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Phospho lipid bilayer nuclear envelope.

Structure and Function of the Cell

Introduction to the cell

Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter.

The cell is the smallest unit of matter that can carry on all the processes of life.

  1. Every living thing, from the tiniest bacterium to the largest whale, is made of one/more cells
  2. Before the C17th, no one knew that cells existed, since they are too small to be seen with the naked eye. The invention of the microscope enabled Robert Hooke, (1665) and Anton van Leuwenhoek (1675) to see and draw the first ‘cells’, a word coined by Hooke to describe the cells in a thin slice of cork, which reminded him of the rooms where monks lived.
  3. The idea that all living things are made of cells was put forward in about 1840 and in 1855 came ‘Cell Theory’ – i.e. ‘cells only come from other cells’ – contradicting the earlier theory of ‘Spontaneous Generation’.

Cell Theory consists of three principles:

  1. All living things are composed of one or more cells.
  2. Cells are the basic units of structure and function in an organism.
  3. Cells come only from the replication of existing cells.

Cell diversity

Not all cells are alike. Even cells within the same organism show enormous diversity in size, shape, and internal organization. Your body contains around 1013 to 1014 cells of around 300 different cell types, which we broadly classify into 4 groups.

Cell size

  1. A few types of cells are large enough to be seen by the unaided eye. The human egg (ovum) is the largest cell in the body, and can (just) be seen without the aid of a microscope.
  2. Most cells are small for two main reasons:
    1. The cell’s nucleus can only control a certain volume of active cytoplasm.
    2. Cells are limited in size by their surface area to volume ratio. A group of small cells has a relatively larger surface area than a single large cell of the same volume. This is important because the nutrients, oxygen, and other materials a cell requires must enter through it surface. As a cell grows larger at some point its surface area becomes too small to allow these materials to enter the cell quickly enough to meet the cell’s need. (= Fick’s Law – something you need to learn well.)

Rate of diffusion α Surface Area x Concentration
                               Difference Distance

Cell shape

  • Cells come in a variety of shapes – depending on their function
  • The neurones from your toes to your head are long and thin
  • Blood cells are rounded disks, so that they can flow smoothly