የባዮሎጂ መግቢያ ፈተና፣ 2015 ዓ.ም. (2023)

The wall of a bacterial cell is built from peptidoglycan (also called murein). No other group of organisms uses this material, so the chemical composition makes the bacterial wall unique. Plant cell walls are made of cellulose and fungal walls of chitin.

Permeability and mechanical support are jobs that cell walls do in many organisms, so they are not unique to bacteria.

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

Thiobacillus (written as Thiolbacillus in the paper) is the sulphur bacterium that oxidizes hydrogen sulphide to sulphate. Plants cannot use hydrogen sulphide, but their roots absorb sulphate from the soil to build proteins.

Desulphovibrio does the opposite: it reduces sulphate back to hydrogen sulphide. Azotobacter and Klebsiella work in the nitrogen cycle, not the sulphur cycle.

Salmonellosis is food poisoning. It comes from eating food or water contaminated with Salmonella bacteria, and it causes diarrhoea, fever and stomach cramps.

Candidiasis is caused by a fungus, not a bacterium. Tuberculosis and pneumonia are spread mainly through the air, not through food.

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

In the lysogenic cycle the viral DNA joins the DNA of the host cell. It stays there quietly and is copied each time the cell divides, so option B is correct.

The other options fail: in the lytic cycle new viruses burst out and kill the host cell, so D is false. The two cycles follow different paths, so C is false. And because lysogeny inserts viral DNA into the host genome, the host cell is genetically altered, so A is false.

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

A virus has no cytoplasm, no ribosomes and no energy system of its own. It can only reproduce by entering a living cell and using that cell's machinery. For this reason viruses are called obligate parasites.

Not every virus harms its host all the time, so "pathogenic" and "harmful" are too strong as general descriptions, and viruses give nothing back to the cell, so they are not mutual partners.

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

A vaccine contains a weakened or killed pathogen, or a harmless piece of one. When it enters the body it triggers the immune system to make antibodies and memory cells. If the real pathogen arrives later, the body recognizes it and fights it off quickly.

The vaccine itself does not kill pathogens; that is the work of the immune system it wakes up. It also cannot prevent functional diseases such as diabetes, and it has nothing to do with healing wounds.