The Fundamental Unit of Life | Class 9 Science

Table of Contents

Notes The Fundamental Unit of Life

In-Text Questions & Answers - Chapter 5 - NCERT Class 9 Science

Exercises Questions & Answers - Ch 5 - The Fundamental Unit of Life

Q1. What is cell – the fundamental unit of life?

Cell is the structural unit of organism. In other words cell is the fundamental unit of life. It is membrane bound and self reproductive. Cell contains cytoplasm and nucleus. Cell is responsible for biological activity and growth of organism.

Q2. What do you mean by unicellular and multicellular organism?

We divide organism into two groups. The groups are i) Unicellular, & ii) Multicellular. If a single cell forms a whole organism, then it is called unicellular organism. For example – Amoeba, Bacteria etc.

Multicellular organisms like humans, animals, plants etc. are constituted by various type of multi cell groups. Human body contains different type of cell groups such as nerve cell, muscle cell, blood cell etc.

Q3. How do you classify cell?

We classify cell into two groups. The groups are a) Prokaryotic cell, & b) Eukaryotic cell.

Q4. What is prokaryotic cell?

Prokaryotic cells do not contain nuclear membrane and membrane bound cell organelles. For example – Bacteria, Gloeocapsa etc.

Q5. What are the properties of prokaryotic cell?

Properties of prokaryotic cell are –
A) Prokaryotic cells are simple and under developed.
B) These cells are smaller in size (1µm -10µm).
C) These cells do not contain nuclear membrane and membrane bound cell organelles such as Mitochondria, Golgi Apparatus etc.
D) Number of chromosome present in prokaryotic cell is one.

Q6. What is eukaryotic cell?

Eukaryotic cells contain well built nucleus and membrane bound cell organelles. For example – animal cells, plant cells etc.

Q7. What are the the properties of the eukaryotic cell?

Properties of eukaryotic cell are –
A) Eukaryotic cells are complex and developed.
B) These are comparatively larger in size (5µm -100µm).
C) These cells contain well built nucleus and membrane bound cell organelles i.e. mitochondria, Golgi apparatus etc.
D) Multiple nos. of chromosome are present in this type of cell.
E) Nucleus of eukaryotic cell has nuclear membrane.

Q8. What are the various parts of a cell?

The fundamental unit of life i.e. cell contains cell wall, plasma membrane, nucleus, cytoplasm, organelles like mitochondria, vacuole, Golgi apparatus etc.

Q9. What is plasma or cell membrane? How cell membrane controls movement of substances through it?

Plasma or cell membrane is the outermost flexible and semi-permeable covering of the cell. This membrane controls and restricts the in and out movement of the substances in cell. Lipids and proteins form cell membrane. Normally substances move from the region of high concentration to the region of low concentration. O₂, CO₂ etc. moves from the region of high concentration to the region of low concentration. This process is also known as diffusion.

Osmosis is the process responsible for movement of water from high density zone to low density zone through this permeable cell membrane. Plant, unicellular organism etc. intake water through osmosis process. Movement of water also depends on the concentration of water in surrounding mediums described in the following.
♠ In case of hypotonic solution where water concentration is higher than the cell, there water goes into the cell and the cell swells up.
♣ In case of hypertonic solution where water concentration is lower than the cell, there water moves out of the cell and the cell shrinks.
♦ In case of isotonic solution where water concentration is same as the cell, there no water movement occurs inside or outside of the cell and size of the cell remains same.

Q10. What is cell wall?

Cell wall is the rigid outer covering surrounding the plasma membrane in the plant cell. Basically it protects soft portion of the cell from outside and gives structural strength to plant cell. Cellulose is the main constituent of cell wall. Bacteria also has cell wall. Sometimes cell contents get contracted after losing water in osmosis process and we call it plasmolysis.

Q11. What is nucleus of a cell? What are the functions of nucleus?

Nucleus is the most condensed part inside the cell. It has double layered covering known as nuclear membrane. Transfer of materials from inside of the nucleus to outside occurs through the pores of nuclear membrane.

♦ In case of prokaryotic cell, nucleus lacks nuclear membrane and only contains nucleic acid. It is also called nucleoid. Nucleus of the eukaryotic cell is well built and has nuclear membrane.

Functions of nucleus –

Nucleus plays crucial role in cell division i.e. reproduction of the cell. Cell division is a process in which a cell (called parent cell) divides and forms two new cells (called daughter cell). In addition, nucleus also determines various functions like developing of cell, maturity, properties, chemical activities etc. during the process of the cell division.

Q12. What are DNA and Gene?

