GCE Advanced Level (A-Levels) Biology Flashcards

24 sample cards from the Biology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Describe anaerobic respiration in animals and in yeast/plants.

   Animals: pyruvate is reduced by reduced NAD to lactate ($\ce{C3H6O3}$), regenerating NAD for glycolysis. Yeast/plants: pyruvate is converted to ethanol and $\ce{CO2}$, also regenerating NAD. Both yield only the 2 ATP from glycolysis.

2. Define gross primary production (GPP) and net primary production (NPP) and give the relationship.

   GPP is the total chemical energy fixed by producers; NPP is the energy left for growth after respiration. $$\text{NPP} = \text{GPP} - R$$ where $R$ is respiratory loss. NPP is available to the next trophic level.

3. Give the equation for efficiency of energy transfer between trophic levels.

   $$\%\,\text{efficiency} = \frac{\text{energy in higher trophic level}}{\text{energy in lower trophic level}} \times 100$$ Transfer is typically only ~10% because energy is lost as heat (respiration), in excretion and in non-consumed parts.

4. Outline the nitrogen cycle's four key processes.

   Nitrogen fixation: $\ce{N2}$ to ammonia by nitrogen-fixing bacteria. Ammonification: decomposers convert organic N to ammonium. Nitrification: nitrifying bacteria oxidise ammonium to nitrite then nitrate. Denitrification: denitrifying bacteria convert nitrate back to $\ce{N2}$ (anaerobic).

5. Define primary succession and explain the role of pioneer species.

   Primary succession is the colonisation of bare, lifeless ground. Pioneer species (e.g. lichens) tolerate harsh conditions and, on dying, form humus/soil, changing the environment so new species can colonise, leading through seral stages to a stable climax community.

6. Describe the structure and function of a reflex arc.

   Stimulus $\to$ receptor $\to$ sensory neurone $\to$ relay neurone (in CNS) $\to$ motor neurone $\to$ effector $\to$ response. It is rapid, automatic and protective because it does not require conscious brain processing.

7. Explain how a resting potential is established in a neurone.

   The sodium-potassium pump actively transports 3 $\ce{Na+}$ out for every 2 $\ce{K+}$ in; the membrane is more permeable to $\ce{K+}$ which diffuses out. This makes the inside negative relative to the outside, giving a resting potential of about $-70\,\text{mV}$.

8. Describe how an action potential is generated and propagated.

   A stimulus opens voltage-gated $\ce{Na+}$ channels causing depolarisation (to about $+40\,\text{mV}$); $\ce{Na+}$ channels close and $\ce{K+}$ channels open causing repolarisation, then hyperpolarisation. Local currents depolarise the next region; in myelinated axons it jumps node to node (saltatory conduction).

9. Explain the all-or-nothing principle and how stimulus strength is coded.

   An action potential only fires if the threshold is reached, and is always the same size regardless of stimulus strength (all-or-nothing). A stronger stimulus is coded by a higher frequency of action potentials and by more neurones firing.

10. Describe synaptic transmission at a cholinergic synapse.

    An action potential opens voltage-gated $\ce{Ca^2+}$ channels; calcium influx causes vesicles to fuse and release acetylcholine into the cleft. ACh binds receptors on the postsynaptic membrane, opening $\ce{Na+}$ channels to depolarise it. Acetylcholinesterase then breaks down ACh.

11. Define homeostasis and explain negative feedback.

    Homeostasis is the maintenance of a stable internal environment within narrow limits. Negative feedback: a deviation from the set point is detected by receptors and triggers responses by effectors that reverse the change, returning conditions to normal.

12. Describe the control of blood glucose by insulin and glucagon.

    High glucose: pancreatic $\beta$-cells release insulin, increasing glucose uptake and glycogenesis (glucose to glycogen), lowering blood glucose. Low glucose: $\alpha$-cells release glucagon, stimulating glycogenolysis and gluconeogenesis, raising blood glucose.

13. Explain the second messenger model of adrenaline/glucagon action.

    The hormone (first messenger) binds a membrane receptor, activating adenylate cyclase, which converts ATP to cyclic AMP (cAMP, the second messenger). cAMP activates protein kinases that catalyse glycogenolysis, releasing glucose.

14. Distinguish type 1 from type 2 diabetes.

    Type 1: $\beta$-cells are destroyed (often autoimmune), so little/no insulin is produced; treated with insulin injections. Type 2: cells become insensitive/lose responsiveness to insulin (receptor down-regulation), usually later onset; managed by diet, exercise and drugs.

15. Describe ultrafiltration in the kidney nephron.

    Blood enters the glomerulus under high hydrostatic pressure (afferent arteriole wider than efferent). Water, glucose, ions and urea are forced through the basement membrane (a molecular filter) into the Bowman's capsule, while blood cells and proteins remain in the blood.

16. Explain how the loop of Henle and ADH control water reabsorption.

    The loop of Henle sets up a high salt concentration in the medulla (counter-current multiplier), lowering water potential. When the body is dehydrated, ADH is released and makes the collecting duct walls more permeable, so more water is reabsorbed by osmosis, producing concentrated urine.

17. Explain how transcription factors and oestrogen control gene expression.

    Transcription factors bind to specific DNA sequences (promoters) to switch transcription on or off. Oestrogen is lipid-soluble and binds an intracellular receptor; the complex acts as a transcription factor, entering the nucleus to bind DNA and stimulate transcription of target genes.

18. Explain how epigenetics regulates gene expression without changing the base sequence.

    Through reversible chemical modifications to DNA and histones: increased DNA methylation and histone deacetylation make chromatin condense, switching genes off; decreased methylation/increased acetylation allow transcription, switching genes on. These marks can be influenced by the environment.

19. Explain how RNA interference (RNAi) regulates gene expression.

    Small interfering RNA (siRNA) or microRNA binds to a complementary mRNA molecule. This leads to the mRNA being broken down or blocked from translation, preventing the production of the corresponding protein (post-transcriptional gene silencing).

20. Describe how a tumour suppressor gene and an oncogene can cause cancer.

    Tumour suppressor genes (e.g. p53) normally slow division or trigger apoptosis; if mutated/hypermethylated and silenced, control is lost. Proto-oncogenes become oncogenes when a mutation makes them permanently active, driving uncontrolled cell division and tumour formation.

21. Outline the polymerase chain reaction (PCR).

    Cycles of: denaturation at ~$95\,^{\circ}\text{C}$ (separate strands), annealing at ~$55\,^{\circ}\text{C}$ (primers bind), and extension at ~$72\,^{\circ}\text{C}$ (Taq polymerase builds new strands). DNA amount doubles each cycle, giving exponential in vitro amplification.

22. Describe how recombinant DNA is made and inserted into a host using restriction enzymes and ligase.

    A restriction enzyme cuts both the desired gene and a plasmid (vector) at the same recognition site, leaving complementary sticky ends. The gene and plasmid are joined by DNA ligase to form recombinant DNA, which is taken up by a host cell (e.g. bacterium) to be expressed.

23. What are marker genes used for in genetic engineering?

    Marker genes (e.g. for antibiotic resistance or fluorescence) are inserted alongside the gene of interest so transformed host cells that took up the recombinant plasmid can be identified and selected from those that did not.

24. Describe how gel electrophoresis separates DNA fragments and its use in genetic fingerprinting.

    DNA fragments are loaded into wells in agarose gel; an electric field draws negatively charged DNA toward the anode. Smaller fragments move faster and travel further, separating by size. Comparing the resulting band patterns of variable repeats (VNTRs) gives a genetic fingerprint unique to an individual.

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