Chapter 7 Cell Membranes Cell Membranes The plasma

Chapter 7 Cell Membranes

Cell Membranes • “The plasma membrane is the edge of life, the boundary that separates the living cell from its surroundings. ” – – Campbell Biology 9 th Edition • “We’re living on the edge, You can’t help yourself from fallin’” – Aerosmith, 1993 2

Basic Membrane Structure • Bi-Layer; 8 nm thick (0. 000008 mm) – Hydrophilic phosphate head – Hydrophobic fatty acid tail • Tight bonded structure – Semi-permeable 3

Basic Membrane Structure • Small non-polar molecules can easily pass across hydrophobic zone – Polar molecules cannot cross hydrophobic area • Large molecules will not fit between phospholipids 4

Review: Diffusion • How will these substances get across a plasma membrane? – O 2 – Testosterone (a sterol lipid) – Glucose – H 2 O – H+ 5

Fluid Mosaic • Not static sheets – Subject to lateral and vertical movement. • Vertical (transverse) movement rare – Hydrophilic heads must cross hydrophobic zone. 6

Endocytosis/Exocytosis • Endocytosis – Engulfment – Formation of an endosome (membrane vesicle) around item outside of the cell • Exocytosis – Secretion – Release of material contained inside of an endosome 7

Endocytosis 8

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Birth of a Protein (Exocytosis) 10

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Antifreeze Glycoproteins • Produced in the pancreas – Found in the Blood, extracellular matrix and cell membranes to prevent freezing 12

Daveson-Danielli Sandwich Hypothesis • The phospholipid bi-layer is coated in proteins, on both sides 13

Trans-membrane proteins • Have hydrophylic and hydrophobic regions – Like phospholipids • Can migrate across the membrane – Fluid mosaic model 14

Lateral movement across a membrane • Researchers fused a mouse and human cell • Resulting cell had Human and Mouse specific proteins mixed across the membrane. 15

Cell Fractionation • Freeze cell; then split open along membrane • Each protein stays intact with one layers 16

Protein Anatomy • Proteins have distinct orientations in the membrane – Specific exterior and interior “Sides” • N-terminus – Outside region – extracellular • C-terminus – Inside region – intracellular • α- helices are hydrophobic – Non-helical regions are typically hydrophilic 17

Membrane structures 18

Roles of membrane proteins • Transport/transmiss ion across the membrane – Signal Transduction – Intercellular recognition • Enzymatic Activity • Adhesion – Intercellular bonding – Extracellular bonding 19

Molecular synthesis & exocytosis 20

Surface Receptors • Extracellular signals are be bound by receptors – Membrane bound proteins • Trigger the cell to respond – Ingest 21

Receptors • Bind specific ligands (signals) • Trigger a cellular response – Signaling – Send information in to the cell 22

Receptor binding 23

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Cell-Cell Signaling • Instead of receiving signals; send them – To neighboring cells – Extracellular signals attached to transmembrane protein • Receptor proteins on other cells bind – Signal transduction – Adhesion • extracellular tags are used to identify cell types 25

Adhesion • Cells can adhere to other cells or an external matrix through transmembrane proteins 26

Cell-cell junctions • Tight Junctions – Cells bond by trans-membrane proteins • Desmosomes – protein anchors inside cell – Stabilize cytoskeleton • Gap junctions – Allow materials to pass between cells 27

Transport across a Membrane • Phospholipid membranes are semi-permeable – Non-polar molecules • e. g. O 2 & CO 2 – Easily pass through hydrophobic region • Transport proteins necessary to pass polar & hydrophilic molecules – H 2 O – Material specific 28

Transport across a Membrane • Passive transport – Molecules travel through trans-membrane protein freely • Active transport – Molecules travel through trans-membrane protein using ATP – Requires energy 29

Diffusion – Passive Transport • Molecules want to disperse to form a equal 30

Diffusion • As table salt is dissolved by water – Individual Na+ & Cl - molecules disperse – Form an even concentration across the entire area 31

Two solutes • Solutes diffuse down the concentration gradient – From high to low • So long as the barrier is permeable to both – Diffuse independently – Until equilibrium 32

Facilitated diffusion • Trans-membrane proteins allow molecules to pass – By creating a channel – Or gateway • Control the rate of movement across the membrane 33

Active Transport • Works against the gradient – Creating higher concentrations on either side of a barrier (membrane) • Unlike passive transport – Requires energy expenditure • “Pump” 34

Na+/K+ Pump – Active Transport 35

Ion Channels • Electrogeneic proteins – Store energy by building up ions across a membrane • Energy release when ions move back across 36

Cotransport • Energy generated by Ion pump used to power another protein • Facilitated diffusion of larger molecules • Active Transport 37

Diffusion - Review 38

Osmosis • Different concentration s of solute • Semipermeable membrane – Only water can move across 39

Osmosis • Water molecules move • Create equal concentration of solute on either side 40

Water Transfer in Cells • Hypertonic – more solute outside cell • Isotonic – equal solute concentrations inside & outside cell • Hypotonic – less solute outside cell 41

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In which direction will water move when a cell is in…. • A hypotonic solution (e. g. pure water)? • A hypertonic solution (e. g. saltwater)? 43

Review: • Why does a wilted plant “revive” when you add water? • When you need IV fluids in the emergency room, are you given water or saline? – Why? 44