Cell - The Unit of Life

What is a cell?

• The cell is the fundamental structural and functional unit of all living beings.
  • It is the smallest part of the body of an organism that is capable of independent existence and of performing the essential functions of life.
  • Every organ in our body—the skin, the brain, the muscle or even the bone—is composed of hundreds of thousands of such cells.
  • Similarly, every part of a plant—the leaf, the flower, the root, and even the wood—is composed of an exceedingly large number of cells.
• Every cell has its own life.
  • Old and weak cells in the body continually die and are replaced by new cells.
  • All organisms including ourselves,  start life as a single cell called the egg.
• Cells are so small (microscopic) that they cannot be seen with the naked eye.
  • It was, therefore, natural that their existence could not be detected by man until he invented magnifying aids in the form of microscopes.

The Invention of the Microscope and the Discovery of the Cell:

• The first microscope was constructed by Dutch scientist Antony van Leeuwenhoek (1632-1723).
  • He was an ordinary public official who ground lenses and made microscopic observations as a hobby.
  • He is said to have constructed 400 microscopes. Basically, all his microscopes consisted of a single biconvex lens and were called simple microscopes.
• Some of these microscopes had considerable magnifying power of up to 200 times.
  • In this microscope, the eye was applied close to the lens on one side and the object was mounted on the needle-like screw point on the opposite side of the lens.
• Robert Hooke (1635-1703), an English scientist, developed a microscope by using two lenses for achieving greater magnification.
  • Such microscopes were later known as compound microscopes.
  • In Hooke’s microscope, the object to be seen was placed on the stage below and light from an oil flame was thrown on it by means of a concave mirror.
  • Hooke examined a thin slice of cork under his microscope and observed that it was made of tiny “box-like” compartments piled up together.
  • This reminded him of the rooms, or cells, of monks in a monastery and so he said that the cork was made up of cells.
  • The cells which Hooke saw were all dead cells and they had only the empty “boxes” or the walls.
• The invention of the electron microscope added further to the unknown facts about cells.
  • It can give a magnification of over 200,000 times as against the ordinary compound microscope which magnifies an object up to a maximum of about 2,000 times.
• The ordinary compound microscope uses light which is bent by glass lenses to magnify the image while the electron microscope uses beams of electrons that are bent by magnets.
  • In 1838, Matthias Schleiden, a German Botanist, announced that every plant is made up of a large number of cells.
  • He added that each of these cells performed various life processes.
  • A year later, Theodor Schwann, a German zoologist, made similar discoveries in animals.
  • He declared that all animals and plants are composed of cells, which serve as the units of structure and function.
  • This, in short, is called the Cell Theory, having been proposed by Schwann and Schleiden in the year 1539.
  • Rudolf Virchow in l858 made an addition to the cell theory by saying that all cells arise from pre-existing cells.

What does the Cell Theory mean?

• Take two examples, a plant such as a mango and an animal such as a frog.
  • Structural Unit - If we take any part of the body of a frog or any part of a  mango plant and examine it under a microscope, it will show a cellular structure.
  • Functional Unit - Any function in the body of the frog or in the mango plant is due to the activity in its cells.
  • For example, the movement of the frog is due to the contractions of muscle cells, food is digested by the enzymes that the cells of the gut secrete, digested food is absorbed by the cells, and absorbed food is used up in cells for various metabolic activities.
  • In a mango plant, photosynthesis occurs in the cells of leaves, the root cells absorb water from the soil, and so on.
  • Cells die and are replaced - The body of the frog, or of the mango tree, is composed of millions and millions of cells.
  • Many of these cells continuously die and are replaced by new ones which are formed by the division of younger cells.
  • The formation of cells from pre-existing cells is a never-ending chain.
  • All life starts as a single cell - The life of the frog and the life of the mango tree started as an egg and as a seed respectively.
  • The egg was a single cell produced by the cells of the ovary of the mother frog.
  • The mango seed had an embryo that also started as a single cell in the ovary of the flowers of the parent mango tree.

Cells: How Numerous?

• Larger an organism, the greater the number of cells in its body.
  • Single-celled: Many small plants and animals are made up of just one single cell.
  • Examples: Bacteria, yeast, amoeba.
  • Few-celled: Some very small plants and animals are made up of relatively few cells—just a few hundred or a few thousand cells.
  • Examples: Spirogyra, Volvox
  • Multi-celled: Most plants and animals we see around us including ourselves, are made up of millions and billions of cells.
  • Examples: Human beings, Mango
    • An average-sized adult human constitutes approximately:
    • 1000 million cells in the whole body.
    • 10,000 million nerve cells in the brain cortex.
    • 5-6 million red blood cells and 7 thousand white blood cells per cubic millimeter of blood.

