Xylem and Phloem
Plants possess a sophisticated vascular system to transport water, minerals, and sugars. This topic examines the structure and function of xylem and phloem, the process of water uptake, and the factors influencing transpiration.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Xylem tissue consists of dead, hollow, lignified cells forming a continuous tube to transport water and minerals upwards from the roots to the leaves.
- Phloem tissue is made of living cells that transport dissolved sugars (translocation) from the leaves to areas of growth or storage, both up and down the plant.
- Root hair cells are adapted for absorption with a large surface area, taking in water via osmosis and mineral ions via energy-requiring active transport.
- Transpiration is the evaporation of water from leaves through pores called stomata, which creates a tension that pulls more water up through the xylem.
- The rate of transpiration is increased by greater light intensity, higher temperature, and more air movement (wind), but is decreased by higher humidity.
- Stomata are pores, typically on the underside of leaves, that open to allow gas exchange (CO2 in, O2 and H2O out) and close to conserve water, controlled by guard cells.
Formulae
rate of transpiration = volume of water / time taken To calculate the rate of water loss or uptake by a plant, typically using data from an experiment with a potometer.
Definitions
- Transpiration
- The loss of water vapour from a plant's surface, primarily through the stomata in the leaves, which drives the movement of water through the plant.
- Translocation
- The movement of dissolved sugars from the leaves (source) to other parts of the plant (sink) for use or storage, occurring in the phloem.
- Xylem
- Plant vascular tissue that transports water and dissolved mineral ions from the roots to the rest of the plant and provides structural support.
- Phloem
- Plant vascular tissue that transports sugars produced during photosynthesis to all parts of the plant where they are needed.
Worked example
A student uses a potometer, as shown in the diagram, to measure the water uptake of a leafy shoot. The capillary tube has a radius of 1 mm. The student observes that an air bubble moves 60 mm along the tube in 30 minutes. What is the rate of transpiration in cm3/hour? (Use the approximation π ≈ 3)
- 1
First, calculate the volume of water taken up.
The volume is that of a cylinder with radius r=1 mm and length l=60 mm.
The formula is V = π × r2 × l.
- 2
Substitute the given values:
V ≈ 3 × (1 mm)2 × 60 mm = 180 mm3.
- 3
Next, convert the volume from mm3 to cm3.
There are 10 mm in 1 cm, so there are 103 = 1000 mm3 in 1 cm3.
Therefore, V = 180 / 1000 = 0.18 cm3.
- 4
The question asks for the rate in cm3 per hour, but the time given is 30 minutes.
Convert the time:
30 minutes = 0.5 hours - 5
Finally, calculate the rate using the formula:
Rate = Volume / TimeRate = 0.18 cm3 / 0.5 hours - 6
To divide by 0.5, you can multiply by 2.
So, Rate = 0.18 × 2 = 0.36 cm3/hour
Answer: 0.36 cm3/hour
Common mistakes
- ×Errors in unit conversion are a major issue. Be very careful converting between mm3 and cm3 (a factor of 1000) and between minutes and hours (a factor of 60). Always double-check the units required in the answer.
- ×Misreading diagrams, particularly for potometers. Ensure you are using the radius, not the diameter, in the volume calculation (V = πr2l). If given the diameter, halve it first.
- ×Confusing the direction of transport in xylem and phloem. Remember Xylem transports water Up from the roots. Phloem can transport food to all parts of the plant.
No-calculator tips
- ✓When calculating the volume of water moved in a potometer (πr2l), using the approximation π ≈ 3 is usually accurate enough to distinguish between the answer choices.
- ✓To divide a number by 0.5, simply multiply it by 2. To divide by 0.25, multiply by 4. This is much faster than long division.
- ✓Mentally converting volume units: moving from mm3 to cm3 means dividing by 1000, so shift the decimal point three places to the left (e.g., 250 mm3 becomes 0.25 cm3).