The Cell Structure and Function NIOS chapter explains the fundamental unit of life. Students learn about prokaryotic and eukaryotic cells, organelles, and their specific roles. Key concepts such as cell division, plasma membrane function, and the importance of cell theory are covered in detail.
With terminal exercise solutions, students can practice and strengthen their knowledge for exams. This chapter is essential for understanding biological processes in higher-level topics of NIOS Class 12 Biology.
Cell Structure And Function NIOS Solutions
1. Justify the statement that the cell wall, although a dead material, influences living processes inside the cell.
The cell wall protects the delicate inner parts of the cell.
- Being rigid, it gives shape to the cell.
- As it is rigid, it does not allow distension of the cell, thus leading to turgidity of the cell, which is useful in many ways.
- It freely allows the passage of water and other chemicals into and out of the cells.
- There are breaks in the primary wall of the adjacent cells through which the cytoplasm of one cell remains connected with the other.
2. Differentiate between the cell wall and cell membrane.
| CELL WALL | CELL MEMBRANE |
| Present only in plant cells | Present in both plant and animal cells |
| It is a non-living layer | It is a living membrane |
| Mainly made of cellulose | Composed of proteins and lipids |
| Thickened by lignin and suberin. | There is no thickening |
| Allows free passage of water and chemicals in and out of the cell | Selectively permeable, allowing only specific materials to pass through |
3. Draw Singer and Nicholson’s model of the cell membrane.
4. Why is the cell membrane vital for the cell?
The cell membrane regulates the movement of substances into and out of the cell. If the cell membrane fails to function normally, the cell dies.
It allows transport of certain substances into and out of the cell, but not all substances, so it is termed ‘selectively permeable’.
5. Draw the structure of mitochondria and chloroplasts as seen by an electron microscope.
Structure of mitochondria
Structure of a chloroplast
6. List the functions of mitochondria and chloroplasts.
- Functions of Mitochondria: Mitochondria are the site where the oxidation of pyruvic acid (a product of glucose breakdown) takes place to release energy, which is stored in the form of ATP. This process is called cellular respiration, and thus mitochondria are called the powerhouse of the cell.
- Functions of Chloroplast: Chloroplasts are the site of photosynthesis (production of sugar, from carbon dioxide and water in the presence of sunlight).
7. Name the self–duplicating cell organelles? Why are they called so?
Mitochondria and chloroplasts are the self-duplicating cell organelles. They both have their own DNA and RNA, which help them perform protein synthesis and replicate themselves without relying on the nucleus.
8. Differentiate between the functions of ER, ribosomes, and Golgi bodies.
| Endoplasmic Reticulum (ER) | Golgi bodies | Ribosomes |
| Provides internal framework,compartment and reaction surfaces, transports enzymes and other materials throughout the cell. | Synthesis and secretion asenzymes. | Site of protein synthesis |
| RER is the site for protein synthesis | participates in the transformation of membranes to give rise to other membrane structures such as lysosomes, acrosomes, and dictyosomes. | |
| SER is the site of steroid synthesis and storescarbohydrates. | Synthesise wall elements such as pectin and mucilage. |
9. Most organelles are membrane–bound. What is the advantage of such an arrangement?
- The plasma membrane in each of these cell organelles regulates the movement of substances in and out of these organelles.
- It protects the contents inside these organelles.
- The membrane protects the integrity and identity of each organelle.
- The membrane of each organelle is unique and operates differently, as on the function of each organelle.
10. Differentiate between the structure and function of the centriole and cilia/flagella.
| Centriole | Cilia | Flagella |
| Are two in number | 100 per cell structure | 1-2 in most cells. |
| Takes part in cell division | Locomotory in function | Locomotory in function |
| Round in shape | Short and elongated | Large and elongated |
| Consists of 9 sets of peripheral triplet tubules | Consists of two central microtubules and nine doublet sets surrounding them | Consists of 9 sets of peripheral duplet microtubulesand 1 set of two singlet tubules in the centre |
11. Why are lysosomes known as “suicidal bags”?
Lysosomes are called “suicidal bags” as enzymes contained in them can digest the cell’s own material when damaged or dead. They contain about 40 different enzymes, which can act on materials that are to be destroyed and also digest the parts of the cell itself. When cells are old, diseased or injured, lysosomes attack their cell organelles and digest them.
12. What are the functions of the nucleus?
- Maintains the cell in a working order.
