Chapter-8: Cell-The Unit of Life

LIVING CELL

Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life. Hence, cell is the fundamental structural and functional unit of all living organisms.

HISTORICAL ACCOUNT

Aristotle proposed that all animals and plants, however complicated but are constituted of a few elements which are repeated in each of them.

The simple microscope was invented by Galileo.

The first compound microscope was made by Robert Hooke (1665).

He examined thin slices of cork under his microscope and observed the honey comb like structures composed of box like compartments which were termed as the cellulae (cells). 

His work was published in his book "Micrographia". Cells were observed prior to Hooke by Malpighi (1661), who called them saccules and utricles.

Leeuwenhoek observed few living cells capable of moving, such as bacteria. protozoa, spermatozoa and red blood corpuscles under his own designed microscope.

Lamarck remarked that "no living being can have life if its constituent parts are not formed by cells". 
Dutrochet concluded that plants and animals were made up of globular cells and the cells are held together by cohesion. 

In 1831, Robert Brown discovered the presence of nucleus in the cells of orchid root. 
Fontana discovered nucleolus in the skin cell of Eel. 

The term nucleolus was given by Bowman.

Colloidal theory of protoplasm explains the nature of protoplasm in the best manner. It is most acceptable theory.

It was proposed by Fischer, according to which conversion of solution into gel and vice versa is due to colloidal nature of cytoplasm.

Presently, this can be better explained as "Multiphasic colloidal system of life".

Protoplasm theory was proposed by Max Shultze (1861).

According to it "cell is an accumulation of living substances which is limited by an outer membrane & possesses a nucleus".

M.J. Schleiden, a German botanist in 1838 stated that "All plants are formed of one or more cells".

Theodore Schwann, a German Zoologist in 1839 stated that "All animals are formed of cells, have nuclei and are enclosed by thin cell membrane instead of thick cell wall as found in plant cells".

Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and their products.

Rudolf Virchow in 1858 observed that new cells arise from pre-existing cells by division i.e., Omnis cellula e cellula.

CELL THEORY

The theory was formulated by Schleiden and Schwann.

The various points of cell theory are:

1. Each cell is made of a small mass of protoplasm having a nucleus and bounded by a cell membrane with or without cell wall.

2. All cells are basically alike in structure and metabolism.

3. Organisms are composed of cells and their products.

4. The functions of an organism is an outcome of activities and interactions of its constituent cells.

But, it did not explain as to how new cells are formed.

Cell theory was first modified in the light of Virchow's findings that cells develop from pre-existing cells i.e.,"Omnis cellula e cellula". It is known as law of cell lineage. Number of other modifications were carried out in cell theory. The modern cell theory is known as cell principle or cell doctrine.

Drawbacks of Cell Theory

Important drawbacks of cell theory are given below:

1. Viruses cannot be explained using this theory.

2. Bacteria and blue-green algae do not have an organized nucleus.

3. Certain fungi, such as Rhizopus, have hyphae composed of a multi-nucleated mass of cytoplasm (coenocyte).

4. Acetabularia (unicelled, marine green algae) has a uninucleated differentiated body (acellular).

5. Sieve tube and mature RBC lack nuclei.

6. Volume of the cell is occupied by a semi-fluid matrix called cytoplasm, and is main arena of cellular activities in both pro and eukaryota.

7. Cells differ greatly in size, shape and activities e.g., Mycoplasma -the smallest cells (0.3 µm length), Bacteria (3 -5 µm), Human RBC (~7.0 µm)

 

AN OVERVIEW OF THE CELL
A typical cell possesses three major elements — outer envelope, genetic material and cytoplasm.

Outer Envelope: A cell is surrounded by an outer membrane called plasma membrane or plasmalemma. It isolates the cell interior. A distinct cell wall lies on its outer side in plant cells. Cell wall provides protection, rigidity and shape to cells.

Genetic Material: It represents hereditary material that not only controls the functioning of the cell but also contains information for forming the whole organism. Genetic material is DNA. In eukaryotes it is enclosed inside the nucleus as chromatin material. The latter appears as chromosomes during cell division. In prokaryotes, the genetic material lies freely inside the cytoplasm as coiled structure called nucleoid.

Cytoplasm: It is semifluid matrix that occupies the interior of cell between nuclear region and outer envelope. Cytoplasm is the area of major cellular or life activities which keep the cell in living state. Certain functions are associated with special cytoplasmic structures called organelles. Organelles are of three types (i) Membrane less, e.g., ribosomes, centrioles, (ii) Single Membranous, e.g., endoplasmic reticulum, Golgi complex, lysosomes, microbodies, sphaerosomes. (iii) Double Membranous, e.g., mitochondria, plastids (in plant cells).

