Contents
Immunology is one of the Biology Topics focused on understanding the immune system and its response to pathogens and diseases.
How is the Division of Labour done in the Cells of Living Organisms?
If you closely study the Figure, you will notice that an organism such as a human being can have cells of different kinds, e.g., sperm, leucocyte (white blood cell), osteocyte (bone cell), muscle cell, nerve cell, fat cell, etc. This is due to the fact that there is a division of labour within multicellular organisms, e.g., human beings. This means that different parts of the human body perform different functions.
The human body has a heart to pump blood, a stomach to digest food, skeletal muscles to perform movement and locomotion, and so on. The heart has a special type of muscle cells called cardiac muscles which contract rapidly, rhythmically, and tirelessly; they never fatigue during the lifetime of an organism. The stomach has special cells such as mucous cells to secrete mucus for lubricating the food, zymogen cells (or Chief cells) to secrete a proenzyme of protein digestive enzyme, the pepsin, called pepsinogen, parietal cells or oxyntic cells to secrete hydrochloric acid (HCl) activating pepsinogen into functional pepsin and also for killing germs of food. Skeletal muscles are striated and voluntary muscles, i.e., their contraction depends on your will or control. Due to this property of skeletal muscle cells, you are able to move your hands and ten fingers in desired ways.
Like the human body, the cell itself has got a division of labour. In fact, each cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, e.g., making of new material in the cell such as protein synthesis by ribosomes, food (glucose/starch) synthesis by chloroplasts, clearing up the waste substances from the cell by the lysosomes, etc. Thus, a cell is able to live and perform its functions because of these organelles. These organelles together constitute the basic building blocks called cells. Quite interestingly, all cells are designed to have the same basic structure, no matter what their function is or what organism they are found in.
Differences between Organs and Organelles
Organs | Organelles |
1. They are found in multicellular organisms. | 1. They are found in all eukaryotic cells. |
2. They are large-sized or macroscopic. | 2. They are very small-sized, either microscopic or submicroscopic. |
3. They may be external or internal to the body of organisms. | 3. They are mostly internal (i.e., intracellular). |
4. The organs are formed of tissues, tissues comprise of cells and cells are formed of organelles. | 4. An organelle is made up of micromolecules and macromolecules. |
5. Organs coordinate to form organ systems, while organ systems form the body of an organism. | 5. Organelles coordinate to produce the cell. |
Cell Shape
The basic shape of the eukaryotic cell is spherical, but the shape of the cell is ultimately determined by the function of the cell. Thus, the shape of the cell may be variable (i.e., frequently changing its shape) or fixed. Variable or irregular shape occurs in Amoeba and white blood cells or leucocytes. In fact, leucocytes are spherical in circulating blood, but in other contractile vacuole water globules conditions they may produce pseudopodia and become irregular in shape. The fixed shape of cells occurs in most plants and animals (including Euglena and Paramecium).
In unicellular organisms, the cell shape is maintained by a tough plasma membrane (e.g., Paramecium) and exoskeleton (e.g., Elphidium or Polystomella). In multicellular organisms, the shape of a cell depends mainly on its functional adaptations and partly on the surface tension, the viscosity of the protoplasm, the mechanical action exerted by adjoining cells, and the rigidity of the cell membrane (e.g., the presence of rigid cell wall in plant cells). Thus, cells may have diverse shapes such as polyhedral (with 8, 12, or 14 sides), spherical (e.g., eggs of many animals), spindle-shaped (e.g., smooth muscle fibre), elongated (e.g., nerve cells), branched (e.g., chromatophores or pigment cells of the skin), discoidal (e.g., erythrocytes or red blood cells) and so on.
Cell Size and Scale
The size of different cells ranges between broad limits. Some plant and animal cells are visible to the naked eye. Most cells, however, are visible only with a microscope, since they are only a few micrometers in diameter. A micrometer (pm) is one-thousandth of a millimeter. The size of cells varies from the very small cells of bacteria (0.2 to 5.0 pm) to the very large eggs of the ostrich (18 cm) (Among the multicellular plants, the largest cell is the ovule of Cycas) Some nerve cells of human beings have a meter-long ‘tail’ or axon. The single marine alga, Acetabularia, measures nearly 10 cm in height. The fiber cells (i.e., sclerenchyma cells) of Manila hemp similarly, are over 100 cm in length.
The prokaryotic cells usually range between 1 to 10 µm. The eukaryotic cells are typically larger (mostly ranging between 10 to 100 µm). The size of unicellular organisms is larger than a typical cell of multicellular organisms. For example, Amoeba proteus is the biggest among the unicellular organisms; its length is 60 pm. The size of typical cells of multicellular organisms ranges between 20 to 30 pm. The smallest cells are those of Mycoplasma gallisepticum, an organism intermediate between virus and bacteria. Their size is about 0.1 µm.
Measurement of Cells
- Millimetre (mm) 1 mm = 1000 µ
- Micrometre (mm) or micron (µ)*
- Nanometre (nm) or millimicron (mm) 1µ = 1000 mµ
- Angstrom (A°): 1A° = 10-1 mµ = 10-4 µ = 10-7 mm
*Micron (µ) is a unit of length in the CGS system, equal to one-millionth of a metre. In SI units it is replaced by the micrometer (µm).
Cell Volume
The volume of a cell is fairly constant for a particular cell type and is independent of the size of an organism. For example, kidney or liver cells are approximately the same size in a bull, horse, and mouse. The difference in the total mass of the organ or organism depends on the number, not on the volume of the cells. Thus, the cells of an elephant are not necessarily larger than those of other tiny animals or plants. The large size of an elephant is due to the larger number of cells present in its body.
Cell Number
The number of cells in most multicellular organisms is indefinite, but the number of cells may be fixed in some multicellular organisms such as rotifers and nematodes. In the nematodes (e.g., Ascaris), the number of cells in various organs is fixed and it is attained by the time hatching takes place. Most growth in size of a nematode results from an increase in cell size. The phenomenon of having a constant and genetically fixed number of cells is called eutely. In eutectic animals, mitosis stops following embryonic development.
The number of cells in multicellular organisms usually is correlated with the size of an organism. Thus, small-sized organism has less number of cells in comparison to large-sized organisms. For example, large-sized organisms such as elephants, whales, camels, neem trees, or banyan trees have countless numbers of cells. In human beings, the number of cells is estimated to be about 100 trillion (1014). [Note: Trillion is a cardinal number. In the numeration system of the USA in a trillion 1 is followed by 12 zeros.]