transport in plants prac notes

Function of the xylem

Transports water and mineral ions and provides support. Water absorbed by the roots is transported through the xylem towards the leaves.

function of phloem

Phloem transports sucrose and amino acids , the movement of these substances through phloem is called translocation

Xylem adaptions (3)

Thick walls containing lignin

Lignin strengthens the walls.

This:

• prevents the xylem vessel from collapsing

• provides mechanical support

No cell contents

The inside of the vessel is hollow.

This provides space for water to move through.

Cells joined end-to-end with no cross walls

The cells form a long continuous tube.

The absence of cross walls means there are no barriers between the cells, allowing water to move continuously through the vessel.

investigate the pathway taken by water through the above-ground parts of a plant.

Principle

A coloured dye is added to water.

The plant takes up the water, and the dye makes the pathway visible.

The stained tissue can then be identified as xylem.

Method

1. Place the cut end of a plant stem into coloured water.

2. Leave the plant for a suitable period.

3. Remove the stem.

4. Cut a thin cross-section through the stem.

5. Examine the section.

6. Identify the stained tissue.

Expected observation

The xylem is stained.

Conclusion

The coloured water has travelled through the xylem, showing that xylem transports water.

Why is this a good practical?

You’re using an observable change to investigate something that cannot normally be seen.

The water itself is invisible once inside the plant, but the dye makes its pathway visible.

Why is the stem cut into a cross-section?

To allow the different tissues in the stem to be observed and the stained xylem to be identified.

What does staining of the xylem show?

It shows that the coloured water has travelled through the xylem.

Why should a thin section be used?

A thin section allows the tissues to be observed more clearly.

What are root hair cells and their functions

Root hair cells are specialised cells found on the surface of roots.

Their function is to absorb:

Water and mineral ions from the soil.

They have long extensions called root hairs.

These provide a large surface area.

Why is this important?

A larger surface area allows more water and mineral ions to be absorbed.

Water pathway

Root hair cells → root cortex cells → xylem → mesophyll cells

Then, once water reaches the mesophyll:

Mesophyll cells → air spaces → stomata → atmosphere

This connects water uptake with transpiration.

Why does water keep moving through the plant?

Water is continuously lost from leaves by transpiration.

When water evaporates from mesophyll cells, it contributes to a transpiration pull.

This pulls water upwards through the xylem.

Water molecules remain together because of forces of attraction between water molecules.

Therefore a continuous column of water can be pulled upwards through the xylem.

PRACTICAL 2 — INVESTIGATING WATER MOVEMENT

You may be asked to investigate how water moves through a plant using coloured water.

What you’re actually investigating:

Independent variable: potentially the treatment/condition being changed

Dependent variable: amount/location of staining or another measurement specified by the experiment

Control variables: things such as plant type, stem size, time and dye concentration where appropriate

Important:

Don’t automatically write the same variables for every question.

Read what the experiment is actually changing.

What is transpiration

Transpiration is the loss of water vapour from leaves.

How does transpiration happen

Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour.

Why does the leaf have a high rate of water loss?

The leaf has a large internal surface area because of the interconnecting air spaces between mesophyll cells.

This provides a large surface area for evaporation.

The size and number of stomata also affect how much water vapour can leave.

Therefore, both:

• large internal surface area

• stomata

are important for transpiration.

Factors affecting transpiration

There are three factors

🌡 Temperature

Increasing temperature generally increases transpiration.

Higher temperature increases the rate of evaporation from mesophyll cells.

Therefore:

Higher temperature → faster evaporation → higher transpiration rate

💨 Wind speed

Increasing wind speed generally increases transpiration.

Moving air removes water vapour from around the leaf.

This maintains a steep diffusion gradient between the inside of the leaf and the atmosphere.

Therefore water vapour diffuses out more quickly.

Higher wind speed → steeper diffusion gradient → faster diffusion → higher transpiration rate

💧 Humidity

Increasing humidity decreases transpiration.

When surrounding air is humid, it contains more water vapour.

This means there is a smaller diffusion gradient between the leaf and atmosphere.

Therefore water vapour diffuses out more slowly.

Higher humidity → smaller diffusion gradient → slower diffusion → lower transpiration rate

What is a photometer

A potometer is an apparatus used to measure the rate of water uptake by a plant shoot.

