Chapter 8: Cell: The Unit Of Life

A cell must have the following structural and functional attributes to be a called a living cell

a) Cell membrane – It is the outer membrane of the cell which is made up of phospholipids primarily.

b) Cytoplasm – The liquid containing the controlled concentration of organic and inorganic compounds. They provide energy and material for reproduction.

c) Nucleus – It contains DNA, the nuclear matrix or the nucleoplasm containing nucleolus and chromatin. They can be also known as the brain of the cell which controls all the activities.

Rudolf Virchow’s modern theory of cell is the cell divides and new cells are formed from the [pre-existing cells. He modified the hypothesis of Schleiden and Schwann to give the cell theory to its final.

1. All living organism is composed of cells and products of cells

2. All cells arise from pre-existing cells

In 1839, a British Zoologist, Theodore Schwann, studied a different type of animal cells and said that cell had a thin layer later known as the plasma membrane. He studied about plant cells and concluded that the presence of cell wall is a unique character of the plant cells.

Yes, extra genomic DNA is present in prokaryotes and eukaryotes. In organisms like bacteria in addition to the genomic DNA, there is a small circular DNA outside the genomic DNA. They are called plasmids. This is a unique phenotypic character to such bacteria.

In eukaryotes, the extra genomic DNA is present in two organelles- Mitochondria and plastids.

It is made up of proteins, carbohydrates and lipids. The lipids are arranged with their hydrophilic polar head directed outwards and non-polar hydrophobic tails directed inwards, providing fluidity to the membrane. This provides the cell membrane with its selective permeability.

a. Non-membrane bound cell organelles – Ribosomes, Centriole, nucleolus, Cytoskeletal structures

b. Single membrane-bound cell organelles – Endoplasmic reticulum, Golgi bodies, lysosomes, vacuoles, Thylakoid.

c. Double membrane-bound cell organelles – Plastids, Mitochondria and nucleus.

Consider a human cell. It has 46 chromosomes and will be constant for all human beings. But the mitochondria present in the human will have a different amount of extrachromosomal DNA. As like in human beings, bacteria will have the same amount of genomic DNA but the extra genomic DNA contained in plastids and mitochondria will be different.

There are two membranes for each mitochondrion. They are the inner and outer membranes. The inner membrane is called the matrix. They have their specific enzymes associated with their functions. They are the sites for aerobic respiration and produce cellular energy in the form of ATP and are called powerhouses of a cell.

Plastids are species-specific or region-specific. They bear some specific pigments, thus imparting specific colours to the plants. According to the types of pigments, plastids are divided into three. chloroplasts, chromoplasts and leucoplasts.

The chloroplasts impart green colour as they contain chlorophyll and carotenoid pigments

The chromoplasts contain fat-soluble carotenoid pigments like carotene, xanthophylls and others are present.

The leucoplasts are colourless plastids of varied shapes and sizes with stored nutrients

a. Centromere – Centromere holds two chromatids or sister chromatids of a chromosome.

b. The cell wall is present in plant cells which gives shape and protection to the cell from mechanical damage.

c. Smooth ER – It is a major site for lipid synthesis. In animal cells, lipid-like steroidal hormones are synthesized at Smooth ER.

d. Golgi apparatus – It is an important site for the formation of glycolipids and glycoproteins.

e. Centrioles – The basal body of cilia or flagella and spindle fibres that give rise to spindle apparatus.

Yes, plastids are interchangeable in their form.

There are three types of plastids – Chloroplasts (green colour), Chromoplasts (red, yellow, orange colour), and Leucoplasts (Colourless). Depending upon different circumstances, these plastids interchange.

For example: Due to the replacement of chloroplast with chromoplasts, the colour of green tomatoes will change from red to blue.

Chapter 8, Cell The Unit of Life, talks about the basic component which is responsible for life existing in all species. It is the smallest and most fundamental unit of life, which can replicate itself in all known living organisms. The largest cell in the female body is the ovum, and the smallest cell in the male body is the sperm. We know that plants and animals are made of cells and can understand all the components within them.