Cell Membranes: Structure and Function – Study Notes

Cell Membranes: Structure and Function

Introduction

Cell membranes are fundamental components of all living cells, providing structural integrity, compartmentalization, and regulation of molecular traffic. This section explores the structure, composition, and functions of biological membranes, with a focus on their roles in prokaryotic and eukaryotic cells.

Cellular Anatomy

Prokaryotic Cells: Archaea and Eubacteria

• Prokaryotes include Archaea and Bacteria.

• They lack internal membranes and membrane-bound organelles.

• All essential processes occur in the cytoplasm or at the plasma membrane.

• Most have a cell wall for structural support and protection.

• Examples: Escherichia coli (bacterium), Halobacterium (archaeon).

Key Terms: Plasma membrane (selective barrier), cytosol (fluid component of cytoplasm), nucleoid (region containing DNA in prokaryotes).

Eukaryotic Cells: Endomembrane System

• Eukaryotes possess extensive internal membranes and organelles.

• The endomembrane system includes:

  • Endoplasmic reticulum (ER): site of protein and lipid synthesis.

  • Golgi apparatus: modifies, sorts, and packages proteins and lipids.

  • Mitochondria: site of cellular respiration and ATP production.

  • Lysosomes: contain digestive enzymes for breakdown of macromolecules.

• Other organelles: nucleus, peroxisomes, vacuoles (in plants and fungi).

Example: Animal and plant cells both have endomembrane systems, but plant cells also contain chloroplasts for photosynthesis.

Membrane Structure

Fluid Mosaic Model

The fluid mosaic model describes the dynamic and heterogeneous nature of biological membranes.

• Membranes are composed of a phospholipid bilayer with embedded proteins, glycoproteins, and glycolipids.

• Lipids and proteins can move laterally within the layer, contributing to membrane fluidity.

• Membrane proteins are not randomly distributed; they have specific arrangements and functions.

• Hydrophilic (water-attracting) heads face outward, while hydrophobic (water-repelling) tails face inward.

Key Terms: Integral proteins (span the membrane), peripheral proteins (attached to membrane surface), cholesterol (modulates fluidity in animal cells).

Membrane Lipids

• Phospholipids: Main structural component; amphipathic molecules forming bilayers.

• Glycolipids: Lipids with carbohydrate groups; involved in cell recognition.

• Sterols (e.g., cholesterol): Stabilize membrane fluidity, especially in animal cells.

• Sphingolipids: Important in nerve cell membranes (e.g., myelin).

Example: The presence of unsaturated fatty acids increases membrane fluidity, while saturated fatty acids decrease it.

Membrane Proteins

• Integral (transmembrane) proteins: Span the bilayer; involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

• Glycoproteins: Proteins with carbohydrate chains; play roles in cell-cell recognition and adhesion.

Membrane Functions

Selective Permeability

• Membranes regulate the movement of substances into and out of the cell.

• Small, nonpolar molecules (e.g., O2, CO2) diffuse freely; ions and large polar molecules require transport proteins.

• Maintains electrochemical gradients essential for cellular processes.

Transport Mechanisms

• Passive Transport: Movement down a concentration gradient; does not require energy.

  • Simple diffusion: Direct movement of small, nonpolar molecules.

  • Facilitated diffusion: Movement via channel or carrier proteins (e.g., aquaporins for water).

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement against a concentration gradient; requires energy (usually ATP).

  • Primary active transport: Direct use of ATP (e.g., Na+/K+ pump).

  • Secondary active transport: Uses energy from an existing gradient (e.g., symport, antiport).

• Bulk Transport: Movement of large particles via vesicles.

  • Endocytosis: Uptake of materials by membrane invagination.

  • Exocytosis: Release of materials by vesicle fusion with the membrane.

Equation (Fick's Law of Diffusion):

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Membrane Specializations

• Lipid rafts: Microdomains enriched in cholesterol and sphingolipids; involved in signaling and trafficking.

• Cell junctions: Structures for cell-cell adhesion (e.g., tight junctions, desmosomes).

• Glycocalyx: Carbohydrate-rich layer on the cell surface; protects and mediates interactions.

Endosymbiosis and Organelle Evolution

Endosymbiotic Theory

• Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence includes double membranes, their own DNA, and similarities to bacteria.

• Endosymbiosis explains the presence of internal membranes in eukaryotic organelles.

Example: Mitochondria share similarities with α-proteobacteria; chloroplasts with cyanobacteria.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Internal Membranes	Absent	Present (endomembrane system)
Organelles	Absent	Present (nucleus, mitochondria, etc.)
Cell Wall	Usually present	Present in plants/fungi; absent in animals
DNA Location	Nucleoid	Nucleus
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of membranes.

• Integral protein: Protein embedded within the membrane, often spanning its entire width.

• Peripheral protein: Protein attached to the membrane surface.

• Glycoprotein: Protein with attached carbohydrate chains.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Endocytosis: Cellular uptake of materials via vesicle formation.

• Exocytosis: Release of cellular contents via vesicle fusion with the plasma membrane.

Additional info: Some details, such as the specific roles of lipid rafts and the full range of membrane proteins, have been expanded for academic completeness.