NCERT Exemplar for Class 11 Biology - Cell The Unit of Life - Free PDF Download

1. A common characteristic feature of plant sieve tube cells and most of mammalian erythrocytes is 

a. Absence of mitochondria 

b. Presence of cell wall

c. Presence of haemoglobin 

d. Absence of nucleus 

Ans: A common characteristic feature of plant sieve tube cells and most mammalian erythrocytes is absence of nucleus as due to absence of nucleus gives erythrocytes more space for binding more Hb. On the other side lack of cell organelles in the sieve tube provide extra space to accomodate more water and ensure fast transportation.

Hence, option (d) is the correct choice.

2. Select one which is not true for ribosome 

a. Made of two subunits 

b. Form polysome 

c. May attach to m₊RNA 

d. Have no role in protein synthesis 

Ans: Ribosomes are present on the endoplasmic reticulum and thus forms a rough endoplasmic reticulum. They are associated with formation of protein.

Hence, option (d) is the correct choice.

3. Which one of these is not a eukaryote? 

a. Euglena 

b. Anabaena 

c. Spirogyra 

d. Agaricus 

Ans: Anabaena is a nitrogen fixing bacteria. It is a prokaryote and belongs to kingdom monera.

Hence, option (b) is the correct choice.

4. Which of the following stains is not used for staining chromosomes? 

a. Basic Fuchsin

b. Safranin 

c. Methylene green 

d. Carmine

Ans: All the stains are used for staining chromosome and nuclei except safranin, it is used as counterstain for endospore staining and gram staining.

Hence, option (b) is the correct choice.

5. Different cells have different sizes. Arrange the following cells in an ascending order of their size. Choose the correct option among the followings 

i. Mycoplasma 

ii. Ostrich eggs

iii. Human RBC 

iv. Bacteria Options: 

a. i, iv, iii & ii 

b. i, iii, iv & ii 

c. ii, i, iii & iv 

d. iii, ii, i & iv

Ans: Mycoplasma is known as the smallest cell, its size is about 0.3 micron. The size of an ostrich egg is about 15.5cm, it is the largest living cell. The size of human RBCs are about 7micron in diameter and the size of the bacteria is about 3-5 micron.

So, the smallest cell is mycoplasma followed by bacteria after that human RBCs and the largest is Ostrich egg.

Hence, option (a) is the correct choice.

6. Which of the following features is common to prokaryotes and many eukaryotes? 

a. Chromatin material 

b. Cell wall present 

c. Nuclear membrane present 

d. Membranes bound sub-cellular organelles present 

Ans: The feature that is common in prokaryotes and many eukaryotes is 'chromatin material present'.

Cell wall is absent in animal cells (eukaryote), Nuclear membrane and membrane bound sub-cellular organelles are absent in prokaryotes.

Hence, option (a) is the correct choice.

7. Who proposed the fluid mosaic model of plasma membrane? 

a. Benda 

b. Schleiden and Schwann 

c. Singer and Nicolson 

d. Robert Brown

Ans: Fluid mosaic model of plasma membrane was given by Singer and Nicolson in early 1970s. It was an improved model of the structure of cell membranes.

Benda had discovered the mitochondria. Schleiden and Schwann had proposed the cell theory. Robert Brown has described the nucleus.

Hence, option (c) is the correct choice.

8. Which of the following statements is true for a secretory cell? 

a. Golgi apparatus is absent 

b. Rough Endoplasmic Reticulum (RER) is easily observed in the cell 

c. Only Smooth Endoplasmic Reticulum (SER) is present 

d. Secretory granules are formed in the nucleus.

Ans: Golgi apparatus is required for packaging secretory granules. Rough Endoplasmic Reticulum (RER) is easily observed in the cell which is involved in secretion and protein synthesis and Smooth Endoplasmic Reticulum (SER) is present along with RER. There are no secretory granules formed in the nucleus.

Hence, option (b) is the correct choice.

9. What is a tonoplast? 

a. Outer membrane of mitochondria 

b. Inner membrane of chloroplast 

c. Membrane boundary of the vacuole of plant cells 

d. Cell membrane of a plant cell

Ans: Tonoplast is a cytoplasmic membrane which surrounds the vacuole and separates it from the cytoplasm in a cell. It regulates the movements of ions around the cell and isolates the substances which may be harmful to the cell.

Hence, option (d) is the correct choice.

