B2.3 Cell specialization

Your body contains hundreds of cell types — nerve, muscle, red blood, skin — yet almost all carry the same genome. How can identical instructions produce such different cells? The answer is differentiation: each cell switches on only the subset of its genes that suit its role. Understanding B2.3 means thinking about cells as members of a larger organisation, building from specialised cells up to tissues, organs and systems, and recognising why being big forces an organism to divide labour among different cells. The link between gene expression and cell function runs through everything in this topic.

Stem cells and differentiation

A stem cell is an unspecialised cell that can both divide repeatedly (self-renew) and differentiate into one or more specialised cell types. Stem cells are described by their potency:

• Totipotent cells (such as the zygote and early embryo cells) can form any cell type, including the placenta.
• Pluripotent cells (embryonic stem cells) can form any cell type of the body.
• Multipotent cells (many adult stem cells, such as those in bone marrow) form a limited range of related cell types.

Differentiation is the process by which a cell develops the specific structure and biochemistry needed for its function. Crucially, differentiation does not change the DNA; instead it changes which genes are expressed. As an organism develops, cells become progressively more specialised and usually lose potency.

Gene expression controls specialization

Every nucleated cell of an organism contains the whole genome, but only a fraction of genes are active in any one cell. Differential gene expression — switching particular genes on or off — is what makes cells different. A gene that is expressed is transcribed and translated to produce its protein; a gene that is switched off is not.

For example, only red blood cell precursors express the genes for haemoglobin in large amounts, and only certain pancreatic cells express the insulin gene. The pattern of active genes determines the proteins a cell makes, and the proteins determine the cell’s structure and function. Chemical signals during development, and factors such as the cell’s position, help decide which genes are switched on.

This is why differentiation is usually considered permanent in normal development: once the expression pattern is established, the cell maintains it.

Adaptations of specialized cells

The form of a specialised cell matches its function — a recurring exam theme. Some standard examples:

• Red blood cells are biconcave and lose their nucleus, giving more room for haemoglobin and a larger surface area for oxygen exchange.
• Sperm cells have a flagellum for swimming and many mitochondria to power it, plus an acrosome of enzymes to penetrate the egg.
• Root hair cells have a long extension that increases surface area for absorbing water and minerals.
• Muscle cells are packed with contractile protein filaments and mitochondria.

In each case the structure can be traced back to the genes the cell expresses, which produce the proteins that build these features.

Surface-area-to-volume ratio and multicellularity

The need for specialisation is closely tied to size. As a cell or organism gets larger, its volume increases faster than its surface area, so the surface-area-to-volume (SA:V) ratio falls. Because exchange of materials and heat happens across the surface but is needed throughout the volume, a low SA:V ratio limits how fast substances can be supplied and wastes removed.

This constraint explains two things. First, why cells stay small. Second, why large organisms must be multicellular and divide labour: specialised cells form tissues (groups of similar cells), tissues form organs, and organs form organ systems. This hierarchy lets large bodies include exchange surfaces — lungs, gills, intestines, roots — that restore an effective surface area for the whole organism. The emergent properties of the organism arise from the cooperation of its specialised parts.