Cell theory and types of cells
Take a drop of water from Lough Corrib or a swab from the inside of your cheek and look at it under the school microscope: you will see cells. Centuries of microscopic observation boil down to an idea as simple as it is powerful: all living matter is organised into cells. From it arise questions that every biologist needs to answer before going any further: 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?
Cell theory is the basis of the characteristic of life called the cellular basis: every living thing is made of one or more cells. Living things show all eleven characteristics of life: organisation, response, respiration, nutrition, excretion, reproduction, growth, metabolism, homeostasis, heredity and cellular basis. The cell is the smallest unit that can carry out all of them, which is why cell theory is so important. How cells are built up into tissues, organs and whole organisms is covered in the next section.
State the three tenets of cell theory and indicate which of these statements contradicts it, reasoning your choice: a) all living things are made of one or more cells;…
State the three tenets of cell theory and indicate which of these statements contradicts it, reasoning your choice: 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.
Levels of organisation
Your body is made of trillions of cells, yet a muscle cell and a nerve cell look very different: how are cells built up into a whole organism?
Definitions: an organelle is a membrane-bound structure inside a eukaryotic cell with a specific function; a cell is the basic structural and functional unit of life; a tissue is a group of similar cells doing the same job; an organ is several tissues working together; an organism is a complete living individual.
Order the following levels of organisation of living matter from lowest to highest complexity and define each with a sentence: organ, cell, tissue, organelle and…
Order the following levels of organisation of living matter from lowest to highest complexity and define each with a sentence: organ, cell, tissue, organelle and organism.
Prokaryotes and eukaryotes
Compare the two cell types side by side:
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Typical size | small, about 1–10 µm | larger, about 10–100 µm |
| Nucleus | none; DNA in the nucleoid | yes, with a double membrane |
| Membrane-bound organelles | absent | present (mitochondria, ER, Golgi and others) |
| Form of DNA | one circular chromosome, free in the cytoplasm | several linear chromosomes inside the nucleus |
| Ribosomes | 70S, small | 80S in the cytosol, larger |
| Examples | bacteria, archaea | animals, plants, fungi, protists |
Classify the following organisms and cells as prokaryotes or eukaryotes, briefly justifying your choice: a cyanobacterium, an amoeba, a skin cell, yogurt bacterium, a…
Classify the following organisms and cells as prokaryotes or eukaryotes, briefly justifying your choice: a cyanobacterium, an amoeba, a skin cell, yogurt bacterium, a yeast and a green alga.
The diagram compares two types of cellular organisation, 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 organisation, 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 organisation 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.
Membrane-bound organelles
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?
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.
In a cell biology laboratory, the effect of two antibiotics on a culture of animal eukaryotic cells is analyzed: cycloheximide (an inhibitor of translation on 80S…
In a cell biology laboratory, the effect of two antibiotics on a culture of animal eukaryotic cells is analyzed: cycloheximide (an inhibitor of translation on 80S ribosomes) and chloramphenicol (an inhibitor of translation on 70S ribosomes). The results show that cycloheximide stops protein synthesis in the cytosol but not inside the mitochondria, whereas chloramphenicol specifically blocks mitochondrial protein synthesis without affecting cytosolic synthesis. Based on these data and the framework of the endosymbiotic theory:
- a) Justify the results observed with both antibiotics and indicate which plant organelle would show a response analogous to that of the mitochondrion when exposed to chloramphenicol.
- b) A researcher claims that mitochondria are fully autonomous bacteria that could survive and divide indefinitely if isolated in a suitable extracellular medium. Reasonably refute their claim by explaining the evolutionary relationship between the mitochondrial genome and the nuclear genome.
- c) Predict what fundamental differences are to be expected between the inner and outer mitochondrial membranes regarding their lipid composition and the presence of porins, and explain how this asymmetry supports the endosymbiosis mechanism proposed by Lynn Margulis.
- d) Point out which specific ancestral bacterial groups molecular phylogenetic analysis points to as the origin of mitochondria and chloroplasts.
The limits of the cell
Viruses are important economically and medically: they cause diseases such as influenza and HPV-related cancers in humans and damage crops and livestock, but they are also used as vectors in biotechnology and vaccine development.
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: Whether you look at Lough Corrib water or a cheek swab, you see cells, and every one of them came from a pre-existing cell. The first cell is thought to have arisen through chemical evolution before any earlier cells existed, the diversity of eukaryotes is explained in part by compartmentalisation and endosymbiosis, and viruses mark the boundary because they lack cellular structure and a metabolism of their own.