Xylem, Phloem, Transpiration & Translocation
Plants use two specialised vascular tissues to transport substances between roots, stems, and leaves. Xylem vessels carry water and dissolved mineral ions upwards from the roots to the leaves in the transpiration stream, while phloem tissue transports dissolved sugars both upwards and downwards by translocation.
- Xylem
- Hollow tubes of dead cells that carry water and minerals upwards.
- Phloem
- Tubes of living cells that transport dissolved sugars in both directions.
- Transpiration stream
- The continuous movement of water from the roots, up the xylem, and out of the leaves.
Xylem vs Phloem Structure, Root/Stem Cross-Sections & Potometer
Switch between the three diagrams examiners test on GCSE Biology papers. Use Hide Labels to practice naming parts from memory.
How to score 3/3 marks on Xylem vs Phloem questions:
- 1. Direction & Substance: Xylem carries water and mineral ions upwards only (roots to leaves). Phloem carries dissolved sugars (sucrose) both up and down (translocation).
- 2. Living vs Dead Cells: Xylem vessels are dead hollow tubes with no end walls. Phloem consists of living cells with perforated sieve plates.
- 3. Energy Requirement: Transpiration pull in xylem is passive (no ATP needed). Translocation in phloem is active and relies on mitochondria in companion cells to supply ATP.
Exam notes: Transpiration Stream, Stomata & Potometer Practicals
- Water travels through a plant via the transpiration stream: Soil → Root hair cells (by osmosis) → Root cortex cells → Xylem vessels in stem (cohesion-tension pull) → Mesophyll cells in leaf → Evaporates into air spaces → Diffuses out through open stomata as water vapour.
- Guard cells control gas exchange and water loss: in bright light, guard cells absorb potassium ions and water by osmosis, become turgid, and curve apart because their inner cell wall is thicker and less elastic than the outer wall, opening the stomatal pore.
- Mineral ions (such as nitrate NO₃⁻ for amino acids/proteins and magnesium Mg²⁺ for making chlorophyll) are at a lower concentration in soil water than inside root hair cells, so they must be absorbed by active transport using ATP from aerobic respiration in root mitochondria.
| Feature | Xylem Vessels | Phloem Tissue |
|---|---|---|
| Substances carried | Water and dissolved mineral ions (e.g. Mg²⁺, NO₃⁻) | Dissolved sugars (sucrose) and amino acids (cell sap) |
| Process name | Transpiration stream | Translocation |
| Direction of flow | One-way only: upwards from roots to stems and leaves | Two-way (bidirectional): from sources (leaves) to sinks (roots/growing shoots) |
| Living or dead cells? | Dead hollow cells with no cytoplasm or nucleus | Living sieve tube elements supported by companion cells |
| End walls between cells | No end walls: forms a continuous open tube | Perforated end walls called sieve plates with pores |
| Wall reinforcement & energy | Thickened with waterproof lignin rings/spirals; passive process (0 ATP) | Cellulose walls (no lignin); active process requiring ATP from companion cells |
Key Concepts & Exam Definitions (11)
- Xylem
- Hollow tubes of dead cells that carry water and minerals upwards.
- Phloem
- Tubes of living cells that transport dissolved sugars in both directions.
- Transpiration stream
- The continuous movement of water from the roots, up the xylem, and out of the leaves.
- Stomata
- Tiny pores on the underside of a leaf that allow gases to exchange and water to evaporate.
- Guard cells
- Cells that open and close the stomata to control water loss.
- Translocation
- The movement of food molecules through the phloem.
- Root hair cell adaptations
- Long hair-like projection increases surface area for fast osmosis of water and active transport of mineral ions; thin cell wall gives a short diffusion pathway; packed with mitochondria to supply ATP for active transport; NO chloroplasts (underground in the dark).
- Guard Cells and Stomata Mechanism
- Kidney-shaped cells in the lower epidermis. When water is plentiful and light is bright, guard cells take in water by osmosis and become turgid; their thicker inner wall bends, opening the stoma for CO₂ uptake. In drought/darkness, they lose water, become flaccid, and close the stoma to conserve water.
- Leaf cross-section tissues (top to bottom)
- 1) Waxy cuticle (waterproof layer reducing evaporation). 2) Upper epidermis (transparent to let light through). 3) Palisade mesophyll (tall cells packed with chloroplasts at the top for photosynthesis). 4) Spongy mesophyll (air spaces for rapid gas diffusion). 5) Lower epidermis with stomata & guard cells (cooler shaded underside reduces water loss).
