Is the living cell simple or complex?

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
complexeukaryotic 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. 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 arent 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
firstmitochondria 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 todays
  ribosomes is the result of an evolutionary
  process.

7. Sequence Which may have appeared firstrRNA 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 wasnt 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?