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CHAPTER 5

MEMBRANE

STRUCTURE

AND TRANSPORT

Prepared by

Brenda Leady, University of Toledo

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Biological Membranes

Basic framework of the membrane is the phospholipid bilayer

Phospholipids are amphipathic moleculesHydrophobic (water-fearing) region faces inHydrophilic (water-loving) region faces out

Membranes also contain proteins and carbohydratesRelative amount of each vary

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Fluid-mosaic model

Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules

Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane

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Proteins bound to membranes Integral membrane proteins

Transmembrane proteins One or more regions that are physically embedded in the

hydrophobic region of the phospholipid bilayerLipid anchors

Covalent attachment of a lipid to an amino acid side chain within a protein

Peripheral membrane proteinsNoncovalently bound to regions of integral membrane

proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids

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Approximately 25% of All Genes Encode Membrane Proteins

Membranes are important biologically and medically Computer programs can be used to predict the number

of membrane proteins Estimated percentage of membrane proteins is

substantial: 20–30% of all genes may encode membrane proteins

This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes

Function of many genes unknown- study may provide better understanding and better treatments

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Membranes are semifluid

Fluidity- individual molecules remain in close association yet have the ability to readily move within the membrane

Semifluid- most lipids can rotate freely around their long axes and move laterally within the membrane leaflet

“Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneouslyFlippase requires ATP to transport lipids from one

leaflet to another

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Factors affecting fluidity

Length of fatty acyl tailsShorter acyl tails are less likely to interact, which

makes the membrane more fluid Presence of double bonds in the acyl tails

Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid

Presence of cholesterolCholesterol tends to stabilize membranesEffects depend on temperature

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Experiments on lateral transport

Larry Frye and Michael Edidin conducted an experiment that verified the lateral movement of membrane proteins

Mouse and human cells were fused Temperature treatment- 0°C or 37°C Mouse membrane protein H-2 fluorescently

labeled 0°C cells- label stays on mouse side 37°C cells- label moves over entire cell

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FRAP

Watt Webb and colleagues used fluorescence recovery after photobleaching (FRAP)

Proteins on the surface of a cell were covalently labeled with a fluorescent chemical

Small area of cell photobleached leaving white spot Over time, bleached molecules within the white spot

spread outward, and the white region filled in with red fluorescent molecules

Indicates that proteins can laterally move in the membrane

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Not all integral membrane proteins can move Depending on the cell type, 10–70% of

membrane proteins may be restricted in their movement

Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally

Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix

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Glycosylation

Process of covalently attaching a carbohydrate to a protein or lipidGlycolipid – carbohydrate to lipidGlycoprotein – carbohydrate to protein

Can serve as recognition signals for other cellular proteins

Often play a role in cell surface recognition Protective effects

Cell coat or glycocalyx - carbohydrate-rich zone on the cell surface shielding cell

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Electron microscopy

Transmission electron microscopy (TEM), uses a biological sample that is thin sectioned and stained with heavy-metal dyes

Dye binds tightly to the polar head groups of phospholipids, but it does not bind well to the fatty acyl chains

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FFEM

Freeze fracture electron microscopy, specialized form of TEM, can be used to analyze the interiors of phospholipid bilayers Sample is frozen in liquid nitrogen and fractured with

a knife Due to the weakness of the central membrane region,

the leaflets separate into a P face (the protoplasmic face that was next to the cytosol) and the E face (the extracellular face)

Can provide significant three-dimensional detail about membrane protein form and shape

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Selectively permeable

Structure ensures …Essential molecules enterMetabolic intermediates remainWaste products exit

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Phospholipid bilayer is a barrier

Hydrophobic interior makes formidable barrier

DiffusionMovement of solute from an area of higher

concentration to an area of lower concentrationPassive diffusion- without transport protein

Solutes vary in their rates of penetration

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Cells maintain gradients

Transmembrane gradientConcentration of a solute is higher on one

side of a membrane than the other Ion electrochemical gradient

Both an electrical gradient and chemical gradient

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Passive transport

Passive transport does not require an input of energy

2 typesPassive diffusion

Diffusion of a solute through a membrane without transport protein

Facilitated diffusion Diffusion of a solute through a membrane with the aid of

a transport protein

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Tonicity Isotonic

Equal water and solute concentrations on either side of the membrane

HypertonicSolute concentration is higher (and water

concentration lower) on one side of the membrane Hypotonic

Solute concentration is lower (and water concentration higher) on one side of the membrane

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Isotonic

Hypertonic

Hypotonic

Outside the cell Inside the cell

The solution and cell are isotonic

The solution is hypertonic to the cell

The solution is hypotonic to the cell

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Osmosis

Water diffuses through a membrane from an area with more water to an area with less water

If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell

Osmotic pressure- the tendency for water to move into any cell

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Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape

Crenation- shrinking in a hypertonic solution

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A cell wall prevents major changes in cell size

Turgor pressure- pushes plasma membrane against cell wallMaintains shape and

size Plasmolysis- plants wilt

because water leaves plant cells

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Agre Discovered That Osmosis Occurs More Quickly in Cells with Transport Proteins That Allow the Facilitated Diffusion of Water Water passively diffuses across plasma membranes Certain cell types allow water to move across the plasma membrane

at a much faster rate than would be predicted by passive diffusion Peter Agre and his colleagues first identified a protein that was

abundant in red blood cells and kidney cells, but not found in many other cell types

CHIP28 Striking difference was observed between frog oocytes that

expressed CHIP28 versus the control Aquaporins Agre was awarded the Nobel Prize in 2003 for this work

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Transport proteins

Transport proteins enable biological membranes to be selectively permeable

2 classesChannelsTransporters

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Channels

Form an open passageway for the direct diffusion of ions or molecules across the membrane

Aquaporins

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Most are gated- open or closeLigand-gated Intracellular

regulatory proteinsPhosphorylationVoltage-gatedMechanosensitive

channels

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Transporters

Also known as carriers Conformational change

transports solute Principal pathway for the

uptake of organic molecules, such as sugars, amino acids, and nucleotides

Key role in export

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Transporter types Uniporter

single molecule or ion

Symporter/ cotransporter

2 or more ions or molecules transported in same direction

Antiporter 2 or more ions or

molecules transported in opposite directions

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PumpCouples

conformational changes to an energy source, such as

ATP-driven pumps ATP hydrolysis can be uniporters,

symporters, or antiporters

Active transport

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Active transport

Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration

Energetically unfavorable and requires the input of energy

Primary active transport Directly use energy to transport solute

Secondary active transport Use pre-existing gradient to drive transport of solute

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ATP-Driven Ion Pumps Generate Ion Electrochemical Gradients Na+/K+-ATPase

Actively transport Na+ and K+ against their gradients by using the energy from ATP hydrolysis

3 Na+ exported for 2 K+ imported into cell Antiporter Electrogenic pump- export 1 net positive charge

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Exocytosis/ Endocytosis

Transport larger molecules such as proteins and polysaccharides, and even very large particles

Exocytosis Material inside the cell, which is packaged into

vesicles, is excreted into the extracellular medium Endocytosis

Plasma membrane invaginates, or folds inward, to form a vesicle that brings substances into the cell

Receptor-mediated endocytosis Pinocytosis Phagocytosis

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