ESAT Biology Mock Module 1 Worked Solutions

Correct Answer: C. $2$ cells, $46$ chromosomes each

Fastest Approach (🚀):
Mitosis: $2$ cells, number unchanged.
$2$ cells of $46$.

Matches Option C.

Step-by-Step Breakdown:

1. What mitosis is for

Growth, repair and replacement of cells, and asexual reproduction. All of those require exact copies, so the chromosome number must be conserved.

2. What happens

During interphase the DNA is replicated, so each chromosome briefly consists of two identical sister chromatids. In mitosis these are pulled apart to opposite poles, and the cell divides once.

One division of a cell whose DNA has been copied once gives two cells, each with the original $46$ chromosomes.

3. Why not 92

The doubling during interphase is temporary. It exists so that each daughter can receive a complete set; counting the chromosomes after replication but before division is what produces the $92$ distractor.

4. Contrast with meiosis

Meiosis has two divisions after a single replication, giving four cells with half the number. Mitosis has one division after one replication, giving two cells with the full number. Counting replications against divisions is what tells the two apart.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. $2$ cells, $23$ chromosomes each — Meiosis Confused
  Correct cell count, but halving the chromosomes is meiosis.
• B. $4$ cells, $23$ chromosomes each — Meiosis Confused
  Both numbers taken from meiosis.
• D. $4$ cells, $46$ chromosomes each — Division Count
  Four cells requires two divisions, which mitosis does not have.
• E. $2$ cells, $92$ chromosomes each — Stage Misread
  Counting the replicated DNA before the cell divides.

Common Mistake (⚠️):
Halving the chromosome number. Halving is meiosis; mitosis conserves the number exactly, which is the entire point of it.

Takeaway (📌):
Mitosis: one replication, one division, two identical diploid cells. Meiosis: one replication, two divisions, four different haploid cells.

Correct Answer: E. Cell wall

Fastest Approach (🚀):
Mitochondria, ribosomes, nucleus and membrane are in both.
Only the cell wall is plant-only.

Matches Option E.

Step-by-Step Breakdown:

1. What both cell types share

Nucleus (holds the DNA), cell membrane (controls what enters and leaves), cytoplasm (site of many reactions), mitochondria (aerobic respiration) and ribosomes (protein synthesis) are present in both animal and plant cells.

2. What only plant cells have

Cell wall — made of cellulose, giving strength and shape.
Chloroplasts — containing chlorophyll, the site of photosynthesis.
Permanent vacuole — filled with cell sap, maintaining turgor.

Only one of those three appears among the options.

3. Why the trap works

Mitochondria are the most-picked wrong answer, because photosynthesis is associated with plants and respiration with animals. But plants respire too — continuously, day and night — so they need mitochondria just as much. A plant only photosynthesises in the light, while it respires all the time.

Matches Option E.

Why the Other Options Are Wrong (❌):

• A. Mitochondria — Common Structure
  Present in both — plants respire as well as photosynthesise.
• B. Cell membrane — Common Structure
  Present in both; plants have a membrane inside the wall.
• C. Nucleus — Common Structure
  Present in both, since both are eukaryotic.
• D. Ribosomes — Common Structure
  Present in both, and in prokaryotes too.

Common Mistake (⚠️):
Choosing mitochondria on the grounds that plants photosynthesise rather than respire. Plants do both, and respire around the clock.

Takeaway (📌):
Plant-only: cell wall, chloroplasts, permanent vacuole. Everything else in the standard list is in both.

Correct Answer: A. Restriction enzymes cut DNA at specific sequences; ligase joins DNA fragments together

Fastest Approach (🚀):
Restriction cuts, ligase joins.

Matches Option A.

Step-by-Step Breakdown:

1. The sequence of steps

1. A restriction enzyme cuts the required gene out of the donor DNA, recognising a specific short base sequence.
2. The same restriction enzyme cuts open a plasmid, so both cut ends match.
3. Ligase joins the gene into the plasmid, forming recombinant DNA.
4. The plasmid — the vector — is inserted into a host cell, usually a bacterium.
5. The host is cultured, and every daughter cell carries the gene and expresses the protein.

2. Why the same enzyme cuts both

Many restriction enzymes cut in a staggered way, leaving short single-stranded overhangs called sticky ends. Using the same enzyme on gene and plasmid makes the overhangs complementary, so they pair up before ligase seals the backbone. That is what makes the joining specific rather than random.

3. What it is used for

Human insulin is produced this way: the human insulin gene is inserted into bacteria, which are then cultured in fermenters. The insulin is identical to the human protein, which avoids the immune reactions that animal-sourced insulin could cause.

Matches Option A.

Why the Other Options Are Wrong (❌):

• B. Restriction enzymes join DNA fragments; ligase cuts DNA at specific sequences — Roles Swapped
  The two roles swapped.
• C. Both cut DNA, at different sequences — Role Confused
  Ligase joins rather than cuts.
• D. Restriction enzymes copy the gene; ligase inserts it into the host — Role Confused
  Copying is done by other enzymes; restriction enzymes cut.
• E. Both join DNA, one in bacteria and one in plants — Role Confused
  Restriction enzymes cut, and neither is restricted to one kingdom.

Common Mistake (⚠️):
Swapping the two enzymes. The names carry the meaning — ligase ligates, which means to tie or bind.

Takeaway (📌):
Restriction enzyme cuts at a specific sequence, leaving sticky ends; ligase joins. The vector carries the gene into the host.

Correct Answer: E. $20\%$

Fastest Approach (🚀):
$\text{A} = \text{T} = 30 \Rightarrow 60\%$ used.
$\text{C}+\text{G} = 40\%$, so $\text{G} = 20\%$.

Matches Option E.

Step-by-Step Breakdown:

1. Use the pairing rules

In double-stranded DNA, adenine always pairs with thymine and cytosine always pairs with guanine. So the amounts are equal in pairs:
$\%\text{A} = \%\text{T}$ and $\%\text{C} = \%\text{G}$

2. Account for all the bases

$\%\text{A}+\%\text{T}+\%\text{C}+\%\text{G} = 100$

With $\%\text{A} = 30$, we have $\%\text{T} = 30$, so those two account for $60\%$.

3. Split the remainder

$\%\text{C}+\%\text{G} = 100-60 = 40$, and since they are equal, each is $20\%$.

4. Check

$30+30+20+20 = 100$ ✓

5. Why it only works for double-stranded DNA

The equalities come from the two strands being complementary. In single-stranded DNA, or in RNA, there is no such constraint and the four bases can be present in any proportions — a distinction worth noticing when a question specifies the strandedness, as this one does.

