Early Days of Cells

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 ${}^{}$m.

• 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.
• Possess 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.

Cell membrane

• 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.

Cytoplasm

• 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.

Cell Factory

Organelles

• 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.

Nucleus

• 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.

Chromosomes

• 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

• 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

• 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

• 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.

Endoplasmic Reticulum

• 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.

Rough ER

• It synthesizes and secretes proteins in the cells.

Smooth ER

• 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

• 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

• 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.

Plastids

• 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