3.9 Transpiration & Translocation

Question 1

What are phloem tubes made up of? (2)

Answer 1

• Columns of living cells called sieve tube elements. These have perforated end-plates to allow stuff to flow through
• Sieve tube elements have no nucleus. This means that they can’t survive on their own, so each sieve tube element has a companion cell. These cells carry out the living functions for both themselves and their sieve cells

Question 2

What do the phloem vessels transport and how? (3)

Answer 2

• Phloem vessels transport food substances (cell sap: liquid mixture of water and sugar) both up and down the stem for growing and storage tissues
• This movement of food substances around the plant is known as translocation. Sugars are usually translocated from photosynthetic tissues e.g. the leaves, to non-photosynthetic tissues, e.g. the roots
• The sugars enter the phloem by active transport. They are then pushed around by water, which enters the phloem by osmosis

Question 3

What are xylem tubes made up of? (2)

Answer 3

• Xylem tubes are made of dead cells joined end to end with no connecting cell walls between them (to create a long tube) and a hole (lumen) down the middle
• The thick side walls are made of cellulose. They’re strong and stiff, which gives the plant support. The cell walls are also strengthened by a material called lignin

Question 4

What is the role of xylem tubes?

Answer 4

Xylem tubes carry water and mineral ions (e.g. nitrates) in aqueous solution from the roots up the stem to the leaves in the transpiration stream

Question 5

Describe the transpiration in a plant (3)

Answer 5

• Transpiration is caused by the evaporation and diffusion of water from a plant’s surface. Most transpiration happens at the leaves
• This evaporation and diffusion creates light shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it
• This in turn means more water is drawn up from the roots, and so there’s a constant transpiration stream of water through the plant

Question 6

What are the 4 factors that can affect the rate of transpiration?

Answer 6

• Temperature
• Light intensity
• Air flow
• Humidity

Question 7

How does light intensity affect the rate of transpiration?

Answer 7

The brighter the light, the greater the transpiration rate. Bright light increases the rate of photosynthesis. This increases the demand for water in the leaves, so more is drawn up from the roots. Bright light also causes the stomata to open to let carbon dioxide in, allowing more water vapour to diffuse out. Stomata begin to close as it gets darker because photosynthesis can’t happen in the dark. When the stomata are closed, water can’t escape.

Question 8

How does temperature affect the rate of transpiration?

Answer 8

The warmer it is, the faster transpiration happens. That’s because when it’s warm the water particles have more energy to evaporate and diffuse out of the stomata. An increase in temperature also increases the rate of photosynthesis and therefore the demand for water - so more is drawn up from the roots.

Question 9

How does air flow affect the rate of transpiration?

Answer 9

If there’s lots of air movement (wind) around a leaf, transpiration happens faster. If the air around a leaf is very still, the water vapour just surrounds the leaf and doesn’t move away. This means there’s a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn’t happen as quickly. If it’s windy, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly, from an area of higher concentration to an area of lower concentration.

Question 10

How does humidity affect the rate of transpiration?

Answer 10

More humid air will decrease the rate of photosynthesis because the large amount of water in the moist air would decrease the concentration gradient meaning less water will diffuse out and we have less evaporation overall

Question 11

Which part of the plant is glucose produced in?

Answer 11

Leaves

Question 12

Describe how to a practical for measuring the loss of mass to estimate transpiration rate (5)

Answer 12

1) Add some damp soil to a plastic sandwich bag. Take a small plant and plant it in the soil, then tie the bag shut around the stem (leaving the leaves out of the bag).
2) Measure the mass of the wrapped up plant and record it.
3) Leave the plant in a well-lit place for 24 hours, then measure its mass again.
4) You should notice that the plant has decreased in mass. It’s assumed this is equal to the mass of the water lost through transpiration.
5) Dividing the mass lost (in grams) by the time taken to lose it (in days) will give you an estimate of the transpiration rate (in g/day). You can also compare how much water a plant loses under different conditions, e.g. in the light versus in the dark. Yo compare them fairly, you need to calculate the percentage change in mass.