Matches Option E.

Why the Other Options Are Wrong (❌):

• A. $30\%$ — Pairing Confused
  Assuming guanine matches adenine, but guanine pairs with cytosine.
• B. $35\%$ — Arithmetic Error
  Splitting the remainder unevenly.
• C. $40\%$ — Not Halved
  Giving $\text{C}+\text{G}$ together rather than guanine alone.
• D. $70\%$ — Pairing Ignored
  Computing $100-30$.

Common Mistake (⚠️):
Assuming the four bases are always $25\%$ each, or forgetting to halve the remaining $40\%$ between cytosine and guanine.

Takeaway (📌):
$\%\text{A} = \%\text{T}$, $\%\text{C} = \%\text{G}$, and all four sum to $100\%$. Two equations and a total settle any base-composition question.

Correct Answer: A. Because its active site has a shape complementary to only one substrate

Fastest Approach (🚀):
Specificity comes from active site shape.

Matches Option A.

Step-by-Step Breakdown:

1. What an enzyme does

An enzyme is a biological catalyst: it speeds up a reaction by lowering the activation energy, and is not used up, so a single enzyme molecule catalyses the reaction many thousands of times. Options A and D each contradict one of those facts.

2. Where specificity comes from

The enzyme's shape — set by the sequence of amino acids in the protein — creates an active site, a region of a particular shape and chemistry. Only a substrate complementary to it can fit and bind to form an enzyme-substrate complex.

This is the lock and key model. The refinement, the induced fit model, says the active site adjusts slightly as the substrate enters, gripping it more tightly, but the requirement for complementary shape is the same.

3. What follows from it

Because the shape does the work, anything that changes the shape destroys the function. High temperature or an extreme pH breaks the bonds holding the protein's structure, the active site no longer fits its substrate, and the enzyme is denatured — permanently, in most cases.

4. Why the other options fail

Enzymes work over a range of temperatures with an optimum, not at one temperature only. And many enzymes occur in several tissues, so location is not what makes them specific.

Matches Option A.

Why the Other Options Are Wrong (❌):

• B. Because it is used up in the reaction and cannot be reused — Catalysis Misstated
  Enzymes are not consumed; one molecule works repeatedly.
• C. Because it can only work at one temperature — Over-restricted
  Enzymes work over a range with an optimum, not at a single temperature.
• D. Because it raises the activation energy of that one reaction — Energy Reversed
  Catalysts lower activation energy.
• E. Because it is present in only one type of cell — Wrong Reason
  Many enzymes occur in several tissues, so location is not the reason.

Common Mistake (⚠️):
Saying an enzyme raises activation energy. It lowers it — that is the entire mechanism by which a catalyst speeds a reaction up.

Takeaway (📌):
Specificity comes from the complementary shape of the active site. Enzymes lower activation energy and are not consumed.

Correct Answer: D. $4$ genetically different cells with $23$ chromosomes each

Fastest Approach (🚀):
Four cells, halved number, all different.

Matches Option D.

Step-by-Step Breakdown:

1. The counts

One replication followed by two divisions gives $4$ cells, each with half the original chromosome number: $\dfrac{46}{2} = 23$.

2. Why halving is necessary

Gametes fuse at fertilisation. If each carried $46$, the zygote would have $92$, and the number would double every generation. Halving in meiosis and doubling at fertilisation keep the species number constant.

3. Why the cells differ

Two independent mechanisms:
Crossing over — homologous chromosomes exchange sections in the first division.
Independent assortment — each pair lines up independently, so maternal and paternal chromosomes are distributed at random.

With $23$ pairs, independent assortment alone gives $2^{23}$ — over eight million — different combinations before crossing over is even considered.

4. Why that matters

This is the origin of genetic variation within a species, and therefore the raw material natural selection acts on. Asexual reproduction, using mitosis, produces none of it.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. $2$ genetically identical cells with $46$ chromosomes each — Mitosis Confused
  Describes mitosis.
• B. $4$ genetically identical cells with $23$ chromosomes each — Variation Denied
  Right counts, but meiotic products are not identical.
• C. $4$ genetically different cells with $46$ chromosomes each — Number Not Halved
  Right count and difference, but the number must halve.
• E. $2$ genetically different cells with $23$ chromosomes each — Division Count
  Two cells is one division, which is mitosis.

Common Mistake (⚠️):
Describing the four cells as identical. Identical daughter cells come from mitosis; the whole biological purpose of meiosis is that they differ.

Takeaway (📌):
Meiosis: four genetically different haploid cells. Variation comes from crossing over and independent assortment.

Correct Answer: E. Nucleus

Fastest Approach (🚀):
Prokaryotic = no true nucleus.

Matches Option E.

Step-by-Step Breakdown:

1. What defines a prokaryote

No membrane-bound organelles at all — no nucleus, no mitochondria, no chloroplasts. The genetic material is a single circular loop of DNA lying free in the cytoplasm.

2. What bacteria do have

Cell wall — present, though not made of cellulose as a plant's is.
Plasmids — small extra rings of DNA, carrying genes such as antibiotic resistance, and the vector used in genetic engineering.
Ribosomes — present but smaller than a eukaryote's, which is precisely why some antibiotics attack bacterial ribosomes without harming ours.
Cytoplasm and a cell membrane — both present.

3. Why the distinction matters

Having no nucleus means the DNA is directly accessible, and plasmids can be taken up from the surroundings. That is why bacteria can share resistance genes between individuals, and why they are the workhorse of genetic engineering.

Matches Option E.

Why the Other Options Are Wrong (❌):

• A. Cell wall — Present in Bacteria
  Present, though not made of cellulose.
• B. Plasmid — Present in Bacteria
  Present — small DNA rings, and the standard vector in genetic engineering.
• C. Ribosomes — Present in Bacteria
  Present, though smaller than eukaryotic ribosomes.
• D. Cytoplasm — Present in Bacteria
  Present in every cell.

Common Mistake (⚠️):
Assuming bacteria have no cell wall because they are not plants. They do have one — it is simply built from different material.

Takeaway (📌):
Prokaryote: no nucleus and no membrane-bound organelles, but a cell wall, ribosomes, a DNA loop and often plasmids.

Correct Answer: C. Early embryonic cells are totipotent, while adult stem cells can form only a limited range of cell types

Fastest Approach (🚀):
Embryonic: unrestricted. Adult: limited range.

Matches Option C.

