Cellular Organelles

Overview

Cells are the fundamental structural and functional units of life, responsible for performing numerous tasks simultaneously such as digestion, nerve signal transmission, blood circulation, protein synthesis, and waste filtration. These diverse functions are possible because cells contain specialized structures called organelles, each dedicated to specific roles. Cells are broadly classified into two categories based on nuclear organization and presence of membrane-bound organelles: prokaryotic and eukaryotic cells. Both types share common components like plasma membrane, cytoplasm, ribosomes, and DNA. Prokaryotic cells lack a well-organized nucleus and membrane-bound organelles but contain structures like mesosomes (plasma membrane infoldings) and sometimes flagella for locomotion. Eukaryotic cells have a defined nucleus, plasma membrane, and various membrane-bound organelles such as endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, vacuoles, lysosomes, and peroxisomes. Advances in microscopy, especially electron microscopy, have been crucial in revealing the detailed structure and functions of these organelles. Understanding the structure and function of individual organelles is essential to comprehend the overall cellular functioning and life processes.

• Cells perform diverse functions essential for organism survival.
• Organelles are specialized cellular structures responsible for specific functions.
• Cells are classified as prokaryotic (without nucleus) and eukaryotic (with nucleus).
• Common components include plasma membrane, cytoplasm, ribosomes, and DNA.
• Prokaryotes have mesosomes and sometimes flagella; eukaryotes have membrane-bound organelles.
• Microscopic advancements enabled detailed study of cellular organelles.
• 📌 Cell: Basic structural and functional unit of life.
• 📌 Organelle: Specialized subunit within a cell with a specific function.
• 📌 Prokaryote: Cell lacking a defined nucleus and membrane-bound organelles.

2.1 Plasma Membrane

The plasma membrane, also called the cell membrane, forms the boundary of the cytoplasm and separates the internal environment of the cell from the extracellular matrix. It regulates the interaction between the cell and its surroundings and is semipermeable, allowing selective passage of substances. The detailed structure of the plasma membrane was elucidated after understanding its chemical composition—mainly lipids and proteins—and with the advent of electron microscopy. The Fluid Mosaic Model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a lipid bilayer with a mosaic of globular proteins embedded within it. The lipid bilayer is mainly composed of phospholipids arranged with hydrophilic heads facing outward and hydrophobic tails inward, creating a quasifluid dynamic structure that allows lateral diffusion of lipids and proteins. Proteins in the membrane are classified as peripheral (attached superficially, involved in cell signaling) and integral (partially or fully embedded, including transmembrane proteins). The membrane's fluidity is crucial for processes like cell division, growth, communication, secretion, and endocytosis. Transport across the plasma membrane occurs via passive transport (diffusion and osmosis), facilitated diffusion (carrier and channel proteins like glucose transporters and ion channels), and active transport (energy-dependent movement against concentration gradients, e.g., Na⁺-K⁺ pump). Coupled transport mechanisms include symport (two molecules in same direction) and antiport (two molecules in opposite directions). Aquaporins are specialized channel proteins facilitating water transport. The plasma membrane's selective permeability maintains cellular composition and homeostasis.

• Plasma membrane separates cytoplasm from extracellular environment.
• Composed mainly of phospholipid bilayer and proteins (integral and peripheral).
• Fluid Mosaic Model describes dynamic lipid-protein arrangement.
• Membrane fluidity allows lateral movement of components.
• Transport mechanisms include passive, facilitated, and active transport.
• Active transport requires ATP, e.g., Na⁺-K⁺ pump; coupled transport includes symport and antiport.
• 📌 Plasma membrane: Semipermeable boundary of the cell.
• 📌 Fluid Mosaic Model: Model describing membrane as fluid lipid bilayer with embedded proteins.
• 📌 Phospholipid: Lipid molecule with hydrophilic head and hydrophobic tail forming bilayer.