In this essay we will discuss about the cell. After reading this essay you will learn about: 1. Definition of Cell 2. Discovery of Cell 3. Cell Theory 4. Modern Cell Theory 5. Limit of Cell Size or Volume 6. Types 7. Compartmentalization for Cellular Life 8. Cell— An Open System 9. Shapes 10. Functions.
- Essay on the Definition of Cell
- Essay on the Discovery of Cell
- Essay on the Cell Theory
- Essay on the Modern Cell Theory
- Essay on the Limit of Cell Size or Volume
- Essay on the Types of Cells
- Essay on Compartmentalization for Cellular Life
- Essay on Cell— An Open System
- Essay on the Shapes of Cells
- Essay on the Functions of Cell Parts
Essay # 1. Definition of Cell:
Cell is a basic unit of life as no living organism can have life without being cellular because 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.
Here 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 cans 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 60 kg human being. About 25% (25 x 1012) of them are found in the blood. A drop of blood contains several million cells. The large sized organisms do not have large sized cells. Instead they possess 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. 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 system results in division of labour or performance of different functions of the body by specialised structures.
Cells are not only the building blocks of the organisms, they are also the functional units of life. Life passes from one generation to the next in the form of cells.
The activities of an organism are actually the sum total of activities of its cells. Each cell of the body possesses the same genetic information though mature cells may become specialized to perform specific functions. A new cell always develops by division of a pre-existing cell.
Cells are totipotent, i.e., a single cell has the ability to form the whole organism. Internally each cell is build up of several organelles. The organelles perform different functions just like the ones carried on by different organ systems of the body.
All life activities of the organism are present in miniature form in each and every cell of its body. Thus, cell is a basic unit of life and structural and functional unit of an organism. It is the smallest unit capable of independent existence and performing the essential functions of life.
Essay # 2. Discovery of Cell:
Work on the study of cell has continued for more than the last three and a half centuries. It required microscopes or instruments with good resolving power and magnification. Techniques like preservation, sectioning, staining and mounting were needed to distinguish various cellular components. Improvement in tools and techniques has continued all this period to enhance our knowledge about the cell.
The first microscope was built by Zacharias Janssen in 1590. It was first modified by Galileo (1610) and then by Robert Hooke (Fig. 8.1). 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 there upon. He took a piece of cork of spanish oak and prepared thin slice by means of sharp pen knife. A deep planoconcave lens was used for throwing light on cork piece. The latter was observed under the microscope.
The piece of cork was found to have a honey comb structure with a number of boxes like compartments, each having a pore and separated from others by diaphragms (Fig. 8.2). 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.
Cells were also observed prior to Hooke, by Malpighi (1661), who called them saccules and utricles. Leeuwenhoek (1673) was first to observe, describe and sketch a free living cell. He observed bacteria, protozoa, spermatozoa, red blood cells, etc. In the beginning of nineteenth century it became clear that the bodies of organisms are made of one or more cells.
Robert Brown (1831) discovered the presence of nucleus in the cells of orchid root. Living semifluid substance of cells was discovered by Dujardin (1835) and named sarcode. Schleiden (1838) found all plant cells to have similar structure— cell wall, a clear jelly-like substance and a nucleus.
Schwann (1838) discovered that animal cells lacked cell wall. Purkinje (1839) and von Mohl (1838, 1846) renamed sarcode or the jelly like substance of the cells as protoplasm (Gk. protos- first, plasma- form).
Cell membrane was discovered by Schwann (1838) but was provided with a name by Nageli and Cramer (1855). Soon various organelles were discovered inside the cells. Electron microscope has elaborated our knowledge about cells.
Essay # 3. Cell Theory:
The theory was jointly put forward by Schleiden and Schwann (1839) in their paper “Microscope Investigations on the similarity of structure and growth in animals and plants.” Cell theory states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organisms.
Formulation of Cell Theory:
Development of cell theory illustrates how scientific methodology operates. It involves observation, hypothesis, formulation of theory and its modification.
Observations were started by Malthias Schleiden (1838), a German botanist who examined a large number of plant tissues. He found that all plant tissues were made of one or the other kind of cells. Therefore, he concluded that cells constitute the ultimate units of all plant tissues.
