Cell Growth and Differentiation

This lesson covers cell growth and differentiation in animals and plants as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how organisms grow, why cells differentiate and the differences between growth and differentiation in animals and plants.

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How Do Organisms Grow?

Multicellular organisms grow by increasing the number of cells and the size of cells. Growth involves:

1. Cell division — mitosis produces new cells.
2. Cell elongation — cells increase in size (particularly important in plants).
3. Cell differentiation — cells become specialised for particular functions.

In animals, growth mainly involves an increase in cell number (through mitosis) followed by differentiation. In plants, growth also involves significant cell elongation, where cells absorb water into their vacuoles and stretch.

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What Is Cell Differentiation?

Differentiation is the process by which a cell changes to become specialised for a particular function. When a cell differentiates, it develops specific structures and features that allow it to carry out its role effectively.

Before differentiation, cells are unspecialised — they have the potential to become many different cell types.

Exam Tip: Differentiation does not mean the cell gains new DNA. All body cells contain the same DNA. During differentiation, different genes are switched on or off, producing different proteins that give the cell its specialised structure and function.

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Differentiation in Animals

In animal cells:

• Most differentiation occurs during early development (in the embryo).
• Once animal cells have differentiated, they generally cannot change into a different cell type.
• In adult animals, cell division is mainly for repair and replacement rather than growth.
• Some cells retain the ability to divide but not to change type (e.g. skin cells, blood stem cells).

Examples of Specialised Animal Cells

Cell Type	Specialisation	Function
Red blood cell	Biconcave disc shape; no nucleus; contains haemoglobin	Carries oxygen around the body
Nerve cell (neurone)	Long axon; branched dendrites; myelin sheath	Transmits electrical impulses over long distances
Sperm cell	Streamlined head; acrosome; many mitochondria; long tail (flagellum)	Swims to and fertilises the egg
Muscle cell	Contains protein fibres that can contract; many mitochondria	Contracts to produce movement

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Differentiation in Plants

In plant cells:

• Differentiation can occur throughout the life of the plant, not just in early development.
• Plants retain meristematic tissue (found at root tips and shoot tips) that contains unspecialised cells capable of dividing and differentiating.
• Plant cells can differentiate, then sometimes de-differentiate and become a different cell type — this is much rarer in animals.

Examples of Specialised Plant Cells

Cell Type	Specialisation	Function
Root hair cell	Long, thin extension (root hair) increasing surface area; no chloroplasts	Absorbs water and mineral ions from the soil
Palisade mesophyll cell	Packed with chloroplasts; located near the upper surface of the leaf	Carries out photosynthesis
Xylem vessel	Dead, hollow, lignified (strengthened) walls; no end walls	Transports water and minerals up the plant
Phloem sieve tube	Living cells with sieve plates; companion cells alongside	Transports dissolved sugars (translocation)

Exam Tip: A key difference between animal and plant differentiation: animals differentiate early in development and then mostly lose the ability; plants can differentiate throughout their entire life from meristematic tissue.

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Growth in Animals vs Plants

Growth can be measured in different ways:

• Height or length — often used for plants and children.
• Wet mass — the mass of an organism including water.
• Dry mass — the mass of an organism after all water has been removed (most accurate measure of growth, but the organism must be killed).

Feature	Animals	Plants
Where growth occurs	All over the body	Mainly at tips (meristems)
Cell division	Throughout life (mainly for repair in adults)	Continuously at meristems
Cell elongation	Limited contribution to growth	Major contribution to growth
Differentiation	Mainly early in development	Throughout life
De-differentiation	Very rare	Can occur

graph TD
    A[Unspecialised Cell] -->|Differentiation| B[Specialised Animal Cell]
    A -->|Differentiation| C[Specialised Plant Cell]
    C -->|De-differentiation| A
    B -.->|Very rare in animals| A

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Measuring Growth

Scientists measure growth using different methods depending on the organism:

• Length — easy to measure but does not account for changes in mass or volume.
• Wet mass — quick to measure but varies with water content (e.g. after drinking or urination).
• Dry mass — most reliable, but destructive (organism must be dried in an oven at 80 °C until constant mass is reached).

Exam Tip: If asked which is the most accurate measure of growth, the answer is dry mass. However, remember the major disadvantage — the organism must be killed, so you cannot track the growth of a single organism over time.

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Summary

• Organisms grow by cell division (mitosis), cell elongation and cell differentiation.
• Differentiation is the process by which cells become specialised for a particular function by switching certain genes on or off.
• In animals, most differentiation occurs early in development; in plants, it continues throughout life from meristematic tissue.
• Plant cells can sometimes de-differentiate, but this is very rare in animal cells.
• Growth is best measured by dry mass, though this requires killing the organism.
• Cell elongation is a major contributor to plant growth.

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Worked Example — Calculating Percentage Increase in Mass

A seedling has a dry mass of 0.25 g on Day 1 and 0.95 g on Day 14. Calculate the percentage increase in dry mass.

Percentage increase = (Final − Initial) ÷ Initial × 100
                    = (0.95 − 0.25) ÷ 0.25 × 100
                    = 0.70 ÷ 0.25 × 100
                    = 280%