Cell Biology Overview
Chapter 6: A Tour of the CELL
Processes Necessary for Sustaining Life
Metabolism and Homeostasis
Interactions
Reproduction, Growth and Development
Adaptation leading to diversity and unity
Properties of Living Things
Order and Hierarchy
Cells as the basic unit of life
Note: Many of these processes occur at the cellular level.
Example of Cellular Development:
Fertilized Egg (Zygote) develops to a 4-cell stage.
Studying Cells
Methods of Study
Visual Observation
Biochemical Analysis of cellular contents
### Size Scales in Microscopy
10$^{-2}$ m (0.01 m)
10$^{-3}$ m (0.001 m)
10$^{-6}$ m (0.000001 m)
10$^{-9}$ m (0.000000001 m)
History of the Microscope
Zacharias Janssen (late 16th century) credited with inventing the compound microscope around 1595.
Functionality: Three draw tubes with bi-convex eyepiece and plano-convex objective lens; magnification ranged from 3x to 10x.
Key Figures in Microscopy History
Robert Hooke (1635-1703): Coined the term "cell" after observing slices of cork.
Antony van Leeuwenhoek (1632-1723): Discovered various cell types including bacteria and sperm cells.
Types of Microscopy
Electron Microscopy (EM)
Scanning EM (SEM): Surface imaging
Transmission EM (TEM): Internal structure imaging
Light Microscopy Techniques
Brightfield: Stained vs. unstained specimens
Confocal: Provides optical slices
Phase Contrast: Enhances contrast without staining
Fluorescence: Uses fluorescent stains
Super-resolution: High-detailed structure imaging
Cell Preparation Techniques
Cell Fixation and Staining: Fix cells to preserve structure and stain for contrast.
Artifacts: Common artifact presence during cell preparation despite careful methods.
Biochemical Analysis of Cellular Components
Cell Fractionation techniques:
Homogenization: Cells broken down into a homogenate
Centrifugation: Using force to separate components:
1,000 g for 10 minutes: Pellet rich in nuclei
20,000 g for 20 minutes: Pellet rich in mitochondria
80,000 g for 60 minutes: Pellet rich in microsomes (membrane remnants)
150,000 g for 3 hours: Pellet rich in ribosomes
Caveat: Isolated organelles may not behave like they do in vivo.
Cell Types: Prokaryotic vs. Eukaryotic
Prokaryotes:
Include Bacteria and Archaea
Simple, smaller cells without membrane-bound organelles.
Unique features: DNA concentrated in the nucleoid region; lacks a true nucleus.
Eukaryotes:
More complex; includes plant and animal cells.
Contains membrane-bound organelles, including a true nucleus.
Eukaryotic DNA housed within a nucleus, surrounded by cytoplasm.
Common Features of All Cells
Plasma Membrane: A phospholipid bilayer defining cell boundaries.
Cytosol/Cytoplasm: Semi-fluid substance where organelles are suspended.
Ribosomes: Sites of protein synthesis.
DNA: Genetic material that directs cellular function.
Surface Area to Volume Ratio
Surface Area Calculation:
Total surface area = Height × Width × Number of Boxes.
Total volume = Height × Width × Length × Number of Boxes.
Surface-to-Volume (S-to-V) Ratio: Surface area / Volume
Importance: S-to-V ratio influences the efficiency of nutrient exchange and waste removal.
Larger cells have lower S-to-V ratios, limiting their size.
Cellular Organelles
General Characteristics
Biological membranes consist largely of phospholipids, but the composition varies by organelle.
Organelles function through unique structures; they selectively regulate passage of materials.
Differences Between Animal and Plant Cells
Animal Cells:
Lysosomes: Digestive organelles.
Centrioles: Important for cell division.
Flagella: Mobility in certain cell types.
Plant Cells:
Chloroplasts: Photo-synthesis organelles.
Central Vacuole: Storage for nutrients and waste.
Cell Wall: Provides structure and support.
Plasmodesmata: Channels for intercellular communication.
