L7 Cell Membrane 1-2.pdf

Plasma Membrane 1- 2 Structure and function Dr Pallav Sengupta Assistant Professor, Physiology College of Medicine February 13, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE Learning objectives On completion of this unit, the student will be able to: Describe the structure of the cell membrane. Describe th...

Plasma Membrane 1- 2 Structure and function Dr Pallav Sengupta Assistant Professor, Physiology College of Medicine February 13, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE Learning objectives On completion of this unit, the student will be able to: Describe the structure of the cell membrane. Describe the significance of the bilipid layer of plasma membrane. Enumerate the membrane proteins and their functions with examples; Describe the significance of Cell Adhesion Molecules and Cytoskeletal proteins. Clinical Case Report 1: Hypothermia Patient Presentation: You are seeing CH, a 72-year-old man who was found in a snowbank in midwinter. CH has Alzheimer disease and wandered away from his home, unnoticed by his family. When brought to the emergency department, CH is confused and irritable. His heart rate is 42/min, respiratory rate 12/min, and core body temperature is 93°F. You begin rewarming treatments. Diagnostic Approach: What is happening at the level of the cell membrane throughout CH’s body during this period of hypothermia? Evolution of Cell/Plasma Membrane Models S.J. Singer G.L. Nicolson Fluid-Mosaic Model ‘Quasi-fluid state’ Cell/Plasma Membrane It forms the boundary for the cytoplasm and other organelles inside the cell and controls the diffusion of substance. 3 2 Proteins 1 Oligosaccharide Bilipid layer 3 3 1 2 2 1 2 2 The main structural components of cell membranes are membrane lipids, along with embedded proteins, carbohydrates, glycoproteins, and glycolipids. What cell membranes do? 1. Serves as a barrier, controlling entry and exit of substances. 2. Facilitates communication and adhesion with other cells. 3. Hub for signal transduction and cellular biochemistry. 4. Regulates cell shape and aids in motility. 5. Compartmentalizes cell, ensuring organelle-specific functions. Types of lipids present in bilipid layer Bilipid Layer Phospholipids make up the majority of the cell membrane, forming the two opposing sheets of the lipid bilayer. Phospholipids are amphipathic, meaning they contain both hydrophilic, ‘water-loving’, and hydrophobic, ‘water-hating’ regions Phospholipid Layer The hydrophilic polar region consists of the phosphate head group built on a carbon backbone that faces the aqueous intracellular or extracellular space. The hydrophobic nonpolar region consists of two fatty acid tails that intermingle within the center of the membrane, forming weak noncovalent bonds with one another to attach the two leaflets together. A phospholipid is compose of a glycerol backbone attached on one end to two fatty acids and the other Attack area end has the esterified phosphoric acid and an organic alcoholic group Due to the amphipathic nature, phospholipids spontaneously form lipid bilayers when placed in an aqueous environment so that the hydrophobic tails are not exposed to the aqueous intracellular and extracellular environments. If phospholipids have small tails, or a single hydrocarbon chain, they may form a small, single-layered sphere known as a micelle. If phospholipids have bulkier tails, or two hydrophobic tails, they may form a hollow droplet known as a liposome. Alterations in phospholipid asymmetry cause a danger signal that can trigger an inflammatory reaction in surrounding cells. Oxidative damage When phosphatidylserine, traditionally an intracellular phospholipid, flips its position to join the outer leaflet, this lipid is recognized by surveilling immune cells that quickly eliminate the cell marked for destruction. Phospholipid varieties and locations Phospholipid varieties Other lipids Other important components for maintaining cell membrane structure are steroids: organic compounds composed of a fourring structure. Cholesterol is a steroid, built from four linked hydrocarbon rings, that has a hydrocarbon tail at one end and a hydroxyl group on the other end. It is oriented parallel to the phospholipid tails so that the hydroxyl group of cholesterol can interact with the phospholipid head groups Cholesterol and Membrane fluidity Cholesterol influences membrane fluidity by forming strong interactions with phospholipids. One of its functions is to reduce the fluidity of the membrane. To do so, it forms strong interactions with phospholipids using its rigid steroid ring structure. At high temperatures (decreases fluidity), cholesterol’s flat, rigid structure limits phospholipid movement. The steroid ring interacts with, and partly immobilizes, the regions of the phospholipid fatty acid chains that are closest to the polar head groups. By decreasing the mobility of the first few CH2 groups in the fatty acid chains, cholesterol makes that region of the lipid bilayer less deformable. Cholesterol and Membrane fluidity At low temperatures (decreases fluidity), cholesterol prevents the hydrocarbon chains of lipids from packing together and forming ordered, crystal-like structures. This interference with packing broadens the phase transition that occurs during freezing, eliminating the rapid change in membrane fluidity that would otherwise occur near the Tm Significance of Membrane fluidity 1. Enables proper membrane protein function and cellular communication. 2. Facilitates lipid and protein movement within the cellular membrane. 3. Allows membrane repair and vesicle formation during endocytosis/exocytosis. 4. Ensures membrane flexibility, adapting to temperature and environmental changes. 5. Supports efficient nutrient absorption and waste product removal. 6. Aids in the immune response by presenting cell surface antigens. 7. Promotes cell division and growth by allowing membrane expansion. Clinical Case Report: Hypothermia Patient Presentation: You are seeing CH, a 72-year-old man who was found in a snowbank in midwinter. CH has Alzheimer disease and wandered away from his home, unnoticed by his family. When brought to the emergency department, CH is confused and irritable. His heart rate is 42/min, respiratory rate 12/min, and core body temperature is 93°F. You begin rewarming treatments. Diagnostic Approach: What is happening at the level of the cell membrane throughout CH’s body during this period of hypothermia? Thinking back to CH, what is the effect of hypothermia on his cell membranes? Discussion You now know that certain conditions can affect the fluidity of the cell membrane, the lipid bilayer that surrounds and protects the cell and regulates the traffic in and out of the cell. At low temperatures, as in CH’s condition, the fluidity of the cell membrane decreases and becomes rigid. This decrease in fluidity decreases the cell’s permeability and possibly restricts the entry of oxygen and glucose. Within the hour, CH’s temperature returns to normal, and you envision his cell membrane’s fluidity returning to normal so that the cell membrane can resume its important functions.