Step-by-Step Breakdown:

1. What a stem cell is

An undifferentiated cell that can keep dividing and can develop into other cell types. Both halves matter: a cell that cannot divide, or has already specialised, is not a stem cell.

2. The grades of potency

Totipotent — can form every cell type including the placenta, so a complete organism. Only the earliest embryonic cells qualify.
Pluripotent — can form any cell type of the body but not a whole organism.
Adult (multipotent) stem cells — restricted to a related family of cell types. Bone marrow stem cells produce blood cells and not, say, neurones.

3. Why plants are different

Plant meristem tissue keeps stem cells throughout life, which is why a whole plant can be grown from a cutting while an animal cannot regenerate a limb. Option D reverses this — both kingdoms have stem cells; plants simply retain more capable ones for longer.

4. Why the distinction is contested

Embryonic stem cells are the most useful medically and the most ethically contested, since obtaining them destroys an embryo. Adult stem cells raise fewer objections but are less versatile. That trade-off is the substance of the debate, and questions often ask for both sides.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. Adult stem cells are totipotent and can form any cell type including a whole organism — Potency Overstated
  Adult stem cells are restricted, not totipotent.
• B. Stem cells are fully differentiated cells that have lost the ability to divide — Definition Reversed
  Stem cells are undifferentiated and dividing — the opposite.
• D. Only plants contain stem cells — Over-restricted
  Animals have stem cells too, in bone marrow among other places.
• E. Stem cells can divide but never differentiate — Definition Denied
  Differentiating is exactly what stem cells do.

Common Mistake (⚠️):
Calling adult stem cells totipotent. Potency decreases as development proceeds, and adult stem cells are already committed to a lineage.

Takeaway (📌):
Stem cells divide and are unspecialised. Totipotent gives a whole organism, pluripotent any body cell, adult stem cells a limited range.

Correct Answer: D. $450$

Fastest Approach (🚀):
$150\times 3 = 450$.

Matches Option D.

Step-by-Step Breakdown:

1. The triplet code

Each amino acid in a polypeptide is specified by a sequence of three bases, called a triplet or codon.

2. Multiply

$150\times 3 = 450$ bases.

3. Sanity-check the direction

There must be more bases than amino acids, since each amino acid needs three. Any answer smaller than $150$ has the multiplication inverted — which rules out Option A immediately.

4. Why three

With four bases, pairs would give only $4^{2} = 16$ combinations, too few for the $20$ amino acids. Triplets give $4^{3} = 64$, comfortably enough. The surplus makes the code degenerate: several triplets code for the same amino acid, which is why many mutations have no effect at all.

5. Why the question says 'minimum'

A real gene also contains a stop codon, and in eukaryotes non-coding introns as well, so the actual gene is longer. The coding sequence alone is $450$ bases.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. $50$ — Inverted
  Dividing by three rather than multiplying.
• B. $150$ — Code Ignored
  Assuming one base per amino acid.
• C. $300$ — Wrong Code Length
  Using pairs of bases rather than triplets.
• E. $900$ — Over-count
  Using six bases per amino acid, perhaps double-counting both DNA strands.

Common Mistake (⚠️):
Dividing by three instead of multiplying, giving $50$. Bases are the smaller unit, so there must be more of them.

Takeaway (📌):
Three bases per amino acid. Multiply going from protein to DNA, divide going the other way, and check the direction by size.

Correct Answer: C. The rate increases to $40\ ^{\circ}\text{C}$, then falls sharply as the enzyme denatures

Fastest Approach (🚀):
Up to $40\ ^{\circ}\text{C}$: more collisions, rate rises.
Above: denaturation, rate falls sharply.

Matches Option C.

Step-by-Step Breakdown:

1. Below the optimum

Raising the temperature gives the enzyme and substrate molecules more kinetic energy. They collide more often and with more energy, so more enzyme-substrate complexes form per second and the rate rises.

2. At the optimum

The rate peaks. This is the temperature at which the enzyme works fastest — around $37\ ^{\circ}\text{C}$ for most human enzymes, which is why body temperature is regulated so tightly.

3. Above the optimum

The extra energy begins to break the bonds holding the protein in shape. The active site changes shape and no longer fits its substrate. The enzyme is denatured, and the rate falls away steeply.

4. Why the curve is not symmetric

The rise is gradual and reversible — cool the enzyme down and it works slowly again. The fall is steep and, for most enzymes, permanent, because denaturation does not reverse. That asymmetry is what makes Option B wrong despite describing the right general trend.

5. On terminology

A denatured enzyme is not 'killed'. It is a protein, not a living thing; its shape has changed and its function is lost.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. The rate increases steadily throughout the range — Denaturation Ignored
  Ignores denaturation above the optimum.
• B. The rate increases to $40\ ^{\circ}\text{C}$ then decreases steadily and symmetrically — Shape Wrong
  Right direction, but the fall is sharp rather than symmetric.
• D. The rate is constant until $40\ ^{\circ}\text{C}$, then rises sharply — Shape Reversed
  Reverses the shape of the curve entirely.
• E. The rate decreases throughout, because heat destroys enzymes — Direction Reversed
  Below the optimum, warming increases the rate.

Common Mistake (⚠️):
Drawing a symmetric curve. The rate rises gradually and falls sharply, because the two sides are governed by different processes.

Takeaway (📌):
Below the optimum, kinetic energy controls the rate; above it, denaturation does. The curve rises gently and falls steeply.

Correct Answer: D. They are genetically identical to the parent, so the whole crop shares its susceptibility to disease

Fastest Approach (🚀):
Cuttings $\Rightarrow$ asexual $\Rightarrow$ mitosis $\Rightarrow$ clones.
No variation, so no resistance anywhere if the parent had none.

Matches Option D.

Step-by-Step Breakdown:

1. Identify the process

A cutting involves one parent and no gametes, so it is asexual reproduction, carried out by mitosis. The offspring are clones.

2. The advantages the grower is buying

Every plant has the desirable characteristics of the parent exactly. It is fast, needs no pollinator, and the crop is uniform in size and ripening time — which matters commercially.

3. The cost

No genetic variation at all. If a new disease can infect the parent, it can infect every plant in the field, and there is no resistant individual anywhere for selection to act on. Uniformity is simultaneously the product and the risk.

4. Why Option D is self-contradictory

Genetically identical offspring cannot each carry a different set of alleles. Identical means identical.