Nucleus
Largest organelle, surrounded by a nuclear envelope, which is continuous with the Endoplasmic Reticulum (ER).
Endoplasmic Reticulum (ER)
Continuous with nuclear envelope:
Rough ER: Studded with ribosomes; involved in protein synthesis.
Smooth ER: Lacks ribosomes; involved in lipid synthesis and metabolism.
Golgi Apparatus
Functionality: Structures formed from flattened membranous sacs.
Cis-face: Receives transport vesicles containing proteins.
Trans-face: Ships proteins to their destination.
Modifies proteins through carbohydrate additions and also manufactures polysaccharides.
Lysosomes
Involved in intracellular digestion and functioning optimally at acidic pH.
Contain hydrolytic enzymes for breaking down cellular waste, bacteria, and destroyed organelles (function example: macrophage engulfing bacteria).
Vacuoles in Plant and Fungal Cells
Types of Vacuoles:
Central Vacuole: Storage and maintaining turgor pressure.
Food Vacuoles: Formed by endocytosis.
Contractile Vacuoles: Regulating osmotic pressure.
Endosymbiotic Theory
Origin of Mitochondria and Chloroplasts:
Semi-autonomous; capable of growth and reproduction.
Contain two membranes and possess their own DNA and ribosomes.
Mitochondria are sites for cellular respiration, while chloroplasts perform photosynthesis.
Peroxisomes
Function: Metabolic compartments that neutralize reactive byproducts (like H2O2) within cells.
Cytoskeleton
Concept: Network of fibers organizing cellular structures and activities.
Major Protein Components:
Microtubules:
Structure: Hollow tubes formed from tubulin dimers (α and β).
Function: Cell shape maintenance, mobility (cilia and flagella), and movement of cellular materials.
Microfilaments (Actin Filaments):
Structure: Two intertwined strands of actin.
Functions: Cell shape maintenance, muscle contraction, and cell motility.
Intermediate Filaments:
Structure: Supercoiled fibrous proteins (e.g. keratin).
Function: Provide mechanical support and maintain cell shape.
Cilia and Flagella
Structure: Specialized microtubule structure involved in locomotion.
Cilia: Shorter, more numerous (e.g. in the respiratory tract).
Flagella: Longer, fewer per cell (e.g. sperm cells).
Movement Mechanism: Undulating motion along axis (cilia) and perpendicular motion (flagella).
Extracellular Structures
Cell Wall:
Found only in plant and fungal cells; helps with protection, shape maintenance, and water regulation.
Extracellular Matrix (ECM):
Space formed by secreted glycoproteins; important for cell signaling and adhesion.
Intercellular Junctions in Plants:
Plasmodesmata: Channels connecting adjacent cells for chemical communication.
Animal Cell Junctions:
Tight Junctions: Prevent leakage across epithelium.
Desmosomes: Provide mechanical strength.
Gap Junctions: Allow chemical and electrical signals to pass between cells (e.g. in heart muscle).
Conclusion
Summary
Understanding cell structures and functions is foundational to biology and highlights the complexity of life at the cellular level.
This chapter provides insights into organelles, their specific functions, key cellular processes, and historical developments in cell biology.
Chapter 6: A Tour of the CELL
Processes Necessary for Sustaining Life
Metabolism and Homeostasis
Metabolism: The sum of all chemical reactions occurring within an organism to maintain life, including anabolism (building up) and catabolism (breaking down).
Homeostasis: The ability of an organism to maintain a stable internal environment despite external changes, crucial for optimal cellular function.
Interactions: Cells interact with their environment and other cells, coordinating activities.
Reproduction, Growth and Development: Cells replicate and differentiate to form tissues, organs, and eventually complex organisms.
Adaptation leading to diversity and unity: Organisms evolve over time, adapting to their environments, leading to a wide variety of life forms, yet all sharing fundamental cellular processes.
Properties of Living Things
Order and Hierarchy: Life is organized into a hierarchical structure, from atoms to molecules, organelles, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystems, and the biosphere.