Cells: How Small?

• Cells are very small and are seen only with a microscope.
  • The smallest cells are the bacteria (0.3-5.0 micrometers), red blood cells (about 7 micrometers) in the human body, etc.
  • The longest cells are the nerve cells.
  • Imagine a nerve cell extending from your fingertip up to the spinal cord inside your backbone.
  • The largest cells are the birds’ eggs (actually the central yellow sphere).
  • The Ostrich egg (before development begins in it) is the largest single cell in the living world today.
  • The white (albumen) of the egg and the egg-shell are extra parts added to the actual egg as it passes down the reproductive tract.
• The smallness of Cells: A Greater Efficiency.
  • Cells generally remain small in size and this is so for these main reasons:
  • Different regions of a cell can communicate with each other rapidly for the cell to function effectively.
  • Cells have a large surface area/volume ratio for greater diffusion of substances in and out of the cell.
    • To understand the second advantage of the surface area/volume ratio imagine a cube with each of its sides measuring 2 mm.
    • The total surface area of this cube will be 2 mm x 2 mm x 6 (surfaces) = 24 sq. mm.
    • Suppose we cut this cube into 8 equal smaller cubes by reducing each side by half its length, then the total surface area of these 8 smaller cubes will be l mm x 1 mm x 6 (surfaces) x 8 pieces = 48 sq. mm, which is double that of the original larger cube.
    • The total volume in both cases still remains the same.

Cell Shapes: To suit Functional Requirements

• Cells vary greatly in shape.
  • These may be disc-like, polygonal, rectangular, cuboid, thread-like, branched, or even irregular.
  • These shapes of cells are often related to the different functions they perform.
  • Human red blood cells are circular and biconcave, to pass through narrow capillaries and transport oxygen.
  • White blood cells are amoeboid (amoeba-like movement, with pseudopodia) that can squeeze out through capillary walls.
  • Nerve cells are long to conduct “impulses” from distant parts of the body to the brain and vice-versa.
  • Muscle cells are long and contractile to pull or squeeze the parts.
  • Guard cells of the stomatal pore in the leaves are bean-shaped to open and close the pore.

Structure of a Cell:

• Various kinds of cells show special differences, yet they all show some basic structural plan which may be expressed in the term  “generalized cell”.
  • A generalized cell consists of three essential parts cell membrane (plasma membrane), nucleus, and cytoplasm.
• Cell organelles (the ‘little organs”): Most parts of a cell have a definite shape, a definite structure, and a definite function.
  • Such parts are called organelles.
  • The organelles have the same status in a cell as the organs have in the entire body of an animal or a plant performing specific functions.
  • Cell organelles are living parts.

Cell Membrane and Cell Wall:

• Each cell is surrounded by a cell membrane or plasma membrane.
  • The cell membrane has fine pores through which substances may enter or leave the cell.
  • The permeability of the cell membrane is selective, i.e. it allows only certain substances to pass through while it prevents others.
  • Plant cells have a cell wall surrounding the cell membrane.
  • The cell wall is made of cellulose, a non-living substance.
  • The cell wall gives shape and a certain degree of rigidity to the cell without interfering with the functions of the cell membrane.
  • The cell wall is freely permeable allowing the substances in the solution to enter and leave the cell without hindrance.

Cytoplasm:

• The cytoplasm is a semi-liquid substance.
  • It occupies most parts of the cell within the cell membrane.
  • Under a compound microscope, it appears to be colorless, partly transparent, and somewhat watery.
  • Many chemical reactions take place in the cytoplasm.
  • Living cytoplasm is always in a state of some movement.
• The following are the cell organelles embedded in the cytoplasm.

Endoplasmic Reticulum:

• The endoplasmic reticulum (ER) is so fine in the structure that its existence is revealed only through an electron microscope.

  • It is an irregular network of double membranes distributed over the entire cytoplasm in a cell.

  • At its outer end, the endoplasmic reticulum is connected to the cell membrane.

  • At its inner end, it is connected to the nuclear membrane. It appears rough when the particle-like ribosomes are attached to it and appears smooth without

    them.

  • It forms the supporting framework of the cell and also serves as a pathway for the distribution of the materials from one part of the cell to the other.

