Transport In Plants

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Notes

Xylem & Phloem

**Xylem vessels** transport water and minerals from roots to stem and leaves, and provide support.
**Phloem vessels** transport food (mainly sucrose and amino acids) from leaves to other parts.
Xylem and phloem are arranged in **vascular bundles** throughout root, stem and leaves.
In cross-section, **xylem is always on the inside** and phloem on the outside.

Root Hair Cells

Root hair cells are found in the **epidermis** of plant roots.
They have **root hairs** that grow between soil particles, increasing surface area.
This adaptation aids **absorption of water and mineral ions** from the soil.

Pathway Taken by Water

Water enters root hair cells by **osmosis**.
It then moves through **root cortex cells** into **xylem vessels**.
Water is carried up the xylem to **leaf mesophyll cells**.
Pathway: root hair cell → root cortex cells → xylem → leaf mesophyll cells.

Transpiration

**Transpiration** is the loss of water vapour from leaves.
Water evaporates from **spongy mesophyll cells** into air spaces.
Water vapour then diffuses out through **stomata**.

Investigating Temperature & Wind Speed

Use a **potometer** to measure transpiration rate.
Cut shoot **underwater** to prevent air entering xylem.
Seal apparatus with **Vaseline** to make it airtight.
Introduce a single **air bubble** in the capillary tube.
Measure distance bubble moves in a set time (e.g. 20 min).
**Higher temperature** or **higher wind speed** increases transpiration rate.
Keep all other factors constant for a fair test.

Explaining Effects of Temperature, Wind Speed & Humidity

**High temperature** increases evaporation and diffusion rate, so transpiration increases.
**High wind speed** removes water vapour near leaf, maintaining concentration gradient, so transpiration increases.
**High humidity** reduces concentration gradient, so transpiration decreases.
If water loss exceeds uptake, **wilting** occurs – cells lose turgor and plant collapses.

Translocation

**Translocation** is the transport of sucrose and amino acids in phloem.
Glucose is converted to **sucrose** for transport.
Phloem cells are joined end-to-end with **holes in end walls** for easy flow.
Movement is from **source** (where produced) to **sink** (where used/stored).
Direction can change: e.g. in spring, storage organs are source; in summer, leaves are source.

Leaf cross-section showing cuticle, epidermis, palisade mesophyll, spongy mesophyll with air spaces, stoma and guard cells.

Practice questions

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1.Which of the following is the main function of xylem vessels?
Easy
- ATransport of sucrose and amino acids
- BTransport of water and minerals
- CTransport of carbon dioxide
- DTransport of oxygen
2.What is the function of root hair cells?
Easy
- ATo carry out photosynthesis
- BTo absorb water and mineral ions from the soil
- CTo transport sucrose to the leaves
- DTo provide structural support
3.Name the two types of transport vessels in plants.
Easy
4.Transpiration is the loss of water vapour from the leaves of plants.
Easy
True or false?
5.Complete the sentence about the pathway of water:
Medium
Water enters the root hair cells, passes through the root cortex, then enters $the ____$ vessels, and finally reaches the leaf mesophyll cells.
6.Match each plant structure with its correct function.
Medium
- Xylem
- Phloem
- Root hair cell
- Absorbs water and minerals
- Transports water and minerals
- Transports sucrose and amino acids
7.Arrange the following steps in the correct order for the pathway of water from soil to leaf.
Medium
- Root cortex cells
- Xylem vessels
- Root hair cells
- Leaf mesophyll cells
8.Which factor increases the rate of transpiration?
Medium
- ALow temperature
- BHigh humidity
- CHigh wind speed
- DLow light intensity

Slides

Xylem & Phloem

Plants have two types of transport vessel: xylem and phloem.
Xylem transports water and minerals from roots to leaves and provides support.
Phloem transports food (sucrose and amino acids) from leaves to other parts.
They are arranged in groups called vascular bundles.

Which vessel transports water and minerals from roots to leaves?

Phloem
Xylem
Both
Neither

Root Hair Cells

Root hair cells are found in the epidermis of plant roots.
They have extensions called root hairs that increase surface area.
This helps absorb water and mineral ions from the soil.

Root hair cells have ____ that increase surface area for absorption.

Pathway of Water

Water enters root hair cells by osmosis.
It then moves through root cortex cells into xylem vessels.
From xylem, water travels to leaf mesophyll cells.
Pathway: root hair cell → root cortex → xylem → leaf mesophyll.

What is the correct pathway of water from the soil to the leaf?

xylem → root hair → cortex → leaf
root hair → cortex → xylem → leaf
cortex → root hair → xylem → leaf
root hair → xylem → cortex → leaf

Transpiration

Transpiration is the loss of water vapour from leaves.
Water evaporates from spongy mesophyll cells into air spaces.
Water vapour then diffuses out through stomata.
It is driven by evaporation and diffusion.

Which of these are true about transpiration?

Water vapour is lost from leaves.
Transpiration occurs only at night.
Water evaporates from spongy mesophyll cells.
Stomata allow water vapour to exit.

Factors Affecting Transpiration Rate

Temperature: higher temperature increases transpiration rate.
Wind speed: higher wind speed increases transpiration rate.
Humidity: higher humidity decreases transpiration rate.
These factors affect the concentration gradient of water vapour.

Which factor decreases the rate of transpiration when it is high?

Temperature
Wind speed
Humidity
Light intensity

Investigating Transpiration with a Potometer

A potometer measures water uptake by a leafy shoot.
Cut shoot underwater to prevent air entering xylem.
Use Vaseline to seal joints and dry leaves before starting.
Introduce an air bubble and measure its movement over time.
Change one factor (temperature or wind speed) at a time.

In a potometer, the movement of $an ____$ over time indicates the rate of transpiration.

Wilting

Wilting occurs when water loss exceeds water uptake.
Cells lose turgor and the plant collapses.
It is a sign of water stress.

What causes wilting in plants?

Too much water in the soil
More water evaporating than available in soil
High humidity
Low temperature

Translocation

Translocation is the transport of sucrose and amino acids in phloem.
It moves from source (where produced) to sink (where used/stored).
Phloem cells have holes in end walls for easy flow.
Direction can change depending on season or growth stage.

Which statements about translocation are true?

Translocation occurs in xylem.
Sucrose and amino acids are transported.
Source is where substances are produced.
Sink is where substances are used or stored.

Summary of Transport in Plants

Xylem: water and minerals upward.
Phloem: sucrose and amino acids from source to sink.
Water enters via root hairs, moves through cortex to xylem.
Transpiration loses water vapour from leaves.
Factors: temperature, wind speed, humidity affect rate.

Which of the following is transported in phloem?

Water
Minerals
Sucrose
Oxygen

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