Biology 1001A Lecture Notes - Lecture 15: Phenylalanine, Phospholipid, Quality Control
Bio1002B Midterm 1
1
Cell Membranes and Signaling
One of the key features of the cell is the plasma membrane which serves as a selectively
permeable barrier. The modern view of the plasma membrane is the fluid mosaic model
- membranes consist of proteins that move around w/in a mixture of lipids that
has the consistency of olive oil. The plasma membrane is also asymmetrical.
Role of Fatty Acids in Plasma Membrane Structure.
• Main membrane component is phospholipids (sterols like cholesterol also help
maintain membrane fluidity).
• Phosphate containing head group - Hydrophilic
• Hydrophobic fatty acid tail
o Lipid bilayers form spontaneously no energy required (hydrophobic
effect- lowest energy state of phospholipids is to form a bilayer, micelle,
or liposome)
Fatty Acid Saturation Levels Affect Membrane Fluidity.
• Saturation determined fluidity of membrane
o More unsaturation=more fluid membrane
o More saturation = less fluid membrane
Temperature Affects Membrane fluidity.
• High Temperatures = Very Fluid (olive oil consistency)
• Low Temperatures = Not Fluid (gelling occurs - butter consistency)
• The more unsaturated a group of lipid molecules, the lower the temperature at
which gelling occurs.
A membrane
can be up to
50% protein!
Hydrophilic Head
(glycerol + phosphate
+ alcohol)
Hydrophobic Tail
(hydrocarbon
chain)
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Bio1002B Midterm 1
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Relationship of Fluidity to Membrane Functions such as Transport.
• Need to maintain fluidity within range to allow protein transport
o Too rigid = no electron transport
o Too fluid = ion leakage, membrane falls apart
• Maintaining membrane fluidity is very important
Adjusting Fatty Acid Composition
• Fatty acids can be
o Saturated- fully saturated with hydrogen, very linear
o Unsaturated- C=C double bond reduced number of hydrogen, results in
kink
• Fatty acids synthesized in saturated form (biosynthesis)
• Desaturases act on saturated fatty acids and can introduce C=C double bonds
• Organisms can have multiple desaturases:
o Enzymes differ in where they make double bond along fatty acid
o Desaturases may work at different temperatures
Relationship of bacterial desaturase expression vs. temperature.
• Amount of desaturase transcript decreases as temperature increases and
increases as temperature decreases
• At low temperatures membranes can maintain fluidity due to the action of
desaturase
Integral/Transmembrane Proteins
• Proteins that transverse the entire lipid bilayer at least once
• These proteins have domains that interact with both sides of the membrane and
the core of the membrane. (The domain that is inside the membrane consists
nonpolar AAs that form the alpha helix - secondary protein structure w/no polar
R-groups b/c interactions are with backbones)
• Given the primary AA sequence of a protein, it is easy to see if a protein is
integral by looking at the hydrophobic region (Ex. Channelrhodopsin is an
integral protein w/7 non-polar domains meaning it crosses the membrane 7
times.
Integral Proteins are
translated on ribosomes
attached to the ER.
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Document Summary
One of the key features of the cell is the plasma membrane which serves as a selectively permeable barrier. The modern view of the plasma membrane is the fluid mosaic model. Membranes consist of proteins that move around w/in a mixture of lipids that has the consistency of olive oil. Role of fatty acids in plasma membrane structure. Fatty acid saturation levels affect membrane fluidity: saturation determined fluidity of membrane, more unsaturation=more fluid membrane, more saturation = less fluid membrane. Temperature affects membrane fluidity: high temperatures = very fluid (olive oil consistency, low temperatures = not fluid (gelling occurs - butter consistency, the more unsaturated a group of lipid molecules, the lower the temperature at which gelling occurs. Relationship of bacterial desaturase expression vs. temperature: amount of desaturase transcript decreases as temperature increases and increases as temperature decreases, at low temperatures membranes can maintain fluidity due to the action of desaturase.