Biology - Molecules and Cells


 Terms and Concepts 



CHAPTER 2 - Introduction to Cells




First of all, don't confuse atoms and cells, which is a common mistake 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 by 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 to move around and interact in, and the simplest of cells is still pretty complex.  Viruses are much smaller than bacteria, but they're not cellular - almost no chemistry occurs inside that tiny transport package.

Relative sizes (use the bar under the picture).

Size restrictions at the upper limit can be trickier to explain.  One leading hypothesis, first discussed in Section 2, 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, the parts that need to get materials in from outside and which produce often-toxic wastes that must leave the cell, increases at a much faster rate than the surface through which both incoming and outgoing materials must move.   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 youd need really good eyesight.  A structure like a chicken egg might be considered by some as a single cell, but it doesnt really fit the rules to be a legitimate cell.  Typically, cells "top out" at about 100 micrometers diameter.

Page on the relationship between surface area and volume.

Various shapes give varied surface area for similar volumes.

And, because biology is full of exceptions to rules:  giant amebas!

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 most water and solutes from moving freely through; various proteins stuck into and through the membrane makes the passage of certain materials possible and controllable.  The exact workings will be covered in more detail in a later section.

Labels vary:
Cell membrane.

Plasma membrane.

Plasmalemma (typoed).

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 solutes that it is thick, almost jellyish.

Cytoplasm can have lots of "stuff" in it.

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.

Intro to DNA.

Getting lots of DNA into a small place.

Proteins are critical to cell function.  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.

Intro to ribosomes (video).




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;  by far more organisms on the planet are prokaryotes than eukaryotes, and most scenarios that could wipe out most eukaryotes would leave many prokaryotes unscathed.

Introduction to prokaryotes (bacteria).

Introduction to eukaryotes.

What makes prokaryotes seem simpler that 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 (dont confuse with an atomic nucleus!), a specialized chamber used as a DNA and RNA processing center in eukaryotes.  Almost all of a eukaryotes chromosomes are "locked away" in the nucleus, behind a double-membraned nuclear envelope through which DNA cannot pass (but RNA can).

Introduction to the nucleus.

Image of a nucleus.

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 do not.  Some prokaryotes may have highly-folded inner membrane structures, but they still lack what are considered membrane-based organelles.  However, recent work has shown that many prokaryotes contain microcompartments, chambers made of proteins (in some ways similar to a lot of viruses) that can change the flow of materials in and out.

More on microcompartments.

And some more.

Intro to 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, with just a few genes on them, plasmids, for some 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 with protein cables all of the various chromosomes so the copies can be separated and pulled to opposite sides of the cell before it divides.

Prokaryote chromosome image.


Breakdown of eukaryote chromosome structure (image).


Terms and Concepts

In the order they were covered.

   Cell size restrictions





General Biology 2 - Molecules and Cells

Copyright 2013 - 2019, Michael McDarby.

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