B7: Plant Organisation
Plant tissues, organs and transport systems
B7 Plant Organisation
📋 Key Concepts
Plants are organised into tissues, organs and transport systems, just like animals. Plant tissues include epidermal, mesophyll, xylem and phloem. Water and minerals are transported through xylem; sugars are transported through phloem. Transpiration drives the movement of water through the plant.
Key Terms
- Epidermal tissue - covers and protects plant surfaces
- Palisade mesophyll - tissue specialised for photosynthesis
- Spongy mesophyll - tissue specialised for gas exchange
- Xylem - transports water and dissolved minerals
- Phloem - transports dissolved sugars (translocation)
- Transpiration - water loss from plant leaves through stomata
- Translocation - movement of sugars through phloem
- Stomata - pores in the leaf for gas exchange
📝 Plant Tissues
Plant tissues are groups of similar cells with the same function. Each tissue in a plant is specialised for its role, from photosynthesis to transport to protection.
| Tissue | Function | Key Features |
|---|---|---|
| Epidermal tissue | Covers and protects surfaces of leaves, stems and roots | Thin, flat cells forming a continuous layer; covered by waxy cuticle on leaves to reduce water loss |
| Palisade mesophyll | Photosynthesis (main site) | Tall, column-shaped cells; packed with chloroplasts; positioned near upper surface of leaf for maximum light |
| Spongy mesophyll | Gas exchange within the leaf | Rounded, loosely-packed cells; large air spaces between cells allow gases (CO₂, O₂, water vapour) to diffuse freely |
| Xylem | Transports water and dissolved minerals from roots to leaves | Dead cells forming continuous hollow tubes; walls strengthened with lignin (waterproof and provides support); no cytoplasm or end walls |
| Phloem | Transports dissolved sugars (translocation) from leaves to growing and storage tissues | Living cells; sieve tube elements with perforated sieve plates; companion cells provide energy for transport |
Example 1
Question: Compare xylem and phloem tissues. (4 marks)
Answer: Xylem transports water and minerals; phloem transports sugars (1). Xylem is made of dead cells forming hollow tubes; phloem is made of living cells (2). Xylem is strengthened with lignin; phloem has sieve plates and companion cells (3). Xylem carries water in one direction (upwards); phloem carries sugars in both directions (up and down) (4).
📝 Plant Organs
Plant organs include roots, stems and leaves. Each organ is made of several tissues working together to perform a specific function.
| Organ | Function | Key Tissues |
|---|---|---|
| Roots | Anchor the plant; absorb water and minerals from soil | Root hair cells (increased surface area for absorption); xylem and phloem (transport) |
| Stems | Support the plant; transport water and sugars between roots and leaves | Xylem (water transport up); phloem (sugar transport both ways); structural tissue for support |
| Leaves | Photosynthesis; gas exchange | Palisade mesophyll (photosynthesis); spongy mesophyll (gas exchange); epidermal tissue with stomata (gas exchange and transpiration) |
Root Hair Cells
- Root hair cells are specialised cells in the root epidermis
- They have a long, thin extension (the "hair") that greatly increases the surface area
- This larger surface area allows more water and minerals to be absorbed from the soil
- The thin wall of the root hair means there is a short diffusion path for water and minerals
Example 2
Question: Explain how root hair cells are adapted for their function. (3 marks)
Answer: Root hair cells have a long, thin projection (root hair) that greatly increases the surface area for absorption of water and minerals from the soil. The thin cell wall creates a short diffusion path, allowing substances to enter quickly. They are also located in large numbers on the root surface, providing a very large total surface area for uptake.
📝 Transpiration
Transpiration is the loss of water vapour from the leaves of a plant through the stomata. It is a continuous process that drives the movement of water from the roots to the leaves through the xylem.
How Transpiration Works
- Water evaporates from the cell walls of the spongy mesophyll cells inside the leaf
- The water vapour diffuses out through the stomata into the surrounding air (down a concentration gradient)
- This creates a water potential gradient from the roots to the leaves
- Water is pulled up through the xylem vessels from the roots to replace the water lost
- This pulling force is called the transpiration stream
Transpiration stream:
Roots (absorb water) → Xylem (transports water up) → Mesophyll cells (water evaporates) → Stomata (water vapour diffuses out)
Water moves UP the plant because water is lost at the leaves and must be replaced.
Roots (absorb water) → Xylem (transports water up) → Mesophyll cells (water evaporates) → Stomata (water vapour diffuses out)
Water moves UP the plant because water is lost at the leaves and must be replaced.
Stomata and guard cells: Stomata are pores on the underside of the leaf, each surrounded by two guard cells. When guard cells are turgid (full of water), the stoma opens, allowing gas exchange and transpiration. When guard cells are flaccid (low on water), the stoma closes, reducing water loss. This is how plants control transpiration rate.
