Microscopes, Cells, and Cellular Structure
Compound Light Microscope vs. Electron Microscope
Definition of a Compound Light Microscope:
A compound light microscope uses visible light and glass lenses to magnify specimens.
It is designed to view living or preserved cells and tissues.
Resolution Limitations:
Compound light microscopes have limited resolution due to the wavelength of visible light.
Definition of an Electron Microscope:
An electron microscope utilizes electrons to achieve much higher magnification and resolution compared to light microscopes.
It is employed to visualize very small structures such as organelles and viruses.
Preparation Limitations:
Specimens must be dead, and preparation methods are complex.
Uses:
Light Microscopes: General viewing of cells and tissues.
Electron Microscopes: Detailed examination of microscopic structures.
Limitations:
Light Microscopes:
Limited to about 1,000x magnification due to the properties of visible light.
Increasing magnification without corresponding resolution enhancements yields a blurry image.
Electron Microscopes:
Requires dead specimens; preparation processes can alter sample characteristics.
Definitions of Magnification and Resolution
Magnification:
The process of enlarging an image compared to the actual object size.
Resolution:
The ability to distinguish two close objects as separate entities and to observe fine details.
Dependence on Magnification:
Resolution is independent of magnification; increasing magnification does not improve detail clarity.
Beyond the upper limit defined by visible light wavelengths, increased magnification results in a blurred image.
Structures Observable with Microscopes
Light Microscopes:
Capable of viewing whole cells and notable structures, including:
Cell shape
Cell walls
Nuclei
Chloroplasts
Vacuoles
Some tissue structures
Living cells can be observed.
Electron Microscopes:
Observable structures beyond the resolution of light microscopes include:
Membranes
Ribosomes
Viruses
Cytoskeletal elements
Fine details of organelles (must be from dead cells).
Comparison of Micrographs:
Transmission Electron Micrographs (TEM):
Produce 2D images by slicing thin sections to show detailed internal structures.
Scanning Electron Micrographs (SEM):
Provide 3D images emphasizing surface textures and external details.
Cell Fractionation
Definition:
A laboratory technique used to separate cellular components (organelles) for individual study.
Usage:
To analyze the structure and functioning of specific organelles such as mitochondria, chloroplasts, or ribosomes.
Process:
Cells are homogenized to break them open and release their internal components.
The mixture undergoes centrifugation at increasing speeds.
Fractions Formed:
Low Speed: Nuclei and large debris.
Medium Speed: Mitochondria, chloroplasts, lysosomes, peroxisomes.
High Speed: Microsomes (ER fragments) and small vesicles.
Very High Speed: Ribosomes and large macromolecules.
Basic Units of Life
Why Cells Are Basic Units of Life:
All living organisms are composed of one or more cells; all vital life functions occur within cells.
Shared Properties:
All cells can perform essential life processes such as metabolism, growth, and reproduction.
Common structures include plasma membrane, cytoplasm, DNA, and ribosomes.
Cell Theory Main Points:
All living organisms consist of one or more cells.
The cell is the fundamental unit of structure and function in living organisms.
All cells arise from pre-existing cells.
Comparing Prokaryotic and Eukaryotic Cells
Prokaryotic Cells:
Smaller, simpler structure.
Lacks a nucleus or membrane-bound organelles; DNA located in a nucleoid region.
Examples include bacteria and archaea.
Eukaryotic Cells:
Larger, more complex structure.
Contains a nucleus and membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).
Examples include plants, animals, fungi, and protists.
Size Limitation of Cells
Giant Cells:
Large cells do not exist due to the disproportionate increase of volume versus surface area as cell size expands.
This results in a decreased surface area-to-volume ratio, making nutrient and waste exchange difficult.
Diffusion efficiency declines over greater distances within larger cells.
To remain efficient, cells either stay small or group as multicellular organisms, where many small cells collaborate.
Plasma Membrane Structure and Functions
Structure:
The plasma membrane is a flexible, thin boundary around the cell, separating internal cellular environments from external surroundings.
Described by the Fluid Mosaic Model:
Composed of a phospholipid bilayer with embedded proteins.
Phospholipids: Hydrophilic heads oriented outward, away from water, with hydrophobic tails facing inward.
Embedded Proteins: Function as channels, carriers, receptors, and enzymes.
Carbohydrates: Assist in cell recognition and communication.
Cholesterol: Affects membrane fluidity in animal cells.
Functions:
Regulates selective permeability (controls entry/exit of substances).
Maintains internal cellular environments.
Facilitates cell communication via receptors.
Provides anchorage for the cytoskeleton and aids cellular adhesion.
Importance of Phospholipids:
Their amphipathic nature drives the self-assembly into a bilayer, forming a stable barrier that prevents most polar and charged substances while allowing some nonpolar molecules to pass freely.
Comparison of Plant and Animal Cells
Common Features: Both plant and animal cells are eukaryotic and include:
Nucleus
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Ribosomes
Cytoskeleton
Plasma membrane.
Additional Features in Plant Cells:
Cell wall for structural support and protection (composed of cellulose).
