Download The Fundamental Unit of Life

Main Concepts
The Fundamental Unit of Life
• The fundamental organisational unit of life is the cell.
• Cells are enclosed by a plasma membrane composed of lipids and proteins.
• The cell membrane is an active part of the cell. It regulates the movement of materials
between the ordered interior of the cell and the outer environment.
• In plant cells, a cell wall composed mainly of cellulose is located outside the cell
membrane.
• The presence of the cell wall enables the cells of plants, fungi and bacteria to exist in
hypotonic media without bursting.
• The nucleus in eukaryotes is separated from the cytoplasm by double-layered membrane
and it directs the life processes of the cell.
• The ER functions both as a passageway for intracellular transport and as a manufacturing
surface.
• The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the
storage, modification and packaging of substances manufactured in the cell.
• Most plant cells have large membranous organelles called plastids, which are of two types
– chromoplasts and leucoplasts.
• Chromoplasts that contain chlorophyll are called chloroplasts and they perform
photosynthesis.
• The primary function of leucoplasts is storage.
• Most mature plant cells have a large central vacuole that helps to maintain the turgidity of
the cell and stores important substances including wastes.
• Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed
of only nucleic acid, and they have only very small ribosomes as organelles.
Exercise
1. Who first observed that living things appear to consist of separate units called cells?
Answer: Robert Hooke first observed that living things appear to consist of separate
units called cells.
2. Who discovered the nucleus in the cell?
Answer: Robert Brown in 1831 discovered the nucleus in the cell.
3. Who coined the term ‘protoplasm’ for the fluid substance of the cell?
Answer: Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the
cell.
4. What is cell theory?
Answer: The cell theory is that all the plants and animals are composed of cells and
that the cell is the basic unit of life.
5. Who presented the cell theory?
Answer: Schleiden (1838) and Schwann (1839) presented the cell theory.
6. When was electron microscope discovered?
Answer: Electron microscope was discovered in 1940.
7. What do you mean by unicellular organism?
Answer: The single cells which constitute the whole organism are called unicellular
organism.
8. Name some unicellular organisms.
Answer: Amoeba Chlamydomonas, Paramoecium and bacteria.
9. What do you mean by multi-cellular organism?
Answer: The cells which group together in a single body and assume different
functions in it to form various body parts are called multi-cellular organism.
10. Name some multi-cellular organisms.
Answer: fungi, plants and animals are multi-cellular organisms.
11. Every multi-cellular organism has come from a single cell. True or false?
Answer: True.
12. On which factors the shape and size of cells depend?
Answer: The shape and size of cells are dependent on the specific function they
perform.
13. What are cell organelles? What are their functions?
Answer: Each cell has got certain specific components within it known as cell
organelles. These organelles together constitute the basic unit called the cell. A cell
is able to live and perform all its functions because of these organelles.
Each kind of cell organelle performs a special function, such as making new
material in the cell, clearing up the waste material from the cell etc.
14. What are the main features of a cell?
Answer: The three main features of a cell are plasma membrane, nucleus and
cytoplasm.
15. Give the definition of plasma membrane. What are its main functions
Answer: plasma membrane is the outermost covering of the cell that separates the
contents of the cell from its external environment.
The plasma membrane allows or permits the entry and exit of some
materials in and out of the cell. It also prevents movement of some other materials.
16. Why is plasma membrane also known as selectively permeable membrane?
Answer: The plasma membrane allows or permits the entry and exit of some
materials in and out of the cell. It also prevents movement of some other materials.
The cell membrane, therefore, is called a selectively permeable membrane.
17. How does carbon dioxide or oxygen move across the cell membrane?
Answer: Carbon dioxide or oxygen move across the cell membrane by a process
called diffusion.
18. Diffusion plays an important role in gaseous exchange between the cells as well as
the cell and its external environment. True or false? Justify your answer
Answer: True.
