Passage of water through a plant

Question 1

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.

Answer 1

(In the root)1. Casparian strip blocks apoplast pathway / only allows symplast pathway2. Active transport by endodermis3. (Of) ions/salts into xylem4. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient(Xylem to leaf)5. Evaporation / transpiration (from leaves)6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure7. Adhesion / water molecules bind to xylem8. (Creates continuous) water column

Question 2

Explain why the values for the pressure in the xylem are negative.

Answer 2

(Inside xylem) lower than atmosphericpressure / (water is under) tension

Question 3

Root pressure moves water through the xylem. Describe what causes root pressure.

Answer 3

1. Active transport by endodermis2. ions/salts into xylem3. Lowers water potential (in xylem)4. (Water enters) by osmosis

Question 4

Describe the two routes water can take through the roots of a plant

Answer 4

Symplast = water travels through the cytoplasmApoplast = water goes through cell walls. When water gets to the endodermis cells, The path is blocked by the casparian strip so the water has to go the symplast pathway

Question 5

Identify four factors that affect transpiration rate and explain how they affect it.

Answer 5

Light = the lighter it is, the faster the transpiration rate. This is because the stomata open when it gets light. When its dark the stomata are usaully closed so theres little transpirationTemperature = The higher the temperature, the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the concentration gradient between the inside and the outside of the leaf making water diffuse out of the leaf fasterHumidity = The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpirationAir movement = The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the concentration gradient, which increases the rate of transpiration

Question 6

Give 5 examples of adaptations for xerophytic plants

Answer 6

1) stomata sunk in pits which trap moist air, reducing evaporation2) curled leaves with the stomata inside, protecting them from wind3) a layer of ‘hairs’ on the epidermis to trap moist air round the stomata, reducing the concentration gradient of water4) A reduced number of stomata, so there are fewer places for water to escape5) waxy, waterproof cuticles on leaves and stems to reduce evaporation

Question 7

Explain what root pressure and the cohesion tension theory do.

Answer 7

Root pressure = When water is transported into the xylem in the roots, it creates pressure and shoves water already in the xylem furthur upwards. This pressure is weak and couldn’t move water to the top of bigger plants by itself. But it helps, especially small plants with leaves still developingCohesion tension theory = water evaporates from the leaves, this creates tension which pulls more water into the leaf. Water molecules are cohesive so when some of them are pulled into the leave, others follow. This means that the whole column of water in the xylem, from the leaves down to the roots, move upwards. Water enters the stem through the roots.

Question 8

A student found the number of stomata per cm2 on the lower surface of a daffodil leaf. He removed a small, thin piece of lower epidermis and mounted it on a microscope slide. He examined the slide using an optical microscope.Explain why it was important that the piece of the epidermis that the student removed was thin.

Answer 8

Single layer of cells / few layers of cellsSo that light that can pass through / cells absorb light

Question 9

Suggest how the student could have used his slide to find the number of stomata per cm2. (3)

Answer 9

Method of determining area of field of view/area seen using microscopeCount number of stomata in field of viewRepeats and calculation of mean

Question 10

The stomata on the leaves of pine trees are found in pits below the leaf surface. Explain how this helps to reduce water loss.

Answer 10

Water vapour accumulates / increased humidity/ reduced air movement (around stomata)Water potential/diffusion gradient reduced