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Testing for reducing sugars:
-All monosaccharides as well as some disaccharides are reducing sugars, and are therefore able to donate e- (reduce) specific reagents.
-By using Benedict's reagent, a clear colour change from blue to brick red can be seen as an insoluble ppt of copper oxide is formed.
1. Add 2cm3 food sample that is being tested (liquid)
2. Add 2cm3 Benedict's reagent
3. Heat mixture in water bath for 5 mins
*You can also test for non-reducing sugars. The difference to the experimental procedure here is that 2cm3 of dilute HCl must be added to the food sample prior to the BR. This HCl hydrolyses the glycosidic bonds, releasing reducing sugars in the process. NaHCO3 will then be added to neutralise excess acid, then the test is run again.
Test for starch: Iodine stains starch blue-black
Polymers and bond dynamics:
-Polymers are molecules which are made up out of repeating units. These repeating units are called monomers.
-Typically, bonds can form between monomers through condensation (where water is excluded and a bond is formed as a result).
-Bonds are broken by adding water to them. This type of bond breaking is known as hydrolysis.
Monomer example: Nucleotide
Polymer example: DNA
Lipids and triglycerides:
-Lipids are made of carbon, hydrogen and oxygen, similar to carbohydrates. However, they are structurally different and carry differing functions.
FATTY ACID
GLYCEROL
FATTY ACID
FATTY ACID
Saturated: No double-bonds in fatty acid tail. Carbons present are 'saturated' with hydrogen. Found in animal fats. (solid as room temp)
Unsaturated: Double-bonds present in the fatty acid tail. Carbons present are not 'saturated' with hydrogen. Found in plants. (liquid at room temp)
Properties of triglycerides:
-Low mass:energy ratio makes them a good storage molecule. Lots of energy stored in small volume.
-High ratio of C:H to C:C bonds, so large amount of energy can be released.
-Large and non-polar means that lipids can be stored inside cells without affecting the WP of the cell.
-Release water when hydrolysed, so important for animals living in arid conditions.
Polysaccharides and carbohydrates:
-Polysaccharides are examples of polymerised carbohydrates. The monomer, or repeating unit to a polysaccharide is a single sugar, a monosaccharide.
-An example of a common monosaccharide is the hexose sugar glucose. These join together through condensation reactions, forming a glycosidic bond.
-Glucose has two isomers (alpha/beta glucose) which differ simply by the orientation of a hydroxyl group. Beta-glucose is used in cellulose, whilst alpha glucose is used in starch (repeating unit)
Disaccharide:
A-Glucose + A-Glucose = Maltose
A-Glucose + Fructose = Sucrose
A-Glucose + Galactose = Lactose
Polysaccharides:
Glycogen = Highly branched form of starch, monomer is A-glucose. Since it is branched, it has both 1,4 and 1,6 glycosidic bonds. Branching mean rapid enzyme activity for rapid hydrolysis/release of glucose into blood. Insoluble means it does not affect WP of the cell.
Cellulose = Long straight chains of beta-glucose, which run parallel to one another and form hydrogen bonds to form microfibrils (strong threads, higher order structure). The strong hydrogen bonds make it easy for plant cell walls to cope with dramatic osmotic pressure changes.
Roles of lipids:
-Lipids are used not only as storage and energy molecules, but also as structural components of every cell.
-The phospholipid bilayer is made up of many phospholipids. It is the hydrophobic nature of the fatty acid tails in these phospholipids which allows the membrane to arrange itself properly in water.
-Glycolipids can be present in this membrane which act as signalling molecules or binding sites for hormones/signalling chemicals.
Testing for lipids:
1. Add 2cm3 of sample to a test tube
2. Add 5cm3 ethanol to the sample
3. Add 5cm3 water and shake gently
4. Formation of white ppt indicates lipids present
*Repeat with a control group with no lipids to make sure solution remain clear!
Importance of protein:
Proteins make up a huge proportion of structures within any organism. They make up all of the enzyme component of cells (as all enzymes are protein), as well as having distinct structural roles in cells such as hair cells. They combine with carbohydrates to form glycoproteins, which act as both signalling molecules as well as binding sites for hormones or chemical messengers.
Primary Structure:
Amino acid monomers (lilac) linked together by planar peptide bonds (dark grey)
Protein folding:
-The primary structure of a protein describes the sequence of amino acids present. This primary structure will form the basis for the folding of the protein.
-The secondary structure describes hydrogen-bond interactions that occur when alpha helices and beta-pleated sheets form from the primary structure.
-The tertiary structure describes the three-dimensional folding of the secondary structure. This will result in the R-groups from residues that are far away from interacting.
-The quaternary structure describes the association of more than one individual polypeptide chain. For example, haemoglobin (Hb) has four polypeptide chains in its quaternary structure.
Protein amino-acid specific interactions:
-Disulphide bridges form between sulphur atoms present in cysteine residues (an amino acid with sulphur present in the R-Group)
-Ionic/salt bridges form between oppositely charged residues such as aspartic acid (negative) and lysine (positive)
Testing for proteins (biruet):
1. Place sample in tube and add equal volume of NaOH (sodium hydroxide).
2. Add a few drops of dilute copper (II) sulphate and mix gently at room temperature.
3. Orange to purple colour change indicates that a protein is present in the sample.
Globular vs Fibrous proteins
Proteins can either fold to form structural components or fold to form shapes which participate in reactions. Two distinct classes of proteins can therefore be brought to light: Globular and Fibrous.
-Globular proteins fold to a more spherical orientation, and therefore form specific binding sites through their tertiary structure shape specificity. These globular proteins therefore typically form enzymes (which are globular and have an active site)
-Fibrous proteins fold to form more long/sheet like structures. Collagen is a triple helix of protein cross-linked to give a high tensile strength. Keratin is another example of a tough protein which folds for strength/structural reasons.
FIBROUS
GLOBULAR
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