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AS Biology – Biological Molecules & Nucleic Acids
Biochemistry Lesson 1
Biochemistry = the study of the molecules that make up living things.
Organic = compounds of carbon, e.g. proteins, fats, carbohydrates & nucleic
acids
Inorganic = everything else, e.g. water & minerals (most important in body)
Dipolar = containing both positive and negative charges.
Condensation reaction = water is removed from the reaction (produced)
Hydrolysis reaction = water is used in the reaction to split bonds
Hydrogen bonds
How they work:
Water molecules consist of 2 hydrogen molecules covalently bonded to 1
oxygen molecule.
The molecules are slightly charged because the oxygen nucleus pulls the
shared electrons away from the hydrogen nuclei, giving the oxygen nuclei
a δ- charge, and the hydrogen nuclei a δ+ charge.
The polarity of water causes attraction between water molecules. This
force of attraction is called a hydrogen bond.
Hydrogen bonds are weak on their own but strong together.
Hydrogen bonds are the reason why water has a cohesive property meaning
molecules of water seem to ‘stick’ together.
Properties of water
Very high specific heat capacity
Meaning it takes a lot of energy to change the temperature of water.
The temperature of water tends not to fluctuate rapidly. This allows water to act
as a habitat for aquatic organisms. They’d not be able to function if the
temperature changed rapidly as enzymes could denature. Less energy used on
temperature control
Ice is less dense than liquid water, so it floats. As well as acting as a habitat for
other organisms, it also insulates the water below and prevents it from freezing.
Therefore, organisms can continue to live under the ice.
High density
,Means ice can float on top, allowing the water underneath to be insulated and
support life.
It also allows organisms to float. New habitat on the ice.
Very high latent heat of vaporisation
Meaning it takes a large amount of heat energy to evaporate water.
This allows organisms to cool themselves without losing a lot of water, e.g.
sweating (heat energy is used to evaporate water from the surface of the skin)
Evaporation is efficient.
Good Solvent
Meaning lots of substances can dissolve in water.
Because it’s such a good solvent, it can be used to transport substances, e.g. in
the blood plasma. The blood plasma contains a large number of dissolved
substances, e.g. glucose.
Water is also used to transport substances in xylem cells, e.g. magnesium.
Water is an excellent medium for metabolic reactions. It’s also able to dilute
toxic substances, e.g. urea and dissolve food for organisms, e.g. oxygen in
water.
How? The water molecules are attracted to the slightly charged particles of the
solute. This disrupts the solute molecules, so they are dissolved.
Cohesive
Due to the hydrogen bonds between water molecules, molecules of water are
attracted.
Cohesion allows long columns of water to travel in xylem tubes, making water
ideal as a transport medium in plants.
Cohesion also causes surface tension where water meets air. It allows the
surface of water to act as a habitat for insects such as pond skaters. Organisms
can hunt for food too.
Metabolic reactions
Water is a reactant in many different reactions, including hydrolysis reactions
and photosynthesis. Water is transparent, allowing photosynthesis below the
surface.
Using a colourimeter to quantify reducing sugar
concentration
, Because the Benedict’s test only gives an estimate for the amount of reducing
sugar, it’s semi-quantitative.
Reducing sugars such as glucose donate an electron to another molecule.
The greater the concentration of reducing sugar, the less blue the concentration
will be.
The change in blueness is hard to know using the eye. So, we quantify the
blueness of the solution by using a machine called a colourimeter.
How it works:
Filter the solid copper oxide
Colourimeter shines a light towards the solution and is detected by a
photoelectric cell.
We shine red light as it’s the complementary colour to blue (opposite end
of spectrum)
More red light will be transmitted if the solution is less blue.
Valid = control variables AND does investigation test what is intended?
Quantitative test:
Take a series of known concentrations of reducing sugar
Add the same volume of unknown liquid to test tube 7, e.g. 5cm3
Mix the tubes and place them in a thermostatically controlled water bath
for 2 minutes to allow glucose to react with Benedict’s, producing a red
precipitate.
Filter the liquid to remove the solid particles.
Plot a calibration curve (of % transmission and glucose concentration)
Use the graph to read off concentration of reducing sugar.
Summary of process to measure change in reducing sugar over time:
1. Take samples at a range of times. Remove the same volume each time.
2. Heat with excess Benedict’s. Colour will change.
3. Filter precipitate.
4. Calibrate a colourimeter using water. Use a red filter.
5. Read transmission or absorbance. More transmission = less sugar.
6. Using known concentrations of the sugar, plot a calibration curve.
7. Plot transmission against reducing sugar concentration.
8. Use graph to read off reducing sugar concentration.
For a non-reducing sugar:
#1 Add hydrochloric acid and heat.
#2 Cool the solution and add sodium hydrogen carbonate to neutralise.
#3 Test for reducing sugar.
Carbohydrates
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