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Plasma Membrane Structure and Function Image from: http://www.biologie.uni-hamburg.de/b-online/ge22/03.gif Cell Size Limitations • The size of cells is limited by the surface area to volume ratio of cells – As cells grow the volume inside of the cell grows faster than the surface area available to diffuse nutrients into the cell and expel wastes out of the cell • Surface area determines rate molecules enter and leave cell • Volume determines the demand for resources needed by the cell (increase in volume requires more resources to keep the cell functioning properly) • Smaller cells have more favorable surface area to volume ratios for exchange with the environment Root hairs Cells of alveoli: Lungs Villi: Small Intestines • You should be able to solve basic surface area and volume equations given to you on the AP Biology exam. • In order for cells to grow, reproduce and maintain dynamic homeostasis, cells must create and maintain an internal environment that is different from the external environment – Cell membranes are selectively permeable to accomplish this goal – Cell membranes separate the internal environment from the external environment FLUID MOSAIC MODEL Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif Click here to See Fluidity Click here to See FLUIDITY 1972- S.J. Singer and G. Nicolson propose membrane is a “mosaic” of proteins and phospholipids that are constantly moving and changing More than just a barrier… • The membranes selective permeability is a direct consequence of the membranes structure consisting of phospholipids, embedded proteins, cholesterol, glycoproteins and glycolipids A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Molecules need to move across membranes in cells OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html Membranes provide a variety of cell functions Cell Membranes are made of PHOSPHOLIPIDS & PROTEINS Amphipathic – Molecules with both hydrophilic and hydrophobic regions phosphate hydrophilic lipid hydrophobic Membrane Proteins • Proteins determine most of membrane’s specific functions – cell membrane & organelle membranes each have unique collections of proteins • Membrane proteins: – peripheral proteins = loosely bound to surface of membrane – integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane protein HYDROPHILIC/HYDROPHOBIC areas determine positions of molecules in cell membranes hydrophobic amino acids – Those with nonpolar side groups – stick in the lipid membrane – anchors the protein in membrane hydrophilic amino acids – Amino acids with charge and polar side groups – stick out in the watery fluid in or out of cell Membrane Carbohydrates • Attached to proteins (glycoproteins) or lipids (glycolipids) • Play a key role in cell-cell recognition – ability of a cell to distinguish neighboring cells from another – important in organ & tissue development – basis for rejection of foreign cells by immune system http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm Semi-permeable membrane • Need to allow passage through the membrane • But need to control what gets in or out – membrane needs to be semi-permeable sugar aa lipid H 2O salt NH3 So what makes a membrane semi permeable? See a movie PHOBIC TAILS in center determine what can pass through What molecules can get through directly? Small non-polar (uncharged) molecules (O2, N2, & CO2) and hydrophobic molecules (fats & other lipids) can slip directly through the phospholipid cell membrane, but… inside cell NH3 lipid O2 outside cell sugar aa salt H 2O What about other stuff? Types of Cell Transport Diffusion • 2nd Law of Thermodynamics governs biological systems – Universe tends towards disorder Animatioin from: http://www.biologycorner.com/resources/diffusion-animated.gif • Diffusion – movement from [higher] [lower] concentration Diffusion will also happen across a cell membrane as long as there is a difference in concentration and the membrane will let the molecule pass through. Example: DIFFUSION IN CELLS http://facstaff.bloomu.edu/gdavis/links%20100.htm O2 automatically moves from HIGHER concentration (in lungs) to LOWER concentration (in blood) CO2 automatically moves from HIGHER concentration (in blood) to LOWER concentration (in lungs) http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html Diffusion of 2 solutes • Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances What if cell needs to move a AGAINST the molecule _________ CONCENTRATION