Download Chapter – 6, the fundamental unit of life: the cell 1. Who discovered

Chapter – 6, the fundamental unit of life: the cell
1. Who discovered cell ?
 The English scientist Robert Hooke observed a thin layer of a bottle cork. He found the structures like
honeycomb. He introduced the name ‘cells’ to these structures.
2. State the contribution of various scientists in the field of cytology.
Some of the scientist who contributed in the field of cytology are Robert Hooke, Leeuwenhoek, Robert
Brown, Purkinje, Schleiden and Schwann, Watson and Crick etc.
Robert Hooke:
Observed the thin layer of cork in primary microscope and introduced the
word ‘cell’.
Leeuwenhoek:
Studied bacterial cells under a simple microscope.
Robert Brown:
Described the characteristics of nucleus in the plant cells.
Purkinje:
Named ‘protoplasm’ for the living substance of the cell.
Schleiden and Schwann:
Observed plant tissue and studied animal tissue. They proposed the cell
theory that all tissues are made up of cells and a cell is the fundamental
unit of life.
Watson and Crick:
Proposed double helical model of DNA molecules.
3. Write a note on Prokaryotic and Eukaryotic cells.
Prokaryotic cell:
 The cell in which nucleus is not developed; nuclear membrane is completely absent and nucleoprotein is
found in mass form is called a prokaryotic cell.
 e.g. cells of bacteria and blue green algae have undeveloped nucleus. The nucleus of prokaryotic cells is
called nucleoid.
Eukaryotic cell:
 The cell in which a clear nuclear membrane and nucleolus are found is called an eukaryotic cell.
 e.g. cells of higher plants and animals have developed nucleus covered by double nuclear membrane.
4. Write down any two characteristics of a Prokaryotic cell.
 In prokaryotic cells (1) nucleoprotein is found in mass form and (2) nuclear membrane is completely
absent.
5. Describe cell membrane.
Cell Membrane:
 Membrane surrounding cytoplasm is called cell membrane. Thus, it is the outer covering of a cell.
 Cell membrane is living, thin, flexible, delicate and selective permeable membrane. It constitutes 75%
phospholipid and 25% of protein, cholesterol and polysaccharide.
 The gases like CO2 or O2 pass through cell membrane by the process of diffusion. The transfer of water
and minerals take place through the cell membrane by the process of osmosis.
 In animal cells it is the outermost covering of the cell while in a plant cell it is covered by cell wall.
6. Explain the process of diffusion of gases in the cells.
 The process of diffusion is responsible for the transfer of gases in the cell. The gases like carbon
dioxide or oxygen pass through cell membrane at the time of diffusion.
 CO2 is produced in the cell in the process of production of energy. Thus the concentration of CO2 in the
cell increases as compared to the concentration outside the cell. Because of this CO 2 from inside of the
cell, diffuses out.
 In the same way, due to increasing concentration of O2 outside the cell, O2 diffuses in the cell. Thus
diffusion plays an important role in gaseous exchange between the cell as well as its external
environment.
7. What is osmosis ?
 When two solutions of different concentrations are separated by semi permeable membrane then the
molecules of solute move from lower concentration to higher concentration. This phenomenon is called
osmosis.
 If plant cell or animal cell is put into sugar or salt solution any one of the following three incidents may
take place:
 If the concentration of water around the cell is more than that in the cell, then water enters into the cell
through osmosis. Such type of solution is known as hypotonic solution. As water enters, the cell swells
up.
 If the concentration of solution is the same inside and outside the cell then there will not be any
exchange. Such type of solution is known as isotonic solution.
 If the concentration of water outside the cell is less than inside the cell, then water moves out of the cell
by the process of osmosis. Such type of solution is known us hypertonic solution. Due to the
phenomenon cell shrinks. This is also called plasmolysis.
8. Explain osmosis with an egg.
 Remove shell of an egg by keeping it in dilute HCl. As the shell of an egg is made up of CaCO3 it reacts
with dil HCl and gets dissolved. A thin outer skin is seen around the egg.
 Now put the egg in clean water and observe after 5 minutes. Water enters into the egg and it swells due
to endosmosis.
 Now place similar deshelled egg in a concentrated salt solution and observe after 5 minute. The egg
shrinks due to exosmosis.
9. Explain: Hypotonic solution.
 When a cell is kept in a solution and if the concentration of solute is less and that of solvent is more in
the solution as compared to the cell sap then the solution in which the cells is kept is called hypotonic
solution.
