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AQA BIOL1 Biology Unit 1 Exam - 16th May 2011 Watch

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    (Original post by sanaindaclub)
    Can someone please explain the mosaic part of the fluid - mosaic model of the phospholipid bilayer.
    It is referred to as mosaic because of the collage of intrinsic proteins that stud the membrane. The diversity if protein channels and carriers, together with their unique capabilities by association with glycoxalyx makes it mosaic like
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    I have the longest exam season of anyone as a current A2 student in my sixth form

    Biology 1: first of any exam, physics 5: last of any exam
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    (Original post by CityTrader)
    It is referred to as mosaic because of the collage of intrinsic proteins that stud the membrane. The diversity if protein channels and carriers, together with their unique capabilities by association with glycoxalyx makes it mosaic like
    'Because the proteins and molecules that are embedded in the phospholipid bilayer vary greatly in size, shape and quantity, like the stones in a mosaic. No two are the same'

    Do you think that would get me the two marks or only one?
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    (Original post by jones_wise)
    Describe how a blood clot may lead to a myocardial infarction. (3 marks)
    Blood clot prevents oxygen and glucose reaching the Muscle of the heart from the cornory artery. Muscle will not be able to respire and will stop beating.
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    (Original post by CityTrader)
    It is referred to as mosaic because of the collage of intrinsic proteins that stud the membrane. The diversity if protein channels and carriers, together with their unique capabilities by association with glycoxalyx makes it mosaic like
    You forgot about extrinsic proteins too. There are intrinsic and extrinsic proteins.
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    HERE ARE THE ANSWERS, SOZ I WAS HAVING LUNCH, AND NASIRA IM A GIRL ..


    Cell and microscopes
    1. Cell-surface membrane (to control the movement of substances in and out of the cell). Nucleus (Acts as the control centre of the cell - retains the genetic material (DNA) responsible for the synthesis of proteins). Mitochondrion (Site of cell respiration and the production of ATP). Lysosome (To digest worn out organelles and material ingested by phagocytic cells). Ribosome (site of protein synthesis). RER (Transport of protein throughout the cell). SER (Synthesis, storage and transport of lipids and carbohydrates). Golgi apparatus (To add carbohydrate to protein and form glycoprotein and secrete it from the cell). Microvilli (To increase the surface area of the membrane for more efficient absorption by epithelial cells lining the small intestine).
    2. Magnification is how many times bigger the image is compared to the original object. Resolution is the minimum distance apart that two objects can be in order for them to appear as separate items (it is the ability to distinguish between very small objects that are very close together).
    3. Because it uses a beam of electrons that has a shorter wavelength than light.
    4. A TEM transmits a beam of electrons through a thin section of the specimen giving 2-D images of the internal structure of cells and organelles. A SEM directs a beam of electrons onto the surface of a specimen and scans them back and forth across the surface to give views of surface structures and produces 3-D images.

    Fractionation and centrifugation
    5. It is the process by which cells are broken up and the different organelles they contain are separated out.
    6. The homogenate is spun in an ultracentrifuge at slow speed for a short time to sediment the heaviest organelles (nuclei). The supernatant is then spun at a higher speed for a longer time to sediment the next heaviest organelle (mitochondria). The process is continued and at each increase in speed, the next heaviest organelle is sedimented and separated out (lysosomes, followed by ribosomes and then broken/fragmented membranes).
    7. (a) To minimise enzyme (lysosome) activity and so prevent damage to the organelles.
    (b) To prevent organelles either bursting or shrinking as a result of osmotic gain of loss of water.
    (c) To maintain a constant pH (for optimum activity of organelles)

    Plasma membranes and lipids
    8. Consists of two layers of phospholipid (lipid bilyer), arranged ‘tail’ to ‘tail’ and interspersed with globular proteins, some of which span the entire lipid bilayer, and some of which only partially penetrate one or other phospholipid layer.
    9. See page 3.


    H O H O
    H C OH HO C R1 H C O C R1
    O O + 3H2O
    H C OH + HO C R2 H C O C R2
    O O
    H C OH HO C R3 H C O C R3
    H H

    10. A saturated fatty acid has no c-c double bonds, whereas, an unsaturated fatty acid has one or more c-c double bonds in its hydrocarbon ‘tail’.
    11. A phospholipid has 2 fatty acid molecules and one phosphate molecule condensed to a glycerol molecule, whereas a triglyceride has only 3 fatty acids molecules condensed to a glycerol molecule.
    12. Add 2/3 cm3 of ethanol to 1 cm3 of lipid sample to be tested and shake to dissolve. Then add 2/3 cm3 of water to the mixture and shake gently. A cloudy-white (milky) colour indicates the presence of lipid.
    13. (1) Transport of large, water-soluble molecules and charged ions (carrier proteins). (2) Act as receptors for hormones (glycoproteins). (3) Serve as recognition sites for antigens (glycolipids).

    Diffusion, osmosis and active transport
    14. (1) Diffusion is the passive movement of substances down a concentration gradient. (2) Facilitated diffusion is the passive movement of substances down a concentration gradient through an intrinsic membrane protein. (3) Osmosis is the diffusion of water from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane.
    15. Rate of diffusion is proportional to Surface Area X Difference in concentration
    thickness of exchange surface
    16. Similar – In all, substances move down concentration gradients and they are all passive processes (requiring no ATP/respiratory energy). Differ – many different substances are transported by diffusion and facilitated diffusion, but only water is transported by osmosis. Facilitated diffusion requires a membrane protein, but diffusion and osmosis do not.
    17. Active transport is the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient) through an intrinsic membrane protein using ATP (respiratory) energy.
    18. (a) Active transport requires ATP (is an active process), a membrane protein and takes place against a concentration gradient, whereas, diffusion is a passive process that takes place down a concentration gradient and requires no membrane protein. (b) Both active transport and facilitated diffusion require a membrane protein but, active transport requires ATP energy and takes place against a concentration gradient whereas, facilitated diffusion is passive (no ATP required) and moves substances down their concentration gradients.
    Proteins
    19. H R O H R O H R O R O
    H N C C OH + H N C C OH H N C C N C C OH + H2O
    H H H H H

    20. Condensation- formation of a bond by elimination of water; Hydrolysis- breaking a bond by addition of water.
    21. Primary structure: the sequence of amino acids joined together by peptide bonds.
    Secondary structure: the coiling and folding of the polypeptide chain due to hydrogen bonding between the peptide groups.
    Tertiary structure: the further coiling and folding of the polypeptide chain into its final 3-D shape due to a variety of bonds between the R-groups, including hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bonds.
    Quaternary structure: the degree of polymerisation of the protein (how many polypeptide chains it has linked together by a variety of bonds (as in tertiary structure).
    22. Add an equal volume of dilute sodium hydroxide solution to the test sample at
    room temperature. Add a few drops (5 drops to 1 cm3) of dilute copper (II)
    sulphate solution and mix it gently. A mauve/violet/purple colour indicates the
    presence of protein. If no protein is present, the solution remains blue.
    23. Globular proteins such as enzymes, hormones, antibodies, albumins & globulins.
    Fibrous proteins such as collagen, myosin and keratin.

