Cell Biology

So what is Cell? Cell is the basic structural and functional unit of living organism. It consists of microscopic mass of protoplasm surrounded by a cell membrane. it is the smallest unit, that can perform all the processes of life. In a unicellular organism, the single cell performs all metabolic functions such as growth, respiration, reproduction, digestion, excretion, etc. In multicellular organisms all the vital activities are the result of combined action of these cells. Hence, the cell is considered as basic unit of life. Robert Hooke(1635-1703) dicovered cell in 1665.
He cut a thin slice of cork and observed it under his microscope. He obsereved many small, hollow, honey-comb like compartments. For each compartment he gave the name cell. Cell is a Latin word derived from cellula meaning small apartment. In 1675, Anton Von Leewenhoek (1632-1723), a Dutch scientist observed and described unicellular bodies like protozoa, bacteria, sperms and red blood cells. He also noticed that cells were not just hollow spaces but contained some organization within them.

The Cell Theory:

In 1838, a German botanist, Matthias Schleiden (1804-1881) concluded that all plants were composed of cells. A year later at 1839 , the German Zoologist Theodor Schwann (1810-1882) came to the same conclusion about animals. Schwann recognised that both plant and animal had cells which had a nucleus and a clear substance around it. He then defined cell as a membrane bound, nucleus-containing structure. He proposed a hypothesis: the bodies of animals and plants composed of cells and products of cells.

The combined views of Schleiden and Schwann ked to the formulation of the Cell Theory. However, this theory was not able to explain how new cwlls are formed. It was Rudolf Virchow (1855) who explained that cells divide and that all new cells must come from pre-existing cells. therefore, some changes have been made in the old theory. The cell theory is also known as the Cell doctrine or Cell principle . It states:

1. All cells arise from pre-existing cells.
2. All living organisms are made up of cells and their products.
3. Cell is the structural and functional unit of life.
4. Cells are hereditary units.
5. All cells are basically alike in chemical composition and metabolic activities.
6. Life passes from one generation to other generation in the form of a living cell.
7. The function of an organism is the outcome of the activities and interactions of the constituent cells.

Drawbacks of Cell Theory: It is one of the most fundamental and universally applied theories except in few cases as ginen below:

1. Virus is a liuving organism but it lacks the cellular organization.
2. Bacteria, blue-green algae are prokaryotes and hence are without true cellular structures.

The Cellular Totipotency: Every living vegetaitive cell of a plant has potentialities to regenrate into a whole plant. this is called cellular totipotency. This concept was given by a German botanist, Haberlandt in 1902. He stated that every living plant cell should be able to regenerate into a whole plant as each cell was derived from the fertilized egg. it must contain the inherent capacity to produce the whole organism.

Steward's Experiment:

In 1950 Steward and his co-workers gave the first experiment evidince of cellular totipotency. Experimental steps:

1. They used small pieces of mature, non-dividing cells of phloem from carrot root.
2. They grew the tissues in a liquid nutrient medium containing coconut water.
3. They seperated cells and cell clusters by shaking the medium gently. The cells started dividing and formed rooting clumps.
4. Then cell clusters were transferred to antother culture tube containing a semisolid medium of the same composition, they developed shoot.
5. New plants are now formed.
6. The new plants were transferred to the pots containing soil which developed into flowering plants.

Thus, from the above experiment, Steward concluded that even mature cells have the capacity to recognize new plants.

Unicellur and Multicellular Organisms:

All living organisms are made up of cells. Some organisms are made from just a single cell, and these are known as unicellular organisms. In a unicellular organism, the single cell performs all metabolic functions such as growth, respiration, digestion, excretion, reproduction, etc. Others including all plants and animals are made from many cells and they are called multicellular organisms. Within a multicellular organism, cells are usually arranged in groups to form tissues. A tissue is an assemblage of specific types of cells having similar structure and function. Several different kinds of tissues may be grouped together to form an organ. Thus, an organ is the assemblage of various types of tissues to serve a specific function such as the leaf of a plant or the eye of an animal. Again in highly evolved organisms several inter-related organs are arranged into some organ systems. For example the respiratory system consist of the organs like nose, larynx, lungs, etc. Thus, various organ systems constitute the body of higher multicellular organisms.

Cells -----> Tissues -----> Organs ----> Organ system ----> Multicellular organisms

Differences between Unicellular and Multicellular Organisms

1. Unicellular organisms are made up of single cell. Eg. Amoeba, Paramecium, Chlamydomonas whereas multicellular organisms are made up of several cells. Cells are usually arranged in groups to form tissues. The tissues form an organ and several organs form organ system.
2. There is no division of labour in unicellular organism. A single cell acts as the body of the organism and performs all life processes whereas in multicellular organism, differnt cells carry out differnt functions. Number of cells work together in a co-ordinate manner in a very effective way. For example, a nerve impulse is transmitted by a series of nerve cells.
3. In unicellular organisms, to perform differnt activities, there are various organelles within a cell. Therefore, the cell has to be sufficiently large to accomodate the numerous organelles. Thus, the cells are comparatively larger in size whereas the size of a multicellular organism does not depend upon the size of the cells. It depends upon the number of cells present in its body. The cells are comparatively smaller.
4. Unicellular organims are unable to exhibit wide range of different functions whereas Multicellular organisms are more efficient and carries out a number of activities with the help of different kind of cells.
5. Unicellular organism are easily susceptible to damage and any injury to cell can lead to death of an organism whereas multicellular organisms have better adaptability capacity in comparison to unicellular organisms. Death of few hundred or thousand cells does not affect much because they are continously replaced.
6. In unicellular organism various life process such as gaseous exchange during excretion, respiration etc. takes place through cell surface whereas in multicellular organisms there are various systems such as circulatory system, respiratory system, excretory system, digestive system to perform such functions.
7. Growth in unicellular organisms occur due to growth of the cell whereas in multicellular organism growth occurs due to multiplication of cells of the body.

