1. Is the living cell simple or complex?
Cells vary in complexity.
Most eukaryotic cells are highly specialized and contain an intricate array of organelles and internal compartments.
Many prokaryotic cells lack internal membranes and organelles except for ribosomes.
However, even prokaryotic cells are complex in their own way.
1. Compare and Contrast Which cells are more complex—eukaryotic or prokaryotic?

2. How do prokaryotes demonstrate complexity?
Like all cells, prokaryotic cells must carry out the processes that sustain life.
For example, a cell must convert food into energy. The cell membranes of most prokaryotes contain an ATP-producing electron transport system that does this.
Some prokaryotes have an internal membrane system that contains chlorophyll and carries out photosynthesis.
2. Review How do some prokaryotes carry out photosynthesis?

3. How do eukaryotes demonstrate complexity?
Specialized eukaryotic cells have organelles, such as cilia and lysosomes, that enable them to carry out specific functions, such as movement and digestion.
Mitochondria are organelles that convert the chemical energy in food to energy the cell can use for life processes.
Chloroplasts are organelles that convert solar energy to chemical energy stored in food.
3. Define What are mitochondria?
(contd.)

4. plant and animal cells contain a variety of organelles
plant and animal cells contain a variety of organelles. Some structures are specific to either plant cells or animal cells only.

5. How did cellular complexity come about?
The fossil record provides few clues about the history of life at the cellular level.
Microscopic fossils generally lack internal detail of cellular structure.
However, careful studies of living cells have helped to answer questions about the origins of cellular complexity.
4. Relate Cause and Effect Why aren’t fossils a good source of information about the evolution of cellular complexity?

6. What is the endosymbiotic theory?
The endosymbiotic theory proposes that eukaryotic cells formed from symbiotic relationships among prokaryotes.
The theory proposes that mitochondria evolved from free-living aerobic bacteria that began to live inside anaerobic prokaryotes.
Chloroplasts evolved from free-living photosynthetic bacteria paired with the earliest eukaryotes.
5. Define What is the endosymbiotic theory?
(contd.)

7. The endosymbiotic theory is diagramed below.
6. Interpret Visuals Which formed first—mitochondria or chloroplasts?

8. What evidence supports the endosymbiotic theory?
Mitochondria and chloroplasts are similar in size to bacteria, have their own genomes, contain ribosomes similar to those of prokaryotes, and are formed by division of preexisting mitochondria and chloroplasts.
The membrane systems of chloroplasts resemble those of photosynthetic prokaryotes.
Some cells today contain endosymbiotic bacteria and algae.

9. What may ribosomes show about the origins of cellular complexity?
Ribosomal RNA (rRNA), not the proteins in ribosomes, carries out the most important tasks of protein synthesis.
This may indicate that the earliest cells produced proteins using RNA alone.
Over time, ribosomal proteins may have been added to the rRNA in ways that helped stabilize the rRNA.
Evidence indicates that the complexity of today’s ribosomes is the result of an evolutionary process.
7. Sequence Which may have appeared first—rRNA or ribosomal proteins?

10. How could the Krebs cycle have arisen?
The Krebs cycle is the second stage of cellular respiration. This complex biochemical cycle requires nine enzymes and a number of other molecules.
The major components, such as some enzymes, were present in cells before aerobic metabolism evolved.
The Krebs cycle may have been built using existing genes and proteins to produce a new biochemical pathway.
8. Use Analogies How might the evolution of the Krebs cycle been similar to using bricks from an old building to construct a new one?

11. How could new enzymes arise?
As an environment changes, organisms may develop new biochemical capabilities through natural selection.
For example, even though nylon wasn’t invented until 1935, some bacteria can now use waste products from its manufacture as a food source.
Scientists found that duplication of an existing enzyme, followed by mutations, gave the bacteria the ability to break down and use waste products produced during the making of nylon.
9. Review How did bacteria develop the ability to use chemicals generated during the manufacture of nylon?

12. How could flagella have evolved in prokaryotes?
Flagella and cilia are structures that produce cellular movement by whipping back and forth or spinning.
Eubacteria and archaebacteria have flagella that are very different biochemically, but both kinds of flagella are assembled from protein subunits that serve other purposes elsewhere in the cell.
This observation suggests that prokaryotes “borrowed” copies of these proteins as flagella evolved.
10. Define What are flagella and cilia?
(contd.)

13. The forward motion provided by cilia (top) or flagella (bottom) is similar to two ways by which oars propel a boat.

14. How could cilia and flagella have evolved in eukaryotes?
Eukaryotic cilia and flagella contain key proteins called tubulin and dynein. Genetic analysis of these proteins indicates that they evolved well before the cilia and flagella themselves.
Both tubulin and dynein are associated with other proteins that change the cell’s shape or produce movement.
This suggests that major protein components of eukaryotic cilia and flagella were present before the structures evolved.

15. Do we fully understand the cell?
No, but evidence suggests that complex cellular structures and pathways were produced by the process of evolution.
However, there are many uncertainties in our current understanding of cellular complexity.
11. Summarize What is the current status of scientific understanding of the complexity of the cell?