[Bio] The fundamental unit of life 

• Robert Hooke discovered cells in 1665, with the help of a self designed [primitive] microscope. He discovered it in a cork piece, which resembled the structure of a honeycomb. Cell is Latin for ‘a little room’.
• %%Knoll and Ruska invented the electron microscope in 1940. Convex lens is used for this.%%
• %%Anton Van Leeuwenhoek in 1674, with the help of an improved microscope, discovered free living cells in pond water for the first time%%.
• %%Robert Brown in 1831 discovered nucleus in the cell. Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell.%%
• %%The cell theory, that all plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden [1883] and Schwann [1839].%% %%The cell theory was further expanded by Virchow in 1855 by suggesting that all cells arise from pre - existing cells.%%
• Each kind of cell organelle performs a special function, such as making new material in the cell, clearing up the waste material from the cell and so on.
• A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit called the cell. All cells are found to have the same organelles, no matter what their function is or what organism they are found in.
• %%Every cell has - plasma membrane, nucleus and cytoplasm.%%

Plasma Membrane

• This 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 some materials in and out of the cell. It also prevents movement of some other materials. ==The cell membrane 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 where its concentration is low.

• Some substances like CO2 [which is cellular waste requires to be exerted 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 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 of concentration 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.

• Water also obeys the laws of diffusion. ==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 selectively permeable membrane to a region of low water concentration till equilibrium is reached.%%

Q. What will happen if we put an animal cell or a plant cell into sugar or salt solution in water ?

Ans. One of the following three things could happen :-

1. 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 eave. The result is that water enters the cell. The cell is likely to swell up.
2. 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 amount going out, so there is no overall movement of water. The cell will stay the same size.
3. 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.@@

Cell Wall

• Plant 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 is mainly 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 wall. This phenomenon is known as plasmolysis. Only living cells, and not dead cells, are able to absorb water by osmosis.==
• Cell wall permit the cells of plants, fungi and bacteria to withstand very dilute [hypnotic] external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building up pressure 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.

Nucleus

• %%The 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 when the cell is about to divide. Chromosomes contain information for inheritance of characters from parents to next generation in the form of DNA [Deoxyribo Nucleic Acid] molecules.
• Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional segments of DNA are called genes.
• In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
• 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.
• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane%%. Such an undefined nuclear region containing only nucleic acids is called nucleoid.%% Such organisms, whose cells lack a nuclear membrane, are called prokaryotes [pro = primitive or primary and karyon = nucleus].
• ==Organisms with cells having a nuclear membrane are called eukaryotes==. Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. Many of the functions of such organelles are also performed by poorly organised parts of the cytoplasm.
• The chlorophyll in photosynthetic prokaryotic bacteria is associated with membrane vesicles [bag like structures] but not with plastids as in eukaryotic cells.

Cytoplasm

• The cytoplasm is the fluid content inside the plasma membrane.
• In prokaryotes, beside the absence of a defined nuclear region, the membrane bound cell organelles are also absent. In eukaryotic cells it has nuclear membrane as well as membrane enclosed organelles.
• Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and us its cell machinery to multiply.

Cell organelles

• Every cell has a membrane around it to keep its own contents separate from the external environment.
• Large 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.
• Examples of cell organelles - Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids.

Endoplasmic Reticulum

• %%The ER is a large network of membrane bound tubes and sheets.%% It looks like long tubules or round oblong bags[vesicles]. The ER membrane is similar in structure to the plasma membrane.
• There are two types of ER - Rough ER and Smooth ER. RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture.
• %%The SER helps in the manufactaure of fat molecules, or lipids, important for cell function. Some of these proteins/lipids help in building the cell membrane. This process is known as membrane biogenesis.%% Some other proteins/lipids function as enzymes and hormones.
• One function of the ER is to serve as channels for the transport of materials [especially proteins] between various regions of the cytoplasm or between the cytoplasm and nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. In liver cells of vertebrates, SER plays a huge role in detoxifying many poisons and drugs.

Golgi Apparatus

• %%The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane bound vesicles [flattened sacs] arranged approximately parallel to each other in stacks called cisterns [cisternae].%%

• Its functions include the storage, modification, and packaging of products in vesicles. It is also involved in the formation of lysosomes

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Lysosomes [suicide sacs]

• Structurally, lysosomes are membrane bound sacs filled with digestive enzymes. These enzymes are made by RER. Lysosomes are a kind of waste disposal system of the cell.
• These 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 complex substances. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.
• When the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as ‘suicide bags’ of a cell.

Mitochondria

• Mitochondria is the powerhouse of the cell. It has two membrane coverings. ==The outer member is porous while the inner member is deeply folded.== These folds increase surface area for ATP [ Adenosine triphopshate] generating chemical reactions. ATP is known as the energy currency of the cell.

• @@It has its own DNA and ribosomes. Therefore, mitochondria are able to make some of their proteins.@@

Plastids

• Plastids are only present only in plant cells. %%There are two types of plastids - chromoplasts [coloured plastids] and leucoplasts [white or colourless plastids]%%
• Chromoplasts containing the pigment chlorophyll are knowns as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contains yellow or orange pigments.
• Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
• There are numerous membrane layers embedded in a materials called the stroma. Plastids also have their own DNA and ribosomes.

Vacuoles

• Vacuoles 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.
• Cell sap is the liquid inside the large central vacuole of a plant. Cell sap serves as the storage of materials and provides mechanical strength, especially in non wooden plants. It has also has vital role in plant cell osmosis.
• Turgidity is a cellular phenomenon of cell swelling because of the absorption of high fluid content. The cells take up water and it gets stored in vacuoles resulting in swelling of cells.
• Many substances are stored in vacuoles such as amino acids, sugars, various organic acids and some organisms.
• In single celled organisms like amoeba, the food vacuole contains the food items that the amoeba has consumed. In some unicellular organisms, specialized vacuoles also play important roles in expelling excess water and some wastes from the cell.
• The cell performs functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.

Cell Division

• The process by which new cells are made is called cell division. There are two main types of cell division - mitosis and meiosis.

• %%The process of cell division by which most of the cells divide for growth is called meiosis.%%

• ==In this process, each cell called the mother cell divides to form two identical daughter cells.== The daughter cells have the same number of chromosomes as the mother cell. It helps in growth and repair of tissues in organisms.

• %%Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilization give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions.%% ==When a cell divides by meiosis it produces four new cells instead of just two cells.==