B9: Plant Organisation
Transpiration, translocation, plant tissues, and stomata
Key Definitions
Transpiration — The loss of water vapour from the leaves of a plant through the stomata when they are open for gas exchange.
Transpiration stream — The continuous movement of water through the plant from the roots to the leaves, driven by transpiration.
Translocation — The transport of dissolved sugars (mainly sucrose) and amino acids through the phloem from source (e.g. leaves) to sink (e.g. roots, fruits, growing shoots).
Stomata — Tiny pores on the underside of leaves, surrounded by guard cells, that allow gas exchange (CO₂ in, O₂ and water vapour out).
Transpiration stream — The continuous movement of water through the plant from the roots to the leaves, driven by transpiration.
Translocation — The transport of dissolved sugars (mainly sucrose) and amino acids through the phloem from source (e.g. leaves) to sink (e.g. roots, fruits, growing shoots).
Stomata — Tiny pores on the underside of leaves, surrounded by guard cells, that allow gas exchange (CO₂ in, O₂ and water vapour out).
Plant Tissues and Their Functions
| Tissue | Function | Key Features |
|---|---|---|
| Epidermal tissue | Covers and protects the plant | Waxy cuticle on upper surface reduces water loss; guard cells control stomata |
| Palisade mesophyll | Photosynthesis | Column-shaped cells packed with chloroplasts; near upper leaf surface for maximum light |
| Spongy mesophyll | Gas exchange within the leaf | Irregular cells with large air spaces; allows CO₂, O₂, and water vapour to diffuse |
| Xylem | Transport of water and minerals; support | Dead, hollow tubes; lignin-strengthened walls; no end walls |
| Phloem | Transport of sugars (translocation) | Living cells with sieve plates; companion cells provide energy |
| Meristem | Growth (cell division) | Unspecialised cells at root/shoot tips; divide by mitosis; can differentiate into any cell type |
Plant Organs
Roots — Anchor the plant; absorb water and mineral ions from the soil (root hair cells increase SA).
Stems — Support the plant; transport water and sugars (vascular bundles containing xylem and phloem).
Leaves — Main organ of photosynthesis; also the site of transpiration and gas exchange.
Vascular bundles run through roots, stems, and leaves. In a stem, the arrangement is typically:
— Xylem on the inside (towards the centre) — transports water upward.
— Phloem on the outside — transports sugars up and down.
— Cambium between them — contains meristem cells that can produce new xylem and phloem.
Stems — Support the plant; transport water and sugars (vascular bundles containing xylem and phloem).
Leaves — Main organ of photosynthesis; also the site of transpiration and gas exchange.
Vascular bundles run through roots, stems, and leaves. In a stem, the arrangement is typically:
— Xylem on the inside (towards the centre) — transports water upward.
— Phloem on the outside — transports sugars up and down.
— Cambium between them — contains meristem cells that can produce new xylem and phloem.
Transpiration and the Transpiration Stream
How transpiration works:
1. Water enters the root hair cells by osmosis from the soil.
2. Water moves through the root cortex cells by osmosis.
3. Water enters the xylem and is pulled upward through the stem to the leaves.
4. Water exits the xylem and moves through the leaf cells by osmosis.
5. Water evaporates from the spongy mesophyll cell surfaces inside the leaf.
6. Water vapour diffuses out of the leaf through the stomata (down a concentration gradient).
Transpiration pull: The loss of water from the leaves creates a negative pressure (tension) that pulls the whole column of water upward through the xylem from the roots. This is a passive process.
1. Water enters the root hair cells by osmosis from the soil.
2. Water moves through the root cortex cells by osmosis.
3. Water enters the xylem and is pulled upward through the stem to the leaves.
4. Water exits the xylem and moves through the leaf cells by osmosis.
5. Water evaporates from the spongy mesophyll cell surfaces inside the leaf.
6. Water vapour diffuses out of the leaf through the stomata (down a concentration gradient).
Transpiration pull: The loss of water from the leaves creates a negative pressure (tension) that pulls the whole column of water upward through the xylem from the roots. This is a passive process.
Exam tip: Transpiration is NOT the same as translocation. Transpiration = water movement through xylem (upward only). Translocation = sugar movement through phloem (both directions). Do not confuse them!
