Early Days of Cells
Cells are the fundamental structural units of living organisms and the basic units of life.
Robert Hooke discovered the cell in the year 1665 from a thin section of cork under a self-made microscope.
World of Cells
Prokaryotic & Eukaryotic cells
Prokaryotic – Size: generally small ( 1-10 µm) Where 1 µm = 10 −6m.
- The nucleus is not well defined and known as a nucleoid.
- Single, circular chromosome.
- Membrane-bound organelles absent. Eg: Bacterial cell
Eukaryotic – Size: generally large ( 5-100 µm).
- The nucleus is well defined and surrounded by a nuclear membrane.
- Possesses more than one and linear chromosome.
- Membrane-bound cell organelles are present. Eg: Human cell.
Cell structure in Eukaryotic cells
- Most eukaryotes are multicellular.
- The nucleolus is present in the nucleus.
- It contains cell organelles such as plastids, mitochondria, Golgi bodies, etc.
- Fungi, plant, and animal cells are eukaryotic.
- The cell membrane separates cells from one another and also the internal contents from the surrounding medium.
- It is porous and allows the movement of substances or materials both inward and outward.
- It 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.
Cell walls in plants
- A cell wall is an additional covering over the cell membrane in plant cells.
- It gives shape and rigidity to these cells.
- It gives a rigid structure to plant cells and that is why they can withstand the stress of high-velocity wind, rain, high temperature etc.
- The various components present within the cell are known as cell organelles.
- All these components are special and specific to their functions.
- For example, mitochondria, lysosomes etc.
- It is generally spherical and located in the centre of the cell.
- The nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
- It uses information in the chromosomes to decide what each organelle should do and the functions of the cell.
- The nucleus contains an even smaller concentrated material called nucleolus.
- The nucleus contains thread-like structures called chromosomes.
- These carry genes and help in inheritance or transfer of characters from the parents to the offspring.
- Chromosomes are composed of DNA in the form of Chromatin and protein.
- Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA molecules.
- DNA molecules contain the information necessary for constructing and organising cells.
Functional segments of DNA are called genes.
Vacuoles, Endoplasmic Reticulum and Golgi Apparatus
- Vacuoles are storage bubbles of irregular shapes which are found in cells.
- The vacuole stores the food, a variety of nutrients that a cell might need to survive or waste.
- In plant cells, vacuoles are much larger than in animal cells and they provide rigidity to the plant cells.
- Golgi Apparatus also termed as Golgi Complex is a membrane-bound nucleus.
- Its main function is to modify proteins.
- These modified proteins are placed in Golgi so that it can transport elsewhere in the cell when required.
- It is a membranous organelle.
- It is found in all eukaryotic cells.
- The ER is divided into two types based on their physical and functional properties viz. Smooth and Rough ER.
- It synthesizes and secretes proteins in the cells.
- The SER is involved in the synthesis of phospholipids, the main lipids in cell membranes and are very important in the process of metabolism.
- They also transport the products of the rough endoplasmic reticulum to other cell parts like the Golgi apparatus.
- Lysosomes are small membrane-bound sac-like structures which release digestive enzymes that break down food.
- They also remove the old and damaged or dead organelles from the cell and thus, protect the cell from further damage.
- Mitochondria is a double membrane-bound sausage shaped organelle.
- Also known as ‘powerhouse of the cell’.
- Mitochondria are the sites of aerobic respiration in the cell and it produces energy in the form of ATP.
- Plant cells have tiny, coloured structures called plastids.
- Plastids carry pigments of various colours like orange, red, green and even colourless.
- The plastids which carry green-coloured pigments i.e., chlorophyll are called chloroplasts.
Plants vs Animals
Comparison of plant and animals cells
|Cell Organelle||Plant Cell||Animal Cell|
|NCERT Solutions for class 8 Science Chapter 8|
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