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Unit 4 Notes: Membrane
Structure
IB Biology
Phospholipids
• The backbone of the membrane is a bilayer
that is produced from large numbers of
molecules called phospholipids.
• Each phospholipid is composed of glycerol (a 3
carbon compound)
• Two of the glycerol carbons have fatty acids
attached. The third carbon is attached to a
polar organic alcohol that includes a bond to a
phosphate group.
Phospholipids
• Fatty acids are non-polar and therefore, not
soluble in water. The organic alcohol is polar and
is soluble in water.
• Because of this structure membranes have two
distinct areas of polarity and water solubility.
• The polar, water soluble portion is referred to as
hydrophilic (water loving).
• The nonpolar, water insoluble portion is referred
to as hydrophobic (water fearing).
Phospholipids
• The fatty acid tails of the phospholipid
molecules do not strongly attract to one
another, so the membrane tends to be fluid
and flexible.
• Animal cells have variable shapes because of
this fluidity.
• The overall structure of the membrane is
maintained by the tendency that water has to
form hydrogen bonds.
Cholesterol
• In order for membranes to function properly,
they must be fluid.
• They tend to have the consistency of olive oil.
• There are cholesterol molecules within the
fatty acid tails in animal cells.
• These molecules determine membrane
fluidity, which changes with temperature.
Cholesterol
• The cholesterol molecules allow the
membrane to function at a wider range of
temperatures than it would if they were not
present.
• Plant cells do not have cholesterol molecules;
they depend on saturated or unsaturated fatty
acids to maintain membrane fluidity.
Proteins
• The proteins create diversity for membrane
function.
• Proteins are embedded in the fluid matrix of
the phospholipid bilayer.
• These proteins create the mosaic effect of the
fluid mosaic model.
• There are two major types of proteins
(integral proteins and peripheral proteins).
Proteins
• Integral proteins have both hydrophobic and
hydrophilic regions in the same protein.
– The hydrophobic region contains non-polar amino
acids, is within the mid-section of the membrane,
and holds the protein in place.
– The hydrophilic region is exposed to the water
solutions on either side of the membrane.
Proteins
• Peripheral proteins do not protrude into the
middle hydrophobic region. They remain
bound to the surface of the membrane.
– These proteins are often anchored to an integral
protein.
Proteins
Membrane protein functions
• There are many different proteins with many
different functions within the cell membrane.
The following are the general functions for
proteins on membranes:
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Hormone binding sites
Enzymatic action
Cell adhesion
Cell-to-cell communication
Channels for passive transport
Pumps for active transport
Membrane protein functions
• Proteins that serve as hormone binding sites
have specific shapes exposed to the exterior
of the cell that fit the shape of the specific
hormone.
– The attachment between the protein and the
hormone cause a change in shape that result in a
message being relayed to the interior of the cell.
Membrane protein functions
• Cells often have enzymes attached to
membranes that catalyze chemical reactions.
– These enzymes may be on the interior or exterior
of the cell.
– They tend to be grouped so that a sequence of
reactions, called a metabolic pathway, may occur.
Membrane protein functions
• Cell adhesion is provided by proteins when they
connect together in various ways
– The connections can either be temporary or
permanent.
– These connections are called junctions; they may
include gap junctions or tight junctions.
• Many cell-to-cell communication proteins are
attached to carbohydrate molecules. They
provide identification for other cells to recognize
different types of cells or different species.
Membrane protein functions
• Some proteins contain channels that span the
membrane so that substances can pass through.
– The transport proteins can include passive transport,
which moves materials through the channel from an
area of high concentration to an area of low
concentration.
– In active transport, the proteins shuttle a substance
from one side of the membrane to another by
changing shape. This process requires energy in the
form of ATP. It does not require a difference in
concentration.
Passive and active transport
• Passive transport
– Passive transport does not require energy in the
form of ATP. It does, however, require a
concentration gradient.
Passive and active transport
• Diffusion
– Diffusion is the movement of particles from a region of
high concentration to an area of low concentration.
– Diffusion often requires a membrane in living things.
– For example, oxygen gas moves outside of the cell to the
inside of the cell to be used for cellular respiration.
– The mitochondria use the oxygen gas when it is within the
cell, thus creating a relatively lower oxygen concentration
inside the cell than outside the cell.
– Oxygen then diffuses into the cell.
– Carbon dioxide will diffuse out of the cell because carbon
dioxide is produced in cellular respiration.
Passive and active transport
• Facilitated diffusion
– Facilitated diffusion is a type of diffusion that
involves a membrane with carrier proteins that
are capable of combining with the substance to
aid in movement.
