The structure of biological membranes makes them fluid and dynamic. MEMBRANE Structure The structure of biological membranes makes them fluid and dynamic. Topic 1.3 IB Biology Miss Werba

ULTRASTRUCTURE OF CELLS TOPIC 1 – CELL BIOLOGY 1.1 INTRODUCTION TO CELLS 1.2 ULTRASTRUCTURE OF CELLS 1.3 MEMBRANE STRUCTURE 1.4 MEMBRANE TRANSPORT 1.5 THE ORIGIN OF CELLS 1.6 CELL DIVISION J WERBA – IB BIOLOGY 2

THINGS TO COVER Statement Guidance U.1 U.2 U.3 A.1 S.1 S.2 S.3 NOS 1.9 Amphipathic properties of phospholipid molecules resulting in formation of bilayers in water. Hydrophobic and hydrophilic properties of phospholipids U.2 Diversity of membrane proteins – inc. structure, position and function U.3 Cholesterol in animal cell membranes A.1 Cholesterol in membranes reduces fluidity and permeability to some solutes S.1 Drawing the fluid mosaic model Can be 2D; Show individual phospholipid molecules and a range of proteins. S.2 Analysis of evidence that led to Davson-Danielli model S.3 Analysis of falsification of the Davson-Danielli model that led to the Singer-Nicholson model. NOS 1.11 Using models as representations of the real world NOS 1.9 Falsification of theories with one theory being superseded by another J WERBA – IB BIOLOGY 3

PHOPHOLIPIDS

PHOSPHOLIPIDS Phospho – contains phosphate Lipid – contains fat U.1 PHOSPHOLIPIDS Phospho – contains phosphate Lipid – contains fat Also contains glycerol, which is a carbohydrate J WERBA – IB BIOLOGY 5

PHOSPHOLIPIDS Phospholipids have a head and 2 tails Head: Tails: U.1 PHOSPHOLIPIDS Phospholipids have a head and 2 tails Head: made from phosphate polar = charged hydrophilic = water-loving Tails: made from fatty acid chains non-polar = uncharged hydrophobic = water-hating J WERBA – IB BIOLOGY 6

U.1 PHOSPHOLIPIDS These are referred to as the amphipathic properties of phospholipids. These properties control how the phospholipids are able to arrange themselves in the membrane. J WERBA – IB BIOLOGY 7

U.1 PHOSPHOLIPIDS If you were given a number of these molecules and some water, how could you arrange them so that the water-loving and water-hating properties of the molecules were respected??? J WERBA – IB BIOLOGY 8

PHOSPHOLIPIDS Phospholipids arrange in a bilayer U.1 PHOSPHOLIPIDS Phospholipids arrange in a bilayer Hydrophobic tail regions face inwards The two hydrophilic head regions associate with the cytosolic (cytoplasm) and extracellular (outside) environments respectively J WERBA – IB BIOLOGY 9

THE FLUID MOSAIC MODEL

Hyperlink for video on next slide: http://youtu.be/w9VBHGNoFrY U.1 S.1 MEMBRANE STRUCTURE (10 nm) Use your space well (⅓–½ page); Use clear, strong lines; Use straight label lines; Make sure lines touch the labelled structure; No shading or colouring; Use a scale bar if appropriate Hyperlink for video on next slide: http://youtu.be/w9VBHGNoFrY J WERBA – IB BIOLOGY 11

U.1 S.1

MEMBRANE FLUIDITY & CHLOESTEROL

U.1 U.3 A.1 PHOSPHOLIPIDS Phospholipids are held together by hydrophobic interactions (weak associations) Hydrophilic / hydrophobic layers restrict entry and exit of substances Phospholipids allow for membrane fluidity / flexibility (important for functionality) Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis) J WERBA – IB BIOLOGY 14

CHOLESTEROL Cholesterol is present in animal cell membranes. U.3 A.1 CHOLESTEROL Cholesterol is present in animal cell membranes. The interactions between it and the phospholipids make the membrane less fluid and more rigid. This makes the membrane less permeable to water-soluble molecules. Cholesterol in the blood has also been associated with coronary heart disease. J WERBA – IB BIOLOGY 15

