Topic 3 - Transport Systems in Plants

Question 1

What is the function of the Xylem ?

Xylem

Answer 1

Xylem tissue transport waster and mineral ions in a solution from the roots of the plants to the leaves.

Question 2

Describe the structure of Xylem

Xylem

Answer 2

• Xylem consist of xylem vessels which are long tube like structurees which are formed from dead cells joined end to end.
• There are no end walls, allowing for water and mineral ions to pass through the middle easily.
• The walls of the cells are made of lignin which is waterproof
• however pits are formed where there is no lignin, this allows water to move from one vessel to an adjacent vessel.

Question 3

What is transpiration ?

Xylem

Answer 3

Loss of water from a plant’s surface

Question 4

What is a transpiration stream ?

Xylem

Answer 4

The movement of water from the roots of the plant to the leaves where it evapourates.

Question 5

How does a transpiration stream work ?

Xylem

Answer 5

• Water evaporates from the stomata when they open , it moves down the concentration gradient by diffusion.
• This creates tension, pulling more water up to the leaves.
• Water molecules are cohesive meaning the column of water in the xylem moves upwards from the roots, enters the stems and up to the leaves where it will diffuse out of.
• Trunk diameter decreases.

Question 6

What 4 things affect transpiration rate ?

Xylem

Answer 6

Wind
Light
Humidity
Temperature

Question 7

How does light affect transpiration rate ?

Xylem

Answer 7

• The lighter it is, the faster the transpiration rate, this is because the stonata open when it gets light to let in carbon dioxide for photosynthesis - allowing for water to diffuse down its concentration gradient into the atmoshphere and evapourate .
• When its dark, stomata close so theres little transpiration.

Question 8

How does temperature affect transpiration rate ?

Xylem

Answer 8

• as temperature increases, water molecules are given more kinetic energy
• so they diffuse out of cells faster, this creates a large concntration gradient causing water to difffuse out of the stomata faster
• increasing transpiration rate.

Question 9

How does humidity affect transpiration rate ?

Xylem

Answer 9

The lower the humidity, the faster the transpiration rate because dry air around the leaf increases the concentration gradient between the leaf and the atmosphere, this increases the transpiration rate.

Question 10

How does wind affect transpiration rate ?

Xylem

Answer 10

The windier it is, the faster the transpirationrate because lots of air movement blows away water molecules from the stomata, increasing the concentration gradient and therfore, the rate of transpiration.

Question 11

What does a potometer measure ?

Xylem

Answer 11

The water uptake of a plant, but it assumes that water uptake is directly linked to water loss (transpiration rate)

Question 12

How would you use a potometer ?

Xylem

Answer 12

1. Cut a shoot underwater to prevent air bubbles from entering the xylem. Cut it diagonally to increase the surface area.
   2.Assemble the potometer underwater and insert the shoot underwater so that no air can enter.
   3.Remove tha apparatus from the water but keep the end of the capillary tube submerged in water. Check tha apparatus is water tight and air tight.
   4.Dry the leaves, allow time for the shoot to acclimatise and then shut the tap.
   5.Remove the end of the capillary tube from the beaker of water until one air bubble has forme, then re submerge it in the beaker.
   6.Record the starting postion of the air bubble, start a stop eatch and record the distance of the bubble every hour for 6 hours. The rate of the air bubble movement is an estimate of the transpiration rate.

Question 13

What is the function of translocation ?

Phloem

Answer 13

The movement of solutes (amino acids and sucrose) from sources to sinks in a plant.

Question 14

What is a source ?

Phloem

Answer 14

Where the solute are loaded onto the phloem ( high concentration), for example the leaf.

Question 15

What is a sink ?

Phloem

Answer 15

Where solutes are removed from the phloem (Low concentration), for example roots.

Question 16

How is the concentration gradient of solutes between sources and sinks maintained ?

Phloem

Answer 16

• Enzymes chnage the solutes at the sink by breaking them down or building them into something else.
• For example in potatos enzymes convert sucrose into stratch at the sinks, this maintains a low concentration of sucrose so that it can continue to move down the concentration gradient from the source to the sink.

Question 17

Explain mass flow theory

Phloem

Answer 17

• Active transport is used to actively load the solutes from the companion cells into seive tubes o the phloem at the source, e.g. the leaf.
• This lowers the water potential of the seive tubes so water moves in by osmosis from the xylem and companion cells.
• This causes a high pressure in the sieve tubes at the source of the phloem.
• At the sink end solutes are removed to be used up.
• This increases the water potential causing water to move out of the cells by osmisis.
• This creates a lower pressure at the seive tubes in the sink end of the phloem.
• This results in a prssure gradient from the source end to the sink end.
• This gradient causes a movement of solutes from the high pressure in the sources to the low pressure at the sinks.
• When the solutes reach the sink, they can be used up (respiration) or stored (starch).

Question 18

How is the rate of translocation Increased ?

Phloem

Answer 18

The higher the concentration of solutes at the source, the higher the rate of translocation.

Question 19

Explain why the mass flow theory is supported

Phloem

Answer 19

• If a ring of bark is removed from a tree brank, a large bludge is formed above it. This buldge has a high concentration of sugars compared to the smaller buldge below the ring. This proves that there is a downward movement of sugars.
• C14 radioactive isotope can be used to track the movement of organic substances in a plant
• Pressure in the phloem can be investiagated using aphins. Sap flows out quicker towards the top of the branch compared to the bottom. This porves that there is a pressure gradient from the sources to the sinks.
• If a metabolic inhibitor is added, transloaction stops. This shows that it requires active transport.

Question 20

Explain why the mass flow theory is rejected

Phloem

Answer 20

• Sugar travels to more than one sink, not just the one with the highest water potential.
• Sieve plated will act as a barrier to solutes. High pressure and speed would be needed to over come this barrier.

Question 21

Explain how you would show mass flow theory by using a radioactive isotope.

Phloem

Answer 21

• surround a plant in a container and pump the 14C radioactive isotope into the conatainer
• The carbon will be incorperated into the plants organic subatsnces produced (photosynthesis sugars) and transported by translocation.
• Movement of the organic substances can be tracked by tracking 14C using an autoradiograph. The plant is placed on photographic film and if there is 14C present, the film will turn black.
• This demonstrates the movement of solutes from sources in the leaves to sinks towards the roots.

Question 22

Why is it a bad assumption that water uptake is equivalent to the rate of transpiration ?

Xylem

Answer 22

• some water will be used to keep cells turgid
• for photosynthesis and other chemical reaction.
• some water will be created from aerobic respiration.