CHAPTER 6 – LIPIDS, MEMBRANES & THE FIRST CELLS
Membrane-forming lipids have a polar, hydrophilic head that interacts with
water molecules when a phospholipid is placed in solution. In addition, a
non-polar, hydrophobic fatty acid tail cannot form hydrogen bonds with the
hydrocarbon tail. This characteristic is called being amphipathic (ie
Phospholipids do not dissolve when they are placed in water. Since they are
amphipathic, they may form one of two structures: micelles or lipid bilayers.
Micelles: tiny droplets created when the hydrophilic heads of phospholipids
face the water and the hydrophobic tails are forced together, away from the
water. Lipids with compact/shorter tails form micelles.
Phospholipid bilayers: when two sheets of phospholipid molecules align.
Lipids with longer tails form bilayers.
Micelles and bilayers form spontaneously (no energy input required,
therefore exergonic). The formation of these structures decreases entropy.
They are much more organized and energetically stable than independent
molecules in solution. However they also have much lower potential energy
than independent molecules. The loss of potential energy outweighs the
decrease in entropy. Overall, free energy in system decreases.
Hydrocarbons are unstable in water because they disrupt hydrogen bonding
in water molecules.
Permeability: tendency of a structure to allow a given substance to pass
Liposome: artificial membrane bound vesicles, formed when shaking agitates
Planar bilayers: artificial membranes used to simulate what happens when a
known ion or molecule is added to one side of a bilayer.
Selective permeability: some substances cross a membrane easily than
others. Small, non-polar molecules (ie O 2 move across bilayers quickly.
Large molecules and charged substances (ie Cl ) cross the membrane slow or
not at all. Small and uncharged molecules (ie H 2) can cross relatively
rapidly, and small polar molecules (ie urea, glycerol) have intermediate
permeability. Lipid bilayers are highly selective.
Theory: charged compounds and large polar molecules can’t pass through
the non-polar hydrophobic tails. The electrical charge of an ion makes it
more stable in solution where they form hydrogen bonds with water
opposed to be interior of membranes where it is electrically neutral.
Two aspects of a hydrocarbon chain affect the way the chain behaves in a
lipid bilayer: number of double bonds (saturation) and length. Double bonds
create a kink in the tail because it no longer has free rotation and is also said
to be unsaturated, meaning fewer than the maximum number of hydrogen
atoms can be bonded. Saturated fats have more C-H bonds which have a higher bond energy than C=C bonds, meaning there is much more chemical
Foods that have unsaturated fats are called polyunsaturated and are said to
Double bonds create a kink in the tail, which produces spaces among the
tightly packed tails. The interior of the membrane is glued less tightly
together, which causes the structure to be more fluid and permeable.
Hydrophobic interactions become stronger as saturated hydrocarbon tails
increase in length. Phospholipids with long, saturated tails form membranes
that are much less permeable than membranes consisting of phospholipids
with shorter, unsaturated tails.
Fluidity of a lipid depends on the length and saturation of its hydrocarbon
chain. Butter consists of saturated lipids, and is solid at room temperature.
Waxes are lipids with extremely long hydrocarbon chains and are stiff solids
at room temperature. Oils are polyunsaturated, with hydrocarbon chains that
contain multiple double bonds and are liquid at room temperature.
At room temperature (approximately 25 degree Celsius) phospholipids in
bilayers are liquid. Fluidity and permeability decrease as temperature
decreases. As temperatures drop, the individual molecules in the bilayer
move slower and as a result pack together more tightly. At very low
temperatures, liquid bilayers begin to solidify.
Membranes are dynamic. The fluid nature allows individual lipid molecules
to move laterally within each layer with an average speed of 2 micrometers
per second at room temperature. Phospholipid molecules move around each
layer while water and small non-polar molecules shoot in and out of the
membrane. How quickly molecules move within and across the membrane is
a function of temperature and the structure of the hydrocarbon tails in the
Dissolved molecules and ions, called solutes, have thermal energy and are in
constant, random motion. Movement of molecules and ions that results from
their kinetic energy is called diffusion. Solutes change position randomly due
to diffusion and tend to move from a region of high concentration to a region
of low concentration. A difference in solute concentrations creates a
concentration gradient. There is a net movement from high concentration to
low concentration. Diffusion along a concentration gradient is spontaneous
process because it results in an increase in entropy.
At equilibrium, molecules move back and forth across the membrane at equal
rates. This means there is no longer a net movement.
