Cell theory, developed in the 19th century by scientists Schleiden, Schwann, and Virchow, states that all living organisms are made of cells, which are the basic units of life that arise from preexisting cells. Modern cell theory extends this by including that cells carry hereditary information, are chemically similar, and perform essential life functions within cells. Additionally, the endosymbiotic theory posits that eukaryotic organelles like mitochondria and chloroplasts originated from ancestral prokaryotic organisms through a symbiotic relationship.

1. Development of Cell Theory
• 1838- German Botanist, Matthias Schleiden,
concluded that all plant parts are made of cells
• 1839- German physiologist, Theodor Schwann, who
was a close friend of Schleiden, stated that all animal
tissues are composed of cells.
• 1858- Rudolf Virchow, German physician, after
extensive study of cellular pathology, concluded that
cells must arise from preexisting cells.
Finally they concluded
1.All organisms are composed of one or more cells.
(Schleiden & Schwann)(1838-39)
2. The cell is the basic unit of life in all living things.
(Schleiden & Schwann)(1838-39)
3. All cells are produced by the division of preexisting
cells. (Virchow)(1858)

2. Cell theory
Cell theory states that living things are composed of one or
more cells, that the cell is the basic unit of life, and that
cells arise from existing cells.
The cell theory describes the basic properties of all cells.
• The three scientists that contributed to the development
of cell theory are Matthias Schleiden, Theodor Schwann,
and Rudolf Virchow.
• A component of the cell theory is that all living things are
composed of one or more cells.
• A component of the cell theory is that the cell is the basic
unit of life.
• A component of the cell theory is that all new cells arise
from existing cells.

4. Modern Cell Theory
Modern Cell Theory contains 4 statements, in addition to
the original Cell Theory:
• The cell contains hereditary information(DNA) which is
passed on from cell to cell during cell division.
• All cells are basically the same in chemical composition
and metabolic activities.
• All basic chemical & physiological functions are carried
out inside the cells.(movement, digestion, etc)
• Cell activity depends on the activities of sub-cellular
structures within the cell(organelles, nucleus, plasma
membrane)
• Cell are classified as Unicellular and Multicellular

5. Endosymbiotic Theory
• In 1831, Scottish botanist Robert Brown (1773–1858) was the first
to describe observations of nuclei, which he observed in plant
cells.
• Then, in the early 1880s, German botanist
Andreas Schimper (1856–1901) was the first to describe the
chloroplasts of plant cells, identifying their role in starch formation
during photosynthesis and they divided independent of the
nucleus.
• Based upon the chloroplasts’ ability to reproduce independently,
Russian botanist Konstantin Mereschkowski (1855–1921)
suggested in 1905 that chloroplasts may have originated from
ancestral photosynthetic bacteria living symbiotically inside a
eukaryotic cell. He proposed a similar origin for the nucleus of plant
cells.
• This was the first articulation of the endosymbiotic hypothesis, and
would explain how eukaryotic cells evolved from ancestral bacteria.

6. • Lynn Margulis (1938–2011), an American geneticist,
published her ideas regarding the endosymbiotic
hypothesis of the origins of mitochondria and
chloroplasts in 1967.
• In the decade leading up to her publication, advances
in microscopy had allowed scientists to differentiate
prokaryotic cells from eukaryotic cells.
• The eukaryotic organelles such as mitochondria and
chloroplasts are of prokaryotic origin.
• This hypothesis was not initially popular, but mounting
genetic evidence due to the advent of DNA
sequencing supported the endosymbiotic theory.
• Additionally, mitochondrial and chloroplast ribosomes
are structurally similar to bacterial ribosomes, rather
than to the eukaryotic ribosomes of their hosts.

7. Endosymbiotic Theory

8. Unicellular organism
Organisms consisting of only one cell carry out all functions of
life in that cell.
• Movement • Reproduction • Sensitivity • Homeostasis • Growth
• Respiration • Excretion • Nutrition
The functions are refined:
• Metabolism - the web of all the enzyme- catalysed reactions in
a cell or organism, e.g. respiration
• Response - Living things can respond to and interact with the
environment
• Homeostasis - The maintenance and regulation of internal cell
conditions, e.g. water and pH
• Growth - Living things can grow or change size / shape
• Excretion – the removal of metabolic waste
• Reproduction - Living things produce offspring, either sexually
or asexually
• Nutrition – feeding by either the synthesis of organic molecules
(e.g. photosynthesis) or the absorption of organic matter

9. Investigation of functions of life in Paramecium
and one named photosynthetic unicellular organism

10. Investigation of functions of life in Paramecium
and one named photosynthetic unicellular organism

11. Multicellular organisms have properties that emerge
from the interaction of their cellular components
• Emergent properties arise from the interaction of component
parts.
• The whole is greater than the sum of its parts.
• Multicellular organisms are capable of completing functions
that individual cells could not undertake - this is due to the
interaction between cells producing new functions.
• Biology uses inductive thinking as it is realised the
importance of emergent properties, whether it be the
interaction of genes, enzymes working together in a metabolic
pathway, or cells forming tissues, different tissues forming
organs, in turn forming organ systems and then the organism
itself.
• At each level emergent properties arise.

12. Multicellular organisms have properties that emerge
from the interaction of their cellular components