The Cell Theory

• All living things are made of cells
   
• Cells are the basic units of life
   
• Cells come only from other cells

Prokaryotes or Eukaryotes

• Prokaryotes - simple cells with no organelles surrounded by membranes.

  Today's prokaryotes are the bacteria.

   
• Eukaryotes - complex cells with organelles surrounded by membranes.

  Except for bacteria, all life on Earth today is composed of eukaryotic cells.

  

Cell Size

• Prokaryotes are the smallest cells. The smallest is only 0.2 micrometers (8 millionths of an inch) in diameter.
• Eukaryotes are up to 10 times larger than prokaryotes.
   
• Plant cells are slightly larger than animal cells, which are about 10 micrometers in diameter.
  
  
• The size of an organism depends on the number of its cells.
   
  • The human body contains several trillion cells.
  • The blue whale is over 100 feet long and weighs 120 tons.
  • The cells of a human and a blue whale are the same size!

Cell Organelles

• Cell membrane surrounds the cell and regulates what moves in or out of the cell. The membrane is selectively permeable, allowing some things to pass through and keeping other things out. Water moves easily through the membrane from an area of high concentration to an area of low concentration. This process is called osmosis.

• Cell wall provides support and protection for bacteria and plant cells. The plant cell wall is composed of cellulose and lignin fibers, the fibers that give plants their "woody" charactteristic.

• Cytoplasm is the thick, aqueous solution of salts surrounding the organelles inside the cell.
   
• The nucleus contains the cell's DNA and directs cell activities. When the cell is not in the process of dividing, the DNA is in the form of long strands called chromatin.
   
• Ribosomes are the sites of protein synthesis. They are the most numerous organells in almost all cells. Most are located on the rough ER in most cells.
   
• The endoplasmic reticulum (ER) is a system of folded sacs and tunnels in the cytoplasm close to the nucleus. Rough ER has ribosomes - smooth ER has few or no ribosomes.
   
• Mitochondria are the energy centers of cells, the sites of cellular respiration.
   
• The Golgi bodies (also called Golgi apparatus) package cellular products for export.
   
• Vacuoles are storage spaces in the cell. Plant cells have large vacuoles for water storage.
   
• Lysosomes are the cell's recycle centers, producing enzymes for digestion.
• Plastids contain pigments, chemical that produce colors. The important plastids in plant cells are the chloroplasts. Human skin cells contain melanoplasts.

Viruses:

• are much smaller and less complex than cells.
• consist of either DNA or RNA enclosed in a protein coat called a capsid.
• do not grow.
• have no nucleus, cytoplasm, or membranes.
• do not carry out cellular functions.
• do not generate metabolic energy.
• are obligate intracellular parasites, meaning that they require a host cell to reproduce.

"Are viruses alive?"