- Co-ordinates the activities of other cell organelles.
- Takes care of repair work.
- Participates directly in cell division to produce genetically identical daughter cells. This division is called mitotic cell division.
- Participates in the production of meiotic gametes and meiospores through another type of cell division called meiotic cell division.
13. List the cell organelles. Write in one line each, about their functions, and explain the division of labour.
- Mitochondria: The site of cellular respiration to generate energy in the form of ATP
- Chloroplast: The site of photosynthesis
- Endoplasmic Reticulum: Forms an internal framework of the cell, a site of protein and steroid synthesis, and transports enzymes.
- Ribosomes: Site of protein synthesis
- Golgi bodies: Transformation of membrane, secretion of enzymes, synthesis of wall elements, etc.
- Lysosomes: Help digest food and waste materials in the cell.
- Peroxisomes: Involved with photorespiration in plant cells.
- Glyoxysomes: Contain enzymes for fatty acid metabolism to convert lipids to carbohydrates during germination.
- Cilia: Help in the movement of cells
- Flagella: Locomotory in function
- Centriole: Involved in cell division to give orientation to the mitotic spindle.
- Basal Bodies: Cilia and flagella originate from this part.
- Nucleus: The Controlling unit of the cell that carries the genetic material.
14. Give the points of difference between
(i) prokaryotic and eukaryotic cells.
(ii) plant and animal cells.
(i) Prokaryotic and Eukaryotic cells.
| Prokaryotic Cell | Eukaryotic Cell |
| Nucleus not distinct, it is in the form of a nuclear zone called ‘nucleoid’. Nuclear membrane absent. | Nucleus distinct, with a well-formed nuclear membrane. |
| Single-membraned cell bodies, like mesosomes present. Endoplasmic reticulum, plastids, mitochondria, microbodies like lysosomes, and the Golgi body are absent. | Double-membraned cell organelles (Chloroplasts, mitochondria, nucleus) and single-membraned (Golgi apparatus, lysosomes, vacuole, endoplasmic reticulum) are present |
| Ribosomes – 70S | Ribosomes – 80S |
| There are no compartments inside the cell | Distinct compartments in the cell, i.e., the cytoplasm and the nucleus |
| There is only one chromosome per cell. | Depending upon the species number of chromosomes per nucleus varies fromtwo to many. |
| The chromosome is circular and remains attached to the cell membrane at one point. | Each chromosome is linear with its two ends free. |
| The chromosome has a single, double-stranded circular DNA molecule and is not associated with histones. | Each chromosome has one linear double-stranded DNA complexed with histones |
| The chromosome lacks a centromere. | Each chromosome has one centromere that divides a chromosome into two arms. However, if the centromere is terminal, the chromosome would have only one arm. |
(ii) Plant and Animal cells.
| Plant Cell | Animal Cell |
| The cellulose cell wall is present external to the cell membrane | No cell wall, the outermost structure is the cell membrane or plasma membrane |
| Vacuoles are usually large. | Generally, vacuoles are absent, and ifpresent, are usually small.. |
| Plastids present. | Plastids absent. |
| The Golgi body is present in the form of unitsknown as dictyosomes. | The Golgi body is well-developed, having 2cisternae |
| Centriole absent. | Centriole present. |
15. Why is the cell termed the structural and functional unit of living organisms?
- A cell is termed the structural and functional unit of life because every organism is made of cells.
- Cells form the building blocks that form tissues, then the organs, the organ system, and ultimately the organism.
- Every cell performs metabolic activities that are essential for sustaining life.
16. Name the following:
(i) The condition in which a cell has the normal paired chromosomes.
(ii) The condition in which a cell contains only one member of each pair of chromosomes.
(iii) The pairing of maternal and paternal chromosomes during meiosis.
(iv) The exchange of parts in homologous (maternal and paternal) chromosomes during prophase I of meiosis.
(v) The point by which a chromosome is attached to the spindle fibre.
(vi) The type of cell division that results in growth.
Ans:
- (i) Diploid
- (ii)Haploid
- (iii) Synapsis
- (iv) Crossing over
- (v) Centromere
- (vi) Mitosis
17. What are the sites of meiosis in a flowering plant and in a sexually reproducing animal?
The sites of meiosis in a flowering plant and in a sexually reproducing animal are microspore mother cells in the anther or male reproductive structure and megaspore mother cells in the ovary.
