• Introduction
• EVOLUTION
• Introduction
• The Theory
• Evidence
• DIVERSITY
• Bacteria
• Fungi
• Protists
• Viruses
• Size
• NUTRITION
• Introduction
• Autotrophy
• Heterotrophy
• REPRODUCTION
• Introduction
• Bacteria
• Fungi
• COMPETITION
• Evidence
• Mechanisms
• 
  
• LINKS
• VLE
• Bio Sciences
• Uni of Sydney

Diversity: Size

While the biology of the groups may differ markedly, one character that they share is their small size. This aspect needs to be considered more closely.

Size has consequences for:

• Metabolic Function
• The Upper Limit
• Surface Area/Volume Ratio
• Movement

Essential Metabolic Functions

All microbes have essential metabolic functions. If the cell is too small, they obtain that function from their host in a parasitic association. The small volume may limit the function of each microbe.

Bacteria smaller than approximately 1 µm are usually found in close symbiotic or parasitic associations with larger cells.

Viruses, the largest of which might be 0.4 µm long, depend entirely on their host for multiplication.

The smallest fungi are Microsporidia. These eukaryotic organisms are less than 1 µm in diameter. They have lost mitochondria and peroxysomes, they lack Golgi and 9+2 ciliary structures. The ribosomes are 70s and their DNA content is less than many bacteria. However, DNA sequences indicate that they are fungi, close to both Ascomycota and Basidiomycota. Most importantly Microsporidia are all absolute parasites. They cause disease in many different organisms. Among the 1,300 species are agents that cause disease in silk worms, algae and plants.

Among the protists are a small group of single-celled heterotrophic parasites.

While some larger microbes are also parasites, and some small microbes are capable of independent living, the underlying mechanisms for selection in this direction will be explored below.

Upper Limit

Most organisms require oxygen. If movement of oxygen is by diffusion, and the oxygen is used as it diffuses through a cell, then the maximum size that a cell might reach is going to be determined by the need for oxygen in each part of the cell: the distance the oxygen has to travel.

Thus larger microbes will probably have a lower rate of respiration, be able to photosynthesise (releases oxygen), or have their essential oxygen requiring activities on the surface of the cell. They may also have an oxygen pump to shift oxygen around the cell, and they may be shaped to enable diffusion of oxygen through the cell. This discussion centres around oxygen. Similar arguments can be made for the removal of waste products.

Question 3

Surface Area/Volume Ratio

All molecules pass through membranes. The rate of movement is determined by the nature of the molecule (charge, size, osmotic potential, etc), the type of wall surrounding the cell membrane, and most importantly, the area of cell membrane in relation to the volume of the contents of the cell.

Small cells have much faster rates of exchange with the environment than larger cells because they have a larger surface area in relation to their volume.

In other words, as the cell size increases, the surface area increases by the square of the radius, and the volume increases by the cube of the radius.

The surface area per unit volume declines as the radius increases. The proportion of the contents that may move through the surface of the cell declines as the cell becomes larger.

Question 4

Question 5

Movement

Because water is lost rapidly through the wall of a microbe, the greatest variety of microbes are found in water. However, in water another problem exists - motion.

Unlike air, liquids cannot be compressed. Deliberate movement of microbes only takes place when the inertia of water can be overcome (drag your hand through water; imagine how difficult it would be for something much smaller). Microbes cannot overcome inertia of water without some form of locomotion. Even then, microbes do not move far, unless within a mass of water. For example, bacteria move through soil because water flows through the air spaces between solid particles.

Locomotion is due to three types of activity:

• Flagella are whip like threads attached to a cell. Some cells have a single flagellum, but others may have two or more. Flagella rotate pushing the cell forward. In effect, the cell moves almost randomly unless the cell is attracted by chemical cues, or light. The directed movement is still highly chaotic. View movie of bacterial motion using a flagellum (308kb).
• Cilia are fine thread-like projections from the surface of the cell. By beating in unison, the cell moves along a surface. View movie of bacterial motion using cilia (596kb).
• The amorphous cell can glide or creep along a surface, using attachment to the surface to direct movement. Some cells 'roll' along the surface, while others have a specific area of the surface that is associated with attachment and movement.