Nucleus has chromosomes which are made up of protein and nucleic acid i.e. DNA (Deoxyribo Nucleic Acid). During cell division chromosome appears as rod –shaped structure. DNA contains information of all biological functions. Functional unit of chromosome is gene which is the carrier and container of the heredity from one generation to next generation. Shape of the DNA in eukaryotic is straight and thread like but round in prokaryotic cell. Generally DNA is present as a part of chromatin material which gets organised into chromosome during cell division.

Diploid and Haploid –

Number of chromosome present in prokaryotic cell is one. But eukaryotic cell contains multiple chromosome and they always stay in pair. Size and shape of chromosome in a pair are same. Generally we term nos. of paired chromosome as Diploid (2n) chromosome. Our body cell contains 23 nos. diploid chromosome (i.e. 23×2=46 chromosome). When chromosome stays in single set in cell, then we term it as Haploid. Gametes have haploid number of chromosome.

Q13. Discuss cytoplasm in detail.

Cytoplasm is semi-liquid, granular and jelly type fluid inside the cell. It is bounded by cell or plasma membrane and formatted into two layers i.e. A) Ectoplasm, & B) Endoplasm. Ectoplasm is the portion adjacent to plasma membrane and is lightly transparent. Endoplasm is the portion around the centre point and is comparatively dense and granular. Hyaloplasm is the fluid in cytoplasm which contains all including cell organelles.

Cytoplasm contains various cell organelles which perform crucial function inside cell. Later we will discuss some important cell organelles like Endoplasmic Reticulum (ER), Plastids, Lysosome, Mitochondria, Golgi Apparatus, Vacuole etc.

Q14. What are the different parts of cell organelles?

Cytoplasm houses cell organelles like Endoplasmic Reticulum (ER), Plastids, Lysosome, Mitochondria, Golgi Apparatus, Vacuole etc.

Q15. Write about endoplasmic reticulum (ER) in detail.

Endoplasmic reticulum (ER) is a network system of membrane bound duct or tubes. It functions as a pathway between various parts in cytoplasm and between cytoplasm and nucleus for movement of materials. ER also helps functioning of various reactions in cell. ER is divided into two types. These are Rough Endoplasmic Reticulum (RER) and Soft Endoplasmic Reticulum (SER).

RER has particles without covering attached to it’s surface and they are the ribosomes. Ribosome especially helps in protein synthesis or manufacturing. The presence of ribosome makes ER looking rough. Protein made by ribosome goes to various parts through ER. SER helps to produce lipids or fat molecules. Both lipids and proteins help in membrane bio-genesis which is the process of building of cell membrane. Also they (lipids and proteins) act as enzymes and hormones in some cases.

Q16. Write about plastids in brief.

It is present in cytoplasm of eukaryotic cell particularly in plant cell. It helps in photosynthesis, pollination, food storage etc. Also it has own DNA and ribosome. Shape of the plastids may be oval, elliptical, star-shaped, ribbon etc. We divide plastids into two categories. These are Chromoplast & Leukoplast. Chromoplast i.e. coloured plastid contains green, yellow, orange pigments. Green plastid helps in photosynthesis in plants. Chromoplast also helps in deciding colour of flower and fruit. Chloroplast is the another name of green plastid. Leukoplast helps in storing fat, protein and starch.

Q17. Discuss lysosome in brief.

Lysosome is membrane bound cell organelle and looks like small bag or sac. It contains RER produced enzymes and helps in digestion or metabolism inside the cell. Due to presence of enzymes it can digest any outside material even old organelles. We also call lysosome as suicide bag because enzymes digest own damaged cell which causes bursting out of lysosome.

Q18. Write about mitochondria in brief.

Mitochondria is double membrane covered cell organelle and scattered in cytoplasm. It has own DNA and ribosome and produces own protein. Generally we term mitochondria as powerhouse of the cell as it releases energy for all activities in the form of Adenosine Tri-Phosphate molecules (ATP).

Q19. Write about Golgi apparatus in brief.

Golgi apparatus or body is a membrane bound cell organelle and looks like flattened sacs. Generally it is organised parallel to each other and stays near nucleus. Golgi apparatus of plant cell is known as dictyosome. Golgi apparatus transmits materials to various parts in the cell.

Q20. Write about centrosome in brief.

Centrosome exists near nucleus and is star-shaped and helps in formation of spindle fibre in cell division of animal. It has two parts – centrioles and centrospheres.

Q21. Write about vacuole in brief.

Vacuole is membrane bound sacs in cytoplasm. In case of plant cell, it functions as storage of cell sap including important materials like amino acids, proteins, organic acids etc. Similarly it acts as storage of food and excretory material in animal cell. Size of vacuole in plant cell is comparatively larger than animal cell. Vacuole performs important functions specially in unicellular organism like expelling waste and water, storing food etc.

Q22. Write about ergastic substances in brief.