Cells as the basic unit of life: The smallest structural and functional unit of an organism, capable of independent existence and performing all necessary life functions.
Note: Many of these processes occur at the cellular level, highlighting the cell's fundamental role in life.
Example of Cellular Development:
Fertilized Egg (Zygote) develops to a 4-cell stage: Illustrates the initial stages of embryonic development through cell division (mitosis).
Studying Cells
Methods of Study
Visual Observation: Using various types of microscopy to visualize cell structures and their organization.
Biochemical Analysis of cellular contents: Involving the isolation and study of molecules and reactions within cells to understand their functions.
Size Scales in Microscopy
m (0.01 m): Centimeters, typically visible to the naked eye (e.g., a large organism or organ).
m (0.001 m): Millimeters, also visible to the naked eye but approaching the limit of unaided vision (e.g., small insects).
m (0.000001 m): Micrometers (µm), the approximate size of most eukaryotic cells (e.g., red blood cells, bacteria). Requires light microscopy.
m (0.000000001 m): Nanometers (nm), the scale for viruses, ribosomes, and molecules. Requires electron microscopy.
History of the Microscope
Zacharias Janssen (late 16th century) credited with inventing the compound microscope around 1595: His invention was a crucial step, enabling early scientists to observe objects at magnifications previously impossible.
Functionality: Three draw tubes with bi-convex eyepiece and plano-convex objective lens; initial models offered low magnification (3x to 10x) but paved the way for more advanced designs.
Key Figures in Microscopy History
Robert Hooke (1635-1703): Coined the term "cell" after observing slices of cork, noting the small, box-like compartments reminded him of monks' cells. His observations were published in Micrographia.
Antony van Leeuwenhoek (1632-1723): Often called the "father of microbiology," he made significant improvements to the simple microscope and was the first to observe and describe various "animalcules," including bacteria, protozoa, and sperm cells, in great detail.
Types of Microscopy
Electron Microscopy (EM): Utilizes a beam of electrons instead of light, offering much higher resolution due to the shorter wavelength of electrons.
Scanning EM (SEM): Used to visualize the surface topography of specimens, providing a 3D-like image by detecting secondary electrons emitted from the sample's surface.
Transmission EM (TEM): Used to examine the internal structure of thin specimens, as electrons pass through the sample, revealing detailed ultrastructure of organelles.
Light Microscopy Techniques: Uses visible light to illuminate specimens. Limits of resolution are around .
Brightfield: The most common type; specimens are often stained to enhance contrast, as unstained biological samples are largely transparent.
Confocal: Employs a laser and pinhole aperture to eliminate out-of-focus light, producing clear optical sections of thick specimens and allowing 3D reconstruction.
Phase Contrast: Converts subtle differences in light phase shifts (caused by variations in cell density) into differences in brightness, enhancing contrast in unstained, living cells.
Fluorescence: Uses specific fluorescent dyes or proteins that bind to cellular components, allowing visualization of specific structures or molecules against a dark background.
Super-resolution: A collection of advanced techniques that overcome the diffraction limit of light microscopy, enabling imaging of structures smaller than with high detail.
Cell Preparation Techniques
Cell Fixation and Staining:
Fixation: Chemically preserves cells by denaturing proteins and cross-linking cellular components, halting metabolic processes and preventing degradation.
Staining: Uses dyes (e.g., hematoxylin, eosin, Gram stain) to selectively bind to different cellular structures, increasing contrast and making them visible under a light microscope.
Artifacts: Structures or features observed in prepared specimens that are not naturally present in the living cell but are introduced during the preparation process (e.g., shrinkages, precipitates, distortions). Careful methods aim to minimize their presence.
Biochemical Analysis of Cellular Components
Cell Fractionation techniques: Aims to separate cellular components while maintaining their function.
Homogenization: Cells are mechanically disrupted (e.g., using a blender or sonication) to break the plasma membrane, releasing organelles into a fluid called homogenate.
