What is a Cell?
- The cell is a basic unit of life as no living organism can have life without being cellular because the cell is a unit of both its structure and function.
- All life begins as a single cell. A number of organisms are made of single cells.
- They are called unicellular or acellular, e.g. Amoeba, Chlamydomonas, Acetabularia, bacteria, yeast.
- A single cell is:
(i) capable of independent existence and
(ii) able to perform all the essential functions of life.
- Anything less than a complete cell can neither lead an independent existence nor perform all the functions of life.
- A multicellular organism is made of many cells.
- A higher animal or plant contains billions of cells. For example, a newly born human infant has 2 x 1012 cells.
- The number increases to 100 trillion (100 x 1012 or 1014) cells in the body of a 60 kg human being. About 25% (25 x 1012) of them are found in the blood.
- A drop of blood contains several million cells. Large-sized organisms do not have large-sized cells. Instead, they possess a higher number of cells.
- In multicellular organisms, cells are building blocks of the body or basic units of body structure. Of course, they become specialized for performing different functions.
- The human body has some 200 types of cells, e.g., erythrocytes, leucocyte types, epithelial cell types, muscle cells, nerve cells, fat cells, cartilage cells, bone cells, connective tissue cells, gland cells, germinal cells, pigment cells, etc.
Cells are grouped into tissues, tissues into organs and organs into organ systems, Occurrence of different types of tissues, organs and organ systems results in the division of labour or performance of different functions of the body by specialized structures.
An Overview of Cell
- The cells of the human cheek have an outer membrane as the delimiting structure of the cell.
- Inside each cell is a dense membrane-bound structure called the nucleus.
- This nucleus contains the chromosomes which in turn contain the genetic material, DNA.
- Cells that have membrane-bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic.
- In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell.
- The cytoplasm is the main arena of cellular activities in both plant and animal cells.
- Various chemical reactions occur in it to keep the cell in the ‘living state’.
- Besides the nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the endoplasmic reticulum (ER), the Golgi complex, lysosomes, mitochondria, microbodies and vacuoles.
- The prokaryotic cells lack such membrane-bound organelles. Ribosomes are non-membrane-bound organelles found in all cells – both eukaryotic as well as prokaryotic.
- Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER.
- Animal cells contain another non-membrane-bound organelle called centriole which helps in cell division.
- For example, 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. Cells also vary greatly in their shape.
- They may be disc-like, polygonal, columnar, cuboid, thread-like, or even irregular.
- The shape of the cell may vary with the function they perform.
The Discovery of Cell
- The first microscope was built by Zacharias Janssen in 1590. It was first modified by Galileo (1610) and then by Robert Hooke.
- Robert Hooke (1635-1703) was a mathematician and physicist. He developed a new microscope with which he studied the internal structure of a number of plants. His work is famous for the study of cork cells.
- In 1665, Robert Hooke wrote a book "Micrographia: or Some Physiological Descriptions of Minutae made by magnifying glasses with observations and enquiries thereupon."
- The chapter which gave birth to cell biology is "Observe XVIII of the schematism or texture of cork and of the cells and pores of some other such frothy bodies".
- He took a piece of cork of Spanish oak and prepared a thin slice by means of a sharp penknife.
- A deep planoconcave lens was used for throwing light on the cork piece. The latter was observed under the microscope.
- The piece of cork was found to have a honeycomb structure with a number of box-like compartments, each having a pore and separated from others by diaphragms Robert Hooke named the compartments as cellulae (singular-cellula) now known as cells (Latin cella hollow spaces or compartments).
- He did not know the significance of these structures and regarded them as passages for conducting fluids.
- Actually, the 'cells' of Hooke were cell walls enclosing spaces left by dead protoplasts. Robert Hooke found that the cells or boxes were not very deep.
- A cubic inch contained 1259,712,000 cells, a square inch 1,66,400 and one inch strip 1080 cells.
- The term "cell" is actually a misnomer as a living cell is neither hollow nor always covered by a wall.
The microscope used by Robert Hooke
- In 1838, Malthias 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 walls 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. This theory, however, did not explain how new cells were formed.
- Rudolf Virchow (1855) first explained that cells are 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.
Fundamental Features of Cell Theory
Five fundamental observations of the cell theory are:
1. All living organisms are composed of cells and their products.
2. Each cell is made of a small mass of protoplasm containing a nucleus in its inside
and a plasma membrane with or without a cell wall on its outside.
3. All cells are basically alike in their chemistry and physiology.
4. Activities of an organism are the sum total of activities and interactions of its
Modern Cell Theory
- It is also known as cell doctrine or cell principle. Modern cell theory states that The bodies of all living beings are made up of cells and their products
- Cells are units of a structure in the body of living organisms. Every cell is made up of a mass of protoplasm, having a nucleus, organelles and a covering membrane.
- Cells are units of function in living organisms, that is, the activities of an organism are the sum total of the activities of its cells.
- While a cell can survive independently, its organelles cannot do so.
- The cells belonging to diverse organisms and different regions of the same organism have a fundamental similarity in their structure, chemical composition and metabolism.
- Life exists only in cells because all the activities of life are performed by cells.
- Depending upon the specific requirements, the cells get modified, e.g., elongated in muscle and nerve cells, loss of nucleus in RBCs or cytoplasm in outer skin cells.
- The growth of an organism involves the growth and multiplication of its cells.
- Genetic information is stored and expressed inside cells.
- Life passes from one generation to the next in the form of a living cell.
- New cells arise from pre-existing cells through division. All new cells contain the same amount and degree of genetic information as contained in the parent cell.
- All the present-day cells/organisms have a common ancestry because they are derived from the first cell that evolved on the planet through a continuous line of cell generations.
- Basically, the cells are totipotent (i.e., a single cell can give rise to the whole organism) unless and until they have become extremely specialized.
- No organism, organ or tissue can have an activity that is absent in its cells.
Question for Introduction to Cell & Cell Theory
Try yourself: What is the main difference between eukaryotic and prokaryotic cells?
Significance of Cell Theory
(i) There is a structural similarity in cells belonging to diverse groups of organisms.
(ii) All the cells perform similar metabolic activities.
(iii) Life exists only in the form of cells.
(iv) Life passes from one generation to the next as cells.
(v) All living beings are descendants of a primitive cell that developed on earth as the first eucaryote and prior to that as the first procaryote.
Shapes of Cells
- The cells vary in their shapes.
- They may be disc-like, polygonal, columnar, cuboid, amoeboid, thread-like or irregular.
- The shape of the cell is related to its position (flat in surface cells, polygonal in the cortex) and function (e.g., RBCs are biconcave to pass through capillaries and carry WBCs are irregular to do phagocytosis, nerve cells are long to conduct impulses, sperms have a tail for motility etc.
- On the basis of the organization of DNA, the cells are of two types-procaryotic and eucaryotic.
- The organisms having procaryotic cells are called prokaryotes.
- They are nowadays placed in a superkingdom called Procaryota.
- Other organisms (having eukaryotic cells) are included in superkingdom Eukaryota.
- Procaryotic cells occur in bacteria, blue-green algae, chlamydiae, Archaebacteria and Mycoplasma or PPLO.
Different shapes of cells found in the human body