Cell Theory and Historical Perspectives

Historical Background of Cell Theory

• Robert Hooke (1665) observed cells in cork and coined the term "cells" due to their resemblance to monastery rooms.
• Anton van Leeuwenhoek (1674) improved microscopes and discovered protozoa and red blood cells, calling microscopic life "animalcules."
• Robert Brown (1831) identified the nucleus and described cytoplasmic streaming, known as Brownian movement.
• Felix Dujardin (1835) demonstrated that cells contain protoplasm, a thick fluid.
• Matthias Schleiden (1838) announced that plants are made of cells, initiating the plant cell theory.
• Theodore Schwann (1839) proposed that animals are also composed of cells, contributing to the animal cell theory.
• Rudolf Virchow (1858) stated that all cells arise from pre-existing cells, solidifying cell theory principles.
• Max Knoll and Ernst Ruska (1932) developed the first transmission electron microscope.
• James Watson and Francis Crick (1953) elucidated DNA structure, marking a pivotal moment in molecular biology.

Cell Theory

• All organisms consist of cells; cells are the fundamental structural and functional units.
• Cells have self-modifying capabilities and originate only from existing cells.

Types of Cells

Prokaryotic Cells

• Lack a true nucleus, typical of bacteria; size ranges from 1 to 10 μm.
• Contain cell wall, plasma membrane, ribosomes, thylakoids, enzymes, and a nucleoid with a single chromosome.

Eukaryotic Cells

• Possess a true nucleus; characteristic of protists, fungi, plants, and animals.
• Comprise plasma membrane, cytoplasm, and a distinct nucleus; organelles are compartmentalized.

Cell Size & Shape

• Mycoplasma is the smallest known cell at 0.0001 mm; giraffe nerve cells can reach 3.0 m in length.
• Human cells vary, with red blood cells measuring 0.00076 mm and liver cells being larger.
• Common cell measurement unit is the micrometer (μm), equal to 1 x 10^-6 meters.
• Smaller cells have a higher surface area-to-volume ratio, impacting efficiency.
• Cell shape influences function; for example, elongated muscle cells allow contraction, while concave blood cells facilitate oxygen distribution.

Plasma Membrane

• Composed of a phospholipid bilayer with proteins in a fluid-mosaic model.
• Functions to provide structural support, protection, and regulate material traffic between the cell interior and environment.

Modification of the Plasma Membrane

• Microvilli increase surface area for absorption.
• Glycocalyx acts as a carbohydrate-protein or carbohydrate-lipid complex for cell interactions.
• Plant cells have a thick cell wall for added support and rigidity.

Water Movement Pathways

• Apoplast Pathway: Water moves through spaces between cells and cell walls.
• Symplast Pathway: Water circulates from cytoplasm to cytoplasm through plasmodesmata.

Vacuoles in Cells

• Sap Vacuole in plants stores resources and maintains turgor pressure.
• Contractile Vacuole in protists expels excess water and waste products.
• Food Vacuole in protozoans facilitates digestion of consumed materials.

Protoplasm

• The living material of the cell that showcases life's properties and activities.

Physical Properties of Protoplasm

• Colloidal and heterogeneous solution; resistant to gravity due to small particle size.
• Exhibits viscosity and can transition between sol and gel states.
• Characterized by transparency and various configurations (reticular, granular, etc.).

Cytoplasm

• Multi-functional cell content external to the nucleus; contains proteins, carbohydrates, and lipids.
• The site of numerous biochemical reactions involving organelles and cellular inclusions.

Cell Structures and Functions

• Cell Wall: Composed of cellulose; provides support and protection (plant cells only).
• Plasma Membrane: Phospholipid bilayer for molecular transit in and out of the cell.
• Nucleus: Contains genetic material for reproduction and protein synthesis regulation.
• Nucleolus: Area for ribosome production.
• Ribosome: Protein synthesis units made of protein and RNA.

Membrane Proteins and Organelles

• Protein Channels: Facilitate the movement of molecules across the membrane.
• Carrier Proteins: Transport substances across membranes in both directions.
• Endoplasmic Reticulum (ER): Site for protein and lipid synthesis, distinguished into rough (with ribosomes) and smooth (without ribosomes).
• Golgi Apparatus: Processes and packages proteins for secretion.
• Lysosomes: Intracellular digestion via digestive enzymes.
• Mitochondria: Cellular respiration powerhouse with inner membrane cristae.
• Chloroplasts: Site of photosynthesis in plant cells, containing thylakoids and grana for light reactions.

Structural Components

• Cilia and Flagella: Microtubule arrangements for cell movement.
• Cytoskeleton: Composed of microtubules and microfilaments; provides cell shape and facilitates movement.