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Membrane Structure & Function
Lecture 3
Fall 2008
Cellular Membranes
Present in all cell types
Function:
• Separates the internal from the external
environment
• Regulate chemical exchanges within the
environment
– Chemical reactions more efficient
• Dynamic selective barrier
• Mosaic of lipids & proteins
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Proteins
Protein
• Complex polymer made from amino acids
• Each protein type has unique 3D shape
• Many different protein types (tens of thousands)
Functions:
• Structural
• Storage
• Muscle movement (Contractile proteins)
• Transport
• Enzymes (catalyze chemical reactions)
• Defense
• Signaling
• Cell-cell communication/recognition
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Proteins
Amino acids
• 20 types
• Basic structure of each
amino acid is the same
• Unique side group
• Only left-handed amino
acids found in living
organisms
See Fig. 5.17
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Proteins
Polypeptide chains
• Amino acids form polypeptide chains
– long chain of amino acids in unique sequence
• Sequence critical
– Change function of protein
See Fig. 5.22
Protein Structure
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4 levels of structure
• Primary
– Unique sequence of
amino acids in polypeptide
chain
•
Secondary
– Folding of polypeptide
chain –hydrogen bonding
•
Tertiary
– Total 3D structure
•
Quaternary
– If composed of more than
one polypeptide chain
Fig. 3.24
Shape sensitive to changes
in the environment
(e.g., heat, ph)
Denaturation: loss of
structure in a protein
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Lipids
Lipid
• Nonpolar
• Hydrophobic
– Does not dissolve
readily in water
Functions
• Energy storage
• Photosynthetic pigments
• Cell-cell signaling
(hormones)
• Waterproof coating
• Act as vitamins
• Plasma membrane
Three types
• Fats
– Dietary fats
• Steroids
– Hormones
• Phospholipids
– Plasma membrane
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Lipids
Phospholipid structure
• Head
–
–
–
–
Hydrophilic (polar)
Glycerol
Phosphate group (negatively charged)
Other polar/charged molecules
• Tail
– Two fatty acids
– Hydrophobic (non-polar)
• Diversity
– Fatty acids
– Groups attached to phosphate group in
head
Fig. 5.13
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Membrane Structure & Function
Phospholipid bilayer
• Phospholipid
– Amphipathic – hydrophobic & hydrophilic region
• Bilayer
– two layered membrane
• Spontaneous self-assembly of phospholipids
• Formation of cell membranes critical for evolution of cells
Fig. 5.14
Membrane Structure & Function
Fluid Mosaic Model
• Dynamic
– Molecules move freely
past one another
• Mosaic
– Many proteins
embedded in
phospholipid bilayer
[Read “Membrane
Modes: Scientific
Inquiry”, pgs. 126-127]
Fig. 7.3
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Membrane Fluidity
Phospholipid movement
• Lateral
• Flip-flop
Protein movement
• Lateral (some)
Temperature & Fluidity
• At lower temperature, membrane
solidifies
• Unsaturated hydrocarbon tails fluid
at lower temps
• Cholesterol acts as temperature
buffer
– At high temps, decrease fluidity
– At low temps, prevents solidification
Fig. 7.5
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Fig. 7.7
Membrane Proteins
Membrane protein
functions
• Transport
• Enzyme activity
• Signal transduction
• Cell-cell
recognition
• Intercellular joining
• Attachment
See Fig. 7.9
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Membrane Proteins
Integral proteins
• Penetrate hydrophobic core
– Transmembrane: span the
membrane
– Partial
• Amphipatic
• Directional
Peripheral proteins
• Not embedded
• Attached to membrane surface
• Associated with integral
proteins
Fig. 7.8
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Cell-Cell Recognition
• Glycoproteins
– Protein + carbohydrate
• Glycolipid
– Lipid + carbohydrate
Fig. 7.9
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The Endomembrane System:
A Closer Look
Membranes
• Distinct inside & outside
faces
• Lipid layers can be
different in composition
• Proteins have specific
orientation
• Asymmetrical
arrangement of proteins,
lipids & associated
carbohydrates in PM
determined by ER & GA
Fig. 7.10
Transport in cells
How do substances move into and out of cells?
1. Path
• Way of getting from one place to another
2. Driving force
• Concentration gradient
• Electrochemical gradient
• Pressure gradient
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Transport in cells: Paths
• Permeability of lipid bilayer
• How do other substances enter/exit cell?
– Proteins
Type of Molecule
Example
Permeability
Water
H 2O
Yes (slow)
Gases
CO2, O2
Yes
Small, uncharged
polar molecules
Ethanol
Yes
Large, uncharged
polar molecules
Glucose
No (slow)
Large charged
molecules
ATP, Amino Acids
No
Ions
H+, K+
No
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Transport in cells: Driving Force
• Concentration gradient
– Variation across space in the concentration of a
dissolved substance, from a region of high
concentration to a region of low concentration
• Electrochemical gradient
– The combined effect of a concentration gradient
and an electrical gradient. Affects the movement of
ions across plasma membranes
• Electrical gradient – differences in electrical charges
across a plasma membrane (e.g., Na+)
• Pressure gradient
– Differences in pressure, from areas of high
pressure to areas of low pressure (e.g.,
cardiovascular system)
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Diffusion
• Tendency of molecules or ions (solutes) to
spread out and equalize their concentrations
• Individual molecules move constantly &
randomly, but overall tendency is directional
• A substance tends to diffuse down its
concentration gradient
• Down = from an area of high concentration to an area of
low concentration
• Spontaneous - does not require input of
energy
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Diffusion
• A substance tends to diffuse down its
concentration gradient
• Continues until dynamic equilibrium reached
– Concentration of molecules the same on each side
of the membrane
– Individual molecules continually in motion
Fig. 7.11
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Diffusion
• Each substance
diffuses down its own
concentration
gradient
• Independent!
