How to Answer Hardy-Weinberg Questions - A-level Biology - with question pack

How to understand and answer Hardy-Weinberg Questions - A-level Biology. Remember to look out for questions where they give you a dominant phenotype frequency -for instance Huntingdons disease, where the frequency of the sufferers is 1 in 100000 - therefore q squared (recessive allele phenotype) is 99999 in 100000.

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Haemoglobin Oxygen Dissociation Curve / Bohr Effect Exam Questions - How to understand the Sigmoid Curve

Lots of Haemoglobin Oxygen Dissociation Curve/ Bohr Shift Questions and Markschemes, suitable for OCR A, AQA - and a brief guide to Understanding them.

The key to understanding dissociation curves is firstly to understand the concept of partial pressure and what would make it change. And to understand cooperative binding.

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How to approach and answer A-level Biology Questions that need you to Analyse Figures, Tables and Images - lots of example past paper questions

How to approach and answer A-level Biology Questions that need you to Analyse Figures, Tables and Images - lots of example past paper questions with the markschemes

DO NOT LOOK at the question and then look at the data to answer the question.

  • Look closely at the graph or table

  • look very carefully at the axes - have they plotted mean or rate or time, mass/volume or concentration ?

  • can you see range bars ?

  • In a table what range is in the replicates when you compare to the mean ?

  • what trends can you observe ? then think about what principle of biology is being shown by the the trends.

  • How would you explain the highest value, the lowest value, the point at which the line crosses the x axis, how would you explain the largest range, how would you change the experiment to reduce the spread in the data ?

Once  you have a coherent understanding of the trends  - only then look at the question.

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Nitrogen Cycle - a quick guide to understanding the concept, and a pack of questions - for A-level Biology

How to understand the Nitrogen Cycle

Nitrogen Cycle : A few tips

Start with dead plants and animals or faeces.  

Remember that decomposers are just heterotrophes respiring dead stuff and producing ammonia from the deamination of amino acids (the keto acid that remains following deamination is respired).

Ammonia then gets oxidised to nitrite  and oxidised to nitrate by nitrifying bacteria  - Nitrosomonas and Nitrobacter , both  aerobic chemoautotrophes.

Nitrates are actively transported into the roots, by a carrier protein which uses ATP from the mitochondria, then via the xylem to the leaves. In the leaves the nitrate is reduced to ammonia  and then used to make (with carbon from photosynthesis)  amino acids and nucleic acids  and hence more plant !.

Growing a crop removes protein and nucleotides from a field (and sells it in Tesco !).  To maintain fertility you need to make an input of nitrogen - this can be organic (dead things and faeces), which feed the decomposers. Or inorganic - made by joining nitrogen and hydrogen in the Haber process to make ammonia (applied to fields as ammonium nitrate), lightening also make nitrogen oxides - but is harder to arrange.

Rhizobium, a symbiotic bacteria,  living in root nodules of legumes, benefits from  an anaerobic environment, the legume makes Leghaemoglobin, a protein with a high affinity for oxygen which therefore prevents oxygen from poisoning the nitrogen reducing enzyme of the bacteria.

Rhizobia use sucrose from the plant for respiration. Rhizobia make ammonia (and hence amino acids) from nitrogen gas, the plants use these amino acids for growth. When legumes die, the nitrogen in the  proteins of the dead plant is made available to other non-leguminous plants by decomposers.

Decomposers and nitrifying bacteria are aerobes so ploughing - which increases the availability of oxygen in the soil - will raise the concentration of nitrates and hence the ability of the plants to make more protein.,

Pseudomonas denitrificans is an anaerobe so thrives in waterlogged soil. Denitrification uses nitrate as an electron acceptor, the nitrate is reduced to Nitrogen gas, thus farmers drain fields to maintain fertility of the soil.

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