CO2 is cellular waste and requires to be excreted out by the cell. It
accumulates in high concentrations inside the cell. In the cell’s external environment,
the concentration of CO2 is low as compared to that inside the cell. As soon as there
is a difference of concentration of CO2 inside and outside a cell, CO2 moves out of
the cell, from a region of high concentration, to a region of low concentration
outside the cell by the process of diffusion. Similarly, O2 enters the cell by the
process of diffusion when the level or concentration of O2 inside the cell decreases.
Thus, diffusion plays an important role in gaseous exchange between the cells as
well as the cell and its external environment.
19. What do you understand by the word ‘osmosis’?
Answer: osmosis is the passage of water from a region of high water concentration
through a semi-permeable membrane to a region of low water concentration.
20. Define hypotonic solution.
Answer: If the medium surrounding the cell has a higher water concentration than
the cell, meaning that the outside solution is very dilute, the cell will gain water by
osmosis. Such a solution is known as a hypotonic solution.
21. When is a cell likely to swell up?
Answer: In case of hypotonic solution, where the medium surrounding the cell has a
higher water concentration than the cell, water molecules are free to pass across the
cell membrane in both directions, but more water will come into the cell than will
leave. The net (overall) result is that water enters the cell and the cell is likely to
swell up.
22. What is isotonic solution?
Answer: If the medium has exactly the same water concentration as the cell, there
will be no net movement of water across the cell membrane. Such a solution is
known as an isotonic solution.
23. What is hypertonic solution?
Answer: If the medium has a lower concentration of water than the cell, meaning
that it is a very concentrated solution, the cell will lose water by osmosis. Such a
solution is known as a hypertonic solution.
24. When does the cell shrink?
Answer: In case of hypertonic solution where the medium has a lower concentration
of water than the cell, water crosses the cell membrane in both directions, but more
water leaves the cell than enters it. Therefore the cell will shrink.
25. When will a cell stay the same size?
Answer: In case of isotonic solution, where there will be no net movement of water
across the cell membrane, Water crosses the cell membrane in both directions, but
the amount going in is the same as the amount going out, so there is no overall
movement of water. The cell will stay the same size.
26. What are the components of plasma membrane?
Answer: The components of plasma membrane are lipids and proteins.
27. How will you observe the structure of the plasma membrane?
Answer: The structure of the plasma membrane can be observed only through an
electron microscope.
28. What is endocytosis? Give example.
Answer: The flexibility of the cell membrane also enables the cell to engulf in food
and other material from its external environment. Such processes are known as
endocytosis. Example: Amoeba.
29. Where is the cell wall situated?
Answer: The cell wall lies outside the plasma membrane.
30. What is the main component of cell wall? What is its main function?
Answer: The plant cell wall is mainly composed of cellulose. Its main function is to
provide structural strength to plants.
31. Define the term plasmolysis.
Answer: When a living plant cell loses water through osmosis there is shrinkage or
contraction of the contents of the cell away from the cell wall. This phenomenon is
known as plasmolysis.
32. What is nuclear membrane? What are its functions?
Answer: The nucleus has a double layered covering called nuclear membrane.
The nuclear membrane has pores which allow the transfer of material from
inside the nucleus to its outside, that is, to the cytoplasm.
33. What does nucleus of a cell contain?
Answer: The nucleus contains chromosomes.
34. What functions does a chromosome perform?
Answer: Chromosomes contain information for inheritance of features from parents
to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
35. What are the two components of chromosome?
Answer: The two components of chromosome are DNA and protein.
36. What is the main function of DNA molecules?
Answer: DNA molecules contain the information necessary for constructing and
organising cells.
37. What are genes?
Answer: Functional segments of DNA are called genes.
38. What is the function of chromatin material?
Answer: The function of chromatin material is whenever the cell is about to divide,
the chromatin material gets organised into chromosomes.
39. What are the main roles of nucleus?
Answer: The nucleus plays a central role in cellular reproduction, the process by
which a single cell divides and forms two new cells. It also plays a crucial part, along
with the environment, in determining the way the cell will develop and what form it
will exhibit at maturity, by directing the chemical activities of the cell.
