Cell structure and functions

Cells:

• The cell is the basic structural and functional unit of life. All living organisms are made up of cells. Cells make tissues, tissues make organs, organs make organ systems and organ systems make a living organism. Thus, the cell is the building block, or the structural unit of the living body.
• Cells were discovered for the first time by Robert Hooke in 1665. He observed a piece of cork under his microscope. He saw that the cork was made up of many small boxes. He called these little boxes cells. These box-like structures were actually dead cells.
• Living cells were observed by Anton Von Leeuwenhoek for the first time using improved microscopes.

Cell Theory

• All living organisms are made up of cells.
• New cells arise from pre-existing cells.
• All cells are similar in structure and function but are not identical.
• The function of an organism depends on the way the cells work or the functions they perform.

Shapes and sizes

• Depending on their function, the cells show a variation in their shapes and sizes.
• They may be oval, spindle-shaped, spherical etc.
• Sizes of cells are measured in micrometer or microns. (1 micrometer = 1×10^-6m)
• Most cells range in size from 5 micrometer to 15 micrometer.
• Bacterial cells are the smallest cells.
• The longest cells in our body are the nerve cells.
• The egg of ostrich is the largest cell- it is a single cell.

Cell Number

Structure of a cell

Cell membrane

• The thin outer layer of the cell is called the cell membrane or the plasma membrane.
• It separates the cell and its contents from the surroundings.
• It protects the inner cell contents.
• It protects the cell and gives it a shape.
• It is selectively permeable and allows water, minerals and other necessary substances to pass through it.
• It controls what passes in and out of the cell

Cell Wall

• Cell wall is present only in plant cells.
• It is an additional wall outside the cell membrane.
• It is thick wall made up of cellulose.
• The cell wall gives shape to the plant cell, offer protection and provide rigidity to the cell.
• The cell wall is important for plants since they do not have a skeleton for support and protection, nor they can move to escape environmental stress.

cytoplasm

• The cytoplasm is a jelly-like substance that makes up most of the inside of the cell.
• The cytoplasm has the nucleus and various other cellular components within it.
• These components are called cell organelles.

Nucleus

• The nucleus is a spherical body present inside the cell. It is the control centre of the cell.
• The nucleus is surrounded by the nuclear membrane and the liquid present inside is called nucleoplasm.
• The nucleus and the cytoplasm together make up the protoplasm.
• A smaller spherical structure present inside the nucleus, called Nucleolus plays an important role in protein synthesis.
• The nucleus also has a thread-like network called chromatin.
• The chromatin organizes itself into rod-like structures when the cell has to divide. These rod-like structures are called chromosomes. They contain the DNA inside them. DNA has the genes that help in transfer of characteristics from parent to the offspring. Therefore, DNA forms the hereditary material in most organisms.
• Note: The nucleus and cytoplasm together make up the protoplasm.

CELL ORGANELLES:

MITOCHONDRIA:

RIBOSOMES:

PLASTIDS:

GOLGI BODIES:

VACUOLES:

ENDOPLASMIC RETICULUM:

CENTRIOLES AND CENTROSOMES:

LYSOSOMES:

CELL DIVISION:

COMPARISON BETWEEN ANIMAL AND PLANT CELLS:

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