የባዮሎጂ መግቢያ ፈተና፣ 2014 ዓ.ም. (2022)

The wall of a bacterial cell is built from peptidoglycan (also called murein), a mesh of sugar chains cross linked by short chains of amino acids. No other group of organisms builds its wall from this material: plant cell walls are made of cellulose and fungal walls of chitin. So what makes the bacterial wall unique is its chemical composition.

Mechanical support is something every cell wall gives, so option D cannot be the unique feature. Permeability is a property of the cell membrane rather than the wall.

Textbook: Biology Grade 12, Unit 2 Microorganisms, section 2.1.1 Structure of Bacterial Cell (page 47).

Thiobacillus is a sulphur oxidizing genus. It gains energy by oxidizing hydrogen sulphide (H₂S) to sulphate, and sulphate is the form of sulphur that plant roots can absorb from the soil.

Desulphovibrio does the opposite job: it reduces sulphate back to hydrogen sulphide. Klebsiella and Azotobacter are nitrogen fixing bacteria, so they work on the nitrogen cycle, not the sulphur cycle.

Salmonellosis is caused by Salmonella bacteria taken in with contaminated food, especially undercooked meat, poultry and eggs. The bacteria multiply in the gut and cause diarrhoea, stomach cramps and fever. The textbook lists salmonellosis among the diseases spread by eating contaminated food.

The other options fail for a different reason each: candidiasis is a fungal infection, while tuberculosis and pneumonia spread mainly through droplets in the air, not through food.

Textbook: Biology Grade 12, Unit 2 Microorganisms, section 2.7 Modes of disease transmission and ways of prevention, Table 2.8 (page 95).

In the lysogenic cycle the virus does not make new virus particles straight away. Its DNA joins the host cell's own DNA and is copied quietly every time the host cell divides. This integrated state is the key feature of lysogeny, so option B is correct.

Option A is wrong because this integration is a genetic change in the host. Option C is wrong because the two cycles follow different paths: the lytic cycle builds new viruses at once and bursts the cell, while the lysogenic cycle hides first. Option D is wrong because release in the lytic cycle destroys the host cell.

Textbook: Biology Grade 12, Unit 2 Microorganisms, section 2.5 Viruses (page 78).

A virus has no ribosomes and no metabolism of its own, so it cannot grow or reproduce outside a cell. It must enter a living cell and use that cell's machinery to copy itself. For this reason viruses are described as complete (obligate) parasites of living cells.

Option A is tempting, but not every viral infection causes disease, so "completely pathogenic" is too strong. Parasitism is the term that applies to all viruses without exception.

Textbook: Biology Grade 12, Unit 2 Microorganisms, section 2.5.1 Characteristics of virus (page 79).

Agglutination (clumping) happens when red blood cells carrying an antigen meet the antibody against that antigen. The tube containing group O blood has both anti A and anti B antibodies, so any sample with A or B antigens would clump at least there.

The unknown sample clumped nowhere. That means its red cells carry neither A nor B antigen, which is exactly the description of group O, the universal donor.

Textbook: Biology Grade 10, Unit 5 Human Biology, section 5.2.1 Blood donation.