Cohesion-Tension Theory in Xylem & Mass Flow Hypothesis of Translocation in Phloem
Vascular plants rely on two specialised mass transport tissues: xylem vessels that draw water and dissolved mineral ions unidirectionally from roots to leaves, and phloem sieve tubes that translocate organic solutes (primarily sucrose and amino acids) bidirectionally from sources to sinks. Water ascent in xylem is a passive physical process driven by solar evaporation at the leaves, explained by the Cohesion-Tension Theory where intermolecular hydrogen bonding holds a continuous column of water under negative hydrostatic pressure. In contrast, phloem loading at the source requires active transport of H⁺ ions to co-transport sucrose into companion cells and sieve tube elements, generating a high hydrostatic pressure gradient that drives mass flow. Examiners frequently test experimental setup rules for potometers alongside ringing and ¹⁴CO₂ tracer evidence for phloem translocation.
- Xylem Structure & Cohesion-Tension Theory
- Xylem vessels consist of dead, hollow cells aligned end-to-end with no end walls (forming continuous tubes) and walls thickened with waterproof lignin rings/spirals that prevent collapse under negative pressure. 1. Water evaporates from mesophyll cell walls and diffuses out of open stomata down a water potential gradient (transpiration). 2. Loss of water lowers the water potential (ψ) of mesophyll cells, pulling water from adjacent cells and xylem vessels. 3. This creates negative pressure (tension/pull) at the top of the xylem. 4. Hydrogen bonds between dipolar water molecules create cohesion (pulling a continuous water column upwards) and adhesion to hydrophilic xylem walls.
- Evidence for Cohesion-Tension & Potometer Precautions
- Tree trunk diameter decreases during the day (when transpiration and negative tension are highest, pulling xylem walls inwards) and increases at night; if a xylem vessel is broken, air is sucked in rather than water leaking out. When setting up a potometer to measure water uptake: cut the shoot underwater at a slant (prevents air locks entering xylem and increases SA), assemble all joints underwater and seal with petroleum jelly so the apparatus is airtight, and dry the leaves.
- Phloem Structure (Sieve Tube Elements & Companion Cells)
- Sieve tube elements are living tubular cells arranged end-to-end with perforated cross-walls called sieve plates; they contain no nucleus, no vacuole, and very few organelles (only a thin peripheral layer of cytoplasm) to reduce resistance to mass flow. Adjacent companion cells are connected via plasmodesmata and contain a nucleus, dense cytoplasm, and numerous mitochondria to supply ATP for active loading of sucrose.
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.
Key Concepts & Exam Definitions (6)
- Xylem Structure & Cohesion-Tension Theory
- Xylem vessels consist of dead, hollow cells aligned end-to-end with no end walls (forming continuous tubes) and walls thickened with waterproof lignin rings/spirals that prevent collapse under negative pressure. 1. Water evaporates from mesophyll cell walls and diffuses out of open stomata down a water potential gradient (transpiration). 2. Loss of water lowers the water potential (ψ) of mesophyll cells, pulling water from adjacent cells and xylem vessels. 3. This creates negative pressure (tension/pull) at the top of the xylem. 4. Hydrogen bonds between dipolar water molecules create cohesion (pulling a continuous water column upwards) and adhesion to hydrophilic xylem walls.
- Evidence for Cohesion-Tension & Potometer Precautions
- Tree trunk diameter decreases during the day (when transpiration and negative tension are highest, pulling xylem walls inwards) and increases at night; if a xylem vessel is broken, air is sucked in rather than water leaking out. When setting up a potometer to measure water uptake: cut the shoot underwater at a slant (prevents air locks entering xylem and increases SA), assemble all joints underwater and seal with petroleum jelly so the apparatus is airtight, and dry the leaves.
- Phloem Structure (Sieve Tube Elements & Companion Cells)
- Sieve tube elements are living tubular cells arranged end-to-end with perforated cross-walls called sieve plates; they contain no nucleus, no vacuole, and very few organelles (only a thin peripheral layer of cytoplasm) to reduce resistance to mass flow. Adjacent companion cells are connected via plasmodesmata and contain a nucleus, dense cytoplasm, and numerous mitochondria to supply ATP for active loading of sucrose.
