Biology Cells Unit of Life Class 8th

Fundamental Unit Of Life

(Q.) Name two major functional regions of a cell.

(Ans)  The two major functional regions of a cell are-
a) Nucleus       b) Cytoplasm

(Q.) Cells come from pre - existing cells. How?

 (Ans)  Cells divide to produce cells of their own kind. Thus, all cells come from pre - existing cells.

(Q.) Who discovered the nucleus in the cell?

 (Ans)  Robert Brown in 1831 discovered the nucleus in the cell.

(Q.) Who discovered the cell?

(Ans)  Robert Hooke discovered the cells.

(Q.) What substances form cell membrane and cell wall?

 (Ans)  Cell membrane is made up of proteins and lipids. Cell wall is made up of cellulose.

(Q.) Name two-cell organelles, which have DNA apart from nucleus.

 (Ans)  1) Plastids     (2) Mitochondria

(Q.) Name the cell organelles that are  found only in plant cell.

 (Ans)  Plastids and cell wall.

(Q.) Name the cell organelle that is found only in animal cell.

 (Ans)  Centriole

(Q.) On what factors do shape and size of cell depend?

 (Ans)  The shape and size of cell depend on the functions they perform.

(Q.) Who coined the term protoplasm?

 (Ans)  J.E. Purkinje

(Q.) What are chromosomes? Where are they present in the cell?

 (Ans)  Chromosomes are visible rod-shaped structures present in the nucleus. They are composed of DNA and protein.Chromosomes contain information for inheritable features from parents to next generation in the form of DNA molecules.

(Q.) Why is the plasma  membrane called a selectively permeable membrane?

 (Ans)  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 in the cell membrane. Therefore, it is called a selectively permeable membrane.

(Q.) Why is the cell called the structural and functional unit of life?

 (Ans)  Cell is called structural and functional unit of life because -
(i) All the living organisms are made up of cells.
(ii) All the functions taking place in the body of organisms are also performed by the cells within themselves.

(Q.) What are multicellular organisms? Give examples.

 (Ans)  The organisms which are made up of many cells and these cells perform various functions of the body are known as multicellular organisms, e.g.fungi, plants, animals.

(Q.) What are unicellular organisms? Give examples.

 (Ans)  Some organisms are made up of a single cell only. They are known as unicellular organisms. e.g. Amoeba, Chlamydomonas, Paramecium and Bacteria.

(Q.) Who presented the cell theory for the first time? What was it?

 (Ans)  Two biologists, Schleiden (1839) and Schwann (1839) presented the cell theory.
This stated that all the plants and animals are composed of cells and the cell is the basic unit of life.

(Q.) Who discovered free-living cells and how?

 (Ans)  Leeuwenhoek with the improved microscope discovered the free-living cells in pond water for the first time.

(Q.) Name the non - living parts of a cell.

 (Ans)  1.Cell wall (plants only)       2.Vacuoles      3.Granules (cell inclusions)

(Q.) Which cell organelle is known as the kitchen of the cell? Why?

 (Ans)  Chloroplast is known as kitchen of the cell as photosynthesis takes place resulting in synthesis of food.

(Q.) How do substances like CO2 and water move in and out of the cell?

 (Ans)  Gases like CO₂ and O₂ move in and out of the cell by diffusion from their higher concentration to lower concentration.
Water enters the cell by endosmosis through semi-permeable plasma membrane from its higher concentration to lower concentration.
Similarly, water moves out of the cell by exosmosis when a cell is placed in a hypertonic solution.

(Q.) Explain the terms.
a) Endocytosis                        b) Plasmolysis

 (Ans)  a) Endocytosis- The flexibility of the cell membrane enables the cell to engulf food and other material from its external environment. Such processes are known as endocytosis.
b) Plasmolysis- 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.

(Q.) Write any four differences between the plasma membrane and cell wall.

(Ans)  Plasma Membrane	Cell Wall
1. It is made up of lipids and protein. 2. It is living. 3. Present in both plant and animal cell. 4. It regulates entry and exit of molecules in and out of the cell.	1. It is made up of cellulose. 2. It is dead. 3. Found exclusively in plant cell. 4. Cell wall provides structural strength to the plants.

(Q.) Define- a) Diffusion   b) Osmosis

 (Ans)  a) Diffusion - It is the spontaneous movement of molecules from region of their high concentration to their lower concentration leading finally to uniform concentrations. It is faster in the gaseous phase than in liquids and solids.
b) Osmosis- It is the movement of solvent (water) molecules from region of its higher concentration to its lower concentration through a semi - permeable membrane. Osmosis is a purely mechanical process by which cells absorb water without spending any amount of energy.

