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Why does water hydrogen-bond with each other extensively?
Because the oxygen is more electronegative than hydrogen and therefore oxygen becomes slightly negative and hydrogen slightly positive. It is a polar molecule. The hydrogen bonds form between the slightly negative oxygen end/pole with a slightly positive hydrogen end/pole of another water molecule.
Why does it take 100 degrees C to evaporate water when H2S is gas at room temp?
Because of hydrogen bonds between water molecules.
Why is water thermally stable - why does it take a lot of energy to increase the temp of water? Why is this important?
- Because hydrogen bonds in liquid water restrict the movement of the molecules.
- Important because large bodies of water, such as lakes and oceans, maintain a constant/stable temperature, even when the atmospheric temp changes suddenly.
What affect does evaporation of water have on its surface?
- As evaporation of water uses up a relatively large amount of energy, when they do evaporate, it can cool surfaces by removing heat.
- eg. Many land-based organisms use evaporation as a cooling mechanism (eg. panting and sweating)
What biological importance does water's properties when freezing have on the environment?
- As water in solid form is less dense than in liquid form (which is unusual, because of hydrogen bonding), so ice floats on water, insulating the water below making it less likely to freeze. - Allows living organisms to survive the winter & live under ice.
- eg. Polar bears rely on floating ice packs to survive; lakes tend not to freeze completely, so as not killing aquatic organisms.
What biological importance does water's cohesive property have?
- Water molecules stick to each other, creating surface tension because of hydrogen bonding.
- Important because cohesion makes long, thin water columns very strong and difficult to break. eg. transport of water up xylem
Describe water's importance as a solvent in metabolic reactions.
Metabolic process in all organisms rely on chemicals being able to react together in solution. eg. respiration and photosynthesis in cytoplasm (made up of 70-95% water)
Describe water as a useful transport medium in organisms.
- Water remains liquid over a large temperature range.
- Movement of materials around organisms (both in cell & in multicellular organisms) requires a liquid transport medium.
- eg. blood in animals and vascular tissues in plants use water as transport medium.
Water's importance in metabolic reactions other than being a solvent?
- Can take part as a reactant in some chemical processes.
- eg. Water molecules used in hydolysis & photosynthesis.
Name cellular process involved in moving liquids into the cell.
- (exo for moving it outside. )
How do you test for starch? Results?
- Add a solution of iodine (in potassium iodide)
- If starch is present, solution changes colour from yellow-brown to blue-black.
How do you test for reducing sugars? Results?
- Benedict's test - heat sample (80oC water bath) with Benedict's solution (alkaline copper sulfate).
- Solution changes colour from Blue to Orange-red.
- Orange-red is precipitate because it comes out of solution.
How do you test for a non-reducing sugar? Results?
- eg. Sucrose (glycosidic bond between glucose & fructose).
- First make sure there are no reducing sugars by doing Benedict's test.
- Boil sample with HCl (hydrolyses sucrose into monosaccharides)
- Cool solution and neutralise it (by NaCO3 etc. alkali)
- Carry out reducing sugar test again.
- Positive result will mean sucrose (non-reducing sugar) was present in original sample (because now monosaccharides fructose & glucose present)
What carbohydrate molecules are reducing sugars?
ALL monosaccharides, and most disaccharides.
How do you test for proteins? Result?
- Biuret test - add biuret reagent to a sample.
- Pale blue to Lilac.
- (Because peptides bonds react with the chemicals)
How do you test for the presence of lipids? Results?
- Use ethanol emulsion test. Mix sample with ethanol (lipids soluble in alcohols) then pour into water in another test tube.
- If lipid is present, a Cloudy White Emulsion will form near top of water - lipid comes out of solution and becomes dispersed as tiny droplets in water.
Describe how a quantitative test can also be done using Benedict's solution.
- The more reducing sugar present, the more precipitate formed, and more Benedict's solution (copper sulfate) used up.
- Filter out precipitates and measure concentration of remaining solution can be measured by using a colorimeter (from this you can estimate conc of reducing sugar in original sampl).
Coloremeter readings do not tell us how much reducing sugar is present, only how much relative to other samples. So to quantify (measure) the amount what needs to be made and how?
- A calibration curve must be made.
- Prepare it by taking range of known conc reducing sugar and carrying out Benedict's test and colorimeter readings then plotting this on a graph against reducing sugar conc.
- You are now ready to take a reading from an unknown sample and estimate its reducing sugar conc by reading off the graph.
What is an assay?
A way of finding out the concentration of a substance by comparing it with known standards like using a calibration curve for glucose conc.
What do you need to make sure you do when carrying out tests on the calorimeter?
- To zero the divice between each reading with an appropriate blank sample (eg. distilled water).
- Use transparent cuvettes to test solution (handle with care to make sure it is completely transparent. eg. no fingerprints)