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Chapter 6

BIOLOGY 1A03 Chapter Notes - Chapter 6: Scanning Electron Microscope, Alkane, Lipid Bilayer

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George Donaldson

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Chapter 6: Lipids, Membranes, and the First Cells
Major Classes and Functions of Lipids
phosopholipids, glycolipids, and some steroids (cholesterol) – membrane structure and function
other steroids – hormones and regulators
triglycerides – food storage
waxes - bee hives and plant leaf surfaces for the protection against H2O loss
Fig 6.2
Fats – consist of glycerol + 3 fatty acids, linked by ester linkages
Fig 6.3
What is an “amphipathic” molecule?
~has both a polar (hydrophillic) region and a nonpolar (hydrophobic) region
Fig 6.4
-phospholipids spontaneously form bilayers in water
Nonpolar -> hydrophobic tails
polar -> hydrophilic heads
Important Concept: Why Are Membranes Importance?
Membranes allow for compartmentalization
~may contain different enzymes
~may produce different products
~may increase metabolic capacity
Functions of the Plasma Membrane (outermost membrane of the cell):
~ functions as a boundary, which separates a living cell's internal environment from its eternal
nonliving environment. It separates “life” from “nonlife”.
~acts as a selective barrier: it keeps damaging compounds out of the cell and allows entry of
compounds needed by the cell
allows only certain molecules to pass through and controls the movement of materials in and
out of the cell
~the enclosed volume serves to increase reaction efficiency and concentrates reactants
b/c plasma membrane sequesters the appropriate chemicals in an enclosed area, reactants
collide more frequently – the chemical reactions necessary for life occur much more efficiently
Important Concept: Why Is Membrane Fluidity Important?

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Membrane fluidity
~permits useful nutrients to be transported into the cell (for energy)
~permits harmful wastes to be transported out of the call
~membrane needs to be fluid and dynamic
Figure 6.8
Figure 6.9
Membrane Fluidity is related to:
~unsaturated (fluid) or saturated (solid) fatty acids
~ # of carbons in fatty acid tails
1. Presence of Unsaturated or Saturated Fatty Acids
- unsaturated hydrocarbon (fatty acid) tails have one or more double bonds
- each double bond forms a kink (a bend), which results in the fatty acid tails spaced apart and polar
heads loosely packed
--> this decreases the strength of interactions between adjacent hydrocarbon tails resulting in a “fluid”
-saturated hydrocarbon (fatty acid) tails have no double bonds and therefore do not have kinks or bends
-hydrocarbon tails are closely packed allowing for interactions extending all along their lengths
-polar heads are tightly packed
--> this increases the strength of interaction between adjacent hydrocarbon tails resulting in the
“viscous” state
Figure 6.10
If no unsaturated fatty acids or lots of saturates fatty acids – low permeability
If several unsaturated fatty acids - high permeability
2. Number of Carbon Atoms in the Fatty Acid Hydrocarbon Tail
- for example, a fatty acid hydrocarbon tail with 16 carbons would be more fluid i comparison to one
with 18 carbons
Which one of the following membranes would be more fluid & more permeable?
1. A membrane composed mainly of shorter, unsaturated phospholipids
2. a membrane composed mainly of longer, saturated phospholipids
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