5. The comparison

Sexual reproduction gives variation, so a population can adapt, but the offspring are not predictable and the process is slower and needs two parents. Growers choose between reliability and adaptability, and cuttings buy the first at the cost of the second.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. They show more genetic variation, so the crop adapts faster to disease — Variation Reversed
  Asexual reproduction produces no new variation.
• B. They are genetically identical, and each carries a different set of alleles — Internally Inconsistent
  Self-contradictory: identical offspring cannot differ in alleles.
• C. They have half the chromosome number of the parent — Wrong Division
  Halving happens in meiosis, which is not involved here.
• E. They are produced by meiosis, so variation is high — Wrong Division
  Cuttings use mitosis, not meiosis.

Common Mistake (⚠️):
Associating 'reproduction' with variation automatically. Variation comes from meiosis and fertilisation, neither of which is involved in taking a cutting.

Takeaway (📌):
Asexual: one parent, mitosis, clones, no variation. Sexual: two parents, meiosis and fertilisation, variation.

Correct Answer: C. $\times 5000$

Fastest Approach (🚀):
$50\ \text{mm} = 50\,000\ \mu\text{m}$.
$\dfrac{50\,000}{10} = 5000$.

Matches Option C.

Step-by-Step Breakdown:

1. The relationship

$\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$

2. Convert to a common unit

$1\ \text{mm} = 1000\ \mu\text{m}$, so $50\ \text{mm} = 50\,000\ \mu\text{m}$.

3. Divide

$\dfrac{50\,000\ \mu\text{m}}{10\ \mu\text{m}} = 5000$

The units cancel, so the answer is written $\times 5000$ with no unit attached — a useful check that the conversion was done.

4. Rearranging it

The same triangle gives the other two questions examiners ask:
actual size $= \dfrac{\text{image size}}{\text{magnification}}$
image size $= \text{actual size}\times\text{magnification}$

5. A sense of scale

Most animal cells are $10$–$100\ \mu\text{m}$ across, bacteria around $1$–$5\ \mu\text{m}$. A light microscope reaches about $\times 1500$; an electron microscope goes far beyond, which is how sub-cellular detail was first seen at all.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. $\times 5$ — Conversion Omitted
  Dividing without converting the units.
• B. $\times 500$ — Conversion Error
  Converting by $100$ rather than $1000$.
• D. $\times 50\,000$ — Incomplete
  Using the image size in micrometres without dividing by the actual size.
• E. $\times 0.0002$ — Inverted
  Dividing actual by image — the reciprocal.

Common Mistake (⚠️):
Dividing $50$ by $10$ without converting, giving $\times 5$. Magnification has no units, so a mismatch never announces itself in the answer.

Takeaway (📌):
Convert to one unit first: $1\ \text{mm} = 1000\ \mu\text{m}$. Magnification is a pure ratio and carries no unit.

Correct Answer: B. In selective breeding humans choose which individuals reproduce; in natural selection the environment determines it

Fastest Approach (🚀):
Same mechanism, different selector: human against environment.

Matches Option B.

Step-by-Step Breakdown:

1. What the two share

Both require pre-existing variation in the population.
Both work by some individuals reproducing more than others.
Both change allele frequencies over many generations.
Neither creates new alleles — that is mutation's job.

2. What differs

In selective breeding a human chooses the parents, for traits people value: milk yield, grain size, temperament. In natural selection the environment does the choosing, and the trait favoured is whatever improves survival and reproduction in that environment.

3. A consequence of the difference

Human-chosen traits need not aid survival, and often hinder it. Breeds with extreme features frequently have health problems and would not persist without human intervention. Selective breeding also narrows the gene pool quickly, since few individuals become parents — which is why inbreeding problems appear in pedigree populations.

4. Why Option A is the most tempting

Neither process creates alleles. New alleles come only from mutation; selection of either kind merely changes how common existing ones are. Getting that straight is what separates a clear answer from a vague one.

Matches Option B.

Why the Other Options Are Wrong (❌):

• A. Selective breeding creates new alleles, whereas natural selection only sorts existing ones — Source Confused
  Neither creates alleles; mutation does.
• C. Natural selection acts within one generation, selective breeding over many — Timescale Wrong
  Both act over many generations.
• D. Selective breeding does not change allele frequencies in the population — Effect Denied
  Changing allele frequencies is exactly what it does.
• E. Natural selection requires no genetic variation to occur — Requirement Denied
  Without variation there is nothing to select between.

Common Mistake (⚠️):
Saying selective breeding 'creates' new characteristics. It concentrates variants that already exist; the source of anything genuinely new is mutation.

Takeaway (📌):
Same mechanism, different selector. Neither creates alleles — both act on variation already present, over many generations.

Correct Answer: C. The genome is the full set of DNA, carried on chromosomes in the nucleus, each of which contains many genes

Fastest Approach (🚀):
Genome is everything; chromosomes carry it; genes are sections of chromosomes.

Matches Option C.

Step-by-Step Breakdown:

1. Order the terms by size

Genome — the entire genetic material of the organism, all of it.
Chromosome — one long DNA molecule, wound with proteins. Humans have $46$ in most cells.
Gene — a section of a chromosome coding for one characteristic. Each chromosome carries thousands.

So genome contains chromosomes, and chromosomes contain genes.

2. Where it sits

In eukaryotic cells the chromosomes are inside the nucleus, which is what makes the cell eukaryotic. A small amount of DNA also sits in mitochondria and, in plants, chloroplasts — but the great majority is nuclear.

3. Test the wrong options

A inverts the nesting. B treats the genome as a single gene. D separates genes from chromosomes, but a gene is part of a chromosome. E would make humans capable of only $46$ characteristics.

4. Why sequencing the genome mattered

Reading the full sequence let researchers locate the genes associated with inherited disorders, trace human migration through shared variations, and design drugs targeted at particular versions of a gene. All of that depends on the genome being the complete set rather than a sample.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. A gene contains many chromosomes, which together make up the genome — Order Inverted
  The nesting is inverted — chromosomes contain genes, not the reverse.
• B. The genome is one gene, and chromosomes are copies of it — Scale Confused
  The genome is the whole set, not a single gene.
• D. Genes are found in the cytoplasm and chromosomes in the nucleus — Location Error
  Genes are sections of chromosomes, so they are in the nucleus too.
• E. Each chromosome carries exactly one gene — Scale Confused
  Each chromosome carries thousands of genes.

Common Mistake (⚠️):
Using 'gene' and 'chromosome' interchangeably. A chromosome carries thousands of genes, and the difference in scale is the point.

Takeaway (📌):
Genome, then chromosome, then gene, in decreasing size. In eukaryotes they sit in the nucleus.

