Plasma Membrane Structure and Function

Transcription

1 Plasma Membrane Structure and Function Chapter 7 Image from: Slide show modified from:

2 MEMBRANE MODELS 1915: Scientists analyzed isolated red blood cell membranes, found them made up of proteins and lipids 10 years later: E. Gorter and F. Grendel reasoned membranes are made of a phospholipid bilayer 1935: Hugh Davson and James Danielli proposed membrane was a sandwich of proteins and phospholipid bilayer with proteins on outside of phopholipids DAVSON-DANIELLI MODEL

3 FLUID MOSAIC MODEL Animation from: Click here to See Fluidity Click here to See FLUIDITY S.J. Singer and G. Nicolson propose membrane is a mosaic of proteins and phospholipids that are constantly moving and changing

4 More than just a barrier Expanding our view of cell membrane beyond just a phospholipid bilayer barrier phospholipids plus

5 A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

6 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

7 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

8 Membranes provide a variety of cell functions

9 Molecules need to move across membranes in cells OUT waste ammonia salts CO 2 H 2 O products IN food carbohydrates sugars, proteins amino acids lipids salts, O 2, H 2 O Image modiified from:

10 Cell Membranes are made of PHOSPHOLIPIDS & PROTEINS Amphipathic Molecules with both hydrophilic and hydrophobic regions phosphate hydrophilic lipid hydrophobic

11 HYDROPHILIC/HYDROPHOBIC areas determine positions of molecules in cell membranes hydrophobic amino acids stick in the lipid membrane anchors the protein in membrane hydrophilic amino acids stick out in the watery fluid in or out of cell

12 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 2 O salt NH 3 So what makes a membrane semi permeable? See a movie

13 PHOBIC TAILS in center determine what can pass through

14 What molecules can get through directly? Small non-polar molar molecules (O 2 & CO 2 ) and hydrophobic molecules (fats & other lipids) can slip directly through the phospholipid cell membrane, but inside cell lipid O 2 NH 3 salt What about other stuff? outside cell sugar aa H 2 O

15 Diffusion 2nd Law of Thermodynamics governs biological systems Universe tends towards disorder Animatioin from: Diffusion movement from [higher] [lower] concentration

16 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.

17 Example: DIFFUSION IN CELLS O 2 automatically moves from HIGHER concentration (in lungs) to LOWER concentration (in blood) CO 2 automatically moves from HIGHER concentration (in blood) to LOWER concentration (in lungs)

18 Diffusion of 2 solutes Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances

19 What if cell needs to move a molecule AGAINST the CONCENTRATION GRADIENT? (LOWER HIGHER) Cell example: Want to put MORE glucose into mitochondria when there is already glucose in there Image from:

20 What if a cell needs to move LARGE or POLAR molecules that can t get through the membrane?

21 What if cell needs to move molecules really? FAST (can t wait for it to diffuse) Cell example: Movement of Na + & K + ions required to send nerve signals

22 Cells need a WAY to HELP molecules across cell membranes that can t go across by themselves

23 Kidspiration by: Riedell

24 Facilitated diffusion Move from HIGH to LOW concentration with aid of membrane transport proteins passive transport no energy needed facilitated = with help

25 Facilitated Diffusion Animation from: 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:

26 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

27 Carriers and Channels are specific inside cell H 2 O aa sugar NH 3 salt outside cell... BUT STILL MOVES FROM [HIGHER] to [LOWER]

28 Active transport Uses energy from ATP to move molecules against concentration gradient Moves from [lower] [higher] Uses protein pumps OR vesicles Animations from:

29 Moves Protons (H + ) across membrane PROTON PUMP EXAMPLES: - Creates acidic condition inside lysosomes - Photosynthesis/respiration use H + gradients to generate ATP See a movie proton pump

30 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:

31 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

32 MEMBRANE POTENTIAL created by electrogenic pumps (proteins that generate voltage by pumping ions) PROTON PUMP Main electrogenic pump in plants, fungi, and bacteria Na + - K + PUMP Main electrogenic pump in animal cells Animation from:

33 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

34 Na + - K + pump sets up MEMBRANE POTENTIAL

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38 ACTIVE transport Requires energy (ATP) Uses vesicles BULK TRANSPORT Watch two video clips about endo/exocytosis endo/exocytosis Animation from:

39 Endocytosis phagocytosis Cell eating large molecules; whole cells pinocytosis Cell drinking Fluids; Small molecules receptor-mediated endocytosis triggered by ligand signal

40 PHAGOCYTOSIS Animation from: Cell eating takes in large molecules; whole cells See phagocytosis in action:

41 Example in cells: WHITE BLOOD CELL ENGULFING BACTERIA using Phagocytosis SEE PHAGOCYTOSIS MOVIE

42 PINOCYTOSIS Cell drinking Takes in fluids; Small molecules

43 Receptor-mediated Endocytosis Ligand-any molecule that binds to a receptor site Binding of ligands allows uptake of specific molecules

44 Receptor-mediated Endocytosis Example in cells: Cholesterol Uptake Low density lipid proteins (LDL s) bind cholesterol and carry it in blood LDL s act as ligands binding receptors on cell surface and cholesterol is taken into cell

45 EXOCYTOSIS Active transport (requires ATP) Uses vesicles Releases substances to outside INSULIN being released by pancreas cells using exocytosis

46 Video: GOLGI BODIES USE EXOCYTOSIS Animation from: See a Golgi movie

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48 Transport summary

49 The Special Case of Water Movement of water across the cell membrane = OSMOSIS

50 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

51 Aquaporins Transport proteins that move water rapidly into & out of cells evidence that there were water channels Peter Agre John Hopkins Roderick MacKinnon Rockefeller

52 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

53 What if there is a difference in concentration but solute molecules can t move across a membrane? WATER will move until concentrations reach equilibrium

54 Animation: See an animation Osmosis1 Animation

55 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

56 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?

57 Animation from: OSMOSIS HYPOTONIC: Concentration outside cell is inside the cell LESS THAN See an animation Osmosis3 Video More water enters than leaves cell so cell will swell and possibly burst Choose Blood Hypotonic link

58 Animation from: 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

59 Remember: Cells try to maintain stable internal conditions = HOMEOSTASIS So an animal cell in ISOTONIC conditions stays same size Water entering = water leaving Video Choose Blood Isotonic link

60 Animal cells = CYTOLYSIS = CRENATION

61 Plant cells Cell wall keeps plant cell from bursting = PLASMOLYSIS

62 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

63 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

64 Loss of water from central vacuole= plants wilt Vacuole full of water gives plant support (turgor pressure) Turgid = very firm Flaccid = limp

65 HYPOTONIC Sitting in the bathtub makes your fingers plump up and get pruny Grocery stores spray water on their veggies to plump them up

66 If cells can t maintain stable internal conditions... damage can result and cells can die.

67 Cell survival depends on balancing water uptake & loss = OSMOREGULATION Paramecium vs. pond water Paramecium is hypertonic H 2 O continually enters cell to solve problem, specialized organelle, contractile vacuole

68 Drink salt water ACTIVELY pump ions OUT; Urinate less frequently Gills ACTIVELY pump ions in; Urinate frequently