The Plasma Membrane and Membrane Potential
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Transcript of The Plasma Membrane and Membrane Potential
The Plasma Membrane and Membrane Potential
Chapter 3
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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academic.brooklyn.cuny.edu
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 faculty.irsc.edu
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