As an organism develops, cells differentiate to form different types of…

The idea

During development, a fertilised egg divides by mitosis to produce many genetically identical cells. As the organism grows, these cells undergo differentiation, meaning they switch on particular genes and become specialised for different roles. This produces the many cell types found in a multicellular organism, such as muscle cells, nerve cells, red blood cells, xylem vessels and palisade mesophyll cells. The process is controlled so that each cell develops the structure needed for its function. In animals, most differentiation happens early in development, while many plants retain the ability to differentiate throughout life. Understanding this sequence helps explain how a single starting cell can give rise to a complex organism with many tissues and organs.

What earns the marks

• A fertilised egg divides by mitosis to produce a ball of genetically identical cells.
• As development proceeds, cells differentiate, meaning they become specialised in structure and function.
• Differentiation involves switching on specific genes so that particular proteins are made.
• Differentiation produces different cell types, such as muscle cells, nerve cells, red blood cells, xylem vessels and palisade mesophyll cells.
• The timing and extent of differentiation differ between animals and plants; most animal cells differentiate early, while many plant cells can differentiate throughout life.
• The result of differentiation is a range of specialised cells that form tissues and organs in a multicellular organism.

Common mistakes and how to put them right

• Thinking that differentiation happens before any cell division.

  Put it right: The error is reversing the sequence, and the correction is that mitosis produces the cells that then differentiate.

• Believing that differentiated cells all contain different genes.

  Put it right: The error is misunderstanding gene content, and the correction is that cells generally keep the same genes but use different ones.

• Assuming that all cells in an organism differentiate at the same time.

  Put it right: The error is ignoring differences between animals and plants, and the correction is that most animal cells differentiate early whereas many plant cells can differentiate throughout life.

Exam tips

• Sequence your answer clearly: mitosis first, then differentiation, then specialised cells forming tissues.
• Name specific cell types to show that you can apply the idea of differentiation to examples.
• If asked about plants, mention that many plant cells retain the ability to differentiate throughout life, unlike most animal cells.