The fundamental unit of life - Biology, Class - IX

1. What is a cell?

Ans.: Cell is a fundamental, structural and functional unit of living organisms. It was discovered by Robert Hook in 1665 while examining a piece of cork in his self made primitive microscope. M. Schleiden and T. Schwann established cell theory, and postulated that cell is the basic unit of life.

2. Write about the modern cell theory.

Ans.: The modern cell theory or cell principle postulates that:

(i) . all living organisms are composed of cells or cell products.

(ii). all living cells arise from pre-existing cells.

(iii). all cells are basically alike in chemical composition and metabolic processes.

(iv). the functions of an organism as a whole is the result of the activities and interactions of the constituent cells, and

(v). a cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane and in many cases cell wall also.

3. What is cell biology?

Ans.: The study of structure of cell and its components is known as cell biology or cytology. Cell Biology deals with the structural as well as the functional aspect of cell.

4. Write the differences between prokaryotic cell and eukaryotic cell.

Ans.:

Prokaryotic cell	Eukaryotic cell
(i). It is unicellular	(i). It is both unicellular and multicellular
(ii). cell size is smaller in size.	(ii). cell size is bigger in size
(iii). DNA is naked	(iii)DNA  is combined with protein
(iv). single chromosomes	(iv).  multiple chromosome
(v).ribosome is smaller, scattered in the cytoplasm	(v). ribosomes is bigger, attached to edoplasmic reticulum or are free.
(vi). Nuclear membrane, nucleolus, Gogi complex, mitochondria, lysosomes, endoplasmic reticulum, chloroplast and vacuoles are absent.	(vi). Nuclear membrane, nucleolus, Gogi complex, mitochondria, lysosomes, endoplasmic reticulum, chloroplast and vacuoles are present.

5. Why is the cell called the structural and functional unit of life?

Ans.: Every cell of the body bears certain basic features common to all, like plasma membrane, nucleus and cytoplasm. These structures are capable of performing all activities inside the cell and can make interaction with its environment. We know that the unicellular forms like amoeba and bacteria with their single celled structure can perform all activities of life. Hence, cell is called structural and functional unit of life.

6. Name the major parts of cell.

Ans.: The major parts of cell are:

(i). Plasma membrane or cell membrane, (ii). Cytoplasm and its contents and (iii). Nucleus.

7. Write about plasma membrane and its functions.

Ans.: The layer that separates the cell content from the external environment is the plasma membrane. OR, Each cell is bounded by an extremely delicate, thin, elastic, selectively permeable, living membrane called plasma membrane. It is the outer living boundary of a cell.

(i). The main function of plasma membrane is to selectively control the entry and exit of substances inside and outside the cell.

(ii). mainly plasma membrane separates the cells from their external environment.

(iii). it protects the cell from injury.

(iv). it bounds the semifluid contents of the cells.

(v). it allows the flow of materials and information between different organelles of the same cell as well as between one cell and other.

(vi). in nerve cells, the plasma membrane takes part in the transmission of impulses.

8. What is diffusion?

Ans.: It is the process of intermixing of molecules of various substances due to random motion. During diffusion, the molecules of a substance move from a region of its higher concentration to a region of its lower concentration.

9. Why is the plasma membrane called a selectively permeable membrane?

Ans.: Plasma membrane is called selectively permeable membrane because it allows only selected substances to pass through it and at the same time prevents the movement of some other materials.

10. What is osmosis?

Ans.: Osmosis is the passage of water from a region of higher water concentration through a semi-permeable membrane to a region of lower water concentration.

11. What is endosmosis?

Ans:. The movement of liquid from outside to inside is known as endosmosis. It takes place in hypotonic solution. E.g. When we put a raisin inside a bowl of water, the raisin swells up after a few minutes because the higher concentration inside the raisin.

12. What is exosmosis?

Ans.: The movement of liquid from inside to outside is known as exosmosis. E.g. when we put the raisin in concentrated solution of salt, the water comes out from the raisin.