 The process of endosmosis takes place by hypotonic solution i.e. solvent enters into the cell.
 Thus the amount of solvent entering the cell is more than that coming out of the cell.
10. Explain: Hypertonic solution.
 When a cell is kept in a solution and if the concentration of solute is more and that of solvent is less in
the solution as compared to the cell sap then the solution in which the cells is kept is called hypertonic
solution.
 The process of exosmosis takes place by hypertonic solution i.e. solvent comes out of the cell.
 Thus the amount of solvent coming out of the cell is more than that entering the cell.
11. Explain: Isotonic solution.
 When a cell is kept in a solution and if the concentration of solute and solvent in the solution is same as
that in the cell sap then the solution in which the cells is kept is called isotonic solution.
 The amount of solvent that enters the cell is same as that moves outside the cell.
12. How do substance like CO2 and water move in and out of the cell ?
 Substance like CO2 and water move in and out of the cell by the processes called diffusion and osmosis
respectively.
Transfer of CO2:
 Carbon dioxide passes through cell membrane at the time of diffusion.
 In diffusion, the substances constantly enter into less concentrated region from more concentrated
region.
 CO2 is produced in the cell in the process of production of energy. Thus the concentration of CO2 in the
cell increases as compared to the concentration outside the cell.
 Because of this CO2 from inside of the cell diffuses out.
Transfer of water:
 If the concentration of water around the cell is more than that in the cell, then water enters into the cell.
 If the concentration of solution is the same inside and outside the cell then there will be no exchange.
 If the concentration of water outside the cell is less than inside the cell, then water moves out of the cell.
(This process is also called plasmolysis).
All the above processes are due to osmosis.
13. Explain cell wall. State the two main functions of cell wall.
 It is present in plant cell as a shield outside the cell membrane.
 It is non – living and freely permeable membrane.
 It is made of cellulose, hemi cellulose and pectin.
Functions of cell wall
 The cell wall permits the entry of substances in the cell.
 It also protects cytoplasm and cell membrane.
 It maintains the shape of the cell.
 It protects the cells from draining away of water from the cell wall.
14. What is Plasmolysis ?
 The process by which water from the cell moves out in the solution in which it is kept and due to this the
cell shrinks is called plasmolysis.
15. Describe an experiment to show that the property of selective permeability is found only in living cell
membrane.
 Peel off the leaf skin of tradescantia and put on the slide.
 A small pigment of green substance, chlorophyll, can be seen through high power microscope.
 Now put concentrated sugar solution or salt solution. Wait for some time.
 Water moves out of the leaf cells due to exosmosis process. The components of the cell separate from
the cell wall. The outer medium being hypertonic, the water comes out because of plasmolysis from the
cell of the leaf.
 Now put some tradescantia leaves into boiling water for some time so that the cells die. Remove the
leaves from the water and put them on the slide. Put simple sugar or salt solution on the leaves. Wait for
some time and observe through microscope. No plasmolysis takes place.
 This means that the property of selective permeability is found only in living cell membrane.
16. Why plasma membrane is called a selectively permeable membrane ?
 The plasma membrane allows only some specific substances to pass through it and prevents the other
substances to pass through it.
 Some material penetrate the membrane readily, other slowly and some other cannot at all penetrate
through the cell membrane.
 Therefore it is called selectively permeable membrane.
17. Write a note on nucleus of a cell.
Nucleus:
 Main and round cellular organelle situated in the middle of the cell.
 Surrounded by double layered nuclear membrane which separates it from cytoplasm.
 Nuclear membrane has many pores and it contains nucleoplasm. The nucleoplasm consists of (1)
nucleolus and (2) chromatin.
Nucleolus:
 The number of nucleolus is one or more but it does not have any membrane. Nucleolus consists of
plenty of protein. It is the site of the origin of ribosome.
Chromatin:
 Chromatin is a thin fibre like mass of interwined chromosomes made up of genetic material DNA
and protein. DNA molecules have information of inherited characteristics. The working component
in DNA is called gene.
 Certain organisms like bacteria do not have clear nuclear region because of the absence of nuclear
membrane. Such unclear nuclear region having nucleic acid is known as nucleoid.