18. List the sub-stages of prophase I.
Sub-stages of Prophase I are Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
19. What is the significance of meiosis?
Significance of Meiosis
- (i) It helps to maintain a constant number of chromosomes in different generations of a species undergoing sexual reproduction.
- (ii) Meiosis occurs during gamete formation (gametogenesis) and reduces the number of chromosomes from diploid (2n) to haploid (n) in the gametes. These haploid gametes fuse to form a diploid zygote during fertilisation. The diploid zygote develops into a normal diploid individual.
- (iii) Meiosis establishes a new combination of characters due to
- (i) mixing of paternal and maternal chromosomes and
- (ii) crossing over during prophase I. As a result, the progeny inherits the traits of both the mother and the father in new gene combinations.
20. Draw a schematic diagram of various stages of the life cycle of a cell.
21. Draw labelled diagrams of various stages of mitosis.
The four stages of mitosis are,
(i) Prophase
(ii) Metaphase
(iii) Anaphase
(iv) Telophase
22. Tabulate the main differences between mitosis and meiosis.
| MITOSIS | MEIOSIS |
| A cell divides only once | There are two cell divisions. The first meiotic division and the second meiotic division. |
| Takes place in somatic cells as well as in reproductive cells, which may be haploid or diploid, or polyploid | Takes place only in diploid germ cells. |
| The duration of prophase is short (a few hours) | Prophase I is comparatively longer. (takes many days). |
| Prophase simple. | Prophase I is complicated, having five sub-stages, namely leptotene, zygotene, pachytene, diplotene, and diakinesis. |
| Synapsis does not occur. | Synapsis of homologous chromosomes takes place during prophase I. |
| No exchange of segments occurs during prophase between two nonsister chromatids of chromosomes. | Exchange of segments during crossing over between non sister chromatids of two homologous chromosomes takes place. |
| Each chromosome consists of two chromatids united by a centromere. | Each bivalent has four chromatids and two centromeres. |
| Chromosomes are duplicated at the beginning of prophase. | In prophase I, chromosomes appear single, although DNA replication has taken place in interphase I. |
| In metaphase, all the centromeres line up in the same plane. | In metaphase I, the centromeres are lined up in two planes, which are parallel to one another. |
| The metaphasic plate is made up of a duplicated chromosome. | The metaphasic plate is made up of paired chromosomes. |
| Centromere division takes place during anaphase. | No centromere divisions occur during Anaphase I; centromeres divide only during Anaphase II. |
| Spindle fibres disappear completely in telophase. | Spindle fibres do not disappear completely during telophase I. |
| Reappearance of nucleoli at telophase | Nucleoli do not appear in telophase I. |
| The chromosome number does not change at the end of mitosis. | There is a reduction in the chromosome number from diploid to haploid. |
| The genetic constitution of the daughter cells is absolutely identical to that of the parent cells. | The genetic constitution of chromosomes in daughter cells is different compared to the parent cells. The daughter cell chromosomes contain a mixture of maternal and paternalgenes. |
| Mitosis is of shorter duration. | Meiosis is of longer duration. |
| It is the basis of growth and repair, and reproduction in vegetatively or asexually reproducing organisms. | It is the basis of maintaining the same chromosome number in different generations of a species reproducing sexually, and provides genetic variation in the progeny. |
23. Why is prophase of meiosis so prolonged and elaborate?
Prophase I of meiosis is a prolonged and elaborate process due to the five substages that happen.
- Leptotene: The chromosomes become distinct and highly coiled.
- Zygotene: Synapsis or the pairing of similar or homologous chromosomes occurs in this stage.
- Pachytene: Crossing-over occurs at the end of pachytene, i.e., break and exchange of parts (genes) occurs between non-sister chromatids (chromatids of a homologous pair) in this stage.
- Diplotene: Formation of chiasmata- the attachment of two non-sister chromatids of a homologous pair occurs in this stage. It is at the chiasmata that the genetic recombination or the exchange of segments of nonsister chromatids (genes) between homologous chromosomes has taken place.
- Diakinesis is when the spindle formation is completed and the nuclear membrane and nucleolus disappear.
24. What is the difference between cytokinesis in an animal cell and cytokinesis in a plant cell?
In animal cells, cytokinesis begins as the cleavage furrow starts from the periphery towards the centre. But in plant cells, the cell plate appears in the centre and grows centrifugally towards the periphery.
