Xylem Transport (9.1)

Question 1

Describe the structure of a leaf.

Answer 1

Epidermis (single layer of cells) - protects the leaf from physical damage and pathogens. Coated with the cuticle which prevents water loss. Palisade mesophyll - a layer of cells containing chloroplasts under the upper epidermis. Site of photosynthesis.Spongy mesophyll - an area of loosely packed parenchyma (cellular tissue) which contains air spaces. Main site of gas exchange. Vascular bundle (including xylem and phloem) are located here.Stomata are located on the lower epidermis of the leaf. Allows gases and water vapor into and out of the leaf.

Question 2

Describe the structure of a stem.

Answer 2

Epidermis (single layer of cells) - protects the stem.Cortex - assists with the transport and storage of materials within the stem.Cambium - circular layer of undifferentiated cells responsible for lateral growth of the stem. Vascular bundles are arranged around the cambium, xylem in the interior and phloem in the exterior. Remember ‘o’ for outside.Pith - assists with the transport and storage of materials within the stem.

Question 3

How is the distribution of vascular bundles in stems different in monocotyledons and dicotyledons?

Answer 3

Monocotyledons have vascular bundles spread through the stem while dicotyledons have vascular bundles in a ring near the outside of the stem.

Question 4

What are xylem?

Answer 4

Specialised structures that function to facilitate the movement of water throughout the plant. i.e. transports water from the roots to the leaves.

Question 5

Describe the structure of the xylem.

Answer 5

Tube composed of dead cells that are hollow to allow for the free movement of water. Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only. The cell wall contains numerous pores, which enables water to be transferred between cells. Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension.

Question 6

What are the different ways in which xylem can be reinforced by lignin?

Answer 6

In spiral vessels, the lignin is present in the form of a helix or coil.In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other.

Question 7

Define transpiration.

Answer 7

The loss of water vapour from the stems and leaves of plants.

Question 8

How is transpiration affected by the level of photosynthesis?

Answer 8

Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis). As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis.

Question 9

How does water move in transpiration steam?

Answer 9

Water is absorbed from the soil into the roots and then into the xylem by osmosis. The forces of cohesion and adhesion cause the water molecules to form a column in the xylem. Water rises through the xylem vessels to the leaf where it evaporates and leaves the plant via diffusion through the stomata.

Question 10

How is water loss regulated in leaves?

Answer 10

The opening and closing of the stomata. Transpiration rates will be higher when stomatal pores are open than when they are closed. When a plant begins to wilt from water stress, dehydrated mesophyll cells release abscisic acid (ABA). This triggers the efflux of potassium ions from the guard cells, decreasing water pressure within the cells and causing them to lose turgor. A loss of turgor makes the stomatal pore close as the guard cells become flaccid and block the opening.

Question 11

Aside from the level of photosynthesis, what other factors affect transpiration rates?

Answer 11

Humidity, temperature, light intensity and wind.

Question 12

What properties of water allow it to pass through the xylem?

Answer 12

Cohesion (the attraction between two particles of the same substance eg. two water molecules) and adhesion (the attraction between two particles of different substance eg. a water molecule and the xylem wall).

Question 13

How can the rate of transpiration be measured?

Answer 13

• Bubble potometer (measuring the distance the water level drops in a graduated tube).- Mass potometer (measuring the reduction in mass of the potometer).

Question 14

What is a potometer?

Answer 14

A piece of apparatus used to measure the rate of water loss from a plant.

Question 15

Describe the transpiration rate practical.

Answer 15

A cut plant stem is sealed into the potometer using a rubber bung or vaseline. An air bubble is introduced to the capillary tube. The distance that the bubble travels shows how much water the stem has taken up.

Question 16

What is the function of a root?

Answer 16

To anchor the plant to the ground, absorb and transport water via the xylem, store food and to transport stored food via the phloem.

Question 17

How are minerals uptaken by roots?

Answer 17

Most minerals are absorbed via active transport. Root cells contain proton pumps that expel H+ ions (stored in the vacuole of root cells) into the surrounding soil.The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient. Negatively charged mineral ions may bind to the H+ ions and be reabsorbed along with the proton.*Minerals may absorb into the root via passive diffusion.

Question 18

How is water uptaken by roots?

Answer 18

Water will follow the mineral ions into the root via osmosis. Once inside the root, there are two pathways for the water.In the symplastic pathway, water travels through the cytoplasm of cells via openings called plasmodesmata. THIS IS SLOW AND TRANSPORTS A LOW VOLUME OF WATER VIA OSMOSIS. In the apoplastic pathway, water travels through the spaces within the cell walls. THIS IS FAST AND TRANSPORTS A HIGH VOLUME OF WATER VIA MASS FLOW.

Question 19

What are xerophytes?

Answer 19

Desert plants. They have high rates of transpiration due to the high temperature and low humidity of desert environments.

Question 20

Describe the adaptations of xerophytes for its environment.

Answer 20

To limit water loss via transpiration they have:- waxy cuticles - small leaves to reduce SA- rolled leaves to reduce stomata exposure- CAM physiology (stomata opens at night, reducing water loss)

Question 21

What are halophytes?

Answer 21

Plants that grow in high salinity eg. marshlands. They lose water as the high intake of salt from the surrounding soil will draw water from plant tissue via osmosis.

Question 22

Describe the adaptations of halophytes for its environment.

Answer 22

To limit the effect of the high levels of salt they have:- altered reproductive cycles (they may flower at times when the salt concentration is low)- cellular sequestration (sequestering toxic ions and salts within the cell wall or vacuoles)- tissue partitioning (concentrating the salt in particular leaves, which then drop off in abscission)- salt excretion (certain parts of the plant may contain salt glands which actively eliminate salt)

Question 23

How can the movement of water up the xylem be modelled?

Answer 23

By using the following apparatus.- Capillary tubing- Filter paper- Porous pots