1. Learning Targets
Outline the cell theory and explain how a single cell demonstrates all the characteristics of living things (e.g. metabolism, response to stimuli, homeostasis, growth, reproduction, evolution, and energy consumption).
Compare the cells of organisms in the Domains Eukarya, Archaea, and Bacteria (eukaryotic vs. prokaryotic cells).
Explain how mitochondria and chloroplasts provide evidence for endosymbiotic theory.
Identify the components of a eukaryotic cell’s endomembrane system and explain how the components demonstrate both continuity and specialization.

2. Cell Theory
The Cell Theory, a foundation of modern biology, states that cells are the fundamental units of life.
What is a theory?
How is a theory different from a hypothesis?
How did scientists, beginning over 350 years ago, develop the cell theory?
The difference between a hypothesis and a theory is a concept we have discussed before. If you do not remember the difference between a theory and a hypothesis, you may want to go back and review this material from Unit 1.
What we know and understand about cells is the result of hundreds of years of scientific inquiry. Modern “cell theory,” therefore, is the result of work from many different scientists. We will look a few contributions here.

3. Hypothesis versus Theory

4. Hypothesis versus Theory
Hypothesis: a testable, tentative explanation for an observed phenomenon in nature.
Theory: According to the National Academy of Sciences:
“In science, a well-substantiated explanation of some aspect of the natural world that can incorporate fact, laws, inferences, and tested hypotheses.”

5. Pathways to Theories and Laws

6. The Cell Theory Emerges
In 1839, Schleiden and Schwann proposed the basic concepts of the modern Cell Theory:
All organisms consists of one or more cells.
A cell is the smallest unit with the properties of life.
Each new cell arises from division of another, preexisting cell.
Each cell passes its hereditary material to its offspring.
The actual identity of the hereditary material mentioned in #4 was not confirmed until 1953 by James Watson and Francis Crick.
Schleiden and Schwann were not the first scientists to observe cells, but they did add their own contributions to what other scientists had discovered. Schleiden was studying plants and Schwann was studying animals. After comparing research with each other, they proposed the basic concepts of modern Cell Theory (listed above).

7. Comparing the Domains Archaea and Bacteria (prokaryotes) with the Domain Eukarya
All cells have several basic features in common
They are bounded by a plasma membrane.
They contain a semifluid substance called the cytosol.
They contain chromosomes in the nucleoid (prokaryote) or nucleus (eukaryote).
They all have ribosomes.
All cells have the features mentioned above:
(The plasma membrane is also called the cell membrane. Cytosol is also referred to as cytoplasm.)
Prokaryotic refers to cells that lack a nucleus. The bacteria cell on the left is a prokaryotic cell. Archaea cells are also prokaryotic. All cells in the domain Eukarya are eukaryotic cells. The chromosomes of these cells are contained within a nucleus. These cells also have more complex membrane-bound organelles such as the mitochondria, chloroplasts, and vesicles.

8. Tree of Life “Prokaryotes”
The terms prokaryotic and eukaryotic are still used by some biologists when describing cells, but many biologists are no longer using these terms. Instead, they are using the domain of the organism as a more accurate description of the cell. (For example, calling a cell a bacteria cell or an archaea cell instead of a prokaryotic cell). Because of changes in the ways that we understand and therefore classify organisms, there will always be changes in the ways that we describe them as well.
“Prokaryotes”

9. General “Prokaryote” Body Plan
Cell wall surrounds the plasma membrane
Made of peptidoglycan (in bacteria) or proteins (in archaea) and coated with a sticky capsule
Flagellum for motion
Pili help cells move across surfaces
Sex pilus aids in sexual reproduction
Capsule
Cell wall
Cytoplasm (with ribosomes
DNA in nucleoid
Flagellum
Pilus
Plasma membrane

10. General “Prokaryote” Body Plan
Cell wall surrounds the plasma membrane
Made of peptidoglycan (in bacteria) or proteins (in archaea) and coated with a sticky capsule
Flagellum for motion
Pili help cells move across surfaces
Sex pilus aids in sexual reproduction
Bacteria and Archaean cells have a plasma membrane (cell membrane) as well as a cell wall. Additionally, there is a sticky capsule that surrounds the cell wall. The DNA of the cell is not contained within a nucleus (this is why they are called prokaryotic). There is one circular chromosome which floats freely in the cytosol, usually found in a specific area of the cell called the nucleoid (not the same as a nucleus).
These cells may contain flagellum or pili for movement or to aid in reproduction.
In some bacterial cells, sex pili allow for the exchange of genetic material between cells (sexual reproduction).
Most reproduction occurs asexually, however.

11. Eukaryotic Cells The endomembrane system
Organelles involved in making, modifying, and transporting biomolecules (proteins, lipids and protein-carbohydrate hybrid molecules)
Membrane is recycled and reused between each of these organelles.
Outer nuclear membrane, vesicles, ER, Golgi apparatus

12. A few facts about mitochondria and chloroplasts
Endosymbiotic Theory
A few facts about mitochondria and chloroplasts
Not produced by the cell (replicate themselves)
Have their own DNA and ribosomes
Have a double-membrane
Similar in size to bacteria cells
Given these facts, where do they come from?

13. Endosymbiotic Theory
The evolution of eukaryotic cells included a symbiotic relationship with energy producing bacteria cells (eventually to become mitochondria and chloroplasts)