RVHS 2. Cellular Transport

River Valley High School 1 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport River Valley High School 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport Name: ( ) Class: 25J__ Date: References Title Authors Biology (9th edition) Campbell and Reece Biological Science 1: Organisms, Energy and Environment Taylor, Green and Stout AS Level Biology Bradfield, Dodds and Taylor Molecular Biology of the Cell (5th edition) Alberts, Johnson, Lewis, Raff, Roberts and Walter H2 Biology Syllabus 9477 (2025) Candidates should be able to use the knowledge gained in the following section(s) in new situations or to solve related problems. Related Topics Concepts Biomolecules of Life and Cellular Transport Structures and roles of carbohydrates, lipids and proteins in living organisms Organelles and Cellular Structure Functions of membrane system and organelles Communication and Equilibrium in Organisms Roles of receptors on cell surface membrane Learning Outcomes 1B. Biomolecules of Life and Cellular Transport 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.

River Valley High School 2 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport Lecture Outline I. Membrane Structure A. Fluid Mosaic Model of Membrane Structure B. Components of Biological Membranes II. Membrane Function III. Transport Across Membranes A. Passive processes 1. Simple Diffusion 2. Facilitated Diffusion 3. Osmosis B. Active processes 1. Active Transport 2. Bulk Transport (Endocytosis, Exocytosis) Websites URL Description https://www.youtube.com/watch?v=nsklF1w4eok This TED-Ed video provides an overview of the structure and function of the membrane and its components. http://moleculesinmotion.com/jmol/boyer_completed/ani mations/membrane_transport/membrane_transport.htm *Note: you would need to use a laptop/desktop and download a chrome extension (e.g. “Flash Player for Chrome”) to view the tutorial. An interactive tutorial on the different mechanisms by which transport across membranes occurs. The tutorial contains animations for better visualisation, as well as quizzes to reinforce concepts.

River Valley High School 3 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport I. Membrane Structure A. Fluid Mosaic Model of Membrane Structure Since 1917, various membrane models have been proposed; and the currently accepted model, though continually being refined, is the fluid mosaic model. This model was proposed by S.J. Singer and G. J. Nicholson in 1972, who described the membrane structure to follow a fluid mosaic model. Evidence for the Fluid Mosaic Model Freeze-Fracture Technique Source: Biology, Pg 127. Evidence for this model came from the freeze-fracture technique, together with electron microscopy. In this technique, a cell is frozen with liquid nitrogen and then “tapped” with a knife. This causes a fracture along the middle of the phospholipid bilayer and split the plasma membrane into two separated layers When the inner surface of each layer of the plasma membrane is viewed under a scanning electron microscope, globular structures (the “bumps”), which are the same size as globular membrane proteins , can be seen scattered throughout. This provided evidence that proteins are interspersed and randomly embedded in the phospholipid bilayer.

River Valley High School 4 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport Structure of the Fluid Mosaic Model Fluid Mosaic Model of Membrane Structure (a) Three-dimensional illustration of the plasma membrane. 1 and 2 are transmembrane proteins. (b) Two-dimensional illustration of the plasma membrane. D shows the phospholipid bilayer. C shows a phospholipid molecule which is made up of a hydrophilic phosphate head A and a hydrophobic hydrocarbon tail B. Source: Biological Science 1: Organisms, Energy and Environment, Pg 142. 1. The fluid mosaic model of biological membrane comprises of phospholipids that moves laterally within the layer, and proteins that moves laterally in the bilayer, making it fluid. Proteins are interspersed and randomly embedded in the phospholipid bilayer giving it a mosaic appearance. 2. The phospholipid bilayer is about 7 nm thick and is asymmetrical. - Two phospholipid layers may differ in phospholipid and protein composition. 3. The hydrophilic phosphate heads of the phospholipids face outwards into the aqueous environments on both sides of the membrane. The non -polar, hydrophobic hydrocarbon tails face inwards and create a hydrophobic core that is shielded from aqueous environment. 4. Proteins may penetrate only part of the membrane, or through the entire length of the membrane. Their hydrophilic regions would protrude out of the membrane, and into the aqueous environments on either side. (b) (a) 1 2

River Valley High School 5 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport B. Components of Biological Membranes Biological membranes are assembled from four main components: 1. Phospholipids 2. Cholesterol (in animal cells only) 3. Proteins 4. Carbohydrates Structure of plasma membrane in an animal cell. Source: Biology, Pg 128. 1. Phospholipids • Phospholipids have a hydrophilic phosphate head and two hydrophobic hydrocarbon tails. • The shape and amphipathic nature of phospholipid molecules cause them to form bilayers spontaneously in aqueous environments. - The hydrophilic phosphate heads face the aqueous environment at each surface of the bilayer. - The hydrophobic hydrocarbon tails are shielded away from water in the interior of the membrane, forming a hydrophobic core that acts as a barrier to prevent the movement of polar molecules and charged ions across membrane .

River Valley High School 6 2025 JC1 H2 Biology Lecture Topic 2: Cellular Transport Fluidity of membranes Movement of phospholipids. Source: Biology, Pg 127. • Phospholipids are held together primaril y by weak hydrophobic interactions . Hence, m ost of the phospholipid molecules and some of the membrane proteins can shift about laterally, in the plane of the membrane. - Flip-flopping transversely across the membrane, however, is rare since it would require the hydrophilic part of the molecule to cross the hydrophobic core of the membrane. • The fluidity of membranes has to be precisely regulated. Certain membrane transport processes and enzyme activities cease when membrane viscosity exceeds a certain threshold level. The fluidity of a phospholipid bilayer depends on four factors: 1. Temperature • As temperature decreases, membrane fluidity decreases. • A membrane remains fluid as temperature decreases, until a particular freezing point where the membrane solidifies. Upon this change of state, known as a phase transition, the phospholipids settle into a rigid, closely packed arrangement. 2. Fatty acid composition - length of hydrocarbon chains • Generally, the longer the fatty acid hydrocarbon chains, the higher the melting point of the membrane. • This is because a longer chain length increases the surface area of contact , thus increasing the tendency of the hydrocarbon tails interacting with one another via hydrophobic interactions. 3. Fatty acid composition - degree of saturation of hydrocarbon chains • Unsaturated fatty acid chains with cis carbon-carbon double bonds have kinks. These kinks hinder the hydrocarbon chain