- Active Loading of Sucrose at the Source
- At the source (e.g. photosynthesising leaf): 1. Companion cells use ATP to actively pump H⁺ ions (protons) out into surrounding cell walls/apoplast, creating a proton gradient. 2. H⁺ ions diffuse back into the companion cell down their concentration gradient via a co-transporter protein, carrying sucrose molecules against the sucrose concentration gradient. 3. Sucrose diffuses from the companion cell into the sieve tube element through plasmodesmata.
- Mass Flow Hypothesis of Translocation (Source to Sink)
- 1. Loading of sucrose into the sieve tube element at the source lowers its water potential (ψ becomes more negative). 2. Water enters the sieve tube element from adjacent xylem vessels by osmosis, increasing hydrostatic pressure at the source. 3. At the sink (e.g. respiring root cells or storage tubers), sucrose is actively unloaded and converted to starch or used in respiration, increasing the sieve tube's water potential (ψ becomes less negative). 4. Water leaves the sieve tube back into the xylem by osmosis, lowering hydrostatic pressure at the sink. 5. Sap moves en masse down the hydrostatic pressure gradient from source to sink.
- Ringing Experiments & Radioactive Tracer (¹⁴CO₂) Evidence
- Ringing (girdling): removing a complete ring of outer bark and phloem around a woody stem (leaving central xylem intact) causes a bulge of tissue fluid rich in sucrose to accumulate above the ring while tissue below the ring withers, proving phloem (not xylem) transports sugars downwards. Tracer experiments: supplying a leaf with radioactive ¹⁴CO₂ results in ¹⁴C-labelled sucrose appearing only in the phloem via autoradiography (X-ray film) or aphid stylet exudate analysis.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Explain how cohesion and adhesion work together in the xylem during transpiration.
Why does the diameter of a tree trunk reach its minimum at midday and its maximum at midnight?
Why must a leafy shoot be cut underwater when setting up a potometer?
How are sieve tube elements and companion cells adapted for translocation?
What is the role of the sink in maintaining the hydrostatic pressure gradient in phloem?
State two pieces of evidence that question or complicate the simple Mass Flow Hypothesis.
Exam-Style Questions & Mark Schemes (4)
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- Q1
Use the cohesion-tension theory to explain how water moves from the xylem in the roots to the atmosphere surrounding the leaves on a warm, sunny day. (5 marks)
Click “Check Mark Scheme” to compare your answer, 1. In light, stomata open and water evaporates from the moist cell walls of spongy mesophyll cells into the leaf air spaces, then diffuses out through the stomata down a water potential gradient into the atmosphere [1 mark]. 2. This loss of water lowers the water potential (ψ) of the mesophyll cells, so water moves from adjacent cells and out of the leaf xylem vessels by osmosis [1 mark]. 3. Removing water from the top of the xylem creates a negative hydrostatic pressure (tension / transpiration pull) at the top of the xylem column [1 mark]. 4. Water molecules are dipolar and form hydrogen bonds with each other (cohesion), pulling the entire continuous column of water upwards through the xylem vessels from the roots [1 mark]. 5. Water molecules also form hydrogen bonds with the hydrophilic cellulose/lignin walls of the narrow xylem vessels (adhesion), helping prevent the column from breaking under gravity [1 mark].
