Question

Biologists think that endosymbiosis gave rise to mitochondria before plastids partly because

(A) the products of photosynthesis could not be metabolized without mitochondrial enzymes.

(B) all eukaryotes have mitochondria (or their remnants), whereas many eukaryotes do not have plastids.

(C) mitochondrial DNA is less similar to prokaryotic DNA than is plastid DNA.

(D) without mitochondrial CO₂ production, photosynthesis could not occur.

Short answer

1. The statement “the products of photosynthesis could not be metabolized without mitochondrial enzymes” isfalse.

2. The statement “all eukaryotes have mitochondria (or their remnants), whereas many eukaryotes do not have plastids” istrue.

3. The statement “mitochondrial DNA is less similar to prokaryotic DNA than is plastid DNA” is false.

4. The statement “without mitochondrial CO₂ production, photosynthesis could not occur” is false.

Step-by-step solution

Endosymbiosis

Endosymbiosis is a type of symbiotic relationship exhibited by certain organisms in which one organism resides inside the other. The endosymbiotic theory has proved that many organisms have evolved through this relationship. It is also said that mitochondria and chloroplasts also evolved through this.

Explanation of option ‘(A)’

In the absence of mitochondria, the products food and energy cannot be metabolized. Due to this, mitochondria are essential for the photosynthetic products to be metabolized. However, this does not prove that mitochondria evolved before plastids.

Thus, the impaired metabolism of photosynthetic products does not prove that mitochondria evolved before plastids.

Therefore, the given statement is false.

Explanation of option ‘(B)’

Most eukaryotes evolved in such a way that they either gained or lost plastids. The organisms that gained plastids were those that required these plastids for photosynthesis, while those that lost these plastids did not perform the photosynthetic process.

However, the mitochondria were essential for metabolizing several products, and hence they remained.

Thus, scientists explained that endosymbiosis produced mitochondria before plastids. This is because all eukaryotes possess mitochondria but not all possess plastids.

Therefore, the given statement is true.

Explanation of option ‘(C)’

The mitochondrial DNA of eukaryotes resembles prokaryotic DNA because they have evolved from prokaryotes. Therefore, the plastids cannot resemble prokaryotic DNA as much as mitochondrial DNA.

Thus, mitochondria have evolved before plastid and not vice versa.

Therefore, the given statement is false.

Explanation of option ‘(D)’

Mitochondria is an important cell organelle that takes up oxygen from the atmosphere and liberates carbon dioxide. However, the presence or absence of carbon dioxide is not an indicatorof whether mitochondria evolved first or did plastids.

Thus liberation of carbon dioxide does not help understand whether mitochondria or plastids evolved first.

Therefore, the given statement is false.