3.1 Cell Theory and Cell Structure

Cell theory forms the foundation of modern biology, explaining how all living things are composed of cells. It highlights cells as the basic unit of life, capable of performing essential functions and reproducing. This theory revolutionized our understanding of life's building blocks.

Cell structure is intricate, with various organelles working together to maintain cellular function. From the protective plasma membrane to the energy-producing mitochondria, each component plays a crucial role in keeping cells alive and thriving. Understanding these structures is key to grasping cellular processes.

Cell Theory Fundamentals

Historical Development of Cell Theory

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• Cell theory states that all living organisms are composed of one or more cells, the cell is the basic unit of structure and organization in organisms, and cells arise from pre-existing cells
• The development of cell theory was based on the work of many scientists over hundreds of years, with technological advancements allowing for new discoveries and understanding
  • Robert Hooke first described cells in 1665 using a primitive microscope to examine cork and honeycomb structure
  • Anton van Leeuwenhoek, considered the "Father of Microbiology," was the first to observe living cells under a microscope in the 1670s
  • Matthias Schleiden in 1838 concluded that all plant tissues are composed of cells and that an embryonic plant arose from a single cell
  • Theodor Schwann in 1839 stated that all animal tissues are composed of cells and that cells are the basic building blocks of all plants and animals

Key Principles of Cell Theory

• All living organisms are composed of one or more cells (unicellular organisms like bacteria, or multicellular organisms like plants and animals)
• The cell is the fundamental unit of structure, function, and organization in all living organisms
  • Cells are the smallest unit of life that can replicate independently
  • All the basic functions of life, such as energy production, protein synthesis, and waste elimination, occur within cells
• Cells arise from pre-existing cells through the process of cell division (mitosis or meiosis)
  • New cells are formed by the division of existing cells, ensuring the continuity of life
  • Rudolf Virchow in 1855 added this third tenet to cell theory, stating that cells cannot spontaneously generate from non-living matter

Cell Organelle Structure and Function

Plasma Membrane and Nucleus

• The plasma membrane is a selectively permeable phospholipid bilayer that controls the movement of substances in and out of the cell and serves as a barrier between the cell and its environment
  • The hydrophobic tails of the phospholipids face inward, while the hydrophilic heads face outward, creating a barrier
  • Proteins embedded in the membrane serve various functions, such as transport, cell signaling, and cell recognition
• The nucleus is the control center of the cell, containing the genetic material (DNA) and directing cellular activities such as protein synthesis and cell division
  • The nuclear envelope is a double membrane that surrounds the nucleus, separating its contents from the cytoplasm
  • Chromatin is the combination of DNA and proteins that make up chromosomes, which condense during cell division
  • The nucleolus is a dense region within the nucleus where ribosomal RNA is synthesized and ribosomal subunits are assembled

Organelles Involved in Protein Synthesis and Transport

• Ribosomes are small organelles composed of rRNA and proteins that are the sites of protein synthesis, either free in the cytoplasm or attached to the rough endoplasmic reticulum
  • Ribosomes translate mRNA into polypeptide chains, which fold into functional proteins
  • Free ribosomes synthesize proteins that function within the cytosol, while bound ribosomes synthesize proteins destined for secretion or insertion into the plasma membrane
• The endoplasmic reticulum (ER) is a network of membranous tubules and sacs that transport materials within the cell, with rough ER studded with ribosomes and smooth ER lacking ribosomes
  • Rough ER is involved in the synthesis, folding, and modification of proteins, as well as the production of phospholipids and steroids
  • Smooth ER is involved in lipid synthesis, carbohydrate metabolism, and detoxification of harmful substances
• The Golgi apparatus is a stack of flattened membranous sacs that modify, package, and sort proteins and other macromolecules for secretion or transport to other parts of the cell
  • Proteins and lipids are modified by the addition of carbohydrates (glycosylation) or other chemical groups
  • The Golgi apparatus sorts and packages the modified products into vesicles for transport to their final destination (lysosomes, plasma membrane, or secretion)

Organelles Involved in Energy Production and Cellular Maintenance

• Mitochondria are the "powerhouses" of the cell, generating ATP through cellular respiration and containing their own DNA, ribosomes, and the enzymes necessary for ATP production
  • The inner membrane of mitochondria is highly folded, forming cristae, which increase the surface area for energy production
  • The matrix, the space within the inner membrane, contains enzymes involved in the citric acid cycle and oxidative phosphorylation
• Lysosomes are membranous sacs containing digestive enzymes that break down worn-out organelles, macromolecules, and foreign invaders
  • Lysosomes maintain cellular homeostasis by recycling old organelles and macromolecules, providing raw materials for new cellular components
  • Lysosomes also play a role in cell defense by digesting pathogens that enter the cell through phagocytosis
• Peroxisomes are small, membranous sacs that contain enzymes involved in detoxification and lipid metabolism
  • Peroxisomes break down fatty acids and detoxify harmful substances like alcohol and hydrogen peroxide
  • Peroxisomes also play a role in the synthesis of plasmalogens, a type of phospholipid abundant in the myelin sheath surrounding nerve cells
• The cytoskeleton is a network of protein filaments (microfilaments, intermediate filaments, and microtubules) that provide structure, support, and movement to the cell and its organelles
  • Microfilaments, composed of actin, are involved in cell motility, muscle contraction, and cell division
  • Intermediate filaments provide mechanical strength and support to the cell
  • Microtubules, composed of tubulin, are involved in intracellular transport, cell division, and the formation of cilia and flagella

