MEMBRANE AND TRANSPORT

1 EJC H2 Biology T2W1 Membrane Transport Membrane Transport 1. Overview of Topic Cells need to regulate the movement of substances into and out of themselves. Substances such as water, oxygen, glucose and minerals are important in the synthesis of new molecules and important cellular processes. According to the fluid mosaic model, cell membranes are selectively permeable due to the nature of the phospholipids and proteins from which they are made. The movement of different molecules depends on the nature of the substances through transport processes such as osmosis, diffusion and active transport. Membranes allow cells to create and maintain internal environments that are different from external environments. Membranes are not just found on the most exterior ends of a cell to provide that barrier between the exterio r and interior. Especially for e ukaryotic cells, there are internal membrane structures that partition the cell into specialised compartments so that cellular processes can occur with optimal activity e.g. chloroplasts and mitochondria. The endomembrane system, consisting of rough and smooth endoplasmic reticulum and Golgi apparatus, is responsible for protein processing and vesicular transport within the cell. 2. Learning Outcomes (j) explain the fluid mosaic model and the roles of the constituent biomolecules (including phospholipids, proteins, glycolipids, glycoproteins and cholesterol) in cell membranes (k) outline the functions of membranes at the surface of cells and membranes within the cell (l) explain how and why different substances move across membranes through simple diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis 3. References Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V. and Jackson, R. B. (2011) Campbell Biology (Ninth Edition) (Pearson Higher Education) ISBN 0321739752

2 EJC H2 Biology T2W1 Membrane Transport Contents 1. Overview of Topic ................................ ................................ ....................... 1 2. Learning Outcomes ................................ ................................ ..................... 1 3. References ................................ ................................ ................................ . 1 4. Types of Membranes ................................ ................................ .................. 2 5. Membrane Structure: The Fluid Mosaic Model ................................ ............ 2 6. Components of Membrane ................................ ................................ .......... 4 Phospholipid ................................ ................................ ................................ ... 4 Cholesterol ................................ ................................ ................................ ..... 5 Protein ................................ ................................ ................................ ............ 6 Carbohydrates ................................ ................................ ................................ 6 7. Membrane Fluidity ................................ ................................ ...................... 7 8. Functions of Membrane ................................ ................................ .............. 9 9. Transport across membrane ................................ ................................ ..... 10 Diffusion ................................ ................................ ................................ ....... 11 Osmosis ................................ ................................ ................................ ....... 13 Active transport ................................ ................................ ............................ 14 Bulk transport ................................ ................................ ............................... 15 4. Types of Membranes There are generally two different types of membranes: External cell membrane that surrounds cells (known as plasmalemma or plasma membrane / cell surface membrane) Intracellular membranes of organelles typically found within eukaryotic cells. 5. Membrane Structure: The Fluid Mosaic Model The membrane is a fluid bilayer of phospholipid molecules in which proteins are dispersed and embedded like a mosaic pattern. This mosaic pattern is not static because proteins are constantly moving in a fluid sea of phospholipids. This is described by the Fluid Mosaic Model.

3 EJC H2 Biology T2W1 Membrane Transport Like a mosaic, the cell surface membrane is a complex structure made up of the following components: Phospholipids, Cholesterol, Glycolipids, Proteins (peripheral and integral) and Glycoproteins. The relative amounts of these components vary from membrane to membrane depending on the function of the membrane. Typically, the membrane is never identical on both sides and is described as asymmetrical. The membrane is fluid and in dynamic motion. Hydrophilic portions of both proteins and phospholipids are maxima lly exposed to water while the hydrophobic portions of proteins and phospholipids are in the non -aqueous environment in the core of the bilayer resulting in a sta ble membrane structure. As the hydrophobic interactions are weak, this allow most membrane lipids and proteins to drift later ally within the plane of the membrane. Flip-flopping across the membrane is rare. Hydrophobic region Hydrophilic region Cholesterol Channel protein- hydrophilic channel acts as a pore through which ions and certain polar molecules can pass Integral protein (Transmembrane protein) Integral protein Glycolipid Glycoprotein Cytoplasm Extracellular fluid Phospholipid bilayer Phospholipid Hydrophilic Phosphate head Hydrophobic fatty acid tails Peripheral protein

4 EJC H2 Biology T2W1 Membrane Transport 6. Components of Membrane Phospholipid It is an amphipathic molecule as it has both hydrophilic and hydrophobic components within its structure. The negatively charged phosphate group can interact with water and therefore hydrophilic while the two hydrocarbon tails are hydrophobic. In an aqueous medium, p hospholipids are able to form stable bilayers spontaneously as t hey tend to line up with polar heads in water and hydrocarbon tails away from water. Phospholipids in the membrane are held together by weak hydrophobic interactions. The formation of bilayers prevents energetically unfavorable interactions of the fatty acid chains with the aqueous medium. The same forces that drive phospholipids to form bilayers also provide a self - healing property. A small tear in the bilayer create s a free edge with water; because this is energetically unfavorable, the lipids spontaneously rearrange to eliminate the free edge. Phospholipid bilayer

5 EJC H2 Biology T2W1 Membrane Transport The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable. Cholesterol Sterols are the most abundant group of steroids made up of a hydroxyl ( -OH) group and a hydrocarbon chain. E.g.: cholesterol. Cholestrol is non -polar, therefore, generally hydrophobic. It also has a carbon sk eleton consisting of 4 interconnected rings. A sterol which h elp maintain fluidity of membrane by preventing close orderly packing of phospholipid molecules when the temperature is low and also has an effect in holding the phospholipids together when the temperature is high ( refer to the Section on Membrane Fluidity).

6 EJC H2 Biology T2W1 Membrane Transport Protein Membrane proteins can be classified based on their positioning in the membrane. Peripheral or extrinsic proteins are loosely attached at the polar surface of the phospholipids They are rich in amino acids with