Chloroplasts for photosynthesis.
Large central vacuole for water storage and maintenance of turgor pressure.
Unique Features in Animal Cells:
Lack cell walls and chloroplasts.
Typically smaller vacuoles; may possess lysosomes (for digestion) and centrioles (for cell division).
Chromosomes and Chromatin
Chromatin:
The material comprising chromosomes, consisting of DNA wrapped around histone proteins.
In non-dividing cells, chromatin appears loose and threadlike, facilitating gene expression.
Chromosomes:
Formed when chromatin condenses and coils during cell division.
Each chromosome is a single long DNA molecule carrying numerous genes.
This condensation facilitates accurate distribution to daughter cells during division.
Key Cellular Structures: Characteristics and Functions
Nucleus:
Structure: Large membrane-bound organelle with a double membrane (nuclear envelope) featuring pores.
Function: Stores DNA and regulates cellular activities.
Cell Type: Eukaryotic (includes plants and animals).
Location: Typically central in the cell.
Nucleolus:
Structure: Dense, spherical entity within the nucleus.
Function: Synthesizes ribosomal RNA (rRNA) and assembles ribosomal subunits.
Cell Type: Eukaryotic.
Location: Inside the nucleus.
Ribosomes:
Structure: Small complexes composed of RNA and proteins; can be free in the cytoplasm or bound to rough ER.
Function: Synthesize proteins.
Cell Type: Present in all cells (prokaryotic and eukaryotic).
Location: Free in cytoplasm or on rough ER.
Endoplasmic Reticulum (ER):
Structure: Network of folded sacs and tubes, with rough ER studded with ribosomes and smooth ER devoid of them.
Function:
Rough ER: Synthesizes and modifies proteins.
Smooth ER: Synthesizes lipids, detoxifies, and stores calcium.
Cell Type: Eukaryotic.
Location: Surrounds the nucleus, extending through the cytoplasm.
Golgi Apparatus:
Structure: Stack of flattened membrane sacs.
Function: Modifies, sorts, and packages proteins and lipids for secretion or delivery.
Cell Type: Eukaryotic.
Location: Near the ER in cytoplasm.
Lysosomes:
Structure: Membrane-bound vesicles containing digestive enzymes.
Function: Breaks down waste, damaged organelles, and macromolecules.
Cell Type: More common in animal cells; some plant cells.
Location: Scattered throughout the cytoplasm.
Vacuoles:
Structure: Large membrane-bound sacs.
Function: Store water, nutrients, and waste, assisting in maintaining turgor pressure in plants.
Cell Type: Plant cells (large central vacuole); smaller in animal cells.
Location: Cytoplasm.
Mitochondria:
Structure: Double membrane; inner membrane has folds (cristae); possesses its own DNA and ribosomes.
Function: Produces ATP via cellular respiration.
Cell Type: Eukaryotic.
Location: Scattered in the cytoplasm.
Chloroplasts:
Structure: Double membrane with thylakoid stacks (grana) and stroma; contains chlorophyll.
Function: Conducts photosynthesis to synthesize glucose.
Cell Type: Found in plant cells and some protists.
Location: Cytoplasm.
Peroxisomes:
Structure: Small, membrane-bound vesicles containing oxidative enzymes.
Function: Breaks down fatty acids and detoxifies substances (e.g., hydrogen peroxide).
Cell Type: Eukaryotic.
Location: Scattered in cytoplasm.
Pathway of Protein and Lipid Synthesis
Pathway of a Protein Produced in the Rough ER:
Rough ER: Ribosomes synthesize proteins while possibly folding them or attaching sugar groups (glycosylation).
Transport Vesicle: Protein packaged into a vesicle that buds off from the ER.
Golgi Apparatus: Vesicle fuses with the Golgi, where the protein is further modified, sorted, and packaged.
Destination: Protein sent to:
Secretion outside the cell
Integration into the plasma membrane
Lysosomes or other organelles.
Pathway of a Lipid Produced in the Smooth ER:
Smooth ER: Synthesizes lipids (e.g., phospholipids, steroids).
Transport Vesicle: Lipids packaged into vesicles.
Golgi Apparatus (optional): Some lipids may be modified or sorted here.
Destination: Lipids sent to:
Plasma membrane to maintain/expand it
Other organelles (e.g., mitochondria, ER)
Secretion (e.g., steroid hormones) in some cases.
Cytoskeleton Overview
Definition: The cytoskeleton is a network of protein fibers present throughout the cytoplasm of the cell.
Functions:
Maintenance of cell shape
Provides mechanical support
Enables cellular movement and intracellular transport
Organizes organelles and facilitates cell division.
Microtubules:
Structure: Hollow tubes formed from tubulin proteins.
Functions:
Maintain cell shape
Act as tracks for organelle and vesicle movement
Form spindle fibers during cell division
Comprise cilia, flagella, and centrioles.
Centrioles:
Structure: Cylindrical arrangements of microtubule triplets.
Function: Organize microtubules during cell division by forming the spindle apparatus.
Location: Present in animal cells.
Flagella:
Structure: Long, whip-like microtubule extensions.