    Enzyme action and properties
    24. It is the energy required to start a reaction.
    25. They lower the activation energy (split the reaction into stages, so providing alternative, lower energy pathways by which the reaction can occur).
    26. (a) (b) (c) (d)









    27. Competitive inhibitor: is similar in shape to the substrate and competes with it for the active site of the enzyme; it binds to the active site and so prevents substrate binding and forming enzyme/substrate complex.
    Non-competitive inhibitor: not similar in shape to substrate and binds to the
    enzyme at a site which is not the active site. Binding alters the shape of the
    active site and so prevents the substrate binding to it and forming
    enzyme/substrate complex.
    28. Denaturation: hydrogen bonds (and ionic bonds) in enzyme molecule have been broken resulting in the loss of tertiary structure and change in shape of the active site so that substrate can no longer bind to active site of enzyme and no enzyme/substrate complex can be formed.

    Carbohydrate biochemistry
    29. A polymer is a large molecule made up of repeating smaller molecules (subunits) called monomers. Polypeptides (proteins), polysaccharides and polynucleotides are important examples.

    30. H H


    HO OH

    31.
    H H H H H H H H
    +
    HO OH HO OH HO O OH

    32. (a) Glucose and fructose; (b) Glucose and galactose.
    33. (a) Benedict’s’ test: Mix together equal volumes (1cm3 each) of test solution and Benedict’s reagent and heat them in a boiling water bath for 2 mins. A brick red precipitate/colour indicates the presence of a reducing sugar such as glucose. If no glucose is present, the solution remains blue.
    (b) Sucrose test: First show that a Benedict’s test is negative. Then hydrolyse
    1 cm3 of sucrose solution by boiling it in 1cm3 of dilute HCl for 5 mins. Neutralise the solution and repeat the Benedict’s test, which should now give a positive result.
    (c) “Iodine solution” test: Add 2 drops of potassium iodide solution to the sample
    and a blue-black colour indicates the presence of starch. If no starch is present
    the solution remains yellow.

    Carbohydrate digestion
    34. Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules. It is necessary in order to absorb food molecules into the body (through cell membranes).
    35. Glucose.
    36. Amylase from the salivary glands and pancreas, followed by maltase located in the cell surface membranes of the epithelial cells lining the small intestine.
    37. Place a solution of carbohydrase enzyme in a well in the centre of the starch agar. Incubate the plate at 25oC for 24 hours. Flood the starch agar with potassium iodide solution. Measure the diameter of the clear zone around the wells (where starch has been digested) as a measure of enzyme activity.
    38. Lactose intolerance is caused by the inability to produce any/sufficient lactase enzyme and so the inability to digest all the lactose (milk sugar) in the diet. Microorganisms in the large intestine convert the undigested lactose (respiration) into gas, which accumulates, and causes bloating, nausea, diarrhoea and cramps.

    Carbohydrate absorption
    39. Very long: large surface area for increased rate of absorption; food takes a long time to pass through and so this allows plenty of time for absorption of digested food products.
    Highly folded with villi: increased surface area leads to increased rate of diffusion.
    Microvilli line the epithelial cells of the villi increasing surface area.
    Villi lined with a single layer of epithelial cells making them thin-walled. This gives short diffusion pathway.
    Villi are well supplied with blood vessels and lacteals, so that absorbed molecules can be carried away and hence maintain a diffusion gradient.
    Villi are able to move and so create currents in the lumen contents, helping to maintain diffusion gradients.
    40. Facilitated diffusion and active co-transport.

    Disease and pathogens and cholera
    41. A pathogen is a microorganism that causes disease e.g. bacteria, viruses and fungi.
    42. The digestive system; the gas exchange system.
    43. (1) By damaging host tissues. (2) By producing toxins.
    44. (1) Keratinised skin. (2) Mucus and cilia (3) acid (4) lysozyme in tears
    45.











    46. Vibrio cholerae. It is a rod-shaped bacterium.
    47. Transmitted by the ingestion of water, or more rarely food, that has been contaminated with faecal material containing the pathogen.
    48. Cholera starts with severe muscle and stomach cramps. Vomiting, fever and then diarrhoea (watery faeces) follows. So much fluid may be lost that the patient becomes very dehydrated and then their circulatory system fails.
    49. The cholera bacterium propels itself through the mucus lining of the intestinal wall. There it produces a toxic protein, part of which binds to the carbohydrate receptors on the cell-surface membrane of the epithelial cells, and part of which enters the epithelial cells where it causes ion channels in the cell-surface membrane to open. This allows chloride ions to flood out of the cells into the lumen of the intestine. This loss of chloride ions from the epithelial cells raises the water potential inside these cells and lowers the water potential in the lumen and so causes water to diffuse from the cells into the lumen by osmosis. In addition, the loss of chloride ions from the epithelial cells establishes a concentration gradient down which ions diffuse into the epithelial cells from the surrounding tissues and blood. This in turn, establishes a water potential gradient that causes water to diffuse by osmosis from the blood and other tissues into the intestine. It is this loss of water from the blood and other tissues, into the intestine that causes the symptoms of cholera, namely, severe diarrhoea and dehydration.
    50. Clean, uncontaminated water supplies/water treatment/chlorination/proper sanitation and sewage treatment/personal hygiene (washing hands after using the toilet)/proper food hygiene.
    51. ORT is a means of treating dehydration involving giving, by mouth, a balanced solution of salts and glucose that stimulates the gut to reabsorb water. ORS contains water, sodium ions, glucose, potassium and other electrolytes such as chloride and citrate ions. Glucose stimulates the uptake of sodium ions from the intestine and provides energy, as it is a respiratory substrate. The sodium ions replace those lost from the body and encourage the use of the sodium-glucose transporter proteins to absorb more sodium ions. As sodium ions are absorbed, so the water potential of the cells falls and water enters the cells by osmosis.