13. Write about cell wall and its functions.

Ans.: Cell wall is a thick, non-living, rigid and permeable covering made up of cellulose. It is present in plant cell only. It was first seen in cork cells by Robert Hooke in 1665.

Functions of cell wall:

(i). cell wall protects cell membrane and the internal structures of the cell.

(ii). it provides rigidity and determines the shape of the plant cell.

(iii). it prevents drying of the cell and helps it to bear unfavourable conditions.

(iv). it prevents the entry of too much watrer inside the cell. This does not allow cytoplasm to become diluted.

(v). it provides mechanical strength to the plant cells.

(vi). it protects the cell against pathogens.

(vii). it counteracts osmotic pressure.

14. What is protoplasm?

Ans.: It is a jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusions remain in colloidal form. Protoplasm includes all the components of the cell including cell membrane.

15. Write about cytoplasm.

Ans.: It is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. It contains about 90% water, 7% proteins, 2% carbohydrates and lipids and 1% inorganic materials, minerals, vitamins, etc. It is the part of the protoplasm which surrounds the nucleus. It occupies the space between the plasma membrane and the nuclear membrane.

15. Write about Endoplasmic Reticulum.

Ans.: The endoplasmic reticulum is a complex network of membranous system in the cytoplasm of eukaryotic cells. It is absent in prokaryotic cells and matures RBCs of mammals. It is of two types: Rough endoplasmic reticulum and Smooth endoplasmic reticulum.

(i). rough endoplasmic reticulum: Bearing ribosomes attached to its surface to assist in the synthesis of proteins and enzymes.

(ii). smooth endoplasmic reticulum: Without having ribosomes attached to its surface and is meant for secreting lipids.

Functions of endoplasmic reticulum:

(i). endoplasmic reticulum provides mechanical support to the cell.

(ii). it forms intracellular transporting system.

(iii). it forms new nuclear membrane after every cell division.

(iv). smooth endoplasmic reticulum along takes part in the synthesis of lipids.

(v). rough endoplasmic reticulum along with ribosomes attached to its surface helps in the synthesis and transport of protein.

16. Write about Golgi complex and its functions.

Ans.: It is formed of interconnected stacks of flattened membranous sacs called cisternae. The complex also bears large vacuoles and cluster of small vesicles. It was discovered by Nobel Laureate Caomillo Golgi in 1898 while he was examining the nerve cells of barn owl.

Functions of Golgi complex:

(i). golgi complex is primarily associated with the secretory activities of the cell.

(ii). it is involved in the synthesis of cell wall, plasma membrane and lysosomes.

(iii). it is involved in the formation of cell plate during cell division.

(iv). dictyosomes in plants serve as temporary storage place for proteins and other synthesized products.

17. Write about ribosomes and its functions.

Ans.: Ribosomes are the smallest cell organelles which can be seen only through electron microscope. They are found in both prokaryotic as well as eukaryotic cells. Ribosomes are dense, round a body which occurs freely in cytoplasm or may remain attached to the surface of endoplasmic reticulum.

Functions of ribosomes: Ribosomes help in the process of protein synthesis. They are, therefore, called the protein factories of the cell.

18. Write about Mitochondria and its functions.

Ans.: Mitochondria present in all eukaryotic cells except mature RBCs of mammals. Each mitochondrion is enclosed by a double membrane of lipoprotein.

Functions of mitochondria:

(i). mitochondria are the sites of cellular respiration, where energy in the form of ATP( Adenosite triphosphate) is formed. That is why they are called the powerhouses of the cell.

(ii). since mitochondria have their own ribosomes and DNA, they can synthesize their own proteins, and they are self-duplicating units.

(iii). mitochondria may store and release calcium when required.

19. Write about lysosomes and its functions.

Ans.: Lysosomes are dark, spherical, single membrane bound sacs containing several digestive enzymes. They are found in the cytoplasm of eukaryotic cells, mostly in animal cells.

