Transpiration & Plant Transport
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Water and Mineral Transport in Plants
- Plants have a specialised transport system to move water and minerals from the soil to all parts of the plant
- transpiration drives this continuous flow of water upward through the plant — it is the loss of water vapour from the surface of plant leaves through stomata
- Pathway: soil → root hair cells → xylem vessels → leaves → atmosphere
The pathway of water through a plant, from root hair cells to the atmosphere
Root Hair Cells
- Root hair cells are specialised cells that absorb water and minerals from the soil
- they have several adaptations that make them highly efficient at absorption
Structure and Adaptations
| Feature | Adaptation | Function |
|---|---|---|
| Long extension | Hair-like projection | Large surface area for absorption |
| Thin cell wall | Permeable membrane | Water and minerals enter easily by osmosis |
| Many mitochondria | Energy-producing organelles | Provide energy for active transport of minerals |
| Large vacuole | Concentrated cell sap | Maintains water potential gradient |
How Root Hair Cells Absorb Water and Minerals
- The general term for water and minerals entering the root hair cell from the soil solution is absorption
- at the molecular level, this absorption happens by osmosis (water) and active transport (minerals) — but if a question just asks for the overall process, "absorption" is the expected answer
- Water enters by osmosis
- movement from high to low water concentration
- Mineral salts are absorbed by active transport
- requires energy, and moves against the concentration gradient
How a root hair cell absorbs water by osmosis and minerals by active transport
Xylem Vessels
- Xylem vessels are long, hollow tubes made of dead cells that transport water and minerals upward from roots to leaves
- their structure is perfectly adapted for efficient water transport
Structure and Function
| Feature | Description | Function |
|---|---|---|
| Hollow tubes | Dead cells with no cytoplasm | No obstruction to water flow |
| Lignin walls | Thick, woody strengthening material | Prevents collapse under pressure; provides support |
| No end walls | Continuous column from roots to leaves | Uninterrupted flow of water |
| Narrow diameter | Thin tubes | Capillary action helps pull water upward |
- Xylem transports water and minerals upward only, from roots to leaves
Transpiration Process
- Transpiration is the process by which water is lost from leaves
- this water loss creates a pulling force that draws more water up from the roots through the xylem
How Transpiration Works
- Water evaporates from mesophyll cells inside the leaf
- Water vapour diffuses out through stomata, tiny pores on the leaf surface
- this creates a "pull" that draws more water up from the roots
The Transpiration Stream
- The transpiration stream is the continuous column of water moving up through the xylem
- Step 1: water evaporates from leaf cells (mesophyll)
- Step 2: water vapour exits through stomata
- Step 3: this creates a "pull" on the water in the xylem
- Step 4: water moves up the xylem, like sucking through a straw
- Step 5: more water is absorbed by root hair cells to replace it
How water evaporates from the mesophyll and diffuses out through the stomata
Stomata and Guard Cells
- Stomata (singular: stoma) are tiny pores on the leaf surface, mainly on the underside
- Guard cells are pairs of specialised cells surrounding each stoma that control opening and closing
Stomatal Opening and Closing
| Condition | Guard Cell State | Stomata | Why? |
|---|---|---|---|
| Daytime / Light | Turgid (full of water) | OPEN | Allow CO₂ in for photosynthesis |
| Nighttime / Dark | Flaccid (lose water) | CLOSED | Reduce water loss (no photosynthesis happening) |
| Hot / Dry conditions | Flaccid | CLOSED | Prevent excessive water loss |
How turgid and flaccid guard cells open and close the stoma
Factors Affecting Transpiration Rate
- Several environmental factors influence the rate at which water is lost through transpiration
- understanding these factors helps explain how plants adapt to different conditions
| Factor | Effect on Rate | Explanation |
|---|---|---|
| Temperature | Higher temperature = faster rate | More evaporation energy; faster diffusion |
| Humidity | Higher humidity = slower rate | Smaller water concentration gradient |
| Wind Speed | Higher wind = faster rate | Removes water vapour from leaf surface, maintaining the gradient |
| Light Intensity | More light = faster rate | Stomata open wider for photosynthesis |
Essential Plant Minerals
- Plants absorb mineral salts from soil through root hair cells to maintain healthy growth
- different minerals have specific roles, and deficiency causes visible symptoms
Key Minerals and Their Functions
| Mineral | Chemical Form | Function | Deficiency Symptoms |
|---|---|---|---|
| Nitrates | NO₃⁻ | Make proteins and DNA for growth | Stunted growth, yellow older leaves |
| Magnesium | Mg²⁺ | Make chlorophyll for photosynthesis | Yellow leaves (chlorosis) |
| Phosphates | PO₄³⁻ | Make DNA and cell membranes; respiration | Poor root growth, purple leaves |
| Potassium | K⁺ | Enzyme activation, photosynthesis | Yellow leaves with dead spots |
- Nitrates are needed to make proteins, for growth
- Magnesium is needed to make chlorophyll, for photosynthesis
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