Theodore Schwann (1838), a German Zoologist, studied different types of animal tissues including development of embryos. He found that animal cells lack a cell wall.
Instead they are covered by a membrane. Otherwise cells of both plants and animals are similar. Schwann defined a cell as membrane en-locked, nucleus containing structure. He also proposed a cell hypothesis — bodies of animals and plants are made of cells and their products.
Schneider and Schwann compared their findings, discussed Schwann’s hypothesis and formulated the cell theory in their joint paper in 1839. The theory proposed that cells are the units of both structure and function of organisms.
Rudolf Virchow (1855) observed that hew cells develop by division of the pre-existing cells— Omnis cellula e cellula (theory of cell lineage or common ancestry). The finding gave cell theory its final shape. Louis Pasteur (1862) further proved that life originated from life. Soon Haeckel (1866) established that nucleus stores and transmits hereditary traits. Cell theory was modified accordingly.
Fundamental Features of Cell Theory:
Five fundamental observations of the cell theory are:
i. All living organisms are composed of cells and their products.
ii. 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.
iii. All cells are basically alike in their chemistry and physiology.
iv. Activities of an organism are the sum total of activities and interactions of its constituent cells.
Essay # 4. Modern Cell Theory:
It is also known as cell doctrine or cell principle.
Modem cell theory states that:
i. The bodies of all living beings are made up of cells and their products.
ii. Cells are units of 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.
iii. 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.
iv. While a cell can survive independently, its organelles cannot do so.
v. The cells belonging to diverse organisms and different regions of the same organism have a fundamental similarity in their structure, chemical composition and metabolism.
vi. Life exists only in cells because all the activities of life are performed by cells.
vii. Depending upon specific requirement, the cells get modified, e.g. elongated in muscle and nerve cells, loss of nucleus in RBCs or cytoplasm in outer skin cells.
viii. Growth of an organism involves the growth and multiplication of its cells.
ix. Genetic information is stored and expressed inside cells.
x. Life passes from one generation to the next in the form of a living cell.
xi. 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.
xii. All the present day cells/organisms have a common ancestry because they are derived from the first cell that evolved on the planet through continuous line of cell generations.
xiii. 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.
xiv. No organism, organ or tissue can have activity that is absent in its cells.
(i) Viruses are acellular and do not have a cellular machinery. Even then they are considered to be organisms.
(ii) In some organisms, the body is not differentiated into cells though it may have numerous nuclei (coenocytes, e.g., Rhizopus).
(iii) Protozoans and many thallophytes have a uninucleate differentiated body (e.g., Acetabularia) which cannot be divided into cells. They are acellular.
(iv) Bacteria and cyanobacteria do not have nucleus and membrane bound organelles.
(v) RBCs and sieve tube cells continue to live without nucleus.
(vi) Protoplasm is replaced by non-living materials in the surface cells of skin and cork.
(vii) Schleiden and Schwann did not know the mechanism of cell formation. Schwann believed cells to develop spontaneously like a crystal. Schleiden thought new cells to develop from cytoblast or nucleus.
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 eukaryote and prior to that as the first prokaryote.
Essay # 5. Limit of Cell Size or Volume:
The factors which set the limit of cell size or volume are:
(i) Nucleocytoplasmic or kern-plasma ratio (ratio of nucleus to cytoplasm) which determines the range of control of metabolic activities by nucleus.
(ii) Ability of oxygen and other materials to reach every part of the cell.
(iii) Ability of waste products to pass to the outside.
(iv) Rate of metabolic activity.
(v) Ratio of surface area to the volume of the cell.
Metabolically active cells are usually smaller due to higher nucleocytoplasmic ratio and higher surface volume ratio. The former will allow the nucleus to have better control of metabolic activities while the latter will allow quicker exchange of materials between the cell and its outside environment.
Surface volume ratio decreases with the increase in cell size or volume as surface increases by the square of the size while volume increases by the cube of the size.