MEMBRANE PROTEINS

MEMBRANE PROTEINS Proteins are also found in the membrane. Integral proteins – are embedded in the membrane Peripheral proteins – attached to the surface Some peripheral proteins are glycoproteins Glycoproteins – have a carbohydrate group attached to them to serve a specific purpose eg. TRACIE J WERBA – IB BIOLOGY 17

MEMBRANE PROTEINS Proteins are also found in the membrane. Integral proteins – are embedded in the membrane Peripheral proteins – attached to the surface Some peripheral proteins are glycoproteins Glycoproteins – have a carbohydrate group attached to them to serve a specific purpose eg. TRACIE J WERBA – IB BIOLOGY 18

U.2 MEMBRANE PROTEINS Remember: TRACIE J WERBA – IB BIOLOGY 19

U.2 MEMBRANE PROTEINS Remember: TRACIE Transport: Protein channels and pumps (eg. Na+/K+ pump) Receptors: Peptide-based hormones (insulin, glucagon, etc) Anchorage: Cytoskeleton attachments and extracellular matrix Cell recognition: MHC proteins and antigens in the immune system Intercellular joinings: Tight junctions and plasmodesmata Enzymatic activity: Metabolic pathways (eg. electron transport chain in cellular respiration) J WERBA – IB BIOLOGY 20

MEMBRANE MODELS

Using models as representations of the real world NOS 1.11 MEMBRANE MODELLING Using models as representations of the real world eg. James Watson & Francis Crick are credited with discovering the structure of DNA by piecing together the research from a number of scientists. They made a model to help them achieve this. 22 J WERBA – IB BIOLOGY

Using models as representations of the real world NOS 1.11 MEMBRANE MODELLING Using models as representations of the real world There are a number of different models of membrane structure. 23 J WERBA – IB BIOLOGY

S.1 S.2 NOS 1.11 NOS 1.9 MEMBRANE MODELS Hugh Davson and James Danielli proposed a model of the plasma membrane in 1935 Their model suggest that a phospholipid bilayer existed, but that it lay between two layers of protein – like a sandwich The proteins in their model were not embedded in the membrane. The pores would have allowed the movement of substances whilst still maintaining an effective barrier to others. J WERBA – IB BIOLOGY Source: http://163.178.103.176/Fisiologia/general/activ_bas_3/Membrane%20Structure%20and%20Function.htm 24

MEMBRANE MODELS The Davson-Danielli model predominated until 1972. NOS 1.11 NOS 1.9 MEMBRANE MODELS The Davson-Danielli model predominated until 1972. It was supported by evidence from electron microscopy. The membranes in these images appeared as two dark lines with a light line between them. As proteins appear dark, and phospholipids appear light – the micrographs supported their model. J WERBA – IB BIOLOGY 25

Falsification of theories with one theory being superseded by another. MEMBRANE MODELLING NOS 1.9 Falsification of theories with one theory being superseded by another. Evidence falsified the Davson-Danielli model. 26 J WERBA – IB BIOLOGY

S.1 S.2 NOS 1.11 NOS 1.9 MEMBRANE MODELS A new technique called freeze-fracturing or freeze- etching was developed. This technique allowed weaknesses in cell structures to be developed. It highlighted areas of weakness in the centre of the membrane. The line of weakness was irregular, indicating structures embedded within the layer. A new model needed to be developed to explain the trans-membrane proteins. J WERBA – IB BIOLOGY 27

S.1 S.2 NOS 1.11 NOS 1.9 MEMBRANE MODELS New discoveries were also made regarding the types of proteins found in the membrane. Different organisms use different proteins to transport different substances across their membranes. All membranes are therefore not the same, as suggested by the Davson-Danielli model. J WERBA – IB BIOLOGY 28

S.1 S.2 NOS 1.11 NOS 1.9 MEMBRANE MODELS The fluid mosaic model was described by Singer and Nicholson in 1972. J WERBA – IB BIOLOGY 29

MEMBRANES Q1. Draw a labelled diagram to show the fluid mosaic structure of a plasma membrane, indicating the hydrophilic and hydrophobic regions. [5] J WERBA – IB BIOLOGY 30