Cell Structure

Cell theory is the basis for the way that biologists  study living things. Cell Theory is the most basic condition for determining if something is living. Modern Cell theory 

1. All living things are made up of cells.

2. Cells are the basic units of structure and function in living things.

3. cells arise only by divisions in previously existing cells


Robert Hooke (1665) 

Hooke was an English scientist who looked at cork through a compound microscope and observed tiny compartments like structures as in “Honey comb”Therefore these structures became known as cells.  He called cork compartments as cellulae.


Anton Van Leeuwenhoek (1674)


He looked at many different things through his microscope (teeth scapings, rainwater, blood) and observed living cells which he called “animalcules”. 

Robert Brown (1838)

Discovered nucleus in cell.

Matthias Schleiden (1838)


Schleiden was German Botanist the viewed plant parts under a microscope and discovered that plant parts are made of cells. He is considered to be the co-founder of cell theory together with Schwann, with whom he consulted.

Theodor Schwann (1839)


Schwann was a German Biologist  who viewed animal parts under a microscope and discovered that animals were made up of cells. He extended Schleiden’s cell theory in plants to animals, stating that all living things are composed of cells.

Rudolph Virchow (1855)

Virchow was a German physician that stated that all living cells come only from other living cells. His major contribution was the idea that pathologic reactions were reactions of cells.

Modern Cell Theory

4.  The cell contains hereditary information which is passed on from cell to cell during cell division.


5.    All cells are basically the same in chemical composition and metabolic activities.


Ans: according to the first principle of cell theory all organisms are composed of one or more cells. Discovery of virus, prions and viroids claim that statement is not so universal. They are not composed of cells rather they cellular or acellular particles. Which do not run any metabolism with in them.

As they show some characters of living organisms as they increase I number and transmit their characteristic to the next generations.



  • Cell wall is a non living structure of plant cell
  • Very strong component of the cell
  • Maintain shape, strength, protection, support to the inner living matter of the cell.
  • Plant cell have a variety of chemical in their cell walls
  • Located outside of the plasma membrane of cell
  • control rate and direction of growth

Structure and chemical composition of cell wall

Primary cell wall: outer layer of plant cell wall is called primary cell wall and cellulose is the most common chemical in it.

Secondary Cell wall: some plant cells like xylem has secondary cell wall  inner to the primary cell wall, thicker than primary cell wall and contain lignin and other chemicals embedded in it

Middle lamella: it is present between adjacent cells as cementing material. It contains waxes, lignin, silica etc.

Plasmodesmata: there are pores in the cell walls of adjacent cells through which their cytoplasm is connected, these pores are known as plasmodesmata.

  • Cell wall of fungi Chitin in their cell wall instead of cellulose
  • Prokaryotes have  Peptidoglycan in their cell walls (a polymer of amino acids and glucose)

Q: Write down functions of cell membrane?

Ans: all prokaryotic and eukaryotic cells have a thin and elastic covering around cytoplasm that is known as cell membrane.

FUNCTIONS: 1. It is semi permeable allowing only a few molecules pass through it, while fencing majority of compounds inside the cell.

2. Maintain internal composition of cell at a constant or nearly a constant level

3. Many organelles of eukaryotic cells are also bounded by cell membranes e.g. mitochondria, chloroplasts, Golgi bodies etc.


Chemically cell membrane mainly composed of proteins and lipids with small quantities of carbohydrates

Q : What is fluid mosaic model of cell membrane structure?


  • Lipids (mostly phospholipids) are aligned in a bilayer. It gives fluidity and elasticity to cell membrane.
  • Proteins may be fully submerged in the lipid bilayer or some of them may stick out into the interior and outside of the cell. 
  • Small amounts of carbohydrates are also found in cell membranes. These are joined with proteins (glycoproteins) or with lipids as (glycolipids).
  • In eukaryotic cells cholesterol is also present in lipid bilayer

Q : What is cytoplasm, its contents & function?

ANS:  Def: cytoplasm is the material between the plasma membrane (cell membrane) and the nuclear envelope.

Structure: cytoplasm is a semi viscous and semi transparent substance. It contains water in which many organic (proteins, carbohydrates, lipids) and inorganic salts are present.

Function:  1. provide spce for proper functioning of cell organelles.

2. act as site for various metabolic activities e.g. glycolysis ( break down of glucose)

Q : What is Cytoskeleton?

ANS: it an important complex and dynamic component of the cell and is invisible under light microscope.

  • It give shape to the cell
  • Anchor the organelles in place    · keep the parts of cell in motion

FILAMENTS:  it has two types of filaments

  1. Microtubule: these are made up of TUBULIN subunits and are used by cell to maintain its shape
    1. Microfilaments: they are made up of ACTIN subunits and form about one third diameter of microtubule. These help the cell in changing shape and holding cellular organelles.

Q: what are cell organelles?

ANS: cell organelles are small structures within the cell cytoplasm that perform specialized functions. There are about dozen type of organelles commonly found in eukaryotic cells. E.g. mitochondria, vacuoles, ribosomes , golgi bodies, centrioles, endoplasmic reticulum.

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