AS Biology Handwritten Notes 2015


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#140 Artificial selection

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Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock. - selective pressure: humans - individuals with desirable features are chosen to interbreed = selective breeding - some of these desirable alleles are passed onto offspring - offspring with the most desirable features are chosen to interbreed - this is repeated over many generations Over many generations, alleles deemed desirable by the breeder increase in frequency, while the 'disadvantageous' ones may completely disappear over time. Dairy cattle - desirable features: docility, fast growth rates, high milk yield - cows with desirable features are chosen to interbreed, and so are their offspring. This is repeated over many generations. Bulls cannot be assessed for milk production as this is a sex-limited trait. Therefore, progeny testing is used: the performance of the bull's femail offpsirng is looked at to see whether or not t...
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#96 Using respirometers

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A respirometer is a device used to measure the rate of respiration of a living organism by measuring its rate of exchange of oxygen and/or carbon dioxide . They allow investigation into how factors such as age, chemicals or the effect of light affect the rate of respiration. There are various different types of respirometer. One type is shown in the diagram. Using a respirometer to measure the rate of uptake of oxygen The organisms to be investigated are placed in one tube, and non-living material of the same mass in the other tube. Soda lime is placed in each tube, to absorb all carbon dioxide. Cotton wool prevents contact of the soda lime with the organisms. Coloured fluid is poured into the reservoir of each manometer and allowed to flow into the capillary tube. It is essential that there are no air bubbles. You must end up with exactly the same quantity of fluid in the two manometers. Two rubber bungs are now taken, fitted with tubes as shown in the diagram. Close the spring clips...
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#41 Transport In phloem

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The movement of substances in phloem tissue is called translocation . The main substances that are moved are sucrose and amino acids , which are in solution in water. These substances have been made by the plant and are called assimilates . Phloem tissue Phloem tissue contains cells called sieve tube elements . Unlike xylem vessel elements, these are living cells and contain cytoplasm and a few organelles but no nucleus. Their walls are made of cellulose. A companion cell is associated with each sieve tube element. Sources and sinks  Vascular plants produce nutrients such as sucrose in their leaves. These nutrients must then be transported to the rest of the shoot or to the root tips, where growth occurs. The leaves are referred to as the source, and the shoot and root tips - sink. A source is an organ that produces more sugar than it requires. That's where assimilates enter the phloem.  A sink is an organ that consumes sugar for its own growth and storage. That's where...
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