• In the year 1665, Robert Hooke was studying slices of cork from the bark of a tree. He was studying it with his self-made microscope.
• He observed that the structure of the cork looked like a 'honey coup'. He could see several tiny compartments adjacent to each other and named each compartment as 'cell' (Like jail cells').
• The cell is a Latin word for 'a tiny room'.

• An organism that consists of only one cell is known as a unicellular organism.
• In other words, the entire organism is made up of only one cell.
• Amoeba, Chlamydomonas, paramoecium, etc. are unicellular organisms.

• When several cells group together to form a single body and to perform specific functions, the organism formed is called a multicellular organism.
• Fungi, plants, animals, humans are all multicellular organisms.

• Every living object is made up of cells.
• For example in humans, cells of similar type unite to form tissues, tissues unite to form organs and organs together form the organ system and finally the whole body.
• This means that cells give structure to the body. Just like a house gets its structure brick-by-brick, the body gets the structure cell-by-cell. Hence, the cell is known as the structural unit of the body and hence life.
• These are specific cells to perform a specific types of functions. For example, muscle cells help in contraction and expansion, bone cells help in bone formation and so on.
• A cell is also able to perform all these functions because of the organelles found in the cells. Thus, the cell is also called the functional unit of life.

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Prokaryotes :

1. Cell membrane or plasma membrane
2. Cell Wall (only in plants)
3. Nucleus
4. Cytoplasm

1. Endoplasmic Reticulum
2. Golgi Apparatus
3. Lysosomes
4. Mitochondria
5. Plastids
6. Vacuoles

• The cell membrane is the outermost covering or says the boundary of the cell. 
• It is present in both animals as well as plant cells.
• It is a living, thin, flexible and delicate membrane. 
• The cell membrane is selectively permeable in nature i.e.
•  it allows only useful substances to enter the cell and removes waste products from the cell.
• Gases like CO2 and O2 continuously move in and out of the cell membrane through diffusion.

• The function of Cell Membrane

• Cell membrane which is an outer covering of the cell takes care of the movement of molecules inside and outside the cell.
• Gases such as CO2 and H₂O continuously move inside and outside the cell through a process called diffusion. Diffusion occurs when the concentration in two adjoining environments differs.
• In diffusion, the substances such as CO2 which are at higher concentration areas move into the area having less concentration.
• CO2 continuously gets accumulated in the high concentration area of the cell. → As a result, the concentration of CO2 increases in the cell compared to the concentration outside the cell.
• So, CO2 from inside the cell starts diffusing out of the cell.
• Similarly, the concentration of O2 increases outside the cell and so O₂ starts diffusing into the cell.
• Thus, diffusion plays an important role in the gaseous exchange between the cells as well as the cell and its external environment.

• Over and above the cell membrane, plant cells have an additional coverage which is known as the cell wall.
• Cell wall lies outside the plasma membrane.

• The cell wall is non-living and it freely permits the entry of substances.
• The cell wall gives mechanical strength to the cell through its structure which is mainly made up of cellulose.
• It also prevents water absorption from the cell through a phenomenon called plasmolysis

• The nucleus is a main, round-shaped organelle situated in the middle of the cell. The nucleus is surrounded by two nuclear membranes which form a nuclear envelope.
• This envelope separates the nucleus from the cytoplasm of the cell.
• The nuclear membrane contains several pores.
• These pores help in transferring the material from inside the nucleus to outside i.e. into the cytoplasm.
• The nucleus also contains a macromolecule called 'chromatin'. Chromatin is further made up of genetic material called DNA and protein.
• Under normal circumstances (i.e. when the cell division is not taking place) chromatin is not visible. When the cell division starts, chromatin starts getting condensed further. The condensed form is called chromosome which can be seen in the microscope.
• DNA is a genetic material that contains hereditary information. -

• The nucleus plays an important role in cellular reproduction.
• It also helps in determining how the cell will develop and what characteristics and form will it display when the cell matures.
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• Chromatin and chromosomes:
• Chromatin is a macromolecule found in the nucleus of a eukaryotic cell. Chromatin can be seen as threads of DNA engulfing protein mass. Chromatin is further made up of genetic material called DNA and protein.
• Under normal circumstances (i.e. when the cell division is not taking place) chromatin is not visible. When the cell division starts, chromatin starts getting condensed further. The condensed form is called a chromosome which can be seen in a microscope.
• Chromosomes can be seen as rod-shaped structures.

• The full form of DNA is Deoxyribonucleic Acid.
• DNA is found in chromosomes (and also in chromatin) of cells.
• The main functional component of DNA is the 'gene'. It contains genetic information which is necessary for the development and organization of cells.
• DNA is the genetic material. It contains hereditary information i.e. information of features that parents will pass to the next generation.