Take three cubic cells which have the surface area of 6 mm2 (6 x 1 x 1), 24 mm2 (6 x 2×2) and 54 mm2 (6×3 x 3) and a volume of 1 ram I3 (1 x 1 x 1), 8 mm3 (2x2x2) and 27 mm3 (3x3x3) respectively (Fig. 8.4). The surface to volume ratio in the three would be 6: 1, 3: 1 and 2: 1.
Therefore, larger cells have lesser surface volume ratio. They tend to become less efficient. All passive cells like eggs are, therefore, larger in size. All active cells are smaller. If larger cells are to remain active, they are either cylindrical in shape or possess several extensions of the cell membrane.
Microvilli are one of such developments. They are found in all those cells which are active in absorption. Membrane infolding’s also occur in transfer cells found in plants in the region of absorption or secretion of nutrients.
Essay # 6. Types of Cells:
A multicellular organism is composed of numerous cells. The cells are of three main types undifferentiated (stem cells), differentiated (post-mitotic cells) and dedifferentiated.
(a) Undifferentiated or Stem Cells:
They are un-specialised cells which usually possess the power of division, e.g., stem apical meristem, root apical meristem, vascular cambium, cork cambium, stratum germinativum of skin, germinal epithelium, bone marrow, etc. Zygote is also an undifferentiated cell.
(b) Differentiated or Post-mitotic Cells:
The cells are specialized to perform specific functions. Differentiation occurs in shape, size, structure and function through an orderly switching on and off of some particular genes of the cells by means of chemicals named as inducers and repressors. It leads to better organisation, division of labour and higher efficiency. Duplication of work is avoided.
(c) Dedifferentiated Cells:
They are differentiated cells which revert to undifferentiated state to take over the function of division. The process by which they lose their specialization is called dedifferentiation. It involves reactivation of certain genes that prevent differentiation, allow limited growth and induce division.
Cork cambium of plants is always produced through dedifferentiation. Dedifferentiation helps in healing of wounds, regeneration in animals, or vegetative propagation in plants. Cell culture experiments are based on this dedifferentiation of cells.
Essay # 7. Compartmentalization for Cellular Life:
Every cell behaves as a compartment because it is completely covered over by a membrane known as plasma membrane or plasma lemma. It may also possess some internal compartments in the form of membrane lined organelles like mitochondria, plastids, lysosomes, Golgi bodies, nucleus, etc. Non membranous organelles occur in both prokaryotic and eukaryotic cells, e.g. ribosomes.
i. Separation from Extracellular Medium:
Plasma membrane of the cell segregates its protoplasm from the extracellular medium. As a result, the protoplasm does not mix with the latter. It allows the cell to maintain its chemical pool, orderliness of structure and reactions in contrast to disorderly distribution and random interaction of molecules in the extra-cellular medium.
ii. Selective Permeability:
A cell is not a closed compartment. Its plasma membrane is selectively permeable, i.e., it allows selective exchange of materials between the cell interior and extracellular medium. The cell is thus able to maintain its internal composition quite different from that of the extracellular medium.
Most cells accumulate inorganic nutrients against their concentration gradient. Sea weeds have iodine in concentration 2 million times the one present in sea water.
Compartmentalization helps the cells to maintain their individuality. However, cells of a multicellular organism do not remain isolated. Cells of plant tissues are often connected with one another through cytoplasmic bridges called plasmodesmata. Junctions occur amongst animal cells.
Cells are able to recognize one another due to presence of specific chemicals on their surface. Thus separated cells of different species of sponges would segregate species-wise if they are allowed to come together. Similar cells of a higher animal would segregate tissue-wise.
vi. Comparatively very large:
A striated muscle cell can be 1-40 mm long and 30-80 pm in thickness. Longest cells of human body are the nerve cells which may reach a length of 90cm.
vii. Intracellular Compartmentalization:
Membrane lined cell organelles act as intracellular compartments. They allow the cells to separate diverse types of chemical reactions.
Essay # 8. Cell— An Open System:
An open system is the one which is separated from its surroundings by a boundary that allows transfer of materials and energy across it. Cell is an open system because it receives a number of materials including energy containing nutrients from outside. It liberates energy as heat and sends out excretions.
There is a wide variation in the size shaped and activities of cells. The smallest cells are those of Mycoplasma. They have a size of 0.1-0.5 pm. Bacteria measure 3-5 pm in length. Viruses are still smaller. They do not have a cellular structure.
