This document is based on the latest CAIE syllabus, over 15 past papers (2018–2025) and official mark schemes. It covers the structure and function of xylem and phloem, water and mineral uptake, transpiration, factors affecting transpiration, experiments, phloem translocation, source-sink relationships, ringing experiments, and all other key topics.
Each concept is presented with standard definitions, key words, structure-function links, examples and exam traps, ready for Paper 2 and Paper 4.
| Feature | Xylem | Phloem |
|---|---|---|
| Function | transports water + dissolved mineral ions; provides mechanical support | transports sucrose + amino acids (process called translocation) |
| Direction of transport | one direction: roots → stem → leaves (upward) | bidirectional: source → sink (can move up or down) |
| Substances transported | water, nitrate, magnesium, other mineral ions | sucrose (organic nutrients), amino acids |
| Cell state | dead cells (no cytoplasm, no nucleus when mature) | living cells (require energy) |
| Cell wall | thickened with lignin, provides support | cellulose cell wall, no lignin |
| End walls | end walls absent, forming a continuous tube | sieve plates present, allow sap to pass |
| Associated cells | none | companion cells + plasmodesmata |
| Organ (cross‑section) | Xylem position | Phloem position | Explanation |
|---|---|---|---|
| Root | central star‑shape (radiating arms) | between the arms of xylem (alternating) | xylem in centre resists tension; phloem at periphery for easy transport to cortex |
| Stem | in vascular bundle near centre (pith side) | in vascular bundle near outside (epidermis side) | xylem inside resists bending; phloem outside for easy distribution |
| Leaf | in vein closer to upper epidermis | in vein closer to lower epidermis | xylem brings water upward; phloem carries sugars downward |
In exams, you are often given a cross‑section diagram and asked to identify xylem (thick‑walled, large lumen) and phloem (smaller cells, sieve tubes + companion cells).
Root hair cells are elongated projections of epidermal cells in the root maturation zone. Their adaptations are:
| Adaptation | How it helps absorption | Explanation |
|---|---|---|
| Root hair | greatly increases surface area | larger surface area → faster absorption |
| Thin cell wall | shortens diffusion distance | water and ions reach the cell membrane more easily |
| Many mitochondria | provide ATP for active transport | mineral ion uptake requires energy |
| Large vacuole | creates osmotic gradient | maintains low water potential, drives water entry |
| High cytoplasmic concentration | maintains low water potential | promotes osmotic water uptake |
Water absorption: root hair cells absorb water by osmosis. The cell sap has high solute concentration (sugars, salts, etc.), so its water potential is lower than that of soil water → water diffuses across the partially permeable membrane into the root hair cell. (Does not require energy.)
Mineral ion absorption: ions are taken up by active transport – requires carrier proteins and energy (ATP) from respiration. This allows uptake against a concentration gradient. After entering, some ions are assimilated into organic compounds, while others are transported upward via the xylem.
Paper 4 answer template: “Why do root hair cells need many mitochondria?” → They use active transport to absorb mineral ions, which requires ATP. Mitochondria are the site of aerobic respiration that produces ATP.
Complete pathway (memorise order – common in Paper 4/6):
soil → root hair cell → root cortex → xylem (vessels) → stem (xylem) → petiole (xylem) → leaf vein (xylem) → mesophyll cells → mesophyll cell walls → air spaces → water vapour diffuses out through stomata (transpiration)
Transpiration: loss of water vapour from leaves, mainly through stomata. Water evaporates from the surfaces of mesophyll cells into the air spaces, then diffuses out through the stomata.
The air spaces in the spongy mesophyll are crucial for gas exchange: water vapour evaporates from moist mesophyll cell walls into the air spaces and then exits via stomata. Stomata open during the day to allow CO₂ entry for photosynthesis, which also lets water vapour escape (major water loss). At night, stomata close to reduce water loss.
Cohesion‑Tension Theory explains how water moves upward in xylem. Key steps (answer chain):
- Stomata open → water evaporates from mesophyll cell walls (transpiration).