Water uptake can be used to estimate the rate of transpiration.

How does a photometer works

A leafy shoot is connected to a capillary tube containing water.

An air bubble is introduced into the capillary tube.

As the shoot takes up water:

the air bubble moves along the capillary tube.

The distance moved by the bubble over a known period of time is measured.

This allows the rate of water uptake to be calculated.

Photometer apparatus

You should recognise:

• leafy shoot

• capillary tube

• air bubble

• water

• reservoir

• scale

• airtight connections

How to set us the photometer

1. Fill the apparatus with water.

2. Attach the leafy shoot securely.

3. Ensure all connections are airtight.

4. Cut the shoot underwater before attaching it where appropriate.

5. Introduce an air bubble into the capillary tube.

6. Allow the apparatus to equilibrate.

7. Record the initial position of the bubble.

8. Start a stopwatch.

9. Record the position of the bubble after a fixed time.

10. Calculate the distance travelled.

11. Calculate the rate of water uptake.

12. Repeat the measurements.

13. Calculate a mean.

Why must the apparatus of the photometer be airtight?

The apparatus must be airtight so that air does not enter and affect the movement of the bubble.

Why cut the stem underwater? (Photometer)

To prevent air entering the xylem and interrupting the continuous column of water.

Why is an air bubble used ?

The bubble acts as a visible marker.

Its movement allows you to measure how much water has been taken up.

You can measure:

Distance moved by bubble ÷ time

🧮 19. Calculating rate

Formula

Rate = change ÷ time

For a potometer:

Rate of water uptake = distance moved by bubble ÷ time

Example

A bubble moves 24 mm in 6 minutes.

Rate:

24 ÷ 6 = 4 mm/min

Investigate the effect of temperature on transpiration

Independent variable

Temperature

Dependent variable

Rate of water uptake

Control variables

Examples:

• species of plant

• number/area of leaves

• size of shoot

• wind speed

• humidity

• light intensity

• time interval

Prediction

Increasing temperature will increase the rate of water uptake.

Explanation

Increasing temperature increases the rate of evaporation from the mesophyll cells, increasing transpiration. This increases the rate at which water is taken up by the shoot.

Wind-speed investigation

Independent variable

Wind speed

Dependent variable

Rate of water uptake

Prediction

Increasing wind speed will increase the rate of water uptake.

Explanation

Moving air removes water vapour from around the leaf, maintaining a steep diffusion gradient. Water vapour therefore diffuses out faster, increasing transpiration and water uptake.

Humidity investigation

Independent variable

Humidity

Dependent variable

Rate of water uptake

Prediction

Increasing humidity will decrease the rate of water uptake.

Explanation

Higher humidity reduces the diffusion gradient between the inside of the leaf and the surrounding air, so water vapour diffuses out more slowly and transpiration decreases.

sources of error

Air leak

An air leak can cause the bubble to move independently of water uptake.

Improvement: ensure all connections are airtight.

Difficulty reading bubble position

The bubble may not be easy to measure precisely.

Improvement: use a clearly graduated capillary tube and take readings carefully.

Plant differences

Different shoots may have different numbers/areas of leaves.

Improvement: use the same shoot where possible, or use shoots of similar size and leaf area.

Environmental conditions

Changes in light, humidity, temperature or wind can affect transpiration.

Improvement: control these variables.

When does a plant wilt

If water loss exceeds water uptake, plant cells lose water and turgor. The cells become less firm, causing the leaves to droop and the plant to wilt.

What’s translocation

Translocation is the movement of sucrose and amino acids in phloem from sources to sinks.

What is a source

A source is a part of the plant that releases sucrose or amino acids.

A mature photosynthesising leaf can be a source because it produces sucrose.

What is a sink

A sink is a part of the plant that uses or stores sucrose or amino acids.

Examples include parts of the plant that are:

• growing

• developing

• storing food

Why can a plant part be a source at one time and a sink at another?

Its role depends on whether it is releasing or using/storing sucrose or amino acids.

For example:

Young leaf

A young leaf requires sucrose for growth.

Therefore it acts as a:

Sink

Mature leaf

A mature leaf produces sucrose through photosynthesis and can release it to other parts.

Therefore it can act as a:

Source

So a plant organ can change between source and sink depending on its developmental stage and function.