Topic 4: Biodiversity and Natural Resources — Edexcel A-Level Biology A (Salters-Nuffield)

Life on Earth has diversified over billions of years through evolution, producing millions of species adapted to diverse environments. This extensive variety is biodiversity. However, human activities now threaten this variety. Examples include habitat destruction (deforestation, urbanisation), pollution, overexploitation (overfishing, hunting), introduction of invasive species, and climate change. These activities reduce populations, cause extinctions, and lower genetic diversity. For instance, deforestation in tropical rainforests destroys habitats, leading to species loss. Conservation efforts aim to mitigate these threats. Understanding this helps explain why biodiversity is declining and why conservation is crucial.

Biodiversity is the variety of life, including species richness, genetic diversity, and ecosystem diversity. Endemism describes species unique to a specific location. Biodiversity within a habitat is measured by species richness (number of species). Within a species, genetic diversity can be estimated using the heterozygosity index H = number of heterozygotes ÷ number of individuals in the population. To compare habitats, use the index of diversity D = N(N - 1) ÷ Σn(n - 1), where N is total individuals and n is individuals per species. Higher D indicates greater diversity. These measures help assess and compare biodiversity.

A niche is the role a species plays in its ecosystem: where it lives, what it consumes, when it is active, and how it interacts with other species. No two species can occupy exactly the same niche in a stable community; competition forces divergence. Adaptations are inherited features that improve survival and reproduction in a particular niche. Behavioural adaptations are actions, such as a kangaroo rat staying in its burrow during the day to avoid desert heat. Physiological adaptations are internal processes, such as the camel producing concentrated urine to conserve water. Anatomical adaptations are structural features, such as the thick, waxy cuticle of a marram grass leaf reducing transpiration. When discussing examples, state the adaptation, classify it, and explain how it raises fitness in that specific environment.

Natural selection is the process by which organisms with inherited characteristics that improve survival and reproduction become more common in a population over generations. Variation arises from mutation and sexual reproduction, producing a range of phenotypes. In a given environment, some individuals have alleles that give a selective advantage, such as better camouflage or disease resistance. These individuals are more likely to survive, reproduce and pass on those alleles. Over many generations, the frequency of advantageous alleles increases, and the population becomes better adapted to its environment. If populations become reproductively isolated and accumulate enough genetic differences, new species can evolve. Evolution is therefore a change in allele frequencies in a population over time, driven by natural selection.

The Hardy-Weinberg equation models allele and genotype frequencies in an ideal population: p + q = 1 for two alleles, and p² + 2pq + q² = 1 for genotypes, where p² is homozygous dominant, 2pq heterozygous and q² homozygous recessive. Calculate q² from the recessive phenotype frequency, take its square root for q, then find p as 1 − q. Compare observed genotype numbers with expected values; a significant deviation signals that allele frequencies are changing, so evolution is occurring. Such change arises when selection, mutation, genetic drift, non-random mating or gene flow act. Reproductive isolation, for example a river separating populations or differing courtship signals, prevents interbreeding, so mutations and selection accumulate independently. Over generations the genetic information diverges; if the populations can no longer produce fertile offspring, new species have formed.

Classification organises the variety of life into groups that reflect evolutionary relationships, using similarities and differences in observable phenotypes, such as anatomy and behaviour, and in genotypes, such as DNA base sequences and amino acid sequences of proteins. It is built around the species concept, in which members of a species interbreed to produce fertile offspring. New data, for example rRNA or genome sequences, are critically evaluated by the scientific community through peer review, replication and debate; accepted evidence can overturn older groupings. Molecular phylogeny, comparing conserved molecules, led to the three-domain system: Bacteria, Archaea and Eukaryota. This replaced the five-kingdom view because it better reflects evolutionary relationships, for example placing Archaea separately from Bacteria despite their prokaryotic appearance.

Plant cells share the eukaryotic ground plan of nucleus, mitochondria, ribosomes and endomembrane system with animal cells, but carry additional structures. A cellulose cell wall lies outside the cell surface membrane; the middle lamella, rich in calcium pectate, cements adjacent walls, and pits are thin regions where plasmodesmata bridge neighbouring cytoplasms for transport and communication. Chloroplasts, bounded by a double membrane, hold stacked thylakoids and stroma for photosynthesis, while amyloplasts are colourless leucoplasts storing starch grains. A large permanent vacuole, bounded by the tonoplast, stores sap and maintains turgor. Comparing with animal cells: animal cells lack walls, chloroplasts, amyloplasts, a large permanent vacuole, plasmodesmata, pits and middle lamella, but may possess centrioles, small temporary vacuoles and glycogen granules instead of starch.