• The fluid content between the nucleus and the plasma membrane is called the cytoplasm. It contains several cell organelles.

• Endoplasmic Reticulum (ER) is a membranous tubular network found in the cytoplasm.
• ER is a system of complex tunnels (tubules) whose one end is connected with the outer membrane of the nucleus whereas the remaining portion spreads out in the cell.
• ER acts as a transmitary system between the cytoplasm and nucleus.

• Rough Endoplasmic Reticulum (RER) In this type of ER, ribosomes are present on the outer surface. This gives a grainy appearance to the ER and hence it is called Rough ER (RER).
• Ribosomes perform the function of synthesizing protein. The protien produced here is sent to the various parts of the cell.

• ER with no ribosomes attached is known as smooth ER.
• Smooth ER does the work of secreting lipids (fat molecules).

• ER works as a base for some of the important biochemical activities of the cells.
• Some of the proteins and lipids of SER helps in building the cell membrane. This building plasma membrane by SER is called 'membrane biogenesis'.
• Some other proteins and lipids function as enzymes and hormones.
• ER also works as a transmit system of material, especially protein between cytoplasm and nucleus.
• SER of liver cells plays a very important role in detoxifying many toxins and drugs from the body of vertebrates.

• The Golgi body or apparatus is a cell organelle named after Camillo Golgi who first described it. 
• Golgi apparatus is a system of membrane-bound vesicles. These vesicles are arranged approximately parallel to each other in the form of flat cisternae.
• The Golgi bodies are often in close connection with the endoplasmic reticulum and together they form another system of the complex cellular membrane systems.

• Golgi apparatus works as a carrier of material produced near the ER. ER sends the material to Golgi bodies and the bodies further transport it to various organelles of cells. Golgi bodies also help to transport material from outside towards the nucleus.
• Thus, the Golgi bodies store modify, sort, pack and dispatch products in the vesicles. →At times, Golgi bodies do the task of making complex sugar from simple sugar.
• Golgi bodies also help in the formation of lysosomes.

• Lysosomes are tiny spherical membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.
• Lysosomes are found in the cytoplasm of cells.

• Lysosomes digest foreign material as well as worn-out cell organelles and thus keep the cells clean.
• When the cell gets damaged, the lysosome may burst and release the enzyme which would then digest the cell. So, the lysosome commits suicide to kill the damaged cell.

• Mitochondria is an ultra minute tubular, rod or round-shaped structure distributed in the cytoplasm
• Each mitochondrion possesses a double membrane covering. 
• The outer membrane is porous whereas the internal membrane is folded. Because of the folded structure of the internal membrane, it has a much larger surface area as compared to the outer membrane.
• The internal folded structure is known as cristae.
• The upper side of cristae is studded with small rounded bodies known as particles or oxysomes.

• Mitochondria do the work of cellular respiration.
• The cell uses this ATP as a source of energy for performing several functions. Mitochondria also produce protein and so it is known as a semi-autonomous organelle.
• Since mitochondria produce energy in the form of protein for the cell, it is also known as a powerhouse of the cell.
• Mitochondria release ATP (Adenosine Triphosphate) molecules.

• The full form of ATP is Adenosine Triphosphate.
• ATP molecules are a form of energy released by mitochondria.
• The energy released is used by the body for performing various activities. So, ATP is known as 'energy currency'.
• The body uses ATP for making new chemical compounds and for mechanical work.

• Plastid is an organelle found only in plant cells (i.e. not in animal cells).
• Plastids contain pigments used in several plant activities.
• Looking at the inside of the plastid one can find several membrane layers embedded in a material called the stroma.
• The external structure of plastids is similar to mitochondria. Just like mitochondria, plastids also have their own DNA and ribosomes.

• These are coloured plastids that impart different colours. 
• For example, the red colour of a tomato, colourful petals of flowers, etc. are due to chromoplast.

• Chloroplasts are green coloured plastids. 
• They also contain orange and yellow pigments. 
• They contain a pigment called chlorophyll. 
• Chlorophyll helps in photosynthesis for producing food. 

• They are white or colourless plastids. 
• These plastids store food in the form of starch, fat (oil) and protein.

• A vacuole is a membrane-bound organelle present in both plant cells as well as animal cells. It is like a storage bag that stores solid and liquid substances called sap. Sap includes amino acids, sugars, various organic acids and some proteins.
• In animals, vacuoles are small-sized where as in plant cells they are very large. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• In single-celled organisms like an amoeba, the food vacuole contains food items that amoeba has consumed.
• In some unicellular organisms, specialized vacuoles perform the important function of expelling excess water and some wastes from the cells.