Fundamentals of Microbiology

Lesson 10. SIZE, SHAPE AND ARRANGEMENT OF PROKARYOTIC CELLS

Module 4. Structure and functions of prokaryotic cells

Lesson 10

SIZE, SHAPE AND ARRANGEMENT OF PROKARYOTIC CELLS

10.1 Prokaryotic Cell

Antony van Leeuwenhoek invented the microscope in the late 1600s, which first showed that all living things are composed of cells. Also, he was the first to see microorganisms. Light microscopes have a limited resolution; magnification of more than about 2000-fold does not improve what one can see. Electron microscopes use electrons instead of light. The short wavelength of electrons allows magnifications much better than visible light. Prokaryotes are much more diverse in both habitat and metabolism than the eukaryotes. However, prokaryotes are not very diverse in body shape or size. Much of their classification into different species is done by examining their internal biochemistry and their DNA. Nearly all prokaryotes are single-celled. Differentiation into different cell types almost never occurs in prokaryotes.

Prokaryotes are simple cells. The DNA is loose in the cytoplasm; there is no separate nucleus. The ribosomes are also in the cytoplasm. In prokaryotes, transcription (synthesis of RNA) and translation (synthesis of proteins) occurs simultaneously. The cell is surrounded by a membrane, but there are no internal membranes. Outside the membrane is a cell wall and sometimes an outer capsule which can have structures projecting form it. Bacteria move using flagella; whip-like hairs similar to the flagellum of a sperm cell.

10.2 Shapes of Bacteria

Prokaryotes display following shapes (Fig. 10.1)

• Spherical, rod like and spiral but variation abound
• Spherical cells called cocci (Greek = berry)
• Cylinders called rods or bacilli (Latin bacillus = walking stick)
• Coma shaped: Vibrio
• Spiral: Spirilli (Greek sprillum = little coil)
• Corkscrew shaped: Spirochete
• Others: Star, Square (discovered on the shores of Red Sea in 1981; 2-4 μm on a side and sometimes aggregated in waffle like sheets)

Bacterial shapes, depending on the organism, can change subtly when cells are growing or existing under different conditions, e.g. a shortening of rods as nutrient concentrations are used up and therefore as growth rates decline. Most bacteria are monomorphic - they maintain the same shape. Nevertheless, some bacteria do not display a constant shape even during growth in an otherwise unchanging, homogeneous environment. Such bacteria are termed pleomorphic to indicate that they do not possess a relatively constant standard shape even under relatively constant, standard conditions shape (e.g. Rhizobium and Corynebacterium).

10.3 Arrangement

Cells are found in distinctive arrangements of group of cells; Occur when cell divide without separating. Cocci can divide in one or more planes leading to different arrangements of cells as follows (Fig. 10.2)

• Division in one plane in pairs (Diplococci) or in chains (Streptococci)
• Division in two planes: Tetrads (4 cells in a cube)
• Division in 3 planes: Sarcinae (8 cells arranged in a cube)
• Random division: Grape like clusters (Staphylococci)

Fig. 10.2 Arrangement of cocci

However, bacilli divide in one plane (Exception: Bacilli arranged in a rosette attached by stalks to a substrate e.g. Caulobacter and spiral not generally grouped together). Like cocci these can be termed as diplo and streptobacilli (Fig. 10.3).

Fig. 10.3 Arrangement of rod shaped bacteria

10.4 Size of Prokaryotic Cells

Prokaryotes are smallest of all living cells (1-5 μm long and 1-2 μm in dia). Some spiral has much larger ‘diameter’ and some cyanobacteria are 60 μm long. The dot of typed ‘i’ can accommodate about 500 bacteria of 1 μm size if placed from end to end. The largest bacterium isolated from sediments of the coast of Namibia (1 mm in dia) Thiomargarita nambiensis, visible to naked eyes, in 1998.

Small is better: Average bacteria 0.5 - 2.0 µm in diameter with a surface area ~12 µm2 and volume (Vol) ~4 µm. Thus surface area (SA) to volume is 3:1 and can reach to 6:1 in case of smaller bacteria (Fig. 10.4). On the other hand typical eukaryote cell SA/Vol is 0.3:1 as eukaroytes need structures and organelles.

Fig. 10.4 Smaller the size; larger the surface area/volume ratio

Large surface area/volume ratio in tiny prokaryotic cells gives a number of advantages as follows:

• Cocci: More resistant to drying
• Rods: More surface area and easily takes in dilute nutrients from the environment
• Spiral: Corkscrew motion and therefore less resistant to movement
• Square: Assists in dealing with extreme salinities