Cell Membranes: Structure, Function, and Origins

Cell Membranes

Introduction to Cell Membranes

Cell membranes are fundamental structures that define the boundaries of all living cells and many organelles. They play a crucial role in maintaining the internal environment of the cell and mediating interactions with the external environment.

• Plasma Membrane: The outer boundary of the cell, separating the living cell from its aqueous environment.

• Thickness: The typical plasma membrane is approximately 8 nanometers (nm) thick.

• Selective Permeability: The membrane allows some substances to cross more easily than others, maintaining homeostasis.

• Hydrophobic vs. Hydrophilic: The membrane's structure is based on the interaction between hydrophobic (nonpolar) and hydrophilic (polar) molecules.

Phospholipids and Membrane Structure

Phospholipid Bilayer

The basic structure of biological membranes is the phospholipid bilayer, which forms the foundation for membrane function and fluidity.

• Phospholipid Structure: Each phospholipid molecule has a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.

• Bilayer Arrangement: Phospholipids arrange themselves in a bilayer, with the hydrophobic tails facing inward and the hydrophilic heads facing outward toward the aqueous environment.

• Structure-Function Relationship: The amphipathic nature (having both hydrophilic and hydrophobic regions) of phospholipids is essential for forming the membrane barrier.

Variations in Phospholipids

Biological membranes may differ in their lipid composition, affecting their properties and functions.

• Fatty Acid Chain Length: Phospholipids can have fatty acid chains of varying lengths, influencing membrane thickness and fluidity.

• Degree of Saturation: The presence of saturated or unsaturated fatty acids affects how tightly the lipids pack together.

• Kinds of Polar Groups: Different polar head groups can be present, contributing to membrane diversity.

Fluid Mosaic Model

General Design of Membranes

The fluid mosaic model describes the structure of cell membranes as a mosaic of various proteins floating in or on the fluid lipid bilayer.

• Fluidity: Fatty acids in phospholipids allow lateral movement of molecules within the membrane, contributing to its fluid nature.

• Influencing Factors:

  • Lipid Composition: Short, unsaturated fatty acid chains increase fluidity.

  • Temperature: Membrane fluidity decreases at lower temperatures.

Membrane Proteins

Proteins are essential components of biological membranes, contributing to their structure and function.

• Integral (Intrinsic) Proteins: Embedded within the phospholipid bilayer, often spanning the membrane.

• Peripheral (Extrinsic) Proteins: Loosely attached to the membrane surface, lacking hydrophobic regions.

• Lipid-Anchored Proteins: Covalently attached to lipids within the membrane.

• Protein Mobility: Some membrane proteins can move laterally within the bilayer, while others are anchored by the cytoskeleton or other cellular structures.

Membrane Carbohydrates

Carbohydrates are found on the outer surface of the plasma membrane and play roles in cell recognition and communication.

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: One or more oligosaccharides covalently bonded to proteins.

• Proteoglycans: Proteins with long, complex carbohydrate chains attached.

Membrane Permeability

Selective Permeability

Biological membranes allow some substances to pass while restricting others, maintaining the internal environment of the cell.

• Simple Diffusion: Small, nonpolar, lipid-soluble molecules (e.g., O2, CO2) can cross the bilayer directly.

• Impermeable to Hydrophilic Molecules: Large or charged molecules (e.g., amino acids, sugars, ions) cannot pass through the hydrophobic core of the membrane unaided.

Facilitated Diffusion and Water Transport

• Facilitated Diffusion: Some molecules cross membranes faster than by simple diffusion by "hitchhiking" with ions or using specific channels.

• Aquaporins: Specialized channel proteins that allow large amounts of water to move along its concentration gradient.

Cell Walls and Intercellular Connections

Structure and Function of Cell Walls

Cell walls provide structural support and act as a permeability barrier in certain organisms.

• Composition: Plant, prokaryotic, and fungal cell walls are composed of complex carbohydrates such as cellulose and chitin.

• Plasmodesmata: Small channels in plant cell walls that connect neighboring cells, allowing the transfer of nutrients, waste, and ions (symplastic pathways).

• Apoplastic Pathways: Molecules can also move through spaces within the cell walls, bypassing the cell membrane.

Origins of Cell Compartmentalization

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells from ancestral prokaryotes, particularly the development of mitochondria and chloroplasts.

• Process: An anaerobic prokaryote lost its cell wall, and the flexible membrane began to fold inward, forming internal compartments such as the nucleus.

• Engulfment: A larger prokaryote engulfed a smaller, autotrophic bacterium, which evolved into the mitochondrion.

• Chloroplast and Mitochondria Evidence:

  • Double membranes (outer membrane may be vesicular in origin)

  • Circular DNA similar to prokaryotic DNA

  • Ribosomes similar to those of prokaryotes (70S type)

Feature	Mitochondria/Chloroplasts	Prokaryotes	Eukaryotic Cells
Membranes	Double	Single	Single (organelles may have double)
DNA	Circular	Circular	Linear (nucleus)
Ribosomes	70S	70S	80S (cytoplasm)

Additional info: The endosymbiotic theory is supported by genetic and biochemical evidence, including similarities in DNA sequences and ribosomal structure between organelles and prokaryotes.