Lymphokines are protein substances produced in small amounts by lymphocytes and move from cell to cell carrying message through the immune system. The most common lymphokine is:

Correct Answer: D

Rationale: Rationale for Correct Answer (D - Interferon): 1. Lymphokines are protein substances produced by lymphocytes. 2. Interferon is a type of lymphokine produced by lymphocytes during viral infections. 3. It helps in regulating the immune response and inhibiting viral replication. 4. Interferon moves from cell to cell carrying messages through the immune system. Summary of Incorrect Choices: A: Vaccine - Not a lymphokine, but a substance used to stimulate the immune system. B: Antigen - Not a lymphokine, but a substance that triggers an immune response. C: Antibody - Not a lymphokine, but a protein produced by the immune system to neutralize pathogens.

Correct Answer: C

Rationale: The correct answer is C: More autoimmune disorders. As individuals age, the immune system weakens, leading to an increased risk of autoimmune disorders where the immune system mistakenly attacks healthy cells. This is more likely in older adults compared to younger adults. High fevers (choice A) are not more likely in older adults, as fever response tends to decrease with age. Fewer infections (choice B) are also less likely in older adults due to a weakened immune response. Greater antibody production (choice D) is not a characteristic of aging, as antibody production may decline with age.

Correct Answer: C

Rationale: The correct answer is C: chloroplasts. Cyanobacteria do not have chloroplasts because they are prokaryotic organisms and do not possess membrane-bound organelles like chloroplasts found in eukaryotic cells. Instead, they perform photosynthesis using thylakoids, which are membrane-bound compartments where photosynthesis occurs. Gas vesicles are also present in cyanobacteria and help regulate their buoyancy. Heterocysts are specialized cells that aid in nitrogen fixation, a process that allows cyanobacteria to convert atmospheric nitrogen into a form that can be used by the organism. Therefore, the absence of chloroplasts in cyanobacteria makes choice C the correct answer.

Correct Answer: A

Rationale: The correct answer is A: liquid water. Water is essential for all cellular functions, including metabolism and nutrient transport. Without water, microorganisms cannot grow or survive. Organic substances (choice B) are important as a nutrient source, but water is indispensable for growth. While oxygen (choice C) and warm temperatures (choice D) can enhance growth for some microorganisms, they are not universally required for all microorganisms to grow.

Correct Answer: B

Rationale: The correct answer is B: carotenoids. Carotenoids are photosynthetic pigments that are found in all prokaryotic and eukaryotic photoautotrophs. They play a key role in capturing light energy for photosynthesis. Chlorophyll c, phycobilins, and phycocyanin are not found in all prokaryotic and eukaryotic photoautotrophs. Chlorophyll c is primarily found in certain algae, phycobilins are specific to cyanobacteria and red algae, and phycocyanin is a type of phycobilin found in cyanobacteria. Carotenoids are the only pigment that is universally present across all photoautotrophs.