 On the basis of presence of nuclear envelope the cells living organisms are classified into (1)
Prokaryotic cell: They are the cells possessing unclear nuclear region without nuclear membrane and (2)
Eukaryotic cell: They are the cells containing nuclear membrane and a well defined nucleus.
18. Name the discoverers of nucleus and DNA.
 The discoverer of nucleus is Robert Brown and that of DNA are Watson and Crick.
19. Explain cytoplasm.
Cytoplasm:
 The semi liquid, colourless part of the cell between cell membrane and the nuclear membrane is called
cytoplasm.
 The mass which is found inside the cytoplasm is called endoplasm and the transparent cell membrane
outside it is called ectoplasm.
 Cytoplasm consists of various cell organelles, insoluble waste and collected substance such as starch,
glycogen and fat. 90% composition of cytoplasm is water.
 It also consists of proteins fibres like tubulin, actin and keratin which are helpful in maintaining the
shape of the cell.
 The cell organelles in cytoplasm are nucleus, mitochondria, chloroplast, golgi body, endoplasmic
reticulum and ribosomes. These organelles do protein synthesis, photosynthesis and ATP synthesis.
20. Name any two major cell organelles of a Eukaryotic cell.
 Endoplasniic Reticulum, golgi body, lysosomes, mitochondria etc are some of the cell organelles of
Eukaryotic cell.
21. Write a note on endoplasmic reticulum.
Endoplasmic Reticulum
 Endoplasmic reticulum is found in all animal and plant cells but absent in red blood cells. It is connected
with the external part of nucleus at one end while the other end is connected with cell membrane. There
are two types of endoplasmic reticulum (1) Rough ER and (2) Smooth ER
 Rough ER: Ribosomes, which carry out protein synthesis, are found on the surface of rough ER.
 Smooth ER: No ribosomes are found on the surface of smooth ER.
 ER are found in three forms. (i) Cisternae, (ii) Vesicle and (iii) Tubules
Cisternae: It is expanded flat, branchless and tubular structure filled with fluid.
Vesicle: It is a round or oval structure and found in cytoplasm freely.
Tubules: It forms a network with cistemae and vesicles which is small and its surface is smooth.
Functions of Endoplasmic reticulum:
 Acts as a transmitary system between cytoplasm and nucleus. Smooth ER synthesis, stores and disperses
glycogen, creates lipids, steroids and toxins in liver cells and protects from harmful medicine.
22. Write a note on golgi bodies.
 Description of golgi body was given by scientist Camilo Golgi.
 In plant cells, they are called dictyosomes. The cells contain different number of golgi.
 The three components found in the micro structure of golgi are: (1) flat bags of cisternae (2) micro
tubules (3) Large vacuoles.
 These components are filled with or without granular substances.
 Cisternae is parallel and arranged in stacked form, flat and dish shaped bags. Sometimes they are
connected with each other and forms network structure. The origin of vesicles and tubules takes place in
the form of bud from cisternae.
 Golgi is not found in the cells of bacteria, blue–green algae, mature sperm and the blood–cells of
mammals.
 The origin of golgi takes place from the membrane of smooth ER.
Golgi Body
 The cisterne of golgi body stores protein synthesised by ribosomes in ER. It also helps to secrete and
transmit by packaging the substances like lipoprotein, glycolipid, phospholipid etc.
23. Write a note on lysosomes.
Lysosomes:
 They are small round bags.
 They produce enzymes for digestion of food particles. Their enzyme is made by rough ER.
 Lysosome performs the functions of intercellular digestion, so they are called digestive bags. It protects
the body from infection by destroying bacteria and virus entering from outside in the cell.
 During disturbance in cellular metabolism destruction of cells take place. The lysosome enzymes come
out and destroy the cell by digesting all the organelles of the cell. Thus lysosome is also called ‘Suicidal
bags’ of the cell.
24. Why lysosome is known as suicide bag ?
 Each lysosome contains lytic enzymes which are responsible for the digestion of all types of organic
molecules.
 They destroy any external molecules or bacteria which are entering in the cell.
 During disturbance in cellular metabolism lysosome enzymes come out and destroy the cell by digesting
all the organelles of the cell.
 Thus they can cause death to the cell. Hence they are called suicide bags.
25. Write a note on mitochondria.
[Draw labelled diagram: Structure of Mitochondria]
Mitochondrion





Mitochondria:
It is ultra–minute, tubular, rod shaped or round body.
It is distributed into cytoplasm.
