The Fundamental Unit of Life Test - 9

Prokaryotic cells do not contain a well-defined nucleus. These cells have only one chromosome scattered in the cytoplasm of the cell. Many important genes of prokaryotes are stored in separate circular DNA structures called plasmids.

Ribosomes are the only cell organelle seen in prokaryotic cell. Ribosomes in prokaryotes are smaller than those in eukaryotes.

Eukaryotic cells contain 80 S ribosomes, which are further divided into two unequal subunits designated small subunit (40S) and large subunit (60S) according to their sedimentation coefficients

In a prokaryotic cell, the nuclear region is poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

On the other hand, organisms with cells having a nuclear membrane are called eukaroytes. Eukaryotic cells are considered complete cells.

Viruses are the link between the living and the non-living because they lack membranes and do not show any characteristics of life until they enter the body of a host.

Peroxisomes are small, membrane-enclosed organelles that contain enzymes involved in a variety of metabolic reaction. They help in removal of toxic substances from the cell. They are surrounded by only a single membrane.

Cell-wall is made up of cellulose - a non-living substance. Hence, cell-wall is a non-living part of the cell. Mitochondria is known as the powerhouse of the cell. They contain enzymes for cellular respiration. During cellular respiration, energy is released in the form of ATP (Adenosine triphosphate). Photosynthesis is performed by chlorophyll pigments present in the Chloroplast (a plastid). Ribosomes are associated with the synthesis of proteins from amino acids. Proteins are required by a cell for functions such as repair or directing chemical processes.

Genes are functional units of heredity that determine the characters of organisms. Diffusion is the process of passage of fluid from a region of high concentration to a region of low concentration. It plays an important role in gaseous exchange between the cells as well as the cell and its external environment. Water also obeys the law of diffusion. The passage of water from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane is called osmosis. The movement of water across the plasma membrane is affected by the amount of substance dissolved in water. Plasmolysis refers to the contraction of protoplast as a result of loss of water from the cell. The shrinkage of cytoplasm occurs due to exo-osmosis in a hypertonic medium. A hypertonic solution is one which has a lesser concentration of water as compared to that inside the cell. During the process, there is a higher external osmotic pressure and a net flow of water from the cell. 

While the nucleic part of the cell had been observed by Leeuwenhoek in 1682, it was Robert Brown who named it the “cell nucleus”. In 1839, Johann Evangelist Purkinje coined the term 'protoplasm' for the fluid substance of a cell. Robert Hooke observed the microscopic structure of the bark of a cork tree and in doing so, discovered and named the cell – the building block of life. The 'Cell theory' was proposed by Matthias Jakob Schleiden and Theodor Schwann.

A prokaryote is a unicellular organism like bacteria that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek language - pro is "before" and karyon is "nut or kernel". Mycoplasma is a genus of bacteria that lack a cell wall around their cell membrane. Without a cell wall, they are unaffected by common antibiotics that target cell wall synthesis. Mycoplasma species are the smallest free-living organisms. Ostrich egg is the largest cell.

Cells are enclosed by a plasma membrane composed of proteins and lipids. It separates the contents of the cell from the external environment. Tonoplast is the membrane which bounds the chief vacuole of a plant cell. It is also known as the 'vacuolar membrane'. It separates the vacuolar contents from the cytoplasm of the cell. Stroma is the part of a tissue or organ that has a connective and structural role. Each of the partial partitions in a mitochondrion formed by infolding of the inner membrane is known as Cristae.

Lysosmes are popularly known as 'suicidal bags'. They play a role in cell death. The main function of the centriole is to help with cell division in animal cells. The centrioles help in the formation of the spindle fibers that separate the chromosomes during cell division (mitosis). The Golgi apparatus is responsible for transporting, modifying and packaging proteins and lipids into vesicles for delivery to targeted destinations. The smooth ER is involved in the synthesis of lipids.