See Fig. 7.11
Transport in cells: Passive
Passive transport
• Passive = no extra energy
(ATP) required
Path
• Through plasma membrane
– Gasses; small, uncharged
polar molecules; water
Driving force
• Simple diffusion
• Concentration or
electrochemical gradients (H+,
glucose)
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Osmosis
Osmosis
• Special case of diffusion
• Diffusion of water across
a selectively permeable
membrane
• Equalizes its
concentration gradient
– Path: plasma membrane
permeable to water
– Driving force:
concentration gradient
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Osmosis
Tonicity
• Ability of a solution to cause a cell to gain or lose water
Hypotonic
• Solution with a lower concentration of solute
• Solution with the higher concentration of water
Hypertonic
• Solution with a higher concentration of solute
• Solution with the lower concentration of water
Isotonic
• The solute concentrations are equal on either side of the
membrane
• No net movement of water across the plasma membrane
Water Balance – Animal Cells
• Isotonic state is the functional norm
• Osmoregulation – to keep cells in isotonic state
with external environment
See Fig. 7.13
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Water Balance – Plant Cells
Functional state for plants - in hypotonic solution
• Turgid (“firm”)
– Turgor pressure – pressure within a plant cell caused
by the pressure of the incoming water vs. the
pressure of the cell wall
– Cell wall prevents plasma membrane from bursting
(Plasmolysis)
See Fig. 7.13
Transport in Cells: Passive
Path
•
Diffusion
– Directly through plasma membrane
How do other molecules get
inside/outside cells?
Transport or carrier proteins
•
Facilitated diffusion
– Through transport proteins
– Highly specific (channels, carriers)
Fig. 7.5
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Transport in Cells: Passive
Driving force
• Concentration or
electrochemical gradients
(H+, glucose)
• No extra energy (ATP)
required
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Transport in Cells: Passive
Channel Proteins
• Provide hydrophilic corridor
– Aquaporin
• A channel protein that facilitate movement of water
across a membrane
– Ion channels
• Specific to ions
• May be gated (open/close in response to electrical
stimulus)
Carrier Proteins
• Change conformation in response to stimulus
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Active Transport
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• Allows movement of a substance against its
concentration or electrochemical gradient
• Cell can maintain an internal environment that is
different from the external environment
• Requires energy (ATP)
Path
• Carrier proteins (Pumps)
– Change conformation (shape) when they bind with
ATP
See Fig.7.17
Driving Force
– Requires energy (ATP)
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Active Transport
Sodium-potassium pump
• Most important pump in animals
• Pumps ions against their steep concentration gradients
• ATP
Fig. 7.16
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Active Transport & Membrane Potential
• Membrane potential
– Voltage across a membrane
– Cytoplasm more negative in charge relative to
extracellular fluid
– Unequal distribution of anions & cations
• Sodium-potassium pump
– Moves 3 sodium (Na+) out of the cell
– Moves 2 potassium (K+) into the cell
– Generates an electrochemical gradient across
membrane = electrogenic pump
Active Transport & Membrane Potential
• Electrochemical gradients
– The combined effect of a concentration
gradient and an electrical gradient. Affects
the movement of ions across plasma
membranes
• Electrical gradient – differences in electrical
charges across a plasma membrane (e.g.,
Na+)
• Due to membrane potential
– Favors passive transport of cations into cell
– Favors passive transport of anions out of cell
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Active Transport & Membrane Potential
Most important pump in plants: proton (H+) pump
• Moves H+ outside cell
• Generates an electrochemical gradient across
membrane
– More positive on the outside of the cell
Fig. 7.19
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Active Transport: Coupled Transport
Paths
• Cotransporters
– Brings two substances into a cell
• Exchangers
– Brings one substance into a cell
and another substance out of a
cell
Driving force
– Uses the movement of one
molecule (H+) going down/with
its concentration gradient to
move another molecule against
its concentration gradient
– Linked to pumps
Fig. 7.19
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Transport of large molecules
Large molecules (e.g., proteins) too big for
transporters
• Use vacuoles formed from plasma membrane
– Exocytosis – secretion of cellular contents to the
outside of a cell by fusion of vacuoles (vesicles) to the
plasma membrane
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Transport of large molecules
Endocytosis
• Uptake of extracellular material by engulfing and
pinching off the plasma membrane to form a small
membrane-bound vacuole (vesicle) in the cell
3 Types of Endocytosis
• Pinocytosis
• Phagocytosis
• Receptor-mediated cytosis
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Transport of large molecules
Pinocytosis (“cellular drinking”)
• Uptake of extracellular fluid by endocytosis
• Low specificity
Fig. 7.20
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Transport of large molecules
Phagocytosis (“cellular eating”)
• The engulfment and uptake of a particle or cell
by an extension of another cell’s plasma
membrane.
• Moderate specificity
Fig. 7.20
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Transport of large molecules
Receptor-mediated cytosis
• Endocytosis triggered by the binding of certain
macromolecules outside the cell to membrane proteins
• High specificity
• Ligand
– molecule that binds specifically to a receptor site of another
molecule
Fig. 7.20
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