(Q.) When and how the cells were discovered?

 (Ans)  In 1665, Robert Hooke observed a thin slice of cork(part of the bark of a tree)under a self - designed microscope and observed astructure like a honeycomb consisting of many little compartments. Robert Hooke called these boxes as cells. Cell is a Latin word usedfor a little room.

(Q.) Name the living parts of a cell.

 (Ans)  1. Plasma Membrane            2.Cytoplasm    3. Endoplasmic reticulam (ER)           a)Smooth ER  (b) Rough ER
4.Mitochondria                       5.Golgi apparatus        6.Ribosomes    7.Lysosomes   8.Centrioles (animals only) 9.Plastids (plants only)

(Q.) Why are Mitochondria known as power house of the cell?

 (Ans)  Mitochondria are called power house of cell because respiration takes place in them and energy is released in the form of adenosine tri-phosphate (ATP). The cell uses this energy to perform various functions such as mechanical work and making of new chemical compound .

(Q.) What are the types of plastids? Write their names and fuctions.

 (Ans)  Plastids are of three types -
i) Leucoplasts- Leucoplasts store starch, protein and fat.
ii) Chromoplasts- Chromoplasts provide colour to fruits and flowers.
iii) Chloroplasts- Green coloured chloroplast is the site of photosynthesis as it contains chlorophyll.

(Q.) Differentiate between prokaryotic and eukaryotic cell.

(Ans)  Feature	Prokaryotic cell	Eurkaryotic
1. Nucleus	Lacks true nucleus. Circular DNA lies naked in the cytoplasm. No chromosomes, nucleolus or nuclear membrane. Nuclecoplasm undifferentiated from cytoplasm.	True nucleus bound by nuclear membrane contains DNA, chromosomes, nucleolus and nuclear membrane present, Nucleoplasm differentiated from cytoplasm.
2. Organelles	Membrane – bound organelles like Golgibodies, plastids, mitochondria and endoplasmic reticular (ER) are absent.	Membrane – bound organelles present.
3. Cell division	Divides by simple fission, spindle is not formed, no mitosis and meiosis.	Divides by mitosis or by meiosis.
4. Examples	Bacteria and cyanobacterica (blue – green algae).	All other organism.

 (Q.) Differentiate between diffusion and osmosis.

Diffusion	Osmosis
1. Diffusion can occur in any medium. 2.The diffusing molecules may be solvent. 3. Semi permeable membrane is not required. 4.It is dependent upon the free energy of the diffusing molecules and independent of molecules of other substances in the system. 5. An equilibrium in the free energy of diffusion molecules is achieved in the system.	1. It occurs only in liquid medium. 2. It involves movement of solvent molecules only. 3. Semi permeable membrane is required. 4. Though it is the diffusion of solvent molecules only yet influenced by the presence of other substances (solutes) in the system. 5.Equilibrium in the free energy of solvent molecules is never achieved.

(Q.)  How does an Amoeba obtain its food?

 (Ans)  Amoeba engulfs its food by forming pseudopodia around the food particle.
These pseudopodia are projections of cell membrane.
Once surrounded by pseudopodia the food becomes part of cell and it forms a food vacuole. This method of engulfing food is termed as endocytosis (phagocytosis).

(Q.) How will you relate nucleus with DNA?

 (Ans)  Nucleus is located in the center of the cell. It is the main cell organelle and controls all the activities of cell. A membrane having pores surrounds it. Its chief compounds are chromatin material and nucleolus. Chromatin material when condenses form knot like structures called chromosomes. Chromosomes contain hereditary units called genes, which in turn are made up of DNA. These DNA are carriers of characters from one generation to another.

(Q.) Write one functions of each of the following cell organelles.
a) Plasma membrane            b) Mitochondria        c) Lysosomes  d) Endoplasmic reticulum    e) Vacuoles

 (Ans)  a) Plasma membrane or cell membrane: It controls the entry and exit of material from the cell to outside and from outside to inside. It also protects the cell.
b) Mitochondria: Cellular respiration takes place resulting in release of energy. Hence, they are also known as powerhouse of the cell.
c) Lysosomes: They contain powerful enzyme that can digest and remove worn out cell organelles.
d) Endoplasmic Reticulum (ER):
Rough Endoplasmic Reticulum (RER)-It synthesizes proteins.
Smooth Endoplasmic Reticulum (SER)- It synthesizes fats and lipids. They are filled with a liquid called cell sap and provide turgidity and rigidity to plant cells.