Exercises: Cell theory and cell types

State the three tenets of cell theory and indicate which of these statements contradicts it, explaining your reasoning: a) all living things are made of one or more cells; b) some cells can arise by spontaneous generation from non-living matter; c) the cell is the functional unit of living things; d) all cells come from a pre-existing cell.

Classify the following organisms and cells as prokaryotes or eukaryotes, briefly justifying your choice: a cyanobacterium, an amoeba, a skin cell, a yogurt bacterium, a yeast, and a green alga.

Order the following levels of organization of living matter from lowest to highest complexity and define each with a sentence: organ, cell, tissue, organelle and organism.

The diagram compares two types of cellular organization, A and B. State which corresponds to a prokaryotic cell and which to a eukaryotic cell, identify the structures marked with ? and explain the implication for the cell of the difference observed in the organization of its genetic material.

Compare a prokaryotic cell and a eukaryotic cell with regard to their typical size, the presence of a nucleus and membrane-bound organelles, and the form of their DNA. Based on this comparison, explain where cyanobacteria carry out photosynthesis if they lack chloroplasts.

The emergence of membrane-bound organelles was a milestone in the evolution of the cell. Explain what advantages dividing the cell interior into membrane-bound compartments offers, and give two specific examples of processes that benefit from it.

In deep-sea sediments, a microscopic entity ("Particle Z") surrounded by a lipid bilayer has been discovered, containing a 1.2 Mb double-stranded DNA genome with genes for synthesizing several amino acids and its own tRNAs. However, it lacks machinery to synthesize ATP, ribosomes, and the ability to divide autonomously outside its cellular host. A science communication headline claims: "New cell discovered that blurs the boundary between viruses and bacteria".

Refute the journalistic headline by rigorously applying the tenets of cell theory and the biological concept of life. Justify why the presence of metabolic genes does not turn this particle into an independent cell, and argue why this type of biological entity is studied within biology.

A group of students attempts to replicate Pasteur's classic experiment: they boil nutrient broth for 10 minutes in a swan-neck flask and leave it undisturbed. After four days, the broth becomes turbid and they detect bacterial proliferation (spore-forming bacilli of the genus Bacillus). One student concludes: "This proves that Pasteur was wrong and that spontaneous generation does occur if the medium is rich enough."

Refute the student's claim by explaining what methodological error they made when replicating Pasteur's design and why the presence of bacterial spores does not validate spontaneous generation. Finally, analyze the apparent paradox posed by the postulate "all cells arise from pre-existing cells" in relation to current scientific theories on the prebiotic origin of the first living cell.