10. Which of the following is not true of a eukaryotic cell? 

a. Cell wall is made up of peptidoglycans 

b. 80S type of ribosomes are present in the cytoplasm 

c. Mitochondria contain circular DNA 

d. Membrane bound organelles are present

Ans: All the given options are true regarding eukaryotic cells except the cell wall is made up of peptidoglycans. Prokaryotic cell wall is made up of peptidoglycans while eukaryotic cell wall is made up of cellulose.

Hence, option (a) is the correct choice.

11. Which of the following statements is not true for plasma membranes? 

a. It is present in both plant and animal cell 

b. Lipid is present as a bilayer in it 

c. Proteins are present integrated as well as loosely associated with the lipid bilayer 

d. Carbohydrate is never found in it 

Ans: The main components of plasma membrane are lipids, proteins, and carbohydrates. Plasma membrane is a phospholipid bilayer which makes a stable barrier between two aqueous compartments. Proteins are embedded within these phospholipid bilayer and carbohydrates are always found on the exterior surface of cells and bound to proteins and forms glycoproteins.

Hence, option (d) is the correct choice.

12. Plastid differs from mitochondria on the basis of one of the following features. Mark the right answer. 

a. Presence of two layers of membrane 

b. Presence of ribosome 

c. Presence of thylakoids 

d. Presence of DNA

Ans: Plastids differ from mitochondria in having thylakoids. Plastids have thylakoids which is a flattened sac which is placed over one another and forms a grana. They are involved in photosynthesis.

Hence, option (c) is the correct choice.

13. Which of the following is not a function of cytoskeleton in a cell? 

a. Intracellular transport 

b. Maintenance of cell shape and structure 

c. Support of the organelle 

d. Cell motility

Ans: Cytoskeleton is the structure found in cytoplasm that is involved in maintaining the shape of the cell and its internal organisation. It provides mechanical support to cells and helps them in carrying  out essential functions.

Hence, option (a) is the correct choice.

14. The stain used to visualise mitochondria is 

a. Fast green 

b. Safranin 

c. Acetocarmine 

d. Janus green

Ans: Janus green is used to visualise mitochondria. It acts as an indicator and changes the colour on the basis of the presence of oxygen present in the mitochondria.

Hence, option (d) is the correct choice.

VERY SHORT ANSWER TYPE QUESTIONS 

1. What is the significance of vacuole in a plant cell?

Ans: The significance of vacuole in a plant cell are:

It plays an important role in homeostasis of the plant cell.

It controls the cell volume and cell turgor.

It regulates the cytoplasmic ions and pH.

It stores the amino acids, sugars, etc.

2. What does ‘S’ refer to in a 70S & an 80S ribosome?

Ans: In 70S & 80S ribosomes, letter S refers to Svedberg's Unit. It is the sedimentation coefficient in the ribosome.

3. Mention a single membrane bound organelle which is rich in hydrolytic enzymes.

Ans: A single membrane bound organelle which is rich in hydrolytic enzymes is Lysosome. It is a vesicular structure formed by the Golgi apparatus.

4. What are gas vacuoles? State their functions?

Ans: Gas vacuoles are also known as air vacuoles. It is the characteristic feature of prokaryotes. It helps bacteria in floating at desired depth. This is done by inflating and deflating the vesicles. In photosynthetic bacteria help in getting optimal light and oxygen.

5. What is the function of a polysome?

Ans: Polysome is a group of ribosomes bound to mRNA. The function of polysomes is to translate mRNA instructions into polypeptides.

6. What is the feature of a metacentric chromosome?

Ans: The feature of the metacentric chromosome is that the centromere is present in the middle of two arms by which four arms are almost equal in length.

7. What is refer-ed to as a satellite chromosome?

Ans: Satellite chromosome is the part of a chromosome which is separated from the rest of the chromosome by a secondary constriction.

SHORT ANSWER TYPE QUESTIONS 

1. Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis. 

Ans: The role of nucleolus in the cells actively involved in protein synthesis are:

It is the chief site of ribosomal RNA synthesis.

It is a colloidal complex that fills the nucleus.

It also combines rRNA with protein to produce ribosomal subunits.

2. Explain the association of carbohydrate to the plasma membrane and its significance.

Ans: Carbohydrates are attached with proteins or lipids of the plasma membrane..

Significance:

It gives distinguishing features to each cell type.

