An Online Introduction to the Biology of Animals and Plants

Key concepts

Section 1

Chapter 4

Microscopes and the Study of Cells 

 
MICROSCOPES
 

 
MICROSCOPES AND THE STUDY OF LIFE 
 

It was soon determined that not only were large organisms made up of huge numbers of tiny cells (multicellular organisms), but tiny organisms existed that were made up of as few as a single cell (unicellular organisms, or single-celled organisms).  And as microscopes and preparation methods improved, it was found that in the unicellular realm, there was a basic either-or difference in organisms:  theirs might be a cell with a nucleus, like cells in multicellular organisms (eukaryotes) or a cell without a nucleus, assumed to be more simple and primitive (prokaryotes) and not found in multicelled forms.

It also became evident that, on the unicellular level, the distinctions between animals and plants were less reliable.  It is not unusual to find unicellular organisms that are mobile and food-gathering like animals but which are also green and photosynthetic.  Were these animals or plants?  Eventually, as we have seen in the multi-Kingdom system, the definition difficulties were addressed by giving the unicellular forms their own Kingdoms, where they could continue to multi-task without upsetting humans and their labeling compulsions.

 
CELL THEORY
 

In 1838, two scientists whose research centered on the microscopic inspection of living things proposed the Cell Theory.  Matthias Schleiden worked with plants and may have come up with most of the hypothesis that Theodor Schwann extended to animals.  The Cell Theory, as it is applied today, says:

- Everything alive is made up of at least one cell.

- Cells can only be made from existing, related cells.

- The smallest single thing that can be considered alive is a cell.

- Cells of all living things are more alike than different.

Everything alive is made up of at least one cell.  The smallest organisms are single-celled, or unicellular.  There is a range of complexity in cells covered later, but a size range in cells also exists, with smaller cells generally being "simpler."  Larger organisms are made up of from several to trillions of cells, and are multicellular.  In these organisms, the cells work together, with different cells doing different jobs to contribute to the function of the whole, each cell being dependent upon the contributions of the others.  As often happens in biology, this is not a situation of either-or;  between unicellular and multicellular are organisms that are unicellular and colonial, with cells that, although they could live independent of each other, they are found banded together and splitting up functions similar to the way it is done in multicellular organisms.

Cells can only be made from existing, related cells.  For a very long time, people believed in spontaneous generation, the idea that many living things could "emerge" from nonliving materials, especially rotting materials.  It fit with what could be observed about the world and was so widely accepted that it was difficult to change.  In fact, some of the earliest developments of what would eventually be Scientific Method were worked out to address this question.  Even after it became accepted that larger organisms do not spontaneously generate, it was accepted that microscopic organisms did.  At the time of Schleiden and Schwann, and even well after, biologists thought that cells could be generated from such nonliving material as boiled chicken broth.  However, the Cell Theory said no, you can only get living cells from cells that are already there.  It was an eventual addition that cells are made from related cells - after germs were discovered, it was thought for some time that our body cells spontaneously generated the bacteria that carried diseases between people, even though bacterial cells are fairly different from ours.  We now know that the bacteria use us as a source of nutrition and as their own little private ecosystems, a place to make lots of offspring bacteria that we are nice enough to pass on to other people.

The smallest single thing that can be considered alive is a cell.  This is a rule that was more absolute back when Schleiden and Schwann first proposed it, since their microscopes limited what they could see below a certain size.  It was almost another hundred years before viruses were discovered, and the bits and pieces inside cells that they could see seemed just that, contributory fragments that would be nothing by themselves.  Today, viruses are controversial because they "break" so many cell rules, and some cell bits are much more "alive" than anyone suspected they would be.

Cells of all living things are more alike than different.  This was not a part of the original theory, although the two researchers seemed to realize over time that what one discovered in plant cells the other was seeing in animal cells.  It was still an era where terms were based more on how things differed than how they were the same, and for over a century any hypotheses that tried to apply rules across the whole of biology were controversial at least partly because of that.

 
CELLS: WHAT ARE THEY?
 

 First of all, don't confuse atoms (or molecules) and cells, which is a common mistake for biology beginners but which is kind of like confusing a grain of sand with a giraffe - they are wildly different sizes (atoms are tiny enough for there to be trillions in a cell) and cells are far more complicated.

There is a range of size in cells, but there seem to be both a lower and upper limit on workable size.  Bacterial cells, which are complex and common but simple compared to our cells, usually range in the 1-5 micrometer-diameter range.  Smaller is rare - one hypothesis is that even a simple cell needs enough room for its important molecules and parts, and the simplest of cells is still pretty complex.  

Size restrictions at the upper limit can be trickier to explain.  One leading hypothesis has to do with the relationship between surface area and volume.  In the case of a round cell, if you double the diameter of the cell, the surface area increases fourfold (the square of 2) but the volume increases eightfold (the cube of two);  as a cell gets bigger, the amount of "guts" it has needing to get materials from outside and producing wastes that must leave the cell increases at a much faster rate than the surface through which both incoming and outgoing materials must move to keep the cell running.   Although there are "tricks" that cells have evolved - for instance, circulation of materials past the surface, or shape changes, like flattening, to increase surface without greatly increasing volume - it still appears that there are limits to how big they can get and still run efficiently.  Very few types of cells are big enough to be seen without a microscope, and even then you'd need really good eyesight.  A structure like a chicken egg might be considered by some as a single cell, but it doesn't really fit the other rules to be a legitimate cell.  Typically, cells "top out" at about 100 micrometers diameter.

