Cytoplasmic Membrane Overview

Cytoplasmic Membrane

• Vital barrier separating the cytoplasm from the environment
• If broken, cell integrity is lost, leading to cell death
• Offers limited protection against osmotic lysis
• Acts as a selective permeability barrier allowing nutrients to enter and waste products to exit

Bacterial Cytoplasmic Membrane

• Structure: phospholipid bilayer with embedded proteins
• Thickness: 8-10 nm
• Consistency: somewhat fluid
• Composition: both hydrophobic (water-repelling) and hydrophilic (water-attracting) components
  • Hydrophobic: fatty acids
  • Hydrophilic: glycerol, phosphate, and other functional groups (e.g., sugars, ethanolamine, choline)
• Fatty acids point inward, creating a hydrophobic environment, while hydrophilic portions face the external environment or the cytoplasm
• Some bacterial species have hopanoids (rigid, sterol-like molecules), strengthening the membrane; different ring structure than sterols
• Embedded proteins often clustered, allowing protein interactions
• Associated proteins include:
  • Integral proteins: significantly embedded, spanning the membrane
  • Peripheral proteins: loosely associated with membrane surfaces; often have a lipid tail that anchors them to the membrane

Archaeal Cytoplasmic Membrane

• Similar structure to bacterial membranes, but different chemistry
• Bacteria and Eukarya use ester linkages between fatty acids and glycerol
• Archaea use ether linkages between glycerol and their hydrophobic side chains (not true fatty acids)
• Archaeal lipid side chains are repeating units of isoprene (5-carbon hydrocarbon)
• Major lipids are either:
  • Phosphoglycerol diethers(20-carbon side chains called phytanyl groups)
  • Diphosphoglycerol tetraethers (40-carbon side chains called biphytanyl groups), which form lipid monolayers (phytanyls covalently linked to form biphytanyl)
• Some archaeal lipids contain rings within their hydrocarbon chains
• Common membrane lipid in Crenarchaeota (major phylum of Archaea) has four C5 rings and one C3 ring
• Rings affect membrane chemical properties and function
• Polar head groups in archaeal lipids can be sugars, ethanolamine, or other molecules
• Hopanoids are not found in archaeal membranes

Cytoplasmic Membrane Functions

• Permeability barrier (gatekeeper): prevents leakage and controls nutrient and waste transport
• Protein anchor: holds transport proteins in place
• Energy conservation and consumption: generates/dissipates the proton motive force
• Cytoplasm composition: contains salts, sugars, amino acids, nucleotides, etc.
• Hydrophobic membrane portion prevents diffusion of these solutes from the cell
• Some small hydrophobic molecules can diffuse through the membrane, but larger polar/charged molecules and ions need transport.
• Water passes freely in both directions, but aquaporins accelerate its movement
• Most substances are transported in/out via transport proteins

Cytoplasmic Membrane Transport Proteins

• Accumulate solutes against concentration gradients using energy, specifically for single molecules or molecule classes
• Essential for biochemical reactions
• Transport systems display saturation effects and specificity for target molecules
• Biosynthesis of transport proteins is highly regulated by the cell, dependent on available resources and concentrations

Explore the structure and functions of the cytoplasmic membrane, focusing on its critical role in maintaining cell integrity and selective permeability. Understand the composition and characteristics of the bacterial cytoplasmic membrane, including its unique phospholipid bilayer and embedded proteins.