Functions of lysosomes:

(i). lysosomes are involved in intracellular digestion.

(ii).during starvation or ageing, the cell digests its own organelles through lysosomal enzymes, a process called autophagy.

(iii). when a cell is destined to die, the lysosomal enzymes digest the whole cell, a process called autolysis. Lysosomes, thus, act as suicide bags.

(iv). lysosomes destroy the worn out cell organelles.

(v). lysosomes destroy foreign substances entering into the cell. They engulf bacteria and viruses that inter the cell.

20. What is plastids? Write its functions.

Ans.: The term ‘plastid’ was given by Haeckel in 1866. Plastids are the largest cell organelles found in plant cells only. They are spherical or discoidal in shape and are enclosed in double membrane. Plastids contain their own DNA.

Functions of plastids:

(i). chloroplasts are photosynthetic organelles. The chlorophyll present in them traps solar energy for the purpose of synthesizing food for the plant.

(ii). plastids help in storing food products like starch, protein and lipid.

(iii). plastids provide colour to flower which in turn attracts insects for pollination.

21. What are Grana and Stroma?

Ans.: Grana are membranous flattened discoidal sacs piled over each other. They contain chlorophyll needed for synthesizing food through photosynthesis and are, there, called the kitchen of the cell.

Stroma is the matrix of the plastid in which lies the grana.

22. Write about centrosomes and its functions.

Ans.: Centrosomes are distinct regions in the cytoplasm close to the nucleus. They do not contain DNA. They are found only in animal cells.

Functions of centrosomes:

(i). during cell division in an animal cell, each of the two centrioles of a centrosome moves to the opposite poles.

(ii). they produce basal bodies from which cilia and flagella arise.

(iii). out of two centrioles in a spermatozoan, the distal one gives rise to axial filament of tail.

23. Write about vacuoles and its functions.

Ans.: Vacuoles are non-living, non-cytoplasmic cell organelles. They are fluid-filled and membrane bound spaces in the cytoplasm. Vacuoles are small in size but more in number in animal cells but in plant cells , they are larger in size and lesser in number.

Functions of vacuoles:

(i). vacuoles are meant for the storage of food, water and other substances.

(ii). they help in the elimination of excess water from the cell, and maintains internal pressure of the cell.

24. Write about nucleus and its major parts.

Ans.: The nucleus is the most important and the largest component of the cell as it is the centre from where all cellular activities are controlled. Nucleus is absent in prokaryotic cells, in mature mammalian RBCs and in sieve tube cells in phloem tissue of a plant.

A nucleus consists of the following five parts:

(i). Nuclear membrane: The nucleus is surrounded on all the sides by a double membrane of lipoprotein. The outer membrane of the nucleus is continuous with the endoplasmic reticulum and may bear ribosomes on its surface, while the inner membrane is smooth.

(ii). Nuclear sap: The nucleoplasm is that part of the protoplasm which is enclosed by nuclear membrane. Nucleoplasm is a viscous fluid containing enzymes, ribosomes, nucleolus and chromatin threads.

(iii). Nucleolus: It is a spherical or oval body present within the nucleus. It is clearly visible in non-dividing cells. During cell division, it disintegrates.

(iv). Chromatin threads: Chromatin threads are intermingled mass of thread-like structures forming chromatin reticulum.

(v). Nuclear matrix: It is present below the nuclear envelope. It is made up of acid protein that function as scaffold for chromatin.

26. Write the functions of nucleus.

Ans.: Functions of nucleus:

(i) nucleus is the centre of all activities of the cell.

(ii). it is a storehouse of genes which are concerned with the development and manifestation of all the body characters.

(iii). nucleus controls cell division.

(iv). nucleus helps in the formation of ribosomes and RNA.

(v). nucleus helps in the transmission of characters from one generation to the next.

(vi). ribosomes are formed in nucleolus part of the nucleus.

