Combined Science,
one step at a time.

Excretion removes metabolic waste and substances in excess of requirements. Egestion removes undigested food, which was never absorbed into cells.

Watch for: Do not repeat the wording of Biology: Characteristics of living organisms. Explain the underlying rule or relationship, apply it to the context and check the command word.

Palisade mesophyll cells contain chloroplasts and are specialised for photosynthesis; root hair cells absorb and red blood cells transport oxygen.

Watch for: Do not repeat the wording of Biology: Cells. Explain the underlying rule or relationship, apply it to the context and check the command word.

A steeper concentration gradient increases net particle movement; larger area, higher temperature and shorter distance also increase diffusion rate.

Watch for: Do not repeat the wording of Biology: Movement into and out of cells. Explain the underlying rule or relationship, apply it to the context and check the command word.

A colour control containing the same food but no Benedict's reagent reveals the sample's background appearance, helping interpretation of the test result.

Watch for: Do not repeat the wording of Biology: Biological molecules. Explain the underlying rule or relationship, apply it to the context and check the command word.

Controlling concentrations prevents changes in collision frequency caused by different particle numbers from confounding the temperature effect.

Watch for: Do not repeat the wording of Biology: Enzymes. Explain the underlying rule or relationship, apply it to the context and check the command word.

Destarching removes pre-existing starch, so starch found after controlled light exposure can be attributed to photosynthesis during the investigation.

Watch for: Do not repeat the wording of Biology: Plant nutrition. Explain the underlying rule or relationship, apply it to the context and check the command word.

Breaking food into smaller pieces increases its surface area, allowing more enzyme-substrate collisions without chemically changing the molecules.

Watch for: Do not repeat the wording of Biology: Human nutrition. Explain the underlying rule or relationship, apply it to the context and check the command word.

Wind carries away water vapour from the leaf boundary layer, preserving a steep diffusion gradient from air spaces to the atmosphere.

Watch for: Do not repeat the wording of Biology: Transport in plants. Explain the underlying rule or relationship, apply it to the context and check the command word.

Platelets are small cell fragments. Reference morphology, consistent magnification and repeated fields improve identification and sampling reliability.

Watch for: Do not repeat the wording of Biology: Transport in animals. Explain the underlying rule or relationship, apply it to the context and check the command word.

Safe waste disposal and sewage treatment prevent faecal pathogens reaching water and food; detailed treatment stages are not required.

Watch for: Do not repeat the wording of Biology: Diseases and immunity. Explain the underlying rule or relationship, apply it to the context and check the command word.

Using the same person, standardised exercise and equal counting intervals controls important variables and permits a valid before-and-after comparison.

Watch for: Do not repeat the wording of Biology: Gas exchange in humans. Explain the underlying rule or relationship, apply it to the context and check the command word.

The control shows changes caused by the apparatus or environment, allowing movement caused by seed respiration to be distinguished.

Watch for: Do not repeat the wording of Biology: Respiration. Explain the underlying rule or relationship, apply it to the context and check the command word.

Symptoms can improve before all susceptible bacteria are eliminated; survivors can multiply, and selection may favour more resistant individuals.

Watch for: Do not repeat the wording of Biology: Drugs. Explain the underlying rule or relationship, apply it to the context and check the command word.

An egg released from an ovary travels through an oviduct, where fertilisation normally occurs before movement towards the uterus.

Watch for: Do not repeat the wording of Biology: Reproduction. Explain the underlying rule or relationship, apply it to the context and check the command word.

Removing a species changes feeding availability in both directions: predators receive less food while organisms it consumed may increase.

Watch for: Do not repeat the wording of Biology: Organisms and their environment. Explain the underlying rule or relationship, apply it to the context and check the command word.

Pollution is a recognised cause of habitat damage, but a sound investigation uses repeated, comparable samples and controls other factors before assigning cause.

Watch for: Do not repeat the wording of Biology: Human influences on ecosystems. Explain the underlying rule or relationship, apply it to the context and check the command word.

During freezing of a pure substance, energy is released while particles rearrange, so temperature can remain constant while liquid and solid coexist.

Watch for: Do not repeat the wording of Chemistry: States of matter. Explain the underlying rule or relationship, apply it to the context and check the command word.

