The Cell, the Unit of Living Things
In the kitchen you cut an onion and, carefully, peel off the transparent film that covers each layer. If you place it on a slide and look at it under a microscope, you see a kind of mosaic of tiny bricks stuck together. Robert Hooke saw something similar in 1665 when he looked at a piece of cork, and that is why he called those little spaces "cells" (small rooms). What do those little bricks have in common with your own body, with a bacterium, and with an axolotl?
Cell Theory
Take samples of a leaf, of mold on bread, and of your saliva, and observe them under a microscope. In all of them you find tiny units that feed, respond to their surroundings, and divide. What rule do all living things on the planet share?
This theory was not formulated by a single person overnight. It was put together over time with better microscopes: Hooke named cells (1665), Leeuwenhoek was the first to observe microscopic living things, and in the mid-19th century Schleiden and Schwann proposed that plants and animals are made of cells. Later Virchow completed the idea: every cell comes from another cell. It is a good example of science in the making: each generation refined what the previous one knew.
All cells, whether from a bacterium or from a person, share the basics. They have a membrane that separates them from the outside and controls what goes in and out. They have cytoplasm, where their reactions take place. And they keep the hereditary information (in the nucleus, in the cells that have one). Explore an animal cell and tap each part to see its function:
State the three basic ideas of cell theory in your own words.
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.
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…
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?
Levels of Organization
A human body has trillions of cells, but they are not a disorderly heap. How are they grouped so that you can run, think, or digest a taco?
Each level has properties that the previous one does not have on its own: a muscle cell contracts, but only an organ like the heart can pump blood.
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…
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.
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.
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.
Unicellular and Multicellular Organisms
In a drop of water from a puddle, microscopic living things swim around and manage on their own. In contrast, in your skin there are millions of cells stuck together and none of them goes off by itself. How are living things organized according to their number of cells?
In a true multicellular organism the cells are specialized (each type does one task) and interdependent: none survives alone, because each depends on the others to receive food, oxygen, and protection.
Colonies and True Multicellular Organisms
Some algae, such as Volvox, form colonies: a sphere of hundreds of similar cells. If you separate their cells, almost all of them can survive and form new colonies, because they are barely specialized. In sea lettuce, on the other hand, the cells of each tissue already carry out different tasks and depend on one another; on their own, they die. That specialization with interdependence is what distinguishes a true multicellular organism.
And what is the use of being multicellular? It makes it possible to reach a larger size, for each type of cell to devote itself to a single task and do it better, and for the organism to keep functioning if one cell dies.
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…
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?
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…
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.
The Size of Cells
A mouse weighs about 20 g and an elephant weighs over 5,000 kg, but under the microscope the cells of their skin are almost the same tiny size. Why don't big living things have giant cells?
Most cells measure between 10 and 100 µm. A micrometer (µm) is one thousandth of a millimeter: in 1 mm there are 1,000 µm. That is why they cannot be seen with the naked eye. A cell needs to exchange substances (nutrients, oxygen, wastes) with the outside through its membrane. To see how size makes a difference, imagine cube-shaped cells:
| Side of the cube | Surface area (membrane) | Volume (interior) | Surface area ÷ volume |
|---|---|---|---|
| 1 | 6 | 1 | 6 |
| 2 | 24 | 8 | 3 |
| 3 | 54 | 27 | 2 |
| 10 | 600 | 1,000 | 0.6 |
As a cell grows, its volume increases much faster than its surface area. A large cell would have too little membrane to feed and clean so much interior.
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…
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…
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.
Summary
Go back to the beginning: the little bricks of the onion are cells, just like those of your body, those of a bacterium, and those of an axolotl. All living things share that unit.