Osmosis and plant transport (CCEA)

Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a selectively permeable membrane.

Water moves down the concentration gradient without needing energy.

A selectively permeable membrane allows some small substances to pass through, but prevents larger substances doing the same.

The cell wall is totally permeable and has no role in controlling what enters and leaves a cell.

The movement of water into or out of a cell depends on the concentration of water surrounding it.

When water moves into an animal cell (eg a red blood cell), it gets bigger.

Eventually, the cell will burst. This is called cell lysis and it happens because there is no cell wall in animal cells to support them.

When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall.

This makes the cell firm or turgid.

If plants do not receive enough water the cells cannot remain turgid and the plant wilts.

Cells that are not turgid are flaccid.

When too much water moves out of a plant cell the vacuole shrinks and the cell membrane pulls away from the cell wall.

Investigate the process of osmosis by measuring the change in length or mass of plant tissue.

Results

Conclusion

The potato cylinders placed in pure water (0.0M) or weak sugar solutions will gain mass as water will have moved from an area of high concentration (outside the potato cells) to an area of lower concentration (inside the potato cells).

The potato cylinders placed in strong sucrose solutions (1.0M) will lose mass as water will have moved from an area of high concentration (inside the potato cells) to an area of lower concentration (outside the potato cells).

The sugar concentration (0.5M) that causes no change in mass is the concentration that is equal to the potato cell.

Transpiration is the evaporation of water from the spongy mesophyll cells, followed by the diffusion of water through the air spaces and out of the stomata.

Most of the water that enters a plant will exit the leaf into the atmosphere.

This provides a continuous supply of water, which is known as the transpiration stream.

Plants use water for:

• photosynthesis
• transpiration
• transport – carry sugar and minerals around the plant
• support – to keep firm and upright

If water availability is limited, plants will reduce transpiration by closing the stomata.

If less water is being lost through transpiration the plant will absorb less water at the roots.

A potometer measures the rate of water uptake in plants.

There are two types of potometer – the bubble potometer and the weight potometer.

As water is transpired from the leaves, the shoot absorbs water from the potometer, causing the air bubble to move.

The distance moved over time indicates the rate of water uptake.

The apparatus is assembled underwater to avoid unwanted air bubbles, and the junction is sealed (eg with petroleum jelly) to prevent air leaks.

A reservoir or syringe resets the bubble position for repeated measurements or testing different environmental factors (eg light, humidity, temperature, wind).

To ensure a fair test, all other factors must remain constant during experiments.

A weight potometer measures water loss by weighing the plant before and after a set time. The difference indicates water loss.

Using a precise balance (eg to two decimal places) improves accuracy.

Aim: To investigate the rate of water loss from a plant under different environmental conditions.

Procedure:

1. Set up the bubble potometer: Fill with water, seal with petroleum jelly, and submerge under water and add plant.
2. Record the initial position of the bubble.
3. Leave the apparatus for 15mins.
4. Record the final bubble position and calculate the rate of water uptake (distance/time).
5. Reset the bubble and repeat under the same conditions to improve reliability.
6. Change environmental factors: adjust light, temperature, or air movement and repeat the measurements.

Results:

Conclusion:

Increased light intensity, air movement and temperature all increase the rate of transpiration.

Aim: To investigate the rate of water loss from a plant under different environmental conditions.

Procedure:

1. Set up a weight potometer and measure the initial mass of the pot and plant.
2. Leave the setup for a set period (eg 12 hours).
3. Measure the new mass of the container and plant.
4. Calculate the rate of mass loss ( mass lost / time).
5. Repeat for reliability.
6. Change one environmental condition (eg temperature or light) and repeat steps 1-5.

Results:

Conclusion:

Increased light intensity, air movement and temperature all increase the rate of transpiration.

Practical 2.2

Aim: To find out if plants lose water through their leaves.

Procedure:

1. Remove four leaves from a plant.
2. Smear petroleum jelly on various sides of the leaf (see table above) to waterproof them.
3. Measure the initial mass of each leaf.
4. Hang the leaves to dry using paperclips and a string.
5. After a set period (24hrs), measure the mass of each leaf again.
6. Calculate the mass loss for each leaf.

Results:

Conclusion:

Leaf A lost the most water since no stomata were covered in Vaseline.

Leaf D lost the least water because all stomata were covered in Vaseline.

As there are more stomata on the underside of the leaf, leaf C loses less water that leaf D because it had more stomata covered allowing more transpiration.