Cytoplasm in eukaryotic cell specially in plant cell contains some ergastic substances or languorous objects except cell organelles .

Q23. Describe different types of cell division in detail.

Cell division is the process to produce new cell. The cell that divides is known as parent or mother cell and newly created cell is known as daughter cell. Cell division is required for the growth of organism. It repairs or replaces injured, old and dead cells and also is responsible for reproduction and creation of gametes. Mitosis, Meiosis and Amitosis are the types of cell division. But among them Mitosis and Meiosis are two main types of cell division.

Mitosis is a process where two daughter cells are formed by single indirect division of nucleus of the mother cell. Daughter cells have same number of chromosome as of mother cell. It has similar properties of mother cell and has similarity in shape with mother cell. Growth of organism, repair of injured cells etc. are resulted through mitosis.

Meiosis is the process where four daughter cells having half number of chromosome of mother cell are formed by two consecutive division of nucleus of the gamete. Gamete is formed by division of cell of the reproductive organs. Then it goes for fertilisation and results in creation of offspring.

Amitosis is another type of cell division where mother cell produces daughter cells through direct division of it’s nucleus. In that case nucleus of mother cell divides and forms two daughter nucleus which further accelerates full daughter cell formation. For example – Amoeba, yeast etc.

In-Text Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life

In-Text Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life | Page No. 59

1. Who discovered cells, & how?
In 1665, Robert Hooke first discovered cell. He put a thin slice of cork (a part of bark of a tree) under a primitive microscope & thoroughly observed. He saw honeycomb-like structures which look like tiny empty compartments. Robert Hooke called those tiny empty compartments as cells.

2.Why is the cell called the structural & functional units of life?

The cell is called the structural & functional units of life because all living plants & animals are made up of cells. The cell also carries out all the biochemical, genetic & physiological functions taking place inside the body of living organisms.

In-Text Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life | Page No. 61

1) How do substances like CO₂ & water move in & out of the cell?

CO₂ is cellular waste. So it should be excreted out of the cell. Concentration of CO₂ inside the cell is higher than the external environment of the cell. So it moves out of the cell as a journey from high concentration zone to low concentration zone.

Basically osmosis is the process responsible for movement of water from high density zone to low density zone through through permeable cell membrane. Plant, unicellular organism etc. intake water through osmosis process.

2) Why plasma membrane is called a selectively permeable membrane?

It is basically the outermost covering of the cell. Also it acts as a separation between the cell & outer environment of the cell. It controls & regulates the in & out entry & exit of some materials of the cell. In addition, it prevents movement of some important material. That’s why plasma membrane is called a selectively permeable membrane.

In-Text Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life | Page No. 63

1) Fill in the gaps in the following table illustrating differences between prokaryotic & eukaryotic cells.

Prokaryotic Cell	Eukaryotic Cell
1) Size : generally small (1-10 µm).	1) Size : generally large (5-100 µm).
2) Nuclear region : .................................................. known as.......................	2) Nuclear region : well defined & surrounded by a nuclear membrane.
3) Chromosome : single.	3) More than one chromosome.
4) Membrane-bound organelles absent.	4) ...................................

Answer:
Prokaryotic Cell	Eukaryotic Cell
1) Size : generally small (1-10 µm).	1) Size : generally large (5-100 µm).
2) Nuclear region : it is very less developed due to lack of nuclear membrane & known as nucleoid.	2) Nuclear region : well defined & surrounded by a nuclear membrane.
3) Chromosome : single.	3) More than one chromosome.
4) Membrane-bound organelles absent.	4) Membrane-bound organelles are present. For example - nucleus, mitochondria etc.

In-Text Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life | Page No. 59

1) Can you name the two organelles we have studied that contain their own genetic material?

Ribosomes & mitochondria are cell organelles which contain their own genetic material i.e. DNA. These organelles are also as strange organelles. Because they can synthesize their own genetic material & protein.

2) If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

It will cause catastrophic effect because cell is the basic functional & structural unit of the body. It will disturb various activities such as respiration, excretion etc. As a result, some life activities may stop & even may cause death.

3) Why are lysosomes known as suicide bags?

Lysosomes contain hydrolytic digestive enzymes. These enzymes are synthesised by endoplasmic reticulum. Also these enzymes have the ability to digest their own cell due to which cell will die. As a result lysosomes are called suicide bags.

4) Where are proteins synthesised inside the cell?

Rough Endoplasmic Reticulum i.e. RER has particles without covering attached to it’s surface. They are called ribosomes. Ribosome especially helps in protein synthesis or manufacturing. This protein goes to various parts through endoplasmic reticulum.

