Bac Sciences Expérimentales Algeria - independent subject practice common mistakes
Bac Sciences Expérimentales Algeria - independent subject practice · Understand the traps and how to avoid them.
Drawing a labelled diagram of a prokaryotic cell or a mitochondrion with labels written as single words ('nucleus', 'ribosome') and no functional annotation, then losing marks in questions that asked for structure–function relationships.
Candidates treat diagrams as art tasks rather than explanatory tools, and past-paper mark schemes often reward annotation of function alongside identification, which candidates do not anticipate.
What to do instead
Adopt a personal rule: every label on a diagram gets one short clause of function (e.g. 'cristae - infoldings that increase surface area for oxidative phosphorylation'). Before drawing, read the command word: 'describe' needs annotation, 'identify' does not.
Confusing magnification with resolution, and writing that an electron microscope 'magnifies better' rather than that its shorter electron wavelength gives higher resolving power, allowing organelles to be distinguished.
The two terms are used interchangeably in everyday speech, and candidates learn microscope comparisons as a list of advantages of EM without grasping the underlying physics.
What to do instead
Distinguish enlargement from resolution: resolution is the ability to separate nearby details. Wavelength, numerical aperture, aberrations, sample preparation and the actual instrument determine achievable resolution; an enlargement alone adds no new detail.
Writing that in mitosis 'chromosomes line up and then split in half' and describing the products as 'two identical cells' without stating that DNA replicates in interphase beforehand and that each daughter receives one chromatid from every chromosome.
Candidates compress the stages into a single vague sentence and omit the replication step, so the numerical and genetic outcomes cannot be credited.
What to do instead
Track DNA and chromosome number separately. In a starting diploid example, S phase duplicates DNA without changing the chromosome count; mitosis separates sister chromatids and normally preserves diploid ploidy. A starting haploid cell instead remains haploid after mitosis. State the starting model rather than assuming every mitotic product is diploid.
Describing meiosis as producing 'four different cells' without explaining that crossing over in prophase I and independent assortment in metaphase I are the two sources of genetic variation, or confusing meiosis I (homologues separate) with meiosis II (sister chromatids separate).
The variation sources are taught as separate topics from the meiotic stages, so candidates do not attach them to the correct phase, and the two divisions become interchangeable in their minds.
What to do instead
Annotate a drawn meiosis: label chiasmata in prophase I as crossing over between non-sister chromatids of homologous chromosomes, label random orientation of bivalents at metaphase I as independent assortment, and mark division I as separation of homologues and division II as separation of chromatids. Practise explaining each variation source in two sentences.
Calculating genetic ratios without showing the parental genotypes and gametes, or writing a monohybrid cross result as '3:1' without stating that this is the expected phenotypic ratio from a cross between two heterozygotes and only when alleles show complete dominance.
Candidates jump straight to the final ratio because they have memorised it, and lose method marks for gamete and offspring genotypes, while also over-generalising the ratio to cases of codominance.
What to do instead
Use a fixed layout every time: parental phenotypes → parental genotypes → gametes (circled) → Punnett square with genotypes → phenotype tally → ratio with the condition stated. Add a one-line check: is dominance complete? If codominance or incomplete dominance applies, the phenotypic ratio follows the genotypic ratio.
Describing water's thermal properties as 'water holds heat well' without naming the mechanism - omitting that hydrogen bonds must be broken before molecules can vaporise, and that the high specific heat capacity buffers temperature changes in cells.
Candidates memorise the list of water properties from textbooks but never link each property to the hydrogen-bond network, so they produce vague statements that cannot earn credit for explaining biological significance.
What to do instead
For each property (high specific heat capacity, high latent heat of vaporisation, cohesion, solvent action), write a three-link chain: hydrogen bonds → physical consequence → biological role (e.g. sweating removes heat because hydrogen bonds must break for molecules to escape). Practise until each chain is automatic.
Describing enzyme rate changes with temperature or pH as 'the enzyme is denatured' at every point beyond the optimum, without distinguishing increased kinetic energy and more enzyme–substrate collisions below the optimum from denaturation of the active site above it.
'Denaturation' becomes a catch-all explanation, and candidates do not separate the two competing effects that produce the characteristic bell-shaped curve.
What to do instead
Explain the actual assay conditions rather than labeling every rate decrease denaturation. Temperature can change reaction kinetics and stability; pH can change protonation and binding without necessarily unfolding the protein. Examine time, substrate depletion, reversibility and structural evidence before assigning a mechanism.
Stating that competitive inhibitors 'change the shape of the active site' and that non-competitive inhibitors are 'reversed by increasing substrate concentration', reversing or merging the two mechanisms.
Candidates learn the names and the word 'inhibit' but not the binding site or the effect of substrate concentration, so the two categories blur under exam pressure.
What to do instead
State the kinetic model. Simple reversible competition increases apparent Km without changing Vmax and can be reduced by increasing substrate. Pure noncompetitive inhibition lowers Vmax without changing Km. Binding location alone does not identify every inhibition type; allosteric binding can produce several kinetic patterns.
Explaining surface area to volume ratio as 'small organisms lose heat faster' without calculating the ratio or connecting it to diffusion distances and the need for a transport system in larger organisms.
Candidates repeat a remembered phrase and do not practise the arithmetic, so they cannot apply the concept to exchange surfaces or to the need for circulatory systems.
What to do instead
Practise calculating SA:V for cubes of side 1, 2 and 4 (6:1, 3:1, 1.5:1) and state the consequence in two steps: as size increases, SA:V falls, so diffusion across the body surface cannot supply the volume, hence specialised exchange surfaces and a transport system. Apply the same reasoning to leaves, alveoli and villi.
In a limiting-reactant problem, the candidate calculates moles of product from each reactant separately and then averages the two results, or selects the reactant with the smaller initial mass as the limiting reactant.
Averaging is a mathematical habit from other contexts; selecting by mass ignores molar mass and stoichiometry. The candidate may also skip the step of dividing moles by the stoichiometric coefficient.
What to do instead
For each reactant, convert mass to moles, then divide by its coefficient in the balanced equation to get the 'reaction extent'. The smallest extent identifies the limiting reactant. Calculate product moles from that reactant only. Always check that the other reactant is in excess.
When using Hess's law or calorimetry data, the candidate forgets to reverse the sign of ΔH when reversing a reaction, or fails to multiply ΔH by the stoichiometric factor when scaling a reaction.
The candidate treats ΔH as an ordinary number rather than a state function that depends on direction and amount. They may also confuse ΔH of formation with ΔH of reaction and use tabulated values without considering the coefficients.
What to do instead
Write each reaction with its ΔH value. When reversing, change the sign; when multiplying by a factor, multiply ΔH by the same factor. Then add the equations and their ΔH values. For formation enthalpies, use ΔH°rxn = Σ nΔH°f(products) − Σ mΔH°f(reactants), being careful with coefficients.
The candidate calculates an equilibrium constant using initial concentrations instead of equilibrium concentrations, or uses the reaction quotient Q and calls it K without evaluating at equilibrium.
The candidate confuses the initial state with the equilibrium state, especially in ICE table problems where they forget to subtract the change. They may also assume that K can be calculated directly from stoichiometry without experimental data.
What to do instead
Always set up an ICE table: write initial concentrations, define the change in terms of x, and express equilibrium concentrations. Substitute only the equilibrium expressions into the K expression. For Q, use current concentrations; compare Q to K to determine direction. Practice problems where the change is not negligible.