- Q2
Explain how sucrose is translocated from photosynthesising cells in a leaf (source) to storage cells in a potato tuber (sink) according to the mass flow hypothesis. (6 marks)
Click “Check Mark Scheme” to compare your answer, 1. At the source, companion cells use ATP to actively pump H⁺ ions out into surrounding cell walls, and H⁺ ions diffuse back in via a co-transporter protein carrying sucrose into the companion cell and through plasmodesmata into the sieve tube element [1 mark]. 2. The high concentration of dissolved sucrose lowers the water potential (ψ becomes more negative) inside the sieve tube element at the source [1 mark]. 3. Water moves from the adjacent xylem vessels (higher ψ) into the sieve tube element by osmosis, creating a high hydrostatic pressure at the source [1 mark]. 4. At the sink (potato tuber), sucrose is actively unloaded from the sieve tube into root/tuber cells and converted into insoluble starch by enzymes [1 mark]. 5. Removal of sucrose increases the water potential (ψ becomes less negative) in the sieve tube at the sink, so water diffuses out of the phloem back into the xylem by osmosis, lowering hydrostatic pressure at the sink [1 mark]. 6. This difference in hydrostatic pressure between the source and the sink drives the mass flow of phloem sap along the sieve tube from source to sink [1 mark].
- Q3
A scientist removed a complete ring of bark (containing the phloem) around the main trunk of a young fruit tree in summer, leaving the woody xylem intact. Describe and explain the effects on the trunk immediately above the ring and on the roots after two weeks. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. A swelling/bulge of tissue develops immediately above the removed ring, while no swelling occurs below the ring [1 mark]. 2. Sucrose and organic solutes translocated downwards from the photosynthesising leaves accumulate above the ring because the phloem pathway has been interrupted [1 mark]. 3. The accumulation of sucrose lowers the water potential (ψ) of tissues above the ring, drawing water in by osmosis and causing the cells to swell and grow [1 mark]. 4. Over two weeks, the roots below the ring cannot receive sucrose for aerobic respiration (since roots cannot photosynthesise), so active transport of ions and root growth cease and the roots eventually die [1 mark].
- Q4
A student used a potometer with a capillary tube of internal diameter 1.0 mm to measure the rate of water uptake by a leafy shoot. In 15 minutes, the air bubble moved 42 mm along the scale. Calculate the rate of water uptake in mm³ h⁻¹ (give your answer to 3 significant figures) and explain why the rate of water uptake does not exactly equal the rate of transpiration. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Radius of capillary tube r = 1.0 ÷ 2 = 0.5 mm; cross-sectional area = πr² = π × (0.5)² = 0.7854 mm² [1 mark]. 2. Volume of water taken up in 15 minutes = Area × distance = 0.7854 × 42 = 32.987 mm³ [1 mark]. 3. Rate per hour (× 4) = 32.987 × 4 = 132 mm³ h⁻¹ (to 3 s.f) [1 mark]. 4. Water uptake is slightly greater than transpiration because a small percentage (~1-2%) of absorbed water is used as a reactant in photosynthesis (photolysis) or retained to maintain cell turgidity and growth [1 mark].
Common Exam Questions (4)
Why do plants transport carbohydrates as sucrose rather than glucose in the phloem?
Sucrose is a non-reducing disaccharide, which makes it much less chemically reactive and less likely to be metabolised by intermediate cells along the transport pathway than glucose, while still being highly soluble in water.
How do aphid stylets help scientists study phloem translocation?
Aphids insert their hollow mouthparts (stylets) selectively into individual phloem sieve tube elements. If the aphid is anaesthetised with CO₂ and its body is severed with a laser, pure phloem sap continues to exude from the stylet for hours due to the high positive hydrostatic pressure inside the sieve tube.
What happens to phloem translocation if a metabolic poison like cyanide or DNP is applied to a leaf?
Cyanide inhibits cytochrome c oxidase in the mitochondrial electron transport chain, stopping aerobic ATP synthesis. Without ATP, companion cells cannot pump H⁺ ions to co-transport sucrose into the sieve tubes, so active phloem loading and translocation stop, whereas xylem transpiration (which is passive) continues unaffected.
How do wind speed, humidity, light intensity, and temperature each affect the rate of transpiration?
Higher light intensity opens stomata to let CO₂ in; higher temperature increases the kinetic energy of water molecules (faster evaporation); higher wind speed blows away humid air from the leaf surface (maintaining a steep ψ gradient); whereas higher external humidity decreases the ψ gradient and slows transpiration.
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