    Lifestyle, correlation and causal relationships
    52. In the context of health, risk is a measure of the probability that damage to health will occur as a result of a given hazard.
    53. Smoking, diet (high-fat, low-fibre, deficient in fruit and vegetables), obesity, lack of physical exercise and exposure to sunlight.
    54. Smoking, high blood pressure, high blood cholesterol levels, obesity, diet (high salt levels, high levels of saturated fats, low levels of dietary fibre) and lack of physical activity (aerobic exercise).
    55. Giving up or not taking up smoking; avoiding becoming overweight, reducing salt intake in the diet, taking regular aerobic exercise, keeping alcohol consumption within safe limits and increasing the intake of dietary fibre and antioxidants in the diet.
    56. A correlation occurs when a change in one of two variables is reflected by a change in the other variable. A correlation does not mean there is a causal link. A causal relationship between two factors means there is a causal link between them, i.e. one factor causes the other.

    Heart structure and function and the biology of heart disease
    57. (a) Right atrium, left atrium, right ventricle and left ventricle.
    (b) Vena cava and pulmonary vein.
    (c) Pulmonary artery and aorta.
    (d) Atrioventricular valves situated between the atria and the ventricles; and the
    semi-lunar valves situated at the base of the arteries above the openings from
    the two ventricles.
    58. Atrioventricular valves: prevent backflow of blood from ventricles into atria when the ventricles contract.
    Semi-lunar valves prevent backflow of blood from arteries into ventricles when ventricles relax.
    59. (a) When pressure in the atrium is raised above the pressure in the ventricle.
    (b) When pressure in the ventricle is raised above the pressure in the atrium.
    (c) When the pressure in the ventricle is raised above the pressure in the artery
    attached to the ventricle.
    (d) When the pressure in the ventricle falls below the pressure in the artery
    attached to the ventricle.
    60. Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute. Cardiac output = heart rate x stroke volume.
    61. Cardiac muscle is myogenic (its contraction is initiated within the muscle itself, not by a nerve impulse from outside, as is the case with other muscles). It contracts when an excitation wave (wave of electrical activity) stimulates it. The SAN (sinoatrial node/pacemaker) generates the excitation wave and so initiates the contraction of cardiac muscle (heart beat). An excitation wave spreads out from the SAN across both atria to the AVN (atrioventricular node), stimulating both atria to contract simultaneously (atrial systole). A band of non-conducting tissue (atrioventricular ring) between the atria and ventricles prevents the excitation wave crossing into the ventricles. The AVN delays the passage of the excitation wave (to allow the atria to finish contracting and so empty completely before the ventricles start to contract) and then conducts it down the septum along Purkyne fibres, which are collectively called the Bundle of His. The excitation wave is conducted down the septum to the apex of the heart and then spreads upwards and outwards over both ventricles causing both ventricles to contract together (ventricular systole) from the base upwards. This squeezes the blood upwards into the aorta and pulmonary artery, which open from the top of the ventricles.
    62. (a) Atheroma is fatty deposits in the walls of the arteries, often associated with high cholesterol levels in the blood.
    (b) An aneurysm is a balloon-like, blood-filled swelling that forms in an artery at a point weakened by atheroma.
    (c) Thrombosis is the formation of a blood clot within a blood vessel that may lead to a blockage.
    63. Myocardial infarction is a heart attack; it results from the interruption of the blood supply to the heart muscle, causing damage to an area of the heart with consequent disruption of its function.
    64. Low-density lipoprotein (LDL) carries cholesterol from the liver to the tissues, including artery walls, which they infiltrate, leading to the development of atheroma and hence heart disease.
    High-density lipoprotein (HDL) transports cholesterol from tissues to the liver for excretion. It helps to protect arteries against heart disease.

    Lung function and the biology of lung disease
    65. The alveolar membranes.
    66. Inspiration:
    External intercostal muscles contract and internal intercostal muscles relax.
    The ribs are pulled upwards and outwards increasing the volume of the thorax.
    Diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.
    The increased volume of the thorax results in decreased pressure in the lungs.
    Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs.
    Expiration:
    Internal intercostal muscles contract and external intercostal muscles relax.
    Ribs move downwards and inwards, decreasing the volume of the thorax.
    Diaphragm muscles relax, making it return to its upwardly domed position, again decreasing the volume of the thorax.
    The decreased volume of the thorax increases the pressure in the lungs.
    The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.
    During normal, quiet breathing, the recoil of the elastic lungs is the main cause of air being forced out. Only under strenuous conditions, such as exercise, do the various muscles play a part.
    67. Pulmonary ventilation is the total volume of air that is moved into the lungs during one minute.
    Pulmonary ventilation (dm3min-1) = tidal volume (dm3) x ventilation rate (min-1)
    68. Walls of alveoli (and capillaries) are very thin (1 cell thick/flattened cells); therefore diffusion distance is very short.
    Millions of alveoli (and pulmonary capillaries) give a very large total surface area.
    Rich blood supply maintains a concentration gradient.
    Ventilation and circulatory systems ensure that a steep concentration gradient of gases to be exchanged is maintained.
    The distance between alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls.
    Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion.
    69. Mycobacterium tuberculosis (human TB); Mycobacterium bovis (bovine TB).
    70. Persistent cough, tiredness, loss of appetite, loss of weight, fever and coughing up blood.
    71. TB is spread through the air by droplets, released into the air when infected individuals cough, sneeze, laugh or even talk. Bovine TB can be spread from infected cows to humans in milk.
    72. Once the bacterium has been inhaled by someone who is not immune to it, infection follows a particular course:
    Bacteria grow and divide within the upper regions of the lungs where there is plenty of oxygen.
    Body’s immune system responds and white blood cells accumulate at the site of infection to ingest the bacteria.
    Leads to inflammation and enlargement of the lymph nodes that drain that area of the lungs. This is called the primary infection, and usually occurs in children.
    In a healthy person there are few symptoms, if any, and the infection is controlled within a few weeks. Some bacteria remain.
    Many years later, these bacteria re-emerge to cause a second infection of TB. This is called post-primary tuberculosis and typically occurs in adults.
    The bacteria destroys the tissues of the lungs, resulting in cavities and, where the lung repairs itself, scar tissue.
    The sufferer coughs up damaged lung tissue containing bacteria, along with blood. Without treatment the TB spreads to the rest of the body and can be fatal.
    73. Vaccination; drug (antibiotic) treatment; better education about TB, particularly the need to complete all courses of drugs; more and better housing; improved health facilities and treatment; better nutrition to ensure that immune systems are not weakened by poor diet.
    74. Pulmonary fibrosis arises when scars form on the epithelium of the lungs causing them to become irreversibly thickened.
    Main symptoms: shortness of breath, especially when exercising; chronic, dry cough; pain and discomfort in the chest; weakness and fatigue.
    Effect on lung function: Thickening of epithelium provides a short diffusion pathway and loss of elasticity makes ventilation difficult so it is hard to maintain a diffusion gradient across the exchange surface.
    75. Asthma: A chronic illness in which there is resistance to air flow to the alveoli of the lungs as a result of the airways becoming inflamed due to an allergic response to an allergen.
    Main symptoms: Difficulty in breathing; a wheezing sound when breathing; a tight feeling in the chest; coughing.
    Effect on lung function: decreases ability to ventilate lungs by inhalation and so makes it difficult to maintain a diffusion gradient across the exchange surface.
    76. Emphysema: The elastin protein in the alveoli walls becomes permanently stretched; the walls are damaged and break down.
    Main symptoms: Shortness of breath; chronic cough; bluish skin colouration.
    Effect on lung function: decreases the surface area available for gas exchange and decreases the ability to ventilate by exhalation.
    77. Smoking; air pollution; genetic make-up; infections; occupation