Function: Propel cells through the environment.
Example: Tail of the sperm cell.
Cilia:
Structure: Short, hair-like microtubule extensions.
Function: Move fluids or materials across the cell surface, or facilitate movement of the cell itself.
Example: Cilia in the respiratory tract clear mucus.
Dynein:
Structure: A motor protein associated with microtubules.
Function: Utilizes ATP to propel cilia and flagella, causing bending and beating actions; also aids transport of vesicles along microtubule tracks.
Summary:
The cytoskeleton can be thought of as the cell's "skeleton and highway system," where microtubules serve crucial roles in structure and transport.
Structures like centrioles, cilia, and flagella assist with cell division and movement, while dynein powers mobilization along microtubules.
Microfilaments and Their Functions
Microfilaments:
Structure: Thin, solid fibers made of the protein actin.
Functions:
Maintain cell shape by resisting tension
Enable cell movement (e.g., amoeboid movement, muscle contraction)
Assist in cytoplasmic streaming (cytoplasm movement within the cell)
Contribute to cell division during cytokinesis (splitting the cell).
Intermediate Filaments
Intermediate Filaments:
Structure: Rope-like fibers composed of various fibrous proteins, such as keratin.
Functions:
Provide mechanical support to the cell
Help maintain cell shape and stabilize organelle positions
Anchor the nucleus and other organelles
Contribute to cell junction stability in tissues, allowing cells to adhere to one another.
Cell Walls and Plasmodesmata
Cell Walls:
Definition: A rigid, protective layer located outside the plasma membrane, providing support, shape, and protection to the cell.
Types of Eukaryotic Cells with Cell Walls:
Plant cells
Fungal cells
Some protists (like algae).
Composition:
Plant Cells: Primarily composed of cellulose.
Fungal Cells: Composed of chitin.
Algae: Composed of cellulose or other polysaccharides.
Functions:
Maintains cell shape
Protects against mechanical stress
Prevents excess water uptake.
Plasmodesmata:
Definition: Small channels within the plant cell wall connecting adjacent plant cells.
Functions:
Facilitates the movement of water, ions, and small molecules between cells
Enables cell-to-cell communication.
Extracellular Matrix of Animal Cells
Extracellular Matrix (ECM):
Definition: A network of proteins and carbohydrates located outside of the plasma membrane in animal cells.
Components:
Collagen: Provides tensile strength and structural support.
Proteoglycans: Gel-like components that resist compression and retain water.
Fibronectin and Other Glycoproteins: Involved in organizing the ECM and connecting it to cells.
Functions:
Provides structural support to tissues
Anchors cells in place
Facilitates communication and signaling between cells.
Integrins:
Definition: Transmembrane proteins bridging the ECM to the cell's cytoskeleton.
Functions:
Anchors cells to the ECM
Transmits signals from ECM, influencing cell behavior and function
Aids cells in sensing and responding to their environment.
Comparison of Intercellular Connections
Plasmodesmata:
Location: Found in plant cells.
Structure: Small channels through the cell wall allowing for intercellular connectivity.
Function: Permits movement of water, ions, and molecules; facilitates communication between cells.
Tight Junctions:
Location: Found in animal cells.
Structure: Proteins that create fused zones between neighboring cell plasma membranes.
Function: Forms a seal to prevent leaks between cells (e.g., in intestinal walls).
Desmosomes:
Location: Found in animal cells.
Structure: Anchoring junctions linked to the cytoskeleton.
Function: Provide strong adhesion between cells; resist mechanical stress (e.g., skin and heart muscle).
Gap Junctions:
Location: Found in animal cells.
Structure: Protein channels (connexons) that allow direct connection between neighboring cell cytoplasms.
Function: Permit the passage of ions and small molecules; facilitate cellular communication (e.g., synchronizing contractions in heart muscle cells).
Emergent Properties in Cellular Function
Definition: Emergent properties refer to characteristics arising from the interaction of individual parts, which alone do not exhibit these properties.
Cellular Illustration of Emergent Properties:
Cells consist of numerous organelles and molecules, each fulfilling specific roles.
Interactions between these components enable growth, reproduction, environmental responsiveness, and metabolic activities—functions that no singular organelle or molecule could achieve alone.
Example: Mitochondria generate energy, ribosomes synthesize proteins, and the nucleus stores DNA, but only collaboratively within a cell can these components sustain life.
Endosymbiont Theory and Supporting Evidence
Definition: The endosymbiont theory suggests that certain organelles (mitochondria and chloroplasts) originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.
Supporting Evidence:
Double Membranes: Indicate the possibility of engulfment due to the presence of two membranes.
Own DNA: Mitochondria and chloroplasts possess circular DNA resembling prokaryotic forms.
Ribosomal Characteristics: Have smaller ribosomes akin to those found in prokaryotes.
Reproductive Methods: Organelles divide independently through binary fission, similar to bacteria.
Genetic Similarity: DNA sequences of these organelles relate closely to particular bacterial groups (e.g., mitochondria to proteobacteria; chloroplasts to cyanobacteria).