OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(l) the production of xylem vessels and phloem sieve tubes as examples of…
Full specification wording
Cell division, cell diversity and cellular organisation: (l) the production of xylem vessels and phloem sieve tubes as examples of distinct, differentiated outcomes derived from a common stem cell in meristems
Meristems hold undifferentiated stem cells that divide by mitosis and then differentiate into specialised tissues.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 12 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Meristems hold undifferentiated stem cells that divide by mitosis and then differentiate into specialised tissues. In the root and shoot apical meristems, a single stem cell lineage can give rise to either xylem vessels or phloem sieve tubes, so the same genetic information produces two distinct outcomes. Xylem vessel elements lose their contents and lignify their walls to form hollow dead tubes carrying water and mineral ions upward. Phloem sieve tube elements keep a living companion cell, lose the nucleus and most organelles, and form sieve plates for translocation of assimilates. Differentiation therefore means selective gene expression and structural change, not a change in the genes present.
What earns the marks
- Meristems contain undifferentiated stem cells that divide by mitosis and are the source of differentiated tissues.
- Xylem vessels and phloem sieve tubes are both derived from a common stem cell in meristems, yet they are structurally and functionally distinct.
- Xylem vessel elements become hollow, lignified and dead, adapted for transporting water and mineral ions.
- Phloem sieve tube elements remain living, lose their nucleus and most organelles, and have sieve plates for translocation of assimilates.
- Differentiation results from selective gene expression and structural change, not from loss or gain of genes.
Common mistakes and how to put them right
Assuming the two cell types arise from different stem cell populations.
Put it right by stating both can derive from a common meristematic stem cell.
Thinking differentiation involves changing the DNA.
Put it right by explaining that the same genes are present but expressed selectively.
Confusing the transport roles.
Put it right by linking xylem to water and mineral ions and phloem to assimilates.
Describing phloem sieve tubes as dead.
Put it right by noting they remain living and are supported by companion cells.
Exam tips
- Read the stem carefully to identify which tissue the question targets before choosing an option.
- Eliminate options that swap xylem and phloem functions or structural features.
- Check whether an option correctly states that both tissues come from meristematic stem cells.
- Watch for distractors claiming genes are lost during differentiation; the genes remain but expression differs.
Try the lesson
The start of the Cell division, cell diversity and cellular organisation: (l) the production of xylem vessels and phloem sieve tubes as examples of distinct, differentiated outcomes derived from a common stem cell in meristems lesson, made from checked teaching notes for this specification point.
Have a go first
Which feature is characteristic of a mature phloem sieve tube element rather than a mature xylem vessel element?
Listen
Meristems hold undifferentiated stem cells that divide by mitosis and then differentiate into specialised tissues. In the root and shoot apical meristems, a single stem cell lineage can give rise to either xylem vessels or phloem sieve tubes, so the same genetic information produces two distinct outcomes. Xylem vessel elements lose their contents and lignify their walls to form hollow dead tubes carrying water and mineral ions upward. Phloem sieve tube elements keep a living companion cell, lose the nucleus and most organelles, and form sieve plates for translocation of assimilates. Differentiation therefore means selective gene expression and structural change, not a change in the genes present. A strong answer earns marks for these points. Meristems contain undifferentiated stem cells that divide by mitosis and are the source of differentiated tissues. Xylem vessels and phloem sieve tubes are both derived from a common stem cell in meristems, yet they are structurally and functionally distinct. Xylem vessel elements become hollow, lignified and dead, adapted for transporting water and mineral ions. Phloem sieve tube elements remain living, lose their nucleus and most organelles, and have sieve plates for translocation of assimilates. A common slip: Assuming the two cell types arise from different stem cell populations. Instead, stating both can derive from a common meristematic stem cell. Exam tip: Read the stem carefully to identify which tissue the question targets before choosing an option.
Key words
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Stem cells in meristems
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Xylem vessel structure and function
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Phloem sieve tube structure and function
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Selective gene expression
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See every definition
- Stem cells in meristems
- Undifferentiated cells held in meristems that divide by mitosis and are the source of differentiated tissues.
- Xylem vessel structure and function
- Elements that become hollow, lignified and dead, adapted for transporting water and mineral ions.
- Phloem sieve tube structure and function
- Elements that remain living, lose their nucleus and most organelles, and have sieve plates for translocation of assimilates.
- Selective gene expression
- The process by which the same genetic information produces distinct outcomes, so differentiation is not a change in the genes present.
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