# Plant Tissues and Transport
Plants, like animals, need transport systems to move substances around their bodies. Unlike animals, plants do not have a heart or blood. Instead, they use two specialised vascular tissues — xylem and phloem — to transport water, minerals, and sugars. Understanding plant transport is a key part of GCSE Biology.
1. Plant Organs
Plants have three main organs:
| Organ | Functions |
|---|---|
| Roots | Absorb water and mineral ions from the soil; anchor the plant |
| Stem | Supports the plant; contains vascular bundles (xylem and phloem) for transport |
| Leaves | Main site of photosynthesis; gas exchange occurs through stomata |
Flowers are also plant organs (for reproduction).
2. Xylem — Water Transport
Xylem tissue transports water and dissolved mineral ions from the roots to the leaves and stems.
Structure
- Made of dead cells arranged end to end, forming long, continuous hollow tubes
- Cell walls are strengthened with lignin (a waterproof, rigid substance) — this provides support and prevents the tube from collapsing
- No cell contents (cytoplasm, nucleus, or end walls removed) — allows uninterrupted flow of water
How Water Moves Through Xylem
The movement of water through a plant follows this pathway:
This process is driven by transpiration (see below).
Transpiration
Transpiration is the loss of water vapour from the surface of a plant, primarily through the stomata in the leaves.
- Water evaporates from the surfaces of spongy mesophyll cells into the air spaces inside the leaf
- Water vapour diffuses out of the leaf through stomata
- This creates a pull (tension) that draws water up through the xylem vessels from the roots — called the transpiration stream
- More water is absorbed by the roots from the soil by osmosis to replace the water lost
Factors Affecting the Rate of Transpiration
| Factor | Effect on Transpiration Rate |
|---|---|
| Temperature ↑ | Increases — water molecules have more energy, evaporate faster |
| Light intensity ↑ | Increases — stomata open wider to allow CO₂ in for photosynthesis, more water escapes |
| Wind speed ↑ | Increases — moves humid air away from leaf surface, maintaining a steep diffusion gradient |
| Humidity ↑ | Decreases — the diffusion gradient between inside the leaf and outside is reduced |
Measuring Transpiration
A potometer is used to measure the rate of water uptake by a plant (as an estimate of transpiration rate).
- It measures the distance a bubble of air moves along a calibrated tube as the plant takes up water
- Rate can be calculated:
3. Phloem — Sugar Transport (Translocation)
Phloem tissue transports dissolved sugars (mainly sucrose) and amino acids from where they are made (sources) to where they are needed (sinks).
Structure
- Made of living cells called sieve tube elements
- Sieve tube elements have sieve plates (perforated end walls) allowing cytoplasm and dissolved substances to flow through
- Each sieve tube element has a companion cell alongside it — the companion cell provides energy (via mitochondria) and controls the sieve tube's functions
- Phloem cells have very little cytoplasm and no nucleus
Translocation
Translocation is the movement of dissolved sugars and other organic substances through phloem.
- Source: Where sugars are produced or released (e.g., leaves during photosynthesis, or storage organs releasing reserves)
- Sink: Where sugars are used or stored (e.g., growing tips, roots, fruits, developing seeds)
Translocation can occur in both directions (up and down the plant), unlike the one-directional flow in xylem.
4. Comparing Xylem and Phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Transports | Water and mineral ions | Dissolved sugars (sucrose) and amino acids |
| Direction | Upward only (roots → leaves) | Both directions (source → sink) |
| Cell type | Dead cells | Living cells (sieve tubes + companion cells) |
| Structure | Hollow tubes with lignified walls; no end walls | Sieve tube elements with sieve plates; companion cells |
| Mechanism | Transpiration pull (passive) | Translocation (requires energy — active process) |
5. Stomata and Guard Cells
Stomata (singular: stoma) are tiny pores on the surface of leaves (mainly on the lower epidermis).
Functions
- Allow carbon dioxide in for photosynthesis
- Allow oxygen out (produced during photosynthesis)
- Allow water vapour out (transpiration)
Guard Cells
Each stoma is surrounded by two guard cells that control its opening and closing.