    Immunology
    78. Chemical products released by pathogens act as attractants, causing phagocytes to move towards pathogens.
    Phagocytes attach themselves to the surface of the pathogen.
    They engulf the pathogen to form a vesicle, known as a phagosome.
    Lysosomes move towards the vesicle and fuse with it.
    Enzymes within the lysosome break down the pathogen into small soluble products.
    The soluble breakdown products of the pathogen are absorbed into the cytoplasm of the phagocyte.
    79. An antigen is a molecule that triggers an immune response by lymphocytes. Antigens are often proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms or diseased cells (cancer cells).
    80. An antibody is a protein produced by B lymphocytes in response to the presence of the appropriate antigen.
    81. Antibodies are proteins. They are made up of 4 polypeptide chains, one pair of which are long and called heavy chains, the other pair being shorter and called light chains. One light chain is attached to one heavy chain by a disulphide bond and, two disulphide bonds attach the two heavy chains to each other. The amino acid sequence in one part of the antibody is unique and called the variable region, whilst the rest of the antibody is the same in all antibodies and is known as the constant region.








    82. Each variable region of an antibody has a binding site with a specific shape. It binds to the complementary shaped antigen to form an antigen-antibody complex. To help the antibody fit exactly around the antigen, it can change shape by moving as if it had a hinge at the fork of the Y-shape. Each antibody can bind to two separate antigens.
    83. B lymphocytes associated with humoral immunity.
    T lymphocytes associated with cell-mediated immunity.
    84. Cell-mediated immunity is the response of T lymphocytes to antigens that are attached to a body cell, e.g. a body cell infected with a viral pathogen; a cancer cell or cells on transplant tissue.
    85. They produce a protein that makes a hole in a cell-surface membrane. These holes make the cell freely permeable to all substances and the cell dies as a result.
    86. Humoral immunity is the immune response involving antibodies released by B lymphocytes into the blood and tissue fluid (the humours of the body).
    87. Plasma cells secrete antibodies directly. Plasma cells survive for only a few days but make prodigious numbers of antibodies during their brief lifespan. The antibodies destroy the pathogen and any toxins it produces and are therefore responsible for the immediate defence of the body against infection. This is known as the primary immune response.
    88. Memory cells live considerably longer than plasma cells. Memory cells do not produce antibodies directly, but circulate in the blood and tissue fluid. When they encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells. These plasma cells produce the antibodies needed to destroy the pathogen, while the new memory cells circulate in readiness for any future infection. Memory cells, in this way, provide long-term immunity against the original infection. This is known as the secondary immune response. It is both more rapid and of greater intensity than the primary immune response and this ensures that the new infection can be repulsed before it causes any harm, or even any symptoms.
    89. Antigenic variability is when the antigens that a pathogen is made of, and those that they produce, are constantly changing.
    90. The antigens of the measles virus come in a single type and so are quickly identified by memory cells when they invade the body on a subsequent occasion, and so are destroyed during the secondary immune response.
    By contrast, influenza and common cold viruses have over 100 different strains. They exhibit antigenic variability. Any subsequent infections are therefore likely to be caused by different varieties of the pathogen and their antigens will not correspond to the antibodies or the memory cells formed during the primary immune response. With no appropriate memory cells to stimulate antibody production, the only means of overcoming the infection is the primary response. The body reacts as though the infection was a new one and the immune response is much slower. In the meantime, symptoms develop-you suffer from another cold!
    91. A vaccine is a preparation containing antigens, derived from a pathogenic organism, which stimulates an immune response.
    92. Active immunity- individuals are stimulated to produce their own antibodies and such immunity is normally long-lasting.
    Passive immunity- antibodies are introduced from outside rather than being produced by the individual, and such immunity is normally only short-lived.
    93. The individuals themselves are not producing antibodies, and those antibodies introduced are quickly broken down and not replaced.
    94. Monoclonal antibodies are highly specific, purified antibodies that are produced from a clone of cells (derived from a single type of B lymphocyte cell) and are complementary to one antigen,
    95. Separation of a chemical from a mixture; cancer treatment; immunoassay (pregnancy testing kit for e.g.); transplant surgery (to minimise rejection).
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    Here are the answers, soz i was having lunch, and nasira im a girl ..
    Have fun
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    (Original post by smit.)
    biology department


    as level


    preparing for the exam

    unit 1
    (biology and disease)


    fundamental questions
    answers

    cell and microscopes
    1. Cell-surface membrane (to control the movement of substances in and out of the cell). Nucleus (acts as the control centre of the cell - retains the genetic material (dna) responsible for the synthesis of proteins). Mitochondrion (site of cell respiration and the production of atp). Lysosome (to digest worn out organelles and material ingested by phagocytic cells). Ribosome (site of protein synthesis). Rer (transport of protein throughout the cell). Ser (synthesis, storage and transport of lipids and carbohydrates). Golgi apparatus (to add carbohydrate to protein and form glycoprotein and secrete it from the cell). Microvilli (to increase the surface area of the membrane for more efficient absorption by epithelial cells lining the small intestine).
    2. Magnification is how many times bigger the image is compared to the original object. Resolution is the minimum distance apart that two objects can be in order for them to appear as separate items (it is the ability to distinguish between very small objects that are very close together).
    3. Because it uses a beam of electrons that has a shorter wavelength than light.
    4. A tem transmits a beam of electrons through a thin section of the specimen giving 2-d images of the internal structure of cells and organelles. A sem directs a beam of electrons onto the surface of a specimen and scans them back and forth across the surface to give views of surface structures and produces 3-d images.

