Download Understanding Polarity

What has happened so far in chemical evolution?
Biological monomers formed from interaction of atmospheric
gases, water and energy
The monomers (ex. nucleotides) stick to clay/sand size particles
at coastal areas of the ocean. As waves wash over them carrying
other nucleotides, they make physical contact, form chemical
bonds and form a chain of nucleotides (RNA or DNA).
Problems with this: polymers not stable, likely to break down as
exposed to harsh conditions of ocean and air.
Not a very efficient way to build polymers (low frequency of
monomers interacting this way in open space of coastal waters)
Cell Membranes are Made up of Lipids
Important Properties of all lipids
Two domains: hydrophobic (H2O X &
Hydrophillic (H2O
).
Hydrophobic domain has varying
length chain of hydrocarbon
groups (C-H)
3 classes: steroids, fats and
phospholipids
Phospholipids
Cell membranes made up of
phospholipids
Individual Phospholipids align
in sheets to form
Lipid Bilayers
(the structure of the cell membrane)
This is spontaneous: Does NOT require energy
How could lipid bilayers (membranes)
spontaneously form?
?
To understand this question,
need to 1st understand polarity
Understanding Polarity
Water is Polar: electrons
are not shared equally
creating a partial
charge on the O and
H atoms
H2 is nonpolar: electrons
are shared equally. There
are no partial charges on
the atoms
Fig. 2.5- also see p25-26
in section 2.2
Understanding Polarity
Molecules with atoms that have different electronegativity
will be polar. The more electronegative atoms attract
the electrons more strongly and will have a partial charge
For example:
H-O & H-N
When the atoms have the same electronegativity, the molecule
Will be nonpolar (equal hogging of electrons). For example:
H-H & C-H
Electronegativity Chart
Understanding Polarity:
Because water is polar it can interact with other
polar molecules like other water molecules by
bonding of partially charged O and H atoms
1 water molecule
Fig 2.12
2 water molecules
together via hydrogen
bonding
Understanding Polarity
Because water is polar it can dissolve ionic molecules
like salt by interacting with the charged Na+ and Cl- ions
Fig 2.12
Understanding Polarity
Nonpolar molecules do not dissolve in water
*Notice all of the C-H bonds*
Understanding Polarity
Nonpolar molecules do not dissolve in water
Understanding Polarity
class of
molecule
ionic
electron
sharing
not shared,
complete
transfer
charge
of atoms
soluble
in water
full
yes
salt (Na+Cl-)
example
polar
not
equally
shared
partial
yes
water, sugar
nonpolar
equally
shared
none
no
lipids
Understanding Polarity
polar head
(hydrophilic)
nonpolar tail
(hydrophobic)
Fig 6.4
What about phospholipids,
which have both
a polar and nonpolar domain?
So what happens to lipids in water?
The polar parts dissolve in water and the nonpolar parts don’t
Result= lipid bilayer
•Polar (hydrophilic) heads interact with water molecules
Nonpolar tails interact
with each other to
avoid water
Fig 6.5
Recall these bilayers form without energy
Individual phospholipids are not stable in water b/c tails
disrupt the very stable hydrogen bonds of water
A second kind of lipid
structure can also form…
Micelles: Tiny droplet of lipids (made up of
one layer). Forms with tails that have shorter
tails (bilayers have longer tails)
Fig 6.5
Soap forms micelles when placed in water
Water filled vesicles made up of lipid bilayers
can form when lipid bilayers are agitated.
Fig 6.7
Artificial vesicles called
liposomes
http://genetics.mgh.harvard.edu/szostakweb/exploringOriginsDownloads/denovoVesicle.mov
This allows for containment of
prebiotic soup components
prebiotic soup
-amino acids
-nucleotides
-sugars
-lipids
-polymers
Wake up time!
