Cell Structure

b) calculate the linear magnifications of drawings, photomicrographs and electron micrographs

Genetics Doubts

explain y Io(gene for blood group o) is recessive to both iA and iB(ie alleles for blood group A and B respectively)..
2.Describe how a single base deletion in the allele Io leads to the synthesis of an inactive protein?


 O is always the recessive. It would barely shows unless it is recessive. As for a single base deletion, point mutation I'll call it. Base deletion will produce frame shift in a code. It might delete the gene coding for the shape of the active site or so, altering the tertiary structure of the protein. The substrate cannot bind to the active site and thus leading to an inactive protein. You look under cystic fibrosis. It's mostly cause by base deletion.

A level Biology Nervous System Doubts

 refractory period

Following the passage of one Action Potential, there is a time delay before the next one can pass. This time delay is known as refractory period and it lasts for a few milliseconds. During this time, the sodium channels in the membrane are closed, preventing the inward movement of Sodium ions. This is known as the absolute refractory period. Another impulse cannot be conducted no matter how large the stimulus is.

or


When an action potential has just occurred at a section of the axon, there is a short period of time when a second action potential can be generated at the same point. This is called the refractory period.

A level Biology-Photosynthesis Doubts

Can somebody tell me how do I write the answer to this question:

Explain the role of NADP in photosynthesis. [6]

Mark Scheme points are as follows:

15 coenzyme ;
16 reduced ;
17 carries protons ;
18 and (high energy) electrons ;
19 from photosystem7light stage ; R photosystem II
20 on thylakoid membrane grans ;
21 to stroma / Calvin cycl~
22 ref. regeneration of NADP ;

 You just need to elaborate these points.
The co-enzyme NADP is used to add hydrogen in CO2 to produces carbohydrate molecule such as glucose. in the light dependent stage,after the photophosphorylation, and at the end of the electron carrier chain, the electron is picked up by PS1 to replace the electron the chlorophyll in PSI had lost in the initial stages. Now these electrons are again passed from PSI to a different chain of carriers to NADP. The NADP also picks up the hydrogen ions from the split water molecules. The NADP becomes reduced now which will be used further in the light independent stage to convert the glycerate 3 phosphate into triose phoshphate which is the 1st carbohydrate to be produced in the photosynthesis.

or


NADP is a co-enzyme that serves as an electron acceptor in the electron transport system during photophosphorylation. The electrons released from the 'excited' chlorophyll in PS1 combine with an electron acceptor and pass down a chain that ends with NADP. During photolysis of water, the protons from the splitting of the water molecule help to form reduced NADP along with the electrons from PS1 that combined with the NADP. The reduced NADP then passes along the electrons on to the calvin cycle. During this cycle, glycerate phosphate is reduced to another 3C sugar called triose phosphate, which is phosphoglyceraldehyde by using the reduced NADP from the light dependent stage as the electron donor and ATP is required. NADP then returns to the light dependent stage to pick up another hydrogen ion. 

A level Enzyme- Doubts

When enzymes are immobilized, what will happen?? i mean..they are all covered by those inert materials..or what else?? any if they are all covered..how do they help in a reaction?



Immobilized enzymes can be recovered easily from a reaction and be used over and over again. The reactant products are not contaminated by the enzyme because the enzyme are held in matrix.

Chemistry Paper 5 Tips

Chemistry Paper 5 Tips

For planning section: 

When asked to draw a diagram, ALWAYS mention the volume of the apparatus being used. The one most often used is gas syringe. I think it is better to limit the volume of syringe to less than 500 cm3 . 
Also, the volume of a commonly used small test tube is around 16 cm3 and that of a boiling tube is about 25 cm3. We always have to keep the volume of the apparatus we are using while choosing the volume of the solution for the planning. 
Saying that we are going to place 50cm3 of aqueous HCl in a test tube is of course not going to please the examiners. 
I think I ended up doing the same in one of the papers where we had to prepare different concentrations of a given solution. I chose volume of water to be added to a beaker to be 250 cm3. However, the ER stated that the volume should be less than 200cm3, that so being since the volume of a commonly used glass beaker is around 250cm3. 
Someone carrying out my exam would have a fully filled beaker. However, we can use greater volume if we state that the volume of the beaker is 400cm3 or 500 cm3 or so on. So, stating the volume of the apparatus is very very important 


Always while measuring volume of solutions, use burette or pipette because they have low PERCENTAGE error. The question of percentage errors are usually asked in P5. The percentage error becomes very unacceptable if we are measuring SMALL volumes of solution or small masses. So, a 3dp balance is much better for measuring masses than a 2dp balance as it would have much less % error when small masses are being measured

