## The Three Domains of Life Life on Earth is broadly categorized into three domains: Bacteria, Archaea, and Eukarya. These domains are distinguished primarily by differences in their genetic material (specifically, ribosomal RNA sequences). * **Bacteria:** This domain encompasses the vast majority of prokaryotic organisms—single-celled organisms lacking a nucleus and other membrane-bound organelles. Bacteria are incredibly diverse and play crucial roles in various ecosystems, from decomposition to nitrogen fixation. * **Archaea:** Archaea are also prokaryotes, but they possess unique genetic and biochemical characteristics that distinguish them from bacteria. Many archaea thrive in extreme environments (extremophiles), such as hot springs or highly saline lakes. * **Eukarya:** This domain includes all organisms with eukaryotic cells—cells containing a membrane-bound nucleus and other organelles. Eukarya encompasses a wide range of organisms, including protists, fungi, plants, and animals. The vast variability within this domain is a key focus in many biological studies. Think of the three domains as three major branches on the tree of life, with the distance between branches reflecting the evolutionary distance based on nucleotide sequences. ## Characteristics of Eukaryotic Cells Eukaryotic cells are fundamentally different from prokaryotic cells in several key aspects: * **Nucleus:** The defining feature of a eukaryotic cell is the presence of a membrane-bound nucleus, which houses the cell's genetic material (DNA). * **Size:** Eukaryotic cells are significantly larger than prokaryotic cells, approximately 1000 times greater in volume. * **Cytoskeleton:** Eukaryotes possess a complex cytoskeleton—a network of protein filaments that provides structural support and facilitates intracellular transport. * **Internal Membranes:** Eukaryotic cells contain an extensive system of internal membranes, forming various organelles such as the endoplasmic reticulum, Golgi apparatus, and mitochondria, each with specialized functions. Imagine a well-organized office (eukaryotic cell) with separate rooms (organelles) for different tasks, compared to a small, open-plan workspace (prokaryotic cell). ## Other Key Features of Cells Beyond the defining characteristics, several other features are common to most cells: * **Size:** Cells typically range from 10 to 100 micrometers (µm) in diameter ($10^{-6}$ m). * **Plasma Membrane:** Cells are enclosed by a plasma membrane, a selectively permeable barrier that separates the cell's internal environment from its surroundings. * **Cell Communication:** Cells communicate with each other through various signaling mechanisms. * **Organization and Compartmentalization:** Cells exhibit a high degree of organization and compartmentalization, with different functions localized to specific regions or organelles. * **Water Content:** Cells are composed of approximately 70% water. * **ATP Dependence:** Many cellular processes require adenosine triphosphate (ATP) as an energy source. * **Chemical Composition:** Cells are primarily composed of sugars, fatty acids, amino acids, and nucleotides. The high water content is crucial for many cellular processes, acting as a solvent and facilitating chemical reactions. ## The Importance of Water Water plays a vital role in cellular function: * **Covalent Bonds:** Water molecules ($H_2O$) are held together by strong covalent bonds. * **Hydrogen Bonds:** Weak hydrogen bonds form between different water molecules, creating a dynamic network. * **Liquid State:** These transient hydrogen bonds allow water to exist as a liquid at ambient temperatures. * **Reaction Medium:** Water serves as the medium for a vast number of chemical reactions within the cell. * **Hydrophobic Effect:** The hydrophobic effect, the tendency of nonpolar molecules to aggregate in water, is crucial for membrane formation and protein folding. Understanding hydrogen bonding is key to understanding many properties of water and its importance in biological systems. ## The Plasma Membrane: A Selectively Permeable Barrier The plasma membrane is a crucial component of all cells: * **Amphipathic Molecules:** The membrane is composed of amphipathic molecules—molecules with both hydrophobic (water-fearing) and hydrophilic (water-loving) regions. * **Phospholipids:** Phospholipids are the primary amphipathic molecules in the membrane, spontaneously forming a bilayer in aqueous environments. * **Permeability:** The plasma membrane must be selectively permeable, allowing the passage of some molecules while restricting others. * **Membrane Proteins:** Membrane proteins, such as ion channels and transporters, facilitate the passage of specific ions and molecules across the membrane. * **Conservation:** The basic structure and function of the plasma membrane are highly conserved across all cellular organisms. Don't confuse hydrophobic and hydrophilic. Hydrophobic molecules repel water, while hydrophilic molecules attract water.