Membrane Structure 2.4 PDF

2.4 CELLS MEMBRANE STRUCTURE Cell Membranes Membranes enclose the contents of a cell, separating internal components from the external environment This allows the internal conditions to be regulated Cell membranes possess two fundamental qualities: Semi-permeability: Only certain material c...

2.4 CELLS MEMBRANE STRUCTURE Cell Membranes Membranes enclose the contents of a cell, separating internal components from the external environment This allows the internal conditions to be regulated Cell membranes possess two fundamental qualities: Semi-permeability: Only certain material can cross Selectivity: Membranes regulate material movement Cell membranes act as barriers and consist of lipid bilayers Cell membranes primarily consist of two biomolecules – Image taken via light microscopy phospholipids (non-polar barrier) and proteins (selectivity) using immunofluorescence B2.1.2 Lipid bilayers as barriers Phospholipids Structure of Phospholipids: Hydrophilic head Consists of a polar head (glycerol and phosphate) Attracted to H2O Consists of two non-polar tails (fatty acids × 2) Hydrophobic tail The head is hydrophilic and the tail is hydrophobic Repelled by H2O A phospholipid is therefore an amphipathic molecule Interstitial Fluid (Extracellular) Arrangement in Membrane: In water, phospholipids form a spontaneous bilayer The tails face inwards, the heads face outwards Cytosolic Fluid (Intracellular) B2.1.1 Lipid bilayers as the basis of cell membranes Phospholipid Bilayer Properties of the Bilayer: The phospholipid bilayer is held together by the weak hydrophobic interactions between the fatty acid tails The presence of a central hydrophobic layer restricts the passage of most water-soluble substances Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility This fluidity allows for the spontaneous breaking and Fatty acid tails are repelled reforming of membranes (endocytosis / exocytosis) by water, forming a bilayer B1.1.12 Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions Membrane Proteins The phospholipid bilayer is embedded with proteins, Integral which can be classified as being integral or peripheral protein Integral Proteins: Permanently attached to the phospholipid bilayer Require detergents to remove (transmembrane) Peripheral Proteins: Temporarily associated with membrane periphery Peripheral Can be dissociated with salts or pH changes protein B2.1.4 Integral and peripheral proteins in membranes Protein Functions Membrane proteins serve a variety of key functions: Junctions – They can function to connect two cells together Enzymes – Fixing enzymes to membranes localizes reactions Transport – Used for facilitated diffusion and active transport Recognition – May function as markers for ‘self’ identification MNEMONIC: Anchorage – Act as attachment points for cytoskeleton / ECM JET RAT Transduction – Function as receptors for peptide hormones Glycosylation Phospholipids and proteins can have carbohydrate chains attached to form either glycolipids or glycoproteins The carbohydrate extends from the extracellular side of the membrane and functions in adhesion and recognition Can serve as an attachment point (e.g. sperm binding) Can act as a recognition point (e.g. ABO blood groups) Glycoproteins and glycolipids also play an important role in Glycocalyx (sugar coat) anchoring cells to a surrounding extracellular matrix (ECM) surrounding an ovum B2.1.9 Structure and function of glycoproteins and glycolipids Fluid Mosaic Model Fluid mosaic model | Cells | Biology | FuseSchool Cell Membrane Structure and Function glycoprotein glycolipid phospholipid peripheral protein integral protein B2.1.10 Fluid mosaic model of membrane structure Topic Connections Carbohydrates Glycoproteins are involved in the process of cell–cell recognition (B1.1.7) Cell Theory Freeze fracturing was used to first visualise membrane proteins (A2.2.3) Cell Respiration (AHL) Membrane-bound enzymes are used in aerobic cell respiration (C1.2.15) Cell Signalling (AHL) Integral proteins function as transmembrane hormone receptors (C2.1.6)