While heating crystals strongly, do not use a simple test tube or any apparatus with sharp edges as they are liable to crack at the sharp edges. For very strong heating to a constant mass, a crucible placed on a pipe clay triangle is appropriate. And of course to measure the mass of the crucible + solid, you would have to place it on a balance. But we wouldn’t want to fry our poor balance by placing a very hot crucible on top of it. So, allow the crucible to cool for a few minutes by placing it on a heat mat. When using a magnesium ribbon or any dirty surface, clean it with SANDPAPER

How to prepare crystals (which is also linked with solubility)

Many methods are mentioned in MS but I found this to be the simplest: CIE usually demands us to prepare crystal after preparing a saturated solution of the crystal.
So, we have to prepare a saturated solution
 first:
 1. Take a fixed volume of water in a beaker of appropriate volume
2. Add the crystal to the water and stir continuously. You have to allow some time for the crystal to dissolve as it is an equilibrium process
3. After 5 min of stirring , if no solid crystals appear, add further mass of crystal
4. And repeat the process until solid appears in the beaker
5. Filter the solution using a filter paper and funnel so that the saturated solution is collected in a beaker(whose mass has been measured previously) beneath the funnel

Now we have a saturated solution in a beaker.
How to get the crystals: We have to place the beaker in a warm water bath. We could use a burner as well but there is a risk of overheating the solution which could decompose the crystal. If the heat is appropriate, the water of the solution should evaporate and we should have dry crystals ready.
  how to measure solubility:

6. Measure the mass of beaker + solution from step 5

7. Subtract the mass of beaker from the mass in 6 to get the mass of saturated solution

8. And evaporate as shown above to get the mass of crystals.

9. Measure the mass of crystal + beaker

10. Subtract mass of beaker from mass in 8 to get the mass of crystals

11. Subtract the mass of crystals from the mass of saturated solution to get the mass of water in the solution

12. I have assumed that all the masses are in grams. So, to get the solubility: Mass of crystal x 100/Mass of water

 When you have to remove moisture from :
1. Surface:
Wash the surface with a stream of propanone. The water gets dissolved in the propanone and repeat it multiple times. Then gently heat the surface to evaporate the propanone from the surface
2. Vapour:

Use dessicants such as:
 ANHYDROUS sulphuric acid
 ANHYDROUS calcium chloride
 Silica gel

You have to pass the vapour from the beaker containing the dessicant
Also, a useful property of soda lime is that it absorbs BOTH water vapour and carbon dioxide




We are also regularly asked to measure enthalpy changes.
Most of us already know that we use a plastic cup and thermometer for this purpose
However this has many disadvantages (asked regularly) and here are some of them with the required measures:
1. Heat loss to the surroundings from the beaker: To avoid this:
a. Cover the plastic cup with a lid
b. Place the cup in a beaker. The air in the beaker acts a good insulator.
c. Use multiple cups so as to thicken the lateral layer of plastic
2. Instability of the cup
a. Place the cup in a glass beaker



3. For exothermic reactions, spray of the solution very likely
a. Use a large beaker to carry out the experiment rather than the small plastic cup (which has a small volume)

b. Put a lid on the top of the beaker (this only MINIMISES the spray, doesn’t prevent it totally)

When we are heating a volume of water in glass beaker, there are two cases of heat loss to consider:
a. Heat loss from the beaker of water
b. Heat loss from the burner heating the volume of water


When preparing a solution of a fixed concentration from a given parent solution of concentration , say, 2.0 mol/dm3
We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 (the concentration cannot be greater than 2 !!!!!!!!!!!!!!)
a. Add 125 cm3 of the parent solution to a volumetric flask of marking at 250 cm3 [1/2 x 250 = 125 cm3]
b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have greater control over the addition process
When required to prepare 250 cm3 of 1 mol/dm3 solution of a crystal of Mr 50g
1. First realize that we need only 250 cm3, not 1 dm3
2. Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid
3. So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER ( not the volumetric flask yet).
4. Also note that the volume of water is less than 250 cm3.
5. Stir properly and if the solid doesn’t dissolve add more water until it fully dissolves
6. Then transfer the solution from the beaker to a volumetric flask. Remember to rinse the beaker with water and transfer the solution to the volumetric flask
7. Stopper the flask and shake properly
8. Finally, add the required volume of water to make the solution upto 250 cm3 USE A FUNNEL OR YOU RISK OVERSHOOTING THE MARK
Titration is accurate because:
1. Standard solution of acid/base is used 2. we obtain concordant titres 3. % error in pipette and burette is very small


