have different D values if one is dominated by one species.

Use the exact formula and counts supplied; do not confuse D with species richness or with the heterozygosity index. Comparing habitats also requires comparable sampling effort.

A niche is the role a species plays in its habitat: what it uses or eats, where and when it is active, and which organisms interact with it. Adaptations are features that improve survival or reproduction in that environment.

Anatomical adaptations are structural; physiological adaptations change internal processes; behavioural adaptations change actions. Two species may appear to share a habitat but cannot occupy exactly the same niche indefinitely without direct competition.

A desert plant’s reduced leaves, water-saving physiology and growth timing each address a different pressure. Together they explain where it can live and how it uses resources, rather than being a list of unrelated traits.

A niche is not simply an address, and an adaptation is not automatically beneficial in every environment. Link each trait to a specific biotic or abiotic pressure and outcome.

Under random mating, a very large population, and no migration, mutation or natural selection, allele frequencies remain stable: p+q=1 and p²+2pq+q²=1. A change over generations suggests one or more assumptions has failed.

For a recessive phenotype, q² is the affected proportion. Take its square root to find q, calculate p=1−q, then find p² and 2pq. Check that the three genotype frequencies add to 1 before interpreting a population change.

If 10% show a recessive phenotype, q²=0.10, q≈0.32 and p≈0.68; p²≈0.46 and 2pq≈0.44. The calculation estimates frequencies under the model—it does not prove the population is at equilibrium.

The Hardy–Weinberg equation is a calculation tool; the principle is the no-change condition. A dominant phenotype combines p² and 2pq, so it cannot be treated as p² alone.

Zoos and seed banks can protect endangered species and genetic diversity when wild conservation is not enough. Their value comes from research, captive breeding or storage, education and possible reintroduction—not from captivity alone.

Seed banks store genetically varied seeds under controlled conditions and can support research or future replanting; zoos can study behaviour, breed animals and prepare releases. Both may suffer from small samples, reduced genetic diversity, limited representativeness or poor survival after reintroduction.

A seed bank collecting from several wild sites preserves more allelic variety than storing one crop line. A captive-breeding programme may restore an endangered species, but release requires suitable habitat, disease checks and animals able to survive in the wild.

Ex situ conservation does not replace habitat protection. Evaluate the objective, genetic diversity, welfare, evidence and reintroduction feasibility rather than counting specimens as success.