Cell Theory: History and Modern Extensions

• The history of cells is part of the cell theory.

Discovery of Cells

• In 1665, Robert Hooke, an English scientist, discovered cells while observing a thin slice of cork.
• He described cells as tiny boxes, similar to a honeycomb.
• Hooke believed that cells existed only in plants and fungi.

Anton van Leeuwenhoek

• In 1673, Anton van Leeuwenhoek used a handmade microscope to observe pond scum, discovering single-celled organisms.
• He referred to these organisms as "animalcules".
• Leeuwenhoek also observed blood cells from various animals, including fish, birds, frogs, dogs, and humans.
• This revealed that cells are present in both animals and plants.

150-200 Year Gap

• Between the discoveries of Hooke and Leeuwenhoek and the mid-19th century, there was limited progress in cell theory.
• This stagnation was likely due to the widely accepted belief in spontaneous generation.
• Examples include the belief that mice came from dirty clothes/corn husks and maggots from rotting meat.

19th Century Advancement

• There was significant doubt surrounding the concept of spontaneous generation.
• Louis Pasteur conclusively disproved spontaneous generation.

Development of Cell Theory

• In 1838, Matthias Schleiden, a German botanist, concluded that all plant parts consist of cells.
• In 1839, Theodor Schwann, a German physiologist and friend of Schleiden, stated that all animal tissues are composed of cells.
• In 1858, Rudolf Virchow, a Russian pathologist, after studying cellular pathology extensively, concluded that cells arise from pre-existing cells.

Three Basic Components of the Cell Theory

• All organisms are composed of one or more cells (Schleiden & Schwann, 1838-39).
• The cell is the basic unit of life in all living things (Schleiden & Schwann, 1838-39).
• All cells are produced by the division of pre-existing cells (Virchow, 1858).

Theory

• A theory is a set of statements or principles.
• These are designed to explain a group of facts or phenomena.
• It is repeatedly tested and widely accepted.
• Theories can be used to make predictions about natural phenomena.

Modern Cell Theory

• It consists of the three basic components of cell theory and four additional statements.
• Cells pass information from cell to cell during cell division using DNA.
• All cells have a similar chemical composition and metabolic activities.
• All cells have basically the same chemical & physiological functions (movement, digestion, etc.).
• Cell activity depends on the activities of structures within the cell (organelles, nucleus, and plasma membrane).

How Cell Theory Has Been Used

• Previously discovered truths about cells, as listed in the Cell Theory, serve as the foundation for advancements.
• It is used in disease/health/medical research such as AIDS, Cancer, Vaccines, Cloning and Stem Cell Research.

Characteristics of Cells

• All cells have common structures:
  • Cell Membrane
  • Ribosomes
  • Cytoplasm
  • Contain DNA found in the nucleus
• Cells are independent units of life.

Prokaryotic Cells

• Prokaryotes do not have a nucleus.
• They are single-celled organisms.
• They have few organelles performing few functions.
• They can eat, respire and reproduce.
• They are smaller than other cells.
• Bacteria are the only known prokaryotes.
• Components of prokaryotic cells include Cytosol, Ribosomes, Nucleoid region, Circular DNA, Plasmid, Cell Membrane, Cell Wall, Capsule, Pili and a Flagellum.

Characteristics of Viruses

• Viruses are not considered living because:
  • They are not made of cells.
  • They need a host cell to reproduce.
• A virus is very small compared to a cell.
• Like cells, viruses contain nucleic acids.

Eukaryotic Cells

• Eukaryotes have a nucleus.
• Eukaryotes can be single or multicellular.
• Eukaryotes have many organelles performing complex functions.
• They are larger than prokaryotic cells.
• Animals, plants, fungi, and protists are made of eukaryotic cells. Components include:
  • Cytosol
  • Nucleus
  • Mitochondria
  • Chloroplast
  • Ribosomes
  • Rough ER
  • Smooth ER
  • Golgi body
  • Vacuoles
  • Cytoskeleton
  • Centriole
  • Cilium and Flagellum
  • Cell membrane
  • Cell Wall

Summary of Differences Between Cells

• Prokaryotic Cells:
  • Are small cells (< 5 mm).
  • Are always unicellular.
  • Have no nucleus or any membrane-bound organelles.
  • Their DNA is circular, without proteins.
  • Ribosomes are small (70S).
  • No cytoskeleton.
  • Cell division is by binary fission.
  • Reproduction is always asexual.
• Eukaryotic Cells:
  • Are larger cells (> 10 mm).
  • Are often multicellular.
  • Have a nucleus and other membrane-bound organelles.
  • DNA is linear and associated with proteins to form chromatin.
  • Ribosomes are large (80S).
  • Always has a cytoskeleton.
  • Cell division is by mitosis or meiosis.
  • Reproduction can be asexual or sexual.

Similarities Between Animal and Plant Cells

• Both have:
  • A nucleus.
  • Ribosomes that make protein.
  • Rough and smooth endoplasmic reticulum.
  • Golgi bodies.
  • Mitochondria.
  • Cytoplasm.
  • Vacuoles that store food, water, and waste products.