Centrifugation: A process that uses centrifugal force to separate cell components based on their size, shape, and density. Heavier and denser components pellet at lower gravitational forces (g) and shorter times.
1,000 g for 10 minutes: Pellets rich in nuclei, the largest and densest organelles.
20,000 g for 20 minutes: Pellets rich in mitochondria, which are smaller than nuclei but still relatively dense.
80,000 g for 60 minutes: Pellets rich in microsomes (fragments of endoplasmic reticulum and Golgi apparatus that re-form into small vesicles).
150,000 g for 3 hours: Pellets rich in ribosomes, the smallest and least dense components separated by this method.
Caveat: Isolated organelles may not behave exactly like they do in vivo (within a living organism) because they are removed from their natural cellular environment and interactions.
Cell Types: Prokaryotic vs. Eukaryotic
Prokaryotes:
Include Bacteria and Archaea, which are the earliest forms of life.
Simple, generally smaller cells (typically in diameter) without membrane-bound organelles.
Unique features: DNA concentrated in the nucleoid region (not enclosed by a membrane); lacks a true nucleus; lack histones associated with DNA (in bacteria); often possess a cell wall and sometimes flagella for motility.
Eukaryotes:
More complex; includes plant, animal, fungi, and protist cells.
Generally larger (typically in diameter).
Contains various membrane-bound organelles, including a true nucleus (housing the DNA), endoplasmic reticulum, Golgi apparatus, mitochondria, etc.
Eukaryotic DNA housed within a nucleus, surrounded by cytoplasm; DNA is organized into chromosomes with associated histone proteins.
Common Features of All Cells
Plasma Membrane: A selectively permeable phospholipid bilayer that forms the boundary of the cell, regulating the passage of substances into and out of the cell.
Cytosol/Cytoplasm: The semi-fluid, jelly-like substance filling the cell, in which organelles and other cellular inclusions are suspended (cytosol refers to the fluid portion, cytoplasm includes the cytosol and organelles).
Ribosomes: Complexes of ribosomal RNA (rRNA) and protein that are responsible for protein synthesis (translation) in both prokaryotic and eukaryotic cells.
DNA: Deoxyribonucleic acid, the genetic material that carries the instructions for building and maintaining an organism, directing cellular function and heredity.
Surface Area to Volume Ratio
Surface Area Calculation: For a cube, total surface area = . For a collection of boxes, it's the sum of exposed surfaces.
Total volume: For a cube, volume = . For a collection of boxes, it's the sum of individual volumes.
Surface-to-Volume (S-to-V) Ratio: Surface area / Volume. This ratio is crucial for cell efficiency.
Importance: A high S-to-V ratio facilitates efficient exchange of nutrients, oxygen, and waste products across the plasma membrane. As a cell grows larger, its volume increases much faster than its surface area, leading to a lower S-to-V ratio. This limits the maximum size a cell can attain, as the surface area may become insufficient to meet the metabolic demands of its larger volume.
Cellular Organelles
General Characteristics
Biological membranes consist largely of phospholipids (forming a bilayer) and proteins; the specific composition (types of lipids and proteins) varies significantly among different organelles, reflecting their diverse functions.
Membranes are selectively permeable, meaning they regulate the passage of materials into and out of organelles and the cell, maintaining distinct internal environments.
Organelles function through unique structures, each adapted for specific biochemical processes.
Differences Between Animal and Plant Cells
Animal Cells:
Lysosomes: Spherical organelles containing hydrolytic enzymes; involved in intracellular digestion of macromolecules, worn-out organelles (autophagy), and foreign substances (phagocytosis).
Centrioles: Cylindrical structures made of nine triplets of microtubules; found in the centrosome and involved in organizing microtubules during cell division (mitosis and meiosis) in animal cells.
Flagella: Long, whip-like appendages (composed of microtubules) extending from the cell surface; primarily used for locomotion in certain cell types (e.g., sperm cells). Cilia are similar but shorter and more numerous.
Plant Cells:
Chloroplasts: Specialized plastids containing chlorophyll; the sites of photosynthesis, where light energy is converted into chemical energy (sugars).
