1.2 Applications and skills

1.2.1 Specialisation and differentiation of cells

  • Differentiation is the process through which cells in multicellular organisms become specialised to perform a certain function.
  • Differentiation occurs during embryonic development.
  • When certain genes are expressed, unspecialised cells, called stem cells, proceed through different pathways to become different types of cell.

differentiationFigure 1.2.1a - Differentiation
A simplified diagram showing how stem cells from an embryo differentiate during embryonic development. Genes are turned on and off at specific times in order to generate different types of cells from unspecialised cells.

Application: Life functions in unicellular organisms

The cells of unicellular organisms do not differentiate. Instead, all of the life functions are performed in a single cell.

ParameciumFigure 1.2.1b – Paramecium
Paramecium, an animal-like protist, has cilia, which beat synchronously, like the oars of a rowboat, in order to propel it through water. Trichocysts play a role in defending against predators.

ChlamydomonasFigure 1.2.1c – Chlamydomonas
Chlamydomonas is a unicellular green algae. It has two long flagella that beat like whips to propel it through water. The pyrenoid may play a role in storing starch.

Life functions in unicellular organisms
Life function Description
Metabolism Both species: Oxygen and carbon dioxide for respiration are exchanged by diffusion through the plasma membrane.

Paramecium: Asexual reproduction, by binary fission (mitosis followed by cytokinesis). Micronucleus controls reproduction. Macronucleus controls all other functions.

Chlamydomonas: Asexually by producing spores, or sexually.


Paramecium: Moves away from sources of light/heat.

Chlamydomonas: Moves towards light, has a photosensitive eye spot.


Paramecium only: Solid wastes are excreted through the anal pore.

Both species: Liquid wastes are pumped out by the contractile vacuole through the cell membrane.


Paramecium: Food is ingested through the oral groove and digested in food vacuoles.

Chlamydomonas: Food is synthesised by photosynthesis in a large U-shaped chloroplast.


Paramecium: Nutrients from food vacuoles are gradually released to the cytoplasm to provide energy for growth.

Chlamydomonas: Starch is broken down by enzymes in cytoplasm to provide energy for growth.

Homeostasis Both species: Excess water is removed by a contractile vacuole.

Application: Therapeutic uses of stem cells

The ability of stem cells to differentiate along different pathways makes them suitable for generating tissues that might be used to treat degenerative diseases.

  • Stem cells from embryonic tissue are pluripotent, meaning they can be induced to differentiate into many types of cell.
  • Adult stem cells are found in the umbilical cord, bone marrow and skin. These can usually be induced to make only certain types of cell. These are called induced pluripotent stem cells (iPSC).
  • When stem cells are bathed in a specific combination of growth factors, they will reproduce to become specialised cells.

Stargardt’s disease

Human trials using stem cells to treat Stargadt’s disease are now underway. Stargardt’s causes degeneration of the cells in the centre of the eye’s visual field, a region called the macula. The treatment uses retinal pigment epithelium cells that were cultured from embryonic stem cells.

treating macular degenerationFigure 1.2.1d – Using stem cells to treat macular degeneration


ALS, or amyotrophic lateral sclerosis, is a degenerative disease that affects the spinal cord and nerves controlling muscle movement. Transplantation trials are underway using adult stem cells induced to form motor neurons. These iPSCs come from skin tissue that has been ‘rejuvenated’ by a specific chemical cocktail in the lab.

how iPS cells are madeFigure 1.2.1e – iPSC
Induced pluripotent stem cells can be cultured from mature adult cells of the skin.

ParameciumFigure 1.2.1f – Paramecium


Figure 1.2.1g – Chlamydomonas

In the lab

Both Paramecium and Chlamydomonas can be found in stagnant pond water. Prepare a slide of some pond water and try to identify the organisms and the structures relevant to the life functions described here. You may need to centrifuge the pond water to concentrate organisms in a small sample.

Language hint

When organisms move towards or away from an external stimulus it is called taxis (pronounced tack-siss, not tack-seez). Paramecia display negative thermotaxis because they move away from sources of heat. What type of taxis do Euglena display?

Course link

Chlamydomonas is one of the algal species that blooms as a result of eutrophication in freshwater. HL students will learn more about eutrophication in 14.1.6.

Nature of Science

Ethical implications: Research involving stem cells is growing in importance, but ethical issues regarding the extraction and cultivation of stem cells from embryonic tissue cannot be ignored.


  • Discuss the pros and cons of using each of the three types of stem cell (embryonic, umbilical cord, and adult).
  • Research a current therapeutic use for stem cells and answer the following questions:
    • Which type of stem cell is used?
    • From where are they harvested?
    • How successful has the therapy been so far?
    • What are the ethical implications of this research?
    • Do the social benefits outweigh the ethical implications?