AS and A Level: Exchange, Transport & Reproduction essays

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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.

359 AS and A Level Exchange, Transport & Reproduction essays

  • Marked by Teachers essays 46
  • Peer Reviewed essays 14
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  1. Marked by a teacher

    Sand Dune Succession Coursework

    5 star(s)

    This could be achieved by seaweed or driftwood lying on the beach. This dune is very unstable and could easily disappear as quickly as it was formed. The next sere is the fore dunes where sea couch grass and marram grass colonise. These plants are drought resistant xerophytes and are capable of withstanding burial by the shifting sand. As they grow up through the sand they trap more of it which results in the dunes increasing in height. The yellow dunes begin to show a greater diversity of plants as conditions become more idealistic.

    • Length: 3125 words
  2. Marked by a teacher

    Biology coursework planning - the effect of lead chloride on the growth of cress seeds

    5 star(s)

    Lead is able to cross the cell membranes via voltage-gated calcium channels. These channels are for the transport of calcium. Lead blocks these channels and causes the inhibition of their activity, preventing calcium being transported. Plants require water for photosynthesis. Photosynthesis provides plants glucose, an energy source, which is needed for the plants to grow. When lead is present in high concentrations in the soil, it decreases the water potential of the soil. It therefore, has a lower water potential than the root cells, causing water to move from a region of higher water potential (root cells)

    • Length: 7403 words
  3. Marked by a teacher

    Investigate the water potential of celeriac.

    5 star(s)

    The test tube will be used to provide a safe environment for the experiment to take place in. Test tube rack - this will be used to hold the test tubes in a steady position when the experiment is carried out. Prediction I predict that as the concentration of the sucrose solution increases, the mass of celeriac will decrease. I think this because, as the concentration of the sucrose solution increases, the water potential of the solution increases in negativity. So therefore as the molarity increases, the water molecules will travel by osmosis from the less negative celeriac to the more negative sucrose solution.

    • Length: 5310 words
  4. Marked by a teacher

    Mr Chips: Investigation to find an isotonic solution for potatoes

    4 star(s)

    At this point the cell wall prevents the cell from bursting and is said to be fully turgid. Turgidity is very important to plants because this is what provides support to make the green parts of plants stand firm. When plant cells are placed in highly concentrated solute solutions they lose water through osmosis. Water is drawn from the cell and the vacuole contracts and shrinks drawing the cell membrane away from the cell wall. The effects of the water loss is called plasmolysis and cells in this condition are said to be plasmolysed or flaccid. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity and as there is no gradient for the molecules to travel down, equilibrium is set.

    • Length: 2458 words
  5. Marked by a teacher

    Comparison of Transport System in Amphibian and Mammalian models

    4 star(s)

    They have small eyes and the mouth is located below the nose. The rats have a touch sensor system that is known as the whiskers on its nasal region called the vibrissa. As for the neck and body, a rat has a short neck with its body consisting of a thorax on its anterior and an abdomen/belly on its posterior. The pectoral region refers to the area where the front legs attach. The ventral surface of the pectoral region consists of the rat's nipples or mammary gland openings. Its tail is long with its anus located on the ventral region of the base of the tail.

    • Length: 2198 words
  6. Marked by a teacher

    An investigation on the effect of temperature on beetroot membrane structure.

    4 star(s)

    The phospholipids consist of hydrophilic heads which point towards the outside environment and the cytoplasm. The hydrophobic tails repel the water and so point in (see fig 3). Two tails that are highly hydrophobic hydrocarbon chains. Hydrophilic heads (negatively charged) (Fig 3 ref 4) My experiment is based on the fact that proteins/ phospholipids denature when heated. When the membrane proteins are damaged the dye in beetroot leaks out of the cell. This is based on the theory that the more the proteins gain thermal energy the more thermal energy is being converted to kinetic energy.

