Sample 1Easy
Who explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula), giving the cell theory its final shape?
- A
Matthias Schleiden
- B
Theodore Schwann
- C
Robert Brown
- D
Rudolf Virchow
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The answer is D.Rudolf Virchow (1855) first explained that cells divide and new cells form from pre-existing cells, and he modified the hypothesis of Schleiden and Schwann into the final cell theory.
- A.
Schleiden observed that plants are composed of different kinds of cells.
- B.
Schwann studied animal cells and proposed that cells have a thin outer layer, the plasma membrane.
- C.
Robert Brown discovered the nucleus.
NCERT: Class 11 Biology, Chapter 8 (Cell: The Unit of Life), Cell Theory
Sample 2Medium
A figure shows a bacterial cell with a cell wall, a plasma membrane and a coiled circular DNA in the cytoplasm. At one point the plasma membrane folds inward to form a mass of vesicles, tubules and lamellae, labelled X. Which option names X and gives one of its functions?
- A
Mesosome; it helps in DNA replication and in distributing DNA to the daughter cells
- B
Mesosome; it is the site of photosynthesis in all bacteria
- C
Plasmid; it gives the bacterium resistance to antibiotics
- D
Nucleoid; it holds the genetic material inside a nuclear envelope
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The answer is A.An infolding of the plasma membrane in a bacterium, made of vesicles, tubules and lamellae, is a mesosome. It helps in cell wall formation, DNA replication and distribution to daughter cells, respiration and secretion, and it increases the membrane's surface area.
- B.
Photosynthetic pigments are carried in chromatophores, membranous extensions found in some cyanobacteria. They are not a function of the mesosome in all bacteria.
- C.
A plasmid is a small circular DNA outside the genomic DNA, not an infolding of the plasma membrane.
- D.
The nucleoid is the naked genomic DNA, with no nuclear envelope. It is not an infolding of the membrane.
NCERT: Class 11 Biology, Chapter 8