Edexcel IGCSE Biology · Spec 2.21
Leaf Tissues, Xylem and Phloem
Covers locate the lower epidermis, xylem and phloem in a leaf, Relate each tissue to its function and Distinguish the substances carried by xylem and phloem.
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Leaf Tissues, Xylem and Phloem
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Explained
Xylem and phloem, and being precise about what each carries
A leaf contains several tissues, and each is defined by what it does rather than by where it sits. Two of them are transport tissues, and they run side by side in the veins.
Xylem
Xylem carries water and dissolved mineral ions from the roots up to the leaves.
It moves in one direction only, upwards, and the movement is driven by transpiration rather than by anything the plant spends energy on. Water evaporating from the leaves pulls the column of water up behind it.
Xylem vessels are made of dead cells with no end walls between them, forming continuous hollow tubes, and their walls are strengthened with lignin. Being dead and empty is what makes them good pipes, and the lignin stops them collapsing under the tension of the water column while also helping to support the plant.
Phloem
Phloem carries dissolved sugars and amino acids from the leaves to wherever the plant needs them, a process called translocation.
It moves in both directions, up and down, because a growing root needs sugar sent down while a developing fruit needs it sent up. That two way movement is a real difference from xylem and questions ask for it.
Phloem is made of living cells with perforated end walls called sieve plates, each supported by a companion cell alongside it. Translocation requires energy from the companion cells, which is why the tissue has to be alive.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for two substances other than water carried in the phloem, and the answers are sucrose or sugar, and amino acids. Beside them are three refusals: ignore glucose, ignore minerals, ignore proteins.
Each of those three is a near miss with a specific reason.
Glucose is the sugar the leaf makes, but it is converted to sucrose for transport, because sucrose is less reactive and does not affect the water balance of the cells in the way glucose would. Glucose is not what travels.
Minerals travel in the xylem, not the phloem. Naming them puts the right substance in the wrong tube.
Proteins are too large to move through the sieve plates. What travels is amino acids, and the plant assembles them into proteins after they arrive.
The examiner reports on two different papers make the same point from opposite ends. One records a response gaining just one mark for sugar solution and advises that candidates should refer to sucrose rather than sugar, to be more precise. The other, on a later paper, notes that many learners gained one mark but few gained both, and that several incorrectly referred to mineral ions and to glucose.
So this is a two mark question that most candidates half answer, and it is answered fully by two words. Sucrose and amino acids.
The other leaf tissues
The upper epidermis is a transparent single layer that lets light through to the cells below, covered by a waxy cuticle that reduces water loss.
The palisade mesophyll is packed with chloroplasts and does most of the photosynthesis.
The spongy mesophyll has large air spaces between its cells, allowing carbon dioxide to reach every cell by diffusion.
The lower epidermis contains the stomata, the pores through which gases enter and leave, each opened and closed by a pair of guard cells. Stomata are mostly on the lower surface, where they are shaded, which reduces evaporation while still permitting gas exchange.
Telling them apart in a diagram
In a leaf cross section the vein appears as a bundle in the middle, with xylem on the upper side and phloem on the lower side.
Xylem is nearer the top surface. That is worth memorising as a fact about the picture, because a labelling question gives you no other way to decide, and the two tissues are drawn adjacent.
In a root or a stem the arrangement differs, so the reliable approach is to identify xylem by its thickened, lignified walls and larger vessels rather than by position alone.
Spec 2.21
What you need to know
- Locate the lower epidermis, xylem and phloem in a leaf
- Relate each tissue to its function
- Distinguish the substances carried by xylem and phloem
Active recall
Quick check
Answer each question before opening the answer.
Which tissue carries water into a leaf?
Xylem
Which two organic substances are transported in phloem?
Sucrose and amino acids
Where are stomata commonly found in a leaf?
In the lower epidermis
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