- Mesophyll cells lose water → lower water potential → they absorb water from xylem vessels.
- Water column inside xylem vessels experiences tension (negative pressure).
- Water molecules are held together by strong cohesion (hydrogen bonds), keeping the water column continuous (does not break).
- Tension is transmitted all the way down the water column to the root xylem.
- Negative pressure in root xylem → water enters the xylem from root hair cells by osmosis, replenishing the water column.
- Result: the whole water column moves upward → transpiration stream.
Cohesion = attraction between water molecules; Adhesion = attraction between water molecules and xylem walls; both maintain the continuous column.
- Internal surface area: the air spaces in spongy mesophyll provide a huge internal surface area for evaporation → high transpiration rate.
- Stomatal size and number: more stomata and larger openings provide more pathways for water vapour diffusion → higher transpiration rate.
Xerophytes (e.g., cacti) have few, sunken stomata to reduce transpiration; hydrophytes (e.g., water lily) have stomata on the upper surface.
| Factor | Change | Effect on transpiration rate | Explanation (Paper 4) |
|---|---|---|---|
| Light intensity | increase | increase ↑ | Stomata open to allow CO₂ for photosynthesis → water vapour escapes. |
| Temperature | increase | increase ↑ | Water molecules gain kinetic energy → faster evaporation from mesophyll cells. |
| Humidity | increase | decrease ↓ | Concentration gradient of water vapour between leaf interior and air is reduced → slower diffusion. |
| Wind speed | increase | increase ↑ | Moving air removes water vapour near the leaf surface, maintaining a steep concentration gradient. |
Leaf size, stomatal density, cuticle thickness also affect transpiration, but IGCSE focuses on the four factors above.
A potometer directly measures water uptake rate. Assuming most water taken up is lost by transpiration (steady state, no growth), water uptake rate ≈ transpiration rate.
- Cut a leafy shoot underwater – prevents air bubbles entering the xylem and blocking water flow.
- Insert the shoot into the potometer and seal joints with petroleum jelly to ensure air‑tightness.
- Allow the shoot to acclimatise, then start recording – measure the distance an air bubble moves or the volume of water taken up per unit time.
- Change the environmental factor – light intensity, temperature, humidity, or wind (use a fan).
- Repeat and calculate mean values – reduce random error.
Transpiration rate = volume of water absorbed per unit time ÷ leaf surface area (or sometimes per unit shoot mass, depending on the question).
Wilting: drooping of leaves and stems due to excessive water loss, occurs when water loss (transpiration) > water uptake.
Cause: when soil water is insufficient or transpiration is too high, roots cannot absorb enough water to replace that lost by leaves → leaf cells lose water.
- Guard cells and mesophyll cells lose water → plasmolysis (protoplast shrinks away from cell wall).
- Cells lose turgor pressure → leaf droops.
- Wilting also causes stomatal closure, further reducing transpiration and photosynthesis.
Translocation: movement of sucrose and amino acids in phloem from sources to sinks.
| Term | Definition | Examples |
|---|---|---|
| Source | part of plant that releases sucrose or amino acids | photosynthesising leaves, germinating seeds (break down starch), storage organs (e.g., tubers) |
| Sink | part of plant that uses or stores sucrose/amino acids | roots (growing), shoot tips (growing), developing fruits (store sugars), storage organs (tubers, seeds) |
Source‑sink change: the same organ can be a sink at one time and a source at another. Example: potato tuber is a sink (receives sucrose from leaves) during the growing season; in spring when it sprouts, it becomes a source (breaks down starch to sucrose to supply new shoots). Another example: leaves are sources during the day but may become sinks at night when they cannot photosynthesise and receive sugars from elsewhere.
Method: remove a ring of bark (phloem) from a tree trunk, leaving the xylem intact (the xylem is inside the wood).
Results:
- Swelling appears above the girdled ring because sugars from leaves accumulate there.
- Tissue below the ring gradually loses sugar and eventually dies.