Animal Cells

• Cannot make their own food.
• Have many lysosomes.
• Are more round shaped.
• Have centrioles.
• Use mitochondria to release energy.
• Have many Golgi bodies.
• Have more extensive cytoskeleton than plant cells.

Animal Cells Continued

• Do not have a cell wall.
• Do not have a large vacuole.
• Do not have chloroplasts.

Nucleus

• Nickname: "The Control Center".
• Function: holds the DNA.
• Parts:
  • Nucleolus: dark spot in the middle of the nucleus that helps make ribosomes.
• The nucleus is the largest organelle and control center of the cell and consists of:
• Nuclear envelope: a double-membrane structure containing pores.
• Nucleoli: synthesize ribosomes.
• Chromatin: threadlike material composed of DNA (genes) & proteins.
• During cell division, chromatin condenses to form chromosomes.

Cytoplasm

• Consists of:
  • Cytosol - liquid portion.
  • Organelles – specialized cellular compartments.
  • Inclusions - chemical substances:
    • Glycogen (muscle & liver cells).
    • Lipid droplets (fat cells).
    • Melanin granules (skin & hair cells).

Mitochondria

• Nickname: "The Powerhouse".
• Function: energy formation.
  • Breaks down food to make ATP.
  • ATP: is the major fuel for all cell activities that require energy.
• Mitochondria are double membrane structures.
• They are the site of cellular respiration (ATP synthesis).
• Contain their own DNA & RNA.
• They are able to replicate themselves.

Ribosomes

• Function: to make proteins.
• Found in all cells, prokaryotic and eukaryotic.
• Ribosomes are dense particles of rRNA and protein
  • Free ribosomes synthesize proteins that function within the cell
  • Attached ribosomes synthesize proteins incorporated into cell membranes or exported outside the cell.

Cytoskeleton

• A network of rods that support the cell.
• Microtubules: thick rods composed of tubulin that form cilia, flagella & centrioles.
• Microfilaments: thin filaments composed of actin. They are involved in muscle contraction and form cleavage furrow during cell division.
• Intermediate filaments: tough protein fibers attached to desmosomes.

Centrioles

• Paired cylindrical bodies composed of microtubules.
• Organize spindle apparatus during cell division.

Cellular extensions

• Microvilli: fingerlike projections of the plasma membrane increase surface area for absorption.
• Cilia: short hair-like projections that propel substances over the surface of cell.
• Flagella: long hair-like projections, propel the cell.

Endoplasmic Reticulum (ER)

• Nickname: "Roads"
• Function: the internal delivery system of the cell.
• Two Types:
  • Rough ER: rough appearance because it has ribosomes and helps make proteins.
  • Smooth ER: no ribosomes and makes fats or lipids.

Endoplasmic Reticulum (ER)

• It has an extensive membrane system.
• Functions:
  • Synthesis of proteins, carbohydrates, and lipids.
  • Storage of synthesized molecules and materials.
  • Transport of materials within the ER.
  • Detoxification of drugs or toxins.
• Rough ER - is studded with ribosomes; abundant in secretory cells.
• Smooth ER - is free of ribosomes:
  • Synthesizes lipids and steroids.
  • Detoxifies drugs/poisons (liver cells).
  • Stores calcium ions (muscle cells).

Golgi Apparatus

• Aka Golgi Body or Golgi complex
• Nickname: The Shippers
• Function: packages, modifies, and transports materials to different locations inside/outside of the cell.
• Appearance: stack of pancakes.
• Golgi apparatus – stack of flattened membranous sacs Packages proteins for secretion from the cell (exocytosis) Packages proteins for incorporation into plasma membrane Forms lysosomes

Lysosomes

• Are circular (but bigger than ribosomes).
• Nickname: "Clean-up Crews”.
• Function: to break down food into particles the rest of the cell can use and to destroy old cells.
• Lysosomes – membranous sacs of hydrolytic enzymes; sites of intracellular digestion Primary lysosome
• Formed by Golgi apparatus and inactive enzymes Secondary lysosome
• Lysosome fused with damaged organelle
• Digestive enzymes activated
• Toxic chemicals isolated

Peroxisomes

• These are membranous sacs of oxidase & catalase enzymes.
• They detoxify alcohol & neutralize dangerous free radicals.

Plant Cells

• Have a cell wall and cell membranes.
• Have a large vacuole unlike the animal cell which only has small vacuoles.
• Have mitochondria to convert sugar to usable energy for the cell.
• Have a few lysosomes.
• Are more rectangular in shape.
• Have chloroplasts to carry out photosynthesis.
• Make sugar to store solar energy.

Vacuoles

• Function: stores water.
• This is what makes lettuce crisp and a plant wilts when there is no water.

Chloroplasts

• Function: Traps energy from the sun to produce food for the plant cell.
• Green in color because of chlorophyll, which is a green pigment.

Cell Wall

• Function: Provides support and protection to the cell membrane. Located outside the cell membrane in plant cells.

Explore Virchow's contribution to the cell theory and the modern extensions beyond the original tenets of Schleiden, Schwann, and Virchow. Understand how cell activity depends on the structures within, and identify key scientists and their contributions to the cell theory.