Test: Cell - The Unit of Life - Assertion and Reasoning - NEET MCQ

Cells are the basic structural and functional unit of organism.

The correct answer is:

1. If both Assertion and Reason are true and Reason is the correct explanation of Assertion

Specialization of cells is indeed useful for an organism because it allows cells to perform specific functions more efficiently, which increases the overall operational efficiency of the organism.

The size and shape of the cell in multicellular organism depends upon the location and function performed by them.

All living organisms have definite shape and size and all show specific individuality with an orderly mannered organisation whereas at the lower level of organisation, both the living and non living are made up of atoms.

Metabolic reactions of a living organism can occur only in a delicately balanced environment in the non-living organisms. The cells are the life supporting chambers which have such a special environment. A living cell keeps its chemical composition steady within its boundary

Number of cells in a multicelluar organism are directly proportional to the size of the body. On the other hand, it is a fact that cell vary greatly in their size. Mycoplasma cells are the smallest, ranging from 0.1 to 0.3μm, whereas human cells, generally range from 20 to 30μm. Nerve cells are the longest.

Cell is an isothermal open system as all the parts of the cell at any given time maintains the same temperature and pressure. Cell is an open system in which materials and energy are transferred between organisms and the exterior environment.

Metabolically active cells are usually smaller due to higher nucleocytoplasmic ratio and higher surface volume ratio. The former will allow the nucleus to have better control of metabolic activities, while the latter will allow quicker exchange of materials between the cells and its outside environment.

Schleiden and Schwann together formulated the cell theory. The theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omins cellua e cellua). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. The present cell theory states that: (i) all living organism are composed of cells and products of cells. (ii) all cells arise from preexisting cells.

They are credited with cell theory but the cells are not always the living unit. Cells die and still remain functional such as horny cells in animal and xylem vessels in plants.

Based on his studies, Schwann proposed the hypothesis that the bodies of animal and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory. Cell theory as understood is (i) All living organism are composed of cells and products of cells and (ii) all cells arise from pre-existing cells.

Eukaryotic cells which have membrane bound distinct structures called organelles like nucleus, endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies and vacuoles. These are found in all protists, plants, animals and fungi. Prokaryotic cells lack such membrane bound organelles. Prokaryotic cells occur in bacteria, archaea, blue-green algae, mycoplasma and PPLO. Genetic material in these cells lies naked in the cytoplasm.

Ribosomes are non-membrane bound organelles found in eukaryotic as well as prokaryotic cells. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles - chloroplasts (in plants) and mitochondria and on rough ER.

Eukaryotic cells have more DNA than prokaryotic cells because in eukaryotic cells complex chromosomes are composed of DNA and histone proteins. But in prokaryotic cells, histone protein is absent.

Most prokaryotic cell particularly the bacterial cells have a chemically complex cell envelope. The cell envelope consists of a tightly bound three layered structure, i.e. the outermost glycocalyx followed by the cell wall and plasma membrane. Although each layer of the envelope performs distinct function, they act together as a single protective unit.

Lipids present in the outer and inner side of the bilayer are commonly different, e.g., lecithin on the outer side and cephalin on the inner side of erythrocyte membrane. Oligosaccoharides are attached to external surface of lipids and proteins of a bio-membrane. They are absent on the inner side.

Cell membrane is semipermeable as it allows continuous flow of selected materials across it as required from time to time. On the other hand, constituent molecules of cell membrane are free to move inside membrane.

With the help of freeze-fracture techniques in electron microscopy, the fluid mosaic model was put forward in 1970 by S.J. Singer and G.L. Nicolson. According to this model, plasma membrane is composed of phospholipids, extrinsic proteins. Selective permeability of plasma membrane can be explained with this model.

Of all the membrane associated enzymes, Na⁺–K⁺ ATPase is one of the most important because of its role in ion transfer across the plasma membrane. This enzyme is dependent on the presence of lipids and is inactivated when all lipids are extracted.

Membrane proteins are classified as integral or peripheral on the basis of ease of extraction. Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane.