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Cell: The Unit of Life – Class 11 Biology notes, important questions and MCQs

NCERT Class 11 Biology, chapter 8. Notes, key terms, important questions with answers and MCQs.

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Chapter overview

The cell is the basic structural and functional unit of life. Prokaryotic cells lack a nucleus and membrane-bound organelles; eukaryotic cells have both.

Cell theory and types

  • Cell theory (Schleiden, Schwann; Virchow added "omnis cellula e cellula").
  • Prokaryotes: nucleoid, 70S ribosomes, plasmids, mesosomes; Gram-positive and Gram-negative bacteria.
  • Eukaryotes: nucleus, 80S ribosomes, membrane-bound organelles.

Organelles

  • Plasma membrane: fluid mosaic model (Singer and Nicolson).
  • Mitochondria (powerhouse, own DNA), chloroplasts (photosynthesis), ER (rough with ribosomes, smooth for lipids), Golgi (packaging), lysosomes (suicide bags), vacuoles.
  • Centrioles (9 + 0 triplets), cilia and flagella (9 + 2), nucleolus makes rRNA.

Key terms

Fluid mosaic model
membrane model with proteins floating in a lipid bilayer
Lysosome
membrane sac of hydrolytic enzymes
Nuclear pore
opening in the nuclear envelope that allows movement of RNA and proteins between nucleus and cytoplasm

Important questions with answers

Give two differences between rough ER and smooth ER.2 marks

Rough ER has ribosomes on its surface and makes proteins for secretion; smooth ER lacks ribosomes and makes lipids and steroids (and detoxifies drugs in liver cells).

What are mesosomes? Give their functions.3 marks

Infoldings of the plasma membrane in bacteria. They help in cell wall formation, DNA replication and its distribution to daughter cells, respiration and secretion, and increase the surface area of the membrane.

Describe the structure and functions of mitochondria and chloroplasts, and give two reasons they are called semi-autonomous organelles.5 marks

Mitochondria: double membrane; inner membrane folded into cristae; matrix contains DNA, 70S ribosomes and Krebs-cycle enzymes; site of aerobic respiration and ATP production. Chloroplasts: double membrane; stroma with enzymes for the Calvin cycle; thylakoids stacked as grana carry chlorophyll and the light reactions; site of photosynthesis. Semi-autonomous: both have their own circular DNA and 70S ribosomes and can make some of their own proteins, and both multiply by fission; but many of their proteins come from nuclear genes.

Quick revision

  • Mitochondria have their own DNA.
  • Cilia show 9 + 2 arrangement.
  • Virchow: cells come from pre-existing cells.

Exam tips

  • Ribosome sizes: prokaryotes 70S (50S + 30S), eukaryotes 80S (60S + 40S).
  • Learn which organelles are double-membraned (nucleus, mitochondria, chloroplast), single-membraned (ER, Golgi, lysosome, vacuole) and non-membranous (ribosome, centriole).

Practice MCQs

  1. Powerhouse of the cell is

    1. mitochondrion
    2. nucleus
    3. Golgi body
    4. ribosome
    Show answer

    Answer: (a) mitochondrion. ATP.

  2. Prokaryotic cells lack

    1. ribosomes
    2. cell membrane
    3. DNA
    4. a well-defined nucleus
    Show answer

    Answer: (d) a well-defined nucleus. Nucleoid only.

  3. Fluid mosaic model was proposed by

    1. Watson and Crick
    2. Singer and Nicolson
    3. Schleiden and Schwann
    4. Robert Brown
    Show answer

    Answer: (b) Singer and Nicolson. 1972.

  4. Ribosomes of prokaryotes are

    1. 70S
    2. 80S
    3. 60S
    4. 40S
    Show answer

    Answer: (a) 70S. 50S + 30S.

  5. Which organelle is called the "protein factory" of the cell

    1. ribosome
    2. lysosome
    3. Golgi body
    4. vacuole
    Show answer

    Answer: (a) ribosome. Site of protein synthesis.

  6. "Omnis cellula e cellula" was given by

    1. Schwann
    2. Brown
    3. Hooke
    4. Virchow
    Show answer

    Answer: (d) Virchow. Rudolf Virchow.

  7. Arrangement of microtubules in a cilium is

    1. 9 + 0
    2. 2 + 9
    3. 9 + 3
    4. 9 + 2
    Show answer

    Answer: (d) 9 + 2. Axoneme.

  8. Glycosylation of proteins and lipids mainly occurs in the

    1. ribosome
    2. mitochondrion
    3. nucleus
    4. Golgi apparatus
    Show answer

    Answer: (d) Golgi apparatus. Packaging and modification.

All chapters of Class 11 Biology

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.