GAC023 Science III: General Science

A pond contains organisms, water, minerals and energy transfers. Understanding it needs several kinds of science, and no single observation answers every question. A plant can be present without proving drinking-water safety. A lower chemical concentration can be useful without proving that a substance has been destroyed.

This guide supports GAC023: biology, classification, ecosystems, Earth resources, chemistry, environmental chemistry and waves. A collaborative investigation 合作探究 assigns clear roles and uses a safe, approved procedure. Its report connects recorded observations with a question, method and bounded conclusion. Follow your actual assignment brief; the companion exercise sheets are original practice, not official assessment papers. Invented case data are labelled separately from cited background facts.

Biology 生物学 studies living systems, including structure, function and change. A cell 细胞 is the basic structural and functional unit of organisms. Prokaryotic 原核 cells have DNA without a membrane-bound nucleus. Eukaryotic 真核 cells typically contain a nucleus and other membrane-bound structures, with specialised exceptions. Do not infer that a structure is absent simply because one poor image does not show it.

Homeostasis 稳态 is regulation of internal conditions within suitable limits as external conditions vary. It does not mean every internal value stays exactly constant. Biological observations should distinguish what is seen, what is inferred and what would need another method.

Historical context and a scale check

Hooke's Micrographia, published in 1665, used “cell” for the tiny spaces observed in cork. This is a selected historical development, not the whole history of cell theory. Sources checked 2 October 2026: Royal Society account and digitised Micrographia.

In the supplied case, a printed cell is 30.0 millimetres long and its stated actual length is 0.0500 millimetres. Use the same units in the ratio:

The magnification is ×600, not a length. Shape and scale alone do not establish growth rate. A permitted growth study needs a specified measure, comparable organisms, conditions and observation period.

Classification 分类 organises organisms using evidence about relationships, including inherited traits and molecular information. A traditional hierarchy is kingdom, phylum, class, order, family, genus and species 物种. Broader groups can contain several groups at the next level. A visible feature helps identification but does not alone establish ancestry.

Binomial nomenclature 双名法 gives a species a genus name and a specific epithet. Capitalise the genus, use lower case for the epithet and italicise both words in print. When handwriting, underline both as required by the task. The epithet alone is not the full species name.

Worked example: one genus, different species

The supplied table places lion, tiger and domestic cat in Felidae. Lion and tiger share Panthera, but their species names are Panthera leo and Panthera tigris. The domestic cat is Felis catus, a different genus within that family. Names checked 2 October 2026 against ITIS records.

A classroom leaf key can separate needle-like leaves from broad leaves, then smooth margins from toothed margins. This identifies prepared cards from stated features. The key's branch order is not evidence that plants evolved in that order.

An ecosystem 生态系统 includes organisms and nonliving conditions. Producers 生产者 make organic material using an energy source. Consumers 消费者 obtain it from other organisms. Decomposers 分解者 break down dead material and waste, helping return nutrients to available forms.

Food-chain arrows point from food to consumer. Energy enters and transfers through the system, with dissipation through respiration and other pathways. It does not disappear from existence or cycle indefinitely back to the producer. Matter can cycle through living and nonliving parts, although a local system can also gain or lose matter across its boundary.

Biodiversity 生物多样性 concerns the variety of life. Species richness and distributions of abundance are possible measures, depending on the question. Counting only one species does not describe all this variety. Diversity may affect ecosystem responses, but it does not guarantee resistance to every disturbance.

Worked example: transfer percentages are case-specific

For the separate meadow record, grass has 5,000 joules in its defined level, grasshoppers receive 600 and frogs receive 60. The efficiency relation is:

Neither result makes ten percent a universal law. Not all material is eaten or assimilated, and energy is used and dissipated. This period's budget contains no population trend, so it cannot predict an exact frog decline the next week.

The rock cycle 岩石循环 connects formation and change. Cooling and solidification form igneous rock; sediment deposition, compaction and cementation can form sedimentary rock; heat and pressure can alter rock into metamorphic rock without melting it. Weathering and erosion produce sediment, while melting and later cooling form another route. These processes are not one compulsory circular sequence for every rock.