For Filtration:
1. If you require a great separation , use a fluted filter paper
2. If you require a quick filtration, use vacuum filtration
Distillation:
1. Distillation is a great way of determining boiling point of a liquid
2. Melting point can be measured using a Mel Temp
Some Random points:
 We do not need to measure the mass of a reagent or volume of reagent that is in excess
 If you want to flush oxygen out of a system, use an inert gas that doesn't oxidise . Very useful when we are conducting a reduction experiment of metal oxides
 If you want to collect water vapour as water droplet, collect the vapour in a beaker placed in ICE BATH
 During magnesium and hydrochloric acid experiment, to avoid loss of gas do not start the experiment by dropping a piece of Mg in the container of acid!!!
 Rather, use a divided flask (I would appreciate it greatly if someone explains what that actually means) or place a test tube containing the magnesium metal in the solution and pull the string to start the reaction
 If a syringe gives wrong volume, it could be because the syringe got stuck during the experiment
 If the percentage difference between the measured value and the true value is more than the maximum apparatus error, this means the experimenter's technique needs modification. If not, the error could be considered to be entirely due to the intrinsic error or the apparatus used
 if the experiment involves plotting a pH against volume curve, the accuracy of the curve can be increased by using data logger interface and computer combined together to plot the graph

For Question 2:
For question 2, explanation in O/N 2010 ER is the best ever:
The ER states:
Two points about data tables are common every session. Firstly, candidates need to label each new column used. These labels require three items – a written label, a correct unit and an expression to be used to calculate the column data. All of these items are required for credit to be awarded (often the expression is omitted). Secondly, the data needs to be correctly quoted, to either a specified number of decimal places or significant figures. In this examination, the data requirement was for 3 significant figures. Usually, the table head and data requirements are given below the table, so candidates should be advised to look there before calculating any data.
In scaling the axes on a graph plot, candidates should always consider if the true origin (0,0) would be a point on the line. If so, it should always be included in the scales since it would always be a reliable, correct point. Two factors need to be taken into account when scaling the axes. The points need to be plotted within at least half of each axis and the scale should be reasonable to facilitate accurate plotting. For example data of 1, 2, 4 or 5 to 1 large square (of 10 small squares) would be reasonable but 3 or 6 or 7 to 1 large square (of 10 small squares) would not. Some candidates would benefit from practice in fitting a reasonable scale into the grid first that uses at least half of each axis.
When referring to the reason for an anomalous result, it should be stressed to candidates that they unambiguously define the point (e.g. Experiment 4) before stating a reason. This is vital when there is more than one anomaly.
For tables:
Each column heading needs three things, a description (e.g. concentration of acid in water), a unit (e.g. /mol dm-3) and an expression to calculate the data (e.g. B x 0.1/20) as required in the text below the table.
Also below the table is stated a requirement for a specific number of significant figures or decimal places. There is a general misunderstanding of the difference between significant figures, decimal places and trailing zeros. In this case the requirement was for three significant figures. There was a frequent misunderstanding of this requirement. For example, 0.0105 (3sig figs.) was given as 0.011 (3dp), even to the extent of seeing 0.0105 written but crossed out and replaced by 0.011 where, presumably, the candidate believes three decimal places is the same as three significant figures. Also a calculated value of 0.05 needs to reported as 0.0500 if three significant figures are required. Care needs to be taken with the reporting of data. The two required columns involved a great deal of calculation. Each column required the calculation of moles of sodium hydroxide, converting this into moles of succinic acid then a concentration of acid. Only the more able candidates had any success here.
Many managed two parts of these calculations but usually not the 2:1 relationship between succinic acid and sodium hydroxide. This may have been due to unfamiliarity with succinic acid or its formula. Quite a significant minority labelled the columns “moles of sodium hydroxide” rather than concentration in each solvent even though this was stated below the table.
For graph plotting:
The scaling of the axes seems to cause misjudgement in the minds of some candidates. There seems to be a determination to cover the whole span of the grid regardless of the implications of the axis scaling necessary to achieve that. An example of difficult scaling is 1 large square being 0.03. This would make each small square (1 mm) 0.003.
This creates difficulties for the candidate in correctly plotting data of the nature of 0.00552 so mis-plotting is often seen when such inconvenient scales are chosen. Another example of difficult scaling was a 1 mm square being 0.00218 on the x-axis and 0.000223 on the y-axis. Such scales require the extensive use of a calculator to adequately plot the data which in the very least may take some time. Some candidates chose a scale for the acid concentration in water that did not accommodate all the points giving the last point outside the grid.
The scaling chosen must adequately cover the available grid but the minimum requirement is for the plot to cover at least half of the grid in both directions. So the candidates must make a compromise between scaling and grid coverage to use at least half of the grid in both directions with a reasonable scale that can be easily used.
Much better scaling for the concentration in the water layer would have been 1 large square for 0.0125 or 0.02 mol dm-3 (depending on which axis) and for the concentration in the ether layer, 1 large square for 0.001 mol dm-3.
Two more factors need to be considered in graph work. Firstly it is the independent variable that is to be plotted on the x-axis and the dependent on the y-axis. Almost uniquely in this particular question either variable could be plotted on either axis. Secondly the data/relationships should always be examined to see if the origin (0,0) is a valid point. If so (as in this question) then the scaling used must include the origin and any line drawn would include the origin since it would be a definite point not subject to any experimental error as the other points may be.
A requirement for determining a slope is the use of construction lines on the plot to produce co-ordinates for the slope calculation. Many candidates did not draw construction lines.
Hope all this helps with question 2
Titration:
Don’t forget to:
- rinse the burette with acid
- rinse the pipette with alkali
- When emptying the pipette into the conical flask, allow it to empty under gravity, and then touch the surface of the liquid with the pipette for approximately one second
- remove funnel before titrating
- add only two drops of indicator
- swirl mixture during titration
- Wash sides of the flask regularly using a wash bottle
- titrate dropwise near end-point
- read the burette accurately (eyes horizontal, bottom of meniscus)
- record burette reading to 2 decimal places (second 0 or 5)
For better observation:
1. Place a white tile under the conical flask
2. Illuminate the burette while taking the reading
Some more random points:
 If asked in an enthalpy experiment what effect does incomplete value of ethanol have on the measured enthalpy change of combustion:
1. Value decreased
2. Because less C=O bonds formed
 Errors in measuring enthalpy change:
1. Maximum temperature difficult to determine (Measure temperatures often or _ use a computer and datalogger to measure the temperature)
2. The temperature of the solution is not uniform (Stir the solution throughout the l _ experiment or use a magnetic stirrer)
 Also, even if there are no experimental error while measuring the enthalpy change of combustion of ethanol , we cannot find the accurate value of the enthalpy. The defintion of enthalpy requires formation of liquid water on combustion but in our experiment the water is being turned into vapour!!!!!!!!
 When asked "Which equipment should be changed so that the accuracy is improved the most?", always choose the equipment which has the highest percentage error in the experiment.
 The volume of a polystyrene cup used in the lab is roughly 150 cm3

