Tiêu đề [ Q1W7 ] GenBio Q1 Reviewer (1).pdf ✅

GENERAL BIOLOGY I S.Y. ‘23 - ‘24 | S1Q1 | PPT AND LECTURE-BASED WEEK 7: CELL MEMBRANE – PPT & lecture-based – 1.0 INTRODUCTION TO CELL MEMBRANE 1.1 C. MEMBRANE : ALSO KNOWN AS ★ Plasma membrane ★ Cytoplasmic membrane ★ Cellular membrane ★ Phospholipid bilayer ★ Plasmalemma 1.2 FUNCTION & CHARACTERISTICS ● Boundary that separates the living cell from its surroundings. ● Selectively permeable barrier - Allows selection and control of substance movement in and out of the cell. ● Provides structural support by maintaining the cell’s shape ● Responsible for cell recognition & communication a) Promotes cell interaction b) Allows the cell to respond to changes in its environment c) Identifies foreign pathogens to initiate immune response. ● Allows cell adhesion, enabling the formation of tissues among multicellular organisms. 1.3 TWO MAIN COMPONENTS ● The cell membrane consists of two main components, namely: a) Lipids (Phospholipids) - Concept 2.0 - 4.3 b) Proteins - Concept 5.0 - 6.6 2.0 C. MEMBRANE : FLUID MOSAIC The Mosaic Structure of Membranes ● The term "mosaic" in the context of the cell membrane refers to the diverse arrangement of various components, including phospholipids, proteins, and other molecules, within the membrane. 2.1 PHOSPHOLIPID ARRANGEMENT PHOSPHOLIPID: FACTS ● Cell membrane is composed primarily of phospholipids, the most abundant lipid in the plasma membrane. ● A phospholipid is made up of two contrasting areas in the cell membrane: a) Phosphate head (hydrophilic) b) Fatty acid tail (hydrophobic) ● Hence, phospholipids are amphipathic molecules, containing hydrophobic (water-hating) and hydrophilic (water-loving) regions. ● Phospholipids are arranged with their hydrophilic heads facing outward towards the aqueous environment, and the hydrophobic tails facing inward, shielded from water. WONRIKA ON TWT (X) | PPT | LECTURE | CAMPBELL BIO 8TH ED. GBIO SEM1 Q1 REVIEWER PAGE 1
GENERAL BIOLOGY I S.Y. ‘23 - ‘24 | S1Q1 | PPT AND LECTURE-BASED WEEK 7: CELL MEMBRANE ● The cell membrane comprises two layers of phospholipids, hence known as “phospholipid bilayer.” PHOSPHOLIPID: MOSAIC STRUCTURE ● Phospholipids contribute to the mosaic nature of the cell membrane by: a) Phospholipids spontaneously arranging themselves in bilayer formation b) Asymmetric distribution of molecules in the cell membrane (due to the different types of phospholipids present) c) Phospholipids serve as an anchor for membrane proteins. 2.2 PROTEIN ARRANGEMENT ● Proteins are scattered across the bilayer; no definite formation. ● The arrangement of these proteins is dynamic, not static. - This allows for the adaptation of the membrane to changing cellular conditions and environmental stimuli. 3.0 TWO MEMBRANE MODELS ● There were specific events that led to the understanding of the cell membrane structure. 3.1. Sandwich Model (1935) - By Hugh Davson and James Danielli 3.2. Fluid Mosaic Model (1972) - By Seymour Singer and Garth Nicolson 3.1 SANDWICH MODEL 1935 ● Davson and Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins, like a sandwich. ● Davson-Danielli model contained assumptions, such as that all membranes had the same structure, thickness, and lipid-protein ratio. ● The model failed to explain the selective permeability of the cell membrane. - e.g. If we place water into the sandwich model, the water WONRIKA ON TWT (X) | PPT | LECTURE | CAMPBELL BIO 8TH ED. GBIO SEM1 Q1 REVIEWER PAGE 2
GENERAL BIOLOGY I S.Y. ‘23 - ‘24 | S1Q1 | PPT AND LECTURE-BASED WEEK 7: CELL MEMBRANE would not completely proceed to the other side since there is a layer of solid protein wherein the water cannot pass through. ● With that, Davson and Danielli made a freeze-fracture experiment to observe the cellular layers of certain components. FREEZE-FRACTURE EXPERIMENT ● Method used to study the arrangement of proteins and lipids in the cell membrane by freezing and fracturing the cell, allowing the interior structures to be revealed. ● By freezing, the cell membrane would solidify, enabling it to be cut into thin layers and its cellular layers to be observed under the microscope. ● In the observation, the extracellular layer appeared to be smoother, whereas the cytoplasmic layer looked rougher and had tiny bumps. - Contradicted Davson-Danielli’s assumption that there was equal protein size and thickness inside and outside the cell membrane. ● Hence, the sandwich model was rejected. PROBLEMS IN THE DAVSON-DANIELLI MODEL ● Various flaws and inconsistencies were subsequently discovered: a) Overlooking the membrane fluidity, wherein lipids could, in fact, move and proteins could change their shape. b) Did not explain membrane asymmetry, where the inner and outer surfaces contain distinct lipids and proteins. c) Falsely explaining membrane protein mobility, wherein research found that proteins are free to transverse the membrane and interact with its many compartments. 3.2 FLUID MOSAIC MODEL 1972 ● Singer and Nicolson presented their own model wherein the proteins, instead of being embedded in between, are proposed to be integrated and scattered across the bilayer. ● Fluidity: The fluid nature allows for the lateral movement of lipids and proteins within the membrane. ● Mosaic: The mosaic structure highlights the presence of diverse and heterogeneous arrangement of proteins, lipids, and other molecules dispersed throughout the membrane. ● Supported by the freeze-fracture experiment. ● Highlighted the dynamic nature of the cell membrane, emphasizing its ability WONRIKA ON TWT (X) | PPT | LECTURE | CAMPBELL BIO 8TH ED. GBIO SEM1 Q1 REVIEWER PAGE 3
GENERAL BIOLOGY I S.Y. ‘23 - ‘24 | S1Q1 | PPT AND LECTURE-BASED WEEK 7: CELL MEMBRANE to adapt to changing cellular and environmental conditions. 4.0 C. MEMBRANE : FLUID MOSAIC The Fluidity of Membranes ● Rather than being a solid structure, cell membranes are fluid. ● Three evidences: 4.1. Movement of phospholipids 4.2. Membrane fluidity 4.3. Presence of cholesterol 4.1 MOVEMENT OF PHOSPHOLIPIDS ● Phospholipids are not stationary; they move across the bilayer that allows fluidity. ● Two types of phospholipid movement: a) Lateral movement - Approximately 10^7 times per second b) Flip-flop movement - Approximately once per month Q: Do membrane proteins move? Yes. 4.2 MEMBRANE FLUIDITY ● Low temperatures have a significant impact on the behavior of fatty acid tails. ● When exposed to extreme cold temperature, the fatty acid tails can be WONRIKA ON TWT (X) | PPT | LECTURE | CAMPBELL BIO 8TH ED. GBIO SEM1 Q1 REVIEWER PAGE 4