    Fractionation and centrifugation
    5. It is the process by which cells are broken up and the different organelles they contain are separated out.
    6. The homogenate is spun in an ultracentrifuge at slow speed for a short time to sediment the heaviest organelles (nuclei). The supernatant is then spun at a higher speed for a longer time to sediment the next heaviest organelle (mitochondria). The process is continued and at each increase in speed, the next heaviest organelle is sedimented and separated out (lysosomes, followed by ribosomes and then broken/fragmented membranes).
    7. (a) to minimise enzyme (lysosome) activity and so prevent damage to the organelles.
    (b) to prevent organelles either bursting or shrinking as a result of osmotic gain of loss of water.
    (c) to maintain a constant ph (for optimum activity of organelles)

    plasma membranes and lipids
    8. Consists of two layers of phospholipid (lipid bilyer), arranged ‘tail’ to ‘tail’ and interspersed with globular proteins, some of which span the entire lipid bilayer, and some of which only partially penetrate one or other phospholipid layer.
    9. See page 3.


    H o h o
    h c oh ho c r1 h c o c r1
    o o + 3h2o
    h c oh + ho c r2 h c o c r2
    o o
    h c oh ho c r3 h c o c r3
    h h

    10. A saturated fatty acid has no c-c double bonds, whereas, an unsaturated fatty acid has one or more c-c double bonds in its hydrocarbon ‘tail’.
    11. A phospholipid has 2 fatty acid molecules and one phosphate molecule condensed to a glycerol molecule, whereas a triglyceride has only 3 fatty acids molecules condensed to a glycerol molecule.
    12. Add 2/3 cm3 of ethanol to 1 cm3 of lipid sample to be tested and shake to dissolve. Then add 2/3 cm3 of water to the mixture and shake gently. A cloudy-white (milky) colour indicates the presence of lipid.
    13. (1) transport of large, water-soluble molecules and charged ions (carrier proteins). (2) act as receptors for hormones (glycoproteins). (3) serve as recognition sites for antigens (glycolipids).

    Diffusion, osmosis and active transport
    14. (1) diffusion is the passive movement of substances down a concentration gradient. (2) facilitated diffusion is the passive movement of substances down a concentration gradient through an intrinsic membrane protein. (3) osmosis is the diffusion of water from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane.
    15. Rate of diffusion is proportional to surface area x difference in concentration
    thickness of exchange surface
    16. Similar – in all, substances move down concentration gradients and they are all passive processes (requiring no atp/respiratory energy). Differ – many different substances are transported by diffusion and facilitated diffusion, but only water is transported by osmosis. Facilitated diffusion requires a membrane protein, but diffusion and osmosis do not.
    17. Active transport is the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient) through an intrinsic membrane protein using atp (respiratory) energy.
    18. (a) active transport requires atp (is an active process), a membrane protein and takes place against a concentration gradient, whereas, diffusion is a passive process that takes place down a concentration gradient and requires no membrane protein. (b) both active transport and facilitated diffusion require a membrane protein but, active transport requires atp energy and takes place against a concentration gradient whereas, facilitated diffusion is passive (no atp required) and moves substances down their concentration gradients.
    Proteins
    19. H r o h r o h r o r o
    h n c c oh + h n c c oh h n c c n c c oh + h2o
    h h h h h

    20. Condensation- formation of a bond by elimination of water; hydrolysis- breaking a bond by addition of water.
    21. Primary structure: The sequence of amino acids joined together by peptide bonds.
    Secondary structure: The coiling and folding of the polypeptide chain due to hydrogen bonding between the peptide groups.
    Tertiary structure: The further coiling and folding of the polypeptide chain into its final 3-d shape due to a variety of bonds between the r-groups, including hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bonds.
    Quaternary structure: The degree of polymerisation of the protein (how many polypeptide chains it has linked together by a variety of bonds (as in tertiary structure).
    22. Add an equal volume of dilute sodium hydroxide solution to the test sample at
    room temperature. Add a few drops (5 drops to 1 cm3) of dilute copper (ii)
    sulphate solution and mix it gently. A mauve/violet/purple colour indicates the
    presence of protein. If no protein is present, the solution remains blue.
    23. Globular proteins such as enzymes, hormones, antibodies, albumins & globulins.
    Fibrous proteins such as collagen, myosin and keratin.

    Enzyme action and properties
    24. It is the energy required to start a reaction.
    25. They lower the activation energy (split the reaction into stages, so providing alternative, lower energy pathways by which the reaction can occur).
    26. (a) (b) (c) (d)



    27. Competitive inhibitor: Is similar in shape to the substrate and competes with it for the active site of the enzyme; it binds to the active site and so prevents substrate binding and forming enzyme/substrate complex.
    Non-competitive inhibitor: Not similar in shape to substrate and binds to the
    enzyme at a site which is not the active site. Binding alters the shape of the
    active site and so prevents the substrate binding to it and forming
    enzyme/substrate complex.
    28. Denaturation: Hydrogen bonds (and ionic bonds) in enzyme molecule have been broken resulting in the loss of tertiary structure and change in shape of the active site so that substrate can no longer bind to active site of enzyme and no enzyme/substrate complex can be formed.

    Carbohydrate biochemistry
    29. A polymer is a large molecule made up of repeating smaller molecules (subunits) called monomers. Polypeptides (proteins), polysaccharides and polynucleotides are important examples.

