HL 1 Biology

2.1c Cell Theory: Differentiation

There are some unicellular organisms that live together in colonies. Within each colony, there are daughter colonies forming inside. Yet these cells are joined (fused) to form a singe cell mass, thus avoids to form into a single organism.

Multicellular organisms contain a single mass of cells that are fused together. Because there are numerous cells, each one is able to specialize specific functions. (E.g. red blood cell transports oxygen around the body, rod cell in retina of eye absorbs light/transmits nerve impulse to brain) Each of the cell type transforms into its own specialized structure, in order to conduct its functions.

Thus differentiationis the development of cells in different ways to perform various functions. Each cell type uses some of the genes in its nucleus and the information in the gene are used to make a protein or other gene products. The “gene is being expressedwhen a gene is being used in a cell. Cells need different genes to develop in specific ways. Although each cell has all the genes to develop in anyway, it is  committed to using only the genes that it needs for it’s development. Once cell has begun to develop in the specific pathway, is fixed/cannot change to a different path.

  • Examples:
    • Heart muscle tissue  (all heart muscle cells are made from protein fibers that helps to pump blood in the heart + contract the cell)
    • Sensory neuron
    • Beta cell in the pancreatic islets


1) There were 11 cells in the nervous system when the larva hatched from the egg.

2) a) About 8 hours elapsed from hatching until the first cell division occurs.

b) Approximately 35 hours elapsed from hatching until the last cell division occurs. 

3)During the development of the nervous system, a total of 9 apoptosis (programmed cell death) occurs. *Apoptosis: particular cells that undergo programmed cell death in order to give a specific feature to the body. Without apoptosis, we would have webbed hands and no space between the fingers.

4)959/11≈ 87. The maximum number of times cell division occurred, to produce an adult nervous system cell is about 87 times.

5) To find the percentage of the cells of an adult C. elegans that are part of the nervous system, I would divide the 78 cells in the adult nervous system by the 959 cells an adult body is made of and multiply the answer by 100%. (78/959)*100%≈ 8.13%


Stem Cells: are cells that has the capacity to self-renew by cell division and differentiation. The human embryo consists entirely of stem cells at first, yet eventually these cells become committed to differentiating in a certain path. Once committed, the cells can divide to about 6 more time, but they are no longer stem cells because all the cells formed will differentiate in the same way.

Despite that, small numbers of these embryonic cells remain as stem cells and still exists in the adult body. Stem cells are found in:

  • many human tissues: bone, marrow, skin and liver. (gives some human tissues powers of regeneration and repair)
  • other tissues: brain, kidney, and heart.  (only allow limited repair)

Great interest in stem cells due to its ability to repair tissue and for treating a variety of degenerative conditions.

  • E.g. Parkinson’s disease; loss of neurons/other cells in nervous system cause multiple sclerosis + strokes. There’s potential to use stem cells to replace these lost cells.

Therapeutic use of stem cells:

  • Bone marrow transplants: use hematopoietic stem cells (HS cells) which are found normally in the bone marrow and they divide continuously, producing new cells which undergo differentiation into red and white blood cells. These HS cells have a large growth potential.
  • HS cells are used in treatment of a variety of  blood disorders like: leukemia, SCID (severe combined immune deficiency), multiple myeloma and lymphoma
    • Lymphoma: cancers of lymphatic system, where patient loses body mass as the lymph nodes swell up.
      • Cells are removed from bone marrow of patient, patient receives chemotherapy drugs to kill dividing cells in cells, cancer+normal cells in bone marrow, and HS cells restore healthy blood cells in bone marrow after transplanted back into patient.
  • Use placenta and umbilical cord of a baby as a source of stem cells. The cord blood which contains many HS cells are obtained and red blood cells are removed from the cord blood. The cord blood can treat patients, especially children with leukemia. If the patient’s tissue type matches with cord blood it is injected into the patient’s blood system in the chest/arm vein. HS cells will divide continuously in the bone marrow and restore healthy production.

Risk of stem cell therapies is only takes one cell that starts to divide out of control and form a tumor.

*AA-CC: 16-18; DBQ p16 • AA-SG: 4


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