1. Which organelle is known as the powerhouse of the cell, producing most of its ATP?
Mitochondria
Mitochondria are where aerobic respiration happens, breaking down glucose to release energy stored as ATP, which is why they are called the powerhouse of the cell.
2. Which organelle contains the cell's genetic material (DNA)?
Nucleus
The nucleus holds the cell's DNA, organised into chromosomes, and controls the cell's activities by controlling which genes are switched on.
3. Which structure is found in plant cells but not in animal cells?
Cell wall
Plant cells have a rigid cell wall made of cellulose surrounding the cell membrane, which gives the cell a fixed shape and support. Animal cells have a cell membrane but no cell wall, so they can change shape.
4. Ribosomes are the site of which process?
Protein synthesis (translation)
Ribosomes read the sequence of codons on mRNA and assemble the matching amino acids into a protein chain. This process is called translation, and it happens on ribosomes whether they float free in the cytoplasm or are attached to the rough endoplasmic reticulum.
5. A cell's rough endoplasmic reticulum looks 'rough' under a microscope because it is studded with which organelle?
Ribosomes
Rough ER has ribosomes attached across its surface, which is what gives it a bumpy, rough appearance. Those ribosomes make proteins and feed them directly into the ER for processing, while smooth ER has no ribosomes and instead builds lipids.
6. Photosynthesis mainly takes place in which organelle?
Chloroplast
Chloroplasts contain chlorophyll, the pigment that captures light energy, which is why photosynthesis happens there rather than in any other organelle.
7. The overall word equation for photosynthesis is
carbon dioxide + water, using light energy, produce glucose + oxygen
Photosynthesis captures light energy to build glucose from carbon dioxide and water, releasing oxygen as a by-product. The second option is actually the equation for respiration, running in the opposite direction, so it is worth checking which way the arrow points.
8. Aerobic respiration is best summarised as
glucose + oxygen produce carbon dioxide + water, releasing energy
Aerobic respiration breaks glucose down using oxygen, releasing carbon dioxide and water as waste products and releasing usable energy as ATP for the cell. The third option describes anaerobic respiration in muscles, which happens only when oxygen is in short supply.
9. Photosynthesis and aerobic respiration are often described as opposite processes because
the products of one (glucose and oxygen) are the reactants of the other
Photosynthesis takes in carbon dioxide and water and produces glucose and oxygen. Respiration takes in glucose and oxygen and produces carbon dioxide and water, which is exactly the reverse, so the two equations mirror each other.
10. A plant is kept in complete darkness for several days. What would you expect for the net exchange of gases through its leaves, and why?
The plant would take in oxygen and release carbon dioxide, since only respiration occurs without light to drive photosynthesis
Respiration happens all the time in every living cell, with or without light, but photosynthesis needs light energy to run. In the dark, photosynthesis stops while respiration carries on, so the leaf's net gas exchange becomes oxygen in and carbon dioxide out, the reverse of what happens in daylight.
11. Diffusion is the movement of particles from an area of
high concentration to low concentration
Diffusion is a passive process: particles naturally spread out from where they are crowded together to where they are more spread out, moving down their concentration gradient with no energy input needed.
12. Osmosis is specifically the movement of
water across a partially permeable membrane, from high to low water concentration
Osmosis is diffusion of water in particular, through a partially permeable membrane, moving from a region where water is more concentrated (a dilute solution) to where it is less concentrated (a more concentrated solution). It needs no energy, since it is water moving down its own concentration gradient.
13. A red blood cell placed in a very salty (hypertonic) solution will
shrink, as water leaves the cell by osmosis
The salty solution outside has a lower water concentration than the inside of the cell, so water moves out of the cell by osmosis, down the water concentration gradient, causing the cell to shrink.
14. Active transport differs from diffusion because active transport
moves particles against their concentration gradient and requires energy (ATP)
Diffusion is passive and only moves particles from high to low concentration. Active transport uses energy from ATP to push particles the other way, from low concentration to high concentration, using carrier proteins in the membrane.
15. Root hair cells absorbing mineral ions from soil, even when the soil has a lower ion concentration than the root cell, is an example of
active transport, since it moves ions against their concentration gradient using energy
Because the ions are moving from a lower concentration (the soil) to a higher concentration (inside the root hair cell), this is movement against the concentration gradient, which only active transport can achieve, using ATP energy released by respiration in the root cell's many mitochondria.
16. Enzymes are best described as
biological catalysts that speed up reactions without being used up
Enzymes are catalysts made of protein: they speed up chemical reactions in the body but come out of the reaction unchanged, so the same enzyme molecule can be used again and again.
17. The 'lock and key' model describes how
an enzyme's active site fits a specific substrate shape
In the lock and key model, an enzyme's active site has a shape that only fits one particular substrate, in the same way a key only fits its matching lock. This is why each enzyme usually only speeds up one specific reaction.
18. As temperature rises above an enzyme's optimum, its rate of reaction
decreases, because the enzyme begins to denature
Heat beyond the optimum temperature breaks the bonds holding the enzyme's shape together, changing the shape of the active site. Once the active site no longer fits the substrate, the enzyme has denatured and the reaction rate falls away.