    • Length: 2104 words
  7. Marked by a teacher

    Human Reproductive System

    4 star(s)

    It transports and store sperm cells that are produced in the testes. It also brings the sperm into maturity, since the sperm that emerge from the testes are immature and incapable of fretilisation. Glans penis The glans penis, commonly referred to as head of the penis, is a sensitive bulbous structure at the end of the penis. It is the conical expansion of the corpus spongiosum. It is anatomically homologous to the glans clitoris in females. Membranous Urethra The membranous urethra is a short portion of the urethra which is situated between the prostatic urethra and spongy urethra. It connects the parts of urethra passing through the prostate gland and the penis.

    • Length: 6606 words
  8. Marked by a teacher

    Reproduction, Gamete formation and fertilisation

    4 star(s)

    A Muscle layer called the Dartos muscle relaxes in hot weather to drop the testes away from the body to keep them cool. In cold weather the muscle contracts to draw up the testes so they do not become too chilled. The spermatic cord suspends each testis within the scrotum, and contains the testicular artery and vein, lymph vessels, nerves, and the sperm-carrying vas deferens. Sperm Production Each testis is a mass of more than 800 tightly looped and folded vessels known as Seminifeous tubules.

    • Length: 1187 words
  9. Marked by a teacher

    Investigation of the effect of different carbohydrate substrates on yeast growth

    4 star(s)

    Maltose will have a slight effect on yeast growth. Sucrose will have the least effect on the yeast growth. Null hypothesis (necessary for statistical model: Chi square) Glucose will have no effect on yeast growth. Maltose will have no effect on yeast growth. sucrose will have no effect on the yeast growth. Variables Independent; glucose, maltose and sucrose (the carbohydrates) Dependent; the growth of the yeast cells (numerical growth/ change in population) is what will be measured and the yeast cell will be counted by a method called: "The Haemocytometer technique" using a light microscope and a counter to count the yeast, use of a microscope will be required due to cell size being no bigger than 10µm.

    • Length: 1512 words
  10. Marked by a teacher

    Transport across plasma membranes

    4 star(s)

    This allows the cell to maintain a constant internal environment. This is more commonly known as osmosis; the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. It is a physical, hydrophilic process in which a solvent moves, across a semi-permeable membrane separating two solutions of different concentrations. Osmosis releases energy but does not require it as it is a passive process. Diffusion is similar to this in the sense that it is also passive, meaning no energy required and it also takes place through the phospholipid bilayer.

    • Length: 880 words
  11. Marked by a teacher

    To investigate the effect of trampling on the frequency of creeping moss - Plan

    4 star(s)

    Plants growing on shallow soils also have less mechanical support than those growing in deep soils. Trees growing in shallow soils are more easily blown over by wind than are those growing in deep soils." (cals.arizona.edu/pubs) Independent Variable: Soil Depth (trampling) Variables: My measured factors are soil pH and soil temperature. My assumed factor is light intensity. My first variable is soil pH. I will be measuring this to find out if it will have any impact on the abundance of creeping moss.

    • Length: 1443 words
  12. Marked by a teacher

    The Role of Carbohydrates in Living Organisms

    4 star(s)

    be used as respiratory substrate, or is converted to starch for storage. Both of these trioses are intermediates in glycolysis, where glucose is broken down by enzymes into pyruvic and lactic acid. Pentoses such as ribose an ribulose possess five carbon atoms. Ribose or deoxyribose make up part of nucleotides and as such give structural support to nucleic acids RNA and DNA. Ribose is a constituent of hydrogen carriers such as NAD, NADP and FAD. Further more it is involved in the synthesis of coenzymes and ATP. The third type of monosaccharides is hexoses like glucose and fructose, which possess six carbon atoms.

    • Length: 624 words
  13. Marked by a teacher

    Affect Of Varying Salt Concentration on Red Blood Cell Haemolysis

    4 star(s)

    Water molecules will diffuse from a solution with a high water potential to a solution with lower water potential down an osmotic gradient. "Osmosis is the diffusion of water molecules from a higher (less negative) water potential to a lower (more negative) potential through a partially permeable membrane" (Molecules and Cells Text book, P66) Water potential plays a key factor during the haemolysis of erythrocytes if the water potential within the cytosol is greater than that of the saline solution to which it has been introduced there will be a net movement of water molecules from the cytosol through the plasma membrane into the saline solution resulting in the shrinking of the red blood cell.