40. What do you mean by nucleoid?
Answer: In some organisms like bacteria, the nuclear region of the cell may be poorly
defined due to the absence of a nuclear membrane. Such an undefined nuclear
region containing only nucleic acids is called a nucleoid.
41. What are prokaryotes?
Answer: In some organisms like bacteria, the nuclear region of the cell may be poorly
defined due to the absence of a nuclear membrane. Such organisms, whose cells lack
a nuclear membrane, are called prokaryotes. Prokaryotic cells also lack most of the
other cytoplasmic organelles present in eukaryotic cells.
42. What are eukaryotes?
Answer: Organisms with cells having a nuclear membrane are called eukaryotes.
43. What is the main characteristic of virus?
Answer: Viruses lack any membranes and hence do not show characteristics of life
until they enter a living body and use its cell machinery to multiply.
44. What are cell organelles?
Answer: Every cell has a membrane around it to keep its own contents separate from
the external environment. Large and complex cells, including cells from multicellular
organisms, need a lot of chemical activities to support their complicated structure
and function. To keep these activities of different kinds separate from each other,
these cells use membrane-bound little structures within themselves. These
structures are called organelles.
45. Name some of the cell organelles.
Answer: Some of the cell organelles endoplasmic reticulum, Golgi apparatus,
lysosomes, mitochondria, plastids and vacuoles.
46. What is endoplasmic reticulum (ER)? How will you classify them?
Answer: The endoplasmic reticulum (ER) is a large network of membrane-bound
tubes and sheets.
There are two types of ER– rough endoplasmic reticulum (RER) and smooth
endoplasmic reticulum (SER).
47. What is the function of ribosome?
Answer: The ribosomes, are the sites of protein manufacture. The manufactured
proteins are then sent to various places in the cell depending on need, using the
endoplasmic reticulum (ER).
48. What is the function of smooth endoplasmic reticulum (SER)?
Answer: The SER helps in the manufacture of fat molecules, or lipids, important for
cell function. SER plays a crucial role in detoxifying many poisons and drugs.
49. What do you mean by membrane biogenesis?
Answer: Smooth endoplasmic reticulum helps in the manufacture of fat molecules,
or lipids, important for cell function. Some of these proteins and lipids help in
building the cell membrane. This process is known as membrane biogenesis.
50. What are the main functions of endoplasmic reticulum (ER)?
Answer: The main functions of endoplasmic reticulum (ER) are
(a) To serve as channels for the transport of materials (especially proteins) between
various regions of the cytoplasm or between the cytoplasm and the nucleus.
(b) As a cytoplasmic framework providing a surface for some of the biochemical
activities of the cell.
51. Who first described Golgi apparatus?
Answer: The Golgi apparatus first described by Camillo Golgi.
52. What is Golgi apparatus?
Answer: Golgi apparatus consists of a system of membrane-bound vesicles arranged
approximately parallel to each other in stacks called cisterns. These membranes
often have connections with the membranes of ER and therefore constitute another
portion of a complex cellular membrane system.
53. What are the functions of Golgi apparatus?
Answer: The functions of Golgi apparatus include the storage, modification and
packaging of products in vesicles. In some cases, complex sugars may be made from
simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the
formation of lysosomes.
54. What are lysosomes? What are its functions?
Answer: Lysosomes are a kind of waste disposal system of the cell.
The functions are--(a) They help to keep the cell clean by digesting any foreign material as well as wornout cell organelles.
(b) Foreign materials entering the cell, such as bacteria or food, as well as old
organelles end up in the lysosomes, which break them up into small pieces.
55. Why lysosomes are also known as the ‘suicide bags’ of a cell?
Answer: When the cell gets damaged, lysosomes may burst and the enzymes digest
their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.
56. What is known as the powerhouses of the cell?
Answer: Mitochondria are known as the powerhouses of the cell.
57. Which is known as the energy currency of the cell?
Answer: ATP (Adenosine triphopshate) is known as the energy currency of the cell.