Size and Shape

Cells differ greatly in size, shape and activities. For example, Mycoplasma, the smallest cell, are only 0.3μm in length while bacteria could be 3 to 5μm. The largest isolated single cell is the egg of an ostrich, Acetabularia, a unicellular green alga is about 10 cm in length.

Cell of alga Caulerpa may be upto one metre. Among multicellular organisms, human red blood cells are about 7.0 μm in diameter, nerve fi bres are the longest, upto 90 cm to few metres.

The upper limit or cell size or cell volume is determined by number of factors like :

(i) Metabolic Activity : Metabolically active cells are small in size while less active ones are large, e.g., sperm (active) and egg (passive).

(ii) Nucleocytoplasmic Ratio: Nucleus controls the metabolic activities of the cytoplasm. A higher nucleocytoplasmic ratio provides more efficient metabolic working. (iii) Surface

Volume Ratio : Active cells possess a higher surface : volume ratio. This occurs in small cells, elongated cells and cells with surface invaginations or ingrowths like microvilli of absorptive 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. e.g., RBCs are biconcave to pass through capillaries and carry O2; WBCs are irregular to perform phagocytosis, nerve cells are long to conduct impulses, sperms have tail for motility etc.

Types of cells

There are two basic types of cells i.e., prokaryotic cells and eukaryotic cells. They are differentiated based on organisation of bio membranes, variety of cytoplasmic organelle and complexity of nuclear material.

Prokaryotic Cells

1. Cell wall present (bacteria) or absent (mycoplasma)

2. A prokaryotic cell is a single membrane system.

3. Cell membrane bears respiratory enzymes.

4. Mesosomes are formed by infolding of cell membrane.

5. Cytoplasm lacks membrane bound organelles

6. Ribosomes (non membrane bound organelle) are70S, lie free in cytoplasm.

7. There are no streaming movements of cytoplasm.

8. Photosynthetic lamellae i.e., thylakoids (if present) occur free in the cytoplasm.

9. Sap vacuoles are lacking. Gas vacuoles may occur.

10. Transcription and translation occur in the cytoplasm.

11. Protein synthesis takes place in cytoplasm only.

12. Cytoskeleton absent.

13. Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid.

14. There is no nucleolus.

15. DNA is closed and circular and without histone core (Polyamines may be present in place of histones)

16. DNA occurs in the cytoplasm only.

17. Plasmids and pili occur in many prokaryotic cells.

18. Flagella, if present, are singlet fibres (9 + 0) and are formed of a protein flagellin.

19. Mitotic spindle is not formed in cell division (Amitotic).

20. Sexual reproduction absent (recombination is present in bacteria)

21. e.g., Bacteria, blue-green algae and mycoplasmas.

Eukaryotic cells

1. Cellulosic cell wall (Plants) or absent (Animals)

2. A eukaryotic cell is a double membrane system.

3. Cell membrane lacks respiratory enzymes.

4. Mesosomes are absent.

5. Cytoplasm contains membrane bound organelles, (endoplasmic reticulum, mitochondria, golgi apparatus, lysosomes and centrosome.

6. Ribosomes are 80 S, may lie free or bound to E.R. and nuclear envelope. (70 S ribosomes are found within mitochondria and chloroplast)

7. Cytoplasm shows streaming movements (cyclosis).

8. Photosynthetic lamellae if present, occur within

9. the chloroplasts.

10. Sap vacuoles are common.

11. Transcription and translation occur in nucleus and cytoplasm respectively.

12. Protein synthesis occurs in the cytoplasm,

13. mitochondria and plastids.

14. Cytoskeleton (microtubule, microfilament and

15. intermediate filaments) present.

16. Nuclear material is enclosed by nuclear envelope to form a nucleus distinct from cytoplasm.

17. One or more nucleoli occur within the nucleus.

18. Nuclear DNA is linear with a histone protein core.

19. DNA occurs in the nucleus as well as in mitochondria and chloroplasts.

20. There are no plasmids and pili in eukaryotic cells.

21. Flagella, if present, are complex, have 9 + 2 pattern of microtubules formed of a protein tubulin.

22.Mitotic spindle is formed in cell division.

23. Sexual reproduction occurs.

e.g., Algae other than blue-green algae, protists, fungi, plants and animals.