Prokaryotic vs Eukaryotic Cells

Structural Differences

• Prokaryotic cells lack a membrane-bound nucleus and other membrane-bound organelles, while eukaryotic cells have a true nucleus and membrane-bound organelles
  • In prokaryotic cells, the DNA is located in the nucleoid region, which is not enclosed by a membrane
  • Eukaryotic cells have a complex endomembrane system, including the endoplasmic reticulum, Golgi apparatus, and lysosomes
• Prokaryotic cells are typically smaller (1-10 µm) than eukaryotic cells (10-100 µm)
  • Bacteria, which are prokaryotic, are usually 1-5 µm in size
  • Animal and plant cells, which are eukaryotic, range from 10-100 µm in size
• Prokaryotic cells have a cell wall composed of peptidoglycan, while eukaryotic cells may have a cell wall composed of cellulose (plants) or chitin (fungi) or lack a cell wall altogether (animals)
  • The peptidoglycan cell wall in prokaryotes provides structural support and protection against osmotic stress
  • Plant cell walls, composed of cellulose, provide structural support and help maintain cell shape
• Prokaryotic cells may have additional structures such as a capsule, pili, and flagella, while eukaryotic cells have specialized structures like cilia and flagella composed of microtubules
  • Capsules, found in some bacteria, are protective layers of polysaccharides that help the cell adhere to surfaces and evade the host immune system
  • Pili are short, hair-like structures that help bacteria adhere to surfaces and facilitate the transfer of genetic material between cells
  • Prokaryotic flagella are composed of the protein flagellin and are used for motility, while eukaryotic flagella are composed of microtubules and are used for motility and sensory functions

Genomic and Metabolic Differences

• Prokaryotic cells have a single circular chromosome of DNA in the nucleoid region, while eukaryotic cells have multiple linear chromosomes contained within the nucleus
  • The prokaryotic genome is usually smaller and less complex than the eukaryotic genome
  • Eukaryotic DNA is associated with histone proteins, forming nucleosomes, which are further packaged into chromatin and chromosomes
• Prokaryotic cells have ribosomes (70S) that are smaller than those found in eukaryotic cells (80S in the cytoplasm)
  • The 70S ribosomes in prokaryotes are composed of a 50S large subunit and a 30S small subunit
  • Eukaryotic cells also have 70S ribosomes in their mitochondria and chloroplasts, reflecting the endosymbiotic origin of these organelles
• Prokaryotic cells lack membrane-bound organelles involved in energy production and metabolism, such as mitochondria and chloroplasts
  • In prokaryotes, energy production and metabolic processes occur in the cytoplasm or on the inner surface of the plasma membrane
  • Eukaryotic cells have specialized organelles like mitochondria for cellular respiration and chloroplasts for photosynthesis (in plants and some algae)

Structure and Function Relationship in Cells

Plasma Membrane and Nucleus

• The plasma membrane's selectively permeable nature allows for the controlled movement of substances, maintaining the cell's homeostasis
  • The phospholipid bilayer structure creates a hydrophobic barrier that prevents the free passage of hydrophilic molecules
  • Membrane proteins, such as channels and carriers, facilitate the selective transport of specific molecules across the membrane
• The nucleus's role as the control center of the cell, housing genetic material, enables it to direct cellular activities and pass on hereditary information during cell division
  • The nuclear envelope, with its pores, regulates the passage of molecules between the nucleus and cytoplasm
  • The organization of DNA into chromatin and chromosomes allows for the compact storage and precise replication of genetic material

Organelles Involved in Protein Synthesis and Transport

• The rough endoplasmic reticulum's association with ribosomes allows for the synthesis and modification of proteins, while the smooth endoplasmic reticulum's lack of ribosomes enables it to synthesize lipids and detoxify substances
  • The rough ER's membranous structure provides a large surface area for the attachment of ribosomes and the synthesis and folding of proteins
  • The smooth ER's tubular structure and lack of ribosomes create a separate compartment for lipid synthesis and detoxification
• The Golgi apparatus's structure of flattened membranous sacs allows for the efficient modification, packaging, and sorting of macromolecules for secretion or transport
  • The cisternae of the Golgi apparatus provide separate compartments for the sequential modification of proteins and lipids
  • The trans face of the Golgi apparatus buds off vesicles containing sorted and packaged macromolecules for transport to their final destination

Organelles Involved in Energy Production and Cellular Maintenance

• Mitochondria's double membrane structure and inner cristae provide a large surface area for the enzymes involved in ATP production, efficiently generating energy for the cell
  • The inner membrane of mitochondria houses the electron transport chain and ATP synthase, which are crucial for oxidative phosphorylation
  • The matrix contains enzymes involved in the citric acid cycle, which generates high-energy electrons for the electron transport chain
• Lysosomes' membranous sac structure containing digestive enzymes enables them to effectively break down cellular waste and foreign materials without damaging the cell itself
  • The single membrane of lysosomes separates the digestive enzymes from the rest of the cell, preventing unwanted degradation
  • The acidic pH within lysosomes provides an optimal environment for the activity of the digestive enzymes
• The cytoskeleton's network of protein filaments provides structural support and enables cell movement and intracellular transport, maintaining the cell's shape and organization
  • Microfilaments' thin, flexible structure allows for the formation of contractile bundles and networks involved in cell motility and division
  • Microtubules' hollow, cylindrical structure provides a rigid scaffold for intracellular transport and the formation of cilia and flagella
  • Intermediate filaments' rope-like structure provides mechanical strength and resistance to shear stress, maintaining cell integrity