Cell Theory (SOL 5.5 & 6.1)

Cell theory was developed in the 1850s.  Two types of cells exist.

  • Prokaryotes – cells with no nucleus or organelles with membranes.
    Bacteria and blue-green bacteria are prokaryotic cells.
  • Eukaryotes – cells that contain a nucleus and organelles surrounded by a membrane.
    The cells of protozoa, algae, fungi, plants, and animals are eukaryotic cells.

Typical Plant Cell

Typical Animal Cell

Plant cells and animal cells are similar, but do not have exactly the same cell parts and shape.  An animal cell will never have a cell wall, which a plant cell may have.

Cells contain organelles.  

Organelles are different components of cells that perform different functions:

Cell membrane

  • This membrane is a complex barrier of lipid molecules separating the cell from its external environment.
  • The molecules that make up this membrane can move apart to allow larger particles to move in or out of the cell.
  • The “selectively permeable” cell membrane regulates what passes into and out of the cell. Some substances, like water, move freely through the cell membrane by a process known as osmosis.  In osmosis, particles move easily from an area of high concentration to an area of low concentration by molecular motion only. Cells can push particles in the opposite direction, from low concentration to high, but it takes energy from the cell to do this.


  • A thick, aqueous solution of salts surrounding the organelles inside the cell membrane.
  • Nutrients and minerals spread through the cytoplasm to all parts of the cell.
  • The constant motion of this gel-like substance is called cytoplasmic streaming.


  • The structure inside the cell that directs cell activities.
  • Contains the DNA of a cell.

Cell wall

  • On the outside of some cells (bacteria and plants) this structure functions for support and protection.
  • There are pores in the cell wall allowing substances to come in contact with the cell membrane.
  • Types of cell walls:
    1. Primary cell wall – formed during cell growth, it is composed of parallel layers of cellulose and pectin. This structure allows the cell to expand as it grows. While it does provide support, it is not nearly as strong as the secondary cell wall.
    2. Secondary cell wall – formed after cell growth stops, it is composed of interwoven cellulose and lignin fibers. This structure is very strong.  It gives plants their “woody” characteristic.


  • The sites of protein synthesis in a cell.
  • These small, spherical structures are the most numerous organelles in almost all cells.
  • Some ribosomes produce protein to be used within the cell and some produce protein that is “exported” to other parts of an organism.

Endoplasmic reticulum

  • A membrane system of folded sacs and tunnels in the cytoplasm.
  • Rough “ER” is covered with ribosomes. It is common in cells that export proteins and directs the proteins flow.
  • Smooth “ER” as few or no ribosomes. It functions as a pathway for molecules to follow.

Golgi Apparatus

  • A stack of membranes or sacs that acts as to prepare substances for export from the cell.
  • Once the Golgi apparatus has enclosed the final product in a vesicle, or pouch, the product is sent through the cell membrane.


  • Respiration centers of a cell.
  • Large organelles scattered through most cells, they are most numerous in cells that use a lot of energy like liver and muscle cells.


  • Digestive centers of a cell.
  • They produce many different types of enzymes and digest things from food particles to a cell’s own worn out parts.


  • Most common in plant cells, they are storage sites within a cell.


  • Pigment producing organelle in cells.

These red blood cells carry oxygen from the lungs to tissues and exchange it for carbon dioxide to carry back to the lungs for exhalation. Unlike most cells, red blood cells have a nucleus only when they are first formed and lose it as they mature and pack themselves with red, oxygen- rich hemoglobin molecules. They must be pliable and fold so they can enter the tiny capillaries that extend to the far reaches of the body.