Question

Cyanobacteria are photosynthetic prokaryotes. Which of the following cell structures are absent in cyanobacteria?
I. Cell membrane
II. Nucleus
III. Ribosomes

(a) I only
(b) II only
(c) III only
(d) I and II only
(e) I and III only

Answer 1

Hint: Prokaryotes are the organisms which have one prokaryotic cell and DNA is bundled in the nucleoid region. They are microscopic single celled organisms that have neither a membrane bound nucleus nor any other specialized organelle.
Cyanobacteria are gram-positive photosynthetic prokaryotes. They are also known as blue-green algae. They have comparatively larger cells than bacteria. It may be unicellular, colonial or filamentous.
Cyanobacteria are one of the most successful organisms which can survive in all possible types of habitats.

Complete answer: We have to look all the options we are provided with-
Cell membrane also called plasma membrane remains attached to the nucleoid by a semicircular group of coiled membranes called lamellosome in cyanobacteria.
In cyanobacteria a true nucleus is absent and the membrane bound structures like Golgi apparatus, plastids, mitochondria, lysosomes, sap vacuoles are also absent.
In the absence of nucleus DNA lies in the coiled shape in the central part of the cytoplasm which is called centroplasm. The coiled DNA is naked and is equal to a single chromosome of higher organisms.
Ribosomes are responsible for the synthesis of proteins. It contains ribonucleic acid (RNA). In cyanobacterial cells, ribosomes are approximately one-third smaller than that of Eukaryotic cells, but they perform similar functions in both the cells.

So, the correct answer is option (B) II only because the nucleus is absent in prokaryotes.

Note: Cyanobacteria reproduce by asexual methods mainly by binary fission. The filamentous forms reproduce by fragmentation. They are very ancient oxygenic photosynthesizers. Many cyanobacteria such as Aulosira, Nostoc, Anabaena, etc. have the ability to fix free nitrogen atmosphere.
Cyanobacteria are a good source of food to several aquatic animals. Some cyanobacterias have larvicidal properties (can be inoculated in ponds to prevent the multiplication of mosquitoes).