19. A student adds the same amount of enzyme to test tubes with increasing substrate concentration. The rate of reaction increases at first but eventually levels off. What explains the levelling off?
The enzyme's active sites are all occupied at once (saturation)
Once every enzyme molecule's active site is busy processing a substrate molecule at the same time, adding more substrate cannot speed the reaction up any further. At that point, the amount of enzyme, not the amount of substrate, is what limits the rate.
20. Extreme pH (for example, strongly acidic conditions) affects enzyme activity because
it changes the shape of the enzyme's active site, so the substrate can no longer bind well
An enzyme's active site shape depends on bonds that are sensitive to pH. A pH far from the enzyme's optimum disrupts those bonds and distorts the active site, so the substrate no longer fits, in much the same way extreme heat denatures an enzyme.
21. In DNA, adenine always pairs with
Thymine
DNA base pairing follows a fixed rule: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).
22. DNA replication is described as 'semi-conservative' because each new DNA molecule contains
one original (old) strand and one newly made strand
When DNA unzips, each old strand acts as a template for building a brand new complementary strand. The result is that every new DNA molecule keeps one old strand and gains one new one, which is why the process is called semi-conservative.
23. During transcription, which molecule is produced from a DNA template?
mRNA
Transcription is the process where RNA polymerase reads one strand of DNA and builds a complementary messenger RNA (mRNA) strand, which then carries the genetic message out of the nucleus.
24. In translation, a ribosome reads mRNA in groups of three bases called
codons
Each group of three mRNA bases is a codon, and each codon codes for one specific amino acid. Transfer RNA molecules carry matching anticodons that pair with the mRNA codons, delivering the correct amino acid to build the protein chain.
25. Transcription pairs each DNA base with its mRNA complement: A with U, T with A, C with G, and G with C. Applying that rule, what mRNA codon is transcribed from the DNA triplet C-G-T?
GCA
Working through the pairing rule base by base: DNA C pairs with mRNA G, DNA G pairs with mRNA C, and DNA T pairs with mRNA A. Put those three mRNA bases in order and the codon is G-C-A.
26. Mitosis produces
two genetically identical diploid cells
Mitosis copies a cell's chromosomes exactly once and then splits the cell in two, so both resulting cells are genetically identical to each other and to the parent cell, and keep the full (diploid) chromosome number. This is how the body grows and repairs tissue.
27. Meiosis produces
four genetically different haploid cells (gametes)
Meiosis involves two rounds of division, producing four cells, each with half the normal chromosome number (haploid). Crossing over and the random way chromosomes are shared out make all four cells genetically different from one another.
28. Which process is responsible for producing sperm and egg cells in humans?
Meiosis
Sperm and egg cells (gametes) need to be haploid, carrying half the usual chromosome number, so that when two of them fuse at fertilisation the full diploid number is restored. Only meiosis halves the chromosome number this way.
29. Crossing over, which increases genetic variation, occurs during
meiosis, when homologous chromosomes exchange sections of DNA
During the first division of meiosis, matching (homologous) chromosome pairs line up next to each other and swap sections of DNA between them. This shuffles the alleles onto new chromosome combinations, adding genetic variation on top of what independent assortment already provides.
30. Why must gametes be haploid (half the normal chromosome number)?
so that fertilisation restores the full diploid chromosome number in the offspring
If gametes kept the full diploid chromosome number, fertilisation would double the chromosome number every generation. Because each gamete only carries half, one set from each parent, fusing them at fertilisation gives the offspring back the correct full number.
31. In genetics, an allele that is expressed even when only one copy is present is called
dominant
A dominant allele only needs one copy to be expressed in the phenotype, masking a recessive allele on the other chromosome. A recessive allele, by contrast, is only expressed when both copies present are recessive.
32. A cross between two heterozygous parents (Aa x Aa) for a single gene gives what phenotype ratio in the offspring?
3 dominant : 1 recessive
A Punnett square for Aa x Aa gives genotypes AA, Aa, Aa and aa, in a 1:2:1 ratio. Since AA and Aa both show the dominant phenotype, three out of four offspring show the dominant trait and one shows the recessive trait, a 3:1 phenotype ratio.
33. In a dihybrid cross between two individuals heterozygous for two genes (AaBb x AaBb), the classic phenotype ratio in the offspring is
9:3:3:1
A dihybrid cross combines two independent 3:1 ratios, one for each gene. Multiplying the possibilities out across a full Punnett square gives 9 showing both dominant traits, 3 showing the first dominant trait only, 3 showing the second dominant trait only, and 1 showing both recessive traits, the 9:3:3:1 ratio.
34. Colour blindness is a sex-linked recessive condition carried on the X chromosome. Why are males much more likely than females to be colour blind?
males have only one X chromosome, so a single recessive allele is enough to show the condition
Males are XY, so they only have one X chromosome and therefore only one copy of the gene, meaning a single recessive allele is enough for the condition to show. Females are XX and need both copies to be recessive to be colour blind, since one dominant allele on either X is enough to mask it.
