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MEMBRANE

STRUCTURE

& FUNCTIONChapter 8

• Cellular membranes are fluid mosaics of lipids and proteins

• Membrane structure results in selective permeability

• Passive transport is diffusion of a substance across a membrane with no energy investment

• Active transport uses energy to move solutes against their gradients

• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

KEY

CONCEPTS

CELLULAR MEMBRANES ARE FLUID MOSAICS OF LIPIDS AND

PROTEINS

• Phospholipids are the most abundant lipid in the plasma membrane

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

MEMBRANE MODELS: SCIENTIFIC INQUIRY

• Membranes have been chemically analyzed and found to be made of proteins and lipids

• Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

Hydrophilichead

Hydrophobictail

WATER

WATER

• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

• Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

• In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

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Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

• Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

• Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

Knife

Plasma membrane Cytoplasmic layer

Proteins

Extracellularlayer

Inside of extracellular layer Inside of cytoplasmic layer

TECHNIQUE

RESULTS

PLASMA MEMBRANE

�Selectively permeable –allows some substances to cross more easily than others

�Fluid mosaic model – fluid structure w/ a “mosiac” of proteins in or attached to a double layer (bilayer) of phospholipids

�Composed primarily of lipids, proteins & carbohydrates

• Not static – held together by hydrophobic interactions ; weaker than covalent bonds

• Lipids and proteins can drift laterally (within the membrane plane)

• Remain fluid as temperatures decrease

• Phospholipids then solidify

• Temp. at which this occurs depends on type of lipid present

“FLUID”

MEMBRANES• Unsaturated phospholipids

– lower temps due to kinks in tails

• Saturated phospholipids packed together more tightly

• Cholesterol (steroid) which is wedged between phospholipids also affected by temp.

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• Warm temp (37C) makes membranes less fluid ; restricts movement of membrane

• Permeability of membrane dependent on fluidity

• Solidified membranes less permeable & enzymes are inactive

• Cells alter lipid composition of their membrane to temp conditions

• Proteins determine the function of the membrane

• Integral proteins

– Penetrate the hydrophobic core of lipid bilayer

– Some are transmembrane –completely span the membrane

– Hydrophobic region consists of nonpolar amino acids

– Hydrophilic region exposed to aqueous solution on either side of membrane

MEMBRANE

PROTEINS

• Peripheral Proteins

– Not embedded in the lipid bilayer

– Appendages that are loosely bound to surface of membrane

PROTEI

N

STRUC

TURE

DETER

MINES

FUNCT

ION

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• Hydrophobic molecules (hydrocarbons, oxygen & carbon dioxide) dissolve in lipid bilayer with ease

• Ions & polar molecules (hydrophilic) blocked by the hydrophobic core of lipid bilayer

• Other polar molecules like glucose, other sugars & water only pass slowly

MEMBRANE

PERMEABILITY

• Proteins that allow ions & some polar molecules to avoid the lipid bilayer

• Ways to aid in moving (translocating) substances across the membrane

– Channel proteins – hydrophilic channels that tunnel molecules across

– Aquaporins – channel water

– Carrier proteins – hold on to molecules & change shape in ways that shuttle them across

TRANSPORT

PROTEINS

• Movement of molecules with no energy being used

• Diffusion – movement of molecules from regions of high concentration to regions of low concentration

• Down the concentration gradient

PASSIVE

TRANSPORT

• Diffusion of water

• Moves from regions of high concentration to low concentration

• Tonicity – ability of a solution to change the shape / size of a cell by gaining or losing water

• Directions of water diffusion depends upon the concentration of solutes in the cell’s environment

OSMOSIS • Isotonic – solute concentration is the same on each side of the plasma membrane

• No net movement of water

• Water flows in and out of cell at same rate

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• Hypertonic – solute concentration is higher outside the plasma membrane

• Water diffuses out of cell at a faster rate

• Causes the cell to shrivel

• Crenated – shrunken cell

• Hypotonic – solute concentration is lower outside the plasma membrane

• Water diffuses into cell at a higher rate

• Cell swells and may burst (lysis)

• If no cell wall (animal, protists) – contractile vacuole

• If cell wall (plant, prokaryotes, fungi & some protists)

– As water diffuses into cell (hypotonic), pressure is exerted on the cell wall – turgid

– As water diffuses out of cell (hypertonic), no pressure of membrane on cell wall – flaccid

– Cell shrivels & plasma membrane pulls away from cell wall - plasmolysis

OSMOREGULATION –

CONTROL OF WATER

BALANCE

• Passive transport with aid of proteins

• Transport proteins are specific –only transport particular substances

• 2 types: carrier proteins & channel proteins

• Channel proteins can be aquaporins(channel water) or gated channels (stimulus causes them to open or close) ; stimulus can be chemical or electrical

• Carrier proteins are triggered by binding & release of transporting molecule to change shape

FACILITATE

DIFFUSION

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ACTIVE TRANSPORT

• Moving molecules against the concentration gradient (low to high)

• Cell must expend energy

• Only involves carrier proteins

• Energy supplied by ATP

• More Na outside cell than inside

• More K inside cell than outside

• Pump cycles 3 Na+ out of cell and 2 K+ into the cell

• Carrier protein receives energy through phosphorylation – high energy phosphate of ATP binds to protein

NA-K PUMP

• Membrane proteins that generate energy by producing a voltage across the membrane

• Translocates positive charge in form of H+

• Energy is stored for other cellular activities

ELECTROGENIC PUMP /

PROTON PUMPS

• A single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes

• A substance that has diffused across a membrane can be used to do work as it moves back in

• Ex. In plants sucrose enters a cell in the company of a H+ ion

COTRANSPORT

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BULK TRANSPORT

• Movement of large molecules or particles across the membrane via vesicles

• Exocytosis – secretion of particles out of the cell ; removal of secretory proteins by the Golgi apparatus

• Endocytosis – cell takes in macromolecules by forming new vesicles from the plasma membrane

• Membrane forms a pocket that sinks inward then pinches off to form a vesicle

• Phagocytosis – “cellular eating” ; cell engulfs large particle forming a vacuole which fusing to a lysosome to be digested

• Pinocytosis – “cell drinking” ; cell gulps droplets of extracellular fluid into vesicles; not specific to substance it transports

• Receptor-mediated endocytosis – coated pits form vesicles when specific molecules (ligands) bind to receptors on the cell surface

3 TYPES OF

ENDOCYTOSIS