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# AS and A Level: Exchange, Transport & Reproduction

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## Five tips on presenting and analysing data from core practicals

1. 1 Raw data should be presented in a table with clear headings. All table column and row headings should contain units and readings should all be recorded to the same decimal place.
2. 2 Raw data should be processed to provide descriptive statistics such as the mean and standard deviation.
3. 3 Present data using the relevant graph type. Ensure that you add error bars showing either standard deviation or standard error.
4. 4 When describing trends and patterns, manipulate data to calculate the size of key changes. For example, absorbance increases by 0.09 absorbance units between 10◦C and 40 ◦C. Preferably express the change as a percentage increase or decrease. Do not simply quote points, eg at 10 ◦C absorbance was 0.01 and at 40 ◦C it had gone up to 0.1 absorbance units.
5. 5 Discuss each phase of the graph. For example if there is a slow increase, followed by a rapid increase, and then the graph levels off and shows a decrease, discuss these four key phases. Do not give detailed descriptions of each small fluctuation. The trends and patterns are the important things.

## Meiosis and Mitosis facts

1. 1 Meiosis generates gametes/sex cells, whereas mitosis is for growth/repair and generates daughter cells identical to the parent cell.
2. 2 During Meiosis chromosome number is halved producing haploid gametes with a single copy of each chromosome. During Mitosis chromosome number is maintained producing diploid daughter cells with maternal and paternal copies of each chromosome, i.e. homologous chromosome pairs.
3. 3 Before mitosis and meiosis all chromosomes are copied as part of interphase. At the end of interphase there are two identical copies of every maternal chromosome and every paternal chromosome, so chromosome number has doubled (i.e. at the end of interphase a human cell contains 46 x 2 = 92 chromosomes). The identical copies of chromosomes are referred to as sister chromatids and they are joined by a centromere.
4. 4 In meiosis, genetic variation is generated by crossing over during prophase 1, and independent chromosome assortment at metaphase 1 and metaphase 2. During crossing over maternal and paternal chromosomes cross each other, and break at points known as chiasma. Maternal and paternal alleles below the chiasma change places so that the paternal chromosome contains maternal alleles and vice-versa. During metaphase 1 and 2, maternal and paternal chromosomes align randomly on one side of the equator. As the maternal and paternal chromosomes can align on either side, different potential chromosome combinations can occur.
5. 5 The events in metaphase, anaphase and telophase are identical in both mitosis and meiosis 1 and 2. In mitosis a single division occurs, whereas in meiosis cells undergo 2 meiotic divisions.
a) During meiosis 1, maternal and paternal sister chromatids are separated so that 1 cell contains both maternal sister chromatids of a pair and the other contains both paternal sister chromatids.
b) During the second meiotic division, sister chromatids are separated. The gametes that result contain only 1 chromosome from each pair, i.e. they are haploid.

## How to evaluate experimental methods

1. 1 When evaluating the reliability of experimental methods, always consider whether all variables other than the independent variable have been adequately controlled. If a variable cannot be controlled has it been monitored to establish any effect it might have?
2. 2 All experiments must be repeated to establish reliability. Has the experiment been repeated at least three times? Preferably you should repeat it more than 3 times.
3. 3 What does the standard deviation suggest about the spread of the data? If the mean is 5, but the standard deviation is 3, readings vary from the mean considerably. This suggests that the mean does not represent the actual readings.
4. 4 How precise are the measurements? If a balance used to measure change in mass only measures to 0.1 g then the reading could be 0.12 g, 0.15g, or 0.18g etc.
5. 5 How accurate are the readings? If equipment is re-used for different repeats for example, cross contamination could affect the accuracy of subsequent readings. If tubes are shaken different amounts, different volumes of gas could be released. Consider all potential sources of error and discuss how the procedure could be improved to reduce these sources of inaccuracy.

• Marked by Teachers essays 46
• Peer Reviewed essays 14
1. ## Investigating the effect of two different antibiotics on bacterial growth

25 degrees Celsius Controls In the experiment the same size paper discs were used, an increase or decrease in the paper disc size will affect the surface area of the paper disc. For e.g. if the size of the paper disc is larger than normal this will lead to more antibiotic being absorbed as there is a larger surface area, resulting in a greater clear zone produced as more bacteria growth will be prevented. Moreover, another important factor was the temperature of the incubator.

• Word count: 1913
2. ## Lymphedema development in the human body

It can progress into a troublesome disorder in time however at first it will seem to come and go. For example in the mornings it may be at minimal stage and during the day time it will become larger.

• Word count: 490
3. ## P6Explain how these dietary nutrients are processed through the digestive system

The proteins that are broken down are then absorbed into the small intestine and into the blood, it helps to build and repair cells and walls of the body. Enzymes from the pancreatic juice break down the protein molecules into amino acids. This helps in the breakdown of food. Amino acids are absorbed into the blood capillaries and transported around the body. Proteins can be digested into the liver where they are stored as fats or glycogen or converted into fatty acids.

• Word count: 1339
4. ## Describe the structure and purpose of the lymphatic system .

Therefore, the lymphatic system drains away all the excess fluid protecting the body from infections and germs like viruses. Bacteria and viruses are constantly spreading throughout the body; however the lymphatic nodes also consist of a certain type of white blood cell. This makes anti bodies which fight off germs such as bacteria and viruses. Fighting of these germs and bacteria prevents the body from infections and the spread of disease. The lymphatic system destroys pathogens and filters waste so that the lymph?s are not destroyed and can carry on with their process.

• Word count: 1649