4.1.5Unit 4: Energy, Environment, Microbiology and Immunity
The Vascular Structure of Plants — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 12 of 28 Unit quiz
Part 1 of 4Roles of the Stem
Stems both hold the plant up and carry materials around it.
Roles of the Stem
Support
- Stems hold leaves in positions where they receive the most light, and display flowers to pollinators.
Flexibility
- Stems must be strong yet flexible so that they can withstand wind and rain without snapping.
Transport
- Water and mineral ions travel up from the roots to leaves in the vascular tissue of the stem.
Distribution of products
- Sugars made in photosynthesis are carried from the leaves to other parts of the plant that need them.
Part 2 of 4Xylem and Sclerenchyma
In a stem the xylem lies towards the inside of each vascular bundle, and the phloem lies closer to the outside.
Xylem function
- Transports water and dissolved mineral ions upwards and also gives structural support to the plant.
Lignified walls
- Lignin makes the walls strong enough to withstand the pressure of the moving column of water.
Dead and hollow
- Xylem vessels are dead cells with no cytoplasm or organelles, so water flows through freely as in a straw.
Pits
- Unlignified regions in the wall allow water and ions to move sideways between neighbouring vessels.
Sclerenchyma
- Bundles of dead cells with lignified walls that give extra support next to the vascular bundles but do not transport substances.
Part 3 of 4Phloem
Phloem transports organic compounds and, unlike xylem, contains living cells.
1
Phloem moves assimilates, mainly sucrose dissolved in water as sap, from sources such as leaves to sinks such as roots; this is translocation.
2
Sieve tube elements are the main conducting cells and line up end to end to form a continuous tube.
3
Sieve tube elements have no lignin and have sieve plates between cells, but they contain very little cytoplasm.
4
Each sieve tube element has a companion cell with a nucleus and dense cytoplasm that controls its metabolism.
5
Companion cells also help load sugars into the sieve tubes and unload them at the sinks, and phloem has no support role.
Part 4 of 4Exam Checklist & Tips
4.1.5 – Key Points for the Exam
- Xylem Vascular tissue that carries water and mineral ions upwards and supports the plant.
- Phloem Vascular tissue that carries sucrose and other assimilates around the plant.
- Translocation Movement of assimilates in the phloem from sources to sinks.
- Companion cell A cell that supports a sieve tube element and loads sugars into it.
- Vascular bundle A group of xylem and phloem running together through the plant.
Common Mistakes
- Stating that xylem carries sucrose and phloem carries water;
- xylem carries water and mineral ions, while phloem carries sucrose.
- Describing mature xylem vessels as living cells with cytoplasm, when they are dead and lignified.
- Saying that phloem moves materials only upwards, when translocation can go up or down the plant.
- Mixing up sieve tube elements with companion cells; the companion cell has the nucleus.
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