NCERT Gist: Cell-Structure And Function | Additional Study Material for UPSC PDF Download

- The basic structural unit of an organ, which is the cell.
- Cells are assembled to make the body of every organism. Human body has trillions of cells which vary in shapes and sizes.
- Different groups of cells perform a variety of functions.
- A billion is a thousand million.
- A trillion is a thousand billion. Organisms made of more than one cell are called multicellular (multi: many; cellular : cell) organisms.
- The number of cells being less in smaller organisms does not, in any way, affect the functioning of the organisms.
- You will be surprised to know that an organism with billions of cells begins life as a single cell which is the fertilized egg.
- The fertilised egg cell multiplies and the number of cells increase as development proceeds.
- The single-celled organisms are called unicellular organisms.
- A single-celled organism performs all the necessary functions that multicellular organisms perform.
- A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces.
- Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues.
- Tissues, in turn, form organs.
- Amoeba has no definite shape, unlike other organisms.
- It keeps on changing its shape.
- Observe the projections of varying lengths protruding out of its body are called pseudopodia.
- A white blood cell (WBC) in human blood is another example of a single cell which can change its shape.
- But while WBC is a cell, amoeba is a full fledged organism capable of independent existence.
- Generally, cells are round, spherical or elongated.
- Some cells are long and pointed at both ends.
- They exhibit spindle shape.
- Cells sometimes are quite long.
- Some are branched like the nerve cell or a neuron.
- The nerve cell receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body.
- Components of the cell are enclosed in a membrane.
- This membrane provides shape to the cells of plants and animals.
- Cell wall is n additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.
- Bacterial cell also has a cell wall. The smallest cell is 0.1 to 0.5 micrometre in bacteria.
- The largest cell measuring 170 mm *130 mm, is the egg of an ostrich.
- Each organ is further made up of smaller parts called tissues.
- A tissue is a group of similar cells performing a specific function.
- The basic components of a cell are cell membrane, cytoplasm and nucleus.
- The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane.
- The membrane separates cells from one another and also the cell from the surrounding medium.
- The plasma membrane is porous and allows the movement of substances or materials both inward and outward. The jelly-like substance between the nucleus and the cell membrane is called cytoplasm.
- Cytoplasm is the jelly-like substance present between the cell membrane and the nucleus.
- Various other components, or organelles, of cells are present in the cytoplasm.
- These are mitochondria, golgi bodies, ribosomes, etc.
- Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
- This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.
- With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus.
- In addition, nucleus contains thread-like structures called chromosomes.
- These carry genes and help in inheritance or transfer of characters from the parents to the offspring.
- The chromosomes can be seen only when the cell divides.
- Gene is a unit of inheritance in living organisms.
- It controls the transfer of a hereditary characteristic from parents to offspring.
- The entire content of a living cell is known as protoplasm.
- It includes the cytoplasm and the nucleus.
- Protoplasm is called the living substance of the cell.
- The cells having nuclear material without nuclear membrane are termed prokaryotic cells.
- The organisms with these kinds of cells are called prokaryotes (pro : primitive; karyon: nucleus).
- Examples are bacteria and blue green algae.
- The cells, like onion cells and cheek cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells.
- All organisms other than bacteria and blue green algae are called eukaryotes, (eu: true; karyon: nucleus). Blank-looking structures in the cytoplasm? It is called vacuole. It could be single and big as in an onion cell.
- Cheek cells have smaller vacuoles. Large vacuoles are common in plant cells.
- Vacuoles in animal cells are much smaller.
- Several small coloured bodies in the cytoplasm of the cells of Tradescantia leaf.
- They are scattered in the cytoplasm of the leaf cells. These are called plastids.
-  They are of different colours.
- Some of them contain green pigment called chlorophyll.
- Green coloured plastids are called chloroplasts. They provide green colour to the leaves. You may recall that chlorophyll in the chloroplasts of leaves, is essential for photosynthesis.