Example 3
Question: Describe the process of transpiration and explain why water moves up through the xylem. (4 marks)
Answer: Water evaporates from the spongy mesophyll cell surfaces inside the leaf. The water vapour then diffuses out through the stomata into the air, down a concentration gradient. This loss of water creates a water potential gradient from the roots to the leaves. Water is pulled up through the xylem vessels from the roots to replace the water lost by transpiration. This continuous upward flow of water is the transpiration stream.
📝 Factors Affecting Transpiration Rate
Four main factors affect the rate of transpiration: temperature, humidity, wind speed and light intensity. Understanding these helps explain why plants lose more water in some conditions than others.
| Factor | Effect on Transpiration Rate | Explanation |
|---|---|---|
| Temperature ↑ | Transpiration rate ↑ (increases) | Higher temperature gives water molecules more kinetic energy, so they evaporate and diffuse faster from the mesophyll cells and through the stomata |
| Humidity ↑ | Transpiration rate ↓ (decreases) | Higher humidity means the air outside the leaf already contains a lot of water vapour, so the concentration gradient between inside the leaf and outside is smaller, reducing diffusion rate |
| Wind speed ↑ | Transpiration rate ↑ (increases) | Wind blows away water vapour from just outside the leaf, maintaining a steep concentration gradient between the inside and outside of the leaf, so diffusion is faster |
| Light intensity ↑ | Transpiration rate ↑ (increases) | Higher light intensity causes the stomata to open wider (for CO₂ entry for photosynthesis), which also allows more water vapour to escape |
Remember: Transpiration is fastest in hot, dry, windy and bright conditions. Transpiration is slowest in cool, humid, still and dark conditions. Think about when a washing line dries fastest - the same conditions apply!
Example 4
Question: A plant is placed in a hot, dry, windy environment. Explain why its transpiration rate will be high. (3 marks)
Answer: The hot temperature gives water molecules more kinetic energy so they evaporate from the mesophyll cells faster. The dry conditions (low humidity) mean there is a steep concentration gradient for water vapour between the inside and outside of the leaf, so water vapour diffuses out faster. The wind blows away water vapour from just outside the leaf, maintaining the steep concentration gradient.
Example 5
Question: Explain why a plant might close its stomata during the hottest part of the day. (2 marks)
Answer: Closing the stomata reduces water loss by transpiration, which helps prevent the plant from wilting and drying out. However, this also limits the amount of carbon dioxide entering the leaf, which reduces the rate of photosynthesis.
📝 Translocation
Translocation is the movement of dissolved sugars (mainly sucrose) and other organic compounds through the phloem from the leaves (where they are made by photosynthesis) to the rest of the plant. It occurs in both directions (up and down).
Key Facts About Translocation
- It moves sugars from source (where sugars are made, e.g. leaves) to sink (where sugars are used or stored, e.g. roots, fruits, growing tips)
- It requires energy from respiration (unlike water transport in xylem)
- The companion cells in the phloem provide the energy needed
- Translocation can go both upwards and downwards in the plant
- It is a continuous process while the plant is photosynthesising
| Feature | Xylem (Water Transport) | Phloem (Translocation) |
|---|---|---|
| What is transported | Water and dissolved minerals | Dissolved sugars (sucrose) |
| Direction | One direction only (upwards) | Both directions (up and down) |
| Energy needed? | No (passive - driven by transpiration) | Yes (active process - requires energy from respiration) |
| Cells alive or dead? | Dead cells | Living cells |
| Process name | Transpiration stream | Translocation |
❓ Practice Questions
Q1: Name the plant tissue responsible for photosynthesis and describe two adaptations of its cells. (3 marks)
Q2: Describe the process of transpiration. (3 marks)
Q3: Explain how increasing wind speed affects the rate of transpiration. (3 marks)
Q4: Compare xylem and phloem in terms of what they transport, the direction of transport, and whether energy is required. (3 marks)
Q5: Explain how root hair cells are adapted for absorbing water from the soil. (3 marks)
Q6: A gardener notices that their plants wilt more on hot, windy days than on cool, still days. Explain this observation using your knowledge of transpiration. (4 marks)
✅ Answers
- Palisade mesophyll. Adaptation 1: Cells are tall and column-shaped, packed with chloroplasts to absorb maximum light for photosynthesis. Adaptation 2: Positioned near the upper surface of the leaf where light intensity is highest. (Also accept: closely packed to maximise the number of photosynthetic cells in the light path.)
- Water evaporates from the cell walls of the spongy mesophyll cells inside the leaf. The water vapour diffuses out through the stomata into the surrounding air, down a concentration gradient. This loss of water creates a pull that draws more water up through the xylem from the roots (the transpiration stream).
- Increasing wind speed blows away water vapour from the air just outside the leaf. This maintains a steep concentration gradient of water vapour between the inside of the leaf (high concentration) and the outside air (low concentration after water vapour is blown away). The steeper gradient means water vapour diffuses out of the stomata faster, increasing the transpiration rate.
