Cells

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Introduction.

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The cell is considered to be the smallest structure in biology that has all the properties of living things and an understanding of cells and the basics of cell structure and function is critical to making sense out of biology.

This image shows a cell in an aquatic plant, Elodea. The clear strand at the tip of the arrow represents a 'conveyor belt' made of proteins that helps to move some of the cell's parts around the cell.


 
 
 

Basic cell structures.

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basic cellAll cells at their essence have at least three things in common:
  • Cell membrane. All cells have a phospholipid based cell membrane. The cell membrane is selectively permeable in that it allows some materials to pass into or out of the cell but not others.
  • Cytoplasm. Cells are filled with a complex collection of of substances in a water based solution. This substance is called cytoplasm. Across all cells there are a number of common features to all cell cytoplasm. For example all cells have ribosomes. Also, in all cells the first steps in cellular respiration take place in the cytoplasm. 
  • DNA. All cells contain DNA(2). In the simplest cells, the DNA is in one loop more loop like structures free in the cytoplasm. In some cells such as those making up our body the DNA is isolated from the cytoplasm in a special structure called a nucleus. Remember not all cells have a nucleus!
Omnis Cellula e Cellula.

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Literally "all cells from preexisting cells". This saying summarizes what has become called the cell theory. Today when we discuss this theory we understand the theory as having least three parts: 

  • A living things are made up of cells. 
  • All cells come from pre-existing cells. 
  • There is no spontaneous generation under current conditions. 
Today these seem self evident, but this theory actually only dates from the mid 19th century. For example, before the mid 19th century many people believed in spontaneous generation. This is the idea that living things can develop from non-living things. People, for instance, used to believe that flies actually developed from rotten meat or that bacteria developed from stagnant water, or that frogs developed from the mud at the bottom of ponds, or that a horse hair put in water would turn into a worm. Slowly, though scientists cleared up these beliefs until now we accept the notion that under current conditions, life does not arise from non life. 

Cell theory is really important because it provided, and still provides one of the great unifying theories in biology: one that says in spite of all the vast diversity of organisms, they are are united at a very fundamental level, namely the presence of cells. 

Types of Cells.
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In the living world there are two basic types of cells, prokaryote and eukaryote cells. 
 

  • Prokaryotic cells include what we commonly refer to as bacteria. Prokaryotic cells have DNA but it is not isolated from the rest of the cell inside of a nucleus. Instead the DNA is a single loop free in the cytoplasm. In addition prokaryotes often have small loops of DNA called plasmids which can be transtered to other cells. 
  • Eukaryotic cells generally are larger and more complex than prokaryotic cells. Eukaryotic cells have a true nucleus containing the DNA as well as various other membrane bound organelles. Some of these organelles are pretty much universal in eukaryotes. These include mitochondria, rough and smooth ER, the nucleus. Other organelles are restricted to one or more kingdoms. For example chloroplasts are restricted to the Kingdoms Protista and Plantae.  


Prokaryotes typically do not have membrane bound organelles but the cell membrane may, as in photosynthetic bacteria, have intricate infoldings to increase surface area for various chemical processes. In addition many bacteria move using a structure called a bacterial flagellum. This is quite different than the flagella found in eukaryotes in that it has a rotating base like a wheel that supplies torque to the rest of the flagellum. 

These cells are a type of prokaryotic cell called cyanobacteria. 

Many eukaryote cells have flagella and cilia which are hair or whip like organelles that move the cell. In all eukaryotes so far as is known these organelles have a similar structure and unlike the the prokaryote flagellum, the eukaryote flagellum moves by using energy internally rather than by torque transfer from the base. 

Eukaryotes such as plants and fungi will have cell walls in addition to the always present cell membrane. 
 