Tips to get A* in A'levels

Do your homeworks regularly and avoid copying from friends. If you do so, you’ll risk your own grades, not the teacher’s! And divide days for each subject, and try doing what has been done in it since you last revised that. On weekends, you can do two subjects per day (or even more if you can), and review all what was done throughout the week. 

Once you’re done with the full chapter, try solving questions from the past papers. After trying them yourself, refer mark schemes and examiner reports. Furthermore, make short notes, chapter-wise, based on the past paper questions. I’d show you a few I made (though, due to shortage of time, I couldn’t do much). This really helps a lot for last-minute revision. In these short notes, you may even add those points which you think you might forget.For maths, regular practice is needed. As for me, after the chapter has been explained, I’d do all the questions in the past papers from that chapter. Again, for this do it yourself, rather than looking up the mark schemes. Solve question from other sources. That helped me a lot. Once I was done with all the chapters, I solved all the papers yearly. 

Tips For  Mcqs
start solving chem mcq paper from the last question and go back wards....
start from Q:40 and go backwards till Q:1
i used to run out of time when solving the paper, and i tried doing this and coudnt believe i was able to complete the paper in about 40 minutes
--leave the calculation questions and do them after finishing the rest...it'll save time.. 
for section B If you are 100% sure that 1 is wrong just mark the answer C
and if you are 100% sure that 2 is wrong , just mark the answer D
If in the end you have time , you can go through the other statements and check...but make sure before doing this, you're more than 100% sure ....[there are questions where it's rather obvious the statement is wrong.. ]
and also if 1 is correct, dont waste your time checking if 2 is correct or not...jump on to statement 3. If you're really sure it's wrong, then D would definitely be wrong, ans will be D
and if you're sure 3 is correct then 2 will definitely be correct, and ans will be A.
May be this'll seem a bit foolish, but trust me, it does get your answers right, and also saves your time. After finishing the paper, if you have enough time, you can check the other statements etc. 
t helped me, and might help you