Look closely at cells
A light microscope makes small structures easier to see. Magnification tells you how much bigger the image is. Resolution tells you how clearly two nearby details can be seen as separate.
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Science in pictures
Look closely, ask a question, then follow the science into a lesson. These illustrations connect cells, materials and physical changes with things you can recognise.
Read the captions, then try the questions before opening the answers.
Biology
Compare two cell types
The left model represents a photosynthetic plant cell. The right represents an animal cell. Colours and sizes are simplified.
- Both show a cell membrane, cytoplasm, a purple nucleus and orange mitochondria.
- The plant model also shows a thick cell wall, green chloroplasts and a large central vacuole. Not every plant cell has chloroplasts.
Does having chloroplasts mean a plant cell does not need mitochondria?
Check your reasoning
No. Photosynthetic plant cells also respire. Mitochondria are involved in aerobic respiration in both plant and animal cells.
Different jobs in blood
Mature human red blood cells are biconcave discs without nuclei. The white cell shown is one type with a lobed nucleus; platelets are small cell fragments.
- The centre of a red blood cell is thinner, not an open hole.
- Red cells carry oxygen using haemoglobin. White cells have roles in defence, and platelets help blood clot.
Would a picture of a few blood cells tell you a person’s blood-cell counts?
Check your reasoning
No. This model shows some shapes and roles. Counting blood components requires a measured sample and a suitable method.
Break a route of infection
Washing hands with soap and water helps remove microbes. Reducing transfer between people can interrupt a route by which an infection spreads.
Keep comparisons fair
Similar packaging can hide which treatment a person receives. This is called blinding. A trial also needs suitable comparison groups, measurements and checks for unwanted effects.
Would identical packaging alone make a clinical comparison fair?
Check your reasoning
No. The groups could differ in other ways. How people are put into groups, how many take part, and how their results are measured also affect the conclusion.
Keep internal conditions steady
Working muscles transfer energy and release heat. The body controls its temperature and water balance as activity changes.
Notice variation
Smooth and wrinkled pea seeds show variation in seed shape. To study how this feature is inherited, scientists count the offspring from known crosses.
A twisted ladder model
DNA has two strands. Paired bases join the strands inside the helix. This model shows the overall arrangement, not a complete chemical structure.
- The outer strands represent sugar–phosphate backbones.
- The rungs represent base pairs. In DNA, A pairs with T and C pairs with G. The colours are model choices.
Where is the information that distinguishes one DNA sequence from another?
Check your reasoning
In the order of the bases. The general double-helix shape can be similar even when base sequences differ.
Species depend on one another
As some insects visit flowers, they transfer pollen. Relationships between species help explain why a change to one population can affect others.
Look for evidence of photosynthesis
Gas bubbles can form around an aquatic plant. Counting bubbles has limits: their sizes can vary, and appearance alone does not identify the gas.
Start with a fair comparison
These potato pieces are not all the same length. For a fair comparison of solution concentration, choose pieces with matching starting dimensions.
Which other conditions should be kept the same when comparing mass changes?
Check your reasoning
Use the same tissue source, soaking time and temperature, with enough solution. Blot surface liquid consistently before weighing and calculate percentage change from each starting mass.
Evidence from the past
Fossils preserve signs of past life. Scientists compare fossils from different rock layers to investigate how living things have changed.
Return materials to a cycle
Decomposers break down dead material. Their activity helps return substances to the environment, where other organisms can use them.
Grow new cells
A growing seedling produces new cells by division, and cells also enlarge. Different cells become specialised for jobs such as absorbing water or transporting substances.
Chemistry
Test material properties
Some materials conduct electricity well. Others are useful insulators. Tests let us compare conductivity, strength and other properties.
Separate an insoluble solid
Filter paper retains insoluble solid as residue. Liquid and dissolved substances can pass through into the filtrate.
Would filtering salty water remove the dissolved salt?
Check your reasoning
No. Dissolved salt passes through ordinary filter paper with the water. A different separation method is needed to recover the salt or collect the water.
Separate a useful mixture
Crude oil is a mixture of hydrocarbons. Fractional distillation separates groups with similar boiling points. Cracking is a different process: it breaks larger molecules into smaller ones.
Water needs treatment
Water from a reservoir may need several treatment stages before supply. Clear appearance does not establish that water is safe to drink.
Follow a gas-producing reaction
An effervescent tablet releases gas as it reacts in water. To compare rates, measure a suitable quantity over time while controlling relevant conditions.
Physics
A change of state
Melting changes the arrangement and movement of particles. The substance remains water; its particles do not become larger as the ice melts.
Measure extension
A load can extend a spring. Extension is the loaded length minus the unloaded length, not the spring’s whole length.
Reduce energy transfer
Insulation across a loft floor reduces energy transfer from rooms below. Air trapped within suitable insulation reduces conduction and limits convection.
Does insulation create energy or stop every transfer?
Check your reasoning
Neither. It reduces the rate of unwanted energy transfer. A heated house still needs energy input and still transfers some energy to its surroundings.
Separate the colours in white light
A prism can spread white light into a spectrum because different wavelengths are refracted by different amounts. The colours were already present in the incoming light.
Reveal a field pattern
Iron filings align with the local magnetic field and reveal a pattern. They do not supply arrowheads showing field direction.
What could you use to find the field direction at a point?
Check your reasoning
A small plotting compass. Its north-seeking end points along the local field direction; outside a bar magnet, field lines are drawn from north to south.
A magnet near a coil
Moving a magnet relative to a coil can change the magnetic field through the coil and induce a potential difference. A stationary arrangement alone does not show induction.
Would the same stationary magnet keep generating a potential difference in a stationary coil?
Check your reasoning
Not if the magnetic field through the coil stays unchanged. Induction requires a change. A complete conducting circuit is also needed for an induced current.
Allow for background counts
Some rocks contain radioactive isotopes. To investigate a sample, measure the background count as well as counts near the sample.
Generate electricity from sunlight
Solar panels generate electricity from sunlight. Their output changes with the light level, so we need to consider how the supply changes over time.
A travelling disturbance
A ripple carries a disturbance across the surface. The pattern travelling outwards does not mean that each piece of water travels outwards with it.
Water vapour and visible mist
Water vapour is an invisible gas. The visible white mist consists of tiny liquid droplets formed as water condenses.