Central Vacuole: A large membrane-bound sac that can occupy 30-80% of the cell volume; stores water, nutrients, pigments, and waste products, and helps maintain turgor pressure against the cell wall.
Cell Wall: A rigid outer layer composed primarily of cellulose (in plants); provides structural support, protection, and prevents excessive water uptake by maintaining turgor pressure.
Plasmodesmata: Channels or pores through the cell wall that connect the cytoplasm of adjacent plant cells, allowing for direct intercellular communication and transport of water, nutrients, and small molecules.
Nucleus
The largest organelle (typically ), surrounded by a double-membraned nuclear envelope (pierced by nuclear pores), which is continuous with the Endoplasmic Reticulum (ER).
Function: Houses the cell's genetic material (DNA) organized into chromosomes, controls gene expression, and is the site of ribosome synthesis (in the nucleolus).
Endoplasmic Reticulum (ER)
An extensive network of membranes, continuous with the outer nuclear envelope and forming a labyrinth of flattened sacs (cisternae) and tubules.
Rough ER: Studded with ribosomes on its cytoplasmic surface; involved in the synthesis, folding, modification, and transport of proteins destined for secretion, insertion into membranes, or delivery to other organelles (like the Golgi apparatus).
Smooth ER: Lacks ribosomes; involved in various metabolic processes including lipid synthesis (e.g., oils, steroids, phospholipids), detoxification of drugs and poisons (especially in liver cells), and storage of calcium ions (e.g., in muscle cells).
Golgi Apparatus
Functionality: Consists of flattened membranous sacs called cisternae, typically arranged in a stack (like a stack of pancakes). Known as the "post office" of the cell.
Cis-face: The "receiving" side, typically located near the ER, where transport vesicles containing proteins and lipids from the ER fuse and empty their contents.
Trans-face: The "shipping" side, where modified and sorted materials are packaged into new transport vesicles that bud off and travel to other destinations within or outside the cell.
Processes: Modifies proteins (e.g., through glycosylation by adding carbohydrates), manufactures certain macromolecules (e.g., polysaccharides like pectin in plant cell walls), sorts and packages materials into vesicles, and targets them to their final destinations.
Lysosomes
Involved in intracellular digestion and functioning optimally at acidic pH (around ), maintained by proton pumps in their membrane.
Contain about 50 different types of hydrolytic enzymes (hydrolyses) for breaking down cellular waste, bacteria engulfed by phagocytosis, and destroyed or worn-out organelles through a process called autophagy.
Function example: Macrophages engulfing bacteria (phagocytosis) use lysosomes to digest and destroy the pathogens.
Vacuoles in Plant and Fungal Cells
Types of Vacuoles: Membrane-bound sacs with diverse functions.
Central Vacuole (in plants): A large, prominent organelle; performs hydrolysis, storage of inorganic ions, water, nutrients, pigments, and waste products; also helps maintain turgor pressure, exerting force on the cell wall.
Food Vacuoles: Formed by endocytosis (phagocytosis) when a cell engulfs food particles; these then fuse with lysosomes for digestion (e.g., in amoebas).
Contractile Vacuoles: Found in many freshwater protists; function to pump excess water out of the cell, regulating osmotic pressure and preventing cell lysis.
Endosymbiotic Theory
Origin of Mitochondria and Chloroplasts: Proposes that these organelles originated from prokaryotic cells (aerobic bacteria and photosynthetic cyanobacteria, respectively) that were engulfed by ancestral eukaryotic cells and established a symbiotic relationship.
Evidence:
Semi-autonomous; capable of growth and reproduction (fission) somewhat independently within the cell.
Contain two membranes (the inner membrane thought to be from the prokaryote, the outer from the host cell's engulfing membrane).
Possess their own circular DNA (similar to bacterial DNA) and ribosomes (similar in size and composition to prokaryotic ribosomes).
Mitochondria are sites for cellular respiration, the process that generates ATP by oxidizing sugars and fats.