Cell Membranes
A. Are made up of single sheets of lipids (monolayers)
B. Are completely nonpolar and therefore do not dissolve in water
C. Have a polar head domain that is not soluble in water
D. Have a polar hydrocarbon tail domain that is not soluble in water
E. Formed when lipid bilayers formed vesicles by remaining motionless
F. Are important because they allow all molecules to enter and leave the cell
G. Allow for individual amino acids to form amino acid polymers (proteins)
more readily than without a cell membrane
Adding all of this to the chemical evolution story…
Simple reduced molecules like H2CO and HCN formed from interaction of
atmospheric gases, water and energy
More complex and more reduced molecules like amino acids, nucleotides,
sugars and lipids formed from H2CO, HCN, atmospheric gases, water and
energy
Nucleotide, sugar and amino acid monomers polymerized to form DNA, RNA,
polysaccharides and protein (requiring energy)
One of these molecules acquired the ability to self-replicate (RNA)
Individual lipids formed lipid bilayers, agitation of the bilayers
resulted in vesicles filled with water and biological molecules
What characteristics are common to all life?
Order: Basic unit of
life is the cell. All organisms
are made up of cells. Other
ex. of order include: symmetry &
tissues (not just bags of molecules)
Respond to environment
(short-term)
Acquire nutrients and Utilize
those nutrients for energy
Evolve: long-term
Response to environment
Grow and
Reproduce
Advantages a Cell
Membrane
•Concentrate reactants so
chemical reactions are
more efficient
Fluorescent staining of
Cell membrane (orange)
Invitrogen.com
•Make the internal environment different
from the external (ex: maintain and internal temp, pH and
other conditions that are favorable for reactions)
•Selectively keep out damaging compounds, admit needed
compounds (nutrients), and allow waste to leave
The cell is surrounded by a membrane so
how are the nutrients allowed in
through a membrane that also keeps the
Riffraff (pathogens, any molecules the
cell doesn’t need) out
Factors that determine if a molecule
will permeate the cell membrane
•Polarity / charge of the molecule
•Size of the molecule
•Properties of the membrane itself
Permeability- measurement of how easily a molecule
can cross the cell membrane
What are the natural barriers to
permeating the membrane?
1. The bilayer structure of the cell membrane creates a barrier for
polar molecules b/c they are repelled by the non-polar membrane
interior
water
Polar (hydrophillic)
heads
Nonpolar (hydrophobic)
tails
Polar (hydrophillic)
heads
water
What are the natural barriers to
permeating the membrane?
2. The densely packed phospholipids (bonds form between
tails of phospholipids) create a barrier for larger molecules b/c they can’t
pass between the phospholipids.
water
Polar (hydrophillic)
heads
Nonpolar (hydrophobic)
tails
Polar (hydrophillic)
heads
water
What are the natural barriers topermeating
the membrane?
Polarity continuum:
Nonpolar (O2, CH4)
Easier passage
Uncharged polar
(water, glucose, amino acids)
Size continuum:
Small (H2O)
Medium
(glucose)
Charged polar
(amino acids, ions such as Na+, Ca2+,
K+, Cl-)
Large (proteins)
Harder passage
•Take home message: The combination of polarity and size determin
permeability (i.e. small polar molecules cross more easily
than medium sized polar molecules).
What are the natural barriers to permeating
the membrane?
Combination of polarity and size determines
permeability
fast
slow
can’t pass
can’t pass
can’t
pass molecules (amino
Polar charged
acids) and ions (Cl-)
(size irrelevant)
Fig
6.8
What are the natural barriers to permeating
the membrane?
3. The types fatty acid (property of the lipid)
like fig 6.9
Unsaturated
Saturated
What are the natural barriers to permeating
the membrane?
4. Longer hydrocarbon tails provide more
opportunities for hydrophobic interactions (bonds)
which decrease permeability
longer
saturated tail
shorter
saturated tail
Fig 6.11
What are the natural barriers topermeating
the membrane?
5. Temperature also effects membrane permeability
a. Phopsholipids constantly
in motion.
b. Mobility increases as
temperature increases
(less rigid membrane)