membranes, osmosis, diffusion

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Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings selective permeability = it regulates what enters and leaves the cell, allowing some substances to cross more easily than others © 2011 Pearson Education, Inc.

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Hydrophilic head Hydrophobic tail WATER WATER Amphipathic molecules - contain hydrophobic and hydrophilic regions Phospholipid bilayer forms the plasma membrane


Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Fluid Mosaic model - membrane is a fluid structure with a “mosaic” of various amphipathic proteins embedded within it

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Lateral movement occurs 1 0 7 times per second. Flip-flopping across the membrane is rare (  once per month). Phospholipids can move within the bilayer lipids, and some proteins, drift laterally


Figure 7.8 Fluid Unsaturated hydrocarbon tails Viscous Saturated hydrocarbon tails Membranes must be fluid to work properly As temperatures cool….. membranes can solidify Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

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Cholesterol within the animal cell membrane Cholesterol In animal cells ……….. cholesterol maintains membrane fluidity at different temperatures In warm temperatures - cholesterol restrains movement of phospholipids At cool temperatures – cholesterol prevents tight packing

Overview: Life at the Edge:

Glyco- protein Carbohydrate Glycolipid Microfilaments of cytoskeleton EXTRACELLULAR SIDE OF MEMBRANE CYTOPLASMIC SIDE Integral protein Peripheral proteins Cholesterol Fibers of extra- cellular matrix (ECM) Proteins determine most of the membrane’s specific functions Peripheral proteins - bound to the surface of the membrane Integral proteins - penetrate the hydrophobic core (fully or partially) Proteins are held in place by cytoskeleton or extracellular matrix

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Enzymes Signaling molecule Receptor Signal “relayed” ATP (1) Transport (2) Enzymatic activity (3) Signal transduction Hydrophilic channel or shuttle Carry out steps of a pathway Receive messenger and relay message Major functions of membrane proteins…….

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Glyco- protein (4) Cell-cell recognition (5) Intercellular joining (6) Attachment to the cytoskeleton and extracellular matrix (ECM) Maintain cell shape and stabilize location Identification tags Cell junctions major functions of membrane proteins…….

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Glyco- protein Carbohydrate Glycolipid Integral protein Peripheral proteins Cholesterol Extra-cellular matrix (ECM) Cells recognize each other by surface molecules on the plasma membrane Carbohydrates bound to lipids = glycolipids Carbohydrates bound to proteins = glycoproteins Carbohydrates on exterior of the plasma membrane can vary among species, individuals, and even cell types in an individual

Figure 7.8:

Membrane WATER Net diffusion Net diffusion Equilibrium Diffusion - tendency of molecules to spread out evenly into the available space Substances diffuse down their concentration gradient (from areas of high concentration to low concentration) Passive transport - diffusion of a substance across a membrane down it’s concentration gradient, thus with no energy investment

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Figure 7.13b Diffusion of two solutes Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium At dynamic equilibrium, molecules cross a membrane at equal rates

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Membrane structure results in selective permeability….. regulating what enters and leaves the cell © 2011 Pearson Education, Inc. Hydrophobic (nonpolar) molecules (O 2 and CO 2 ) can dissolve in the lipid bilayer and can pass through the membrane rapidly Polar and large molecules (sugars) can not cross the membrane easily or quickly

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Figure 7.14 Lower concentration of solute (sugar) Higher concentration of solute Sugar molecule H 2 O Same concentration of solute Selectively permeable membrane Osmosis Osmosis - diffusion of water across a selectively permeable membrane Water diffuses from higher H2O concentration to area of lower H2O concentration

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Hypotonic solution Osmosis Isotonic solution Hypertonic solution Animal cell H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled Turgid (normal) Flaccid Plasmolyzed Isotonic solution - Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution - Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution - Solute concentration is less than that inside the cell; cell gains water Tonicity - ability of a solution to cause a cell to gain or lose water

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Figure 7.16 Contractile vacuole Osmoregulation – regulation of solute concentrations and water balance - a necessary adaptation for living organisms The protist Paramecium, is lives in hypotonic pond water environment, has a contractile vacuole to pump out excess water

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Hypotonic solution Osmosis Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O Cell wall Turgid (normal) Flaccid Plasmolyzed Cell walls help plants maintain water balance hypotonic solution - swells until the wall opposes uptake; the cell is now turgid (firm) Isotonic - there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt hypertonic - cells lose water; membrane pulls away from the wall, a usually lethal effect called plasmolysis

Figure 7.13b:

Transport Proteins: allow passage of hydrophilic substances across the membrane A transport protein is specific for the substance it moves © 2011 Pearson Education, Inc. Channel proteins - creates a hydrophilic channel that certain molecules or ions can use as a tunnel Carrier proteins - bind to molecules and change shape to shuttle them across the membrane Facilitated diffusion - transport proteins speed the passive movement of molecules across the plasma membrane and it is passive because items are moved down their concentration gradient…..and it does not require energy

Membrane structure results in selective permeability…..regulating what enters and leaves the cell :

Figure 7.17 EXTRACELLULAR FLUID CYTOPLASM Channel protein Solute Solute Carrier protein Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane……….. passive – no energy required Channel proteins include: Aquaporins – channel proteins that facilitate diffusion of water Ion channels that open or close in response to a stimulus ( gated channels )

Figure 7.14:

Figure 7.17 Solute Carrier protein Carrier proteins - undergo a change in shape that translocates the solute-binding site across the membrane Passive – no energy required, carry substances down concentration gradient

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Active transport: uses energy to move solutes against the concentration gradients © 2011 Pearson Education, Inc. Active transport allows cells to maintain concentration gradients different from their surroundings

Figure 7.16:

Figure 7.18-1 EXTRACELLULAR FLUID [Na  ] high [K  ] low [Na  ] low [K  ] high CYTOPLASM Na  Na  Na  1 sodium-potassium pump = type of active transport system Active transport is also performed by specific proteins embedded in the membranes

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Figure 7.18-2 EXTRACELLULAR FLUID [Na  ] high [Na  ] low CYTOPLASM Na  Na  Na  1 2 Na  Na  Na  P ATP ADP requires energy - usually in the form of ATP

Transport Proteins::

Figure 7.18-3 EXTRACELLULAR FLUID [Na  ] high [Na  ] low CYTOPLASM Na  Na  Na  1 2 3 Na  Na  Na  Na  Na  Na  P P ATP ADP [Na  ] high [Na  ] low

Figure 7.17:

Figure 7.18-6 EXTRACELLULAR FLUID [Na  ] high [K  ] low [Na  ] low [K  ] high CYTOPLASM Na  Na  Na  1 2 3 4 5 6 Na  Na  Na  Na  Na  Na  K  K  K  K  K  K  P P P P i ATP ADP

Figure 7.17:

Active transport allows cells to maintain concentration gradients different from their surroundings

Active transport: uses energy to move solutes against the concentration gradients:

How Ion Pumps Maintain Membrane Potential Voltage is created by differences in the distribution of positive and negative ions on either side of a membrane Membrane potential - the voltage difference across a membrane © 2011 Pearson Education, Inc.

Figure 7.18-1:

Electro-Chemical gradient drives the diffusion of ions across a membrane……thru a combination of chemical and electrical force chemical force - the effect of ion’s concentration gradient electrical force - the effect of the membrane potential (voltage) on the ion’s movement

Figure 7.18-2:

CYTOPLASM ATP EXTRACELLULAR FLUID Proton pump H  H  H  H  H  H          electrogenic pumps = transport protein generates voltage across a membrane helps store energy that can be used for cellular work proton pump - main electrogenic pump of plants, fungi, and bacteria sodium-potassium pump is the main electrogenic pump of animal cells

Figure 7.18-3:

ATP H  H  H  H  H  H  H  H  Proton pump Sucrose-H  cotransporter Sucrose Sucrose Diffusion of H          Cotransport - when active transport of a solute indirectly drives the transport of other solutes Plants use cotransport – use the gradient of H+ ions generated by proton pumps to drive active transport of nutrients into the cell

Figure 7.18-6:

Figure 7.19 Passive transport Active transport Diffusion Facilitated diffusion ATP

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Large molecules (polysaccharides and proteins) cross the membrane in bulk via vesicles…………… requires energy © 2011 Pearson Education, Inc. Exocytosis - transport vesicles migrate to the membrane, fuse with it, and release their contents out of the cell Endocytosis - the cell takes in macromolecules by forming vesicles from the plasma membrane 3 types of endocytosis……… Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis

How Ion Pumps Maintain Membrane Potential:

Figure 7.22a Pseudopodium Solutes “Food” or other particle Food vacuole CYTOPLASM EXTRACELLULAR FLUID Pseudopodium of amoeba Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis Phagocytosis phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle

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Figure 7.22b Pinocytosis vesicles forming in a cell lining a small blood vessel Plasma membrane Vesicle Pinocytosis Pinocytosis - molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

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Figure 7.22c Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis Receptor Receptor-Mediated Endocytosis Ligand Coat proteins Coated pit Coated vesicle Coat proteins Plasma membrane receptor-mediated endocytosis - binding of ligands to receptors triggers vesicle formation

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