58. Write briefly on the composition of Mitochondria.
Answer: Mitochondria have two membrane coverings instead of just one. The outer
membrane is very porous while the inner membrane is deeply folded. These folds
create a large surface area for ATP-generating chemical reactions.
59. Why Mitochondria are called strange organelles?
Answer: Mitochondria are strange organelles in the sense that they have their own
DNA and ribosomes. Therefore, mitochondria are able to make some of their own
proteins.
60. What is so special about plastids?
Answer: Plastids are present only in plant cells.
61. What are the two types of plastids?
Answer: The two types of plastids are– chromoplasts (coloured plastids) and
leucoplasts (white or colourless plastids).
62. What is chloroplast? Why it is important?
Answer: Plastids containing the pigment chlorophyll are known as chloroplasts.
Chloroplasts are important for photosynthesis in plants.
63. What does a Leucoplast do?
Answer: Leucoplasts are primarily organelles in which materials such as starch, oils
and protein granules are stored.
64. Plastids are similar to mitochondria. Why?
Answer: Plastids are similar to mitochondria in external structure. Like the
mitochondria, plastids also have their own DNA and ribosomes.
65. What are Vacuoles?
Answer: Vacuoles are storage sacs for solid or liquid contents.
66. What are the main functions of Vacuoles?
Answer: The main functions of Vacuoles are--(a) In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the
cell.
(b) Many substances of importance in the life of the plant cell are stored in vacuoles.
These include amino acids, sugars, various organic acids and some proteins.
(c) In single-celled organisms like Amoeba, the food vacuole contains the food items
that the Amoeba has consumed
(d) In some unicellular organisms, specialised vacuoles also play important roles in
expelling excess water and some wastes from the cell.
Multiple Choice Questions:
1. Which of the following can be made into crystal?
(a) A Bacterium
(b) An Amoeba
(c) A Virus
2. A cell will swell up if
(d) A Sperm
3.
4.
5.
6.
7.
8.
(a) The concentration of water molecules in the cell is higher than the concentration
of water molecules in surrounding medium
(b) The concentration of water molecules in surrounding medium is higher than
water molecules concentration in the cell
(c) The concentration of water molecules is same in the cell and in the surrounding
medium
(d) Concentration of water molecules does not matter
Chromosomes are made up of
(a) DNA
(b) protein
(c) DNA and protein
(d) RNA
Which of these options are not functions of Ribosomes?
(i) It helps in manufacture of protein molecules
(ii) It helps in manufacture of enzymes
(iii) It helps in manufacture of hormones
(iv) It helps in manufacture of starch molecules
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (iv) and (i)
Which of these is not related to endoplasmic reticulum?
(a) It behaves as transport channel for proteins between nucleus and cytoplasm
(b) It transports materials between various regions in cytoplasm
(c) It can be the site of energy generation
(d) It can be the site for some biochemical activities of the cell
Following are a few definitions of osmosis. Read carefully and select the correct
definition
(a) Movement of water molecules from a region of higher concentration to a region
of lower concentration through a semi permeable membrane
(b) Movement of solvent molecules from its higher concentration to lower
concentration
(c) Movement of solvent molecules from higher concentration to lower
concentration of solution through a permeable membrane
(d) Movement of solute molecules from lower concentration to higher concentration
of solution through a semi permeable membrane.
Plasmolysis in a plant cell is defined as
(a) break down (lysis ) of plasma membrane in hypotonic medium
(b) shrinkage of cytoplasm in hypertonic medium
(c) shrinkage of nucleoplasm
(d) none of them
Which of the following are covered by a single membrane?
(a) Mitochondria
(b) Vacuole
(c) Lysosome
(d) Plastid
9. Find out the false sentences
(a) Golgi apparatus is involved with the formation of lysosomes
(b) Nucleus, mitochondria and plastid have DNA; hence they are able to make their
own structural proteins
(c) Mitochondria is said to be the power house of the cell as ATP is generated in
them.
