Membrane Structure and Function Chapter 7

The plasma membrane  selectively permeable Overview: Life at the Edge

Phospholipids most abundant lipids in plasma membrane amphipathic = hydrophobic and hydrophilic regions – Polar head – Hydrocarbon tails Cell membranes

Phospholipid

Hydrophilic head WATER Hydrophobic tail WATER Membrane Models: Scientific Inquiry

1935Davson and Danielli - bilayer model 1972 Singer and Nicolson - fluid mosaic model – membrane mosaic of proteins dispersed within bilayer Current model: mosaicism

TECHNIQUE Extracellular layer Knife Proteins Inside of extracellular layer RESULTS Inside of cytoplasmic layer Cytoplasmic layer Plasma membrane

Freeze-fracture splits membrane along middle of bilayer (lipid layer is weak) proteins

Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Proteins embedded in bilayer

Phospholipids in membrane move laterally within bilayer rarely flip flop Lipids, proteins, may move laterally The Fluidity of Membranes (a) Movement of phospholipids Lateral movement (  10 7 times per second) Flip-flop (  once per month)

RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour

1. Type of phospholipid Membrane fluidity affected by: Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro- carbon tails

2. Temperature cool  gel – Tightly packed tails warm  fluid

3. Cholesterol Stabilizes membrane fluidity with changing temperature FYI Cholesterol can compose ½ of the membrane Bacterial cell membranes do not contain cholesterol Plant cells do not contain much

Cholesterol (c) Cholesterol within the animal cell membrane

Mosaic of proteins embedded in lipid bilayer Proteins determine most of membrane’s functions Membrane Proteins and Their Functions

Membrane Proteins Peripheral proteins – bound to _____________ of membrane

Integral proteins – penetrate hydrophobic region – Transmembrane proteins N- terminus C-terminus

ex. Insulin receptor 1. Receptor proteins for signal transduction

Hydrophilic core Ex. Aquaporins 2. Channel proteins for passage of molecules hydrophilic core

Ex. Glucose transporter s huttles glucose across membrane 3. Transport proteins

(a) Transport (b) Enzymatic activity (c) Signal transduction ATP Enzymes Signal transduction Signaling molecule Receptor

(d) Cell-cell recognition Glyco- protein (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Glycoproteins – Carbohydrates attached to proteins mucin 4. Cell-cell recognition

Ex. gap junctions allow passage of ions and small molecules from cell to cell 5. Intercellular joining proteins

6. Extracellular matrix proteins 7. Membrane enzymes

– Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Six major functions of membrane proteins :

Plasma membrane regulates cell molecular traffic Selective permeability

Hydrophobic molecules dissolve in bilayer and pass through membrane rapidly – O2, CO2, NO, steroids, nanoparticles O2 CO2 Permeability of Lipid Bilayer Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

U-M scientists used a manmade nanoparticle less than 5 nm. They are small enough to cross cell membranes. One nanometer equals one-billionth of a meter, which means it would take 100,000 nanometers lined up side-by-side to equal the diameter of a human hair. Folate molecules on the nanoparticle bind to receptors on cancer tumor cell membranes and the cell immediately internalizes it, because it thinks it's getting folate, which it needs to grow. Folate is a vitamin But while it's bringing folate across the cell membrane, the cell also draws in the methotrexate, a poison that will kill it. Nanoparticles can cross cell membranes!

Hydrophilic (polar) molecules do not cross easily – sugar, water, ions

molecules move randomly A. Diffusion Molecules diffuse down their concentration gradient from high to lower concentration until equilibrium Passive transport = no energy used

(b) Diffusion of two solutes Net diffusion Equilibrium What molecules can diffuse across the cell membrane? Answer: O2, CO2, urea

B. Osmosis is diffusion of water across a selectively permeable membrane Water diffuses across membrane from region of higher water concentration to the region of lower water concentration until equilbrium

Lower concentration of sugar) H2OH2O Higher Concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis

Tonicity =ability of solution to cause cell to gain or lose water Isotonic solution: Solute concentration same as in cell – no net water movement across membrane Hypertonic solution: Solute concentration greater than inside cell – cell loses water Hypotonic solution: Solute concentration is less than inside cell – cell gains water Water Balance of Cells Without Walls

Hypotonic solution (a ) Animal cell H2OH2O Lysed H2OH2O H2OH2O Normal Isotonic solution H2OH2O Shriveled Hypertonic solution Solution type? Isotonic, hypotonic, hypertonic?