    30. H h


    ho oh

    31.
    H h h h h h h h
    +
    ho oh ho oh ho o oh

    32. (a) glucose and fructose; (b) glucose and galactose.
    33. (a) benedict’s’ test: Mix together equal volumes (1cm3 each) of test solution and benedict’s reagent and heat them in a boiling water bath for 2 mins. A brick red precipitate/colour indicates the presence of a reducing sugar such as glucose. If no glucose is present, the solution remains blue.
    (b) sucrose test: First show that a benedict’s test is negative. Then hydrolyse
    1 cm3 of sucrose solution by boiling it in 1cm3 of dilute hcl for 5 mins. Neutralise the solution and repeat the benedict’s test, which should now give a positive result.
    (c) “iodine solution” test: Add 2 drops of potassium iodide solution to the sample
    and a blue-black colour indicates the presence of starch. If no starch is present
    the solution remains yellow.

    Carbohydrate digestion
    34. Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules. It is necessary in order to absorb food molecules into the body (through cell membranes).
    35. Glucose.
    36. Amylase from the salivary glands and pancreas, followed by maltase located in the cell surface membranes of the epithelial cells lining the small intestine.
    37. Place a solution of carbohydrase enzyme in a well in the centre of the starch agar. Incubate the plate at 25oc for 24 hours. Flood the starch agar with potassium iodide solution. Measure the diameter of the clear zone around the wells (where starch has been digested) as a measure of enzyme activity.
    38. Lactose intolerance is caused by the inability to produce any/sufficient lactase enzyme and so the inability to digest all the lactose (milk sugar) in the diet. Microorganisms in the large intestine convert the undigested lactose (respiration) into gas, which accumulates, and causes bloating, nausea, diarrhoea and cramps.

    Carbohydrate absorption
    39. Very long: Large surface area for increased rate of absorption; food takes a long time to pass through and so this allows plenty of time for absorption of digested food products.
    Highly folded with villi: Increased surface area leads to increased rate of diffusion.
    Microvilli line the epithelial cells of the villi increasing surface area.
    Villi lined with a single layer of epithelial cells making them thin-walled. This gives short diffusion pathway.
    Villi are well supplied with blood vessels and lacteals, so that absorbed molecules can be carried away and hence maintain a diffusion gradient.
    Villi are able to move and so create currents in the lumen contents, helping to maintain diffusion gradients.
    40. Facilitated diffusion and active co-transport.

    Disease and pathogens and cholera
    41. A pathogen is a microorganism that causes disease e.g. Bacteria, viruses and fungi.
    42. The digestive system; the gas exchange system.
    43. (1) by damaging host tissues. (2) by producing toxins.
    44. (1) keratinised skin. (2) mucus and cilia (3) acid (4) lysozyme in tears
    45.



    46. Vibrio cholerae. It is a rod-shaped bacterium.
    47. Transmitted by the ingestion of water, or more rarely food, that has been contaminated with faecal material containing the pathogen.
    48. Cholera starts with severe muscle and stomach cramps. Vomiting, fever and then diarrhoea (watery faeces) follows. So much fluid may be lost that the patient becomes very dehydrated and then their circulatory system fails.
    49. The cholera bacterium propels itself through the mucus lining of the intestinal wall. There it produces a toxic protein, part of which binds to the carbohydrate receptors on the cell-surface membrane of the epithelial cells, and part of which enters the epithelial cells where it causes ion channels in the cell-surface membrane to open. This allows chloride ions to flood out of the cells into the lumen of the intestine. This loss of chloride ions from the epithelial cells raises the water potential inside these cells and lowers the water potential in the lumen and so causes water to diffuse from the cells into the lumen by osmosis. In addition, the loss of chloride ions from the epithelial cells establishes a concentration gradient down which ions diffuse into the epithelial cells from the surrounding tissues and blood. This in turn, establishes a water potential gradient that causes water to diffuse by osmosis from the blood and other tissues into the intestine. It is this loss of water from the blood and other tissues, into the intestine that causes the symptoms of cholera, namely, severe diarrhoea and dehydration.
    50. Clean, uncontaminated water supplies/water treatment/chlorination/proper sanitation and sewage treatment/personal hygiene (washing hands after using the toilet)/proper food hygiene.
    51. Ort is a means of treating dehydration involving giving, by mouth, a balanced solution of salts and glucose that stimulates the gut to reabsorb water. Ors contains water, sodium ions, glucose, potassium and other electrolytes such as chloride and citrate ions. Glucose stimulates the uptake of sodium ions from the intestine and provides energy, as it is a respiratory substrate. The sodium ions replace those lost from the body and encourage the use of the sodium-glucose transporter proteins to absorb more sodium ions. As sodium ions are absorbed, so the water potential of the cells falls and water enters the cells by osmosis.

    Lifestyle, correlation and causal relationships
    52. In the context of health, risk is a measure of the probability that damage to health will occur as a result of a given hazard.
    53. Smoking, diet (high-fat, low-fibre, deficient in fruit and vegetables), obesity, lack of physical exercise and exposure to sunlight.
    54. Smoking, high blood pressure, high blood cholesterol levels, obesity, diet (high salt levels, high levels of saturated fats, low levels of dietary fibre) and lack of physical activity (aerobic exercise).
    55. Giving up or not taking up smoking; avoiding becoming overweight, reducing salt intake in the diet, taking regular aerobic exercise, keeping alcohol consumption within safe limits and increasing the intake of dietary fibre and antioxidants in the diet.
    56. A correlation occurs when a change in one of two variables is reflected by a change in the other variable. A correlation does not mean there is a causal link. A causal relationship between two factors means there is a causal link between them, i.e. One factor causes the other.

