Cell - The Fundamental Unit of Life
Synopsis
- The cell is the basic unit of life. Cells were first discovered by Robert Hooke in 1665 by the observation of a cork slice under microscope. The cell theory was proposed by Schleiden and Schwann.
- Rudolf Virchow proposed that all cells originate from pre-existing cells. The outermost covering that separates the contents of the cell from its external environment is called the plasma membrane, which is selectively permeable and is made up of lipids and proteins.
- Plant cells have a rigid outer covering in addition to the plasma membrane. It is called the cell wall and is composed of cellulose.
- The Nucleus is covered by a double-layered nuclear membrane and is filled with nucleoplasm. It has chromosomes that are composed of DNA and proteins.
- Cells which lack a nuclear membrane and cell membrane bound organelles are called
Prokaryotes and cells having a nuclear membrane are called Eukaryotes.
- The Cytoplasm is the fluid content inside the plasma membrane of a cell.
- The Endoplasmic reticulum (ER) is a large network of membrane bound tubes and sheets.
There are two types of ER - RER (site of lipid-protein synthesis) and SER (site of synthesis and detoxification of poisons and drugs).
- Lysosomes are saclike, small, spherical, single membrane bound vesicles which contain enzymes. They keep the cell clean by digesting the foreign materials entering the cell. They are otherwise known as the suicidal bags of the cell
- The Golgi complex is a stack of flattened sacs meant for the secretion of substances.
- Mitochondria are the power houses of the cell which generate ATP that provide energy required for various chemical activities in a cell.
- Ribosomes are responsible for protein synthesis in a cell. A chain of ribosomes is termed as a polyribosome or polysome.
- Plastids are present only in plant cells. There are three types of plastids - chloroplasts, chloroplasts (coloured) and leucoplasts (white or colourless).
- Vacuoles are storage structures for solid or liquid content which are filled with cell sap.
- Diffusion is the movement of a gaseous substance from a region of higher concentration to a region of lower concentration.
- Osmosis is the movement of water molecules from a solution of lower concentration to a solution of higher concentration across a partially permeable membrane.
- The pressure of the water in the vacuole which keeps the plant cell firm or turgid is called the turgor pressure.