    • Length: 3109 words
  14. Marked by a teacher

    Investigating how sugars are metabolised by yeast

    4 star(s)

    * Repeat this exercise 3 times for each sugar for reliability. * Calculate the difference between start and end volume to find out the amount Carbon Dioxide produced. Background Information Respiration is the release of energy from [1]glucose or other organic substances. Energy is required for growth, repair, movement and other [2]metabolic activities. There are two main types of respiration, aerobic and anaerobic. Aerobic respiration takes place in the presence of oxygen. This is the normal type of respiration that takes place in all living cells. However, in some cases respiration takes places in the absence of oxygen and this is called anaerobic respiration.

    • Length: 1461 words
  15. Marked by a teacher

    Osmosis. Aim: To find the molarity of potato tubers cell sap. BIOLOGICAL KNOWLEDGE

    4 star(s)

    This reduces the concentration of free water molecules. Therefore, as the amount of solute increases, the number of free water molecules decrease. Osmosis takes place due to the difference in the concentration of free water molecules on either side of the semi-permeable membrane. It continues as long as the concentration in both sides of the membrane is equal. The sugar molecules on the right have 'captured' half the water molecules. There are more free water molecules on the left of the membrane than on the right, so water will diffuse rapidly from left to right across the partially permeable membrane by osmosis.

    • Length: 5993 words
  16. Marked by a teacher

    Homeostatic Control of Blood Glucose Levels

    4 star(s)

    Essentially, blood glucose levels are controlled by the pancreas. It is in this organ that areas of cells called the Islets of Langerhans exist, which consist of alpha cells and beta cells. These cells monitor blood glucose and secrete the hormones glucagon and insulin respectively. Glucagon and insulin regulate blood glucose levels through their antagonistic and opposite effects in a system of negative feedback. The most important effects of insulin work to lower blood glucose levels, which may be too high following a large, carbohydrate heavy meal for example and are as follows.

    • Length: 1123 words
  17. Marked by a teacher

    Describe the molecular structure of starch (amylase), glycogen and cellulose, and relate these structures to their functions in living organisms.

    4 star(s)

    form. The chemical and physical properties of the two isomerism are the same. Many enzymes will however only act on one type. Glucose easily forms stable ring structures and most molecules exist as rings rather than a chain. Carbon atom no 1 however may combine with oxygen atoms on carbon no 5 to form a ring (two further isomers). ? D(+) Glucose ß D(+) Glucose Alphadextroglucose Betadextroglucose Monosaccharides are sugars. They dissolve easily in water to form sweet solutions (saccharide refers to sweet or sugar). Monosaccharides have the general formula (CH2O)n and consist of a single sugar molecule (mono).

    • Length: 3400 words
  18. Marked by a teacher

    A Colorimetric Method for the Estimation of Glucose in Solution.

    4 star(s)

    A clean pipette and pipette filler was used to transfer 2cm of potassium permanganate solution into each boiling tube. A stopwatch was started the exact moment the potassium permanganate was added to the acidified glucose solutions. A time was recorded, in seconds for the complete decolourisation of the potassium permanganate solutions. Results Concentration of glucose (%) Start time (seconds) Finish time (seconds) Elapsed time (seconds) 10 90 562 472 9 80 578 498 8 70 590 520 7 60 605 545 6 50 614 564 5 40 629 589 4 30 635 605 3 20 963 943 2 10 1787 1777 1 0 2486 2486 Results of Unknowns Name of Unknown Start time (seconds)

    • Length: 930 words
  19. Marked by a teacher

    An Investigation into the Water Potential Of Root Vegetables.

