Differentiation (OCR A-Level Biology A): Revision Notes

Differentiation

What is differentiation?

In multicellular organisms, specialized cells develop from stem cells. Stem cells are unspecialized cells that lack the specific adaptations needed for particular functions. Through differentiation, these cells undergo structural and biochemical changes that allow them to perform specialized roles.

Erythrocyte differentiation

Erythrocytes (red blood cells) transport oxygen throughout the body and assist in carbon dioxide removal. These cells form from stem cells located in bone marrow through a process called erythropoiesis.

Multipotent stem cells

Bone marrow stem cells are multipotent, meaning they can differentiate into several different cell types. However, early in the differentiation pathway, specific changes occur that restrict the developmental potential of the cell. Once these changes happen, the cell can only develop into one particular cell type - in this case, an erythrocyte.

Stages of erythropoiesis

Structure and function in erythrocytes

Differentiation creates structural adaptations that enable erythrocytes to transport oxygen efficiently:

Biconcave disc shape: The cell's double indentation increases surface area compared to a sphere of the same volume. Since oxygen diffuses across the cell membrane, greater surface area allows more rapid gas exchange.

High haemoglobin concentration: The cytoplasm contains approximately 280 million haemoglobin molecules. This protein binds oxygen in areas of high oxygen concentration (the lungs) and releases it where oxygen levels are low (respiring tissues).

Absence of nucleus and organelles: Mature erythrocytes lack a nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. This absence creates additional space for haemoglobin, increasing the cell's oxygen-carrying capacity.

Elastic membrane: The flexible cell membrane allows erythrocytes to deform as they squeeze through narrow capillaries (which may be smaller in diameter than the cell itself).

Neutrophil differentiation

Neutrophils are white blood cells that originate from the same multipotent stem cells in bone marrow that produce erythrocytes. The differentiation changes are less extensive than in erythrocytes but remain significant.

The main structural modifications during neutrophil differentiation include:

• Formation of a lobed nucleus through indentations of the nuclear membrane
• Accumulation of numerous granules throughout the cytoplasm
• These granules are lysosomes containing hydrolytic enzymes

Structure and function of neutrophils

Neutrophils serve as the first line of cellular defense at infection sites. They leave the bloodstream by squeezing between capillary wall cells, gather around foreign materials in tissues, and destroy pathogens through phagocytosis and enzyme secretion.

Their structural adaptations support this immune function:

Flexible shape: The cell can change shape readily, allowing it to penetrate gaps between capillary endothelial cells and form pseudopodia (cytoplasmic projections) to engulf microorganisms.

Abundant lysosomes: The numerous lysosomes produce digestive enzymes that break down engulfed pathogens and foreign materials.

Flexible nuclear membrane: The nuclear membrane shows greater flexibility than in most cells, presumably assisting the cell's movement through tiny gaps in capillary walls. While the functional advantage of the lobed nuclear structure remains unclear, it may result from the membrane's enhanced flexibility.

Xylem and phloem differentiation

In plants, the transport tissues - xylem vessels and phloem sieve tubes - differentiate from stem cells in the cambium. The cambium is an example of a meristem: undifferentiated plant tissue capable of producing new cells. Other meristems exist at shoot tips and root tips.

The cambium's position differs between stems and roots, but in both organs it produces phloem cells toward the outside and xylem cells toward the inside.

Regulation of xylem and phloem production

Although cambium cells appear similar, their position determines their fate:

• Cells toward the outside produce only phloem
• Cells toward the inside produce only xylem

Differentiation processes

Xylem formation: Cells destined to become xylem lose their cytoplasm and deposit lignin in their cell walls. Lignin provides structural strength and waterproofing. The end walls between adjacent cells may disappear entirely, creating continuous tubes for water transport.

Phloem formation: Differentiating phloem cells undergo partial loss of cytoplasm and organelles. Sieve plates develop at cell ends - these perforated structures allow fluid movement between cells while maintaining some separation.