Chapter 7 Membrane Structure and Function
Fluid mosaic model of cell membranes (S.J. Singer and G.L. Nicolson-1972)
 Phospholipid layer-is a bilayer, two molecules thick
 Amphipathic-condition where a molecule has a hydrophilic and a hydrophobic region.
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Proteins are individually embedded in the phospholipid bilayer.
Hydrophilic portions of both proteins and phospholipids are exposed to water resulting in a
stable membrane structure.
Hydrophobic portions of proteins and phospholipids are in the non-aqueous environment
inside the bilayer.
The membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids.
The Fluid Quality of Membranes
 Membranes are held together by hydrophobic interactions.
 Most membrane lipids and some proteins can drift laterally within the membrane.
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Molecules rarely flip transversely across the membrane because hydrophilic parts would have to
cross the membrane’s hydrophobic core.
Phospholipids move quickly along the membrane’s plane (averaging 2 µm per second)
Membrane proteins drift more slowly than lipids. (David Frye and Michael Edidin Figure 7.6)
Some membrane proteins are attached to the cytoskeleton and cannot move far.
Membranes solidify if the temperature decreases to a critical point. Critical temperature is lower
in membranes with a greater concentration of unsaturated phospholipids.
Membranes must be fluid to work properly. If a membrane becomes solid, permeability
changes my result and enzymes may be deactivated.
Organisms adapt to cold temperatures by altering membrane lipid composition (e.g. winter
wheat increases the concentration of membrane unsaturated phospholipids and some
hibernating animals enrich membranes with cholesterol).
Proteins of the Membrane ( Figure 7.9 page 129)
1. Integral proteins are inserted into the membrane so that their hydrophobic regions are
surrounded by the hydrocarbon portions of phospholipids. They may be:
a. Unilateral- reach only partway across membrane
b. Transmembrane- with hydrophobic midsections between hydrophilic ends exposed on
both side of the membrane.
2. Peripheral proteins are not embedded but are attached the membrane’s surface.
a. May be attached to integral proteins.
b. On cytoplasmic side, may be held by filaments of cytoskeleton.
3. Functions of membrane proteins:
a. Transport-transmembrane proteins are used in passive and active transport
b. Enzymatic activity- may be part of a metabolic pathway
c. Signal transduction-proteins in membrane may be receptors that relay a message from
the environment into the cell.
d. Cell-cell recognition-some membrane proteins recognize glycoproteins tags of other
cells.
e. Intercellular joining-may form gap junctions or tight junctions between adjacent cells
f. Attachment to ECM and cytoskeleton-proteins attached to cytoskeleton helps
maintain cell shape and one attached to ECM can coordinate extracellular and
intracellular changes
Membranes are Bifacial
 Two lipid layers may differ in lipid composition.
 Membrane proteins have distinct directional orientation.
The molecules that start
out on inside face of ER
end up on the outside
face of the plasma
membrane.
Cell-Cell Recognition-the ability of a cell to determine if other cells it encounters are alike or different
 Is crucial in the functioning of an organism because it is the basis for:
a. the sorting of animal embryos’ cells into tissues and organs
b. the rejection of foreign cells by immune system
 Cell markers on external surface of cell membrane include the following:
a. Glycolipid-carbohydrate bonded to a lipid of cell membrane
b. Glycoprotein-carbohydrate bonded to membrane protein
 Cell markers vary from species to species, between individuals of same species and among cells
in same individual.
Traffic of Small Molecules across a phospholipid bilayer
 Nonpolar Molecules-hydrophobic-pass easily-examples hydrocarbons, O2 (mass =32 Daltons),
CO2 (44 Daltons), N2, Steroids
 Polar Molecules-hydrophilic
a. small polar examples: H2O (18 Daltons) ethanol (46 Daltons) can pass but do not pass
quickly.
b. large polar example Glucose (180 Daltons) will NOT pass easily (very slow)
c. Ions (even small ones) will NOT pass easily (examples: Na+, H+)
Transport Proteins
 Glucose and ions can pass through the cell membrane by avoiding the hydrophobic core of the
bilayer. They pass through Integral transport or carrier proteins. Transport proteins carry
specific molecules or ions across the membrane.
 Channel proteins- have a hydrophilic channel (acts as a tunnel) that certain molecules or ions
can use to pass through the membrane. (example: Aquaporins that transport water across the
cell membrane which allow up to 3 X 109 water molecules per second to pass through the
membrane.
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Carrier Proteins- hold onto molecules and change shape to move the molecules across the
membrane
Types of Transport Proteins
a. Uniport- carries a single solute across the membrane in one direction
b. Symport- carries two solutes at the same time in the same direction
c. Antiport – exchanges two solutes by transporting them in opposite directions.
Diffusion and Passive Transport
Diffusion-the net movement of a substance down a concentration gradient. Results from intrinsic
kinetic energy of molecules (Brownian movement0
Concentration gradient – regular, graded concentration change over a distance or across a membrane
Net directional movement – overall movement away from center of concentration which results from
random movement of molecules in all directions.