Osmoregulation= control of water balance Ex. Paramecium videoParamecium video – pond water is _____________ to the protista – contractile vacuole How do cells deal with changing external water concentrations?

Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

Isotonic solution  no net movement of water into cell – flaccid (limp) Water Balance in plants (cell wall)

Hypotonic solution  cell (vacuole) swells – cell wall opposes uptake  turgid (firm)

Hypertonic  lose water; membrane pulls away from wall – plasmolysis (lethal)

Hypotonic solution (b ) Plant cell H2OH2O Turgid (normal) H2OH2O H2OH2O Isotonic solution Flaccid H2OH2O Plasmolyzed Hypertonic solution

C. Facilitated Diffusion: Passive Transport aided by proteins Channel proteins Carrier proteins What molecules use facilitated diffusion to cross membrane? Answer: glucose, sodium ions, chloride ions, water

EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Solute Carrier protein (b) A carrier protein

Energy (ATP) required to move solutes against their gradients (from lower to higher conc. !) Pumps are membrane proteins Ex. sodium-potassium pump (an enzyme) FYI: All animals Nobel prize 1997 (Jens Skou) Uses 1/3 of cells total energy production Provides driving force for other cell processes (secondary transport, volume, gradients) Active transport

EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + [Na + ] low [K + ] high CYTOPLASM Cytoplasmic Na + binds to the sodium-potassium pump. 1 Examine the figure: Na+ high outside cell K+ low Na+ low inside cell K+ high According to diffusion ?

1. Cytoplasmic Na+ binds to pump

Na + binding stimulates phosphorylation by ATP. Na + ATP P ADP 2 2. ADP phosphorylated to ATP What is ATP?

Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. Na + P 3 Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. Na + P 3 3. Na+  out of cell shape change of pump Against conc. grad.

K + binds on the extracellular side and triggers release of the phosphate group. P P K+K+ K+K Extracellular K+ binds to pump ATP used

Shape change K+K+ K+K K+  inside cellPump animationPump animation Step by step

Passive transport Diffusion Facilitated diffusion Active transport ATP Review the difference passive vs active transport NO ATP IS USED WHICH ONE REPRESENTS THE CHANNEL PROTEIN?

a.Maintain membrane potential = voltage difference across membrane  Inside of cell more electronegative than out  = negative membrane potential Why do cells need pumps?

EXTRACELLULAR FLUID H+H+ H+H+ H+H+ H+H+ Proton pump H+H+ H+H+ + + H+H+ – – – – ATP CYTOPLASM –

– chemical = concentration gradient – electrical = membrane potential b. Maintain electrochemical gradients

c. Cotransport. Cotransport Ex. Sucrose transporter sodium driven

Proton pump – – – – – – ATP H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Diffusion of H + Sucrose-H + cotransporter Sucrose

Exocytosis – To secrete products from cell – Vesicles fuse with membrane Bulk transport

2. Endocytosis cell takes in macromolecules by forming vesicles from membrane a. Phagocytosis – for large particle Vesicle fuses with lysosome to digest particle

PHAGOCYTOSIS CYTOPLASM EXTRACELLULAR FLUID Pseudopodium “Food” or other particle Food vacuole Food vacuole Bacterium An amoeba engulfing a bacterium via phagocytosis (TEM) Pseudopodium of amoeba 1 µm

b. Pinocytosis – for fluids/small molecules PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated pit Ligand Coated vesicle c. receptor-mediated endocytosis, ligand binds to receptor  vesicle