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At the end of this course on Cell Structure and Function, you will be able to:

A cell is the smallest unit of living organisms. Cell structure and function is a fundamental topic in Biology. A cell is composed of structures called organelles, which perform diverse functions. For example, mitochondria is responsible for cellular respiration and chloroplast for photosynthesis.

The cell was first discovered and named by Robert Hooke in 1665. He remarked that it looked similar to cellula (small rooms), which monks inhabited, thus coining the name “cell” for his discovery. Hooke observed the dead cell walls of plant cells (cork) as it appeared under the microscope.

Cork shaped cell observed by Robert Hooke under the microscope
Figure 1. Cork-shaped cell (Robert Hooke, Micrographia, 1665)

Each cell acts as a self-contained and self-maintaining unit. It takes nutrients in, converts these nutrients into energy, carries out specialized functions, and reproduces as required.

Every living thing has cells within its body. The composition, distribution, and the number of cells present in an organism determine its type. Unicellular organisms are those made of a single cell, whereas multicellular organisms contain many (sometimes millions of) cells.

The table below shows basic differences in unicellular and multicellular organisms.

CharacteristicsUnicellularMulticellular
Number of cellsOneMore than one
OrganizationSimpleComplex
Division of laborAt organelle level therefore low level of operational efficiencyAt cellular, tissue, organelle, and organ level therefore the operational efficiency is very high
ExposureExposed to the environmentContains cell wall no cell organelles inside the cell are not exposed
SizeSmallLarge
Extreme environmentMore likely to be foundRare
FunctionSingle cell performs all life processesDifferent cells specialized for to carry out specific functions
ReproductionAsexualMainly Sexual
InjuryCan lead to death of the organismNo death and lost parts can be replaced in some multicellular organisms.
Life spanShortLong because of division of labor
ExampleAmoeba

Amoeba

neuronNerve Cell

Cell theory

Cell theory is a widely accepted explanation of the relationship between cells and living things. Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow developed the classical cell theory. Today, cell theory is considered to be a foundation of biology. According to this theory:

  • All living organisms are composed of one or more cells.
  • The cell is the basic unit of structure and organization in organisms.
  • Cells arise from pre-existing cells.

Modern cell theory

Over time, new discoveries and concepts have refined and modified the scientific view of cells to form the modern cell theory. The following are the important concepts of this theory:

  • The cell is the fundamental unit of structure and function in living things.
  • All cells come from pre-existing cells by division.
  • Energy flow occurs within cells.
  • Cells contain hereditary information (DNA), which is passed from cell to cell during cell division
  • All cells are basically the same in chemical composition.
  • All known living things are made up of cells.
  • Some organisms are unicellular, made up of only one cell.
  • Other organisms are multicellular (composed of countless number of cells).
  • The activity of an organism depends on the total activity of independent cells.

Some well-known exceptions to the theory are given below:

  • Although viruses are living organisms, they are not made up of cells.
  • The first cell did not originate from a pre-existing cell.
  • Mitochondria and chloroplasts have their own genetic material. They both reproduce.independently from the rest of the cell.

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