Topic 4- TRANSPORT IN PLANTS

Question 1

Describe the structure of xylem and how they function.

Answer 1

The xylem starts off as living tissue. The first xylem to form are called the protoxylem, these cells can stretch and grow with the plant, the cellulose microfibrils within their cell wall are parable to the stem allowing for them to withstand the pressure of the growing plant. As the cells mature the cell wall lays down more lignin until they are fully lignified, making them impermeable to water. Now know as metaxylem, the process of lignifying the cells kills them. The end walls of the cells break down forming one long hollow tube. Water moves out of the xylem through pits. Support is gained through the parenchyma, sclerenchyma and collenchyma

Question 2

Describe the structure of phloem and how they function.

Answer 2

Phloem are made up of two separate elements. The sieve tube element, they are very long tubes that run the length of the plant which are joined end to end. The cells do not become lignified and so remain living. The walls between the cells become perforated to form sieve plates, during this process the nucleus, tonoplast and other organelles break down. The second element is the companion cell, this cell is very active with lots of mitochondria providing atp and sucrose to the sieve tube element, they are connected via plasmodesmata.

Question 3

Describe the apoplast pathway of water

Answer 3

The water is taken up by the root hair cells and moves through the cellulose cell walls of the plant. They continue to move this way until they hit the endodermis which contains the casperian strip which is impermeable. The water is briefly forced into the cytoplasm of the cell. The nitrate ions within the water move into the the xylem through transporter proteins. This allows for a large water potential gradient between the water in the cells and the xylem.

Question 4

Describe the symplast pathway of water

Answer 4

In the symplast pathway water moves through the plasmodesmata which connects all of the cell walls within the plant. They then reach the endodermis, casperian strip, release nitrate ions them move into the xylem.