Chapter – 8
Cell : The unit of Life
Q1. Which of the following is not correct?
(a). Robert Brown discovered the cell.
(b). Schleiden and Schwann formulated the cell theory
(c). Virchow explained that the cells are formed from pre-existing cells.
(d). A unicellular organism carries out its activities within a single cell.
Answer:
(a) is not correct. All the others are correct.Robert Brown discovered the nucleus and not the cell. Cells were first observed by Robert Hooke when he was studying the cork cells of plants. Metthi as Schleiden and Theodore Schwann( 1938) proposed the cell theory which was later modified by Rudolf Virchow(1855). The cell theory states that1. All living organisms are composed of cells and products of cells.2. All cells arise from pre-existing cells. ( This was suggested by Rudolf Virchow)
Q2. New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic material
Answer:
(c) . New cells generate from pre-existing cells.According to Rudolf Virchow, the new cells arise from pre-existing cells by their division.
Q3. Match the following Column I Column II
(a) Cristae
(i) Flat membranous sacs in stroma
(b) Cisternae
(ii) In foldings in mitochondria
(c) Thylakoids
(iii) Disc-shaped sacs in Golgi apparatus
Answer:
The correct matching is
(a)- (ii),
(b)- (iii),
(c)- (i)Column I Column II
(a) Cristae (ii) In foldings in mitochondria
(b) Cisternae (iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids (i) Flat membranous sacs in stroma
Q4. Which of the following is correct:
(a) Cells of all liv ing organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.
Answer:
The correct statement is
(c) In prokaryotes, there are no membrane-bound organelles. All other statements are incorrect as cells of some organism can lack a nucleus (e.g. RBC ), animal cells do not possess a cell wall and cells are formed from pre-existing cells.
Q5. What is a meso some in a prokaryotic cell? Mention the functions that it performs.
Answer:
Meso some is a complex membranous structure formed by the infoldings of the plasma membrane in prokaryotic cells. The functions performed by mesosome are as follows:1. Meso somes play important roles in cell wall formation, DNA replication etc.2. Meso somes are folded structures, this quality helps to increase the surface area of the plasma membrane to carry out enzymatic activities.3. Mesosome also helps in cellular respiration and secretion.
Q6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Answer:
Neutral solutes are lipid soluble. These move across the plasma membrane by directly crossing through the lipid bilayer. Their rate of movement across the plasma membrane depends on concentration gradient and lipid solubility of neutral solutes. No polar molecules can not move across the membrane in the same manner as neutral solutes. The polar molecules require carrier proteins in order to pass through the membrane. The carrier proteins are integral membrane proteins that have an affinity for some specific molecules and during transport, these carriers carry these molecules to the other side of the membrane.
7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
Answer:
The two double membrane-bound organelles are mitochondria and chloroplast.Characteristics of Mitochondria-1. Mitochondria are cylindrical shaped cell organelles which can be stained by Janus green and observed in the microscope. MItochondria consists of 60-70% proteins, 25-35 % lipids, 5-7% RNA, DNA etc.2. Mitochondria are encircled by two membranes i.e. an outer membrane and inner membrane. These two membranes are separated by per-mitochondrial space. The outer membrane is smooth and porous in nature. POrins are present in the outer membrane.3. The inner membrane of mitochondria is invaginated into folds that are called cristae. Electron transport chain and oxysomes are found in the inner membrane.4. Inside the mitochondria, a semi-fluid matrix is present which consists of protein particle, ribosomes, RNA, DNA and enzymes that are used in the Krebs cycle, amino acid synthesis etc.5. Mitochondria are semi-autonomous organelles as they possess their own DNA and protein synthetic machinery. The DNA is of mitochondria is naked and circular.
8. What are the characteristics of prokaryotic cells?
Answer:
Characteristics of prokaryotic cells Prokaryotic cells are those cells which do not have a well-defined nucleus. The most important characteristics of prokaryotic cells are as follows:1. The nucleus of prokaryotic cells is not well defined i.e. it does not possess a nuclear envelope. The DNA of these cells lies freely in the cytoplasm. The DNA is naked and variously coiled. Extrachromosomal DNA Is also found in these cells.2. Prokaryotic cells are devoid of membrane-bound cell organelles.3. The cell wall is present in prokaryotic cells of bacteria and cyanobacteria, however, it is absent in mycoplasma.4. Instead of chloroplasts, chromatophores are present in prokaryotes to carry out photosynthesis.5. Ribosomes of prokaryotic cells are of 70S type.
Q9. Multicellular organisms have division of labour. Explain.
Answer:
Division of labour refers to the differentiation of certain components or parts to perform specific functions to cause an increase in the efficiency and survival rate of that organism. Multicellular organisms are composed of millions of cells. All these cells are different from each other with respect to their structure and function. These cells together are responsible for the survival of the organism. The various necessary functions are executed via the division of labour. In simple words, we can say that in multicellular organisms the functions essential for the survival of the organism are divided and allotted to different tissues and tissue systems. All these tissues and their systems work in a coordinated manner. For example, every cell of a multicellular organism cannot obtain food from outside. The organism requires a system for obtaining food, its digestion and distribution. Therefore, a digestive system and system of transport are also required. So, there are tissues responsible for digestion and transportation. Similarly, tissues are there for excretion, respiration etc. Similarly, certain cells of the body take over the function of reproduction. Others take part in repair and replacement of worn out or injured portions. Hence, multicellular organisms show the division of labour.
Q10. Cell is the basic unit of life. Discuss in brief.
Answer:
A living organism is composed of various organ and organs systems. These organ systems are made up of tissues that in turn are composed of cells. A cell is the smallest unit of life which can survive on its own and perform all the essential functions necessary for its survival. It not only supports the structure of an organism but its functions also. Due to this reason cell is the basic structural and functional unit of life.
Q11. What are nuclear pores? State their function.
Answer:
Nuclear pores are the areas in the nuclear membrane where two membranes fuse with each other. The nuclear pores allow the transfer of only some specific substances across the nuclear membrane. For example, they allow RNA and proteins to move in and move out of the membrane.
Q12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Answer:
Both lysosome and vacuole are endomem branous structures with only a single membrane, IN case of vacuoles, the membrane is called tonoplast. Despite being endomem branous structures, the function of lysosome and vacuole differs considerably. The lysosome consists of hydrolytic enzymes that hydrolyze various substances except for cellulose. These can even digest worn contents of their own cell if required. Due to this reason, lysosomes are also called suicidal bags. On the other hand, vacuoles are non-cytoplasmic sacs that store substances. For example, sap vacuole store sap with organic and inorganic substances and by this maintains the osmotic pressure and turgidity of the cell. Similarly, food vacuole store food and gas vacuoles store metabolic gases.