(d) Cytoplasm is called as protoplasm
10. Find out the correct sentence
(a) Enzymes packed in Lysosomes are made through RER (rough endoplasmic
reticulum)
(b) Rough endoplasmic reticulum and smooth endoplasmic reticulum produce lipid
and protein respectively
(c) Endoplasmic reticulum is related with the destruction of plasma membrane
(d) Nucleoid is present inside the nucleoplasm of eukaryotic nucleus.
11. Which cell organelle plays a crucial role in detoxifying many poisons and drugs in a
cell?
(a) Golgi apparatus
(b) Lysosomes
(c) Smooth endoplasmic reticulum
(d) Vacuoles
12. The proteins and lipids, essential for building the cell membrane, are manufactured
by
(a) rough endoplasmic reticulum
(b) golgi apparatus
(c) plasma membrane
(d) mitochondria
13. The undefined nuclear region of prokaryotes are also known as
(a) nucleus
(b) nucleolus
(c) nucleic acid
(d) nucleoid
14. The cell organelle involved in forming complex sugars from simple sugars are
(a) endoplasmic reticulum
(b) ribosomes
(c) plastids
(d) golgi apparatus
15. Which out of the following is not a function of vacuole?
(a) Storage
(b) Providing turgidity and rigidity to the cell
(c) Waste excretion
(d) Locomotion
16. Amoeba acquires its food through a process, termed
(a) exocytosis
(b) endocytosis
(c) plasmolysis
(d) exocytosis and endocytosis both
17. Cell wall of which one of these is not made up of cellulose?
(a) Bacteria
(b) Hydrilla
(c) Mango tree
(d) Cactus
18. Silver nitrate solution is used to study
(a) endoplasmic reticulum
(b) golgi apparatus
(c) nucleus
(d) mitochondria
19. Organelle other than nucleus, containing DNA is
(a) endoplasmic reticulum
(b) golgi apparatus
(c) mitochondria
(d) lysosome
20. Kitchen of the cell is
(a) mitochondria
(b) endoplasmic reticulum
(c) chloroplast
(d) golgi apparatus
21. Lipid molecules in the cell are sythesized by
(a) smooth endoplasmic reticulum
(b) rough endoplasmic reticulum
(c) golgi apparatus
(d) plastid
22. Cell arises from pre-existing cell was stated by
(a) Haeckel
(b) Virchow
(c) Hooke
(d) Schleiden
23. Cell theory was given by
(a) Schleiden and Schwann
(b) Virchow
(c) Hooke
(d) Haeckel
24. The only cell organelle seen in prokaryotic cell is
(a) mitochondria
(b) ribosomes
(c) plastids
(d) lysosomes
25. Organelle without a cell membrane is
(a) ribosome
(b) golgi apparatus
(c) chloroplast
(d) nucleus
26. 1 µm is
(a) 10–6 m
(b) 10–9 m
(c) 10–10 m
(d) 10–3 m
27. Lysosome arises from
(a) endoplasmic reticulum
(b) golgi apparatus
(c) nucleus
(d) mitochondria
28. Living cells were discovered by
(a) Robert Hooke
(b) Purkinje
(c) Leeuwenhoek
(d) Robert Brown
29. Select the odd one out
(a) The movement of water across a semi permeable membrane is affected by the
amount of substances dissolved in it.
(b) Membranes are made of organic molecules like proteins and lipids
(c) Molecules soluble in organic solvents can easily pass through the membrane.
(d) Plasma membranes contain chitin sugar in plants
Answers: 1. (c)
5. (c)
9. (a)
13. (d)
17. (a)
21. (a)
25. (a)
29. (d)
2. (b)
6. (a)
10. (a)
14. (d)
18. (b)
22. (b)
26. (a)
3. (c)
7. (b)
11. (c)
15. (d)
19. (c)
23. (a)
27. (b)
4. (c)
8. (b)
12. (a)
16. (b)
20. (c)
24. (b)
28. (c)