Biology

Chapter 04 Cell Structure and Function

NIOS Class 12 Biology Intext Solutions Chapter 4

INTEXT QUESTIONS 4.1

1. From where do new cells arise?

Ans: Pre-existing cells

2. Name the scientists who proposed the ‘cell theory’.

Ans: Schleiden and Schwann

3. Name an organelle that a plant cell has but an animal cell does not.

Ans: Chloroplast/plastid

4. Give two points of difference between a prokaryotic cell and a eukaryotic cell

Ans: Prokaryote – naked nucleus, no membrane-bound cell organelles

Eukaryote – distinct nucleus with a cell membrane, membrane-bound cell organelles present

INTEXT QUESTIONS 4.2

1. Define diffusion and osmosis.

Ans:

Diffusion: Movement of molecules from a region of higher concentration to a region of lower concentration.
Osmosis: Movement of water molecules across a semipermeable membrane from a region of higher concentration to the region of lower concentration.

2. What does active transport mean?

Ans: Movement of molecules against a concentration gradient, that is region of lower to higher concentration by expending energy.

3. Give one point of difference between phagocytosis and pinocytosis.

Ans:

Intake of solid particles (phagocytosis)
Intake of fluid droplets (pinocytosis)

4. Match the following :

(i) hydrophilic end	(a) cell wall
(ii) microfibrils	(b) inner ends of lipids
(iii) fluid-mosaic model	(c) fluid droplets
(iv) hydrophobic end	(d) outer ends of lipids
(v) pinocytosis	(e) Nicholson and Singer

Ans: (i) – d (ii) – a (iii) – e (iv)– b (v) – c

5. Give two functions of the plant cell wall.

Ans:

Protection/shape
Provide communication between cells through plasmodesmata

INTEXT QUESTIONS 4.3

1. What is a cell organelle?

Ans: Membrane-bound bodies in the cytoplasm

2. Name the chemical which provides energy trapped in its bonds to the cell.

Ans: ATP

3. Which part of the chloroplasts is the site of light reaction?

Ans: Grana

4. Name the sac-like structure which forms the grana?

Ans: Thylakoids

5. Why is the mitochondrion called the “power house” of the cell?

Ans: Because energy, as ATP, is generated and stored in mitochondria

6. Which organelle contains enzymes for cellular respiration?

Ans: Mitochondria

7. State two similarities between mitochondria and chloroplasts.

Ans:

Both are semiautonomous of Life
Both contain DNA, or both contain ribosomes

8. Which plastid imparts colour to flower petals?

Ans: Chromoplasts

9. Which plastid is green in colour?

Ans: Chloroplast

10. Why are mitochondria and chloroplasts called semi-autonomous?

Ans: They have their own DNA for the production of more copies of themselves by self-duplication but cannot lead an independent life outside the cell/ cytoplasm.

INTEXT QUESTIONS 4.4

1. Given below is a list of functions; relate them to their respective organelles:

(a) synthesis of some enzymes

(b) synthesis of steroids

(c) storage of carbohydrates

(d) Intracellular transport

(e) Synthesis of proteins

Ans: (a) Golgi body (b) ER, (c) amyloplasts (d) ER, (e) ribosomes

2. Name the equivalent structure of Golgi body in plants. Mention two differences between their structures.

Ans: In plant cells Golgi body are present in the form of units known as dictyosomes. Here, they are many in number and are scattered throughout the cell.

3. Mention any two advantages of the extensive network of endoplasmic reticulum.

Ans: (i) internal framework, (ii) transport of cellular substances

4. What are the three places where ribosomes occur in a cell?

Ans: Cytoplasm, ER, Nucleolus; chloroplasts, mitochondria

5. Name the membrane system that connects the nuclear membrane with the cell membrane?

Ans: ER

INTEXT QUESTIONS 4.5

1. Why are lysosomes called suicidal bags?

Ans: Lysosomes are called “suicidal bags” as enzymes contained in them can digest the cell’s own material when damaged or dead.