Electron micrographs reveal plant cell ultrastructure at resolutions far beyond the light microscope. To recognise organelles, first establish scale and magnification from the scale bar, then identify the rigid, often rectangular cell wall and the middle lamella as a darker line between adjacent walls. Chloroplasts appear as large oval bodies with stacked thylakoid membranes (grana) and a granular stroma; amyloplasts look similar in shape but lack thylakoids and contain smooth, electron-lucent starch grains. The vacuole is a large clear area bounded by the thin tonoplast. Plasmodesmata appear as narrow channels crossing the wall at pits. Compare with animal cell images, which lack walls and chloroplasts but may show centrioles and small vesicles.

Starch and cellulose are both polymers of glucose but differ in monomer type, linkage and chain shape, so their structures suit different functions. Starch is made from α-glucose joined by α-1,4 glycosidic bonds, with α-1,6 branches in amylopectin. Its helical, branched chains are compact and expose many ends for rapid hydrolysis to glucose for respiration. Cellulose is made from β-glucose joined by β-1,4 glycosidic bonds. Each β-glucose is inverted 180° relative to its neighbour, so the chain is straight and unbranched. Parallel chains form hydrogen bonds between hydroxyl groups, holding them in flat sheets that stack into microfibrils. These hydrogen bonds are numerous, giving cellulose high tensile strength. Starch is storage in plants; cellulose is structural in cell walls.

Plant cell walls contain cellulose microfibrils embedded in a matrix. The microfibrils are arranged at angles to resist tension, and secondary thickening adds lignin and extra cellulose. In xylem vessels, cells die to form hollow tubes; lignin in secondary walls makes them rigid and waterproof, so they withstand negative pressure and transport water and mineral ions. Sclerenchyma fibres are elongated dead cells with thick lignified secondary walls, providing tensile strength and support. These properties are exploited by humans: xylem and sclerenchyma fibres from plants such as flax and hemp are used for rope, textiles, paper and composite materials. The strength, flexibility and resistance to decay depend on the arrangement and thickening of cellulose microfibrils.

Compare three plant tissues across structure, stem position and function. Sclerenchyma fibres have thick lignified walls, small lumens and dead cells at maturity; they sit as caps outside the phloem and provide support. Xylem vessels are wide, lignified, hollow tubes formed from dead cells with no end walls; they lie towards the stem interior and give support while transporting water and mineral ions upward. Phloem sieve tubes are living cells with sieve plates and companion cells; they occupy the outer vascular bundle and translocate organic solutes such as sucrose in either direction. All three are elongated and grouped in vascular bundles, but differ in wall thickness, cell viability and role.

Water is essential to plants as a solvent for metabolic reactions, a transport medium in xylem and phloem, a reactant in photosynthesis and hydrolysis, and a source of turgor pressure for support and cell expansion. Inorganic ions are absorbed from soil water by active transport. Nitrate (NO₃⁻) supplies nitrogen for amino acids, proteins, nucleic acids and chlorophyll; deficiency stunts growth and causes chlorosis. Calcium ions (Ca²⁺) are needed for cell wall structure as calcium pectate, membrane permeability and enzyme regulation; deficiency damages growing tips and causes stunted growth. Magnesium ions (Mg²⁺) are central to chlorophyll; deficiency causes chlorosis between veins. For example, a plant in nitrate-poor soil cannot synthesise enough chlorophyll, so leaves yellow and growth slows.

Drug testing has moved from observation and traditional remedies to controlled scientific protocols. William Withering tested digitalis from foxglove for dropsy, preparing a variable herbal mixture sometimes called digitalis soup; his observations suggested benefit but lacked controlled comparison. Contemporary testing uses double blind trials, where neither participants nor assessors know who receives the treatment, reducing bias. A placebo is an inactive treatment given to a control group, allowing comparison of real effects against expectation. Three-phased testing progresses from safety and dosage in small groups, to efficacy and side effects in larger patient groups, then to larger-scale comparison with existing treatments. Together these features improve reliability, validity and ethical standards.