Ribosomes: Sites of Protein Synthesis

• The ribosomes are numerous small granules either scattered freely in the cytoplasm or attached to the membranes of the endoplasmic reticulum.
• These are the ‘factories’ for the synthesis of proteins.

Mitochondrion: The cell’s energy producers

• The mitochondria *(*mitochondrion) are spherical, rod-shaped, or thread-like {mitos: thread) bodies.
  • These are minute double-walled bags with their inner walls produced into finger-like processes projecting inwards (called cristae).
  • Mitochondria are the sites where cellular respiration occurs to release energy.
  • This energy is stored in the form of an energy-rich compound ATP (adenosine triphosphate) and is used in various metabolic functions of the cell, and in turn, of the body.
  • Some people call the mitochondria “powerhouses of the cell”.

Golgi apparatus — The delivery system of the cell

• The Golgi apparatus occurs in the form of granules, filaments, or rods which are supposed to be originated from the endoplasmic reticulum.
• These are very small vesicles of different shapes and are generally located near the nucleus.
• The Golgi complex consists of many small groups of hollow tubular structures with membranous walls and is associated with some minute vesicles and vacuoles.
• It is concerned with the secretions of the cell including enzymes, hormones, etc.

Lysosomes — The Intercellular Digestive Sites

• Lysosomes are small vesicles of different shapes containing some digestive enzymes.
  • Their enzymes destroy and digest foreign substances around them.
  • They digest the stored food during starvation of the cell.
• Many damaged cells are rapidly destroyed or dissolved by their own lysosomes and hence these are also called “suicide bags”.

Centrosome and centrioles:

• A centrosome is found only in an animal cell.
  • It is a clear area of cytoplasm close to the nucleus, (from which spindle fibers develop during cell division both in mitosis and meiosis).
  • The centrosome contains two centrioles which are short bundles of microfilaments arranged at right angles to each other (that is why they always appear in this shape in the microscopic view of the cell). [There are no centrosomes and centrioles in plant cells].

Plastids:

• Plastids are found only in plant cells.
• These are special organelles in different shapes—oval, spherical and disc-shaped.
• Depending upon the color they impart plastids are classified as leucoplasts, chromoplasts, and chloroplasts.
• Leucoplasts: (leuco: white) are colorless plastids.
  • They have no pigment.
  • They store starch.
  • The cells of potatoes have lots of leucoplasts in them.
• Chromoplasts: (chromo: color)
  • These are variously colored plastids—yellow, orange, and red.
  • They are mostly present in the petals of flowers and in fruits, and the coloring substances (pigments) associated with them are xanthophyll (yellow) and carotene (orange-red).
  • Some coloring pigments such as blue, violet, and purple are not associated with plastids; instead, they remain dissolved in the cell sap and give that color to the plant structure.
  • Such pigments are called anthocyanins.
• Chloroplasts: (chloro green).
  • These are green-colored plastids.
  • They have a green-colored pigment called chlorophyll.
  • Chloroplasts are abundant in parts exposed to light, e.g. leaves.
  • They also have other pigments such as orange and yellow, but these pigments are masked by large quantities of chlorophyll.
  • Their function is to trap solar energy and absorb carbon dioxide for the manufacture of starch and sugar during photosynthesis.
  • Chloroplasts contain DNA and have the capacity to divide.

Cell Inclusions:

• Granules: There are many small particles in the cytoplasm, these particles are believed to contain food materials, such as starch, glycogen and fats.
• Vacuoles: These are certain clear spaces in the cytoplasm.
  • They are filled with water and various substances in solution.
  • In plant cells the vacuoles are usually quite large and the liquid which they contain is called cell-sap.
  • An animal cell does not have such prominent vacuoles, and the vacuoles are fewer in number.

