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.

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 protein for export is transcribed in the nucleus, translated on an rER-bound ribosome, moved in vesicles to the Golgi and packaged for its destination. A free ribosome instead makes many proteins used in the cytoplasm.

1. A transport vesicle buds from the rER and carries the protein to the cis face of the Golgi apparatus.
2. Golgi cisternae modify the protein, for example by adding carbohydrate groups, and sort it.
3. A secretory vesicle buds from the trans face, moves to the cell-surface membrane and fuses with it.
4. Exocytosis releases the enzyme outside the cell.

A pancreatic cell follows this route to produce digestive enzymes. The enzyme is packaged before release, so it does not digest cellular material while being transported.

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.

A TEM thin section shows internal ultrastructure. The same organelle can appear circular, elongated or partial depending on where and at what angle it was sectioned; use surrounding structures and the scale bar to test the identification.

A light microscope uses glass lenses and visible light, so living or stained cells can be viewed but fine ultrastructure is unresolved. Electron microscopes use electromagnets and electrons with a much shorter wavelength, giving higher resolution. TEM shows thin internal sections; SEM scans surfaces.

Many specimens have little natural contrast. Coloured stains absorb visible light in selected structures; heavy-metal stains scatter electrons in electron microscopy. Differential uptake makes boundaries or components distinguishable, but preparation can introduce artefacts.

Keep units consistent: $\text{magnification}=\dfrac{\text{image size}}{\text{actual size}}$. A scale bar remains valid if an image is resized, whereas a printed magnification label may not.

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.

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.

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.

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.

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.

1. The pollen tube grows down a chemical gradient and enters the ovule through the micropyle.
2. The tube releases the two male nuclei into the embryo sac.
3. One male nucleus fuses with the egg nucleus to form a diploid zygote.
4. The other fuses with the two polar nuclei to form the triploid primary endosperm nucleus, which develops into nutritive tissue.

The tube provides a protected route that does not require motile sperm to cross dry air. The two nuclear fusions coordinate embryo formation with a food supply.

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.

1. Cut the final few millimetres of a young root tip.
2. Treat it with warm dilute hydrochloric acid for the specified time, then rinse.
3. Place the meristem on a slide, add acetic orcein or another suitable chromosome stain and tease the tissue apart.
4. Lower a coverslip and press vertically through folded paper to spread one cell layer.
5. Start at low power, then identify stages at higher power in several fields of view.

Prophase shows condensing chromosomes; metaphase shows them at the equator; anaphase shows sister chromatids separating; telophase shows two chromosome groups forming nuclei.

Use roots of the same age/region, consistent acid and staining times, and systematic fields. Wear eye protection and handle acid, stain, glass and heating equipment safely.

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 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.

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.

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.

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.

The ABO blood-group gene has $I^A$, $I^B$ and $i$ alleles in the population. $I^A$ and $I^B$ are codominant, while each is dominant to $i$, producing four phenotype categories from several genotypes.

For a polygenic trait, many allele combinations make small additive contributions. Recombination and random fertilisation create numerous combinations, while factors such as nutrition or sunlight shift expression. The resulting values often form continuous variation, such as height or skin pigmentation.

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.

First locate the cell wall and cell interior. Then trace membranes continuously: the tonoplast surrounds the vacuole, while the cell-surface membrane lies just inside the wall. Use the scale bar to reject structures of an impossible size and compare repeated examples in neighbouring cells.

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.

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.

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.

1. Cut a very thin transverse section with a sharp blade on a safe cutting surface, or use a prepared slide.
2. Mount it in water and add a suitable stain if required; lower the coverslip at an angle.
3. Scan at low power and draw clean, single, unbroken boundaries for each tissue region.
4. Keep shapes and proportions faithful, use no shading or cell detail, add a title and magnification or scale bar, and label with ruled lines.
5. At higher power, draw representative cells separately and record wall, lumen and visible contents.

Use sections from equivalent positions and the same orientation, stain and magnification when comparing specimens. Repeat sections because thickness and cutting angle can distort apparent proportions.

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.

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.

1. Select fibres of the same material, age and length; condition them at the same temperature and humidity.
2. Measure diameter at several positions and calculate a mean cross-sectional area, $A=\pi(d/2)^2$, if the fibre is approximately circular.
3. Secure the fibre between clamps without damaging or slipping it.
4. Add known masses gradually until it breaks and record the breaking mass.
5. Convert mass to force using $F=mg$, then calculate $F/A$.
6. Repeat with several fibres, identify anomalies and compare means with spread.

Independent variable: fibre type or treatment. Dependent variable: breaking force or tensile strength. Control fibre length, diameter-selection rule, hydration, temperature, loading rate and clamp arrangement.

Under suitable conditions, a culture typically passes through lag, exponential, stationary and decline phases. Stationary phase can result from nutrient depletion, waste accumulation or oxygen limitation rather than every cell stopping metabolism at once.

A clear zone around an extract on an inoculated agar plate is preliminary evidence of antimicrobial activity. It does not by itself identify the active molecule, show how it acts, establish a safe human dose or prove clinical effectiveness. Extraction solvent and concentration also affect the result.

1. Grind equal masses of each plant part with equal volumes of the same solvent for the same time, then filter.
2. Disinfect the bench, work near a suitable flame only if the approved protocol requires it, use sterile equipment and minimise the time culture vessels are open.
3. Spread the same bacterial suspension evenly over sterile nutrient agar.
4. Place equal sterile paper discs loaded with equal extract volumes; include a solvent-only negative control and, if available, a standard antimicrobial positive control.
5. Tape the lid as specified by the school protocol, incubate inverted at a safe approved temperature (commonly no more than 25 °C in school work) for a fixed time, and do not reopen the plate.
6. Measure inhibition-zone diameter in two perpendicular directions and calculate a mean; repeat plates and compare means with variation.

Keep bacterial strain and density, agar depth, disc size, extract concentration, disc spacing, incubation time and temperature constant. Randomise disc positions where several treatments share a plate.

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.

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.

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.

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.

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.

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.

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.

1. A barrier separates populations geographically, ecologically, behaviourally or temporally.
2. Mutation introduces variation and existing alleles occur at different starting frequencies.
3. Different selection pressures favour different phenotypes; genetic drift may also alter small populations.
4. Allele frequencies diverge over generations.
5. Pre-zygotic or post-zygotic barriers evolve, so populations no longer produce fertile offspring together: speciation has occurred.

The Hardy-Weinberg model gives a no-evolution baseline: $p+q=1$ and $p^2+2pq+q^2=1$ for two alleles. Constancy requires a large population, random mating, no mutation, no migration and no selection. A repeatable frequency change when sampling error is controlled shows at least one assumption is not met.

If a recessive phenotype has frequency $q^2=0.09$, then $q=0.30$, $p=0.70$ and the expected heterozygote frequency is $2pq=0.42$. These calculations describe allele/genotype frequencies; biological evidence is still needed to identify the evolutionary cause.

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.