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Membranes

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Outline

• Phospholipid Bilayer• Fluid Mosaic Model• Membrane Proteins• Diffusion• Facilitated Diffusion• Osmosis

– Osmotic Balance• Bulk Transport• Active Transport

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Phospholipid Bilayer

• Phospholipid has two fatty-acid chains attached to its backbone.

– One end is strongly nonpolar while the other end is strongly polar.

polar head oriented toward water and nonpolar tails oriented away from water

bilayer is stable because water’s affinity for hydrogen bonding never stops

Fatty acidPhosphorylated

alcohol

Polar(hydrophilic) region

Nonpolar (hydrophobic) region

Fatty acidGLYCEROL

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Fluid Mosaic Model

• Plasma membrane is composed of both lipids and globular proteins.

– Membrane proteins are not very soluble in water.

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Cell Membrane

• Cell membrane components– phospholipid bilayer– transmembrane proteins– interior protein network– cell surface markers– extra cellular matrix (ECM)

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Membrane Proteins

• transporters• enzymes• cell surface identity markers• cell adhesion proteins• attachments to cytoskeleton

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Plasma Membrane Proteins

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Membrane Protein Structure• Anchoring proteins in the bilayer• Transmembrane proteins

Cytoplasmic sideof cell membrane

Cytoskeletalproteins

Junctionalcomplex

100 nm

Actin

Spectrin

Linkerproteincomplex

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Membrane Protein Structure• Transmembrane proteins

– single-pass anchors– multiple-pass channels and carriers– pores

Phospholipids

Polar areasof protein

Cholesterol

Nonpolar areas ofprotein

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)-helices in thecell membrane

H+

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Solutions

• Intra cellular fluid (ICF) - within cells• Extra cellular Fluid (ECF) - outside cells

– Inter cellular = tissue fluid = interstitial fluid– Plasma = fluid portion of blood

• Composition of fluids change as substances move between compartments

– nutrients, oxygen, ions and wastes move in both directions across capillary walls and cell membranes

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Selective Permeability of Membrane

• Lipid bilayer– permeable to nonpolar, uncharged molecules --

oxygen, CO2, steroids

• Transmembrane proteins act as specific channels– small and medium polar & charged particles

• Macromolecules unable to pass through the membrane

– vesicular transport

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Passive Transport

• Diffusion– Random motion causes a net movement

of substances from regions of high concentration to regions of low concentration – down their concentration gradient. AnimationAnimation

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Diffusion Rates

• Factors affecting diffusion rate through a membrane

– temperature - temp., motion of particles– molecular weight - larger molecules move

slower– steepness of concentrated gradient - difference, rate

– membrane surface area - area, rate– membrane permeability - permeability,

rate

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Selective Membrane Transport

• Cell transport channels are selectively permeable, as only certain molecules are allowed to enter.

– Ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins.

ion channels

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Facilitated Diffusion

• Carriers transport ions and other solutes across the plasma membrane.

– Facilitate movement by physically binding molecules on one side of the membrane, and releasing them on the other down their concentration gradient. animationanimation

essential characteristicsspecificpassive (no ATP) transport saturation

Outside of cell

Inside of cell

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Osmosis• In an aqueous solution, both water and solutes diffuse down

concentration gradients.• Net movement of water through a selectively permeable

membrane from an area of high water concentration to an area of lower water concentration

• Only occurs if membrane is permeable to water but not to certain solutes

Aquaporins = channel proteins specialized for osmosis

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Osmotic Pressure

• Amount of hydrostatic pressure required to stop osmosis

• Osmosis slows due to filtration of water back across membrane due to increased hydrostatic pressure

• AnimationAnimation

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Tonicity

• Tonicity - ability of a solution to affect fluid volume and pressure within a cell

– depends on concentration and permeability of solute• Hypotonic (hypoosmotic) solution

– low concentration of nonpermeating solutes (high water concentration)

– cells absorb water, swell and may burst (lyse)• Hypertonic solution (hyperosmotic)

– has high concentration of nonpermeating solutes (low water concentration)

– cells lose water + shrink (crenate)• Isotonic (isoosmotic) solution = normal saline

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Maintaining Osmotic Balance

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Osmotic Pressure

• Hydrostatic pressure - pressure of the cytoplasm pushing out against the cell membrane

• Osmotic pressure - pressure that must be applied to stop the osmotic movement of water across a membrane

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Bulk Transport

• Endocytosis - enveloping food

– phagocytosis - material taken in is in particulate form

– pinocytosis - material taken in is in liquid form

– receptor-mediated - transport of specific molecules

• Exocytosis - discharge of material from vesicles at the cell surface

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Endocytosis / Exocytosis

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Active Transport

• Active transport involves the expenditure of energy to move substance against their concentration gradient.

– involves highly selective protein carriers within the membrane

sodium-potassium pump• Secondary active transport

– coupled transport - using energy stored in a gradient of a different molecule

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Sodium-Potassium Pump

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Secondary Active Transport

• Uniporter– carries only one solute at a time

• Symporter– carries 2 or more solutes simultaneously in same

direction (cotransport)• Antiporter

– carries 2 or more solutes in opposite directions (countertransport) (animationanimation)

sodium-potassium pump brings in K+ and removes Na+ from cell

• Any carrier type can use either facilitated diffusion or active transport (primary or secondary)

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Digitalis

• Slows the sodium pump, which lets more Na+ accumulate in the heart muscle cells.

• Less Na+ concentration gradient across the membrane

• Na+/Ca+2 antiporters slow down so more Ca+2 remains inside the cardiac cells

• Strengthening the force of contraction• Balance between concentration of Na+ and

Ca+2 in cytosol & extracellular fluid is important

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Summary

• Phospholipid Bilayer• Fluid Mosaic Model• Membrane Proteins• Diffusion• Facilitated Diffusion• Osmosis

– Osmotic Balance• Bulk Transport• Active Transport

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