Factors Affecting Transpiration Rate
| Factor | Effect on Transpiration Rate | Explanation |
|---|---|---|
| Temperature ↑ | Rate increases | Higher temperature gives water molecules more kinetic energy, so they evaporate from mesophyll cells faster; also increases the concentration gradient between inside and outside the leaf |
| Humidity ↑ | Rate decreases | High humidity means the air outside the leaf is already saturated with water vapour, reducing the concentration gradient between inside and outside the leaf, so water vapour diffuses out more slowly |
| Wind speed ↑ | Rate increases | Wind carries water vapour away from the leaf surface, maintaining a steep concentration gradient between inside and outside the leaf; faster wind = steeper gradient = faster diffusion |
| Light intensity ↑ | Rate increases | Light causes guard cells to become turgid, opening the stomata wider; more open stomata means more water vapour can escape; also increases photosynthesis so more gas exchange needed |
Example 1: Explaining the Effect of Temperature on Transpiration
On a hot day, water molecules inside the leaf have more kinetic energy and evaporate more rapidly from the spongy mesophyll cell surfaces. This increases the water vapour concentration inside the leaf air spaces, creating a steeper concentration gradient between the inside of the leaf and the outside air. As a result, water vapour diffuses out through the stomata faster, increasing the transpiration rate.
Example 2: Explaining the Effect of Humidity on Transpiration
On a humid day, the air outside the leaf already contains a lot of water vapour. This reduces the concentration gradient between the water vapour inside the leaf (high) and outside the leaf (also high). With a smaller gradient, water vapour diffuses out of the stomata more slowly, so the transpiration rate decreases. In very humid conditions, transpiration almost stops.
Example 3: Explaining the Effect of Wind on Transpiration
In still air, water vapour that diffuses out of the stomata builds up around the leaf surface, reducing the concentration gradient and slowing transpiration. Wind carries this water vapour away, maintaining a steep concentration gradient between inside and outside the leaf, so water vapour continues to diffuse out rapidly. Stronger wind removes water vapour faster, increasing the transpiration rate.
Translocation
Translocation is the transport of dissolved sugars (mainly sucrose) and amino acids through the phloem:
— Source — Where sugars are made or released (e.g. leaves during photosynthesis, or storage organs like potato tubers when sprouting).
— Sink — Where sugars are used or stored (e.g. growing root tips, developing fruits, storage organs being filled).
— Movement is in both directions (up and down) depending on the needs of the plant.
— Translocation requires energy (ATP) from companion cells — it is an active process, unlike transpiration.
— The phloem is made of living cells (unlike xylem which is dead).
— Source — Where sugars are made or released (e.g. leaves during photosynthesis, or storage organs like potato tubers when sprouting).
— Sink — Where sugars are used or stored (e.g. growing root tips, developing fruits, storage organs being filled).
— Movement is in both directions (up and down) depending on the needs of the plant.
— Translocation requires energy (ATP) from companion cells — it is an active process, unlike transpiration.
— The phloem is made of living cells (unlike xylem which is dead).
Example 4: Source and Sink in Translocation
In summer, the leaves (source) produce glucose by photosynthesis, which is converted to sucrose and transported via the phloem to growing root tips (sink) where it is used for growth, and to developing fruits (sink) where it is stored. In spring, when leaves have not yet grown, stored starch in potato tubers or root stores (now the source) is broken down to sucrose and transported upward through the phloem to growing shoots (sink).
Stomata and Guard Cells
Guard cells control the opening and closing of stomata:
— When guard cells are turgid (full of water), they swell and the stomata open — allowing CO₂ in for photosynthesis and water vapour out (transpiration).
— When guard cells are flaccid (low water), they become limp and the stomata close — reducing water loss by transpiration.
— Stomata typically open in the light (for photosynthesis) and close in the dark.
— In drought conditions, stomata close to conserve water, even though this reduces CO₂ intake and slows photosynthesis.
Guard cell shape: The inner wall of each guard cell is thicker and less flexible than the outer wall. When water enters, the outer wall stretches more, causing the guard cell to bend outward and open the stoma.
— When guard cells are turgid (full of water), they swell and the stomata open — allowing CO₂ in for photosynthesis and water vapour out (transpiration).
— When guard cells are flaccid (low water), they become limp and the stomata close — reducing water loss by transpiration.
— Stomata typically open in the light (for photosynthesis) and close in the dark.
— In drought conditions, stomata close to conserve water, even though this reduces CO₂ intake and slows photosynthesis.
Guard cell shape: The inner wall of each guard cell is thicker and less flexible than the outer wall. When water enters, the outer wall stretches more, causing the guard cell to bend outward and open the stoma.
