MPSC Notes - Cell Structure - Cell Organelles

1. What is cell theory?

Cell theory asserts that the cell is the constituent unit of living beings.

Before the discovery of the cell, it was not recognized that living beings were made of building blocks like cells.

The cell theory is one of the basic theories of Biology.

2. Are there living beings without cells?

Viruses are considered the only living beings that do not have cells. Viruses are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membranes and cell organelles nor do they have self-metabolism.

3. In 1665 Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under the microscope presented small cavities similar to pores which were filled with air. Based on later knowledge, of what were the walls of those cavities constituted? What is the historical importance of that observation?

The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation led to the discovery of the cells, a fact only possible after the invention of the microscope. In that work, Hooke established the term “cell”, now widely used in Biology, to designate those cavities seen under the microscope.

4. What are the two big groups into which cells are classified?

Cells can be classified as eukaryotic or prokaryotic.

Prokaryotic cell is that without a delimited nucleus. Eukaryotic cells are those with nucleus delimited by membrane.

5. Do bacteria cells have a nucleus?

In bacteria the genetic material is dispersed in the cytosol and there is no internal membrane that delimits a nucleus.

6. Are there any bacteria made of more than one cell?

There are no pluricellular bacteria. All bacteria are unicellular prokaryotic.

7. What is the plasma membrane of the cell? What are its main functions?

The plasma membrane is the outer membrane of the cell, it delimits the cell itself and a cell interior with specific conditions for the cellular function. Since it is selectively permeable, the plasma membrane has an important role for the passage of substances inwards or outwards.

8. What are the chemical substances that compose the plasma membrane?

The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipds, amphipathic molecules, are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non-polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.

9. What is the difference between plasma membrane and cell wall?

Plasma membrane and cell wall are not the same thing. Plasma membrane, also called cell membrane, is the outer membrane common to all living cells and it is made of a phospholipid bilayer, embedded proteins and some appended carbohydrates.

Because cell membranes are fragile, in some types of cells there are even outer structures that support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also present an outer cell wall made of peptidoglycans and other organic substances.


10. What are the main respective constituents of cell walls in bacteria, protists, fungi and plants?

In bacteria the cell wall is made of peptidoglycans; among protists algae have cell walls made of cellulose; in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods); in plants, the cell wall is made of cellulose too.