Mr. Briner Unit 1.3 Membrane structure ASM IB DP Biology Unit 1.3 Membrane structure

Cell membranes Structure The structure of cell membranes make them fluid and dynamic Maintains its shape and integrity Does not break Allows some molecules to move through it Can form and absorb vesicles

1.3.u1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Cell membranes Structure Phospholipids are amphipathic Have hydrophobic and hydrophilic regions

Cell membranes Structure Hydrophobic fatty acid tails Repel water and form the middle layer of the membrane. Hydrophilic phosphate heads Attract water and form the outer layers of the membrane.

Cell membranes Structure When put in water, phospholipids have emergent properties of organization Keep the heads ‘wet’ and the tails ‘dry’

Cell membranes Structure Can form micelles – lipoprotein complexes! Can form liposomes – vesicles!

Cell membranes Vesicle transport In cells, phospholids form bilayers Bilayer made of phospholipids and others Examples: Cell membrane Membrane bound organelles (Eukaryotes only) Nuclear membrane Mitochondria Chloroplasts RER Golgi apparatus Vesicles. Etc…

Cell membranes

Cell membranes Vesicle transport Membrane structure: Lipids move laterally (around each other) in a membrane But flip-flopping across the membrane is rare

Cell membranes Vesicle transport Membrane structure: Unsaturated hydrocarbon tail of phospholipids have kinks Due to double bonds Keeps the molecules from packing together too closely Enhancing membrane fluidity

Cell membranes

1.3.u3 Cholesterol is a component of animal cell membranes.

1.3.a1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Cell membranes Vesicle transport Membrane structure: Cholesterol reduces membrane fluidity and add stiffness Reducing phospholipid movement Also hinders solidification at low temperatures Bad analogy Imagine a room full of people wearing fluffy sweaters. Its is crowded, but people can slip past each other. Now sprinkle the crowd with people wearing Velcro suits…

Cell membranes Vesicle transport Membrane structure: Cholesterol Has polar and non-polar regions So crosses one layer

Cell membranes Vesicle transport Membrane structure: Cholesterol Prevents phospholipids from being to close Reduces permeability of polar molecules and ions

Cell membranes Vesicle transport Membrane have to be: Fluid enough for cells to move Fluid enough to let materials cross Stiff enough to prevent entrance of some materials Stiff enough to hold cell together

Cell membranes Vesicle transport Membrane structure: Phospholipid molecules can move in the horizontal plane Phospholipid exchange in the vertical plane does not occur Cholestoral reduces fluidity

Cell membranes Vesicle transport Membrane structure:

1.3.u2 Membrane proteins are diverse in terms of structure, position in the membrane and function.

Cell membranes Structure Integral proteins: Peripheral proteins: Embedded in the phospholipid of the membrane Can be monotopic (penetrating one layer) or polytopic (penetrating both layers) Peripheral proteins: Attached to the phospholipid surface Can be monotopic or just attach to surface

Cell membranes Structure Transport: protein channels and pumps Receptors: for hormones, etc. Anchorage: cytoskeleton and extracellular matrix Cell recognition: MHC proteins and antigens Intracellular joints: junctions between cells Enzymes: metabolic pathways / electron transport

Cell membranes Structure Glycoproteins: Proteins with markers for recognition Identify cells as belonging to your body Receptors for certain types of hormones Receptors for identifying ‘invaders’ by immune system cells

Cell membranes

Cell membranes

Cell membranes Function of membrane proteins Channel proteins: Integral proteins that allow movement of large substances from one side of the membrane to the other Like tunnels through the membrane Enzymes: Integral proteins that metabolize reactions For example, ATP synthetase

Cell membranes Function of membrane proteins Receptor proteins: Proteins with a surface outside of the cell Can detect external signals Changing the function of the cell (ie. hormones) Involved in cell or substance recognition (ie. immune system) Electron carriers Proteins involved in the movement of electrons (ie. in photosynthesis)

Cell membranes Function of membrane proteins Receptor proteins:

Cell membranes Structure Membrane proteins Hormone binding sites Receptors Immobilized enzymes Cell adhesion Cell-to-cell communication Marker proteins - glycoproteins Channels for passive transport Pumps for active transport

1.3.s3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Cell membranes Structure Models of the cell membrane Singer-Nicolson fluid mosaic model Proposed by Singer-Nicholson in 1972 Accepted today Replaced the Davson-Danielli model

Cell membranes Structure Models of the cell membrane Singer-Nicolson fluid mosaic model

Cell membranes Structure Singer-Nicolson fluid mosaic model Phospholipids in a bilayer Can move laterally but not flip Peripheral proteins either inside or outside Integral proteins can go through the bilayer Membrane is a fluid mosaic Bendable and has a mix of phospholipids and proteins Proteins can move laterally But not flip

Cell membranes Structure Evidence for the S-N fluid mosaic model Membrane proteins of two cells tagged with red and green fluorescent markers Cells fused together Red and green markers mixed So proteins can move around the membrane!

Cell membranes Structure Evidence for the S-N fluid mosaic model

1.3.s2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

Cell membranes Structure Davson-Danielli model Popular before S-N model Based on electron micrograph of membrane

Cell membranes Structure Davson-Danielli model Proteins usually dark and phospholipids light Believed proteins covered the whole phospholipid membrane No proteins in the membrane

Cell membranes Structure Davson-Danielli model = wrong!

Cell membranes Structure Davson-Danielli model = wrong!

Cell membranes Structure Davson-Danielli model Proteins were found to exist inside the membrane Photo of membrane interior was uneven, suggesting proteins and lipids mixed there Demonstrated the D-D model was wrong

1.3.s1 Drawing of the fluid mosaic model.

MAJOR SOURCES Mr. Briner ASM Thank you to my favorite sources of information when making these lectures! John Burrell (Bangkok, TH) www.click4biology.info Dave Ferguson (Kobe, JA) http://canada.canacad.ac.jp/High/49 Stephen Taylor (Bandung, IN) www.i-biology.net Andrew Allott – Biology for the IB Diploma C. J.Clegg – Biology for the IB Diploma Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate Howard Hugh’s Medical Institute – www.hhmi.org/biointeractive Mr. Hoye’s TOK Website – http://mrhoyestokwebsite.com And all the contributors at www.YouTube.com