ICSE Class 9 Biology Chapter 02 Cell The Unit of Life

Cell: The Unit Of Life

Unit-1 Basic Biology

Syllabus: The cell, a unit of life, protoplasm, basic difference between prokaryotic and eukaryotic cell; differences between an animal cell and a plant cell.

A basic understanding of the cell theory, structure of plant and animal cell with functions of various cell organelles. (Protoplasm, Cytoplasm, Cell Wall, Cell Membrane, Nucleus, Nucleolus, Mitochondria, Endoplasmic Reticulum, Ribosome, Golgi bodies, Plastids, Lysosomes, Centrosome and Vacuole). Major differences between a prokaryotic and eukaryotic cell. Difference between a plant cell and an animal cell should be mainly discussed with respect to cell wall, centrosome, vacuoles and plastids.

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 which 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 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 a considerable magnifying power up to 200 times. One of Leeuwenhoek's microscopes is shown in Fig. 2.1. 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 (Fig. 2.2 A) 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 (Fig. 2.2 B) and observed that it was made of tiny "boxlike" 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 ordinary compound microscope (Fig. 2.3) of today is a greatly improved design of the original Hooke's microscope.

The invention of the electron microscope (Fig. 2.4) added further to the unknown facts about cells. It can give a magnification to 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 which are bent by magnets.

Cell Theory

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 1839. Rudolf Virchow in 1858 made an addition to the cell theory by saying that all cells arise from pre-existing cells.

The Cell Theory states three major points.

1. The cell is the smallest unit of structure of all living things.

2. The cell is the unit of function of all living things.

3. All cells arise from pre-existing cells.

What Does The Cell Theory Mean?

Take two examples, a plant such as 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, movement of the frog is due to the contractions of muscle cells, food is digested by the enzymes which 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. 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 which also started as a single cell in the ovary of the flowers of the parent mango tree.

Teacher's Note

Understanding cells helps us appreciate why we need healthy food and sleep - our cells are constantly dividing and repairing, requiring proper nutrition and rest to function optimally.

Cells - How Numerous?

Larger an organism, 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 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 millimetre of blood.

Cells - How Small?

Cells are very small and are seen only with a microscope.

Smallest cells are the bacteria (0.3-5.0 micrometre), red blood cells (about 7 micrometre) in the human body, etc.

Longest cells are the nerve cells. Imagine a nerve cell extending from your finger tip up to the spinal cord inside your backbone.

Largest cells are the birds' eggs (actually the central yellow sphere). Ostrich egg (before development begins in it) is the largest single cell of the living world today. The white (albumen) of the egg and the egg-shell are extra parts added on to the actual egg as it passes down the reproductive tract.

Smallness Of Cells: A Greater Efficiency

Cells generally remain small in size and this is so for two main reasons.

(i) Different regions of a cell can communicate with each other rapidly for the cell to function effectively.

(ii) Cells have a large surface area / volume ratio for greater diffusion of substances in and out of the cell.

To understand this second advantage about 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 1 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.

Small size of cell presents a larger surface area / volume ratio.

The larger surface area relative to volume of the cell ensures greater diffusion of

nutrients into the cell,

metabolic wastes from the interior to the outside of the cell,

respiratory gases i.e. oxygen into the cell and carbon dioxide out of the cell

Any damage to the cell, can be easily repaired.

Teacher's Note

Just as a larger surface area on a cooling fin helps heat dissipate faster, smaller cells are more efficient at exchanging nutrients and waste - similar to how smaller organisms survive in harsh environments through higher surface-to-volume ratios.

Cell Shapes - To Suit Functional Requirement

Cells vary greatly in shape (Fig. 2.5). 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 "impulse" 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 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 "generalised cell".

A generalised cell consists of three essential parts: (1) cell membrane (plasma membrane), (2) nucleus and (3) cytoplasm. Fig. 2.6 shows the structure of a generalised animal cell and of a generalised plant cell as seen under a compound microscope.

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.

Parts of a cell:

Living Parts

Cell membrane

Non-living Parts

Cell wall (only in plant cell)

In the Cytoplasm

1. Endoplasmic reticulum

1. Granules

2. Mitochondria

2. Vacuoles

3. Golgi apparatus

3. Fat droplets

4. Ribosomes

5. Lysosomes

6. Centrosome (only in animal cell)

7. Plastids (only in plant cell)

In the Nucleus

1. Nuclear membrane

1. Nucleoplasm

2. Nucleoli

3. Chromatin fibres

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 (Fig. 2.6). 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 solution to enter and leave the cell without hindrance.

