Call Us: US - +1 845 478 5244 | UK - +44 20 7193 7850 | AUS - +61 2 8005 4826

The endoplasmic reticulum

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

Internal organisation

  1. Cells contain a variety of internal structures called organelles
  2. An organelle is a cell component that performs a specific function in that cell
  3. Just as the organs of a multicellular organism carry out the organism’s life functions, the organelles
    of a cell maintain the life of the cell
  4. There are many different cells; however, there are certain features common to all cells
  5. The entire cell is surrounded by a thin cell membrane
    All membranes have the same thickness and basic structure
  6. Organelles often have their own membranes too – once again, these membranes have a similar structure
  7. The nucleus, mitochondria and chloroplasts all have double membranes, more correctly called envelopes
  8. Because membranes are fluid mosaics, the molecules making them up – phospholipids and proteins – move independently. The proteins appear to ‘float’ in the phospholipids bilayer
  9. Membranes can thus be used to transport molecules within the cell e.g. endoplasmic reticulum.
  10. Proteins in the membrane can be used to transport substances across the membrane
    – e.g. by facilitated diffusion or by active transport.
  11. The proteins on the outside of cell membranes identify us as unique.

Prokaryotes vs. Eukaryotes

Organisms whose cells normally contain a nucleus are called Eukaryotes; those (generally smaller) organisms whose cells lack a nucleus and have no membrane-bound organelles are known as Prokaryotes.

Typical organismsBacteriaProtoctista, fungi, plants, animals
Typical size≈1-10 µm≈10-100 µm (sperm cells) apart from the tail, are smaller)
Type of nucleusNuclear body
No nucleus
Real nucleus with nuclear envelope DNA circular (ccc DNA) linear molecules (chromosomes) with histone proteins
Cytoplasmatic structureVery few structuresHighly structured by membranes and a cytoskeleton
Cell movementFlagellae/cilia made of flagellinFlagellae and cilia made of tubulin
MitochondriaNone1 – 100 (though RBC’s have none)
ChloroplastsNoneIn algae and plants
OrganizationUsually single cellsSingle cells, colonies, higher multicellular organisms with specialized cells
Cell divisionBinary fission (simple division)Mitosis (normal cell replication)
Meiosis (gamete production)