lecture 2: Fundamentals of Cell Biology: Membrane Structure

Model Organisms for Study

• Arabidopsis thaliana: A model for plant biology, extensively used to study plant genetics, developmental processes, and responses to environmental stresses such as drought and disease. Its small genome and short life cycle facilitate rapid generations.

• Saccharomyces cerevisiae: A model for yeast and basic cellular processes, including cell division, metabolism, and aging. Its eukaryotic nature allows for the study of complex biological pathways and is a vital tool in biotechnology and genetics.

• Caenorhabditis elegans: A nematode used for developmental biology studies. Its transparent body allows for the observation of its 959 somatic cells, making it a powerful system for studying development, neurobiology, and cell death. This organism has a well-mapped neural circuitry and is instrumental in studying the effects of genes on behavior.

• Drosophila melanogaster: A fruit fly used in genetics and developmental studies, especially in the understanding of inheritance patterns, gene function, and evolutionary biology. Its rapid life cycle and the availability of extensive genetic tools make it a key organism in biology research.

• Zebrafish: A vertebrate model for developmental biology and genetics. Its transparent embryos enable researchers to watch the development of organs and organ systems in real-time. Zebrafish are particularly useful in studying vertebrate development, disease mechanisms, and drug discovery.

• Mouse: A mammalian model for studying complex biological systems, including genetics, development, and disease. Mice share a high degree of genetic and physiological similarity with humans, making them essential for medical research, particularly in areas like cancer, diabetes, and neurological disorders.

Basics of Membrane Structure

• The plasma membrane: Encloses the cell, defining its boundaries while maintaining differences between the cytosol and the extracellular environment. The plasma membrane is crucial for transport, communication, and overall cell integrity. Membranes of organelles (e.g., nucleus, ER, mitochondria) also maintain unique environments conducive to their specific functions.

Different functions require specialized membrane compositions that enable selective permeability and interactions with proteins and lipids.

• General Membrane Structure

  • 3. Lipid Bilayer: Composed primarily of phospholipids that create a semi-permeable barrier, preventing free passage of polar and charged molecules.

  • 5. Membrane Proteins: Integral to the structure and function, facilitating transport, acting as receptors, and providing structural support to the membrane. Most of the mass in membranes is lipid (approx. 50%); the rest is membrane proteins that perform various functions, such as ATP synthesis for energy and signal transduction for cellular communication.

• Membrane Lipids

  • Glycerophospholipids, Sphingolipids, and Sterols: Major lipids in cell membranes, phospholipids spontaneously form bilayers in aqueous environments due to their amphiphilic nature.

  • Fluid Nature of Lipid Bilayer: The lipid bilayer acts as a two-dimensional fluid, with lipid fluidity influenced by composition; for instance, unsaturated fats increase membrane fluidity, allowing for muscle contraction and neuronal activity by facilitating the movement of membrane proteins and vesicles.

• Amphiphilic Nature of Lipids: Lipid molecules have hydrophilic (polar) heads that interact with the aqueous environment and hydrophobic (nonpolar) tails (fatty acids) that avoid water. This composition influences the packing and fluidity of membranes, playing a crucial role in numerous biological processes.

Types of Phospholipids

• 3. Glycerophospholipids: The main lipids in animal membranes formed from glycerol, fatty acids, and a phosphate group.

• 5. Sphingolipids: Built from sphingosine; serve as major components of myelin sheath in nerve cells, playing a role in cell signaling and recognition.

• 7. Sterols (e.g., Cholesterol): Help to stabilize membrane fluidity and structure by fitting between phospholipid molecules and affecting membrane dynamics, further influencing membrane protein interactions.

• Lipid Bilayer Formation: Spontaneous bilayer formation minimizes hydrophobic tail exposure to water, promoting the formation of closed structures essential for cellular function.

Self-sealing property: Lipids demonstrate a self-healing ability, rapidly reorienting to eliminate free edges, which is vital for maintaining cell integrity.

Fluidity of the Lipid Bilayer

• Dependent on numerous factors including temperature and lipid composition (e.g., double bonds and hydrocarbon chain length), which can lead to phase transitions between liquid and gel states with temperature changes, crucial for membrane functionality.

Asymmetry of the Lipid Bilayer

• Exhibits different lipid compositions in inner and outer leaflets; this asymmetry is important for membrane functionality as it influences the localization of proteins and signaling pathways. Specific proteins bind lipid head groups, maintaining this asymmetry via phospholipid translocators, which transport lipids between leaflets.

• Synthesis of Phospholipids: Primarily occurs in the endoplasmic reticulum (ER). The synthesis process includes activation of fatty acids within the ER, followed by their addition to glycerol phosphate forming phosphatidic acid, and head group modifications lead to diverse phospholipid species essential for cellular functions.

• Phospholipid Transport:

  • Scramblases: Catalyze the nonselective flip-flop of phospholipids between leaflets, contributing to membrane remodeling in response to stimuli.

  • Flippases: Facilitate specific lipid movements to maintain bilayer asymmetry, crucial for signal transduction and membrane integrity.

Membrane Proteins and Their Functions

• Membrane proteins provide specific tasks and characteristics to membranes, essential for cellular communication and transport.

  • Transmembrane proteins: Pass through the bilayer, can be single or multi-pass, serving as gateways for molecules to enter or exit the cell.

  • Peripheral proteins: Indirectly associated with the membrane surface; often involved in signaling pathways and maintaining the cell’s shape.

• Glycosylation: Many proteins have carbohydrate structures attached for recognition and signaling, contributing to cell-cell communication and the immune response.

• Protein Mobility and Interactions: Membrane proteins can rotate and diffuse within the plane of the membrane, allowing for dynamic interactions necessary for signaling and function. Detergents can solubilize proteins for purification by disrupting bilayer structures, aiding in the study of individual protein functions.