Passive and active transport
• Osmosis
– Osmosis is the passive movement of water across a
semi-permeable membrane.
– Osmosis requires a concentration gradient.
– The semi-permeable membrane only allows certain
substances to pass through the membrane.
– The concentration gradient allows the movement to
occur based on a difference in solute concentrations
on either side of the semi-permeable membrane.
Passive and active transport
– Hyperosmotic (hypertonic) solutions have a higher
concentration of solutes on the outside of the
membrane than inside the membrane.
– Hypo-osmotic (hypotonic) solutions have a lower
concentration of solutes on the outside of the
membrane than inside the membrane.
– Iso-osmotic (isotonic) solutions have equal
concentrations of solutes on both sides of the
membrane.
– Passive transport will continue until there is an equal
concentration of substances on both sides of the
membrane, which is called equilibrium.
Passive and active transport
Simple
Substances other than water move
diffusion
between phospholipid molecules or
through proteins which possess channels.
Facilitated
Non-channel protein carriers change
diffusion
shape to allow movement of substances
other than water.
Osmosis
Only water moves through the
membrane using aquaporins which are
proteins with specialized channels for
water movement.
Passive and active transport
• Size and charge
– Substances that can move passively through the
membrane are affected by two factors, size and
charge.
– Small substances that are non-polar move easily
across the membrane.
– Large substances that are polar do not move easily
across the membrane.
Passive and active transport
– Gases such as oxygen, carbon dioxide, and
nitrogen are examples of small non-polar
substances.
– Ions such as chloride ions, potassium ions, and
sodium ions have a hard time moving across the
membrane passively.
– Molecules such as water and glycerol are small,
uncharged polar molecules that can move fairly
easily across the membrane.
Passive and active transport
• Active transport and the cell
– Active transport requires an input of energy in the
form of ATP.
– Active transport moves substances against a
concentration gradient, which allows the cell to
maintain interior concentrations of molecules that
are different from the exterior environment.
Passive and active transport
– For example, animal cells have a much higher
concentration of potassium ions than the exterior
environment, whereas sodium ions are more
concentrated outside the cell.
– The cell maintains a proper level of these ions by
pumping potassium ions into the cell and sodium
ions out of it.
– An input of energy and a membrane protein must
be involved for this active transport to occur.
Passive and active transport
• The sodium-potassium pump
– A specific protein binds to three intracellular sodium ions.
– Binding of these sodium ions causes phosphorylation by
ATP.
– The phosphorylation causes the protein to change its
shape, thus expelling sodium ions to the exterior.
– Two extracellular potassium ions bind to different regions
of the protein and cause the release of the phosphate
group.
– Loss of the phosphate group restores the protein’s original
shape thus causing the release of potassium ions into the
intracellular space.
Passive and active transport
• The sodium-potassium pump illustrates the
importance of proteins in active transport of
particular substances and how important ATP is in
active transport.
Endocytosis and exocytosis
• Endocytosis and exocytosis are necessary to move
large materials into and out of the plasma membrane.
• Endocytosis allows macromolecules into the cell and
exocytosis allows macromolecules to leave the cell.
• Membranes are fluid, therefore the phospholipid
molecules are not closely packed together due to the
loose connections between fatty acid tails. The
membrane is also stable because of the hydrophilic and
hydrophobic properties of the different regions of
phospholipid molecules.
Endocytosis and exocytosis
• Endocytosis occurs when a portion of the plasma
membrane is pinched off to enclose
macromolecules.
– The pinching involves a change in the shape of the
membrane.
– The result is the formation of a vesicle that enters the
cytoplasm of the cell.
– The ends of the membrane reattach because of the
hydrophobic and hydrophilic properties of the
phospholipids and the presence of water.
– If the membrane weren’t fluid, this would not occur.
Endocytosis and exocytosis
• Exocytosis is the reverse of endocytosis.
– Exocytosis usually begins in the ribosomes of the
rough ER and progresses through a serious of four
steps until the substance that is produced is
secreted to the external environment.
Endocytosis and exocytosis
1. Protein produced by the ribosomes of the ER enters
the lumen of the ER.
2. Protein exits the ER and enters the cis side of the
Golgi apparatus; a vesicle is then involved.
3. As the protein moves through the Golgi apparatus, it
is modified and exits the trans face inside a vesicle.
4. The vesicle with the modified protein moves to and
fuses with the plasma membrane, which results in
the secretion of the contents from the cell.
Endocytosis and exocytosis