Nucleus;

• Nucleus is a small spherical mass located somewhat in the center of the cytoplasm.
  • It has a delicate nuclear membrane which is filled with a relatively dense nucleoplasm.
  • In the nucleoplasm there are certain threadlike structures called chromatin fibers. During cell division the chromatin fibers become thick and ribbon-like.
    • These fibers are then called chromosomes.
  • Cells in which nuclear membrane is absent are called Prokaryotic cells (pro-primitive; karyon-nucleus).
  • They have nuclear material called chromatic fibers which occur freely in the cytoplasm.
    • Example: bacteria.
  • Cells in which double nuclear membrane is present are called Eukaryotic cells (eu: true; karyon : nucleus).
  • Example: all organism other than bacteria.
• Each nucleus also has, at least, one nucleolus in it.
  • Some cells may have more than one nucleolus.
  • The number of nucleoli in a cell is fixed.
  • The nucleolus participates in protein synthesis.
• The number of chromosomes is definite in each species.
  • Every human body cell has 46 (23 pairs) chromosomes.
• Chromosome numbers of some other common animals and plants are as follows:
  • Ascaris (round worm)                             2
  • Garden pea                                            14
  • Onion                                                    16
  • Maize                                                    20
• The chromosomes carry the genetic characters from the parents to the offspring through the union of the egg of the female and the sperm of the male.
  • Chromosomes are made of chromatin, which is composed of hereditary units called genes.
  • Genes are made of a complex chemical substance DNA  (deoxyribonucleic acid).
  • Genes and not the number of chromosomes determine the characteristics of a species.
• Lion, tiger and the house cat all have 38 chromosomes but they look different due to their different genes located on these chromosomes.

Protoplasm:

• Biologists have been using the term “protoplasm” for a long time.
• By this they mean the living substances in an organism.
• This living substance or the protoplasm is contained in the cells.
• The protoplasm has been described as a translucent fluid somewhat colorless, greyish or brownish.
• The chemical composition of protoplasm is very complex.
• It varies a little from one cell to another, although the common elements included in the composition of protoplasm, such as carbon, hydrogen, oxygen, nitrogen, sulphur, iron and phosphorus, are the same in all cells.
• These elements are in the form of specific compounds such as water, proteins, carbohydrates, fats and mineral salts.
• It is also true to say that it is impossible to make an accurate chemical analysis of protoplasm because it ceases to be protoplasm as soon as it is removed.

Some Examples of Cellular Activity:

• All organisms, grow due to the growth in size and increase in the number of cells. Such growth is the production of more body substance and cell substance.

• Repair of an injury or regeneration of a lost part (as the tail of a lizard) is due to cell divisions.

• Movement of the body is due to contractility of the cells or the cellular parts.

  • For example: Animals walk, run, jump, swim or fly with the help of muscles (contractile cells) which move the bones (formed of cells and cell secretions).
  • Even the flow of blood in blood vessels and the passage of food in the gut are the result of muscle cell contractions.
  • Feathers which help the birds to fly are also the products of cell.
  • The drooping of leaves of the sensitive plant on touching and their subsequent recovery to stand out is due to the activity of the cells at the base of the leaves.
  • In plants, the bending movements of roots towards water or gravity, or movements of stems towards light or away from gravity are due to the activities of cells (unequal growth in the cells).
  • Closing of stomata of leaves at night and the opening of petals in a flower are all due to changes in the water content of their cells.

• Feeding and nutrition has many steps and each step is the result of cellular activities.

  • Sensory cells on the tongue taste the food and muscle cells of the jaws and of the tongue help in chewing and swallowing.
  • The cells of the digestive glands secrete enzymes to digest food.
  • The cells of the inner mining of the intestines absorb digested food.
  • Extra food is stored as fat in fat cells and as glycogen in liver cells.

• Circulation of blood and movement of other fluids in the body are through forces set up by contraction of muscle cells of the heart or other parts.

• Respiratory gases are transported from the lungs to other parts of the body by blood cells.

• The body protects itself from disease germs through certain cells (W.B.C.) which devour the germs or which give out antibodies and antitoxins to kill them or to neutralize their effects.

• We see, hear. smell, taste or feel the sensation of touch, pain, heat, cold, etc. through sensory cells.

  • The brain orders muscles to contract or a gland to secrete through its cells (response) The memory and the capacity to solve problems are also due to the activity of the cells.

• We maintain our body heat (thermo-regulation) by cellular activity and we cool it when hot by sweating from gland cells in the skin.

• All organisms produce their young ones (eggs or babies in animals, or the seeds in plants) through the activity of cells (eggs and sperms).

• In plants, the absorption of’ water and nutrients is through root cells. The stem cells conduct the food and water to different parts of the plant.

• Light is trapped by the leaf cells containing chloroplasts to produce food.

• Flowers attract insects by thief color contained in the petal cells, or by nectar secreted by the cells.

• The mango seed produces a mango plant and a hen’s egg produces a hen and similarly, the transmission of parental features to their young ones inheritance) is also dependent on what the germ cells (egg and sperm) early with them.