    4 star(s)

    Once the isotonic solution is found for each vegetable, its water potential can be found by using a reference graph, which shows the water potential for different concentrations of sucrose. The two vegetables I will use will be a potato and a swede. Swedes are considered a sweet root vegetable and so its sucrose concentration is probably quite different from that of a potato because the sucrose gives the vegetable its sweet taste. Method To carry out this investigation I will need the following apparatus: * * Two boiling tube racks * Ten boiling tubes * 200ml of 1mol sucrose

    • Length: 2161 words
  20. Marked by a teacher

    Determining the Water Potential of Sweet Potato Tissue

    4 star(s)

    of a solution. This value is therefore always negative and the symbol for solute potential is ?s. WATER POTENTIAL is a measure of the tendency of water molecules to move from one place to another. The symbol for this is ?. PRESSURE POTENTIAL is the contribution made by pressure to increase the ? of a solution. It is therefore positive, and its symbol is ?p. The more solute the more negative the ? becomes: the ? of pure water is therefore zero. As the solute molecules prevent the water molecules leaving a solution, the ? of the solution will be a negative value.

    • Length: 4708 words
  21. Marked by a teacher

    Investigate a factor which can affect the process of osmosis in a living plant tissue

    4 star(s)

    For this investigation I will be looking at the affects of osmosis on plant tissue. The cytoplasm of a plant cell and the cell SAP in its vacuole contain salt, sugars and proteins, which effectively reduce the concentration of the free water molecules inside the cell giving a low water potential. The cell wall is freely permeable to water and dissolved substances but it is the cell membrane will acts as the partially permeable membrane. For this experiment the easiest plant cells to use would be that of a potato (chips) and the solution I will use is Sucrose solution.

    • Length: 2618 words
  22. Marked by a teacher

    The Effect Of Temperature On The Permeability Of The Cell Membrane

    3 star(s)

    Temperature has an affect on membranes. In this experiment I will be looking at how temperature has the effect on membranes and what factors increase and decrease it. The cell membrane controls the substances moving into and out of the cell. The structure of the cell membranes is proteins floating in it. The proteins span the membrane and touch the inside and outside of the cell. The cell membrane is between 6-8nm thick and contains many distinct molecules. The fatty acid tails are non-polar which is difficult for polar molecules/ions to pass through them.

    • Length: 5375 words
  23. Marked by a teacher

    The transport system in plants moves water soluble molecules by vascular tissue. There are two types of vascular tissue which are Xylem and Phloem.

    3 star(s)

    Most importantly the tissues do not use any energy or transport mechanism to move the water based molecules however the water or water based substance itself has cohesive properties. The cohesive properties of water cause molecules to attract to each other and the surface tension in the vessels keeps it moving up towards the leaves. As the water on the leaves is evaporating or being used in a process it causes water to move up the plant and more water to be taken in by the root's which is also seen as an ongoing cycle.

    • Length: 484 words
  24. Marked by a teacher

    Effect of temperature on membranes

    3 star(s)

    The phospholipid is made of two parts that is the polar hydrophilic (water-loving) heads and the non-polar hydrophobic (water-hating) fatty acids tails. In the cell membrane, the hydrophilic heads point into the water on both external surfaces of the bilayer while the hydrophobic tails are protected in the middle. Various types of protein which are embedded (integral proteins) or attached on the surface of the bilayer (peripheral proteins) are found within the bilayer. There are also cholesterol present in the bilayer to strengthen it and make it more flexible and less permeable to water-soluble substances.

    • Length: 3706 words
  25. Marked by a teacher

    The effect of caffeine on heart rate

    3 star(s)

    This made the results valid as caffeine was the only variable that was manipulated in the experiment. Ethics: Since this experiment involves living organisms, ethical issues were raised. Consideration has been taken into account and the following guidelines were used: * Make sure the Daphnia is kept in a natural environment comfortably * Don't leave them in the solution of caffeine for too long * As they are God's creatures, some religions believe that humans were given responsibility to look after the world.

    • Length: 1220 words

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