- Xylem transports water and dissolved minerals; phloem transports dissolved sugars (sucrose). Xylem transports in one direction only (upwards from roots to leaves); phloem transports in both directions (up and down). Xylem transport does not require energy (it is passive, driven by transpiration); phloem transport requires energy from respiration (it is an active process).
- Root hair cells have a long, thin projection (the root hair) which greatly increases the surface area for absorption of water from the soil. The thin wall of the root hair creates a short diffusion path, allowing water to enter the cell quickly by osmosis. There are many root hair cells on each root, providing a large total surface area for water uptake.
- On hot, windy days, the transpiration rate is very high. The high temperature gives water molecules more kinetic energy so they evaporate faster from the mesophyll cells. The wind blows away water vapour from outside the leaf, maintaining a steep concentration gradient so water vapour diffuses out of the stomata faster. The plant loses water faster than it can absorb it from the soil, so the cells lose turgor pressure and the plant wilts. On cool, still days, the transpiration rate is low, so the plant can replace water as fast as it is lost and remains turgid.
🧭 Board Focus — AQA
Spec 8461. Two 1h45 written papers (100 marks each), Foundation/Higher tiers. Ten required practicals are examined inside the papers — learn the method, variables and graph work for each. Paper 1 covers Cell biology, Organisation, Infection and response, Bioenergetics; Paper 2 covers Homeostasis, Inheritance, Variation and evolution, Ecology.
Always check: specification details change — confirm paper structure and set texts against the current specification for your exam year.
🎯 Exam Tips
- Learn the difference between transpiration (water loss through stomata) and translocation (sugar transport through phloem)
- Xylem = UP only, dead cells, no energy; Phloem = BOTH directions, living cells, energy needed
- For transpiration questions, always mention the concentration gradient and diffusion
- When explaining factors, say what happens to the concentration gradient (steeper = faster transpiration)
- Root hair cells increase surface area - this is the most common answer for this topic
- Remember stomata close to reduce water loss, but this also reduces CO₂ entry and therefore photosynthesis
- Translocation requires energy from respiration - this is a key difference from xylem transport
- Use the "source to sink" terminology for translocation in Higher tier questions
🔢 Maths Skills
Mathematical Skills
Transpiration rate = water loss ÷ time
Often measured using a potometer: rate = distance moved by bubble ÷ time
Often measured using a potometer: rate = distance moved by bubble ÷ time
Maths Example
In a potometer, the air bubble moves 48 mm in 20 minutes. Calculate the rate of water uptake.
Rate = 48 ÷ 20 = 2.4 mm/min
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Plants absorb water through their leaves Correct: Water is absorbed through root hair cells — the leaves are where water exits via transpiration
2. Wrong: Xylem and phloem do the same thing Correct: Xylem transports water and minerals upwards only (passive, dead cells); phloem transports sugars in both directions (active, living cells)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the adaptations of xylem and phloem and how the transpiration stream works.
Xylem vessels are made of dead cells forming hollow tubes, strengthened with lignin which makes them waterproof and provides structural support. This allows water and dissolved minerals to flow freely upwards from roots to leaves without obstruction. Phloem is made of living cells — sieve tube elements with perforated sieve plates allow dissolved sugars to flow through, and companion cells provide energy (ATP) for active transport of sugars (translocation). The transpiration stream works because water evaporates from the spongy mesophyll cell surfaces inside the leaf. Water vapour diffuses out through the stomata down a concentration gradient. This loss of water creates a pull that draws more water up through the xylem from the roots, replacing the water lost. This continuous flow is the transpiration stream.
Mark scheme: 1 mark for xylem adaptations (dead/hollow/lignin); 1 mark for xylem function; 1 mark for phloem adaptations (living/sieve plates/companion cells); 1 mark for phloem function; 1 mark for transpiration mechanism (evaporation + diffusion); 1 mark for transpiration stream pull
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student measured transpiration rate under different conditions:
| Condition | Rate (arbitrary units) |
|---|---|
| 20 °C, still air, humid | 3 |
| 20 °C, still air, dry | 5 |
| 30 °C, still air, dry | 9 |
| 30 °C, windy, dry | 14 |
| 30 °C, windy, dry, dark (stomata closed) | 2 |
(a) Explain the increase from 5 to 9 units.
(b) Explain why the rate drops to 2 in the final condition despite high temperature and wind.
Answers: (a) Increasing temperature from 20 °C to 30 °C gives water molecules more kinetic energy, so they evaporate from mesophyll cells faster, increasing the transpiration rate. (b) In the dark, stomata close to reduce water loss. With stomata closed, water vapour cannot diffuse out of the leaf, so even though temperature and wind favour high transpiration, the closed stomata block the exit route for water vapour, drastically reducing the rate.
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