Saturday, 5 January 2019

Class 8th : Science - Chapter 16 : The Cell


Chapter 16 : The Cell
Q1. Define a cell?
Ans. A cell is a tiny living part of an organism consisting of many organelles such as, mitochondria, chloroplast, nucleus, Golgi complex, ribosomes etc. it is a basic structural unit in living organism and is called as the building block of body.

Q2.  Who discovered the cell?
Ans. The cell was discovered by an English Scientist, Robert Hook in 1665. He discovered it by observing the thin slices of cork under his crude microscope (Simple, microscope consisting of biconcave lens enclosed in two metal plates)
Q3. Give three examples of unicellular organisms.
Ans. Three examples of unicellular organisms are: Amoeba (Protozoa), Paramecium (Protozoa) and Chlamydomonas (Green algae).
Q4. Answer the following questions:
i. Why cells could not be observed before 17th century?
Ans. Most cells are smaller in size, due to this smaller size these cells could not be seen before 17th century because no instrument was invented that could be used see these small structures.
ii. Why cork could not be observed as such by Hook?
Ans. Cork is a solid structure, so Hook was unable to observe it under his crude microscope. He then made slices of cork and observed them under his microscope.
iii. Where did Hooke demonstrate his observations on cork slice?
Ans. Hook demonstrated his observations on cork slices to scientist at Royal Society of London.
iv. Name the outermost layer of animal cell.
Ans. Cell membrane is the outermost layer of animal cell. It protects and provides shape to the cell. It also allows materials to enter and leave the cell through tiny holes.
v. Name the layer which is present outside the plasma membrane in plant cell?
Ans. Cell wall is the layer which is present outside the plasma membrane in plant cell. It is made of a stiff, non-living material called cellulose. It protects the cell. Cell wall is lacking in animal cells.
vi. Where are chromosomes present in a cell?
Ans. Chromosomes are present in the nucleus of the cell.
vii. Name the cell part which has tiny holes?
Ans. Cell membrane or plasma membrane and nuclear membrane have tiny holes. These tiny holes allow the cell parts in the exchange of substances or materials.
viii. Name the cell organelles which are found in a plant cell?
Ans. Mitochondria, Chloroplast, Nucleus, Endoplasmic Reticulum, Golgi Complex, Ribosomes, Lysosomes etc.
ix. Name the cell having branched structures?
Ans. Nerve cell is the cell that have branched structures. A nerve cell is long and has thread like projections, as it has to convey messages to different parts of the body.
x. Which cell can be observed under unaided eye?
Ans. The cells that are bigger in size can be observed with unaided eyes. For example, an Ostrich egg is the largest animal cell that can be seen with naked eyes. In plants, an Algae, Acetabularia has a one big cell that can be seen with naked eyes.
Q5. Mention the functions of the following;
i. Cell membrane   ii. Chromosome
Ans. i. Functions of Cell membrane: It is a very thin skin covering of cell.
i. It protects the cell.
ii. It provides shape to the cell.
iii. It allows materials to enter and the cell through tiny pores.
ii. Functions of chromosomes: Chromosomes are thread like structures.
i. They inherit the characters from one generation to another.
ii. They protect the genetic material (DNA) from being damaged during cell division.
Q6. Why are the following important to a plant cell?
i. Cell wall    ii. Chloroplast     iii. Mitochondria    iv. Nucleus
Ans. i. Cell Wall: Cell wall is important to the plant cell because it provides rigidity to it and makes it to stand erect. It also provides protection to the cell.
ii. Chloroplast: It is important to the plant cell because it helps a plant to make food. It has a green pigment called chlorophyll, which in presence of sunlight with CO­2 and water helps in the manufacture of food in plants.
iii. Mitochondria: It is important to the cell because it provides energy to the cell and performs the functions of respiration.
iv. Nucleus: Nucleus is important to the cell because it is responsible for the transmission of characters from one generation to another. It controls all the life functions taking place in the cell. It is the boss of the cell.
Q7. Draw an outline diagram of an animal cell. Label its different parts.