Correct Answer: A. pH $2$

Fastest Approach (🚀):
Stomach is acidic $\Rightarrow$ low pH optimum.

Matches Option A.

Step-by-Step Breakdown:

1. Reason from the location

The stomach secretes hydrochloric acid, giving a pH of about $2$. An enzyme working there must have an active site that holds its shape in strong acid, so its optimum is around pH $2$.

2. What pH does to an enzyme

Moving away from the optimum in either direction disrupts the bonds maintaining the protein's shape. The active site distorts, the substrate no longer fits, and the enzyme denatures. Unlike temperature, the fall is roughly symmetric about the optimum, since acid and alkali both disrupt the structure.

3. The contrast worth knowing

Trypsin is also a protease but works in the small intestine, where bile and pancreatic secretions make conditions slightly alkaline. Its optimum is around pH $8$. Two enzymes doing the same job on the same substrate have completely different optimum pH values, set by where they operate.

4. Why Option E is worth ruling out explicitly

pH affects enzymes as strongly as temperature does. The digestive system relies on it: food leaving the acidic stomach must be neutralised by bile before intestinal enzymes can work at all.

Matches Option A.

Why the Other Options Are Wrong (❌):

• B. pH $7$ — Location Ignored
  Neutral — correct for many enzymes, but not for a stomach enzyme.
• C. pH $9$ — Wrong Enzyme
  Alkaline — this is closer to trypsin's optimum in the small intestine.
• D. pH $14$ — Implausible
  Extremely alkaline; no digestive enzyme works there.
• E. pH does not affect enzymes — Effect Denied
  pH affects enzyme activity strongly.

Common Mistake (⚠️):
Assuming every enzyme has a neutral optimum. Most human enzymes do work near pH $7$, but the digestive enzymes are precisely the exceptions.

Takeaway (📌):
An enzyme's optimum pH matches where it works: pepsin about $2$ in the stomach, trypsin about $8$ in the small intestine.

Correct Answer: D. $\tfrac12$, determined by the father

Fastest Approach (🚀):
Mother gives $\text{X}$ always; father gives $\text{X}$ or $\text{Y}$.
$\tfrac12$ male, determined by the father.

Matches Option D.

Step-by-Step Breakdown:

1. Set up the cross

Mother $\text{XX}$ produces only $\text{X}$ gametes.
Father $\text{XY}$ produces $\text{X}$ and $\text{Y}$ gametes in equal numbers.

2. The four equally likely outcomes

$\text{X}\times\text{X} = \text{XX}$ (female)
$\text{X}\times\text{X} = \text{XX}$ (female)
$\text{X}\times\text{Y} = \text{XY}$ (male)
$\text{X}\times\text{Y} = \text{XY}$ (male)

Two of the four are male, so $P(\text{male}) = \tfrac24 = \tfrac12$.

3. Which parent decides

Every child receives an $\text{X}$ from the mother regardless. The sex is settled entirely by whether the sperm carried $\text{X}$ or $\text{Y}$ — so the father determines it, despite the historical assumption otherwise.

4. Each child is independent

A couple with three daughters still has probability $\tfrac12$ for the next child. The gametes carry no memory, and expecting the ratio to 'correct itself' is the gambler's fallacy in biological dress.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. $\tfrac12$, determined by the mother — Parent Confused
  Right probability, wrong parent — the mother contributes only X.
• B. $\tfrac14$, determined by the father — Ratio Error
  $\tfrac14$ would need both parents to be heterozygous for the same thing.
• C. $\tfrac34$, determined by the father — Ratio Error
  $\tfrac34$ is the dominant phenotype ratio from a different cross.
• E. $\tfrac12$, determined equally by both parents — Parent Confused
  Both contribute a chromosome, but only the father's varies.

Common Mistake (⚠️):
Reading the four Punnett boxes as four children rather than four equally likely outcomes for each child, and then expecting exactly half of any real family to be male.

Takeaway (📌):
$\text{XX}\times\text{XY}$ gives a $1:1$ sex ratio, decided by the father's gamete, and independently for each child.

Correct Answer: B. Cell, tissue, organ, organ system

Fastest Approach (🚀):
Cells build tissues, tissues build organs, organs build systems.

Matches Option B.

Step-by-Step Breakdown:

1. Define each level

Cell — the basic unit, for example a muscle cell.
Tissue — a group of similar cells working together, for example muscle tissue.
Organ — several tissues working together for one function, for example the stomach, which contains muscle tissue, glandular tissue and epithelial tissue.
Organ system — organs working together, for example the digestive system.

2. Test the order

Each level is composed of many of the one below, so the order is fixed: cell, tissue, organ, organ system.

3. The level above

Organ systems together make the organism. The specification stops at organ systems, but the pattern continues, and in ecology it carries on outwards: organism, population, community, ecosystem.

Matches Option B.

Why the Other Options Are Wrong (❌):

• A. Cell, organ, tissue, organ system — Order Error
  Organs placed before tissues, but organs contain tissues.
• C. Tissue, cell, organ, organ system — Order Error
  Tissues placed before cells, but tissues are made of cells.
• D. Organ system, organ, tissue, cell — Direction Reversed
  Largest to smallest — the question asked for the reverse.
• E. Cell, tissue, organ system, organ — Order Error
  Organ system placed before organ.

Common Mistake (⚠️):
Putting organs before tissues. An organ contains several tissue types, so it cannot be the smaller unit.

Takeaway (📌):
Cell, tissue, organ, organ system — each made of many of the previous. The same nesting logic runs outwards through ecology.

Correct Answer: D. A few bacteria already carried a resistance mutation; these survived, reproduced, and came to dominate the population

Fastest Approach (🚀):
Mutation first, at random. Antibiotic selects afterwards.

Matches Option D.

Step-by-Step Breakdown:

1. Get the order right

Random mutation produces genetic variation in the bacterial population before any antibiotic is present. A few individuals happen to carry an allele conferring resistance.

The antibiotic is then applied. Non-resistant bacteria die; the resistant few survive.

Those survivors reproduce, and since bacteria divide rapidly, the resistant allele becomes common within days.

2. Why the order is the whole answer

Options A and B reverse it. The antibiotic does not cause the mutation and the bacteria do not respond to need — the variation is already there, generated at random, and the antibiotic merely determines which variants leave descendants.

3. Why 'need' is never an explanation

Organisms do not acquire traits because they would be useful. This is the Lamarckian error, and it is the single most common misconception in evolution questions. Mutations happen regardless of whether they help; the environment decides afterwards which ones persist.

