FUNDAMENTAL UNIT OF LIFEWhile examining a thin slice of cork, Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments.Cork is a substance which comes from the bark of a tree. This was in the year 1665 when Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells. Cell is a Latin word for ‘a little room’.STRUCTURAL ORGANIZATION OF CELLPLASMA MEMBRANE OR CELL MEMBRANEThis is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of somematerials in and out of the cell . It also prevents movement of some other materials.The cell membrane, therefore, is called a selectively permeable membrane.Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.There is spontaneous movement of a substance from a region of high concentration to a region where its concentration is low.For example, some substance like CO2(which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. As soon as there is a difference of concentration of CO2 inside and outside a cell,CO2 moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion.Similarly, O2 enters the cell by the process of diffusion when the level orconcentration of O2 inside the cell decreases.Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.The movement of water molecules through such a selectively permeable membrane is called osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.What will happen if we put an animal cell or a plant cell into a solution of sugar or salt in water?One of the following three things could happen:If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink. Thus, osmosis is a special case of diffusion through a selectively permeable membrane.The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.CELL WALLPlant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall ismainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as Plasmolysis.Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building uppressure against the cell wall. The wall exerts an equal pressure against the swollen cell. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.NUCLEUSThe nucleus has a double layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, that is, to the cytoplasm.The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein.The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells. It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.Organisms, whose cells lack a nuclear membrane, are called prokaryotes. Organisms with cells having a nuclear membrane are called eukaryotes.CYTOPLASMThe cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.CELL ORGANNELELarge and complex cells including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures called ‘organelles’ within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope.Examples of cell organelles are : endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.They are important because they carry out some very crucial functions in cells.ENDOPLASMIC RETICULUM (ER)The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane.There are two types of ER– rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER looks rough under a microscope because it has particles calledRibosomes attached to its surface.The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent toVarious places in the cell depending on need, using the ER.The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis.FUNCTIONS OF ER:Serves as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. In the liver cells of the group of animals called vertebrates SER plays a crucial role in detoxifying many poisons and drugs.GOLGI APPARATUSThe Golgi apparatus,was first described by Camillo Golgi. It consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.FUNCTIONS :The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.Its functions also include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes.LYSOSOMESLysosomes are a kind of waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell. Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.MITOCHONDRIAMitochondria are known as the powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanicalwork.Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP-generatingchemical reactions.Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.PLASTIDSPlastids are present only in plant cells. There are two types of plastids – Chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids). Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma.Plastids are similar to mitochondria in external structure. Like the mitochondria, Plastids also have their own DNA and ribosomes.VACUOLESVacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles.The central vacuole of some plant cells may occupy 50-90% of the cell volume.In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins. In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.EUKARYOTIC CELL DIAGRAMCOMPARISON OF PLANT AND ANIMAL CELLPLANT CELLANIMAL CELL