CELLS SHARE SOME FEATURES, beyond basic life functions, no matter the type of cell:

They all use the same sort of phosphate-and-lipid barrier, called the cell membrane or plasma membrane or plasmalemma, to separate their insides from their surroundings.  The lipid (fatty / oily) nature of the membrane blocks water and dissolved particles from moving freely through; various proteins stuck into and through the membrane makes the passage of certain materials possible and controllable.

They all need an internal environment where molecules and ions must be free to float around and react, so all cells have a watery plasm, commonly called cytoplasm, with enough stuff dissolved in it that it is thick, almost jellyish.

They share some basic functions that use the same structures.  Since all cells use DNA as a code carrier, it is reasonable that the DNA is similar.  All cells have DNA bound together with proteins in the form of at least one chromosome.  Chromosomes themselves can vary in form and number.

Proteins are critical to cell function - cell chemistry is almost completely dependent upon the workings of various proteins.  DNA carries codes for proteins in genes.  Proteins need to be made using the code.  Ribosomes, an acorn-shaped collection of proteins and RNA, are cell structures where proteins are made, and are found in all cells.  Ribosomes are a type of organelle, internal structures that have specialized functions within a cell.

CELL TYPES
 

There are many ways to split living things into categories;  as mentioned above, the most obvious division is between the prokaryotes (also spelled "procaryotes") and eukaryotes ("eucaryotes").  In general, prokaryotes are supposed to be much more like the very first cells and much simpler than eukaryotes.  Just remember that "simpler" is one of those labels that means little to the organisms;  far more organisms on the planet are prokaryotes than eukaryotes, and the majority of scenarios that could wipe out most eukaryotes would leave many prokaryotes barely bothered.

What makes prokaryotes seem simpler than eukaryotes is the fact that several structures found in eukaryotes are not found in prokaryotes.  The first one that gets mentioned, because it was easily seen even in the early microscopes, is the cell nucleus (don't confuse with an atomic nucleus!), a specialized chamber used as a DNA and RNA processing center in eukaryotes.  Almost all of a eukaryote's chromosomes are "locked away" in the nucleus, behind a double-membraned nuclear envelope through which DNA cannot pass (but RNA, a code carrier, can).

Discussing the nuclear envelope leads to another difference:  eukaryotes contain several types of special membrane-enclosed chambers (membrane-based organelles), barriers, and passageways, while most prokaryotes have nothing like this.  Some prokaryotes may have highly-folded inner membrane structures, but they still should lack what are considered membrane-based organelles.

Another difference is the form of chromosomes:  prokaryotes have single loop chromosomes, a single strand of DNA with all of the basic gene codes on it (they can make little supplement DNA loops, called plasmids for some commonly-used parts of the chromosome, too), while eukaryotes have multiple 2-ended chromosomes, usually in matched pairs.  The processes involved in getting copies properly into offspring cells when a cell divides are different - copies of prokaryote chromosomes are "hooked" to the inside of the cell membrane, one to each side of where the cell will divide, while eukaryote division involves getting the nuclear envelope out of the way, then setting up all of the various chromosomes so the copies can be separated and pulled to opposite sides of the cell before it divides.

 
LANGUAGE AND MODERN MICROSCOPY
 

You may find a lot of terminology in the sciences difficult to remember.  Much of the origins of modern science goes back to a time where all of the various Europeans working in different disciplines agreed that Latin and some related Greek would be their common language for writing and naming things.  This tradition continued into the 19th Century, when the idea of publishing in a dead language lagged (the primary languages of Science, or at least Biology, became German, English and eventually Russian);  the tradition of basing names on Latin faded more gradually, but by the middle of the 20th Century most new terms were being made by scientists in their native languages or something close to it.  Electron microscopy and other sciences, most notably ecology, were advanced mostly in Britain and the U.S. and are therefore full of English or Latin-English hybrid terms that you may find easier to remember, and the more advanced / newer (in microscopy, the smaller structures) terms have a greater-and-greater English component.  This means that in many ways learning the basics may be much tougher than learning the advanced concepts.  Maybe.

   Some additional informational links:

A Page of links demonstrating visually some aspects of microscopy.
 

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

Resolution of magnifying devices
Compound microscopes
Marcello Malpighi (1660)
Robert Hooke (1665)
Antony van Leeuwenhoek
Simple microscope
Light microscope
Electron microscope
Transmission microscope
Staining
Sectioning
Scanning microscope
Multicellular vs Unicellular
Prokaryote vs Eukaryote
Cell Theory (1838)
Colonial organisms
Size limits on cells
Cell membrane
Cytoplasm
DNA
Chromosomes
Ribosomes
Nucleus
Nuclear envelope
Eukaryote / prokaryote differences 
Plasmid 

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