Plasma Membrane

Introduction to Cell Membrane

Overview

  • Every cell, whether prokaryotic or eukaryotic, is surrounded by a plasma membrane (cell membrane or plasma lemma).

  • Functions:

    • Maintains internal and external environment by controlling molecule and ion transport.

    • Prevents loss of useful substances and facilitates removal of toxic byproducts.

    • Acts as a semi-permeable or selectively permeable membrane.

  • Thickness: Approximately 70-100 Å, composed of lipids and proteins.

History of Cell Membrane

Early Discoveries

  • Karl W. Nageli (1817-1891) showed that cell membranes are semi-permeable and vital for osmotic functions.

  • Introduced terminology:

    • "Zellen membrane" before 1855.

    • "Plasma membrane" in 1855 for membranes formed by cytoplasm.

Structural Models of the Cell Membrane

Key Contributions

  • Lipid Bilayer Model (Overton, Gorter, Grendel): Proposed that the membrane consists of a lipid bilayer.

    • Overton (1902) observed lipid-soluble substances pass through membranes, suggesting lipid composition.

    • Gorter and Grendel (1926) concluded membranes are double layers based on erythrocyte studies.

Protein-Lipid-Protein Hypothesis (Danielli and Davson)

  • Proposed existence of proteins on the membrane surface altering surface tension.

  • Suggested a lipid bilayer with proteins on both surfaces.

  • Electron microscopy later revealed fine structures of membranes, with thickness estimates of 6-10 nm.

Unit Membrane Model (Robertson)

  • Robertson (1953) described membranes as trilaminar with two dark layers separated by a light layer in electron micrographs.

  • Emphasized similar structure across all biological membranes.

Fluid Mosaic Model (Singer and Nicolson)

  • Proposed in 1972 that membranes are mosaics of lipids and proteins with fluidity.

  • Membrane proteins are mobile and embedded within the lipid bilayer.

  • Distinguishes between extrinsic (peripheral) and intrinsic (integral) proteins.

Composition of Cell Membrane

Primary Components

  • Lipids: Form the basic structure with phospholipids, cholesterol, and glycolipids.

  • Proteins: Integral and peripheral proteins play various roles, including transport and signaling.

  • Carbohydrates: Present as glycoproteins and glycolipids, contributing to cell recognition and signaling.

Membrane Fluidity

Factors Influencing Fluidity

  • Temperature Effects: Increasing temp raises fluidity, while lower temps reduce it.

  • **Lipid Composition:

    • Saturated fatty acids reduce fluidity; unsaturated fatty acids increase it due to kinks in tails.

    • Cholesterol modulates fluidity, making membranes more stable under varying temperatures.

Transport Mechanisms

Types of Transport

  • Passive Transport: Simple diffusion, facilitated diffusion (e.g., with transport proteins).

  • Active Transport: Requires energy to move substances against concentration gradients (e.g., Na+/K+ pump).

  • Endocytosis/Exocytosis: Involves vesicle formation for uptake or release of large molecules.

Specific Transport Processes

  • Phagocytosis: Engulfing large particles (e.g., pathogens) by immune cells.

  • Pinocytosis: Uptake of fluids and small molecules.

  • Receptor-Mediated Endocytosis: Selective uptake of specific molecules based on receptor interaction.

Conclusion

  • The cell membrane is a complex structure vital for maintaining homeostasis and enabling communication and transport within the cell.