- Nitrate vs Magnesium Ion Deficiency in Plants
- Nitrate (NO₃⁻) ions are needed to make amino acids and proteins for growth, deficiency causes stunted growth. Magnesium (Mg²⁺) ions are needed to make chlorophyll: deficiency causes yellow leaves (chlorosis) and reduced photosynthesis.
- Potometer Practical Precautions
- Cut the plant shoot underwater and slanted (prevents air bubbles entering and blocking the xylem); assemble the potometer underwater; seal all joints with Vaseline/petroleum jelly so it is airtight; dry the leaves before starting so humidity around stomata isn't artificially high.
Quick-Fire Flashcards (10)
Click any card to check the answer, then mark it as Known to track your progress.
What substance strengthens xylem vessels?
Which process involves the movement of sugars in a plant?
What effect does increased temperature have on transpiration?
How does wind affect the transpiration rate?
Where are most stomata found?
Why do root hair cells have lots of mitochondria but NO chloroplasts?
Why are most stomata located on the underside (lower epidermis) of a leaf?
What causes yellow leaves (chlorosis) in a plant, and why?
Why must a plant shoot be cut underwater when setting up a potometer?
Explain how companion cells help phloem sieve tubes carry out translocation.
Exam-Style Questions & Mark Schemes (6)
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- Q1
Explain how transpiration occurs in a plant.
Click “Check Mark Scheme” to compare your answer, Water evaporates from cells inside the leaf and diffuses out through the stomata. This creates a shortage of water in the leaf, drawing more water up from the roots through the xylem vessels.
- Q2
Describe three factors that increase the rate of transpiration.
Click “Check Mark Scheme” to compare your answer, Higher temperature (water evaporates faster), lower humidity (steeper concentration gradient), and higher wind speed (blows water vapor away).
- Q3
Compare the structure and function of xylem and phloem.
Click “Check Mark Scheme” to compare your answer, Xylem is made of dead, hollow cells strengthened by lignin; it transports water and minerals upwards only. Phloem is made of living cells with pores in the end walls; it transports dissolved sugars in both directions.
- Q4
In a potometer experiment, the air bubble moved 42 mm in 15 minutes under a desk lamp. Calculate the rate of water uptake in mm/min and explain why this is slightly less than the true volume of water absorbed by the plant. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1) Rate = distance ÷ time = 42 mm ÷ 15 min = 2.8 mm/min [1 mark]. 2) A small fraction of the water absorbed by the shoot does not evaporate from the leaves, it is used as a reactant in photosynthesis (to make glucose) [1 mark] and to keep cells turgid / support growth [1 mark].
- Q5
Explain how the structure of xylem vessels is adapted for transporting water up a tall tree. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1) Cells are dead and hollow with no cytoplasm or organelles, so nothing impedes water flow [1 mark]. 2) End walls have completely broken down to form a continuous unbroken tube from roots to leaves [1 mark]. 3) Walls are thickened with rings/spirals of waterproof lignin, which strengthens the tube and prevents it collapsing under negative tension pressure [2 marks].
- Q6
Explain why a plant growing in waterlogged soil may show signs of mineral deficiency even though the soil is rich in nitrate ions. [3 marks]
Click “Check Mark Scheme” to compare your answer, Waterlogged soil lacks air spaces, so root hair cells do not get enough oxygen [1 mark]. Without oxygen, the root cells cannot carry out aerobic respiration and produce very little ATP [1 mark]. Since mineral ions (nitrates) are absorbed against their concentration gradient by active transport, which requires ATP, the plant cannot absorb enough nitrates [1 mark].
Common Exam Questions (5)
Name the tubes that transport water up the stem of a plant.
Xylem. Xylem vessels are hollow tubes formed from dead cells joined end to end, strengthened with rings or spirals of lignin, that transport water and dissolved mineral ions from the roots up the stem to the leaves.
Is xylem transpiration or translocation?
Xylem carries water and mineral ions upwards via the transpiration stream, driven by water evaporating from the leaves. Phloem transports dissolved sugars (sucrose) in both directions by translocation.
Name the process that transports dissolved sugars around a plant.
Translocation. It takes place in the living phloem tissue, moving dissolved sugars produced by photosynthesis in the leaves to growing meristems and storage organs.
What does phloem transport?
Phloem transports dissolved sugars (mainly sucrose) and amino acids from the leaves to other parts of the plant for immediate respiration or storage as starch.
What is transpiration?
Transpiration is the loss of water vapour from plant leaves by evaporation from the surfaces of spongy mesophyll cells followed by diffusion out through the stomata, which pulls a continuous column of water up through the xylem.
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