Chapter 8: Cell: The Unit of Life Short Questions | biology Class 11 CBSE

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Chapter 8: Cell: The Unit of Life Short Questions | biology Class 11 CBSE | ByteNotes.in

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Question 1

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State the two postulates of the modern cell theory as understood today.

All living organisms are composed of cells and products of cells.
All cells arise from pre-existing cells — a principle expressed as Omnis cellula-e cellula.
Rudolf Virchow (1855) completed the cell theory by adding the second postulate, which Schleiden and Schwann had not explained.

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Question 2

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What is a plasmid? How does it differ from the genomic DNA of bacteria?

A plasmid is a small circular DNA molecule present in bacterial cells outside the main genomic (chromosomal) DNA.
Unlike the single circular genomic DNA, plasmids are much smaller and are not essential for normal bacterial survival.
Plasmid DNA confers unique phenotypic characters to bacteria such as resistance to antibiotics, and is used in recombinant DNA technology.

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Question 3

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Distinguish between integral and peripheral membrane proteins.

Peripheral proteins lie on the surface of the plasma membrane and are loosely associated with it.
Integral proteins are partially or totally buried within the phospholipid bilayer and are tightly bound.
This classification is based on the ease with which the proteins can be extracted from the membrane.

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Question 4

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What is osmosis? How does it differ from simple diffusion?

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher concentration to a region of lower water concentration.
Simple diffusion involves movement of neutral solutes along their concentration gradient without the need for a carrier protein or energy.
Osmosis is specifically restricted to water movement, while simple diffusion applies to neutral solutes; both are forms of passive transport requiring no ATP.

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Question 5

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State the role of the middle lamella in plant tissues.

The middle lamella is a layer composed mainly of calcium pectate located between the cell walls of adjacent plant cells.
It holds or glues neighbouring cells together, providing structural integrity to plant tissues.
It may be traversed by plasmodesmata, which are cytoplasmic connections that facilitate communication between adjacent cells.

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Question 6

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Explain why the Golgi apparatus is said to remain in close association with the endoplasmic reticulum.

Proteins synthesised on ribosomes of the rough ER are packaged into vesicles that bud off from the ER.
These vesicles fuse with the cis (forming) face of the Golgi apparatus, delivering materials for modification and packaging.
This functional continuity — ER producing and Golgi modifying and dispatching proteins — necessitates their physical proximity within the cell.

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Question 7

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What are inclusion bodies in prokaryotic cells? Give two examples.

Inclusion bodies are reserve material storage structures found free in the cytoplasm of prokaryotic cells, not bounded by any membrane.
Examples include phosphate granules, cyanophycean granules, and glycogen granules stored in the cytoplasm.
Gas vacuoles, which are also inclusion bodies, are found in blue-green, purple, and green photosynthetic bacteria and help in buoyancy.

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Question 8

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Why are mitochondria referred to as the 'powerhouses' of the cell?

Mitochondria are the sites of aerobic respiration, where organic molecules are oxidised to release energy.
The energy released is stored in the form of ATP (adenosine triphosphate), the universal energy currency of the cell.
Because they generate ATP — the primary energy source for all cellular activities — they are appropriately called the powerhouses of the cell.

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Question 9

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Distinguish between grana and stroma lamellae in a chloroplast.

Grana are stacks of flattened membranous sacs called thylakoids, arranged like piles of coins within the stroma of the chloroplast.
Stroma lamellae are flat membranous tubules connecting the thylakoids of different grana to each other.
Chlorophyll pigments are located in the thylakoids of grana, making grana the site of light-dependent reactions, while stroma lamellae provide structural connectivity.

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Question 10

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State the sedimentation values of eukaryotic and prokaryotic ribosomes and their respective subunits.

Prokaryotic ribosomes are 70S, composed of a 50S large subunit and a 30S small subunit.
Eukaryotic ribosomes are 80S, composed of a 60S large subunit and a 40S small subunit.
The 'S' unit stands for Svedberg's unit, which is an indirect measure of the density and size of the ribosome.

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Question 11

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What is a polysome? Explain its functional significance.

A polysome (or polyribosome) is a chain of multiple ribosomes simultaneously attached to a single mRNA molecule.
Each ribosome on the chain independently translates the mRNA, producing multiple copies of the same protein at the same time.
This arrangement greatly increases the efficiency of protein synthesis in both prokaryotic and eukaryotic cells.

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Question 12

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Name the four types of bacteria based on their shape and give one example of each shape.

Bacillus (rod-like shape), Coccus (spherical shape), Vibrio (comma-shaped), and Spirillum (spiral-shaped) are the four basic bacterial forms.
These different shapes are possible because all prokaryotic cells share a fundamentally similar internal organisation despite morphological diversity.
The shape of a bacterium is determined primarily by its rigid cell wall, which provides structural support and prevents bursting or collapsing.

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Question 13

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What are nuclear pores and what function do they perform?

Nuclear pores are minute openings in the nuclear envelope formed by the fusion of its two parallel membranes.
They serve as passages through which RNA and protein molecules move in both directions between the nucleus and the cytoplasm.
This bidirectional transport is essential for gene expression — mRNA moves out to ribosomes for translation, while regulatory proteins move into the nucleus.

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Chapter 8: Cell: The Unit of Life | Short Questions

State the two postulates of the modern cell theory as understood today.

What is a plasmid? How does it differ from the genomic DNA of bacteria?

Distinguish between integral and peripheral membrane proteins.

What is osmosis? How does it differ from simple diffusion?

State the role of the middle lamella in plant tissues.

Explain why the Golgi apparatus is said to remain in close association with the endoplasmic reticulum.

What are inclusion bodies in prokaryotic cells? Give two examples.

Why are mitochondria referred to as the 'powerhouses' of the cell?

Distinguish between grana and stroma lamellae in a chloroplast.

State the sedimentation values of eukaryotic and prokaryotic ribosomes and their respective subunits.

What is a polysome? Explain its functional significance.

Name the four types of bacteria based on their shape and give one example of each shape.

What are nuclear pores and what function do they perform?

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