Covered by double membrane covering.
The outer membrane is porous but the internal membrane is folded.
This folded structure is known as criseae in which at the upper side F1 microparticles or oxysomes are
present.
 The internal cavity of mitochondria is filled with intercellular substance which is cells gel like protein
and possesses ribosomes of small size, atoms of DNA and phosphates.
 It creates its own protein so it is a semi – autonomous organelle. The mitochondria are not found in the
red blood cells of mammals and bacteria.
Functions of Mitochondria:
 Cellular respiration takes place in mitochondria.
 It transforms carbohydrate and lipid from the cells into CO2 and H2O. Because of oxidation, it gets free
energy and ATP is formed.
 In mitochondria the synthesis of ATP takes place and hence it is called "Power House" of the cell.
26. Which organelle is known as power house of the cell ? Why ?
 In mitochondria, cellular respiration takes place and it is responsible for the energy production in the
cell.
 It transforms carbohydrate and lipid from the cells into CO 2 and H2O. Because of oxidation, it gets free
energy and ATP is formed .
 Hence mitochondrion is called power house of cell.
27. Describe various types of plastids.
 Plastids: Plastids is found in almost all the plant cells. There are three types of plastids.(1) Chloroplast:
Green colour plastids, (2) Chromoplast: Except green colour plastids and (3) Luecoplast: colourless
plastids.
Chloroplasts:
[Draw labelled diagram of structure of Chloroplast.]
Chloroplast
 They are found in green coloured algae and higher level plants.
 The green pigment found in chloroplast is called chlorophyll.
 Chloroplast absorbs the light energy and uses it for preparing food for the plants. It produces food by
photosynthesis and so it is called the kitchen of the cell.
 It is covered with double membrane. Chloroplast has its own genome and ribosomes are found.
Chloroplast has self – replication ability and divides independent.
 The two specific area of chloroplast are: (1) Grana and (2) Stroma.
Grana:
 Every granum is a stack of membrane bounded, flattened discoid sacs like structures containing the
molecules of chlorophyll and it is called thylakoid.
 Thylakoid contains the chlorophyll, which is the main functional unit of chloroplast.
 It also consists of enzymes of photosynthesis, atoms of starch, DNA and ribosomes.
 It is the place of photoreaction of photosynthesis.
Stroma:
 It is a common substance, in which grana are found. It is the place of dark reaction.
Chromoplast: It imparts different colours to flowers from which the insects are attracted for pollination.
Leucoplast: It stores food in the form of starch, fat and protein.
28. Write a note on vacuoles.
 Vacuoles are filled with fluid or hard substance and they are covered with a membrane. They are one
type of storage bags.
Vacuoles in plant cell:
 In plant cell they are big, specific and permanent.
 In a mature plant, they occupy 90% of the cell space.
 As it is located in the middle of the cell, the nucleus and other organelles of the cell are pushed towards
cell wall.
 The membrane which covers the vacuoles is called tonoplast.
 The vacuoles contain water, sugars, amino acids, proteins and minerals.
 They keep the plant cells rigid and turgid.
Vacuoles in animal cell:
 In animal cells, the vacuoles are small and temporary.
 They contain water, glycogen and protein.
 Vacuoles perform the function of osmoregulation in lower organisms. In Amoeba and paramecium
vacuoles digest food.
 In animals, vacuoles perform the functions of digestion and osmoregulation.
29. State the chief function of following organelles. (A) Mitochondria, (B) Lysosomes, (C) Golgi body, (D)
Ribosomes and (E) Chloroplast.
(A) Chief function of mitochondria:
It transforms carbohydrate and lipid from the cells into CO 2 and
H2O and release free energy and ATP is formed.
(B) Chief function of lysosomes:
It performs the functions of intercellular digestion. It protects the
body from infection by destroying bacteria and virus entering from
outside in the cell.
(C) Chief function of golgi body:
It stores protein synthesised in ribosomes in ER in the cistern. It
helps to secrete and transmit the substances like lipoprotein,
glycolipid, phospholipid etc.
(D) Chief function of ribosomes:
It acts as a transmitary system between cytoplasm and nucleus.
Smooth ER synthesize, stores and disperses glycogen, lipids,
streoids and toxins in liver cells and protects from harmful
medicine.
(E) Chief function of chloroplast:
It absorbs the light energy and uses it for preparing food for the
plants.