BIO 1140 Lecture Notes - Lecture 2: Alpha Helix, Histidine, Sickle-Cell Disease

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Topic 2 Lectures
In learning objectives highlighted sections are important, those are ones that people tend to
struggle with!
*Structure, how it affects where the macromolecule can reside in the cell, and how it influences
its function.
Carbohydrates are water-soluble, the plasma membrane is liposoluble, so carbohydrates cannot
be imbedded in the membrane because it won’t be happy there – unless it’s attached to
something, therefore carbohydrates do not contribute to membrane fluidity. Carbohydrates are
however involved in cellular recognitions and communication, providing a source of chemical
energy, and structural aspects of outer membrane.
Q: Given the relative electronegativities of the elements listed, a molecule with the formula CH4
would have which type of bonds? O=3.44 N=3.04 C=2.55 H=2.20
A: the difference in electronegativity between C and H is relatively low, uneven distribution of
charge is what creates polarity. Here, charge is distributed relatively evenly, so the bonds will be
nonpolar covalent bonds.
Why does nitrogen contribute to polarity in the molecule CH3NH2? Lone pair of electrons on
nitrogen, so a more negative area than the rest of the molecule, contributing to the
electronegativity and polarity.
*Know which are the relevant atoms that contribute to electronegativity/polarity in biology N,
O, C, S, P
*most electronegative, to the right and to the top of the periodic table
DEFINITELY need to determine polarities of amino acids NOT memorize.
Determining the polarities of amino acids helps us to know:
Solubility in water
Know where they can be in the cell
How it is going to fold
The ‘backbone’ is made up of an amino group – carbon, hydrogen, and carboxyl group. That
carbon is an asymmetrical carbon every group that is attached to it is different. Every single
one of the amino acids has the SAME backbone (see diagram)
The amino group and the carboxyl group both contribute electronegativity to the backbone,
contribute roughly the same amount of electronegativity. Same in all 20 amino acids, all have the
same contributions to electronegativity almost cancels each other out.
In comparing 2 amino acids and determining which is more polar look at the rest of the
molecule, besides the backbone. The ‘R’ group, the radical group.
The radical group is what matters when determining whether an amino acid is polar or
nonpolar ignore the backbone.
Glycine nonpolar (H has low negativity)
Tyrosine polar (has benzene ring that is evenly distributed, but O is very
electronegative and is going to pull down on that benzene)
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