35. A colour-blind man (his genotype is X carrying the recessive allele, paired with Y) has children with a woman who is a carrier but not colour blind herself. What fraction of their sons is expected to be colour blind?
1/2
Sons always inherit their Y chromosome from their father and their single X chromosome from their mother. Since the carrier mother has one normal X and one recessive X, she passes the recessive X to half her children on average, so about half of her sons (who have only that one X to rely on) are expected to be colour blind.
36. Natural selection is best described as
individuals with advantageous traits are more likely to survive and reproduce
Natural selection works on differences that already exist between individuals: those with traits suited to their environment tend to survive and reproduce more, passing those traits on, while individuals do not change their own traits during their lifetime.
37. Variation within a population, which natural selection acts on, is originally caused by
mutations and sexual reproduction (shuffling of alleles)
New alleles arise through mutation, and sexual reproduction shuffles existing alleles into new combinations. Together these are the source of the variation that natural selection then acts on.
38. 'Survival of the fittest' in evolution means the fittest individuals are those that
leave the most surviving offspring
In biology, fitness is measured by reproductive success, not strength, size or lifespan. An individual that survives a long time but leaves no offspring has zero evolutionary fitness, while one that reproduces successfully passes its alleles on to the next generation.
39. Two populations of the same species become separated by a mountain range and, over many generations, evolve into two separate species that can no longer interbreed. This is an example of
allopatric speciation, caused by geographic isolation
Allopatric speciation happens when a geographic barrier, such as a mountain range, physically separates two populations. Cut off from interbreeding, the two populations accumulate different mutations and face different selection pressures until they become too different to interbreed even if reunited.
40. A population of moths has two colour forms, pale and dark. After industrial soot darkens the tree trunks they rest on, dark moths become far more common within a few generations, while pale moths become rare. What best explains this shift?
Birds preferentially ate the more visible pale moths, so dark moths survived and reproduced more, passing on the dark allele
Both colour forms already existed in the population before the soot appeared. Once dark trunks made pale moths stand out to predators, birds ate more pale moths than dark ones, so more dark moths survived to reproduce and pass on the dark allele, shifting the population's colour over generations. Individual moths never changed colour themselves; the proportion of each colour in the population changed.
41. An ecosystem is best described as
a community of living things interacting with each other and their physical environment
An ecosystem includes both the living organisms (the community) and the non-living physical environment they interact with, such as soil, water and climate, all considered together.
42. A species' 'niche' refers to
the role it plays and resources it uses within its habitat
A niche is broader than just where a species lives (that is its habitat). It describes the whole role a species plays, including what it eats, what eats it, and how it uses the resources available to it.
43. Primary succession begins on
bare rock or newly formed land with no soil, such as after a volcanic eruption
Primary succession starts from bare ground where no soil or living things exist yet, such as fresh volcanic rock. Secondary succession, by contrast, starts on ground that already has soil, such as a cleared field or a burnt forest, so it happens much faster.
44. During ecological succession, pioneer species such as lichen are eventually replaced by other species because
pioneer species change the environment (for example, by building up soil), making conditions suitable for new species to outcompete them
Pioneer species like lichen can survive harsh, bare conditions, and as they grow and die they help break down rock and build up a thin layer of soil. That improved soil then lets other plants establish, and those plants often outcompete the pioneers for light and space, so the community changes over time.
45. A climax community in ecological succession is best described as
a relatively stable, long-lasting community that stops changing significantly unless disturbed
Succession is a sequence of communities replacing one another over time, and it ends at the climax community: a stable, mature stage that stays much the same for as long as conditions remain undisturbed, such as an old-growth forest.
46. Homeostasis is best described as
the maintenance of a stable internal environment despite external changes
Homeostasis is the body's ongoing job of keeping internal conditions, such as temperature and blood glucose, steady, even when the outside environment or the body's activity is changing.
47. Homeostasis in the body typically works through
negative feedback loops that correct changes back towards a set point
Negative feedback detects a change away from the normal set point and triggers a response that pushes conditions back towards normal, which is how the body corrects both rises and falls in things like temperature and blood glucose.
48. When blood glucose rises after a meal, the pancreas releases
insulin, which causes cells to take up glucose and lowers blood glucose
Insulin signals cells, especially in the liver and muscles, to take up glucose from the blood and store it as glycogen, which brings blood glucose back down towards its normal level.
49. If blood glucose falls too low, the pancreas releases glucagon, which
causes the liver to break down stored glycogen into glucose, raising blood glucose
Glucagon has the opposite effect to insulin: it signals the liver to break down its stored glycogen back into glucose and release it into the blood, raising blood glucose back up towards normal. Insulin and glucagon working against each other is what keeps blood glucose stable.
50. On a hot day, the body cools itself partly through vasodilation, which is
widening of blood vessels near the skin, increasing blood flow so more heat can radiate away
Vasodilation widens the blood vessels close to the skin's surface, bringing more warm blood near the surface where heat can radiate away into the surrounding air. This works alongside increased sweating, since evaporating sweat also removes heat, to bring body temperature back down.