    Heart structure and function and the biology of heart disease
    57. (a) right atrium, left atrium, right ventricle and left ventricle.
    (b) vena cava and pulmonary vein.
    (c) pulmonary artery and aorta.
    (d) atrioventricular valves situated between the atria and the ventricles; and the
    semi-lunar valves situated at the base of the arteries above the openings from
    the two ventricles.
    58. Atrioventricular valves: Prevent backflow of blood from ventricles into atria when the ventricles contract.
    Semi-lunar valves prevent backflow of blood from arteries into ventricles when ventricles relax.
    59. (a) when pressure in the atrium is raised above the pressure in the ventricle.
    (b) when pressure in the ventricle is raised above the pressure in the atrium.
    (c) when the pressure in the ventricle is raised above the pressure in the artery
    attached to the ventricle.
    (d) when the pressure in the ventricle falls below the pressure in the artery
    attached to the ventricle.
    60. Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute. Cardiac output = heart rate x stroke volume.
    61. Cardiac muscle is myogenic (its contraction is initiated within the muscle itself, not by a nerve impulse from outside, as is the case with other muscles). It contracts when an excitation wave (wave of electrical activity) stimulates it. The san (sinoatrial node/pacemaker) generates the excitation wave and so initiates the contraction of cardiac muscle (heart beat). An excitation wave spreads out from the san across both atria to the avn (atrioventricular node), stimulating both atria to contract simultaneously (atrial systole). A band of non-conducting tissue (atrioventricular ring) between the atria and ventricles prevents the excitation wave crossing into the ventricles. The avn delays the passage of the excitation wave (to allow the atria to finish contracting and so empty completely before the ventricles start to contract) and then conducts it down the septum along purkyne fibres, which are collectively called the bundle of his. The excitation wave is conducted down the septum to the apex of the heart and then spreads upwards and outwards over both ventricles causing both ventricles to contract together (ventricular systole) from the base upwards. This squeezes the blood upwards into the aorta and pulmonary artery, which open from the top of the ventricles.
    62. (a) atheroma is fatty deposits in the walls of the arteries, often associated with high cholesterol levels in the blood.
    (b) an aneurysm is a balloon-like, blood-filled swelling that forms in an artery at a point weakened by atheroma.
    (c) thrombosis is the formation of a blood clot within a blood vessel that may lead to a blockage.
    63. Myocardial infarction is a heart attack; it results from the interruption of the blood supply to the heart muscle, causing damage to an area of the heart with consequent disruption of its function.
    64. Low-density lipoprotein (ldl) carries cholesterol from the liver to the tissues, including artery walls, which they infiltrate, leading to the development of atheroma and hence heart disease.
    High-density lipoprotein (hdl) transports cholesterol from tissues to the liver for excretion. It helps to protect arteries against heart disease.

    Lung function and the biology of lung disease
    65. The alveolar membranes.
    66. Inspiration:
    External intercostal muscles contract and internal intercostal muscles relax.
    The ribs are pulled upwards and outwards increasing the volume of the thorax.
    Diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.
    The increased volume of the thorax results in decreased pressure in the lungs.
    Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs.
    Expiration:
    Internal intercostal muscles contract and external intercostal muscles relax.
    Ribs move downwards and inwards, decreasing the volume of the thorax.
    Diaphragm muscles relax, making it return to its upwardly domed position, again decreasing the volume of the thorax.
    The decreased volume of the thorax increases the pressure in the lungs.
    The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.
    During normal, quiet breathing, the recoil of the elastic lungs is the main cause of air being forced out. Only under strenuous conditions, such as exercise, do the various muscles play a part.
    67. Pulmonary ventilation is the total volume of air that is moved into the lungs during one minute.
    Pulmonary ventilation (dm3min-1) = tidal volume (dm3) x ventilation rate (min-1)
    68. Walls of alveoli (and capillaries) are very thin (1 cell thick/flattened cells); therefore diffusion distance is very short.
    Millions of alveoli (and pulmonary capillaries) give a very large total surface area.
    Rich blood supply maintains a concentration gradient.
    Ventilation and circulatory systems ensure that a steep concentration gradient of gases to be exchanged is maintained.
    The distance between alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls.
    Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion.
    69. Mycobacterium tuberculosis (human tb); mycobacterium bovis (bovine tb).
    70. Persistent cough, tiredness, loss of appetite, loss of weight, fever and coughing up blood.
    71. Tb is spread through the air by droplets, released into the air when infected individuals cough, sneeze, laugh or even talk. Bovine tb can be spread from infected cows to humans in milk.
    72. Once the bacterium has been inhaled by someone who is not immune to it, infection follows a particular course:
    Bacteria grow and divide within the upper regions of the lungs where there is plenty of oxygen.
    Body’s immune system responds and white blood cells accumulate at the site of infection to ingest the bacteria.
    Leads to inflammation and enlargement of the lymph nodes that drain that area of the lungs. This is called the primary infection, and usually occurs in children.
    In a healthy person there are few symptoms, if any, and the infection is controlled within a few weeks. Some bacteria remain.
    Many years later, these bacteria re-emerge to cause a second infection of tb. This is called post-primary tuberculosis and typically occurs in adults.
    The bacteria destroys the tissues of the lungs, resulting in cavities and, where the lung repairs itself, scar tissue.
    The sufferer coughs up damaged lung tissue containing bacteria, along with blood. Without treatment the tb spreads to the rest of the body and can be fatal.
    73. Vaccination; drug (antibiotic) treatment; better education about tb, particularly the need to complete all courses of drugs; more and better housing; improved health facilities and treatment; better nutrition to ensure that immune systems are not weakened by poor diet.
    74. Pulmonary fibrosis arises when scars form on the epithelium of the lungs causing them to become irreversibly thickened.
    Main symptoms: Shortness of breath, especially when exercising; chronic, dry cough; pain and discomfort in the chest; weakness and fatigue.
    Effect on lung function: Thickening of epithelium provides a short diffusion pathway and loss of elasticity makes ventilation difficult so it is hard to maintain a diffusion gradient across the exchange surface.
    75. Asthma: A chronic illness in which there is resistance to air flow to the alveoli of the lungs as a result of the airways becoming inflamed due to an allergic response to an allergen.
    Main symptoms: Difficulty in breathing; a wheezing sound when breathing; a tight feeling in the chest; coughing.
    Effect on lung function: Decreases ability to ventilate lungs by inhalation and so makes it difficult to maintain a diffusion gradient across the exchange surface.
    76. Emphysema: The elastin protein in the alveoli walls becomes permanently stretched; the walls are damaged and break down.
    Main symptoms: Shortness of breath; chronic cough; bluish skin colouration.
    Effect on lung function: Decreases the surface area available for gas exchange and decreases the ability to ventilate by exhalation.
    77. Smoking; air pollution; genetic make-up; infections; occupation

    immunology
    78. Chemical products released by pathogens act as attractants, causing phagocytes to move towards pathogens.
    Phagocytes attach themselves to the surface of the pathogen.
    They engulf the pathogen to form a vesicle, known as a phagosome.
    Lysosomes move towards the vesicle and fuse with it.
    Enzymes within the lysosome break down the pathogen into small soluble products.
    The soluble breakdown products of the pathogen are absorbed into the cytoplasm of the phagocyte.
    79. An antigen is a molecule that triggers an immune response by lymphocytes. Antigens are often proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms or diseased cells (cancer cells).
    80. An antibody is a protein produced by b lymphocytes in response to the presence of the appropriate antigen.
    81. Antibodies are proteins. They are made up of 4 polypeptide chains, one pair of which are long and called heavy chains, the other pair being shorter and called light chains. One light chain is attached to one heavy chain by a disulphide bond and, two disulphide bonds attach the two heavy chains to each other. The amino acid sequence in one part of the antibody is unique and called the variable region, whilst the rest of the antibody is the same in all antibodies and is known as the constant region.