 

pgd created 6/30/98, revised 06/17/02 
 

Why most cells are small.
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If you think about it for a bit you'll release that most cells are small: in fact too small to be seen by the unaided eye. The average cell in our body is about 50 micrometers(0.05mm) in diameter. Indeed, if you were to average the diameter of all the cells on the planet, the average would certainly be far less than that because most of the cells on this planet are bacteria and the average bacterial cell is 3-5 micrometers in diameter. 

Why most cells are small has to do with simple geometry more than anything else: specifically the relationship between surface area to volume as a cell gets bigger. First, as a cell gets larger, the volume of the cell increases more rapidly than the the surface area if the cell maintains its same shape. Thus following the diagram, imagine a cell that's a cube 1mm on a side. Its volume is 1mm3 and its surface area is 6mm2. But suppose the cell grows to 2mm on a side. Now its volume is 8mm3 and the surface area 24mm2. The volume has increased eightfold but the surface area has increased only four fold. It turns out that in general, the surface area increases in proportion to the square of the width and volume as the cube of the width. See that geometry class did come in useful! 

What does this have to do with the size of cells? Everything that the cell needs or has to get rid of has to go through the cell membrane, the amount of which relates to the surface area. Therefore, the cell's ability to either get substances from the outside or eliminate waste is related to the surface area. Secondly, how much food and other material from the outside and how much waste the cell has to get rid of, is related to the volume. 

Therefore, as a cell gets bigger there will come a time when its surface area is insufficient to meet the demands of the cell's volume and the cell stops growing. 

There are ways to get around this problem. Bird eggs and frog eggs are much larger than typical cells, but they have a store house of food and also rapidly divide to give rize to multicelled embryos. In fact this multicellular embryo is a good illustration of another way cells get around the surface area to volume problem: they divide. In the third part of the diagram I've taken the 2mm width "cell" and divided it's volume into eight 1mm width cells. Notice that the surface area is much higher, giving more surface for obtaining nutrients, gas exchange, etc. 

Another way to get around limitations of surface area is to make the cell long and thin or skinny and flat. Notice when I make the cell thin, even though I keep the volume 8mm3, the cell's surface area becomes 133mm2, a vast increase. This technique is used by many protists as well as certain cells in your body such as nerve cells and muscle cells, both of which are long and skinny. 

One thing this problem illustrates is that living things are shaped by basic mathemetical principles in that the types of adaptations that arise through evolution are constrined in many cases by basic geometry! 

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pgd

Organelles

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Organelles are well defined large scale structures that have a particular set of functions in the cell. Some organelles such as cilia, centrioles, and ribosomes and the membrane infoldings of some prokaryotes are not isolated from the cytoplasm of the cell. But in eukaryotes, many of the organelles are what are called "membrane bound" organelles: that is, organelles completely surrounded by a plasma membrane, or even a double membrane. 

Indeed, the concept of membrane bound organelles is so important that many texts restrict the definition of organelles to mean membrane bound structures within a cell. However, this leaves out important structures in prokaryotes such as the bacterial flagella and membrane infoldings found in bacteria.* Also, people who study bacteria usually consider such non membrane bound structures as the ribosome to be organelles. So here we will not restrict organelles to membrane bound structures. 

Be that as it may, membrane bound organelles are extremely important in the organization of eukaryotic cells. These organelles allow different sets of chemical reactions to be separated from each other so that they do not interfere. Its much like a chemical factory where the different chemicals are kept in separate vats and the different reaction pathways involved in manufacturing compounds are kept isolated from each other. In the compartmentalization of the cytoplasm by membrane bound organelles not only prevents interference between different reaction pathways, but allows the cell to provide radically different environments that allow each reaction to operate most efficiently. 

*If your instructor insists on defining organelles as being membrane bound, humor him or her: the concept is what's important and membrane bound organelles are extremely important from a eukaryote point of view. 

The map of an animal cell describes some of the main organelles found in animal cells. Note that plant cells and some other cells have optional structures not found in animal cells. 

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pgd revised 02/02/00
 
 
 

 pgd Created 6/21/99 revised 06/17/02