4. Why this matters practically

It explains why finishing a course of antibiotics matters — stopping early leaves the partially resistant survivors to multiply — and why over-prescription accelerates resistance across whole populations.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. The bacteria developed resistance because they needed it to survive the antibiotic — Need-Driven
  The Lamarckian error — traits are not acquired because they are needed.
• B. The antibiotic caused mutations that made the bacteria resistant — Cause Reversed
  The antibiotic selects among mutations; it does not cause them.
• C. All the bacteria became slightly resistant at the same time — Variation Denied
  Selection works because individuals differ, not because all change together.
• E. The bacteria learned to resist the antibiotic and passed the knowledge on — Lamarckian
  Bacteria do not learn, and learned traits are not inherited.

Common Mistake (⚠️):
Saying the bacteria 'developed' or 'became' resistant in response to the antibiotic. Mutation is random and prior; selection is what the antibiotic does.

Takeaway (📌):
Variation first by random mutation, selection second by the environment. Nothing acquires a trait because it needs one.

Correct Answer: C. The protein is unchanged, because several triplets code for the same amino acid

Fastest Approach (🚀):
$64$ codons, $20$ amino acids $\Rightarrow$ code is degenerate.
Most substitutions are silent.

Matches Option C.

Step-by-Step Breakdown:

1. Why substitutions are often silent

There are $4^{3} = 64$ possible triplets but only about $20$ amino acids, so most amino acids are specified by several different triplets. Changing the third base of a codon very often produces a triplet coding for the same amino acid, so the protein is identical and nothing changes.

2. When a substitution does matter

If the new triplet codes for a different amino acid, the protein's shape may change. That matters most if the altered amino acid is in the active site of an enzyme or at a binding site — in which case the protein may lose its function.

3. Why Option B describes a different mutation

Insertions and deletions shift the reading frame, so every triplet after the change is misread and every subsequent amino acid is altered. A substitution replaces one base with another, so the frame is preserved and only that one codon is affected. Confusing the two is the standard error.

4. Why Option E is wrong

Most mutations have no effect; of those that do, most are neutral or harmful. Beneficial mutations are rare — but they are the raw material of evolution, which is why they matter far beyond their frequency.

Matches Option C.

Why the Other Options Are Wrong (❌):

• A. The protein is always non-functional, because the sequence has changed — Over-general
  Overstates it — most substitutions leave the protein unchanged.
• B. Every amino acid after the change is altered — Wrong Mutation Type
  Describes a frameshift, caused by insertion or deletion.
• D. The gene can no longer be transcribed at all — Over-general
  Transcription is not usually prevented by a single base change.
• E. The mutation is always beneficial to the organism — Over-general
  Beneficial mutations are rare, not the norm.

Common Mistake (⚠️):
Treating every mutation as damaging. Most substitutions change nothing at all, and the reading-frame catastrophe belongs to insertions and deletions, not substitutions.

Takeaway (📌):
Substitution: one codon affected, often silent. Insertion or deletion: the reading frame shifts and everything downstream changes.

Correct Answer: D. Lipase breaks lipids into fatty acids and glycerol

Fastest Approach (🚀):
Lip-ase $\to$ lipids $\to$ fatty acids and glycerol.

Matches Option D.

Step-by-Step Breakdown:

1. The three pairs

Amylase — breaks down starch (amylose) into sugars, ultimately glucose. Made in the salivary glands and the pancreas.
Protease — breaks down proteins into amino acids. Pepsin in the stomach, trypsin in the small intestine.
Lipase — breaks down lipids into fatty acids and glycerol. Made in the pancreas, acting in the small intestine.

2. Why the names help

Each is named after its substrate, with the ending -ase marking it as an enzyme. Amyl- for amylose, prote- for protein, lip- for lipid. Every wrong option here pairs an enzyme with a substrate belonging to a different one.

3. Where bile fits

Bile is not an enzyme. It emulsifies fats into small droplets, increasing the surface area available to lipase, and it neutralises stomach acid so that intestinal enzymes meet a suitable pH. It speeds digestion without catalysing anything itself — a favourite examiner distinction.

4. Why break them down at all

Starch, protein and lipid molecules are too large to be absorbed across the wall of the small intestine. Digestion reduces them to small soluble molecules that can pass into the blood.

Matches Option D.

Why the Other Options Are Wrong (❌):

• A. Amylase breaks proteins into amino acids — Substrate Swapped
  Amylase acts on starch, not protein.
• B. Lipase breaks starch into sugars — Substrate Swapped
  Lipase acts on lipids, not starch.
• C. Protease breaks starch into glucose — Substrate Swapped
  Protease acts on protein, not starch.
• E. Amylase breaks lipids into fatty acids and glycerol — Substrate Swapped
  Amylase acts on starch, not lipids.

Common Mistake (⚠️):
Giving 'fatty acids' alone as the products of lipid digestion. Both fatty acids and glycerol are produced, and answers omitting glycerol are marked incomplete.

Takeaway (📌):
Amylase to sugars, protease to amino acids, lipase to fatty acids and glycerol. Bile emulsifies but is not an enzyme.

Correct Answer: B. A plant with genotype $\text{Tt}$ is tall and heterozygous

Fastest Approach (🚀):
$\text{Tt}$: two different alleles $\Rightarrow$ heterozygous.
Dominant present $\Rightarrow$ tall.

Matches Option B.

Step-by-Step Breakdown:

1. The definitions

Gene — a section of DNA coding for a characteristic.
Allele — a version of that gene, here $\text{T}$ or $\text{t}$.
Homozygous — two identical alleles, $\text{TT}$ or $\text{tt}$.
Heterozygous — two different alleles, $\text{Tt}$.
Genotype — the alleles present.
Phenotype — the characteristic actually shown.
Dominant — expressed whenever present, even in a single copy.
Recessive — expressed only when there is no dominant allele, so only in the homozygous recessive.

2. Apply them

$\text{TT}$: homozygous dominant, tall.
$\text{Tt}$: heterozygous, tall — because one $\text{T}$ is enough.
$\text{tt}$: homozygous recessive, short — the only genotype that is short.

3. Check each option

A is wrong: $\text{Tt}$ is tall.
B is correct on both counts.
C is wrong: $\text{TT}$ has two identical alleles, so it is homozygous.
D is wrong: $\text{Tt}$ is tall as well.
E is wrong twice: $\text{tt}$ is short, and carrying a recessive allele does not make it expressed.