- When turgid (full of water): Guard cells swell and curve, opening the stoma → gas exchange and transpiration occur
- When flaccid (lost water): Guard cells become straight and close the stoma → reduces water loss
- Stomata generally open during the day (for photosynthesis) and close at night (to conserve water)
6. Leaf Structure — Adaptations for Photosynthesis
The leaf is adapted for efficient photosynthesis and gas exchange:
| Layer | Adaptation |
|---|---|
| Waxy cuticle | Transparent (lets light through); waterproof (reduces water loss) |
| Upper epidermis | Transparent; allows light to pass through to mesophyll |
| Palisade mesophyll | Tall, column-shaped cells packed with chloroplasts; near the top of the leaf to absorb maximum light |
| Spongy mesophyll | Irregular cells with large air spaces between them; allows efficient gas exchange (CO₂ and O₂ diffusion) |
| Lower epidermis | Contains stomata for gas exchange |
| Vascular bundles | Contain xylem (water supply) and phloem (sugar transport) |
7. Root Hair Cells — Absorption
Root hair cells are specialised for absorbing water and mineral ions from the soil.
Adaptations
- Long hair-like extension — increases the surface area enormously for absorption
- Thin cell wall — short diffusion distance
- Large permanent vacuole — maintains a low water potential inside the cell
Water Absorption
Water enters root hair cells by osmosis — moving from the soil (higher water potential / more dilute) to the cell (lower water potential / more concentrated cell sap).
Mineral Ion Absorption
Mineral ions (e.g., nitrates, phosphates) are present in the soil at low concentrations. They are absorbed by active transport — this requires energy from respiration because ions move against their concentration gradient (from low concentration in soil to higher concentration in the root).
Worked Example
Question: A student uses a potometer to measure the rate of water uptake by a plant. In 10 minutes, the air bubble moves 35 mm. Calculate the rate of water uptake.
Solution:
Practice Questions
- Name the two types of transport tissue in plants and state what each transports. (4 marks)
- Explain how transpiration causes water to move up through a plant. (4 marks)
- State three factors that increase the rate of transpiration. (3 marks)
- Explain the difference between xylem and phloem in terms of cell structure. (4 marks)
- Describe how guard cells control the opening and closing of stomata. (3 marks)
Answers
Want to check your answers and get step-by-step solutions?
Exam Tips
- Xylem = dead, hollow, lignin, water UP. Phloem = living, sieve plates, companion cells, sugars BOTH WAYS.
- Transpiration is NOT the same as evaporation — transpiration specifically refers to water loss from plant surfaces through stomata.
- Remember: mineral ions are absorbed by active transport (not osmosis or diffusion) because they move against the concentration gradient.
- A potometer measures water uptake, not transpiration directly. Some water taken up is used in photosynthesis.
Frequently Asked Questions
Why does transpiration increase in windy conditions?
Wind blows away the layer of humid air surrounding the leaf surface. This maintains a steep concentration gradient for water vapour between the inside of the leaf (high humidity) and the outside (low humidity), so water diffuses out faster.
Do all leaves have stomata on the lower surface only?
Most leaves have more stomata on the lower epidermis to reduce direct exposure to sunlight (which would increase water loss). However, floating aquatic plants (like water lilies) have stomata on the upper surface. Some plants have stomata on both surfaces.
Why is translocation an active process?
Translocation in phloem requires energy because sugars must be actively loaded into sieve tube elements at the source (against a concentration gradient). This energy comes from the companion cells, which have many mitochondria for aerobic respiration.
Summary
- Plants transport water via xylem (dead tubes, lignin walls, passive transpiration pull) and sugars via phloem (living sieve tubes with companion cells, active translocation).
- Transpiration drives water movement upward; its rate depends on temperature, light, wind, and humidity.
- Stomata (controlled by guard cells) regulate gas exchange and water loss.
- Root hair cells absorb water by osmosis and minerals by active transport.
- Leaves are adapted for photosynthesis with palisade mesophyll, spongy mesophyll, and vascular bundles.