Exercises Questions & Answers | NCERT Class 9 Science Science Chapter 5 | The Fundamental Unit of Life | Page No. 67

1) Make a comparison & write down ways in which plant cells are different from animal cells.
Plant Cell	Animal Cell
Plant cell is rectangular & fixed.	Animal cell is round & irregular.
Nucleus exists on one side of the cell.	Nucleus exists in the center of the cell.
Plant cell has cell wall. It also contains plastids.	Both cell wall & plastids are absent in animal cell.
It has large & few central vacuoles.	Animal cell has small & numerous central vacuoles.
Cilia is absent.	It has the presence of cilia.
It rarely contains lysosomes. Also it does not contains centrosomes.	It contains both lysosomes & centrosomes.
Plant cell can synthesize amino acids, vitamins & co-enzymes.	Animal cell can not synthesize amino acids, vitamins & co-enzymes.

2) How a prokaryotic cell different from a eukaryotic cell?
Prokaryotic Cell	Eukaryotic Cell
Size is generally small (1-10 µm).	Size is generally large (5-100 µm).
Nucleus is very less developed. It lacks of nuclear membrane. Also it is called nucleoid.	Nucleus is well defined. It also has nuclear membrane.
This cell has single chromosome.	This cell has more than one chromosome.
Membrane-bound organelles are absent.	Membrane-bound organelles are present.
Cell division is happened by fission or budding.	It follows meiosis or mitosis process for cell division.

3. What would happen if the plasma membrane ruptures or breaks down?

If the plasma membrane breaks down, then there will be no exchange of materials between the cell & outside environment. As a result, protoplasm will disappear gradually. The cell will die finally.

4. What would happen to the life of a cell if there was no Golgi apparatus?

Golgi apparatus is a membrane bound cell organelle and looks like flattened sacs. Generally it is organised parallel to each other and stays near nucleus. Golgi apparatus of plant cell is known as dictyosome. Golgi apparatus transmits materials to various parts in the cell. If cell has no Golgi apparatus, then it will lack network to transmit processed nutrition. Hence, there will be no cellular life.

5. Which organelle is known as the powerhouse of the cell? Why?

The powerhouse of the cell is mitochondria. It releases energy in the form of ATP molecules. That energy helps to perform various chemical activities. Also ATP is known as the energy currency of the cell.

6. Where do the lipids & proteins constituting the cell membrane get synthesised?

Both lipids & proteins constituting the cell membrane get synthesised at SER & RER gradually. RER has particles without covering attached to it’s surface and they are the ribosomes. Ribosome especially helps in protein synthesis or manufacturing. Protein made by ribosome goes to various parts through ER. SER helps to produce lipids or fat molecules. Both lipids and proteins help in membrane bio-genesis which is the process of building of cell membrane.

7. How does an Amoeba obtain it’s food?

Amoeba is unicellular. It has finger-like extension over it’s surface which is called pseudopodia. That finger-like extensions fuse to form food vacuole. Food vacuole performs the digestion of food. The digested food diffuses into the cytoplasm. On the other hand the undigested food moves on & thrown out. This process is known as endocytosis. Basically endocytosis is the process in which flexibility of the cell membrane helps the cell to engulf in food & other material from it’s external environment.

8. What is osmosis?

Osmosis is the process responsible for movement of water from high density zone to low density zone through this permeable cell membrane. Plant, unicellular organism etc. intake water through osmosis process.

9. Carry out an osmosis experiment as ⇒
Take four peeled potato halves & scoops each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cups in a trough containing water. Now
i) Keep cup A empty
ii) Put one teaspoon sugar in cup B
iii) Put one teaspoon salt in cup C
iv) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups & answer the following.

a. Explain why water gathers in the hollowed portion of B & C.

Water gathers in the hollowed portion of B & C through osmosis process. As the water concentration in the surrounding medium is high, so water gathers in B & C.

b. Why is potato A necessary for this experiment?

The cells of potato have a selectively permeable membrane through which osmosis occurs. As a result entry & exit of some materials in & out of the cell happens. So potato A is required.

c. Explain why water does not gather in the hollowed out portion of A & D.

Cup A is empty. There is no difference in concentration of water. So no osmosis happens. Hence no water gathers in A. Potato D is boiled. So it’s permeable membrane is totally destroyed. Therefore, there is no scope for osmosis process. Hence no water gathers in D.

10. Which type of cell division is required for growth & repair of body? Which type is involved in formation of gametes?

Basically mitosis is a process where single indirect division of nucleus of the mother cell forms two daughter cells. Daughter cells have same number of chromosome as of mother cell. It has similar properties of mother cell and has similarity in shape with mother cell. Growth of organism, repair of injured cells etc. are resulted through mitosis. Meiosis is used for sexual reproduction & produces haploids cells different to each other & the parent cell. So meiosis results in formation of gametes.

To Be Continued