Osmosis: movement of water. It only occurs when solutions are separated by
membranes that are permeable to some molecules but not others (semi
permeable membrane). Water undergoes a net movement from the region of
low concentration of solute/high concentration of water to the region of high
concentration of solute/low concentration of water. The movement is
spontaneous, driven by the increase in entropy achieved when solute
concentrations are equal on both sides of the membrane. Hypertonic solution: Outside solution containing more solutes than the
solution on the other side of a membrane. Ex. Outside solution will come into
vesicle and cause it to swell.
Hypotonic solution: Outside solution containing less solute than the solution
on the other side of membrane. Ex. Inside solution will come out of vesicle to
outside solution, causing it to shrivel.
Isotonic: Outside solution has same amount of solutes as inside solution. Ex.
The cell maintains its shape.
Amphipathic proteins can be inserted into lipid bilayers and affect the
permeability. Because amino acids can have a series of non polar amino acids
in the middle of its primary structure but polar or charged amino acids on
both ends, the non polar amino acids would be stable in the interior of a
bilayer while the polar or charged amino acids would be stable alongside the
polar heads and surrounding water. Also, because secondary and tertiary
structures are varied and complex, it is possible for proteins to form tubes
and function as some sort of channel or pore across a lipid bilayer.
Fluid mosaic membrane: Singer and Nicolson proposed that membranes are
a mosaic of phospholipids and different types of proteins. Overall structure is
dynamic and fluid.
Integral membrane proteins (transmembrane proteins): Some proteins that
span the membrane and have segments facing both the interior and exterior
Peripheral membrane proteins: proteins found on only one side of the
membrane. They are often attached to an integral membrane protein.
Membrane proteins are in constant motion, diffusing through the oily film.
Detergents are small amphipathic molecules that tend to form micelles in
water. They break up bilayers when their hydrophobic tails interact with
hydrophobic tails of lipids. Detergent molecules displace membrane
phospholipids and end up forming water-soluble detergent-protein
complexes. This is an effective way to isolate membrane proteins so they can
be purified and studied.
Transport proteins: channels, transporters and pumps.
Facilitated diffusion via channel proteins: Ions move from regions of high
concentration to regions of low concentration via diffusion and also flow
from areas of like charge to areas of unlike charge. When ions build up on
one side of a membrane, they establish a combined concentration and
electrical gradient. Ions move in response to a combined concentration and
electrical gradient, called an electrochemical gradient. The movement of ions
creates an electrical current.
An ion channel is a peptide or protein that makes lipid bilayers permeable to
Cells have many different types of channel proteins in their membranes that
allow it to admit a particular type of ion or small molecule. Example:
aquaporins (“water pores”) allow water to cross the plasma membrane over
10x faster than it does in its absence. Aquaporins and ion channels are gated channels, meaning they open or close
in response to the binding of a particular molecule or to a change in the
electrical charge on the outside of the membrane.
Passive transport: the movement of substances through channels does not
require a lot of energy. It is powered by diffusion along an electrochemical
Facilitated diffusion via carrier proteins: Facilitated diffusion is aided by the
presence of specialized proteins in the membrane. Facilitated diffusion can
also occur through carrier proteins (transporters) that change shape during
GLUT-1 facilitates glucose diffusion into a cell. It is a transmembrane
transport protein with its binding site facing outside the cell. It works when
glucose binds to GLUT-1 from outside the cell. A conformational change
results, transporting glucose into the interior by its concentration gradient
and then released inside.
Active transport by pumps: transport against an electrochemical gradient.
This task requires energy because the cell must counteract the entropy loss
that occurs when molecules or ions are concentrated. A phosphate group
usually provides the energy from ATP. The result of a lost phosphate group
turns ATP into ADP. When one of the phosphate groups from ATP binds to a
protein, two negative charges are added to the protein. These charges repel
other charges on the protein’s amino acids. The protein’s potential energy
increases and shape changes.
First pump discovered with the sodium-potassium pump or ATPase. Three
binding sites within the protein have a high affinity for sodium ions, which
are inside the cell. They bind to these sites and a phosphate group from ATP
binds to the protein and changes its shape. The sodium ions diffuse to the
exterior of the cell. In this conformation, the protein has binding sites, which
have a high affinity for potassium ions. Two potassium ions bind to the pump
and a phosphate group drops off the protein. The protein changes back to its
original shape and the potassium ions diffuse into the cell. This movement of
ions can occur even if an electrochemical gradient exists that favours the
outflow of potassium and the inflow of sodium. By exchanging three sodium
ions for two potassium ions, the outside of the membrane becomes positively
charged relative to the inside. In this way, an electrical and chemical gradient
are set up across the membrane.