- The swollen part contains high sugar concentration (test with Benedict's solution).
Conclusion: Phloem transports organic nutrients (the swelling above shows that sugar is moving downward from leaves and is blocked by removing phloem). Xylem water transport is not affected because xylem is inside the wood.
Supporting evidence: Radioactive tracers – provide radioactive CO₂ to a leaf; the plant produces radioactive sugars, and their movement can be traced by autoradiography, confirming the phloem pathway.
| Cell type | Features | Adaptations |
|---|---|---|
| Sieve tube element | living, no nucleus, no vacuole, thin cytoplasm | low‑resistance transport over long distances |
| Companion cell | many mitochondria, nucleus, connected to sieve tube via plasmodesmata | supplies ATP for active processes and maintains sieve tube functions |
Translocation requires ATP for active loading and unloading. Companion cells provide ATP to sieve tubes.
- Sucrose is produced in source leaf mesophyll cells.
- Active loading into companion cells (requires ATP and carrier proteins).
- Sucrose moves from companion cells to sieve tubes via plasmodesmata.
- Sieve tubes transport sucrose to sink organs.
- Unloading at the sink may be active or passive, depending on concentration gradient.
Purpose: to observe the pathway of water through a plant.
Method: place a leafy shoot in water containing eosin or food colouring, leave in light. After several hours, observe the leaf veins and leaf blade.
Result: colour appears only in the xylem of veins, because xylem is specialised for water transport. This demonstrates that water moves up through the xylem and follows the veins.
| Wrong idea | Correct statement | Sub‑topic |
|---|---|---|
| Xylem transports sucrose, phloem transports water | Xylem transports water and minerals; phloem transports sucrose and amino acids | 8.1 |
| Phloem transport is one‑way (only downwards) | Phloem transports bidirectionally, depending on source‑sink positions | 8.1, 8.7 |
| Water must pass through cytoplasm to reach xylem | Most water moves between cells (apoplast pathway); some moves through cells (symplast) | 8.2, 8.3 |
| Potometer directly measures transpiration rate | Potometer measures water uptake rate, used as an estimate of transpiration | 8.5 |
| Plants absorb water by active transport | Water enters root hairs by osmosis, no direct energy cost | 8.2 |
| Transpiration occurs only during the day | Transpiration is highest during the day, but a small amount may occur at night through the cuticle | 8.3 |
| Translocation occurs in all plants | IGCSE focuses on flowering plants; other plant groups may differ | 8.7 |
| Ringing experiment proves xylem transports water | The experiment proves phloem transports organic nutrients; xylem water transport is unaffected because xylem is inside the wood | 8.7 |
| Wilting increases turgor pressure | Wilting cells lose turgor pressure (turgor decreases) | 8.6 |
- State the functions of xylem (water + minerals + support) and phloem (sucrose + amino acids)
- Identify xylem and phloem positions in root, stem and leaf cross‑sections
- Describe the adaptations of root hair cells (root hair, thin wall, many mitochondria, large vacuole, high solute concentration)
- Explain why root hair cells require many mitochondria (active transport of minerals needs ATP)
- State the pathway of water from soil to atmosphere
- Define transpiration and explain the role of mesophyll cells, air spaces and stomata
- Explain the cohesion‑tension theory (transpiration pull, cohesion, continuous water column)
- Describe the effects of light intensity, temperature, humidity and wind speed on transpiration rate
- Describe how a potometer works and the procedure for measuring transpiration rate
- Explain wilting (water loss > uptake) and the cellular changes (plasmolysis, loss of turgor)
- Define source and sink; give examples; explain how an organ can change from sink to source
- Compare sieve tube elements and companion cells (structure and function)
- Describe the ringing experiment and its conclusions (phloem transports sugars)
- Describe the dye experiment to trace water movement (xylem pathway)
This document covers all IGCSE Biology Topic 8 content for the 2026–2028 syllabus, based on 15+ past papers and official mark schemes. Detailed explanations and model answers are included. For other topics, please refer to the corresponding notes.