Microscopy and the Discovery of Cells

History of Cell Observation

The development of microscopy in the 1600s and 1700s enabled scientists to observe cells for the first time, laying the foundation for modern cell biology.

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

• 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 achieve up to ~1000x life-size magnification.

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

  • Scanning Electron Microscope (SEM): Focuses electrons onto the surface of a specimen, producing three-dimensional images.

  • Transmission Electron Microscope (TEM): Focuses electrons through a specimen, revealing internal structures.

Parameters and Limitations of Microscopy

• Magnification: Ratio of an object's image size to its real size.

• Resolution: Measure of image clarity; the minimum distance between two distinguishable points.

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

Cell Types and Basic Features

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural characteristics.

• Prokaryotic Cells:

  • Include Bacteria and Archaea.

  • No nucleus; DNA is located in a nucleoid region.

  • No membrane-bound organelles.

  • Usually smaller than eukaryotic cells.

• Eukaryotic Cells:

  • Include Protists, Fungi, Animals, and Plants.

  • DNA is enclosed in a nucleus.

  • Contain membrane-bound organelles.

  • Usually larger than prokaryotic cells.

Basic Features of All Cells

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

• Cytosol: Semi-fluid substance in which organelles are suspended.

• Chromosomes: Carry genes in the form of DNA.

• Ribosomes: Synthesize proteins.

Nucleus and Ribosomes

Nucleus: Information Central

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

• Chromatin: DNA and histone proteins forming chromosomes.

• Nuclear Envelope: Double lipid bilayer that encloses the nucleus, separating DNA from cytoplasm.

• Nuclear Pores: Regulate entry/exit of molecules through the nuclear envelope.

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

Ribosomes: Protein Factories

• Use information from DNA to make proteins.

• Composed of ribosomal RNA and protein.

• Carry out protein synthesis in two locations:

  • Free ribosomes (in cytosol)

  • Bound ribosomes (on endoplasmic reticulum or nuclear envelope)

The Endomembrane System

Components and Functions

The endomembrane system regulates protein traffic and performs metabolic functions in the cell.

• Nuclear Envelope

• Plasma Membrane

• Endoplasmic Reticulum (ER): Biosynthetic factory with two regions:

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

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

• Golgi Apparatus: Shipping and receiving center; modifies, sorts, and packages products of the ER.

• Lysosomes: Digestive compartments containing hydrolytic enzymes to break down macromolecules.

• Vacuoles: Diverse compartments for storage and maintenance; include food vacuoles, contractile vacuoles, and central vacuoles (in plants).

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria

Mitochondria are the sites of cellular respiration, a metabolic process that uses oxygen to generate ATP from organic molecules.

• Found in nearly all eukaryotic cells.

• Contain a double membrane, their own DNA, and ribosomes.

• Grow and reproduce independently within the cell.

• Structure:

  • Smooth outer membrane (protective barrier).

  • Inner membrane folded into cristae, creating two compartments:

    • Intermembrane space (between inner and outer membranes).

    • Mitochondrial matrix (site of metabolic steps of cellular respiration and location of mitochondrial DNA and ribosomes).

Chloroplasts

Chloroplasts are the sites of photosynthesis, converting light energy and CO2 into organic compounds. Found in plants and algae.

• Contain green pigment chlorophyll and other enzymes for photosynthesis.

• Structure:

  • Thylakoids: Membranous sacs stacked to form a granum.

  • Stroma: Internal fluid containing DNA, ribosomes, and enzymes.

Endosymbiont Theory

This theory proposes that mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancestral eukaryotic cell, leading to a symbiotic relationship.

• Supported by similarities between mitochondria/chloroplasts and bacteria (e.g., double membranes, own DNA, ribosomes).

Other Organelles and Structures

Peroxisomes

• Specialized metabolic compartments bound by a single membrane.

• Contain enzymes that remove hydrogen atoms from substrates and transfer them to oxygen, producing hydrogen peroxide (H2O2), which is then converted to water.

The Cytoskeleton: Support and Motility

Roles of the Cytoskeleton

The cytoskeleton is a network of fibers that organizes structures and activities in the cell, providing mechanical support and facilitating movement.

• Organizes cell's structures and activities.

• Maintains cell shape.

• Anchors many organelles and molecules.

Components of the Cytoskeleton

• Microtubules: Hollow rods made of tubulin; shape and support the cell, serve as tracks for organelle movement, separate chromosomes during cell division.

• Microfilaments: Thin rods built from actin; bear tension, help maintain cell shape, involved in muscle contraction.

• Intermediate Filaments: Fibers with diameters in the middle range; reinforce cell shape, anchor organelles, more permanent than other cytoskeletal elements.

Cell Surfaces and Junctions

Cell Walls of Plants

• Extracellular structure distinguishing plant cells from animal cells.

• Composed of cellulose microfibrils embedded in polysaccharides and protein.

• Protects the plant cell, maintains its shape, and 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) in the plasma membrane.

• Functions: Bind cells together, communication, and other functions.

Cell Junctions

Neighboring cells in tissues often adhere, interact, and communicate through specialized junctions.

• Plasmodesmata: Channels that perforate plant cell walls, allowing passage of water, solutes, proteins, and RNA.

• Tight Junctions: Prevent leakage of extracellular fluid between animal cells (e.g., in the bladder).

• Desmosomes: Fasten cells together into strong sheets, providing mechanical stability (e.g., in muscle cells and skin).

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

Summary Table: Prokaryotic vs. Eukaryotic Cells