GRADIENT? _______________ (LOWER HIGHER) Cell example: Want to put MORE glucose into mitochondria when there is already glucose in there Image from: http://www.biologyclass.net/mitochondria.jpg What if a cell needs to LARGE or ______ POLAR move _____ molecules that can’t get through the membrane? http://www.d.umn.edu/~sdowning/Membranes/membraneImages/jpegimages/diffusionmedium.jpg What if cell needs to move FAST molecules really _______? (can’t wait for it to diffuse) Cell example: Movement of Na + & K+ ions required to send nerve signals http://www.steve.gb.com/images/science/neuron.png WAY to Cells need a ____ HELP molecules across ____ cell membranes that can’t go across by _______ themselves ___________ Facilitated diffusion • Move from HIGH to LOW concentration with aid of membrane transport proteins – passive transport – no energy needed – facilitated = with help – Examples: • Way glucose enters cells • Also how charged and polar molecules enter the cell Facilitated Diffusion Animation from: http://bio.winona.edu/berg/ANIMTNS/facdifan.gif Carrier Proteins Grab molecule, undergo conformational change, flip to other side Channel Proteins Create passageway for substances to pass through May be gated or not: open in response to chemical or electrical signals Animation from: http://www2.uic.edu/~myilma1/ionchannel.gif Gated channels • open only in presence of stimulus (signal) – stimulus usually different from transported molecule • ex: ion-gated channels when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell • ex: voltage-gated channels change in electrical charge across nerve cell membrane opens Na+ & K+ channels Carriers and Channels are specific inside cell H2O aa sugar NH3 salt outside cell . . . BUT STILL MOVES FROM [HIGHER] to [LOWER] Active transport Uses energy from ATP to move molecules against concentration gradient – Moves from [lower] → [higher] – Uses protein pumps OR vesicles Animations from: http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.html http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif PROTON PUMP Moves Protons (H+) across membrane EXAMPLES: - Creates acidic condition inside lysosomes - Photosynthesis/respiration use H+ gradients to generate ATP See a movie proton pump Active transport SODIUM-POTASSIUM PUMP Sets up difference in charge across membranes Pumps 3 Na+ out and 2 K+ in Makes cells more + outside more - inside See a movie about Na+ - K+ pump Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif All cells have voltages across their membranes = membrane potential • Cytoplasm inside cell is more negative than extracellular fluid outside • TWO FORCES drive diffusion • Concentration gradient • Electrical force • Ions move DOWN the electrochemical gradient Favors the passage of cations (+) into cells MEMBRANE POTENTIAL created by electrogenic pumps (proteins that generate voltage by pumping ions) PROTON PUMP Main electrogenic pump in plants, fungi, and bacteria http://fig.cox.miami.edu/~cmallery/150/memb/electrogenic.jpg Na+- K+ PUMP Main electrogenic pump in animal cells Animation from: http://www.lionden.com/cell_animations.htm Electrogenic pumps can be coupled to actively transport other substances = COTRANSPORT Ex: Cells pump H+ out of cell then use the diffusion of H+ back into cell down its gradient to drive the uptake of sucrose into cells http://faculty.southwest.tn.edu/rburkett/GB1-os29.jpg Na+ - K+ pump sets up MEMBRANE POTENTIAL http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2 BULK TRANSPORT • ACTIVE transport • Requires energy (ATP) • Uses vesicles Watch two video clips about endo/exocytosis endo/exocytosis Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis “Cell eating” large molecules; whole cells “Cell drinking” Fluids; Small molecules triggered by ligand signal Example in cells: WHITE BLOOD CELL ENGULFING BACTERIA SEE PHAGOCYTOSIS MOVIE http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x28.jpg EXOCYTOSIS • Active transport (requires ATP) • Uses vesicles • Releases substances to outside INSULIN being released by pancreas cells using exocytosis Video: http://www.southtexascollege.edu/tdehne/BC_ShockwaveAnimations/07SWF-TourOfTheCell/07-16-EndomembraneSystem.swf GOLGI BODIES USE EXOCYTOSIS Animation from: http://www.franklincollege.edu/bioweb/A&Pfiles/week04.html See a Golgi movie http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg Transport summary Water Transport The Special Case of Water The Special Case of Water Movement of water across the cell membrane = OSMOSIS Osmosis is diffusion of water • Water is very important, so we talk about water separately • Diffusion of water from high concentration of water to low concentration of water – across a semi-permeable membrane – Passive – Uses no energy Aquaporins 1991 | 2003 • Transport proteins that move water rapidly into & out of cells – evidence that there were water channels Peter Agre Roderick MacKinnon John Hopkins Rockefeller TONICITY- ability of a solution to cause a cell to lose or gain water • Refers to the concentration of SOLUTES • Is a RELATIVE term, comparing two different solutions Solute-substance that is dissolved Solvent- substance solute in dissolved in Solution = solute + solvent What if there is a difference in concentration but solute molecules can’t move across a membrane? WATER will move until concentrations reach equilibrium Animation: http://www.ouhscphysio.org/humanphys/animations/osmosis1.swf See an animation Osmosis1 http://faculty.etsu.edu/currie/images/osmosis1.jpg Animation Concentration of water • Direction of osmosis is determined by comparing total solute concentrations – Hypertonic - more solute, less water – Hypotonic - less solute, more water – Isotonic - equal solute, equal water water hypotonic hypertonic net movement of water • Pressure potential: – pressure exerted by the rigid cell wall that limits further water uptake. • Solute potential – The effect of solute concentration. – Pure water at atmospheric pressure has a solute potential of zero. – As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. – As solute is added, the water potential of a solution drops, and water will tend to move into the solution. In beaker B, what is the water potential of the distilled water in the beaker, and of the beet core? – Since water potential = solute potential (-0.4) + pressure potential (0.2), – water potential = -0.2 (water left the beet core) Solute concentration Lower outside than inside Equal outside and inside Greater outside than inside HYPOTONIC ISOTONIC HYPERTONIC What will happen to an animal cell placed in different solutions? Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis3.swf OSMOSIS See an animation Osmosis3 Video Choose Blood Hypotonic link HYPOTONIC: Concentration outside cell is ________________ inside the cell LESS THAN More water enters than leaves cell so cell will ___________________ swell and possibly burst Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis4.swf OSMOSIS See an animation OSMOSIS 4 HYPERTONIC: Concentration outside cell is ____________________ inside cell GREATER THAN More water leaves cell than enters so cell ____________ shrinks Video Choose Blood Hypertonic link Remember: Cells try to “maintain stable internal conditions = HOMEOSTASIS ____________________ http://bioweb.wku.edu/courses/biol121/Osmosis/Osmosis.asp So an animal cell in ISOTONIC conditions stays same size Water entering = water leaving Video Choose Blood Isotonic link Animal cells = CYTOLYSIS = CRENATION http://www.stchs.org/science/courses/sbioa/metenergy/bloodcells.gif http://www.stchs.org/science/courses/sbioa/metenergy/aplantturgor.gif Plant cells Cell wall keeps plant cell from bursting made of cellulose rather than phospholipids and is external the cell membrane. Provides structural barrier for some substances to the internal environment. Managing water balance • Isotonic – animal cell immersed in isotonic solution • blood cells in blood • no net movement of water across plasma membrane • water flows across membrane, at same rate in both directions • volume of cell is stable Osmosis… .05 M .03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell http://www.biology4kids.com/files/cell_vacuole.html Loss of water from central vacuole= plants “wilt” Vacuole full of water gives plant support (turgor pressure) Turgid = very firm Flaccid = limp HYPOTONIC Sitting in the bathtub makes your fingers plump up and get “pruny” Grocery stores spray water on their veggies to “plump them up” If cells can’t maintain “stable internal conditions” . . . damage can result and cells can die. http://www.the-aps.org/education/lot/cell/Quiz.htm Cell survival depends on balancing water uptake & loss = OSMOREGULATION http://www.microscopy-uk.org.uk/mag/imgjun99/vidjun1.gif Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell to solve problem, specialized organelle, contractile vacuole Drink salt water ACTIVELY pump ions OUT; Urinate less frequently Gills ACTIVELY pump ions in; Urinate frequently Kidspiration by: Riedell