The smallest virus has a volume of 7.0 x 10-7 µm3. The smallest mycoplasma has a volume of 1.0 x 10-3 µm3 while the smallest bacterium possesses a volume of 2.0 x 10-2 µm3. Unicellular eukaryotes have a size of 1-1000 µm.
Sporozoite of Plasmodium is only 2 µm long. Cells of multicellular eukaryotes have a size range of 5-100 µm. Among multicellular organisms, human erythrocytes (RBC) are about 7 µm in diameter. Some lymphocytes are still smaller (6 µm). Cells of kidney, liver, skin and intestine are 20-30 pm in diameter.
Muscle and nerve cells are comparatively very large. A striated muscle cell can be 1-40 mm long and 30-80 µm in thickness. Longest cells of human body are the nerve cells which may reach a length of 90cm.
Amongst plants, large cells occur in many algae. Intermodal cells of Char a are 1—10 cm in length. Acetabularia (Fig. 8.4B), a unicellular alga, is up to 10 cm in length. It is differentiated into rhizoid, stalk and cap. Plant fibres are still longer— 4 cm in Cotton, 55 cm in Ramie, 30-90 cm in Jute and over a metre in Hemp.
In general, eggs are large sized cells because they store food for partial or complete development of the embryo. Human egg is slightly over 0.1 mm or 100 µm in diameter. It has a volume 1.4 x 106 µm3 or 0.1 million times that of the human sperm (1.7 x 101 µm3 tables 8.1). Avian eggs are the largest. Hen egg is 60 x 45 mm with a volume of 5.0 x 10 µm13 while the egg of Ostrich is 170 x 150 mm with a volume of 1.1 x 1015 – µm3.
Essay # 9. Shapes of Cell:
The cells vary in their shapes. They may be disc like, polygonal columnar, cuboid, amoeboid, thread like or irregular. The shape of cell is related to its position (flat in surface cells, polygonal in cortex) and function (e.g., RBCs are biconcave to pass through capillaries and carry 02 ; WBCs are irregular to do phagocytosis, nerve cells are long to conduct impulses, sperms have tail for motility etc. ; Fig. 8.5).
On the basis of organisation of DNA, the cells are of two types prokaryotic and eukaryotic. The organisms having prokaryotic cells are called prokaryotes. They are now- a-days placed in a super kingdom called Prokaryote. Other organisms (having eukaryotic cells) are included in super kingdom Eukaryote. Prokaryotic cells occur in bacteria, blue green algae, chlamydiae, Archaebacteria and Mycoplasma or PPLO.
Essay # 10. Functions of Cell Parts:
i. Cell Wall:
Shape, rigidity and protection to cell.
ii. Plasma Membrane:
Regulation of substances leaving or entering a cell.
(a) Endoplasmic Reticulum — Cytoskeleton, channelization, synthesis of fats, steroids, proteins, formation of vacuoles and vesicles,
(b) Ribosomes— Protein synthesis,
(c) Mitochondria— Krebs cycle, amino acid synthesis, fatty acid synthesis,
(d) Chloroplasts— Photosynthesis,
(e) Amyloplasts— Storage of starch,
(f) Golgi Apparatus— Storage, secretion, excretion, wall synthesis, some chemical transformations, membrane transformation, lysosome formation,
(g) Centrioles— Formation of astral poles, flagella.
(h) Lysosomes— Separation and storage of hydrolytic (digestive) enzymes, digestion, autophagy.
(i) Sphaerosomes— Metabolism, storage and synthesis of fats,
(j) Glyoxysomes— Glyoxylate cycle, conversion of fat to carbohydrates,
(k) Peroxisomes— Photorespiration, peroxide metabolism.
(l) Microtubules— Cytoskeleton, formation of spindle and flagella.
(m) Microfilaments— Holding of membrane proteins, controlling cleavage and cyclosis.
(n) Vacuole— Osmotic pressure, storage.
Carrier of hereditary information, control of cell metabolism, cell differentiation, synthesis of DNA and RNA, formation of ribosomes, control of reproduction.