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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Practising A-level Biology past paper questions with Graphs

Multiple Choice Questions - really good practice for OCR A A-level Biology

Good practice if you are struggling with data analysis questions

Pack of challenging past paper questions with lots of Data Analysis and application - A-level Biology past paper questions - good for OCR and AQA specifications

Terrific exam question - combining autosomal linkage and Chi-Squared. Give it a go if you have ambition.....

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.

This is an outstanding synoptic question from a Pre-U Biology Paper. Covers Protein Synthesis, Enzymes, Heart Disease, Respiration ....

Pack of past paper questions on Osmosis and Water potential - I have tried to include one question of each type

How to improve your answers to questions about practicals in A-level Biology exam questions.

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Neurones and Action Potentials give an opportunity for examiners to get you to explain a graph (in terms of the flow of Sodium and Potassium ions through voltage gated channel) and to link that explanation to first year concepts of facilitated diffusion and active transport.

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A level Biology Questions by topic - Kidney Questions with markschemes, useful for OCR, AQA, Eduqas

Here are lots of good questions on the kidney. I expect the kidney to appear frequently in the new style papers as they are very synoptic, give opportunities to require calculation and get students to analyse and explain data.

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