5 Membranes & Transport

• Learning Outcomes

  • Explain the fluid mosaic model and the roles of the constituent biomolecules (including phospholipids, proteins, glycolipids, glycoproteins and cholesterol) in cell membranes
  • Outline the functions of membranes at the surface of cells and membranes within the cell
  • Explain how and why different substances move across membranes through simple diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis
• There are generally two different types of membranes: External cell membrane that surrounds cells (known the cell surface or plasma membrane) and Intracellular membranes of organelles typically found within eukaryotic cells
• References


  • Reece, K., Jane, B. Taylor, Martha, R., Simon, Eric, J., Dickey, Jean, L., Hogan (2020) Campbell Biology (12th Edition) (Pearson Higher Education) ISBN-10 0135188741
  • Raven, P., Johnson, G., Mason, K., Losos, J. and Duncan, T. (2022) Biology (13th Edition) (McGraw-Hill) ISBN-10 1264097859
  • Alberts, B., Heald, R., Johnson, A., Morgan, D., Raff, M., Roberts, K. and Walter, P. (2022) Molecular Biology of the Cell (7th Edition) (W. W. Norton & Company) ISBN-10 0393884821
• The Fluid Mosaic Model

  The membrane is a fluid bilayer in which both phospholipid molecules and some proteins are free to move laterally. The random arrangement of proteins embedded in the membrane resemble
• Cell membrane asymmetry


  The membrane is never identical on both sides and is described as asymmetrical
• Cell membrane


  The structure that surrounds cells, also known as the cell surface or plasma membrane
• Formation of stable bilayers by phospholipids in an aqueous medium

  Phospholipids line up with charged, hydrophilic phosphate heads facing water and non-polar, hydrophobic fatty acid tails away from water, resulting in a hydrophobic core in the bilayer
• Fluid Mosaic Model

  The membrane is a fluid bilayer in which phospholipid molecules and some proteins are free to move laterally, resembling a mosaic pattern
• Saturated fatty acid tails


  Make the membrane less fluid due to maximum interaction of the fatty acid tails with each other
• Cholesterol

  An amphipathic molecule mainly characterized by a non-polar carbon skeleton consisting of 4 interconnected rings
• Membrane proteins


  Serve functions such as transport, enzymes, receptor sites for cell signaling, attachment to cytoskeleton and extracellular matrix
• Phospholipids are amphipathic molecules with hydrophilic and hydrophobic components
• Membrane fluidity

  Phospholipids in the membrane are held together by weak hydrophobic interactions, allowing for membrane fluidity
• Classification of membrane proteins

  • Peripheral or extrinsic proteins
• Unsaturated fatty acid tails

  Make membranes more fluid due to the presence of C=C (carbon double bonds) causing kinks in the fatty acid tail, preventing close packing of phospholipid molecules
• Intracellular membranes


  Membranes typically found within eukaryotic cells
• Components of the cell surface membrane

  • Phospholipids
  • Cholesterol
  • Glycolipids
  • Proteins (peripheral and integral)
  • Glycoproteins
• Regulation of membrane fluidity by cholesterol

  At low temperatures, cholesterol prevents phospholipid molecules from being too closely packed and solidifying, maintaining membrane fluidity. At high temperatures, cholesterol prevents phospholipid molecules from moving too far apart, restricting membrane fluidity and stabilizing the membrane structure
• Synthesis and embedding of membrane proteins

  1. Membrane proteins are synthesised by bound ribosomes at the rough endoplasmic reticulum and are embedded in the membrane of the rough ER

  2. Glycosylation (i.e. addition of carbohydrates to the proteins) can occur to form glycoproteins

  3. A transport vesicle with the glycoproteins embedded on its membrane buds off from rough ER and brings the glycoproteins to the cis face of the Golgi apparatus, where further chemical modifications may occur

  4. A secretory vesicle with the glycoproteins embedded on its membrane buds off from the trans face of the Golgi apparatus and transports the glycoproteins to the cell surface membrane

  5. The secretory vesicle fuses with the cell surface membrane, and the glycoproteins are now embedded in the cell surface membrane
• Carbohydrates in membranes tend to be covalently associated with either lipids or proteins forming glycolipids and glycoproteins respectively
• Freeze Fracturing Technique is an electron microscope technique which shows the existence of proteins among the phospholipid bilayer
• Peripheral or extrinsic proteins

  • Loosely attached at the surface of the phospholipid bilayer
  • Rich in amino acids with hydrophilic side chains which permit interaction with surrounding water and the charged hydrophilic phosphate groups of the phospholipid bilayer
• Membranes act as selective barriers for regulating the passage of substances in and out of cells or organelles due to their high composition of lipids and presence of specific integral transport proteins
• Types of membrane proteins

  • Peripheral or extrinsic proteins
  • Integral or intrinsic proteins
• Integral or intrinsic proteins


  • Either partially embedded or span across the phospholipid bilayer entirely (known as transmembrane protein)
  • Contain both hydrophilic and hydrophobic regions which interact with the charged phosphate heads and the non-polar fatty acid tails of the phospholipids respectively
  • In the case of transport proteins, some function as carrier proteins while some function as channel proteins
• Membrane proteins get embedded in the cell surface membrane

  Other transmembrane proteins such as channel proteins and carrier proteins are embedded in the same way
• Membranes allow separate compartments to be formed within cells, maintaining a constant internal environment within each compartment
• In 1972, Singer and Nicolson proposed the Fluid Mosaic Model through the freeze fracture technique, which accounted for the amphipathic character of proteins
• Selectively permeable suggests the presence of a certain degree of control over what passes through the membrane via transport proteins based on various factors. Partially permeable membrane allows some particles to pass through based on size
• Membranes serve as a point of communication between cells and their surrounding environment, with proteins at the membrane acting as receptor sites for recognising external stimuli such as hormones
• Integral transport proteins

  Allow hydrophilic substances to pass through the hydrophilic channel of transmembrane proteins (e.g. channel proteins) to avoid the hydrophobic core of the bilayer
• Two broad categories of movements across membranes: passive (substances move down a concentration gradient) and active (substances move against a concentration gradient through transport proteins in the membrane)
• Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. ATP is not required as it is a passive process
• Terminology clarification: selectively permeable for biological membranes VS partially permeable for artificial membranes (e.g. Visking tubing)
• Membrane permeability


  Allows small molecules to pass through while preventing large or polar molecules (e.g. glucose and amino acids) and charged particles/ions (e.g. Na+, H+) from getting across
• Passive processes include simple diffusion, facilitated diffusion, and osmosis, while active processes include active transport and bulk transport (endocytosis, exocytosis)
• Movement of substances across membranes is vital for the cell to obtain nutrients, excrete waste substances, secrete useful substances, generate electrochemical gradients, maintain suitable pH and ion concentration for enzyme activity
• Membranes are sites of chemical reactions due to the presence of enzymes and proteins that are organised and localised in a sequential manner for specific biochemical reactions to take place efficiently
• Water molecules can move across the membrane through transient pores or aquaporins. Osmosis continues until dynamic equilibrium is achieved between the two regions
• Facilitated diffusion


  Movement of a substance across a membrane via transport proteins from a region of high concentration to a region of low concentration, down a concentration gradient with no expenditure of ATP