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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.

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  1. Peer reviewed

    the heart

    3 star(s)

    Main: The heart is a muscular organ that it part of the cardiovascular system. It is located between the lungs, behind the sternum and is approximately the size of a fist. The heart is protected by a membrane called the pericardium which surrounds the heart and secretes a fluid that reduces friction when the heart beats. The atria's job is to receive blood and the ventricles job is to be filled with blood and send it to everywhere in the body. There are three layers which make up the wall of the heart. The outer layer is epithelial tissue, the middle layer is cardiac muscle and the inner layer is connective tissue.

    • Word count: 1292
  2. Peer reviewed

    The Various Roles Played By Proteins

    3 star(s)

    Intrinsic proteins lie across the whole membrane. They help ions and molecules which are water soluble to cross the membrane at this point, therefore controlling what goes into and what comes out of the cell. Another function of proteins in the cell membrane is to 'detect signals from the outside of cells and relay them to the inside' (Phillipallan,nov 1999); these are known as receptor proteins (sometimes with sugars attached) and their job is to alter a cells behaviour to enable signal molecules like hormones to bind specifically to their own receptor.

    • Word count: 889
  3. Peer reviewed

    Effect of Caffeine on the Heart Rate of Daphnia

    3 star(s)

    It is the most commonly used mind-altering drug in the world. When used in moderation, caffeine acts as a mild stimulant to the nervous system, blocking the neurotransmitter adenosine and resulting in a feeling of well-being and alertness. Also, it is found that caffeine binds to receptors on the surface of heart muscle cells which leads to an increase in the level of cAMP inside the cells (by blocking the enzyme that degrades cAMP), mimicking the effects of epinephrine (which binds to receptors on the cell that activate cAMP production). cAMP acts as a "second messenger," and activates a large number of protein kinase A (PKA; cAMP-dependent protein kinase).

    • Word count: 4926
  4. Peer reviewed

    Osmosis. Finding the water potential of potato tuber cells

    3 star(s)

    I know form prior work, investigation and study that the water potential will be around 0.4M. So I have decided to use 6 different solutions that are on either side of this figure. Table 1 below shows which molarity's I will be using. Table 13 Molarity (mol dm3) Water Potential (kPa) 0.00 �0 0.05 -130 0.10 -260 0.15 -410 0.20 -540 0.25 -680 0.30 -860 0.35 -970 0.40 -1120 0.45 -1280 0.50 -1450 0.55 -1620 0.60 -1800 0.65 -1980 0.70 -2180 0.75 -2370 0.80 -2580 0.85 -2790 0.90 -3000 0.95 -3250 1.00 -3500 Making the solution Of these 7 solutions I will have to make myself form the 1 mole stock sucrose solution.

    • Word count: 1094
  5. Peer reviewed

    Types of diffusion

    3 star(s)

    Plants use osmosis to take up water into their roots because they have a large surface area and the water coming through the permeable membrane generates the pressure needed to send the water up the plant to the vital areas. Simple diffusion is a process of diffusion which does not require a special protein channel, facilitated diffusion (or facilitated transport) is a process of diffusion, a form of passive transport, where molecules diffuse across membranes, with the assistance of transport proteins.

    • Word count: 796
  6. Free essay

    Movement of Nutrients in Plants

    3 star(s)

    In some cases, the ions may enter the plant against a concentration gradient. The ions are selectively absorbed by active transport, therefore energy is used. Mineral ions are taken up by the root hairs and other surface cells in the young parts of the root. Active transport occurs across the plasma membranes of the root hairs and the cortex cells. The ions move inwards via the symplast, through plasmodesmata. Once inside the vessels and tracheids, the ions are carried up the stem, along with the water in the transpiration stream.

    • Word count: 944

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