Chapter 3
The Plasma Membrane and
Membrane Potential
Objectives
• Know the composition of the plasma membrane
• Understand the functions of the plasma membrane
• Explain how the various forms of membrane transport
work
• Know the functions of the sodium-potassium pumps
• Know what a membrane potential is and how it is
established
The Plasma Membrane
• Surrounds all living cells
• Composed of a phospholipid bilayer
– Polar hydrophilic heads on the outside, nonpolar
hydrophobic tails on the inside
– Trilaminar appearance
Phospholipid Bilayer
Composition of Plasma Membrane
• Trilaminar structure
• Composition includes
–
–
–
–
Phospholipids
Proteins
Cholesterol
Carbohydrates
• Fluid mosaic model
– Proteins and cholesterol embedded
Bilayer Function
• Provides structure and fluidity to the
membrane
• Prevents hydrophilic substances from crossing
the membrane
Membrane Protein Function
• Transmembrane (integral) proteins
– Channels
• Leak or gated
– Carrier (transport)
• Selectively transport substances across membrane
– Docking-marker acceptors or receptors
– Membrane-bound enzymes
– Cell adhesion molecules (CAMs)
• Caherins
• Integrins
Membrane Carbohydrate Function
• Function as “self” markers
• Allow cells to identify themselves as belonging to
you
• Allows cells to identify cells of the same type
• Used during tissue formation to ensure that the
same type of cells are being used
– Also ensure that tissues do not overlap
Cell Adhesion
• Plasma membrane involved in cell adhesion
– Three ways: CAMs, ECM, and specialized junctions
• Extra cellular matrix
– The “glue” that holds the cells together
– Network of fibrous proteins embedded in gel-like fluid
• Collagen, elastin, fibronectin
– Secreted by fibroblasts
– Cellular regulation and protection
• Specilaized junctions
– Desmosomes
– Tight junctions
– Gap junctions
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Cell Adhesions
• Tight junctions
• Gap junctions
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Membrane Transport
• Selective permeability
– Plasma membrane controls what enter and exits
the cell
– Determined by two properties
• Size
• Solubility in lipids
– Can be unassisted or assisted
Diffusion
• Diffusion (simple diffusion)
– Net movement from an area of higher concentration to an area of
lower concentration
– Does not require energy, passive process
• Fick’s law of diffusion
– Effects of factors that influence the rate of diffusion
•
•
•
•
•
Magnitude of concentration gradient
Surface area of membrane
Lipid solubility of substance
Molecular weight
Distance across membrane
Osmosis
• Osmosis
– Diffusion of water across a selectively permeable membrane
•
•
Osmotic pressure , osmolarity (milliosmoles/L) 300 mOsm normal in body fluids
Hydrostatic pressure
– Aquaporins – protein channels that allow water the diffuse in and out
of cell
– Tonicity refers to the effect the solution will have on cell volume
• Hypertonic
– Water out of cell, cell shrinks
• Hypotonic
– Water into cell, cell swells
• Isotonic
– Water movement is at equilibrium, cell retains its normal shape
Osmosis
Effects of Tonicity
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Assisted Membrane Transport
• Two types
– Carrier mediated
• Transport of small hydrophilic molecules
• Require a conformational change of as transport
protein
• Depends on: specificity, saturation (Tm), competition
– Vesicular transport
• Transport of large molecules or many molecules at a
time
• Formation of vesicles needed
Facilitated Diffusion and Active
Transport
• Facilitated diffusion
– Similar to simple diffusion, but requires a carrier
– Glucose and amino acids
• Active transport
– Moves substances against their concentration
gradient
• Requires energy
• Primary and secondary active transport
Sodium Potassium Pump
Secondary Active Transport
• Depends on primary active transport
• Symports
– SGLT
• Antiports
Bulk Passage
• Endocytosis
– Phagocytosis
– Pinocytosis
– Receptor-mediated
• Exocytosis
Membrane Potential
• Separation of opposite
charges across the
plasma membrane
– Occur in thin areas
adjacent to the
membrane
• Electrical potential
measured in mV
• Na+, K+, A- responsible
for maintaining resting
membrane potential
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Membrane Potential
• K+ more concentrated in the ICF
– If K+ diffuses out, the ICF becomes more negative
– K+ attracted by negative charge, moves into cell
– Equilibrium is reach, membrane potential will equal -90mV
• Na+ more concentrated in the ECF
– Diffuses into the cell
– Inside becomes more positive
– Equilibrium potential of Na equals 60mv
• Must consider both at the same time
– Resting membrane potential typically -70mV
– K+ has more influence because membrane more permeable to it