Important Questions for Class 11 Biology- Cell : The Unit of Life

A cell is the structural and functional unit of life. Each cell is surrounded by a cell membrane which separates the internal and external environments of the cell. The interior environment of a cell is known as the cytoplasm which consists of cellular machinery and structural elements. The nucleus is present at the centre of the cell which contains all the hereditary information of an organism. Proteins, carbohydrates, sugars and starch are the organic molecules present in the cell.

Questions

Q.1. What is the importance of a vacuole in a plant cell?
A.1. The vacuole is a membrane-bound space in the cytoplasm of a plant cell. It contains sap, water, excretory products and other materials not useful for the cell. Vacuoles occupy 90% of the cell volume during osmosis. They maintain the turgor pressure against the cell wall thereby maintaining the shape of the cell and cell fluid balance.
Q.2. What is a satellite chromosome?
A.2. The chromosomes that have an additional or secondary constriction at the distal part of the arm formed by a chromatin thread are known as satellite chromosomes. These appear as an outgrowth or a small fragment. These are also known as marker chromosomes. The chromosomes 13, 14, 15, 16, 21, and 22 are satellite chromosomes.
Q.3. Match the following:

Column I Column II
Cristae Fat membranous sacs in the stroma
Cisternae Infoldings in mitochondria
Thylakoids Disc-shaped sacs in Golgi apparatus

A.3. Cristae- Infoldings in mitochondria Cisternae- Disc-shaped sacs in Golgi apparatus Thylakoids- Fat membranous sacs in the stroma
Q.4. State the characteristics of prokaryotic cells.
A.4. The characteristics of a prokaryotic cell are as follows:

  • A prokaryotic cell is surrounded by a cell membrane.
  • Mitochondria, chloroplast and nucleus are not present.
  • The DNA is circular and not associated with basic proteins.
  • The cytoplasm is filled with dense granules most of which are ribosomes.
  • The thylakoids are scattered in the chloroplast, and not placed in the form of stacks.

Q.5. Multicellular organisms exhibit division of labour. Comment.
A.5. The cells of a multicellular organism are organised to form tissues such as bones, blood etc. The tissues collectively form an organ such as kidney, liver, etc. and these organs organize to form an organ system, for eg., digestive system, circulatory system etc. Every single cell has its own function. Different organs and organ systems carry out different functions which help in the proper functioning of an individual.
Q.6. Why is cell the basic unit of life?
A.6. A human body is made up of trillions of cells. There are various organisms with a single cell such as amoeba. All cells divide to form new cells and possess several organelles to carry out various life functions. The cells of an organism carry the same genetic material which carries all the hereditary information of an individual. That is why it is called the basic unit of life.
Q.7. What do you mean by plasmids? What role do they play in bacteria?
A.7. A plasmid is an autonomously replicating, extra-chromosomal, circular, double-stranded DNA found in the cytoplasm of the bacterial cell. These usually remain separate from the chromosome.
Role- Plasmids act as vectors to transfer important genes and in recombination experiments. It also helps in bacterial conjugation. For eg., pBR322
Q.8. Describe the cell theory in brief.
A.8. The cell theory is based on the following postulates:

  • The cell is the basic structural and functional unit of life.
  • All living organisms are made up of cells.
  • All cells arise from pre-existing cells.

Q.9. Differentiate between Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum.
A.9.

Rough Endoplasmic Reticulum (RER) Smooth Endoplasmic Reticulum (SER)
Ribosomes are attached to their surface. Ribosomes are not attached to their surface.
Formed of cisternae and a few tubules. Formed of vesicles and tubules.
It participates in the protein and enzyme synthesis. Takes part in the synthesis of glycogen, lipids, and steroids.
May develop from the nuclear envelope. May develop from RER.
It is internal. It is peripheral.
It is connected to the nuclear envelope. It is connected to the plasmalemma.
Lacks detoxification enzymes. Contains detoxification enzymes.
Gives rise to lysosomes. Gives rise to sphaerosomes.

Q.10. What is the biochemical composition of the plasma membrane?
A.10. The plasma membrane comprises of the following biochemical components:

  • Lipids- 20-79%
  • Proteins- 20-70%
  • Carbohydrates- 1-5%
  • Water- 20%

Q.11. What is a mesosome?
A.11. Mesosome is formed by the extension of the plasma membrane into the cell in prokaryotes. It facilitates cell wall formation, DNA replication, and distribution of DNA to the daughter cells. It also helps in respiration and secretion and increases the surface area of the plasma membrane and enzymatic content.
Q.12. What are histones? What role do they play?
A.12. Histones are alkaline proteins found inside the nucleus of eukaryotic cells. They package the DNA into structural units called nucleosomes. They are the main proteins in chromatin.
Q.13. What does “S” stand for in the 70S and 80S ribosome?
A.13. “S” is the Svedberg’s unit for sedimentation coefficient. It depicts the rate of sedimentation of a cell during ultracentrifugation. Heavier the cell structure, higher is the sedimentation coefficient.
Q.14. Why are mitochondria known as the “powerhouse of the cell”?
A.14. Mitochondria carry out the process of anaerobic respiration and generate ATP for cell functioning. That is why it is called “powerhouse of the cell”.
Q.15. What is the fluid mosaic model of the plasma membrane?
A.15. The fluid mosaic model was proposed by Singer and Nicholson. It explains the structures and functions of various cell membranes. According to this model, the protein molecules are embedded in the lipid bilayer. This lipid bilayer gives elasticity and fluidity to the cell membrane. For more information on Cells or any Biology article, keep visiting BYJU’S website. You can also download BYJU’S app for further reference.

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