They are involved in cell recognition such as blood groups as different blood groups have different antigens.

3. Comment on the cartwheel structure of centriole.

Ans: Centrosomes consist of two centrioles, they are surrounded by pericentriolar materials  and both the centriole in centrosome lies perpendicular to each other. This organisation appears like a 'cart wheel'.

4. Briefly describe the cell theory.

Ans: Cell theory was proposed by Schleiden and Schwann. This theory states that:

All living organisms are made up of cells and their products.

Rudolf Virchow proposed that cells are formed from pre-existing cells.

Cells are the basic unit of life.

5. Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).

Ans:

6. Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?

Ans: Plasma membrane consists of lipids, proteins and carbohydrates. Carbohydrates are attached with proteins or lipids.

Lipids are arranged in bilayer with protein embedded in the matrix. Lipids have one hydrophilic polar head which is present on the outer side and another hydrophobic tail which is present on the inner side of the membrane.

7. What are plasmids? Describe their role in bacteria?

Ans: Plasmids are extrachromosomal DNA present in the bacteria.

It is used to transfer information from one cell to another.

It is used to monitor bacterial transformation with foreign DNA.

It provides unique phenotypic characteristics to bacteria.

8. What are histones? What are their functions?

Ans: Histones are proteins which are alkaline in nature. It is present in the nucleus of a eukaryotic cell.

DNA gets coiled around the histone protein as a result a very long strand of DNA gets fit into small space.

LONG ANSWER TYPE QUESTIONS 

1. What structural and functional attributes must a cell have to be called a living cell? 

Ans: The structural and functional attributes must a cell have to be called a living cell are:

1. Cell membrane- It is the outer membrane of the cell, it is selectively permeable so that allows only selected material to pass through it.

2. Cytoplasm- It is a semi-fluid matrix and occupies the volume of the cell. It contains various materials which are necessary for continuation of life as well as various chemical reactions occur in it to keep the cell in the living state.

3. Genetic material- Both the eukaryotic and prokaryotic cells have genetic material. It is responsible for the carrying of hereditary characters from one generation to another.

2. Briefly give the contributions of the following scientists in formulating the cell theory

a. Robert Virchow 

Ans: Rudolf Virchow- He made valuable contributions to cell theory. He proposed that a cell is always formed from pre-existing cells.

b. Schleiden and Schwann 

Ans: They are the scientists who proposed the cell theory for the first time. In this theory they told that-

1. All living cells are made up of one or more cells.

2. Cells are the basic unit of life.

3. Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.

Ans: Yes, extra genomic DNA is present in both prokaryotes and eukaryotes.

In prokaryotes, extra genomic DNA is present in the form of plasmid. Plasmid is an extrachromosomal circular DNA, it imparts certain unique characteristics to bacteria like resistance to antibiotics.

In eukaryotes, extra genomic DNA is present in mitochondria and plastids because of this they both have self replication capabilities.

4. Structure and function are correlatable in living organisms. Can you justify this by taking plasma membrane as an example?

Ans: Yes, structure and function are correlatable in living organisms. Plasma membrane provides a boundary to the cell contents.

1. It is done by the ability to segregate the internal environment of the cell with the external environment.

2. Plasma membrane is semi-permeable, this helps the plasma membrane to allow exchange of only selected materials.

Protein molecules are present which serve as a channel for active transport.

Lipid layer allows the molecules to travel through osmosis.

5. Eukaryotic cells have organelles which may

a. not be bound by a membrane 

b. bound by a single membrane 

c. bound by a double membrane 

Group the various subcellular organelles into these three categories.

Ans: 

(a) Not be bound by a membrane- Ribosome

(b) Bound by a single membrane- Lysosome

(c) Bound by a double membrane- Mitochondria, Plastids, Nucleus, Endoplasmic Reticulum

6. The genomic content of the nucleus is constant for a given species whereas the extra chromosomal DNA is found to be variable among the members of a population. Explain.

Ans: The genomic content of the nucleus is constant for a given species because each species has its own unique set of characters. For example; Somatic cells of human beings contains 23 pairs of chromosomes, this chromosome number gives unique set of characters to human beings, but in the case of extra genomic content of nucleus, the number can be variable because they are not responsible for transferring traits instead they are only responsible for replication of a cell organelle in which they are present.