Renewable 可再生 resources can be replenished on a relevant timescale. Non-renewable 不可再生 resources are not replenished fast enough on human-use timescales. Renewable does not mean unlimited: demand can exceed replacement, and ecosystem needs also matter.

The water cycle 水循环 includes evaporation, condensation, precipitation and movement or storage through runoff, infiltration and groundwater. A local pond is open to inputs and losses. A report should combine physical layout, dated chemical measurements and non-destructive biological records, citing their sources and methods.

Worked example: renewal versus extraction

In the simplified supplied budget, inputs are 30 cubic metres, outlet loss is 18 and evaporation is 7. Other changes are excluded by assumption:

With a proposed additional extraction of 12 cubic metres:

That period's estimate does not establish long-term sustainability, initial storage or permission to extract. Reeds do not prove drinking-water safety. A rock photograph does not establish every rock's type or exact age.

An atom 原子 has a nucleus and electrons. Its atomic number 原子序数 is its proton count, while its mass number 质量数 counts protons and neutrons. Most nuclei contain both, but hydrogen-1 has one proton and no neutron. Isotopes 同位素 of an element have the same proton count and different neutron counts.

The periodic table 元素周期表 orders elements by atomic number. In an introductory main-group model, outer electron arrangements help explain similarities within groups. The elements are not identical, and the simplified rule should not be extended without care to every element.

Ionic bonds 离子键 are attractions between oppositely charged ions. Electron transfer can explain their formation; it is not the attraction itself. Covalent bonds 共价键 involve shared electron pairs. These are explanatory models, not instructions to handle reactive materials.

Worked example: keep proton number unchanged

Sodium-23 has 11 protons and mass number 23:

Its neutral atom has 11 electrons. Losing one gives Na⁺ with ten electrons and the same eleven protons. Sodium-24 has thirteen neutrons but remains sodium. Neutral chlorine has seventeen electrons; gaining one gives Cl⁻ with eighteen. Their opposite charges explain attraction.

Combustion 燃烧 of carbon-containing fuels can release carbon dioxide. The greenhouse effect 温室效应 involves absorption and emission of thermal infrared by gases including carbon dioxide. Acid rain 酸雨 involves atmospheric reactions of sulfur dioxide and nitrogen oxides forming acidic compounds. These are different mechanisms, with different evidence and responses.

Pollution 污染 depends on a substance or agent, its amount, pathway and effects. Chemistry can help monitor and treat it, but moving material into another output is not necessarily destroying it. Assess benefits alongside energy use, residual material and appropriate waste handling. Sources checked 2 October 2026: EPA acid-rain explanation and NASA radiation budget.

Worked example: a reduction can still miss a target

An invented 10-litre treatment record falls from 12.0 to 3.0 milligrams per litre. At unchanged volume:

The initial mass was 120 milligrams, so ninety are retained on the filter under the stated assumptions. The concentration reduction is seventy-five percent, but the final 3.0 still exceeds the fictional task target of 2.0 milligrams per litre. This is not a real safety standard. Students analyse the supplied data and do not discharge chemicals.

A wave 波 transfers energy through a travelling disturbance without carrying each medium particle along with it. Frequency 频率 counts complete oscillations per second. Wavelength 波长 is the spacing of successive points in the same phase. Speed 波速 relates them through the wave equation. Amplitude is maximum displacement from equilibrium, not crest-to-trough height.

Transverse waves 横波 oscillate perpendicular to propagation; longitudinal waves 纵波 oscillate parallel to it. The direction drawn on a page alone does not decide the type. A string point may oscillate vertically while the disturbance moves horizontally.

The electromagnetic spectrum 电磁波谱 runs through radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Electromagnetic waves do not require a material medium. They share the same vacuum speed while differing in frequency and wavelength; mechanical waves do not all have that speed.

Worked example: choose the equation for the question

The supplied string wave has frequency 5.0 hertz and wavelength 0.40 metres:

Its period is the time per complete oscillation:

An amplitude of 0.030 metres gives a crest-to-trough height of 0.060 metres. Neither is the 0.40-metre wavelength. A marked point oscillates about equilibrium; no supplied observation establishes that changing amplitude changes frequency.