Chloroplasts perform photosynthesis, converting light energy into chemical energy.
Similar size to bacteria.
Peroxisomes
Function: Specialized metabolic compartments enclosed by a single membrane; they detoxify various harmful substances by transferring hydrogen atoms from substrates to oxygen, producing hydrogen peroxide ().
They then convert into water and oxygen using the enzyme catalase, neutralizing these reactive byproducts within cells.
Example: In the liver, peroxisomes detoxify alcohol and other harmful compounds. In plants, they are involved in converting fatty acids to sugars.
Cytoskeleton
Concept: A complex, dynamic network of protein fibers extending throughout the cytoplasm of eukaryotic cells, organizing cellular structures and activities, providing mechanical support, maintaining cell shape, and enabling cell motility and intracellular transport.
Major Protein Components:
Microtubules:
Structure: Hollow cylinders (about in diameter, lumen) formed from 13 parallel protofilaments, each made of tubulin dimers (α-tubulin and β-tubulin). They have distinct plus and minus ends.
Function: Maintain cell shape (resist compression), cell motility (as components of cilia and flagella), chromosome movements during cell division (forming the spindle fibers), and organelle movements (acting as tracks for motor proteins like kinesin and dynein).
Microfilaments (Actin Filaments):
Structure: Solid rods (about in diameter) consisting of two intertwined strands of actin protein. They also have plus and minus ends.
Functions: Maintain cell shape (bear tension), muscle contraction (interacting with myosin), amoeboid movement (pseudopodia), cytoplasmic streaming (in plant cells), and formation of the cleavage furrow during cytokinesis in animal cells.
Intermediate Filaments:
Structure: Fibrous proteins (about in diameter) supercoiled into thicker cables; made of various proteins (e.g., keratins in epithelial cells, lamins in the nucleus), making them a more diverse and permanent class of cytoskeletal elements.
Function: Provide mechanical support and maintain cell shape (e.g., reinforcing the nuclear envelope, anchoring organelles), more involved in bearing tension. They are particularly important in cells that experience mechanical stress.
Cilia and Flagella
Structure: Both are specialized locomotor appendages composed of microtubules, typically arranged in a "" pattern, meaning nine doublets of microtubules arranged in a ring with two single microtubules in the center, enclosed by the plasma membrane. They arise from a basal body (similar to centrioles).
Cilia: Shorter (typically ), more numerous per cell; often move in a coordinated back-and-forth fashion, creating a current (e.g., in the respiratory tract to sweep mucus, or on protozoa for feeding/locomotion).
Flagella: Longer (typically ), fewer per cell (usually one or a few); move in an undulating, wave-like motion along their axis to propel the cell (e.g., sperm cells, many protists).
Movement Mechanism: Both types move due to the bending of microtubules, powered by the motor protein dynein, which walks along adjacent microtubule doublets. Cilia exhibit a power stroke and recovery stroke, while flagella exhibit a propeller-like motion.
Extracellular Structures
Cell Wall:
Found only in plant (composed of cellulose, hemicellulose, pectin) and fungal cells (composed of chitin); located exterior to the plasma membrane.
Helps with protection against mechanical stress and pathogens, maintains cell shape, and prevents excessive water uptake (turgor).
Extracellular Matrix (ECM):
Found in animal cells; a complex network of secreted glycoproteins (like collagen, fibronectin) and proteoglycans that fills the space between cells.
Important for cell signaling (integrins link ECM to cytoskeleton), cell adhesion, migration, differentiation, and mechanical support to tissues.
Intercellular Junctions in Plants:
Plasmodesmata: Small channels (about in diameter) that perforate the cell walls of adjacent plant cells, connecting their cytosols and allowing for the passage of water, small solutes, and even some proteins and RNA molecules for chemical communication.
Animal Cell Junctions:
Tight Junctions: Formed by proteins (e.g., claudins, occludins) that press adjacent cell membranes together, creating a watertight seal; prevent leakage of extracellular fluid across a layer of epithelial cells (e.g., in the stomach lining or bladder).