MOLECULES MOVE FROM AREAS OF GREATER CONCENTRATION TO AREAS OF LESSER
CONCENTRATION.
A substance diffuses down its OWN concentration gradient and is not affected by the gradients of
other substances.
Passive transport-diffusion across a biological membrane from greater concentration to lesser
concentration without the use of cellular energy.
Hyperosmotic (Hypertonic)- a solution with a greater solute concentration compared to another
solution (usually inside the cell)
CELL
Salt=5%
Water =
95%
ENVIRONMENT OUTSIDE THE CELL IS HYPERTONIC.
Salt = 10%
Water = 90%
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Hypoosmotic (Hypotonic)- a solution with a lower solute concentration compared to inside the cell.
CELL
Salt=5%
Water =
95%
ENVIRONMENT OUTSIDE THE CELL IS HYPOTONIC.
Salt = 3%
Water = 97%
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Isotonic solution – a solution with equal solute concentration outside the cell as compared to inside the
cell.
CELL
Salt=5%
Water =
95%
ENVIRONMENT OUTSIDE THE CELL IS HYPOTONIC.
Salt = 5%
Water = 95%
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WATER BALANCE OF CELLS WITHOUT WALLS (ANIMAL CELLS)
 In isosmotic environment-- NO NET movement of water (cell will remain stableHOMEOSTASIS)
 In hyperosmotic environment  cell WILL LOSE water and crenate (shrivel)
 In hypoosmotic environmentcell WILL GAIN water, swell, perhaps lyse.
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ANIMAL CELLS PREVENT EXCESSIVE LOSS OR GAIN OF WATER BY:
Living is an isosmotic environment
Osmoregulation in a hypoosmotic or hyperosmotic environment.
WATER BALANCE OF CELLS WITH WALLS (PROKARYOTES, SOME PROTISTS, FUNGI, AND
PLANTS)
 In hyperosmotic environment cell WILL LOSE water and plasmolyze.
 In hypoosmotic environment cell WILL GAIN water, Central vacuole swells, cell has turgor
pressure
 In isosmotic environment NO NET MOVEMENT of water, cells are flaccid (limp, wilted).
HYPOTONIC
ISOTONIC
HYPERTONIC
FACILITATED DIFFUSION- Diffusion of solutes across a membrane with the help of transport proteins.
 Passive (because molecules move down their concentration gradient).
 Helps move polar molecules and ions (which would have trouble crossing the phospholipid
layer of cell membrane).
PROPERTIES WHICH TRANSPORT PROTEINS SHARE WITH ENZYMES:
 They both can be saturated with solute.
 They are both specific.
 They can be inhibited by molecules that resemble their normal solute/or substrate.
ONE MAJOR DIFFERENCE:
 Enzymes catalyze chemical reactions and transport proteins do not.
ACTIVE TRANSPORT
 Energy is required.
 Transport proteins pump molecules against their concentration gradient.
 The energy comes from ATP.
EXAMPLES: Sodium-potassium pump (major electrogenic pump in animals) STUDY! KNOW! AND
PROTON PUMP (major electogenic pump of plants, fungi and bacteria).
 Electrogenic pump – A transport protein that generates voltage across a membrane.
 Voltages created by electrogenic pumps are sources of potential energy.
Sodium-Potassium Pump
Proton Pump and Co-transport
Proton Pump
MEMBRANE POTENTIAL AND ION TRANSPORT
Membrane potential - voltage across membranes
 The cell’s inside is negatively charged as compared to outside.
 Membrane potential favors movement of cations into cells and anions out of cells.
TWO FACTORS DRIVE PASSIVE TRANSPORT OF IONS ACROSS THE MEMBRANES:
 Concentration gradient of ion
 Effect of membrane potential on ions.
ELECTROCHEMICAL GRADIENT – The diffusion gradient resulting from the combined effects of
membrane potential and concentration gradient.
FACTORS WHICH CONTRIBUTE TO PLASMA MEMBRANE’S NEGATIVE CHARGE ON THE INSIDE:
 Negatively charged proteins in cytoplasm.
 Selective permeability to various ions.
 Sodium-potassium pump (3 Na+ out for every two K+ in)
ENDOCYTOSIS AND EXOCYTOSIS – FORMS OF ACTIVE TRANSPORT TO MOVE LARGE
MOLECULES ACROSS THE CELL MEMBRANE.
EXOCYTOSIS
Cell secretes macromolecules by fusion of vesicles
with plasma membrane.
Vesicles usually budded from the ER or Golgi and
then migrate to the plasma membrane.
Used by secretory cells to export products.
ENDOCYTOSIS
Cell takes in macromolecules by forming vesicles
derived from plasma membrane.
Vesicle forms from a localized region of the
plasma membrane that sinks inward : pinches off
into cytoplasm.
Used by cells to incorporate extracellular
substances.
THREE TYPES OF ENDOCYTOSIS: (READ descriptions on page 139 in textbook)
 Phagocytosis
 Pinocytosis
 Receptor-mediated endocytosis.