Cell Membrane Structure and Function

iZ İstanbul Zaim Universitesi

Cell Membrane Structure and Function

  • Course Code: MBG 101 - General Biology

  • Lecturer: Dr Esra YILDIRIM SERVİ

Course Outline

  1. Derse giriş (Introduction to the Course)

  2. Hücre Turu (A Tour of the Cell)

  3. Hücre Membran Yapısı ve İşlevi (Cell Membrane Structure and Function)

  4. Metabolizmaya Giriş (An Introduction to Metabolism)

  5. Hücre Solonumu ve Fermentasyonu (Cellular Respiration and Fermentation)

  6. Fotosentez (Photosynthesis)

  7. Hücre İletişimi (Cell Communication)

  8. Ara Sınav (Midterm)

  9. Hücre Döngüsü (The Cell Cycle)

  10. Prokaryotik Hayat (Prokaryotic Life)

  11. Bitki Formu ve İşlevi (Plant Form and Function)

  12. Hayvan Formu ve İşlevi (Animal Form and Function)

  13. Genetik (Genetics)

  14. DNA Araçları ve Biyoteknoloji (DNA Tools and Biotechnology)

  15. Final (Final Exam)

Overview: Life at the Edge

  • The plasma membrane is the boundary separating the living cell from its surroundings.

  • Exhibits selective permeability, allowing some substances to cross more easily than others.

Cellular Membrane Composition

  • Cellular membranes are characterized as fluid mosaics of lipids and proteins.

  • Phospholipids: Abundant lipids in the plasma membrane, classified as amphipathic (containing both hydrophobic and hydrophilic regions).

  • Fluid Mosaic Model: Describes membranes as fluid structures with a mosaic of various embedded proteins.

Membrane Models: Scientific Inquiry

  • Chemically analyzed to consist of proteins and lipids, leading to the realization of a phospholipid bilayer structure.

  • Historical Models:

    • 1935 Davson-Danielli Sandwich Model: Proposed a sandwich-like structure with phospholipid layers between globular proteins.

    • 1972 Singer-Nicolson Model: Revised to state that membranes consist of a mosaic of proteins within the bilayer, with only hydrophilic regions exposed to water.

Freeze-Fracture Studies

  • A specialized technique that splits membranes, revealing the internal structure consistent with the fluid mosaic model.

Membrane Fluidity

  • Within the plasma membrane, phospholipids can move laterally.

  • Factors Affecting Fluidity:

    1. Unsaturated vs. Saturated Hydrocarbon Tails:

    • Unsaturated (kinked) tails increase fluidity.

    • Saturated tails increase viscosity by packing closely.

    1. Cholesterol:

    • Moderate temperatures reduce fluidity by limiting phospholipid movement, while low temperatures prevent solidification.

Membrane Structure and Function

Components of the Membrane

  • Membrane structure includes lipids, proteins, and carbohydrates.

  • Types of Membrane Proteins:

    • Peripheral Proteins: Bound to the membrane surface.

    • Integral Proteins: Penetrate the hydrophobic core, with transmembrane proteins spanning the membrane.

Functions of Membrane Proteins

  • Six major functions:

    1. Transport: Provide pathways for substances across the membrane.

    2. Enzymatic Activity: Enzymes integrated into the membrane facilitate reactions.

    3. Signal Transduction: Membrane proteins respond to external signalling molecules, transmitting messages inside the cell.

    4. Cell-Cell Recognition: Glycoproteins function as identification markers for cellular interaction.

    5. Intercellular Joining: Join adjacent cells through various junctions (like gap and tight junctions).

    6. Attachment to Cytoskeleton and ECM: Help maintain cell shape and stabilize membrane protein location.

Membrane Carbohydrates

  • Play crucial roles in cell-cell recognition.

  • Carbohydrates may bond covalently to lipids (glycolipids) or proteins (glycoproteins).

Synthesis and Sidedness of Membranes

  • Membranes exhibit distinct inner and outer faces, with asymmetrical distributions of proteins and lipids established during synthesis by the endoplasmic reticulum (ER) and Golgi apparatus.

Selective Permeability of Membranes

  • Plasma membranes must facilitate material exchange, controlling what enters and exits the cell.

Permeability of the Lipid Bilayer

  • Hydrophobic (nonpolar) molecules pass through quickly, while polar molecules (like sugars) do not.

Transport Proteins

  • Facilitate movement across membranes:

    • Channel Proteins: Create hydrophilic channels for specific ions or molecules.

    • Aquaporins: Specialized for water transport.

    • Carrier Proteins: Bind to molecules, changing shape to shuttle substances across.

Passive Transport Concepts

  • Diffusion: Movement of molecules to equilibrate concentrations, requiring no energy (passive transport).

  • Osmosis: Water's diffusion across a selectively permeable membrane.

Tonicity and Water Balance

  • Tonicity:

    • Isotonic: Equal solute concentration; no water movement.

    • Hypertonic: Higher external solute concentration; cell loses water.

    • Hypotonic: Lower external solute concentration; cell gains water.

  • Osmoregulation: The management of water balance within organisms.

Facilitated Diffusion

  • Involves transport proteins for passive transport of substances across membranes.

Active Transport

  • Requires energy, moving substances against concentration gradients via specific proteins.

Sodium-Potassium Pump

  • Example of active transport, maintaining ion gradients essential for cellular functions.

Bulk Transport Mechanisms

  • Exocytosis: Process where transport vesicles fuse to the plasma membrane to release contents outside the cell.

  • Endocytosis: Process of taking in macromolecules by forming vesicles from the plasma membrane, including three types:

    1. Phagocytosis: Engulfing larger particles.

    2. Pinocytosis: Uptake of extracellular fluid.

    3. Receptor-Mediated Endocytosis: Specific ligand-receptor interactions trigger vesicle formation.