Plant Organ Systems and Transport
Plant Organs
Plants are organised into organs and organ systems just like animals:
- Roots — anchor the plant and absorb water and mineral ions from the soil
- Stem — supports the plant and transports substances between roots and leaves
- Leaves — the main site of photosynthesis
- Flowers — reproductive organs
Leaf Structure
The leaf is an organ adapted for photosynthesis and gas exchange:
| Layer | Features and function |
|---|---|
| Waxy cuticle | Waterproof layer on upper surface; reduces water loss |
| Upper epidermis | Transparent; lets light through to the palisade layer |
| Palisade mesophyll | Packed with chloroplasts; main site of photosynthesis; near upper surface for maximum light |
| Spongy mesophyll | Air spaces between cells allow gases (CO₂ and O₂) to diffuse easily; some photosynthesis occurs here |
| Lower epidermis | Contains stomata (pores) controlled by guard cells |
| Vascular bundle | Contains xylem (water in) and phloem (sugars out) |
Stomata and Guard Cells
Stomata are tiny pores mostly on the underside of leaves. They allow:
- CO₂ to diffuse in for photosynthesis
- O₂ to diffuse out (waste product of photosynthesis)
- Water vapour to escape — this is called transpiration
Guard cells control the opening and closing of stomata:
- In light (daytime) — guard cells absorb water by osmosis, become turgid, and bend apart → stoma opens → gas exchange occurs
- In darkness (night) — guard cells lose water, become flaccid → stoma closes → reduces water loss when photosynthesis cannot occur
Transport in Plants — Xylem
Xylem transports water and dissolved mineral ions from the roots to the leaves.
The movement of water through a plant follows this pathway:
Soil → root hair cells → root cortex → xylem → stem → leaves → stomata → atmosphere
The loss of water from the leaves is called transpiration. This creates a continuous column of water being pulled up through the xylem — the transpiration stream.
Factors affecting the rate of transpiration:
| Factor | Effect on transpiration rate |
|---|---|
| Temperature | Higher temp = faster evaporation = faster transpiration |
| Humidity | Higher humidity = lower concentration gradient = slower transpiration |
| Wind speed | Higher wind = carries water vapour away = faster transpiration |
| Light intensity | More light = stomata open wider = faster transpiration |
Required Practical: Transpiration Rates Using a Potometer
Aim: Investigate the effect of one factor (e.g. wind speed, light, temperature) on the rate of transpiration.
Method:
1. Set up a potometer — a piece of apparatus that measures water uptake by a plant shoot
2. Cut the shoot underwater to prevent air entering the xylem
3. Record how far the air bubble in the capillary tube moves in a set time under different conditions (e.g. with and without a fan for wind)
4. The distance moved ÷ time = rate of water uptake (a proxy for transpiration rate)
Note: A potometer measures water uptake, not transpiration directly, but most water taken up is lost through transpiration.
Transport in Plants — Phloem
Phloem transports dissolved sugars (mainly sucrose) and amino acids from the leaves (where they are made by photosynthesis) to the rest of the plant. This is called translocation.
Key differences from xylem transport:
| Feature | Xylem | Phloem |
|---|---|---|
| Substance transported | Water and minerals | Dissolved sugars and amino acids |
| Direction | Roots → leaves (upward) | Leaves → rest of plant (up and down) |
| Cells alive or dead? | Dead | Alive |
| Process name | Transpiration stream | Translocation |
| Energy required? | No (passive — driven by transpiration) | Yes (active process — companion cells use energy) |
Root Hair Cells and Absorption
Root hair cells are adapted for absorbing water and mineral ions:
- Water is absorbed by osmosis — the soil solution is more dilute than the cell sap in the root hair cell
- Mineral ions (e.g. nitrates, magnesium) are absorbed by active transport because the concentration of minerals is usually higher in the root hair cell than in the soil
Exam Tips
- Know the transpiration pathway from soil to atmosphere
- In the potometer practical, cutting the stem underwater is essential — always state why (prevents air bubbles blocking the xylem)
- Translocation is an active process (uses energy from companion cells), whereas the transpiration stream is passive