The plant cell shown in Fig. 2.6 also shows portions of the cell walls of six surrounding cells. A thin middle layer (shown as a thick dark line) holds the two adjacent cells together.

Cotton, jute and coconut fibres are the cell walls of their dead cells.

Cytoplasm

Cytoplasm is a semi-liquid substance. It occupies most part of the cell within the cell membrane. Under a compound microscope, it appears to be colourless, 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:

1. Endoplasmic Reticulum

The endoplasmic reticulum (ER) is so fine in structure that its existence is revealed only through an electron microscope (Fig. 2.7). It is an irregular network of double membranes distributed over the entire cytoplasm in a cell.

At the its outer end endoplasmic reticulum is connected with the cell membrane.

At the its inner end it is connected with 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.

2. Ribosomes - The 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.

3. Mitochondria - The Cell's Energy Producers

The mitochondria (sing. 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 cell 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 as "power houses of the cell".

4. 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 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 connected with the secretions of the cell including enzymes, hormones, etc.

5. Lysosomes - The Intracellular Digestive Centres

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 the "suicide bags".

6. Centrosome And Centrioles

A centrosome is found only in an animal cell. It is a clear area of cytoplasm close to nucleus. (from which spindle fibres 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 cell. [There are no centrosome and centrioles in plant cells].

7. Plastids

Plastids are found only in plant cells. These are special organelles in different shapes-oval, spherical and disc-shaped. Depending upon the colour they impart plastids are classified as leucoplasts, chromoplasts and chloroplasts.

(a) Leucoplasts (leuco: white) are colourless plastids. They have no pigment. They store starch. Cells of a potato have lots of leucoplasts in them.

(b) Chromoplasts (chromo: colour) These are variously coloured plastids-yellow, orange and red. They are mostly present in petals of flowers and in fruits, and the colouring substances (pigments) associated with them are xanthophyll (yellow) and carotene (orange-red).

Some colouring pigments such as blue, violet and purple are not associated with plastids; instead, they remain dissolved in the cell-sap and give that colour to the plant structure. Such pigments are called anthocyanins.

(c) Chloroplasts (chloro: green). These are green coloured plastids. They have green coloured 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.

Some people describe the chloroplasts as "kitchen" of the cell. It is a wrong analogy. In kitchen we cook the food to make it suitable for eating and do not produce it whereas the chloroplasts produce the food.

Green turns into Red!

Raw tomatoes and unripe chillies are green (due to chlorophyll). During ripening the chlorophyll degenerates and the masked red (carotene) takes over.

Non-living substances or Cell Inclusions

1. Granules. There are many small particles in the cytoplasm, these particles are believed to contain food materials, such as starch, glycogen and fats.

2. 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 the most important part of the cell.

It regulates and coordinates various life processes of the cell.

It plays an important part in cell division.

It contains factors (genes) which determine heredity.

Nucleus is a small spherical mass located somewhat in the centre 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 fibres. During cell division the chromatin fibres become thick and ribbon-like. These fibres are then called chromosomes (Fig. 2.8). Cells in which nuclear membrane is absent are called Prokaryotic cells (pro-primitive; karyon-nucleus). They have nuclear material called chromatin fibres which occur freely in the cytoplasm e.g. bacteria. Cells in which double nuclear membrane is present are called Eukaryotic cells (eu: true; karyon: nucleus), e.g. 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

Honey-bee - 32

Lion - 38

Mouse - 40

Wheat - 42

Potato - 48

Chimpanzee - 48

Monkey - 54

Chicken - 78

Dog - 78

Sugarcane - 80

Crayfish - 200

Some insects - more than 1000

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).

DNA - Fingerprinting

Like the fingerprints, the DNA pattern helps in ascertaining the identity of a person and hence the term DNA fingerprinting. This technique can even testify the parentage of an individual. In a woman's murder case of Delhi in July 1995 the DNA from her unidentifiable charred dead body was matched with the DNA from the body cells of her parents to confirm that they really were the father and mother of the murdered woman. That was one of the earliest cases. Now, DNA-fingerprinting has become very common.

Genes and not the number of chromosomes determines 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.

You can revise your understanding about the various cell parts by going through Table 3.1 which summarizes the various parts of a cell, their main characteristics and chief functions.

Teacher's Note

Understanding the nucleus and chromosomes helps explain why children resemble their parents - they inherit genetic information through DNA, similar to how software code copies from one computer to another.

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