27. Write the differences between plant cell and animal cell.

Ans.: Plant Cell Animal Cell

(i). Plant cell is comparatively larger than animal cell. (i). Animal cell is usually smaller than plant cell.

(ii). Plant cell is usually rectangular in shape. (ii). Animal cell is generally oval in shape.

(iii). In plant cell cell wall formed of cellulose is present outside the cell membrane. (iii). In animal cell cell wall is absent.

(iv). In a mature plant cell, generally a single large centrally located vacuole is found. (iv). In animal cell vacuole is small and many in number and scattered.

(v). In plant cell plastids are present in the cytoplasm. (v). In animal cell plastids are absent except the protozoan Euglena.

(vi). In plant cell Centrioles and centrosomes are absent but polarcaps are present. (vi). In animal cell possesses centrioles and centrosomes.

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

Ans.: In the absence of Golgi apparatus the following problems will arise:

(i). the secretory activities of the cell will cease to occur.

(ii). the broken membranes like those of lysosomes, cell wall, plasma membnrane will not get repaired.

(iii). acrosome formation in sperm will not take place, causing inabil,ity of sperms to enter into egg.

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

Ans.: Mitochondrion is the site of cellular respiration where energy in the form of ATP is generated as a consequence of oxidation of food substances. That is why it is called powerhouse of the cell where energy is generated.

30. Where do the lipids and proteins constituting the cell membrane get synthesized?

Ans.: The lipids present in plasma membrane are synthesized in Golgi complex, while the proteins are formed on endoplasmic reticulum bny ribosomes. However, the glycoprotein and glycolipid attached to outer plasma membrane are also synthesized in Golgi complex.

31. How does an Amoeba obtain its food?

Ans.: Amoeba is a holozoic animal. The food is engulfed from external environment through the process of phagocytosis with the help of pseudopodia.

32. Why are lysosomes known as suicide bags?

Ans.: Lysosomes are known as the suicide bags of the cell because they contain digestive enzymes capable to digest the whole cell when the situation so demands. Since, a cell in itself contains an organelle through which it can kill itself, it is called suicide bag of the cell.

33. How do substances like carbon dioxide and water move in an out of the cell? Discuss.

Ans.: Carbon dioxide can move across the cell membrane by a process called diffusion. When the CO2 concentration of the cell increases in comparison to the CO2 concentration outside the cell or vice-versa, there occures an spontaneous movement of CO2 from a region of higher concentration to a region where its concentration is low. Water also follows the law of diffusion. The spontaneous movement of water molecules through plasma membrane which is selectively permeable is called osmosis. However, the dissolved substances present in water affects the movement of water across plasma membrane. Hence, osmosis is the passage of water from a region of higher water concentration through a semipermeable membrane to a region of lower water concentration.

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

Ans.: Plasma membrane at times may get ruptured or breaks down, especially during the process of endocytosis and exocytosis which later on are repaired by Golgi complex.

35. Carry out the following osmosis experiment:

Take four peeled potato halves and scoop each oone out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup 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 there for two hours. Then observe the four cups and answer the following:

(i). explain why water gathers in the follow portion of B and C

(ii). why is potato A necessary for this experiment?

(iii). explain why water does not gather in the hollow portion of A and D.

Ans.: (i). In the hollow portions of B and C water gathers because the medium surrounding the potatoes has a higher water concentration then the sugar and salt present in them. Hence, according to the laws of osmosis water from a region of higher water concentration moves through semi-permeable membrane to a region of lower water concentration, i.e., sugar and salt solutions.

(ii). Potato A is necessary to show that if there is no concentration difference inside and outside the cell then osmosis does not occur.

(iii). In the hollow portion of A water does not gather because water concentration inside potato cell and the surrounding medium is the same so there is no net movement of water across the cell membrane of potato.

In the hollow portion of D, though the water concentration is lower than the surrounding, water does not pass across the cell membrane of potato to enter into the hollow cup because after boiling, the cell membrane lost its permeability.