    82. Each variable region of an antibody has a binding site with a specific shape. It binds to the complementary shaped antigen to form an antigen-antibody complex. To help the antibody fit exactly around the antigen, it can change shape by moving as if it had a hinge at the fork of the y-shape. Each antibody can bind to two separate antigens.
    83. B lymphocytes associated with humoral immunity.
    T lymphocytes associated with cell-mediated immunity.
    84. Cell-mediated immunity is the response of t lymphocytes to antigens that are attached to a body cell, e.g. A body cell infected with a viral pathogen; a cancer cell or cells on transplant tissue.
    85. They produce a protein that makes a hole in a cell-surface membrane. These holes make the cell freely permeable to all substances and the cell dies as a result.
    86. Humoral immunity is the immune response involving antibodies released by b lymphocytes into the blood and tissue fluid (the humours of the body).
    87. Plasma cells secrete antibodies directly. Plasma cells survive for only a few days but make prodigious numbers of antibodies during their brief lifespan. The antibodies destroy the pathogen and any toxins it produces and are therefore responsible for the immediate defence of the body against infection. This is known as the primary immune response.
    88. Memory cells live considerably longer than plasma cells. Memory cells do not produce antibodies directly, but circulate in the blood and tissue fluid. When they encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells. These plasma cells produce the antibodies needed to destroy the pathogen, while the new memory cells circulate in readiness for any future infection. Memory cells, in this way, provide long-term immunity against the original infection. This is known as the secondary immune response. It is both more rapid and of greater intensity than the primary immune response and this ensures that the new infection can be repulsed before it causes any harm, or even any symptoms.
    89. Antigenic variability is when the antigens that a pathogen is made of, and those that they produce, are constantly changing.
    90. The antigens of the measles virus come in a single type and so are quickly identified by memory cells when they invade the body on a subsequent occasion, and so are destroyed during the secondary immune response.
    By contrast, influenza and common cold viruses have over 100 different strains. They exhibit antigenic variability. Any subsequent infections are therefore likely to be caused by different varieties of the pathogen and their antigens will not correspond to the antibodies or the memory cells formed during the primary immune response. With no appropriate memory cells to stimulate antibody production, the only means of overcoming the infection is the primary response. The body reacts as though the infection was a new one and the immune response is much slower. In the meantime, symptoms develop-you suffer from another cold!
    91. A vaccine is a preparation containing antigens, derived from a pathogenic organism, which stimulates an immune response.
    92. Active immunity- individuals are stimulated to produce their own antibodies and such immunity is normally long-lasting.
    Passive immunity- antibodies are introduced from outside rather than being produced by the individual, and such immunity is normally only short-lived.
    93. The individuals themselves are not producing antibodies, and those antibodies introduced are quickly broken down and not replaced.
    94. Monoclonal antibodies are highly specific, purified antibodies that are produced from a clone of cells (derived from a single type of b lymphocyte cell) and are complementary to one antigen,
    95. Separation of a chemical from a mixture; cancer treatment; immunoassay (pregnancy testing kit for e.g.); transplant surgery (to minimise rejection).


    Here are the answers to the questions posted above! Hope you find them useful, and sorry if you can't see some of the answers :/ the alignment kinda messed up when i copied and pasted them! Good luck to everyone though, all the best for tomorrow!
    guessing ur from qe??
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    Learn the mark schemes and recognise the type of questions and answers they want and just spit out the bull**** and don't be surprised when you get that A.

    In the textbook it even says "The factual content of these sections is not required for examination purposes," this is obviously about the How Science Works questions, but they equate to alot of marks in Biology papers. Its all about technique.
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    (Original post by sanaindaclub)
    Can someone please explain the mosaic part of the fluid - mosaic model of the phospholipid bilayer.
    It's studded with many different size and shape proteins in the same way as stones in a mosaic.
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    (Original post by Ramin Gorji)
    Blood clot prevents oxygen and glucose reaching the Muscle of the heart from the cornory artery. Muscle will not be able to respire and will stop beating.
    Correct. That would be enough to get all three marks.
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    Can I have the answers via PM please thanks
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    INSANITY514 - just scroll up x
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    Still looking for the disadvantages of SEMs & TEMs... Anyone?
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    (Original post by nosta)
    Still looking for the disadvantages of SEMs & TEMs... Anyone?
    Disadvantage:

    -Has to be in a vacuum, thus organic specimens have to be dead.
    - Can't produce colour image.

    TEMs: Can't produce 3d image; Specimen has to be very thin.

    SEM: Has a lower resolution than a TEM.
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    (Original post by nosta)
    Still looking for the disadvantages of SEMs & TEMs... Anyone?
    Both create artefacts too due to preparation.
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    (Original post by nosta)
    Still looking for the disadvantages of SEMs & TEMs... Anyone?
    advantages of TEM:
    - has a high resolution

    disadvantages of TEM:
    - specimen has to be cut reallyyyy thin so electrons can pass through
    - complicated staining process has to be carried out
    - image can only be seen in one/two colours
    - final image is 2D
    - final image may contain artefacts due to staining process
    - has to be carried out in a vacuum due to beam of electrons

    advantages of SEM:
    - produces a 3D image
    - specimen does not have to be extremely thin

    disadvantages of SEM:
    - has a low resolution sompared to TEM
    - also has to be carried out in a vacuum


    i think thats all??
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    Are you guys answering questions together? IF SO YAY
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    I am so stressed about this exam :/ I have tried to revise as much as I can but my brain wont take in anymore, I am really worried.

    If immunology comes up as on of the longer q's, possible question they could ask?
 
 
 
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