Cell Theory Prokaryotic

8.1 The Cell — What is the Unit of Life?

When you look at very thin slices of an onion under a microscope you observe a large number of small compartments very similar to the honeycomb. You observe a similar thing in a piece of cork tissue. These small compartments are called cells. The cell is the fundamental structural and functional unit of all living organisms. Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus. The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

8.2 Cell Theory

In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, Theodore Schwann (1839), a British zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the 'plasma membrane'. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.

On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory. However, this theory did not explain as to how new cells were formed.

In 1855, Rudolf Virchow first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understood today is:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells (Virchow).

8.3 An Overview of Cell

You have already learnt about the structure of cell in classes IX and X. Let us re-look at a typical cell. A typical eukaryotic cell consists of a cell membrane enclosing the protoplasm. The protoplasm is differentiated into the cytoplasm and the nucleus. The cytoplasm contains various non-living and living structures called cell inclusions and cell organelles.

The size, shape and activities of cells differ greatly. Mycoplasmas, the smallest cells, are only 0.3 µm in length while bacteria could be 3 to 5 µm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter. Nerve cells are some of the longest cells.

8.4 Prokaryotic Cells

The prokaryotic cells are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells. They may vary greatly in shape and size. The four basic shapes of bacteria are: bacillus (rod-like), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).

Though the prokaryotes have a considerable variation in form and function, all prokaryotic cells have a cell envelope and their cytoplasm is not compartmentalised. Further, while they lack a well-defined nucleus, they possess genomic DNA double-stranded, circular, and packed in a region called the nucleoid. In addition to the genomic DNA, many bacteria have small circular DNA outside the genomic DNA called plasmids. The plasmid DNA confers certain unique phenotypic characters to bacteria like resistance to antibiotics. Genomic DNA is the main DNA of the bacteria, whereas plasmids are used to monitor bacterial transformation with foreign DNA.

8.4.1 Cell Envelope and its Modifications

Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope. The cell envelope consists of a tightly bound three-layered structure i.e., the outermost glycocalyx followed by the cell wall and then the plasma membrane. Although each layer of the envelope performs distinct function, they act together as a single protective unit.

Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram. Bacteria that take up the gram stain are called Gram-positive and the others that do not are called Gram-negative bacteria.

The glycocalyx could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule. The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.

The plasma membrane is selectively permeable in nature and interacts with the outside world. This membrane is similar structurally to that of the eukaryotes. A special membranous structure is the mesosome which is formed by the extensions of the plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content. In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.

8.4.2 Flagella, Pili and Fimbriae

These are surface structures of the bacterial cell but they do not take part in cell division. Pili and fimbriae are surface structures of the bacteria but do not play a role in bacterial movement. Pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria they are known to help attach the bacteria to rocks in streams and also to the host tissues.

Bacterial cells may have flagella. Bacterial flagellum is composed of three parts — filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. Compare the eukaryotic flagellum which is structurally very different.

8.4.3 Ribosomes and Inclusion Bodies

In prokaryotes, ribosomes are associated with the plasma membrane of the cell. They are about 15 nm by 20 nm in size and are made of two subunits — 50S and 30S units which when present together form 70S prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins.

Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

8.5 Eukaryotic Cells

The eukaryotes include all the protists, plants, animals and fungi. In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane-bound organelles. Eukaryotic cells possess an organised nucleus with a nuclear envelope. They have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes.

All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in almost all plant cells (except in lower forms).

8.6 Worked Examples