The cell, the unit of living things

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

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

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?

Explain what a tissue is and give two examples from the human body, stating what their cells do.

What is an organ? Name three organs of the human body and briefly explain what function each one performs.

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.

Explain the difference between a unicellular and a multicellular living thing, and give an example of each type.

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?

Your body has highly specialised cells: the neurone transmits signals, the red blood cell carries oxygen, and the muscle cell contracts. Explain what it means that cells "share out the work" in a multicellular living thing and what advantage this brings.

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.

The sciatic nerve of a giraffe contains axons of individual neurons that can measure more than 1,5 m in length. Given this fact, a student claims: "If it measures a meter and a half, that neuron is no longer microscopic and we could see it with the naked eye on a table as if it were a sewing thread."\na) Knowing that the thickness of that axon is barely 0,01 mm (10 micrometres), refute the student's mistake: explain why it is still a microscopic cell despite its great length.\nb) Compare that elongated neuron with an imaginary spherical cell that had exactly the same volume: which of the two will have a larger surface area to exchange substances with its environment? Justify why the elongated shape is advantageous for such an extensive cell.