Q8. Mention three differences between plant cell and animal cell.
Ans. Three differences between plant cell and animal cell are as under:
Plant Cell
Animal Cell
1. It has a rigid cell wall.
1. Cell wall is lacking.
2. Chloroplast is present.
2. Chloroplast is absent.
3. Vacuole is present in large size.
3. Vacuole is absent, if present it is smaller in size.
Q9. What features are possessed by both plant cells and animal cells?
Ans. Features possessed by both plant cells and animal cells are as under:
i. Plasma membrane is present in both.
ii. Nucleus is present in both.
iii. Both cells contain mitochondria.
iv. Both cells contain the organelles like, endoplasmic reticulum, Golgi complex, lysosomes and ribosomes.
Q10. Why are nerve cells long? Why do these cells have projections?
Ans. Nerve cells are long, branched and have thread-like projections because their function is to convey messages to all parts of the body.
Q11. Why are mitochondria known as the ‘power house of the cell’?
Ans. Mitochondria are called the ‘power house of the cell’ because they provide energy to the cell and also helps in respiration.
Q12. Which four basic elements constitute more than 90% of protoplasm?
Ans. Carbon, Nitrogen, Hydrogen and Oxygen are the four basic elements that constitute more than 90% of protoplasm.
Q13. Write in brief about the variation in shape and size of cells.
Ans. Variation in shape and size of the cells depends upon the function performed by them. In unicellular organisms all the functions of the body is carried out by a single cell, so its shape and size may vary from the cells of the multicellular organisms in which work is divided to many cells.
Q14. Name the different cell organelles and the functions of these organelles?
Ans. Organelles occur in the cytoplasm of a cell. The names and functions of these organelles are given as under:
i. Mitochondria: It provides energy to the cell and performs the functions of respiration.
ii. Chloroplast: It is only present in plant cells. It contains a green pigment called chlorophyll that helps a plant in the manufacture of food by the process of photosynthesis.
iii. Nucleus: Nucleus plays an important in the transmission of characters from one generation to another. It controls all the functions taking place inside the cell.
iv. Endoplasmic reticulum: It is a network of membranes and provides a large surface area for life functions to take place.
v. Golgi complex: It collects and distributes the substances made in the cell synthesis and secretion of many materials.
vii. Lysosomes: They contain enzymes which help in breaking down or destroying the various materials. These are also called suicidal bags.
Q15. What is meant by protoplasm? How does it differ from cytoplasm?
Ans. Protoplasm is a liquid consisting of cytoplasm, cell membrane and nucleus. Protoplasm is called ‘the physical basis of life’. It is a colloidal, complex, elastic, viscous, granular and colourless substance present in the cell. It contains many chemical elements like hydrogen, carbon, oxygen, nitrogen etc.
It is different from the cytoplasm because most of the functions take place in cytoplasm as it contains many tiny structures called organelles which perform the various life functions.
Q16. Fill in the blanks, using the words given below:
(mycoplasma, microscope, nucleus, ostrich egg, cytoplasm, lysosome, mitochondria, Robert Hook, plant, cell membrane)
i. The lysosomes are also called as suicide bags.
ii. The term ‘cell’ was given by Robert Hook.
iii. The instrument used to see tiny objects is called a microscope.
iv. Smallest cell is that of a mycoplasma.
v. An Ostrich egg is a cell that can be seen without a microscope.
vi. The cell membrane, cytoplasm and nucleus are parts of a cell.
vii. Energy is produced in mitochondria.
viii. Cell wall is present in plant cells.
Q17. Write ‘True’ or ‘False’ in front of the statement given below:
i. Most of a cell is nucleus. (True)
ii. Only the nucleus of a cell represents the protoplasm. (False)
iii. Most cells are microscopic. (True)
iv. All living organisms are made of cells. (True)
v. Every cell has cytoplasm. (True)
vi. All cells in a multicellular organism can live independently. (False)
vii. The outermost covering in an animal cell is called cell wall. (False)
Q18. Match the statements in Column A with those in Column B:
Column A                    Column B
1. Cell                          c. Unit of living body.
2. Nucleus                   d. Boss of the cell.
3. Cell wall                  a. Outermost covering in plant cells.
4. Chloroplast              e. Photosynthetic unit.
5. Cytoplasm               f. Jelly-like substance between cell membrane and nucleus.
6. Organelles              b. Tiny structures inside cells.
Q19. Label the different parts of the plant cell.
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