4. The consequence

Two genotypes share one phenotype, which is why a tall plant's genotype cannot be read off its appearance — and why the test cross in the next question exists.

Matches Option B.

Why the Other Options Are Wrong (❌):

• A. A plant with genotype $\text{Tt}$ is short — Dominance Ignored
  $\text{Tt}$ contains a dominant allele, so it is tall.
• C. A plant with genotype $\text{TT}$ is heterozygous — Term Confused
  $\text{TT}$ has two identical alleles, so it is homozygous.
• D. Only $\text{TT}$ plants are tall — Genotype Overlooked
  $\text{Tt}$ is tall as well.
• E. A plant with genotype $\text{tt}$ is tall because it carries the recessive allele — Dominance Reversed
  $\text{tt}$ is short; a recessive allele is expressed only in the absence of the dominant.

Common Mistake (⚠️):
Treating the recessive allele as having no effect unless it is 'stronger'. Dominance is not about strength: one dominant allele produces enough functional protein for the phenotype.

Takeaway (📌):
Only the homozygous recessive shows the recessive phenotype. Two genotypes, $\text{TT}$ and $\text{Tt}$, give the same appearance.

Correct Answer: A. Water leaves the cell, because the solution has a lower water potential

Fastest Approach (🚀):
Concentrated solution $\Rightarrow$ low water potential.
Water moves down the gradient, so it leaves the cell.

Matches Option A.

Step-by-Step Breakdown:

1. Get the terminology the right way round

Water potential measures the tendency of water to move. Pure water has the highest water potential, set at zero, and adding solute lowers it. So a concentrated sugar solution has a low (very negative) water potential.

2. Apply the rule

Water moves by osmosis from higher to lower water potential, through a partially permeable membrane. The cell contents have a higher water potential than the concentrated solution outside, so water moves out of the cell.

3. What that does to a plant cell

The cell loses water, the vacuole shrinks, and the membrane pulls away from the cell wall. The cell is plasmolysed and the plant wilts. In a dilute solution the opposite happens: water enters, the cell becomes turgid, and the wall stops it bursting — which is why plant cells survive in pure water while animal cells lyse.

4. Why Option C is wrong twice over

Osmosis is the movement of water, not of solute. Sugar moving down its own gradient would be diffusion, and in any case the membrane is only partially permeable.

5. Why Option E is wrong

The cell wall is fully permeable — it is the cell membrane that is partially permeable and controls what crosses.

Matches Option A.

Why the Other Options Are Wrong (❌):

• B. Water enters the cell, because the solution has a higher water potential — Potential Reversed
  Right reasoning structure, but a concentrated solution has a lower water potential, not higher.
• C. Sugar enters the cell by osmosis, down its concentration gradient — Wrong Substance
  Osmosis moves water, not solute.
• D. Water leaves the cell, because the solution has a higher water potential — Internally Inconsistent
  Correct direction reached by contradictory reasoning.
• E. Nothing happens, because the cell wall is impermeable to water — Wrong Structure
  The cell wall is fully permeable; the membrane is the selective barrier.

Common Mistake (⚠️):
Saying water moves from low to high concentration without saying concentration of what. Water moves towards higher solute concentration, which is lower water potential — the two phrasings sound opposite and mean the same thing.

Takeaway (📌):
Adding solute lowers water potential. Water always moves down the water potential gradient, and osmosis moves water only.

Correct Answer: E. A scar from an injury

Fastest Approach (🚀):
Blood group and eye colour: genetic.
Height and mass: both. Scar: environment only.

Matches Option E.

Step-by-Step Breakdown:

1. Sort the options

Purely genetic: blood group and eye colour are determined by alleles, essentially unaffected by surroundings.

Both: height and body mass have a strong genetic component but are also shaped by nutrition, exercise and health. These are the largest category and the reason the question says entirely.

Purely environmental: a scar arises from an injury. No allele codes for it, and it appears only because of an external event.

2. Why it is not inherited

Acquired characteristics are not passed on. A scar affects body cells only, not the DNA in gametes, so a child is not born with a parent's scar.

3. Why that matters for evolution

Only genetic variation can be acted on by natural selection across generations, because only it is heritable. Environmental variation may affect whether an individual survives, but it leaves no trace in the next generation — which is precisely why the Lamarckian account of evolution fails.

Matches Option E.

Why the Other Options Are Wrong (❌):

• A. Human height — Both Causes
  Influenced by both genes and nutrition.
• B. Body mass — Both Causes
  Influenced by both genes and lifestyle.
• C. Blood group — Purely Genetic
  Determined entirely by alleles.
• D. Eye colour — Purely Genetic
  Determined by alleles.

Common Mistake (⚠️):
Choosing height because nutrition affects it. Nutrition does affect it, but genes set the range, so height is a both-causes characteristic rather than a purely environmental one.

Takeaway (📌):
Genetic variation is inherited and drives evolution. Environmental variation is acquired and is not passed on. Most characteristics involve both.

Correct Answer: A. $\tfrac14$

Fastest Approach (🚀):
$\text{Tt}\times\text{Tt} \Rightarrow 3$ tall $: 1$ short.
Short $= \tfrac14$.

Matches Option A.

Step-by-Step Breakdown:

1. Draw the grid

Each parent produces $\text{T}$ and $\text{t}$ gametes in equal numbers, giving four equally likely combinations:

$\text{TT}$, $\text{Tt}$, $\text{Tt}$, $\text{tt}$

2. Read the genotype ratio

$1\ \text{TT} : 2\ \text{Tt} : 1\ \text{tt}$

3. Convert to phenotypes

$\text{TT}$ and $\text{Tt}$ are both tall, so tall $:$ short $= 3:1$, and the proportion short is $\tfrac14$.

4. Read the question carefully

The two ratios are different and both are asked for in exams. $\tfrac14$ is short; $\tfrac34$ is tall; $\tfrac12$ is heterozygous. All three appear here as options, and picking the wrong one is far more common than mis-drawing the grid.

5. What 'expected' means

A cross producing four offspring will not reliably give exactly one short plant. The ratio is a probability for each offspring independently, and only approaches $3:1$ over large numbers — which is why Mendel counted thousands of plants.

Matches Option A.

Why the Other Options Are Wrong (❌):

• B. $\tfrac12$ — Wrong Category
  The proportion that are heterozygous, $\text{Tt}$.
• C. $\tfrac34$ — Phenotype Swapped
  The proportion that are tall.
• D. none — Outcome Denied
  $\text{tt}$ does occur, in one box of four.
• E. $\tfrac13$ — Ratio Error
  Three categories misread as three equally likely outcomes.