Desmosomes (Anchoring Junctions): Act as "rivets," fastening cells together into strong sheets; intermediate filaments (e.g., keratin) anchor them into the cytoplasm, providing mechanical strength to tissues subject to stretching (e.g., muscle cells, skin cells).
Gap Junctions (Communicating Junctions): Formed by protein channels (connexons) between adjacent cells, allowing for direct passage of small molecules (ions, sugars, amino acids) and electrical signals between cells, facilitating rapid communication (e.g., in heart muscle for coordinated contractions, or in embryos).
Conclusion
Summary
Understanding cell structures and functions is foundational to biology and highlights the incredible complexity and elegance of life at the cellular level.
This chapter provides insights into the diverse organelles, their specific functions and interconnections, key cellular processes (like metabolism, reproduction, and adaptation), and historical developments in cell biology, from early microscopy to modern biochemical analysis. The detailed structure-function relationships within cells underpin all aspects of organismal biology.
Chapter 6: A Tour of the CELL
Processes Necessary for Sustaining Life
Metabolism and Homeostasis: Chemical reactions to maintain life (anabolism/catabolism) and stable internal environment.
Interactions: Cells coordinate with environment and other cells.
Reproduction, Growth and Development: Cells replicate and differentiate.
Adaptation: Evolution leading to diverse life forms with shared cellular processes.
Properties of Living Things
Order and Hierarchy: Organization from atoms to biosphere.
Cells as the basic unit of life.
Note: Many processes occur at the cellular level.
Example of Cellular Development:
Fertilized Egg (Zygote) develops to a 4-cell stage.
Studying Cells
Methods of Study
Visual Observation: Microscopy.
Biochemical Analysis: Study of molecules and reactions.
Size Scales in Microscopy
m (0.01 m): Centimeters (visible).
m (0.001 m): Millimeters (visible limit).
m (0.000001 m): Micrometers (eukaryotic cells); requires light microscopy.
m (0.000000001 m): Nanometers (viruses, ribosomes, molecules); requires electron microscopy.
History of the Microscope
Zacharias Janssen (late 16th century): Credited with inventing the compound microscope (~1595), 3x to 10x magnification.
Key Figures in Microscopy History
Robert Hooke (1635-1703): Coined "cell" after observing cork slices.
Antony van Leeuwenhoek (1632-1723): First to observe bacteria, protozoa, and sperm cells.
Types of Microscopy
Electron Microscopy (EM): Uses electron beam for higher resolution.
Scanning EM (SEM): Surface topography (3D-like).
Transmission EM (TEM): Internal structure.
Light Microscopy Techniques: Uses visible light (resolution limit ~).
Brightfield: Stained specimens.
Confocal: Optical slices, 3D reconstruction.
Phase Contrast: Enhances contrast in unstained living cells.
Fluorescence: Uses fluorescent dyes for specific structures.
Super-resolution: Overcomes diffraction limit for high detail (<).
Cell Preparation Techniques
Cell Fixation and Staining: Preserves cells, enhances contrast.
Artifacts: Preparation-induced features; minimized by careful methods.
Biochemical Analysis of Cellular Components
Cell Fractionation: Separates components by function.
Homogenization: Disrupts cells into homogenate.
Centrifugation: Separates components by size/density.
1,000 g for 10 min: Nuclei.
20,000 g for 20 min: Mitochondria.
80,000 g for 60 min: Microsomes (ER/Golgi fragments).
150,000 g for 3 hours: Ribosomes.
Caveat: Isolated organelles may not behave as in vivo.
Cell Types: Prokaryotic vs. Eukaryotic
Prokaryotes: Bacteria, Archaea. Simple, small (), no membrane-bound organelles, DNA in nucleoid.
Eukaryotes: Plants, animals, fungi, protists. Complex, larger (), membrane-bound organelles (true nucleus).
Common Features of All Cells
Plasma Membrane: Selectively permeable phospholipid bilayer.
Cytosol/Cytoplasm: Semi-fluid substance, suspends organelles.
