Exercises: The cell, the unit of living things

State the three basic ideas of cell theory in your own words.

The pyramid represents the five levels of organisation of living things, from the simplest (base, level 1) to the most complex (apex, level 5), but their names have been rubbed out. Place these terms — organism, cell, organ, tissue, and organ system — in their corresponding level and give an example of each level in the human body.

What is the smallest unit that can be considered alive? Explain why an isolated organelle, such as a mitochondrion, is not a living thing.

A person is made of trillions of cells. Describe the full chain of levels of organisation from a neurone to the organism, naming each intermediate level with an example.

Classify these living things as unicellular or multicellular: a bacterium, an amoeba, a mushroom, an earthworm, and an oak tree. What advantages do you think being multicellular offers?

Almost all cells are microscopic. Explain why it is advantageous for cells to be small, thinking about how they exchange substances with their surroundings across their membrane.

A classmate claims: "For a baby elephant to grow into a 5000 kg adult, its cells must become thousands of times larger than those of a newborn mouse."\na) Refute their claim by explaining why cells cannot grow indefinitely (think about the difficulties a giant cell would face in having nutrients and oxygen reach its entire interior across the membrane).\nb) Explain what cellular mechanism actually allows an organism to increase in size without its cells ceasing to be microscopic.

In a laboratory, the following experiment is carried out: the cells of a colony of the microscopic alga Pandorina are separated individually and, on the other hand, the cells of the tissue of a complex multicellular alga (such as kelp) are also separated. All the isolated cells are placed in containers with water and nutrients.\na) The cells of Pandorina are able to survive and form new colonies on their own, whereas the cells of the complex alga die shortly after. Justify this result by explaining what true multicellular organisms have (cellular specialization and interdependence) that a colony lacks.\nb) From this, deduce whether a colony can be considered a single multicellular living being or a cluster of unicellular beings.

Observe the following historical paradox: in 1665, Robert Hooke was the first to use the word "cell" when looking at a thin slice of cork, but almost two centuries had to pass until the first principle of cell theory was formulated ("all living things are composed of cells").\na) Explain what Hooke actually observed in dead cork and why that image did not allow him to discover the living interior of a cell or to understand that animals were also made of cells.\nb) If today you were given an unknown sample under the microscope and you wanted to prove that it comes from a living organism and not from inert matter, what cellular characteristics or components would you look for to justify it?