Common Mistake (⚠️):
Giving $\tfrac34$ — the proportion that are tall. Read which phenotype the question asked for before choosing.

Takeaway (📌):
$\text{Tt}\times\text{Tt}$: genotypes $1:2:1$, phenotypes $3:1$. Short is $\tfrac14$, heterozygous is $\tfrac12$.

Correct Answer: B. Active transport moves substances against a concentration gradient and requires energy from respiration

Fastest Approach (🚀):
Against the gradient, and it costs energy.

Matches Option B.

Step-by-Step Breakdown:

1. Diffusion

Movement of particles from a region of higher concentration to one of lower concentration — down the gradient. It happens spontaneously and needs no energy input, because the particles are already moving randomly.

2. Active transport

Movement against the gradient, from lower to higher concentration. That cannot happen spontaneously, so it needs energy released by respiration, and it requires carrier proteins in the membrane.

3. Why cells bother

Root hair cells take up mineral ions from soil where the ions are more dilute than inside the cell. Diffusion would move them the wrong way, so only active transport can do it. The same applies to glucose absorption in the small intestine once the concentration in the gut falls below that in the blood.

4. The consequence worth knowing

Because active transport depends on respiration, anything that stops respiration stops it. A cell deprived of oxygen, or poisoned with a respiratory inhibitor, loses the ability to take up ions against a gradient — while diffusion continues unaffected.

Matches Option B.

Why the Other Options Are Wrong (❌):

• A. Active transport moves substances down a concentration gradient and requires energy — Direction Wrong
  Down the gradient is diffusion, which needs no energy.
• C. Active transport moves only water, whereas diffusion moves only solutes — Substance Confused
  Osmosis moves water; active transport moves solutes.
• D. Active transport is faster than diffusion but neither requires energy — Energy Denied
  Active transport certainly requires energy.
• E. Active transport occurs only in plant cells — Over-restricted
  It occurs in animal cells too, notably in the gut and kidney.

Common Mistake (⚠️):
Saying active transport requires energy but moves down the gradient. Movement down a gradient is spontaneous and free; the energy is precisely what buys movement the other way.

Takeaway (📌):
Diffusion and osmosis are passive and go down the gradient. Active transport goes up it, needs carrier proteins, and needs energy from respiration.

Correct Answer: B. $\text{Tt}$

Fastest Approach (🚀):
Any short offspring $\Rightarrow$ tall parent carries $\text{t}$.
$\text{Tt}\times\text{tt} \Rightarrow 1:1$. ✓

Matches Option B.

Step-by-Step Breakdown:

1. Test both possibilities

If the tall parent were $\text{TT}$:
$\text{TT}\times\text{tt}$ gives $\text{Tt}$, $\text{Tt}$, $\text{Tt}$, $\text{Tt}$ — all tall, no short offspring at all.

If the tall parent were $\text{Tt}$:
$\text{Tt}\times\text{tt}$ gives $\text{Tt}$, $\text{Tt}$, $\text{tt}$, $\text{tt}$ — a $1:1$ ratio, half tall and half short.

2. Match to the observation

Half the offspring are short, which matches the second cross. The tall parent was $\text{Tt}$.

3. The reasoning that settles it in one line

A short plant is $\text{tt}$, so it received a $\text{t}$ from each parent. One of those came from the tall parent, which therefore carried a $\text{t}$ — so it cannot have been $\text{TT}$.

4. Why cross with the recessive

The homozygous recessive contributes only $\text{t}$ gametes, so it hides nothing. Whatever appears in the offspring came from the unknown parent, which makes the offspring a direct readout of its genotype. That is exactly what a test cross is for.

Matches Option B.

Why the Other Options Are Wrong (❌):

• A. $\text{TT}$ — Contradicts Data
  $\text{TT}\times\text{tt}$ would give no short offspring at all.
• C. $\text{tt}$ — Contradicts Stem
  The parent is stated to be tall, so it cannot be $\text{tt}$.
• D. Cannot be determined from this cross — Under-reading Data
  The short offspring settle it decisively.
• E. $\text{TT}$ or $\text{Tt}$, equally likely — Contradicts Data
  $\text{TT}$ is ruled out by the short offspring.

Common Mistake (⚠️):
Concluding the genotype cannot be determined. A single short offspring is decisive, because it requires a recessive allele from each parent.

Takeaway (📌):
Test cross against $\text{tt}$: all dominant offspring means the parent was homozygous; a $1:1$ split means it was heterozygous.

Correct Answer: E. It is half as large

Fastest Approach (🚀):
Small: $\dfrac{6}{1} = 6$. Large: $\dfrac{24}{8} = 3$.
Halved.

Matches Option E.

Step-by-Step Breakdown:

1. The small cube

surface area $= 6\times 1^{2} = 6\ \text{cm}^{2}$
volume $= 1^{3} = 1\ \text{cm}^{3}$
ratio $= 6:1$

2. The large cube

surface area $= 6\times 2^{2} = 24\ \text{cm}^{2}$
volume $= 2^{3} = 8\ \text{cm}^{3}$
ratio $= 24:8 = 3:1$

3. Compare

$3$ is half of $6$, so the larger cube's ratio is half that of the smaller. In general, for a cube of side $L$ the ratio is $\dfrac{6L^{2}}{L^{3}} = \dfrac{6}{L}$ — inversely proportional to the length.

4. Why this governs the whole of physiology

Diffusion supplies the surface, but the volume is what has to be supplied. As an organism grows, the demand outruns the supply. That is why single-celled organisms can rely on diffusion alone, and why anything larger needs a transport system, and gas exchange surfaces that are folded and flattened — alveoli, gills, villi, root hairs — to restore the ratio artificially.

Matches Option E.

Why the Other Options Are Wrong (❌):

• A. It is twice as large — Direction Reversed
  Backwards — the larger cube has the smaller ratio.
• B. It is a quarter as large — Scaling Error
  Applying the cube factor rather than the linear one.
• C. It is the same — Scaling Ignored
  Area and volume scale by different powers, so the ratio must change.
• D. It is four times as large — Volume Ignored
  The area ratio alone, ignoring the volume.

Common Mistake (⚠️):
Assuming the ratio rises because the larger cube has more surface. It has more surface, but disproportionately more volume, and the ratio is what matters for diffusion.

Takeaway (📌):
For a cube of side $L$ the ratio is $\dfrac{6}{L}$. Bigger always means a smaller ratio, which is why large organisms need transport systems.