Stem Cells in Animals & Plants (OCR A Level Biology): Revision Note

Stem cells in the bone marrow

• Examples of distinct types of differentiated cell that arise from a common stem cell within bone marrow include:

  • erythrocytes

  • neutrophils

• Stem cells in bone marrow are multipotent adult stem cells

Erythrocytes

• Erythrocytes are red blood cells, specialised to aid oxygen transport around the body

• Red blood cells lack a nucleus so they cannot perform mitosis to give rise to new cells; this means that new erythrocytes develop from bone marrow stem cells in order to maintain the red blood cell count in the blood

• The process by which erythrocytes arise from stem cells includes changes such as:

  • loss of the nucleus

  • a shape change to form a biconcave disc

  • increased production of haemoglobin protein

  • an increase in membrane flexibility

Stem cells in bone marrow differentiate to give rise to erythrocytes

Neutrophils

• The same stem cells that form erythrocytes also give rise to neutrophils, a type of white blood cell that is specialised to destroy pathogens by phagocytosis

• Changes that occur as stem cells differentiate into neutrophils include:

  • indentations form in the nucleus, giving it a lobed structure

  • production of hydrolytic enzymes increases

  • lysosomes that contain hydrolytic enzymes accumulate

  • membrane flexibility increases

Meristems

• Plant stem cells are found in regions where growth is taking place; these tissues are known as meristems

• Examples of distinct types of differentiated cell that arise from a common stem cell within plant meristems include:

  • xylem

  • phloem

• The specialised cells that make up xylem and phloem are located in a meristem known as cambium

• Cambium cells are found between regions of xylem and phloem tissue in plant stems and roots

The cells that make up xylem and phloem tissue arise from the same type of stem cell within a meristem known as cambium

Xylem and phloem

• Xylem cells are specialised to allow the transport of water and minerals; changes that occur when cambium cells differentiate into xylem cells include:

  • the deposition of lignin in cell walls

  • loss of cytoplasm

  • loss of end walls

• Phloem tissue is specialised to allow the transport of assimilates; changes that allow cambium cells to differentiate into phloem sieve tubes include:

  • a reduction in cytoplasm volume

  • loss of some organelles

  • end walls develop into sieve plates