7. Justify the statement, “Mitochondria are power houses of the cell” 

Ans: Mitochondria is known as the powerhouse of the cell because it generates ATP which is the energy currency of the cell. It is used for all cellular activities. Thus, it is known as the powerhouse of the cell.

8. Is there a species specific or region specific type of plastids? How does one distinguish one from the other? 

Ans: Yes, plastids are species or region specific. They are found in plants and in euglenoids.

There are different types of plastids which impart different colours. Thus, based on colour, plastids are categorised into three types:

(i) Leucoplast- It is colourless plastid. It is further divided into three types: Amyloplast (it stores starch), Elaioplast (It stores fats), Aleuroplast- (It stores proteins).

(ii) Chromoplast- It imparts all the different colours to the plant part except green colour. All the different colours of fruits, flowers are all because of this plastid.

(iii) Chloroplast- It imparts green colour as it contains chlorophyll which is a green pigment.

9. Write the functions of the following 

a. Centromere 

Ans: It facilitates proper segregation of chromosomes.

b. Cell wall 

Ans: It provides rigidity to the plant cell and maintains structure.

c. Smooth ER 

Ans: Smooth ER organelle is a specialised sub-unit of a cell with a specific purpose. The smooth ER's primary purpose is to produce cellular products such as hormones and lipids. It also transports those products around the cell and throughout the organism.

d. Golgi Apparatus 

Ans: It is involved in the synthesis of lipids, metabolism of carbohydrates, regulation of calcium concentration and drug drug detoxification.

e. Centrioles 

Ans: It plays an important role in organising microtubules that act as the skeletal system of the cell.

10. Are the different types of plastids interchangeable? If yes, give examples where they are getting converted from one type to another.

Ans: Yes, the different types of plastids are interchangeable. This interchange takes place according to the stage of the development.

For example; 

1. Flowers are modified shoot which means initially it is green in colour but later have different colours, this is due to conversion of chloroplast into chromoplast.

2. Many fruits are green due to the presence of chloroplast when they are raw but after ripening many of them change their colour because chloroplast changes to chromoplast.

The cell is the smallest, the most essential and most complex of all living beings. The basic unit of living organisms is called a cell. Every cell is a building block for the formation of other cells. These cells are called organelles. All the organelles in a cell are found together. The smallest unit within the organelle is called a unit. The smallest organelle in the cell is called the mitochondrion. The structure of the cell is also called cell architecture. The cell architecture represents the organization of cells and helps to maintain their function. The cellular architecture depends on the type of cell and the species of the cell.

The Structure and Function of Cell

Every cell has a complex organization. It has different organelles within it. Some organelles perform one specific function and some perform different functions. To understand the structure and function of a cell we have to understand the structure of the cell and its various organelles. 

The cell comprises two main regions. One is the plasma membrane and the other is the cell nucleus. The plasma membrane helps the cell to perform its function. The plasma membrane has different membrane channels. The ions that move inside the cell are also known as electrically charged particles. They move inside the cell from outside to inside and vice versa. The movement of ions inside the cell is called ion flow. 

To maintain the concentration of ions inside the cell, ions must be removed from the cell and must be replaced with other ions. To remove the ions from the cell, the membrane channels open and close and ions enter and leave the cell. These channels are called channels. The process of replacing the ions with other ions is called transmembrane transportation of ions.

Every cell has certain properties which maintain the function of the cell. Cells have a certain structure that helps to maintain the function of the cell. A cell can be divided into many parts. The parts of the cell that help to maintain its function are mentioned here.

Ribosomes: The ribosomes are the key components of the cell. They are responsible for protein synthesis. The proteins which play a key role in maintaining the function of the cell are called proteins. The ribosomes help to synthesize these proteins by translating the genetic material called mRNA into proteins. Therefore, they help to maintain the function of the cell.

Lysosomes: These are the components of the cells that help to break down the waste materials present in the cell. These enzymes digest the protein and break down other waste materials and in this way, the cell keeps the balance of the cell.

Peroxisomes: These are the components of the cell that play a role in lipid metabolism. Lipids are important components of the cell. These are the small components that help to maintain the structure of the cell.

Vesicles: These are the components of the cell that help to maintain the function of the cell. It takes the other materials to the different areas of the cell and helps in their functioning.

Mitochondria: Mitochondria are the components of the cell which help to take the required materials for energy. The cell uses the energy stored in the mitochondria to function.