Chapter 5: Organizing Principles: Lipids, Membranes, and Cell
Compartments
5.1 Structure of Cell Membranes
● Lipids are the main component of cell membranes. They have
properties that allow them to form a barrier in an aqueous
environment.
○ Proteins are often embedded in or associated with the
membrane, where they perform important functions such as
transporting molecules.
○ Carbohydrates can also be found in cell membranes, usually
attached to lipids (glycolipids) and proteins (glycoproteins).
● The major types of lipid found in cell membranes are phospholipids,
made up of a glycerol backbone attached to a phosphate group and
two fatty acids.
○ The phosphate head group is hydrophilic because it is polar,
enabling it to form hydrogen bonds with water.
○ The two fatty acid tails are hydrophobic because they are
nonpolar and do not form hydrogen bonds with water.
○ Molecules with both hydrophilic and hydrophobic regions in a
single molecule are called amphipathic.
● In an aqueous environment, amphipathic molecules spontaneously
arrange themselves into various structures in which the polar head
groups on the outside interact with water and the nonpolar tail groups
come together on the inside away from water.
● The shape of the structure is determined by the bulkiness of the head
group relative to the hydrophobic tails.
, ○ Lipids with bulky heads and a single hydrophobic fatty acid tail
are wedge-shaped and pack into spherical structures called
micelles.
○ Lipids with less bulky head groups and two hydrophobic tails
form a bilayer. A lipid bilayer is a structure formed of two layers
of lipids in which the hydrophilic heads are the outside surfaces
of the bilayer and the hydrophobic tails are sandwiched in
between, isolated from contact with the aqueous environment.
● The bilayer structure forms spontaneously, dependent solely on the
properties of the phospholipid, as long as the concentration of free
phospholipids is high enough and the pH of the solution is similar to
that of a cell.
○ The pH ensures that the head groups are in their ionized
(charged) form and thus suitably hydrophilic.
○ If phospholipids are added to a test tube of water at neutral
pH, they spontaneously form spherical bilayer structures
called liposomes that surround a central space.
● Lipids freely associate with one another because of extensive van der
Waals forces between their fatty acid tails.
○ These weak interactions are easily broken and re-formed, so
lipid molecules are able to move within the plane of the
membrane very rapidly.
○ Lipids can also rapidly rotate around their vertical axis, and
individual fatty acid chains are able to flex, or bend.
○ Membranes are dynamic: they are continually moving, forming,
and re-forming during the lifetime of a cell.
, ● Membrane lipids are able to move in the plane of the membrane
(fluid membrane).
○ The strength of the van der Waals interactions between the
lipids’ tails depends on the length of the fatty acid tails and the
presence of double bonds between neighboring carbon atoms.
○ The longer the fatty acid tails, the more surface is available to
participate in van der Waals interactions. The tighter packing
that results reduces lipid mobility.
○ Saturated fatty acid tails, are straight and tightly packed—
reducing mobility.
○ The double bonds in unsaturated fatty acids introduce kinks in
the fatty acid tails, reducing the tightness of packing and
enhancing lipid mobility in the membrane.
● At temperatures typically found in a cell, cholesterol decreases
membrane fluidity because the interaction of the rigid ring structure of
cholesterol with the phospholipid fatty acid tails reduces the mobility
of the phospholipids.
● At low temperatures, cholesterol increases membrane fluidity
because it prevents phospholipids from packing tightly with other
phospholipids. Cholesterol helps maintain a consistent state of
membrane fluidity by preventing transitions from a fluid to solid.
● Although lipids are free to move in the plane of the membrane, the
spontaneous transfer of a lipid between layers of the bilayer, known
as lipid flip-flop, is very rare.
● Membrane proteins serve different functions:
○ Transporters- move ions or other molecules across the
membrane.
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