The covalent bonds inside molecules are strong, but the forces between separate simple molecules are weak and require relatively little energy to overcome.

Watch for: Do not repeat the wording of Chemistry: Atoms, elements and compounds. Explain the underlying rule or relationship, apply it to the context and check the command word.

A molecular formula states the number and type of atoms in one molecule; H₂O therefore contains two H atoms and one O atom.

Watch for: Do not repeat the wording of Chemistry: Stoichiometry. Explain the underlying rule or relationship, apply it to the context and check the command word.

Hydrogen and chlorine require controlled collection and suitable tests; direct inhalation, flames near mixed gases and electrical contact are unsafe.

Watch for: Do not repeat the wording of Chemistry: Electrochemistry. Explain the underlying rule or relationship, apply it to the context and check the command word.

Bond breaking is endothermic and bond making is exothermic; the reaction is exothermic when bond formation releases the larger amount.

Watch for: Do not repeat the wording of Chemistry: Chemical energetics. Explain the underlying rule or relationship, apply it to the context and check the command word.

Keeping temperature, reactant amount, surface area and apparatus constant allows rate differences to be attributed to concentration.

Watch for: Do not repeat the wording of Chemistry: Chemical reactions. Explain the underlying rule or relationship, apply it to the context and check the command word.

Partial evaporation creates a concentrated solution; cooling forms crystals that are separated and dried without excessive heating.

Watch for: Do not repeat the wording of Chemistry: Acids, bases and salts. Explain the underlying rule or relationship, apply it to the context and check the command word.

Matched quantities and starting-colour controls allow colour changes to be attributed to displacement rather than dilution or reagent colour.

Watch for: Do not repeat the wording of Chemistry: The Periodic Table. Explain the underlying rule or relationship, apply it to the context and check the command word.

R and S contain both oxygen and water, whereas dry air lacks water and boiled water under oil lacks oxygen; dissolved salt can speed corrosion in S.

Watch for: Do not repeat the wording of Chemistry: Metals. Explain the underlying rule or relationship, apply it to the context and check the command word.

Sedimentation allows larger particles to settle and filtration removes remaining suspended solids; carbon and chlorine have different treatment roles.

Watch for: Do not repeat the wording of Chemistry: Chemistry of the environment. Explain the underlying rule or relationship, apply it to the context and check the command word.

The carbon–carbon double bond in an alkene undergoes addition with bromine, removing the bromine colour; an alkane does not under these test conditions.

Watch for: Do not repeat the wording of Chemistry: Organic chemistry. Explain the underlying rule or relationship, apply it to the context and check the command word.

Controlling acid conditions and apparatus isolates carbonate identity; an airtight gas syringe provides quantitative data and repeats reveal random variation.

Watch for: Do not repeat the wording of Chemistry: Experimental techniques and chemical analysis. Explain the underlying rule or relationship, apply it to the context and check the command word.

Timing many oscillations makes the total interval longer, reducing the percentage effect of reaction-time uncertainty; dividing by the number gives the mean period.

Watch for: Do not repeat the wording of Physics: Motion, forces and energy. Explain the underlying rule or relationship, apply it to the context and check the command word.

The escaping particles have above-average kinetic energy, so the particles left behind have a lower mean kinetic energy and therefore a lower temperature.

Watch for: Do not repeat the wording of Physics: Thermal physics. Explain the underlying rule or relationship, apply it to the context and check the command word.

In a transverse wave the disturbance is at right angles to the direction of energy propagation; electromagnetic waves and seismic S-waves fit this model.

Watch for: Do not repeat the wording of Physics: Waves. Explain the underlying rule or relationship, apply it to the context and check the command word.

Cells and batteries transfer chemical energy electrically, generators convert kinetic input to electrical output, and motors convert electrical input to kinetic output.

Watch for: Do not repeat the wording of Physics: Electricity. Explain the underlying rule or relationship, apply it to the context and check the command word.

Core collapse leaves a neutron star or black hole, while expelled material forms a nebula enriched with elements that can contribute to later stars and planets.

Watch for: Do not repeat the wording of Physics: Space physics. Explain the underlying rule or relationship, apply it to the context and check the command word.