Single Cell Dynamics and Membrane Structure

Introduction

This section explores the fundamental differences between prokaryotic and eukaryotic cells, the structure and function of biological membranes, and the dynamic nature of single cells. Understanding these concepts is essential for grasping cell biology and the molecular basis of life.

Major Cell Types: Prokaryotes vs. Eukaryotes

Definitions and Key Features

• Prokaryotic Cells: Cells lacking a true nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Cells with a true nucleus and various membrane-bound organelles. Examples include Animals, Plants, Fungi, and Protists.

Basic Features of All Cells:

• Plasma membrane: Encloses the cell, separating it from the environment.

• Cytosol: Semifluid substance inside the cell.

• Chromosomes: Carry genetic information.

• Ribosomes: Synthesize proteins.

Comparison Table:

Cellular Membranes: Structure and Function

Plasma Membrane Overview

The plasma membrane is a selectively permeable barrier that protects the cell, maintains its internal environment, and regulates the movement of substances in and out.

• Protection: Shields the cell from external threats.

• Compartmentalization: Establishes a distinct internal environment.

• Transport Regulation: Controls entry and exit of molecules.

Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of proteins floating in or on a fluid lipid bilayer.

• Phospholipid Bilayer: Forms the basic fabric of the membrane; amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Integral Proteins: Embedded within the bilayer, often spanning the membrane.

• Peripheral Proteins: Loosely attached to the surface of the membrane.

• Cholesterol: Stabilizes membrane fluidity, especially in animal cells.

Key Terms:

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Phospholipid: Major component of cell membranes, consisting of a glycerol backbone, two fatty acid tails, and a phosphate group.

Membrane Proteins: Types and Functions

• Integral Proteins: Span the membrane; involved in transport, signaling, and cell recognition.

• Peripheral Proteins: Attached to the membrane surface; play roles in signaling and maintaining cell shape.

Functions of Membrane Proteins:

• Transport of molecules

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to cytoskeleton and extracellular matrix

Membrane Fluidity and Experimental Evidence

Membrane components are not static; they move laterally within the bilayer, contributing to membrane fluidity. Experimental techniques such as Fluorescence Recovery After Photobleaching (FRAP) demonstrate this lateral movement.

• FRAP: A region of the membrane is bleached with a laser, and the recovery of fluorescence indicates movement of unbleached molecules into the area.

• Protein Mobility: Experiments mixing mouse and human cell membranes show proteins can diffuse and intermingle over time.

Orientation and Biosynthesis of Membrane Components

Membrane asymmetry is established during biosynthesis and maintained throughout the life of the cell. Specific phospholipids and proteins are oriented differently on the inner and outer leaflets of the bilayer.

• Asymmetry: Certain lipids and glycoproteins are found predominantly on one side of the membrane.

• Biosynthesis: Membrane components are synthesized in the endomembrane system and delivered to the plasma membrane via vesicles.

Examples and Applications

Immune Cell Dynamics

Neutrophils (a type of white blood cell) chase and engulf bacteria, demonstrating single cell dynamics and the importance of membrane structure in cell movement and recognition.

Membrane Proteins in Health and Disease

• HIV Infection: HIV uses specific membrane proteins (e.g., CD4 and CCR5) to enter host cells. Individuals lacking CCR5 are resistant to infection.

• SARS-CoV-2: The virus responsible for COVID-19 hijacks membrane proteins to infect cells.

Summary Table: Key Vocabulary

Key Equations

• Surface Area to Volume Ratio: Important for cell size and efficiency

Additional info:

• Woese's 3 Domains of Life: Bacteria, Archaea, Eukarya (inferred from context).

• Membrane fluidity is affected by temperature, lipid composition, and cholesterol content.

• Phospholipids in animal cell membranes include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin.