AP Biology :2.10 Origins of Cell – Exam Style questions with Answer- FRQ

Ans:

Mammalian cells contain between 800 and 2,500 mitochondria.
Erythrocytes (red blood cells) contain none. The range in the number of
mitochondria is from as few as one or two to as many as a half million!
The mitochondria and chloroplasts as obligate cellular symbiotes is an
example of co-evolution (see answer 99) that is also a parallel
diversification.
If the AP Biology exam asks for four pieces of evidence, provide only four.
They will give you points only for the first four if you provide more, so
giving more will only waste time. Choose the four you can explain best.
Because there’s a lot of evidence to support the endocytic origin of the
mitochondria (and chloroplasts), and the AP Biology readers are not
grading this answer, more than four are provided here:
(1) Mitochondria (and chloroplast) are similar in size and shape to
many prokaryotic cells.
Strength of the evidence: Suggestive, but not definitive “proof.”
(2) Endocytosis wraps the engulfed particle within a membrane derived
from the plasma membrane of the engulfing cell. Mitochondria have
two membranes, inner and outer membranes, which are structurally
and biochemically (composition) different. The inner membrane is
highly convoluted (into cristae), like that of the plasma membrane of
an aerobic prokaryote. The lipid and protein composition of the
inner membrane is more similar to a eubacteria than a eukaryote.

(3) The mitochondria (and chloroplasts) can reproduce semi-
autonomously through binary fission (not mitosis). The number of

mitochondria in a cell can change throughout the cell’s lifetime
regardless of whether it has undergone any cell division.
Strength of the evidence: Suggestive, but not definitive “proof.”
(4) The mitochondria, like eubacteria, have their own circular
chromosome that lacks histones and introns.
Strength of the evidence: Pretty convincing.
(5) Many of the genes from the original mitochondria have been
transferred to the nucleus (it’s called endosymbiotic gene transfer,
and it makes the mitochondria dependent on their host cell).
Strength of the evidence: Pretty convincing if the method for
determining the nuclear genes originated from the mitochondria.

(6) The mitochondrion has its own ribosomes. The large subunit of the
mitochondrial ribosome is more similar to the prokaryotic ribosome
than the ribosomes in the cytosol.
Strength of the evidence: Pretty convincing.
(7) Antibiotics that work by blocking protein synthesis in eubacteria
(like streptomycin) also block protein synthesis in mitochondria (and chloroplasts) but not cytosolic ribosomes.
Antibiotics that work by blocking RNA polymerase in eubacteria
block RNA synthesis in mitochondria (and chloroplasts) but not in
the eukaryotic nucleus (rifampicin).
Inhibitors of eukaryotic protein synthesis (like the diphtheria toxin)
do not inhibit protein synthesis in the mitochondria (or chloroplast).
Strength of the evidence: Pretty convincing.

The Oxygen Catastrophe of 2.3 BYA
Aerobic metabolism (and multicellularity) evolved after the oxygen
gas released by cyanobacterial photosynthesis significantly
accumulated in the atmosphere. Most organisms could not tolerate
the oxidizing atmosphere. This “oxygen catastrophe” caused many
prokaryote species to become extinct.
The original endocytosis of the mitochondria occurred between 1.7
and 2 billion years ago and was not followed by its digestion. It’s
possible that the organelle initially served to detoxify oxygen for its
host by converting it to water. Because the reduction of oxygen to
water is spontaneous, any cells that could use oxygen to drive electron
transport chains would certainly have had an advantage.
The free energy change from the spontaneous, exergonic movement
of electrons from organic molecules to oxygen is very large, meaning
that a lot of energy is available to couple the synthesis of ATP.