YIJC [H2] CI1.4 Membranes and Cellular Transport (N)

2023 JC1 BIOLOGY LECTURE NOTES CORE IDEA 1: THE CELL AND BIOMOLECULES OF LIFE TOPIC 1.4: MEMBRANES & CELLULAR TRANSPORT Learning Outcomes You should be able to: (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 Textbooks and References Campbell, Urry, Cain, Wasserman, Minorsky, Reece (2018) Biology – A Global Approach (11 th Edition)(Global Edition) Chapter 8: Cell Membranes pg. 196 – 211 (Pearson Publication) ISBN-10 1-292-17043-3 Note: This textbook is available in our library. You may wish to borrow them to supplement your reading when necessary. Contents Introduction 1.1 Fluid mosaic model of membrane structure 1.2 Roles of the constituent biomolecules in membrane 1.2.1 Phospholipids 1.2.2 Proteins 1.2.3 Glycolipids & Glycoproteins 1.2.4 Cholesterol 1.3 Functions of membranes 1.3.1 At surface of cells 1.3.2 Within the cell 2.0 Transport across membrane 2.1 Passive transport 2.1.1 Simple diffusion 2.1.2 Osmosis 2.1.3 Facilitated diffusion 2.2 Active transport 2.3 Bulk transport 2.3.1 Endocytosis 2.3.2 Exocytosis H2

2 Introduction The following question should help you frame your learning: 1. How do cells regulate the movement of substances into and out of themselves, and what are the implications of such movements? Biomolecules make up cells and cells regulate many cellular processes, including the movement of substances into and out of themselves, through membranes. Phospholipids, cholesterol, carbohydrates and proteins are important components in biological membranes. Cells need to regulate the movement of substances into and out of themselves. Substances such as water, oxygen, glucose and minerals are important in t he synthesis of new molecules and important cellular processes. According to the fluid mosaic model, cell membranes are partially 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.

3 Learning Outcome (j): Explain the fluid mosaic model and the roles of the constituent biomolecules (including phospholipids, proteins, glycolipids, glycoproteins and cholesterol) in cell membranes 1.1 Fluid mosaic model of membrane structure Practices of Science ● As early as 1915 scientists were aware that the structure of membranes isolated from cells included proteins and lipids. ● Early theories were mostly concerned with phospholipids forming a bilayer with proteins, forming thin layers on the exterior and interior of the bilayer. ● The Davson–Danielli model, proposed by Hugh Davson and James Danielli in 1935, used this lipid bilayer model, suggesting there was a thin layer of globular protein on both sides. ● In 1972, Seymour J. Singer and Garth L. Nicolson proposed that proteins are inserted into the phospholipid bilayer and do not form a layer on the phospholipid bilayer surfaces. ● There were several reasons why Singer and Nicolson proposed a model that was different from the Davson–Danielli model: ○ Not all membranes are identical or symmetrical, as the first model implied. ○ Membranes with different functions also have a different composition and different structure, as can be seen with an electron microscope. ○ A protein layer is not likely because it is largely non-polar and would not interface with water, as shown by cell studies. ● Much of the evidence used to change the Davson –Danielli model was gathered with the use of the electron microscope. ● Since 1972 further evidence has been gathered about membranes, and slight changes to the Singer–Nicolson model have been made. Fig. 1.1.1 Freeze-Fracture of Phospholipid Bilayer This demonstrates that scientific knowledge is reliable and durable, yet subject to revision in the light of new evidence.

4 ● The current agreed model for the cellular membrane is the fluid mosaic model. ● All cellular membranes, whether cell surface membrane or organelle membranes, have the same general structure. Fig. 1.1.2 The Fluid Mosaic Model of Cell Membrane ● The cell surface membrane is made almost entirely of protein and lipid, together with a small and variable amount of carbohydrate. ● The cell surface membrane is described as fluid because: ○ phospholipid molecules are capable of rapid lateral and rotational (“flip-flop”) movement within the monolayer, as they are held together only by relatively weak hydrophobic interactions. Fig. 1.1.3 Lateral and flip-flop movement of phospholipids in the membrane ○ membrane proteins can also move slowly although most are immobile due to anchorage to cytoskeleton.

5 A group of enzymes called flippase, floppase and scramblase catalyses the lateral and rotational movement. Fig. 1.1.4 Flippase, floppase and scramblase PE: Phosphatidylethanolamines (PE) are a class of phospholipids found in biological membranes. PE are found in the inner (cytoplasmic) leaflet of the lipid bilayer. PS: Phosphatidylserine (PS) is a phospholipid and is a component of the cell membrane. ABC transporter: The ATP-binding cassette transporters (ABC transporters) consist of multiple subunits, one or two of which are transmembrane proteins and one or two of which are membrane-associated ATPases. The ATPase subunits utilize the energy of adenosine triphosphate (ATP) binding and hydrolysis to provide the energy needed for the translocation of substrates across membranes, either for uptake or for export of the substrate. Experiment: Larry Frye and Michael Edidin, at Jonhn Hopkins University, labelled the cell surface membrane proteins of a mouse cell and a human cell with two different green and red fluorescent dyes. The two cell types were fused to form a hybrid cell with a continu ous membrane. Using a microscope, they observed the labelled proteins on the hybrid cell and found that the hybrid cell membrane had initially distinct regions of green and red dye. Results: In less than an hour, the two colours were intermixed. Conclusion: The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the cell surface membrane.

6 ● The cell surface membrane is also described as mosaic because: ○ the proteins of various sizes are randomly embedded in or attached to the bilayer. ○ the phospholipid composition differs in the 2 monolayers - asymmetrical distribution. 1.2 Roles of the constituent biomolecules in membrane Fig. 1.2.1 Biomolecules in membrane

7 1.2.1 Phospholipids Fig. 1.2.1.1 Structure of phospholipid FYI: This phospholipid is a phosphatidylcholine. ● The membrane is a bilayer produced from huge numbers of phospholipids. ○ Each phospholipid is composed of a glycerol (three-carbon compound). ○ Two of the glycerol carbons have fatty acids. ■ Fatty acids are non-polar because they are not able to form hydrogen bonds with water molecules. ○ The third carbon is attached to a highly polar organic alcohol that includes a bond to a phosphate group. ■ The organic alcohol with phosphate is highly polar because hydrogen bonds readily form between the phosphate head and water molecules.

8 ■ This structure means that membranes have