Section 6.1: Biologists Use Microscopes and Biochemistry to Study Cells

Brief History of Microscopes

The development of microscopes revolutionized biology by allowing scientists to observe cells and microorganisms. Early microscopes were simple magnifying glasses, evolving into compound microscopes with greater magnification and resolution.

• Burning Glass & Magnifying Lens: Used in ancient times for basic magnification.

• Flea Glass & First Compound Microscope: Developed in the 16th century, offering less than 10x magnification.

• First Microscopic Science: In the 17th century, compound light microscopes reached up to 270x magnification, enabling the first scientific studies of microscopic life.

• 18th Century Microscopes: Became popular among the wealthy for scientific amusement and study.

• 19th and Early 20th Century: Reflecting compound light microscopes were developed, still used in fieldwork without electricity.

Modern Microscopes

Modern microscopy includes advanced light and electron microscopes, each with unique capabilities for studying cells.

• Compound Light Microscope: Magnifies up to 1000x, suitable for viewing stained or live cells.

• Dissecting Microscope: Used for low-magnification observation of larger specimens.

Other Types of Light Microscopy

• Deconvolution Microscopy: Uses software to remove blur and increase contrast, allowing clearer images of live cells.

• Super Resolution Technology: Enhances detail beyond the limits of traditional light microscopy without requiring electron microscopes.

Electron Microscopes

Electron microscopes use beams of electrons for much higher resolution than light microscopes, revealing ultrastructural details of cells.

• Scanning Electron Microscope (SEM): Produces 3D-like images by detecting reflected electrons from the specimen's surface.

• Transmission Electron Microscope (TEM): Uses transmitted electrons to visualize internal structures, producing 2D images.

TEM vs SEM

• TEM: Reveals internal cell structures in two dimensions.

• SEM: Shows surface details in three dimensions.

Atomic Force Microscopes (AFM)

AFMs can visualize objects at the nanometer scale, such as DNA, proteins, viruses, and nanoparticles, by physically scanning the surface with a fine needle and detecting atomic forces.

Importance of Microscopes and Optical Technology

• Enabled the development of microbiology, modern medicine, and epidemiology.

• Applications extend to astronomy (telescopes), photography, and satellite imaging.

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components for individual study.

1. Homogenize tissues to break open cells and release organelles.

2. Centrifuge to pellet large organelles.

3. Transfer supernatant and repeat centrifugation at higher speeds to pellet smaller components.

Section 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cell Volume vs. Surface Area

As cells increase in size, their volume grows faster than their surface area, limiting the efficiency of material exchange and communication.

• Surface Area: The cell membrane area available for diffusion.

• Volume: The total internal content of the cell.

• Large cells have a lower surface-area-to-volume ratio, making diffusion less efficient.

• Adaptations such as microvilli increase surface area for absorption (e.g., in intestinal cells).

Cell Sizes

Cell size varies widely depending on species and function. For example, bacteria are much smaller than most eukaryotic cells.

Microvilli in Intestines

Microvilli are finger-like projections that increase the surface area of intestinal cells, enhancing nutrient absorption.

Adaptations to Cell Size Limitations

• Nerve cells are elongated to maximize surface area for rapid communication.

• Bacterial cells are much smaller in volume compared to human cells, allowing efficient diffusion.

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on their internal structure.

• Prokaryotes: Have a single circular chromosome, lack a membrane-bound nucleus, and have fewer organelles.

• Eukaryotes: Possess multiple linear chromosomes within a membrane-bound nucleus and contain various organelles.

Section 6.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

Nucleus and Nucleolus

The nucleus contains most of the cell's genetic material, while the nucleolus is the site of ribosomal RNA synthesis.

• Nucleus: Enclosed by a nuclear envelope with pores for mRNA export; contains DNA organized as chromosomes.

• Nucleolus: Region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

Nuclear Envelope and Lamina

• Nuclear Envelope: Double membrane that encloses the nucleus and contains pores for molecular transport.

• Nuclear Lamina: Protein network lining the inner surface of the nuclear envelope, providing structural support and anchoring DNA.

Ribosomes

Ribosomes are complexes of RNA and protein that carry out protein synthesis (translation). They consist of large (28S) and small (18S) subunits.

Chromosomes and Chromatin

• Chromosomes: Condensed structures of DNA and protein that carry genetic information.

• Chromatin: The less condensed form of DNA and proteins found in the nucleus.

Section 6.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that regulates protein and lipid synthesis, modification, and transport.

