RAVEN & JOHNSON CHAPTER 5 CAMPBELL CHAPTER 8 Membrane Structure & Function

Membrane Structure Lipids (phospholipids) & proteins  They are amphipathic – both hydrophobic & hydrophilic Fluid-mosaic model  (Singer & Nicolson, 1972)

Fluidity Held by weak hydrophobic interactions Movement is lateral

Fluidity Membrane must be fluid to work properly Cholesterol is wedged into phospholipid tails – restrains movement in warm temp (less fluid), but also lowers solidifying temp (more fluid)

Fluidity

Mosaic Embedded proteins (over 50 types) determine specific function of the membrane Integral proteins  Transmembrane  Hydrophobic (  -helical amino acids) & hydrophilic regions

Mosaic Peripheral proteins  On the surface of membrane

Protein Functions

Cell Recognition Crucial Determined by surface molecules (carbohydrates) on plasma membrane

Cell Recognition Oligosaccharides (polysaccharide fewer than 15 units) vary for different types of cells Used as markers to distinguish  Example – A, B, AB, O

Molecular Movement Hydrophobic molecules move across easily  Hydrocarbons, CO 2, O 2 Hydrophilic have trouble moving through hydrophobic core (ions, polar molecules, water, glucose, sugars)  Need transport proteins (facilitated diffusion)

Molecular Movement Proteins are specific to molecule it transports Function: move molecules across the membrane that would not be able to do so otherwise Two types:

Molecular Movement

1) Channel Proteins Water or small ions If water, called aquaporins If needs electrical or chemical stimulus to open & close, called gated channel

Molecular Movement 2) Other carrier proteins  Some transport proteins change shape to transport molecules

Molecular Movement Review: Passive transport (diffusion) Active transport Concentration gradient Equilibrium Osmosis Hypertonic, Hypotonic, Isotonic ( & what happens to the cell)

Molecular Movement

Animal cells need isotonic environment If not, cells must adapt for osmoregulation (water balance)  Ex – Paramecium’s contractile vacuole

Active Transport Against concentration gradient – requires energy (ATP) Ability to have cell with concentrations very different from surroundings  Ex) Animal cell has much K + and little Na + compared to surroundings

Active Transport Accomplished by membrane proteins  ATP transfers P to transport protein, protein changes shape, molecule moves in or out Ex) Sodium-Potassium pump  One ATP pumps three Na + out and two K + in

Active Transport

Cotransport Coupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient

Exocytosis & Endocytosis For macromolecules Exocytosis – cell secretes macromolecules by fusion of vesicles with plasma membrane

Exocytosis & Endocytosis Endocytosis – cells takes in macromolecules by forming new vesicles Three types:  Phagocytosis (particles)  Pinocytosis (any liquid)  Receptor-mediated endocytosis (specific)

Exocytosis & Endocytosis

Receptor-mediated endocytosis Proteins embedded in membrane – specific receptor sites (area where they are – coated pits) Extracellular substances (ligands) bind to sites – triggers vesicle formation Process enables cell to acquire bulk quantities of specific substances (cholesterol)

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