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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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