Cell Structure and Microscopy

History and Development of Microscopy

The study of cells began with the invention of microscopes in the 1600s, allowing scientists to observe structures too small for the naked eye.

• Robert Hooke: First recorded person to observe cell walls.

• Anton van Leeuwenhoek: Created his own microscopes and observed 'animalcules' (microorganisms).

Types of Microscopes

• Light Microscope (LM): Uses visible light passed through a specimen; can magnify up to ~1000x life-size.

• Electron Microscopes (EM): Used to study subcellular structures; uses beams of electrons for higher resolution.

• Scanning Electron Microscope (SEM): Focuses electrons onto specimen surface for 3D images.

• Transmission Electron Microscope (TEM): Passes electrons through specimen for internal structure details.

Parameters and Limitations of Microscopy

• Magnification: Ratio of image size to real size.

• Resolution: Clarity of image; minimum distance between two distinguishable points.

• Contrast: Difference in brightness between light and dark areas.

Cell Types and Classification

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: No nucleus; DNA in nucleoid region; no membrane-bound organelles; usually smaller.

• Eukaryotic Cells: DNA in nucleus; contain membrane-bound organelles; usually larger.

Examples of Cell Types

• Prokaryotes: Bacteria, Archaea

• Eukaryotes: Protists, Fungi, Animals, Plants

Basic Features of All Cells

• Plasma Membrane: Selective barrier; allows passage of oxygen, nutrients, and waste.

• Cytosol: Semi-fluid substance where organelles are suspended.

• Chromosomes: Carry genetic information.

• Ribosomes: Synthesize proteins.

Cellular Components and Their Functions

Nucleus: Information Central

The nucleus contains most of the cell's DNA and is the site of genetic information storage and processing.

• Chromatin: DNA and histone proteins forming chromosomes.

• Nuclear Envelope: Double lipid bilayer separating DNA from cytoplasm.

• Nuclear Pores: Regulate entry/exit of molecules.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes: Protein Factories

• Use DNA information to make proteins.

• Composed of ribosomal RNA and protein.

• Protein synthesis occurs in cytosol (free ribosomes) and on the endoplasmic reticulum/nuclear envelope (bound ribosomes).

Endomembrane System

The endomembrane system regulates protein traffic and performs metabolic functions.

• Includes: Nuclear envelope, plasma membrane, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles.

• Directly connected or communicate via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Studded with ribosomes; secretes proteins and glycoproteins, distributes proteins via transport vesicles, membrane factory for cell.

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae).

• Modifies products of ER, manufactures macromolecules, sorts/packages materials into transport vesicles.

Lysosomes

• Membranous sac of hydrolytic enzymes; digests macromolecules.

• Recycles cell's own organelles and macromolecules.

Vacuoles

• Food Vacuoles: Formed by phagocytosis.

• Contractile Vacuoles: Pump excess water out of cells.

• Central Vacuole: Stores water and ions; found in many plant cells.

Energy Conversion Organelles

Mitochondria

Mitochondria are the sites of cellular respiration, generating ATP by using oxygen and organic molecules.

• Found in all eukaryotes.

• Contain double membrane, own DNA, and ribosomes.

• Grow and reproduce independently.

• Inner membrane folded into cristae, creating intermembrane space and mitochondrial matrix.

Chloroplasts

Chloroplasts are the sites of photosynthesis, converting light energy into chemical energy.

• Found in plants and algae.

• Contain double membrane, own DNA, and ribosomes.

• Structure includes thylakoids (stacked into grana) and stroma (fluid containing DNA, ribosomes, enzymes).

Endosymbiont Theory

• Mitochondria and chloroplasts display similarities with bacteria, suggesting they originated from engulfed prokaryotic cells.

• Early eukaryotic cells engulfed non-photosynthetic and photosynthetic prokaryotes.

Other Organelles

Peroxisomes

• Specialized metabolic compartments bound by a single membrane.

• Carry out oxidation reactions, breaking down fatty acids and detoxifying harmful substances.

Cytoskeleton: Support and Motility

Roles of the Cytoskeleton

The cytoskeleton is a network of fibers that organizes cell structures and activities, providing mechanical support and maintaining cell shape.

• Anchors many organelles and molecules.

• Involved in cell movement and division.

Components of the Cytoskeleton

• Microtubules: Hollow rods of tubulin; shape/support cell, guide organelle movement, separate chromosomes during division.

• Microfilaments: Thin rods of actin; bear tension, help cell shape, compose microvilli, involved in muscle contraction.

• Intermediate Filaments: Middle-range diameter; reinforce cell shape, anchor organelles, more permanent than other fibers.

Cell Surfaces and Junctions

Cell Walls

• Extracellular structure distinguishing plant cells from animal cells.

• Composed of cellulose microfibrils in polysaccharides and protein.

• Protects, maintains shape, prevents excessive water uptake.

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins (collagen, proteoglycans, fibronectin).

• ECM proteins bind to cell-surface receptor proteins (integrins).

• Functions: Bind cells, communication, other functions.

Cell Junctions

• Neighboring cells adhere, interact, and communicate via junctions.

• Plasmodesmata: Channels in plant cell walls for water, solutes, proteins, RNA.

• Tight Junctions: Prevent substances from moving between cells (e.g., bladder).

• Desmosomes: Hold cells together, provide mechanical stability (e.g., muscle, skin).

• Gap Junctions: Provide communication between animal cells.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Key Equations

• Magnification:

• Resolution: