Introduction to cells
Joanna George-Johnson, Contributor

Cells are usually called the building blocks of life. This is because the smallest unit or entity that can be considered as living is a cell.

For an entity to be considered living, it must exhibit all the characteristics of living things. A cell, therefore, is the smallest unit that can respire, grow, reproduce, feed, move, excrete and respond to stimuli. Therefore, from now, when you think of a cell, think of a living thing.

Parts of a typical Cell

A typical cell contains various 'bodies' floating within it in a watery substance called cytoplasm. These 'bodies' are called organelles.

Organelles are present in both plant and animal cells:

• Nucleus: Often the central portion of the cell. It directs the cell's activities, and stores genetic information
  
  
• Mitochondrion: These are referred to as the powerhouses of the cell, as they break down sugar to produce energy (ATP).
  
  
• Vacuoles: Fluid-filled spaces in the cell, surrounded by membranes. In animal cells they are small and temporary, while in plant cells they are large and often permanent.
  
  
• Cytoplasm: Jelly-like matrix made mainly of water in which substances are dissolved. The cell organelles are suspended in it.
  
  
• Cell membrane: Outer lining of the cell, which controls what enters and leaves the cell.
  
  
• Ribosome's: small round bodies which produce proteins, and are often attached to the endoplasm reticule
  
  
• Endoplasm Reticule: System of membranes, which transports substances made by the cell.
  

Organelles present in plant cells and never in animal cells

• Cell wall: the outermost covering of the cell, which is made of cellulose. It gives rigidity to the cell.
  
  
• Chloroplast: These are oval bodies present in the cytoplasm, which contain chlorophyll, a green pigment. This pigment traps solar energy for use in photosynthesis.

Specialisation

Just as larger living things tend to have different characteristics depending on their nature or way of life, cells also exhibit variations in shapes and features depending on the job they do. This is called specialisation.

Examples:

Sperm: Because of its job, it contains a tail for movement and contains more mitochondria than a stationary cell to provide energy.

Neurones (nerve cells) are longer so they reach many parts of the body. They have dendrites (projections coming from them so they can receive information from many areas at once).

Palisade cells contains more chloroplast than any other plant cell because most of the photosynthesis takes place in these cells.

Xylem Vessels have reinforced walls so they can withstand the pressure of moving water through them. They are hollow so water can move through them freely.

Cellular organisation of living things

As cells are only the building blocks of life, they come together in an organised way to form larger and more complex biological structures:

• Tissue: A group of identical cells working together.
  
  
• Organ: A group of tissues, each with different structures and functions working together.
  
  
• Organ systems: A group of different organs working together.
  
  
• Organism: A group of organ systems.

Avoid These!

• When asked to give functions of the various organelles, do not say that the mitochondria are the 'powerhouse' or the nucleus is the 'brain' of the cell. It is clichéd and is only used in lower school to explain what it does.
  
  
• Remember that even though plant and animal cells contain vacuoles, the ones in plants are permanent, so when referring to plants use the term 'large central'.
  
  
• Please note the distinction between an organ and a tissue. Many students tend not to get the difference!
  
  
• Note that the cell wall is made of a carbohydrate, while the cell membrane is made up mainly of lipids.

Written by: Joanna George-Johnson and company.