Ribosomes: Protein synthesis sites.
DNA: Genetic material.
Surface Area to Volume Ratio
Importance: High S-to-V ratio (Surface area / Volume) ensures efficient exchange; limits cell size as volume increases faster than surface area.
Cellular Organelles
General Characteristics
Biological membranes (phospholipids, proteins) vary by organelle.
Selectively regulate material passage; each organelle has unique structure for specific processes.
Differences Between Animal and Plant Cells
Animal Cells:
Lysosomes: Digestive organelles (acidic pH, hydrolytic enzymes).
Centrioles: Organize microtubules for cell division.
Flagella: Locomotion.
Plant Cells:
Chloroplasts: Photosynthesis.
Central Vacuole: Storage, turgor pressure.
Cell Wall: Structural support, protection (cellulose).
Plasmodesmata: Intercellular communication channels.
Nucleus
Largest organelle (); enveloped by nuclear envelope (continuous with ER).
Houses DNA, controls gene expression, site of ribosome synthesis.
Endoplasmic Reticulum (ER)
Network of membranes continuous with nuclear envelope.
Rough ER: Ribosomes present; protein synthesis, folding, transport for secretion/membranes.
Smooth ER: No ribosomes; lipid synthesis, detoxification, calcium storage.
Golgi Apparatus
Stack of flattened sacs (cisternae); "post office" of the cell.
Cis-face: Receives vesicles from ER.
Trans-face: Ships modified proteins/lipids to destination.
Modifies proteins (e.g., glycosylation), manufactures polysaccharides, sorts and packages materials.
Lysosomes
Intracellular digestion (optimal at acidic pH ).
Contain ~50 hydrolytic enzymes; break down waste, bacteria, worn-out organelles (autophagy).
Vacuoles in Plant and Fungal Cells
Central Vacuole: Storage, turgor in plants.
Food Vacuoles: Formed by endocytosis, fuse with lysosomes for digestion.
Contractile Vacuoles: Pump excess water in protists, regulate osmotic pressure.
Endosymbiotic Theory
Origin of Mitochondria and Chloroplasts: Engulfed prokaryotes became organelles.
Evidence: Semi-autonomous, two membranes, own circular DNA/ribosomes, similar size to bacteria.
Mitochondria: Cellular respiration. Chloroplasts: Photosynthesis.
Peroxisomes
Metabolic compartments; detoxify substances by producing (hydrogen peroxide), then convert it to water and oxygen via catalase.
Cytoskeleton
Network of protein fibers providing support, shape, motility, and transport.
Major Protein Components:
Microtubules: Hollow tubes (tubulin dimers, ); cell shape, motility (cilia/flagella), chromosome/organelle movement.
Microfilaments (Actin Filaments): Two intertwined actin strands (); cell shape (tension), muscle contraction, cell motility.
Intermediate Filaments: Supercoiled fibrous proteins (); mechanical support, cell shape (e.g., keratin, lamins).
Cilia and Flagella
Structure: Microtubule-based locomotor appendages ( pattern), arise from basal body.
Cilia: Shorter (), numerous, coordinated back-and-forth motion.
Flagella: Longer (), fewer, undulating wave-like motion.
Movement Mechanism: Dynein motor protein causes microtubule bending.
Extracellular Structures
Cell Wall: In plant/fungal cells; protection, shape, water regulation.
Extracellular Matrix (ECM): In animal cells; secreted glycoproteins/proteoglycans; cell signaling, adhesion, mechanical support.
Intercellular Junctions in Plants:
Plasmodesmata: Channels connecting plant cytosols for communication.
Animal Cell Junctions:
Tight Junctions: Prevent leakage across epithelial layers.
Desmosomes: Provide mechanical strength, rivet cells together.
Gap Junctions: Allow chemical/electrical signals between cells (e.g., heart muscle).
Conclusion
Summary
Understanding cell structures and functions is foundational to biology, highlighting complexity and elegance. This chapter covers organelles, their functions, key cellular processes, and historical developments.