NCERT Gist: The Fundamental Unit of Life | Additional Study Material for UPSC PDF Download

- Each such cell has got certain specific components within it known as cell organelles.
- 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.
- If we study a cell under a microscope, we would come across three features in almost every cell; plasma membrane, nucleus and cytoplasm.

Plasma Membrane or Cell 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, 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.
- Diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.
- Water also obeys the law of diffusion.
- The movement of water molecules through such a selectively permeable membrane is called osmosis.
- 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.
- 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.
- 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.
- 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.
- Absorption of water by plant roots is also an example of osmosis.
- The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
- However, we can observe the structure of the plasma membrane only through an electron microscope.
- 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.
- 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 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.
- 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 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.
- 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 entangled mass of thread like structures.
- Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.
- The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.
-  An undefined nuclear region containing only nucleic acids is called a nucleoid.
- Such organisms, whose cells lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary; karyote -karyon = nucleus).
- Organisms with cells having a nuclear membrane are called eukaryotes.
- The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.
- The cytoplasm is the fluid content inside the plasma membrane.
- The significance of membranes can be illustrated with the example of viruses.
- Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.
- Important examples of cell organelles are: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.

Endoplasmic Reticulum

- A large network of membrane-bound tubes and sheets.
- 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 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 manufacture of fat molecules, or lipids, important for cell function.
- 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 the nucleus.
- SER plays a crucial role in detoxifying many poisons and drugs.

Golgi Apparatus

- Its functions 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.
- Also involved in the formation of lysosomes.

Lysosomes

- A kind of waste disposal system of the cell.
- Help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
- Able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.

Mitochondria

- known as the powerhouses of the cell.
- ATP (Adenosine triphopshate) molecules is known as the energy currency of the cell.
- The body uses energy stored in ATP for making new chemical compounds and for mechanical work. 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-generating chemical reactions.
- Strange organelles in the sense that they have their own DNA and ribosomes.
- Able to make some of their own proteins.

Plastids

- present only in plant cells.
- There are two types of plastids - chromoplasts (coloured plastids) and leucoplasts (white or colourless 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. Like the mitochondria, plastids also have their own DNA and ribosomes.

Vacuoles

- Storage sacs for solid or liquid contents.
- Small sized in animal cells while plant cells have very large vacuoles.
- 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.