Bacteria are prokaryotes that reproduce by binary fission, so growth means an increase in cell number rather than cell size. Population growth requires nutrients such as a carbon source, nitrogen for proteins and nucleic acids, plus minerals including phosphate and sulfur. Water activity must be high enough for enzyme activity and transport, and most species grow best near neutral pH, though some tolerate acid or alkaline conditions. Temperature matters because enzymes have an optimum; below it metabolism slows, above it enzymes denature. Oxygen requirements differ: obligate aerobes need oxygen for aerobic respiration, obligate anaerobes are poisoned by it, and facultative anaerobes can switch. In a closed culture, numbers rise through lag, log, stationary and death phases as nutrients fall and toxic waste accumulates.

Plant fibres and starch can replace oil-based plastics, supporting sustainability. Fibres from flax, hemp or jute are strong, biodegradable and renewable; they can be woven into textiles, used in composites for car panels or moulded into packaging. Starch from maize or potato can be processed into bioplastics such as polylactic acid (PLA) or thermoplastic starch. These products reduce reliance on finite crude oil, lower carbon emissions if grown and processed sustainably, and biodegrade, cutting plastic pollution. However, they compete with food crops for land and water, and some need industrial composting. Sustainability means meeting present needs without compromising future generations, so using renewable, biodegradable plant materials can help, provided life-cycle impacts are considered.

Zoos and seed banks conserve endangered species and genetic diversity through scientific research, captive breeding, reintroduction and education. Zoos maintain captive populations, study genetics, health and behaviour, and breed individuals to increase numbers. Reintroduction programmes release captive-bred animals into suitable habitats, but success depends on habitat quality, disease and adaptation. Seed banks store seeds at low temperature and moisture, preserving genetic diversity of plants cheaply and long-term; some seeds need cryopreservation. Education raises public awareness and funding. Evaluation must weigh strengths, such as protecting species from extinction and enabling research, against limitations like inbreeding, cost, and animals losing natural behaviours. Both methods are complementary, not replacements for protecting habitats in situ.

In this core practical you prepare a stained transverse section of a herbaceous stem and use a light microscope to locate three tissues. Sclerenchyma fibres form caps outside the phloem, with thick lignified walls and small lumens. Phloem sieve tubes sit in the outer vascular bundle, with sieve plates and companion cells. Xylem vessels lie towards the stem interior, wide and lignified, carrying water and mineral ions. Scan at low power to find a vascular bundle, then increase magnification to identify each tissue by wall thickness, lumen size and position. Draw a labelled plan and a high-power detail, recording magnification.

Core Practical 7 investigates how the absence of specific mineral ions affects plant growth and appearance. You grow plants in culture solutions that each lack one mineral, such as nitrate, calcium or magnesium, alongside a complete control. Prepare solutions with all essential ions except the one being tested, using distilled water and clean apparatus to avoid contamination. Measure growth over time by counting leaves, measuring shoot height or recording leaf colour. Deficiency symptoms appear: nitrate deficiency causes stunted growth and chlorosis; calcium deficiency damages growing tips; magnesium deficiency causes chlorosis between veins. Control plants remain healthy. Record observations and relate symptoms to ion functions.

Tensile strength is the maximum force a fibre withstands per unit cross-sectional area before breaking. You clamp a fibre between a fixed point and a newton meter or masses, increase the load steadily, and record the breaking force. Measure fibre diameter with a microscope and eyepiece graticule, calibrating against a stage micrometer, then calculate cross-sectional area using A = πr². Tensile strength = breaking force ÷ cross-sectional area, in N m⁻² or Pa. Fibres vary, so test several samples and calculate a mean. Control length, moisture and loading rate. Compare fibres such as flax, hemp or cotton; wider fibres usually need greater force but may have lower strength per unit area.

This practical tests whether plant extracts inhibit bacterial growth. You prepare a lawn of a safe bacterium on agar, then place discs soaked in plant extract, a solvent control and a positive control such as a disinfectant. Clear zones around discs show inhibition; measuring zone diameter gives a quantitative comparison of antimicrobial activity. Aseptic technique protects you and keeps the culture pure: disinfect the bench, flame the loop and bottle neck, lift the lid minimally, work near a Bunsen flame, and never open a dish containing a grown culture. Seal plates with tape, incubate at a temperature that discourages human pathogens, and autoclave all waste. Controls matter because the solvent alone may inhibit growth, and replicates reduce random error.