• Endoplasmic Reticulum (ER): Divided into rough (with ribosomes) and smooth (without ribosomes) regions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Vesicles: Transport materials between organelles and to/from the cell membrane.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes glycoproteins (proteins with carbohydrate groups) that are often secreted.

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, and stores calcium ions.

Golgi Apparatus and Vesicles

• Golgi Apparatus: Receives proteins from the ER, modifies them, and directs them to their final destinations.

• Vesicles: Small membrane-bound sacs that transport substances within the cell.

• Lysosomes: Specialized vesicles for intracellular digestion.

Vacuoles

• Vacuoles: Large vesicles with diverse functions, such as storing water, nutrients, pigments, or toxins.

• Contractile Vacuoles: Found in freshwater protists, pump out excess water.

• Central Vacuole: In plant cells, stores water and nutrients, maintaining cell rigidity.

Section 6.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Endosymbiosis

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells, forming a mutually beneficial relationship.

• Both organelles contain their own DNA and ribosomes, supporting their evolutionary origin from prokaryotes.

Mitochondria

• Function: Convert glucose into ATP, the cell's main energy currency.

• Cristae: Infoldings of the inner membrane that increase surface area for ATP production.

• Mitochondrial Matrix: Contains enzymes, ribosomes, and mitochondrial DNA.

Chloroplasts

• Function: Use sunlight to synthesize sugars via photosynthesis.

• Thylakoids: Membranous sacs where light-dependent reactions occur.

• Granum: Stack of thylakoids.

• Stroma: Fluid surrounding thylakoids, containing DNA, ribosomes, and enzymes.

Peroxisomes

• Break down fatty acids and detoxify harmful substances.

• Produce hydrogen peroxide (H2O2) as a byproduct, which is neutralized by catalase.

Section 6.6: The Cytoskeleton Is a Network of Fibers That Organizes Structures and Activities in the Cell

Cytoskeleton

The cytoskeleton provides structural support, facilitates cell movement, and organizes organelles within the cell.

• Microtubules: Hollow rods that maintain cell shape, enable organelle movement, and are involved in cell division.

• Microfilaments: Thin filaments (actin) involved in cell shape changes, muscle contraction, and cell division.

• Intermediate Filaments: Provide mechanical support and anchor organelles.

Centrioles and Centrosomes

• Centrioles: Cylindrical structures within centrosomes, important for cell division and forming the base of cilia and flagella.

• Centrosome: Microtubule-organizing center in animal cells.

Cilia and Flagella

• Cilia: Short, hair-like projections for movement or moving substances across cell surfaces.

• Flagella: Long, whip-like structures for cell movement.

• Basal Body: Anchors cilia and flagella to the cell.

Motor Proteins

• Proteins that use energy (usually from ATP) to move along cytoskeletal fibers, transporting cellular cargo.

• Dynein: A motor protein that powers cilia and flagella movement.

Muscle Movement

• Muscle contraction is explained by the sliding filament theory, where actin and myosin filaments slide past each other.

Movement of Cytoplasm

• Cytoplasmic Streaming: Circular flow of cytoplasm within cells, aiding in the distribution of materials.

• Pseudopodia: Temporary extensions of the cell membrane formed by cytoplasmic streaming, used for movement and feeding in some cells.

Section 6.7: Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Cell Wall

• Provides protection and structural support for plant, fungal, and some protist cells.

• Primary Cell Wall: Thin and flexible.

• Secondary Cell Wall: Thicker and more rigid.

• Middle Lamella: Pectin-rich layer that glues adjacent cells together.

Extracellular Matrix (ECM)

• In animal cells, the ECM provides structural support and mediates cell signaling.

• Collagen: Main structural protein forming fibers outside cells.

• Proteoglycans: Protein-polysaccharide complexes forming a network.

• Fibronectin: Glycoprotein that attaches cells to the ECM.

• Integrins: Cell surface receptors that bind ECM components.

Cell Junctions

• Adherens Junctions: Mechanically attach cells to each other.

• Tight Junctions: Seal adjacent cells to prevent leakage of extracellular fluid.

• Desmosomes: Fasten cells together into strong sheets.

• Gap Junctions: Provide cytoplasmic channels for direct communication between adjacent cells.

Section 6.8: A Cell Is Greater Than the Sum of Its Parts

All cellular components work together in a coordinated manner to maintain life. The integration of structures and functions allows cells to perform complex biological processes essential for growth, reproduction, and response to the environment.