2. List the usefulness of intracellular digestion by lysosomes

Ans: Importance of intracellular digestion by the lysosomes

Help in the nutrition of the cell by digesting food, as they are rich in various hydrolysing enzymes which enable them to digest almost all major chemical constituents of the living cell.
Help in defence by digesting germs, as in white blood cells.
Help in cleaning up the cell by digesting damaged material of the cell.
Provide energy during cell starvation by digestion of the own parts of the cells (autophagic, auto: self; phagos: eat up).
Help sperm cells enter the egg by breaking through (digesting) the egg membrane.
In plant cells, mature xylem cells lose all cellular contents by lysosomal activity.
When cells are old, diseased or injured, lysosomes attack their cell organelles and digest them. In other words, lysosomes are autophagic, i.e. self-devouring.

3. What is the function of peroxisomes in plant cells

Ans: Peroxisomes are involved in photorespiration in plant cells. They also bring about fat metabolism in cells.

INTEXT QUESTIONS 4.6

1. Why cannot the cell survive without the nucleus?

Ans: The Nucleus controls all the functions of the cell as it has the hereditary information

2. Explain the following terms:

(a) chromatin network……

(b) chromosomes ..

Ans:

(a) Chromosomes are present as a network when not dividing, that is, at early interphase or Go-stage (Differentiation stage during development)

(b) Bearers of hereditary information as genes on them

3. What is the function of the nucleolus in the cell?

Ans: Site of RNA synthesis

INTEXT QUESTIONS 4.7

1. What is the importance of water in a living cell.

Ans:

(i) It is a universal solvent, and most chemical reactions of the cell occur in an aqueous medium.

(ii) It is a constituent of protoplasm

2. Which is the basic molecule in starch?

Ans: Glucose

3. What is a peptide bond, and where will you find it?

Ans: –NHCO–, between amino acids in a polypeptide, found in proteins

4. Which is the most energy-rich biomolecule in living organisms?

Ans: ATP

5. What are nucleotides?

Ans: Building blocks of nucleic acids, each containing a pentose sugar, a nitrogenous base and a phosphate.

INTEXT QUESTIONS 4.8

1. Explain in one sentence

(i) Interphase ……..

(ii) Synthetic-phase …..

(iii) Dividing-phase

Ans:

(i) Interphase – The interval between two successive cell divisions is termed interphase (the phase at which the cell is not dividing).
(ii) Synthetic phase – DNA is synthesised in this phase.
(iii) Dividing phase – Mitosis in somatic cells or meiosis in the germ cells takes place.

2. What is the full form of the following in the cell cycle?

(i) Gl ….

(ii) S …….

(iii) G2 …..

(iv) M-Phase

Ans:

(i) First growth phase
(ii) Synthetic phase;
(iii) Second growth phase
(iv) Mitosis/meiotic phase.

INTEXT QUESTIONS 4.9

1. Name the stage of cell cycle during which chromatin material is duplicated.

Ans: S-shape of Interphase;

2. Is the number of chromosomes reduced in the daughter cells during mitosis? Yes/no?

Ans: No

3. Name the stage in nuclear division described by each of the following sentences:

(i) disappearance of the nuclear membrane

Ans: Late Prophase.

(ii) The nuclear membrane and nucleolus reappear

Ans: Late Telophase

(iii) The centromere divides, and the chromatids move to opposite poles due to the shortening of spindle fibres.

Ans: Anaphase

(iv) The chromosomes arrange themselves at the equatorial plane of the spindle with the spindle fibres attached to the centromeres.

Ans: Metaphase

INTEXT QUESTIONS 4.10

1. Name the sub-stage of Meiosis I in which the :

(i) Homologous chromosomes pair

Ans: Zygotene (prophase I)

(ii) Tetrads are formed.

Ans: Pachytene

(iii) Homologous chromosomes begin to move away from each other.

Ans: Diplotene

2. Sites of meiosis in flowering plants, are :

Ans: Microspore/pollen mother cell in anthers and megaspore mother cell in the ovule.

3. Rearrange the following stages of meiosis I in their proper sequence : zygotene, pachytene, leptotene, metaphase-I diakinesis, anaphase-I, telophase-I.

Ans: Leptotene, zygotene, pachytene, diplotene, diakinesis, metaphase I, telophase I.

4. Mention two major points in which Meiosis I differs from Meiosis II.

Ans: Reduction in chromosome number to half in Meiosis II; exchange of genetic material in Meiosis I.