The lipid bilayer and proteins involved in passive transport and active
transport enable cells to create an internal environment that is much
different from the external one. Membrane proteins allow ions and molecules
to cross the plasma membrane even though they are not lipid soluble.
CHAPTER 7 – INSIDE THE CELL
Prokaryotic cells: Phospholipid bilayer and proteins that span the bilayer or
attach to one side. Inside membrane, contents are called cytoplasm.
Hypertonic relative to surrounding environment. Water enters the cell via osmosis and makes cell’s volume expand. This pressure is resisted by a cell
Glycolipids: lipids that contain carbohydrate groups.
Most dominant structure is the single circular chromosome that consists of a
large DNA molecule (contains information) and small number of proteins
Chromosomes contain DNA that contains genes. Prokaryotic chromosomes
are found in localized area of the cell called the nucleoid. To fit in the cell, the
DNA double helix coils itself with the aid of enzymes to form a highly
compact structure, called plasmids. Plasmids contain genes but are physically
independent of the main cellular chromosome.
Ribosomes: manufacture proteins. Flagella: rote to power swimming in
aquatic species. Both lack a membrane.
Bacterial ribosomes: complex structures consisting of three RNA molecules
and 50+ proteins. They are organized into a large subunit and small subunit.
A cell may contain 10 000 of these.
Not all bacteria species have flagella. When present, they are few in number
and on surface of cell. Over 40 proteins are involved in building and
controlling bacterial flagella. At top speed, flagellar movement can drive a
bacteria cell through water at 60 cell lengths per second.
Prokaryotes lack a nucleus but they do have membrane bound internal
Organelle: membrane bound compartment in the cytoplasm that contains
enzymes or structures specialized for a particular function.
Bacteria and archaea contain long thin fibres for structural support. Some
species have protein filaments that help maintain cell shape called the
Location of DNA Internal Cytoskeleton Overall Size
Bacteria and In nucleoid (not Extensive Limited in extent, Usually small
Archaea membrane bound), internal relative to relative to
plasmids common membranes only eukarotes. eukaryotes.
Eukaryotes Inside nucleus Large numbers of Extensive- Most are
(membrane bound), organelles, many usually found larger than
plasmids rare types of throughout prokaryotes.
organelles. volume of cell
Eukaryotic Cells: ranges from uni cellular species to plants and animals.
Organelles in eurkaryotes can separate incompatible chemical reactions. Also
the efficiency of chemical reactions is increased.
Two advantages of compartmentalization of eukaryotic cells: incompatible
chemical reactions can be separated (ie fatty acids can be synthesized in one organelle while excess or damaged fatty acids are degraded and recycled in
another); the efficiency of chemical reactions is increased (ie substrates for
reactions can be localized and highly concentrated, if substrates are used up
in part of organelle they can be replaced , groups of enzymes that work
together can be clustered on internal membranes therefore increasing speed
and efficiency of reaction).
Bacteria and archaea = machine shops, eukaryotic cells = industrial factories.
Nucleus: highly organized, enclosed in double membrane called nuclear
envelope that is studded with pores and inside surface is associated with
fibrous proteins that form a lattice-like sheet called nuclear lamina that
stiffens structure; chromosomes are attached to nuclear lamina, contains
distinct region called nucleolus where RNA molecules found in ribosomes are
manufactured and small and large subunits are manufactured.
Cytoplasm: everything inside plasma membrane excluding nucleus. Cytosol:
fluid portion of cytoplasm.
Ribosomes: found scattered throughout cytosol, comprised of two subunits
(large and small – large composed of three RNA molecules while small is
composed of one RNA molecule – both have protein as well, neither enclosed
by membrane), synthesize proteins
Rough ER: ribosomes are associated with rough ER (studded on surface),
responsible for synthesizing proteins to be inserted into plasma membrane,
secreted to cell exterior, shipped to lysosome; ER is continuous with outer
membrane of nuclear envelope; interior of sac like structure called lumen,
where newly manufactured proteins undergo folding; these proteins have
variety of functions – messengers, membrane transport proteins, enzymes.
Golgi Apparatus: prod