Flow of Energy through a Cell:

All systems need a constant supply of energy for their existence and maintenance. If they are not provided with energy supply, they disorganise and disintegrate leading to death - the highest state of entropy. Thus, a cell also requires a continous flow of energy to carry out its life processes. This energy is obtained either from sunlight or from chemical compounds of food. Cells bearing chlorophyll trap solar energy and synthesize food having chemical energy in the process of photosynthesis. The cells of other non-green organisms, animals and decomposers also get regular supply of the energy through food chain. The food energy is oxidised in non-green cells of plants and animals to liberate energy for all life activities. The energy is available in the form of adenosine-triphosphate (ATP). This form of energy can be easily stored , transferred and transformed in a living system. The flow of information in a cell takes place by two routes:

1. Intrinsic Information / Genetic Information: DNA molecules control intrinsic activities by synthesizing protein molecules. DNA transcribes the stores information into RNA molecules, which translate the genetic information into specific proteins. The proteins then perform and regulate all the activities of the cell. DNA trancription_> RNA translation_> Proteins ---------> regulate all the activities of the cell
2. Extrinsic Information: It is the flow of information through hormones. Some cells or specific glands secrete hormones which are transported to the other cells or organs to regulate their activities.

Structure of Prokaryotic Cells and Eukaryotic Cells:

Two types of cells are found in living organisms:

1. Prokaryotic Cell: [Pro: primitive; Karyotic: nucleated] A prokaryotic cell is a relatively simple cell in term of its structure. The cell with incipient nucleis is called prokaryotic cell. In other words an organised nucleus is absent. The chromatin material is present as a single circular structure formed of DNA fibres. DNA molecule lies freely in the centre of the cytoplasm in the form of a tangled mass, which is called the nuclear body. The nuclear body lacks nuclear membrane and nucleoli. Plastids, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes are absent. Photosynthetic pigments are found in photosynthetic lamellae or chromatophores. Other lamellae are mesosomes, which carry on respiration. Ribosomes are only 70s type. Prokaryotic cells are found only in bacteria and blue green algae.
2. Eukaryotic Cell: [Eu: well or good; Karyotic: nucleated] These are well develpoed cells with organised nucleus, i.e. the nucleus contains nuclear membrane and nucleolus. All membrane bounded structures such as plastids, mitochondria, endoplasmic reticulum, golgi apparatus etc. are found in eukaryotic cell. Eukaryotic cells are found in all living organisms except bacteria and blue-green algae.

Differences between Prokaryotic cell and Eukaryotic cell:

1. In prokaryotic cell, an organised nucleus is absent. Nucleus is represented by packed mass of DNA fibres, which lacks nuclear membrane and nucleoli. In Eukaryotic cell an organised nucleus is present. It contains chromatin network, nuclear membrane and nucleoli.
2. In prokaryotic cell, DNA is naked i.e. it lacks protein association whereas DNA is covered with histone proteins to form chromosome.
3. Prokaryotic cell occurs only in lower organisms such as bacteria and blue green algae whereas Eukaryotic cell occurs in all living organisms except bacteria and blue green algae.
4. In prokaryotic cell, cell wall is made up of peptidoglycan or mucopeptide or murein. The mucopeptide is made up of amino sugars and amino acids. In Eukaryotic cell, cell wall mainly consists of cellulose. Amino sugars and amino acids are absent.
5. Prokaryotic cell has a single envelope system whereas Eukaryotic cell has double envelope system. First membrane is the cell membrane and second membranes are the envelopes of the nucleus and other cell organelles.
6. All membranes bound structures such as chloroplast, mitochondria, endoplasmic reticulum, Golgi complex , lysosomes, etc. are absent in Prokaryotic cell whereas all membrane bound structures are present in Eukaryotic cell.
7. In Prokaryotic cell, the photosynthetic pigments occur in flattened sacs, called lamellae. The lamellae are not enclosed by membranes. In Eukaryotic cell, the lamellae are enclosed within double membranes and are called chloroplast.
8. In Prokaryotic cell, mesosomes are the site of respiration whereas Mitochondria is the site of respiration in Eukaryotic cell.
9. Cytoplasm of Prokaryotic cell contains 70s type of ribosomes whereas cytoplasm of Eukaryotic cell contains 80s type of ribosomes, whereas the 70s ribosomes are present in cell organelles.
10. In Prokaryotic cell, the cyclosis movement of protoplasm is not observed whereas the cyclosis movement is observed in Eukaryotic cell.

Differences between Plant cell and Animal cell:

1. The plant cell is characterized by the presence of a cell wall outside the cell membrane whereas the cell wall is absent in animal cell. It is enclosed by a cell membrane.
2. Plant cell contains plastids, i.e. chloroplast, chromoplast and leucoplast whereas there is no plastids in animal cell.
3. Centriole is usually absent in plant cell whereas centriole is present in animal cell.
4. A mature plant cell contains a large central vacuole whereas are vacuoles are either small or absent in animal cell.