Cell theory and types of cells
Three centuries of microscopic observation boil down to an idea as simple as it is powerful: all living matter is organized into cells. Picture a lab in your high school: a teacher puts a drop of pond water under the microscope, and it is full of tiny swimmers. Where did the first cell come from? Why are there cells that are so different from one another? Where is the boundary between what is alive and what is not?
Cell theory
When you examine a drop of stagnant water, a piece of cork or a blood sample with a lens, you always see microscopic units able to work on their own: what is the minimum structure common to all living matter?
The levels of organization of living matter rank complexity above the cell: the cell is the basic unit; several specialized cells form a tissue; several tissues that work together form an organ; and the set of organs with a general function makes up an organ system, up to the complete organism. Below the cell is the organelle, which is no longer a unit with a life of its own: it needs the whole cell in order to function.
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…
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.
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…
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.
Prokaryotes and eukaryotes
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…
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.
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…
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.…
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.
Compartmentalization
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…
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.
The endosymbiotic theory
A bacterium has its genetic material free in the cytoplasm, but any animal cell encloses its own in a nucleus and also contains organelles with a double membrane and DNA of their own: how did these complex internal compartments arise? (This is a core idea in Unit 2 of AP® Biology.)
The limits of the cell
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…
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…
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.
Back to the beginning: The first cell is thought to have arisen through chemical evolution before any earlier cells existed, eukaryotic diversity was shaped by compartmentalization and endosymbiosis, and viruses mark the boundary because they lack cellular structure and a metabolism of their own.