Learn the principles of cell theory and the basic structure of cells. Includes practice questions for biology exams.
One nucleus per cell is the usual condition, but variations in nuclear number occur. Some specialised mature cells, including erythrocytes of many mammals and sieve-tube cells of vascular plants, lack a nucleus.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Mitochondria are not easily visible unless specifically stained. Their number varies according to the physiological activity of the cell, so the more active cell is likely to contain and reveal more mitochondria.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
The observations combine the two central principles of cell theory: living organisms are composed of cells and products of cells, and new cells arise through division of pre-existing cells.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
The bacterial cell wall determines cell shape and supplies strong structural support that prevents bursting or collapsing. Damage to it would therefore affect shape and stability without converting ribosomes, DNA organisation or surface structures into different components.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
The primary wall grows in a young cell, but this capacity diminishes with maturity. The secondary wall is deposited on the inner side, towards the plasma membrane. Plasmodesmata may traverse the walls and connect neighbouring cytoplasms.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
A chloroplast is surrounded by two membranes, of which the inner membrane is relatively less permeable. The space enclosed by the inner membrane is called the stroma.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
The cell wall gives shape, protects against mechanical damage and infection, assists cell-to-cell interaction and restricts undesirable macromolecules. Movement between the nucleus and cytoplasm occurs through nuclear pores.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Schwann proposed that the bodies of animals and plants are composed of cells and products of cells. His work with Schleiden formed the basis of the cell theory.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Matthias Schleiden examined many plants and concluded that they are composed of different kinds of cells that form plant tissues. He and Theodore Schwann later formulated the cell theory.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Chromoplasts contain fat-soluble carotenoid pigments such as carotene and xanthophylls. These pigments may give plant parts a yellow, orange or red colour.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Some mature cells lack a nucleus. Examples include erythrocytes of many mammals and sieve-tube cells of vascular plants.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
The endoplasmic reticulum divides the intracellular space into a luminal compartment within the ER and an extra-luminal compartment represented by the cytoplasm.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
