BIOL 150 Chapter Notes - Chapter 5: Peptidoglycan, Antimicrobial Resistance, Chloroplast
Chapter 5: Organizing Principles: Lipids, Membranes, and Cell Compartments.
5.1 STRUCTURE OF CELL MEMBRANES
➔Cells are defined by membranes.
◆ Membranes physically separate cells from their external environment.
◆Membranes define spaces within many cells that allow them to carry out their
diverse functions.
➔Lipids are the main component of cell membranes.
◆They have properties that allow them to form a barrier in an aqueous (watery)
environment.
◆Proteins are often embedded in or associated with the membrane--they perform
important functions, (transporting molecules).
◆Carbohydrates can also be found in membranes, usually attached to lipids
(glycolipids) and proteins (glycoproteins).
Cell membranes are composed of two layers of lipids.
➔Major type of lipid found in cell membranes are phospholipids.
◆Most are made up of glycerol backbone attached to a phosphate group and two
fatty acids.
◆Phosphate head group is hydrophilic (water-loving)-- it is polar, enabling it to
form hydrogen bonds with water.
◆Two fatty acid tails are hydrophobic (water-fearing)--nonpolar, do not form
hydrogen bonds with water.
◆Amphipathic- Molecules with both hydrophilic and hydrophobic regions ina
single molecule.
➔In an aqueous environment, amphipathic molecules (phospholipids), behave in an
interesting way.
◆They 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.
●Arrangement results from the tendency of polar molecules (like water) to
exclude nonpolar molecules or nonpolar groups of molecules.
◆Shape of the structure is determined by the bulkiness of the head group relative to
the hydrophobic tails.
●ie) lipids with bulky heads and a single hydrophobic fatty acid tail are
wedged-shaped and pack into spherical structures--micelles.
●ie) lipids with less bulky head groups and two hydrophobic tails form a
bilayer.
○Lipid bilayer--structure formed of two layers of lipids, hydrophilic
heads are the outside surfaces of the bilayer, hydrophobic tails are
sandwiched in between, isolated from contact with the aqueous
environment.
◆Bilayers form closed structures with an inner space since free edges would expose
the hydrophobic chains to the aqueous environment.
●Small tears in a membrane are rapidly sealed by the spontaneous
rearrangement of the lipids surrounding the damaged region, due to the
tendency of water to exclude nonpolar molecules.
Cell membranes are dynamic.
➔Lipids freely associate with one another because of extensive van der Waals forces
between their fatty acid tails.
◆Weak interactions are easily broken and re-formed.
●Lipid molecules are able to move within the plane of the membrane,
sometimes very rapidly.
➔Lipids can also rapidly rotate around their vertical axis, individual fatty acid chains are
able to flex, or bend.
◆Result: membranes are dynamic--continually moving, forming, and reforming
during the lifetime of a cell.
➔Membrane is said to be fluid- membrane fluids are able to move in the plane of the
membrane.
◆Degree of the membrane fluidity depends on which types of lipid make up the
membrane.
◆In a single layer of the lipid bilayer--strength of the van der Waals interactions
between the lipids’ tails depends on the length of the fatty acid tails--the more
surface is available to participate in van der Waals interactions.
◆Tighter packing that results tends to reduce lipid mobility.
●Saturated fatty acid tails (no double bonds) are straight and tightly packed.
○Reduced mobility.
◆Unsaturated fatty acids introduce kinks in the fatty acid tails- reducing the
tightness of packing and enhancing lipid mobility in the membrane.
➔Cholesterol--a major component of animal cell membranes.
◆30% by mass of the membrane lipids.
◆Amphipathic (hydrophilic and hydrophobic).
●Hydrophilic region- hydroxyl group (-OH).
●Hydrophobic region- consists of 4 interconnected carbon rings with an
attached hydrocarbon chain.
●Structure allows cholesterol to insert into the lipid bilayer-- head group
interacts with the hydrophilic head group of phospholipids.
○Ring structure participates in van der Waals interactions with the
fatty acid chains.
◆Increases or decreases membrane fluidity depending on temperature.
●Temperature found in cell-- it decreases it.
○Interaction of the rigid ring structure of cholesterol with the
phospholipid fatty acid tails reduces mobility of the phospholipids.
●Low temperatures--increases it.
○Prevents phospholipids from packing tightly with other
phospholipids.
◆Helps maintain a consistent state of membrane fluidity by preventing dramatic
transitions from a fluid to solid state.
➔Specific types of lipid (sphingolipids) sometimes assemble into defined patches--lipid
rafts.
◆Cholesterol and other membrane components (proteins) appear to accumulate in
some of these regions.
➔Membranes are not always a uniform fluid bilayer, can contain regions with discrete
components.
➔Spontaneous transfer of a lipid between layers of the bilayer (lipid flip-flop), is very rare.
◆Flip-flop requires the hydrophilic head group to pass through the hydrophobic
interior of the membrane.
●There is little exchange of components between the two layers of the
membrane- allows the two layers to differ in composition.
Proteins associate with cell membranes in different ways.
➔Most membranes contain proteins as well as lipids.
◆ie) proteins represent as much as 50% by mass of the membrane of a red blood
cell.
➔Membrane proteins serve different functions.
◆Transporters- moving ions or other molecules across the membrane.
◆Receptors- allow the cell to receive signals from the environment.
◆Enzymes- catalyze chemical reactions.
◆Anchors- attach to other proteins and help to maintain cell structure and shape.
➔Membrane proteins can be classified into 2 groups depending on how they associate with
the membrane.
◆Integral membrane proteins- permanently associated with the cell membranes,
cannot be separated from the membrane experimentally without destroying the
membrane itself.
●Transmembrane proteins- span the entire lipid bilayer, most common
integral membrane, composed of 3 regions.
○Two hydrophilic regions.
○One protruding from each face of the membrane.
○A connecting hydrophobic region that spans the membrane.
Document Summary
Chapter 5: organizing principles: lipids, membranes, and cell compartments. Membranes physically separate cells from their external environment. Membranes define spaces within many cells that allow them to carry out their diverse functions. Lipids are the main component of cell membranes. They have properties that allow them to form a barrier in an aqueous (watery) environment. Proteins are often embedded in or associated with the membrane--they perform important functions, (transporting molecules). Carbohydrates can also be found in membranes, usually attached to lipids (glycolipids) and proteins (glycoproteins). Cell membranes are composed of two layers of lipids. Major type of lipid found in cell membranes are phospholipids. Most are made up of glycerol backbone attached to a phosphate group and two fatty acids. Phosphate head group is hydrophilic (water-loving)-- it is polar, enabling it to form hydrogen bonds with water. Two fatty acid tails are hydrophobic (water-fearing)--nonpolar, do not form hydrogen bonds with water.