Exam tip: Stomata are mainly on the underside of the leaf, not the top. This reduces water loss because the lower surface is cooler and less exposed to direct sunlight and wind. When asked about guard cells, mention turgid = open, flaccid = closed.
Practice Questions
1. Foundation Describe the transpiration stream in plants.
Water is absorbed by root hair cells by osmosis, moves through root cortex cells, enters the xylem, and is pulled upward through the stem to the leaves. Water evaporates from spongy mesophyll cell surfaces inside the leaf, and the water vapour diffuses out through the stomata. The loss of water from the leaves creates a pull (tension) that draws the whole column of water upward.
2. Foundation Explain how increasing temperature affects the rate of transpiration.
Higher temperature gives water molecules more kinetic energy, so they evaporate faster from the mesophyll cell surfaces inside the leaf. This increases the concentration of water vapour inside the leaf, creating a steeper concentration gradient between the inside and outside, so water vapour diffuses out through the stomata faster — increasing the transpiration rate.
3. Higher Explain the difference between transpiration and translocation.
Transpiration is the movement of water (and dissolved minerals) upward through the xylem from roots to leaves, driven by evaporation and the transpiration pull — it is a passive process. Translocation is the movement of dissolved sugars (sucrose) and amino acids through the phloem from source to sink, in both directions — it is an active process requiring energy from companion cells. Xylem cells are dead; phloem cells are living.
4. Higher Explain how guard cells open and close stomata.
When guard cells absorb water, they become turgid. The inner wall is thicker and less flexible than the outer wall, so the outer wall stretches more, causing the guard cells to bend outward and open the stoma. When guard cells lose water, they become flaccid and the stomata close. In the light, guard cells pump in potassium ions, water follows by osmosis, and the stomata open for gas exchange. In the dark or during drought, the reverse happens and stomata close to conserve water.
5. Foundation Name three plant organs and state the function of each.
1) Roots — anchor the plant and absorb water and mineral ions from the soil. 2) Stems — support the plant and transport water and sugars through vascular bundles. 3) Leaves — carry out photosynthesis, transpiration, and gas exchange.
🔢 Maths Skills
Mathematical Skills
Calculating transpiration rate from data: rate = mass of water lost ÷ time. For example, if a plant loses 3.2 g of water in 4 hours, the rate = 3.2 ÷ 4 = 0.8 g/hour. You may also need to interpret transpiration rate graphs — compare rates under different conditions (temperature, humidity, wind speed, light intensity) and explain the differences using concentration gradients.
⚠️ Common Misconceptions
Watch Out!
Students often think plants absorb water through their leaves. Wrong: Water enters the plant through the leaves Correct: Water is absorbed through root hair cells by osmosis from the soil
Students often think xylem and phloem do the same thing. Wrong: Xylem and phloem both transport the same substances Correct: Xylem transports water and mineral ions upward only; phloem transports dissolved sugars in both directions (translocation)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain xylem and phloem adaptations and describe the transpiration stream.
Xylem is adapted to transport water and minerals from roots to leaves: it consists of dead cells forming hollow tubes with no cytoplasm, allowing free water flow; walls are strengthened with lignin, which provides structural support and waterproofing; there are no end walls between cells, creating a continuous column. Phloem is adapted to transport dissolved sugars (translocation): it consists of living cells with sieve plates that have pores allowing sap to flow; companion cells provide energy (ATP) for active transport of sugars into and out of the phloem. The transpiration stream is the continuous movement of water through the plant: water enters root hair cells by osmosis, moves through root cortex cells, travels up the xylem, evaporates from spongy mesophyll cell surfaces, and water vapour diffuses out through stomata. The loss of water from leaves creates a transpiration pull that draws the whole water column upward.
Mark scheme: 1 mark for each xylem adaptation explained (max 2); 1 mark for each phloem adaptation explained (max 2); 1 mark for describing the transpiration stream pathway; 1 mark for explaining transpiration pull
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Interpret transpiration rate data at different conditions. For example, a potometer shows water uptake of 0.5 cm³/h at 20°C, 1.2 cm³/h at 30°C, and 0.2 cm³/h at 20°C with high humidity. Explain: at 30°C, higher temperature gives water molecules more kinetic energy, increasing evaporation and the concentration gradient, so transpiration rate increases. With high humidity, the air outside the leaf already has high water vapour content, reducing the concentration gradient, so transpiration rate decreases.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.