B2.3 Cell specialization

Question 1

Describe the importance of stem cells in differentiation. [3]

Answer 1

a. Stem cells are undifferentiated / not fully differentiated / unspecialised cells;
b. Embryo cells are stem cells;
c. Stem cells can differentiate in many / all ways / are pluripotent / totipotent;
d. Differentiation involves expressing some genes but not others;
e. Stem cells can be used to repair / replace tissues / heal wounds;

Question 2

Outline the importance of morphogens in the development of organisms

Answer 2

The concentration of the morphogen in each cell is important as it determines a series of subsequent signals (cascades) . Responses to these signals determines the direction and extent of cell growth and development. Ultimately, morphogens help direct the formation of all the tissues and organs in the body. The expression of such gradients also controls LENGTH of body structures such as toes and fingers, the location of the nose and other body parts.

Question 3

Distinguish betwen totipotent, pluripotent, multipotetent ad unipotent

Answer 3

Totipotent cells can differentiate into all cell types, including embryonic tissues and extra-embryonic tissues such as the placenta (e.g. the zygote and early embryo cells). Pluripotent cells can differentiate into all body cell types derived from the three germ layers but cannot form extra-embryonic tissues (e.g. embryonic stem cells). Multipotent cells can differentiate into a limited range of related cell types within a particular tissue or organ (e.g. bone marrow stem cells forming blood cells). Unipotent cells can differentiate into only one specific cell type, but retain the ability to self-renew (e.g. skin stem cells forming keratinocytes).

Question 4

List the differences between embryonic stem cells and adult stem cells

Answer 4

Undifferentiated embryonic cells capable of continual cell division and developing into all the cell types of an adult organism. However, adult stem cells that are undifferentiated are only capable of cell division to give rise to a limited range of cells within a tissue, for eg: blood stem cells give rise to red and white blood cells and platelets only.

Embryonic stem cells make up the bulk of the embryo as it begins development. However, adult stem cells occur only in the growing and adult body, within most organs; they replace dead or damaged cells, such as in bone marrow, brain and liver

Question 5

Explain the importance of the surface area to volume ratio as a factor limiting cell size. [7]

Answer 5

a. As size increases both surface area and volume increase, but volume increases
more / ratio of surface area to volume decreases as size of cell increases;
b. Rate of metabolism is a function of its mass to volume ratio;
c. Surface area limits / affects the rate at which substances can enter (or leave) the
cell;
d. Volume determines the rate at which material is produced / used;
e. Oxygen / nutrients / substances will take too long to diffuse into / out of the centre
of the cell if it is too big;
f. Excretory products would take too long to be eliminated;
g. Heat will take too long to be eliminated;
h. Example of cell adaptation to increase the ratio of surface area: volume e.g. root
hair cell / microvilli of intestinal epithelium;

Question 6

Define what is a stem cell

Answer 6

Stem cells are self-replicating, a stem cell can divide endlessly to produce more stem cells. Stem cells are also undifferentiated, a stem cell has not developed specialized features that would commit it to one particular role. It retains the capacity to differentiate along different pathways.

Question 7

Describe what is a stem cell niche (giving 2 examples)

Answer 7

A stem cell niche provides a microenvironment with conditions needed either for the stem cells to remain inactive and undifferentiated over long periods of time for them to proliferate rapidly and differentiate.

Bone marrow- Bone marrow contains haematopoietic stem cells that produce huge numbers of red and white blood cells and platelets each day. This is made possible by a generous supply of blood carrying oxygen, amino acids and other nutrients.

Hair follicles- pores in the skin that hold hairs. Stem cells at the base of each hair divide repeatedly to generate the many cells needed for hair growth. Blood capillaries supply the necessary nutrients.