Transcript Is the living cell simple or complex?

TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
Is the living cell
simple or complex?
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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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How do prokaryotes
demonstrate complexity?
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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 ATPproducing 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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How do eukaryotes
demonstrate complexity?
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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.)
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
plant and animal cells contain a variety of organelles. Some structures are
specific to either plant cells or animal cells only.
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How did cellular complexity
come about?
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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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
What is the
endosymbiotic theory?
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The endosymbiotic theory proposes that eukaryotic
cells formed from symbiotic relationships among
prokaryotes.
The theory proposes that mitochondria evolved from freeliving 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.)
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
The endosymbiotic theory is diagramed below.
6. Interpret Visuals Which formed first—mitochondria or chloroplasts?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
What evidence supports the
endosymbiotic theory?
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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.
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
What may ribosomes show about
the origins of cellular complexity?
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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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How could the Krebs cycle
have arisen?
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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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How could new enzymes arise?
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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?
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How could flagella have evolved
in prokaryotes?
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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.)
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
The forward motion provided by cilia (top) or flagella (bottom) is similar to two ways by which oars propel a boat.
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
How could cilia and flagella have
evolved in eukaryotes?
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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.
TEKS 7G: Analyze and evaluate scientific explanations concerning the complexity of the cell.
Do we fully understand the cell?
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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?