Cell Types and Domains

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two fundamental types based on their internal structure: prokaryotic and eukaryotic. This distinction is central to understanding cell biology and the evolutionary relationships among organisms.

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Examples include Bacteria and Archaea.

• Eukaryotic cells possess a membrane-bound nucleus and various organelles. Examples include plants, animals, fungi, and protists.

• Organisms are divided into three domains: Bacteria (prokaryotic), Archaea (prokaryotic), and Eukarya (eukaryotic).

Prokaryotic Cell Structure

Components and Functions

Prokaryotic cells are typically unicellular and have a simpler structure compared to eukaryotes. Despite this simplicity, they possess specialized features for survival and adaptation.

• Contain at least one chromosome (usually circular DNA).

• Have numerous ribosomes for protein synthesis.

• Some prokaryotes, especially photosynthetic species, have internal membrane complexes for energy conversion.

Cell Wall and External Structures

The cell wall provides structural support and protection. In bacteria, it is primarily composed of peptidoglycan, a polysaccharide. Some bacteria also have an outer membrane made of glycolipids.

• Flagella: Long filaments that rotate to propel the cell.

• Fimbriae: Needle-like projections that promote attachment to surfaces or other cells.

Eukaryotic Cell Structure

Cell Size and Complexity

Eukaryotic cells are generally larger and more complex than prokaryotic cells. They contain a variety of organelles, each with specialized functions.

• Organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and chloroplasts (in plants and algae).

• Cell size varies widely, from microscopic cells to large eggs.

The Plasma Membrane

The plasma membrane is a critical structure that separates the cell's internal environment from the external surroundings. It is composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Maintains homeostasis by regulating the entry and exit of substances.

• Contains proteins for transport, signaling, and structural support.

Cytoplasm and Cytoskeleton

The cytoplasm is the region between the plasma membrane and the nuclear envelope, where many metabolic reactions occur. The cytoskeleton is a network of protein fibers that maintains cell shape, secures organelles, and enables movement.

• Flagella and cilia are extensions of the cytoskeleton used for movement.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs involved in storage and transport. The central vacuole in plant cells regulates water concentration and provides structural support.

The Nucleus and Nucleolus

The nucleus houses the cell's DNA and directs the synthesis of ribosomes and proteins. The nucleolus is the site of ribosomal RNA synthesis and ribosome assembly.

The Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins. It includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and transport vesicles.

Ribosomes

Ribosomes are responsible for protein synthesis. They can be found free in the cytoplasm or attached to the rough ER.

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules. The rough ER (RER) has ribosomes and modifies proteins, while the smooth ER (SER) synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

The Golgi apparatus is a series of flattened sacs that modify, sort, and package proteins and lipids for transport.

Lysosomes and Peroxisomes

Lysosomes contain digestive enzymes for breaking down waste. Peroxisomes carry out oxidation reactions and detoxify harmful substances.

Mitochondria

Mitochondria are the site of ATP production, the cell's main energy-carrying molecule. They have a double membrane and their own DNA.

Chloroplasts

Chloroplasts are found in plant and algal cells and are responsible for photosynthesis. They convert light energy, carbon dioxide, and water into glucose.

Endosymbiosis Theory

This theory proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells, leading to a mutually beneficial relationship.

Cell Walls and Extracellular Matrix

Cell walls provide structural support and protection. Plant cell walls are made of cellulose, bacterial cell walls of peptidoglycan, and fungal cell walls of chitin. Animal cells lack cell walls but have an extracellular matrix composed of glycoproteins and collagen.

Membrane Structure and Transport

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which highlights its dynamic nature and the presence of various proteins, lipids, and carbohydrates.

• Phospholipids form the bilayer, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates are attached to lipids (glycolipids) or proteins (glycoproteins) and play roles in cell recognition.

Passive Transport

Passive transport involves the movement of substances across the membrane without energy input. It includes diffusion, osmosis, and facilitated diffusion.

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Tonicity: Describes the solute concentration of a solution relative to the cell (hypotonic, hypertonic, isotonic).

• Facilitated diffusion: Requires protein channels for transport.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient. It includes ion pumps and bulk transport mechanisms such as endocytosis and exocytosis.

• Ion pumps: Move ions across membranes to maintain electrochemical gradients.

• Endocytosis: Uptake of large particles or fluids (phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Release of substances from the cell.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Key Equations

• Diffusion Rate:

• Osmosis:

Additional info:

• Cell membranes are selectively permeable, allowing only certain molecules to pass based on size, charge, and solubility.

• Endosymbiosis theory is supported by the presence of DNA and ribosomes in mitochondria and chloroplasts, similar to bacteria.