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.

With which instrument were cells discovered? Explain why we cannot see them with the naked eye and why their discovery came so late in history.

When you get a small cut on your skin, the wound closes after a few days. Explain, using cell theory, what the cells of the area have done to repair it and what would happen if they could not divide.

Explain in your own words what the phrase "every cell comes from another cell" means and what consequence it has: all present-day living things are linked by a chain of cell divisions stretching far back in time.

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?

In the 19th century, Louis Pasteur designed an experiment to test whether microorganisms arose by "spontaneous generation" or whether Virchow's idea held true ("all cells arise from pre-existing cells"):\n1. He boiled nutrient broth in two flasks to eliminate any existing living cells.\n2. Flask A had an "S"-curved neck (swan neck), open to the air but with a curve where dust accumulated.\n3. Flask B had its neck broken off, leaving it vertically open.\na) Predict what will happen in each flask according to the hypothesis of spontaneous generation and what would happen according to Virchow's principle.\nb) In the actual experiment, only the broth in flask B became full of microbes. Justify how this result proves that microorganisms do not appear out of nowhere and validates cell theory.

The three fundamental principles of cell theory state that the cell is the structural (all living things are made of cells), functional (the cell performs the three vital functions), and origin unit (every cell comes from a pre-existing cell).\na) Analyze the structure and way of life of a virus in light of these three principles: state precisely which ones it fails to meet and justify why.\nb) A classmate claims: "Since viruses have genetic material and can multiply inside us causing diseases, cell theory is wrong in not considering them living beings." Refute their claim by explaining why viruses are considered "obligate molecular parasites" and not independent cells.

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.

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.

Describe the full chain of levels of organisation in an oak tree, from a cell in a leaf to the whole tree, naming each level.

Choose two organ systems of the human body (for example, the digestive system and the respiratory system), name two organs of each, and explain what task each system performs as a whole.

A medical team analyzes four biological samples obtained from the human body and records their observations:\n- Sample 1: An individual myocyte (muscle cell) capable of contracting in response to stimuli.\n- Sample 2: A sheet of cardiac muscle tissue made up of thousands of synchronized myocytes.\n- Sample 3: The complete heart, formed by muscular, connective, and nervous tissue, and blood vessels.\n- Sample 4: The circulatory system as a whole (heart, arteries, veins, and capillaries).\na) Order the four samples from the simplest to the most complex level of organization and explain what new function appears when going from sample 2 to sample 3 that sample 2 cannot perform on its own.\nb) If a toxin were to exclusively destroy the contractile structure of all cardiac myocytes, such as the one in sample 1, predict and justify in a cascade manner how this failure would affect the levels represented by samples 2, 3, and 4, and the survival of the organism.

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.

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.

If you pass a living marine sponge through a fine sieve, its cells separate into a liquid suspension, but after a few hours they recognize each other, reaggregate, and reconstruct a functional sponge. In contrast, if you did the same with the muscle tissue or heart of a fish, the isolated cells would be unable to reconstruct the organ and would die.\na) Justify why this happens by explaining the difference between the cellular organization level of a sponge and that of an animal with true tissues.\nb) Based on this experiment, would you consider it correct to state that the sponge has organs or systems? Support your answer based on the hierarchy of levels of organization.

The yolk of a hen's egg is a single giant cell that can be seen perfectly with the naked eye. Explain why it is an exception to the rule that cells are microscopic and what it contains so that the chick can develop.

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.