Chapter 9: Transport in plants

Question 1

What are 2 structures of the xylem vessel that facilitates its functions?

Answer 1

No cross-walls nor protoplasm, to enable water and dissolved mineral salts to move easily from roots to other parts of the plant. This is to conduct water and dissolved mineral salts from the roots to the leaves more easily.

The xylem vessel possess lignin deposits in the inner walls Lignified wall prevents collapse of the vessels, providing more mechanical support to the plant to allow it to stand upright to get maximum sunlight.

Question 2

Why are companion cells full of mitochondria?

Answer 2

So that the companion cell can undergo aerobic respiration to release energy for active transport of sucrose and amino acids into phloem vessels

Question 3

Describe 3 structures of Sieve tube elements

Answer 3

1. Made up of elongated Sieve tube cells which lack nuclei and possess thin cytoplasm layer
2. Joined end-to-end to form Sieve tube elements which sieve plates in-between them (to create high pressure)
3. Sieve plates are cross-walls with many small pores

Question 4

What are 2 structures of phloem tissues that facilitate its function?

Answer 4

1. The phloem tissue has reduced cytoplasm(thin cytoplasm) to reduce resistance to flow of the bidirectional transport of sucrose and amino acids
   2 The cross-walls are perforated to allow rapid flow of sucrose and amino acids bidirectionally.

Question 5

What is a structure of a companion cell that facilitates its function?

Answer 5

Companion cells are narrow and have numerous mitochondria. This is to undergo aerobic respiration to release energy for loading sugars and amino acids into phloem sieve tubes, and release energy for metabolic activities of sieve tube cells to ensure its survival (Active transport of sucrose and amino acids into sieve tube cells)

Question 6

What is translocation?

Answer 6

Translocation is the bidirectional transport of sucrose and amino acids from the leaves to other parts of the plant via the phloem

Question 7

Briefly state and describe 3 translocation studies

Answer 7

1. Aphid study
   a. Aphids feed on sucrose and amino acids
   b. Possesses proboscis that penetrates into phloem
2. Ringing experiment
   a. Swelling observed above the cut point.
   b. Cut point removes phloem
   c. Results in accumulation of sucrose and amino acids, followed by the build-up of water in cells.
   d. Plant eventually dies as a lack of sucrose and amino acids will cause the roots to die, leading to no uptake of water and mineral salts. Thus the plant cannot photosynthesise and will die.
3. Radioactive carbon isotope
   a. Radioactivity found in the phloem.
   b. Shows that during photosynthesis, radioactive carbon is used to make glucose which is then converted to sucrose which is transported through the phloem.

Question 8

Describe how a molecule of water moves from the soil to the xylem and up the plant to the leaves.

Answer 8

1. The root hair cell has lower water potential than the soil
2. Water molecules would move into the root hair cell by osmosis.
3. This causes water potential of the root hair cell to now be higher than neighbouring cells
4. Osmosis occurs, water moves from root hair cell into neighbouring cell
5. Repeated osmosis occurs through the cells till water enters the xylem.
6. Water in the xylem moves upwards to the leaves due to transpiration pull
7. Caused by water molecules leaving the aerial parts of the plant by transpiration

Question 9

Explain 2 feature of the root hair cell that facilitates its function

Answer 9

1. The root hair cell has a tubular outgrowth between soil particles. This leads to an increased surface are to volume ratio. As such there is an increased rate of absorption of water and mineral salts into the plant.
2. The root hair cell has a cell membrane. This allows osmosis to occur to increase the rate of absorption of water and mineral salts into the plant (?)

Question 10

How are mineral salts absorbed via the root hair cell

Answer 10

Mineral salts can be absorbed via diffusion and active transport.

Active transport - There is a higher concentration of mineral salts in the cell sap of the root hair cell, compared to the soil solution which has comparatively low concentrations of mineral salts. Hence energy released from the mitochondria in root hair cell is used to transport mineral salts into the xylem via active transport.

Question 11

Define transpiration

Answer 11

Transpiration is the loss of water vapour through the aerial parts of the plant to the surroundings

Question 12

What are the 4 functions of transpiration?

Answer 12

1. Transpiration pull
2. Maintains turgidity of the plant
3. Cools down the plant through evaporation of water from cell surface
4. Needed for photosynthesis

Question 13

Describe how water moves from the xylem to out of the leaf

Answer 13

1. Water moves from within the xylem to the spongy mesophyll by osmosis because there is higher water potential inside the xylem that inside the spongy mesophyll cell.
2. Water from inside the spongy mesophyll cell moves onto the surface of the cell via osmosis due to higher water potential inside the cell than on it surface
3. Due to higher water potential inside cell than on its surface, water on the surface of the spongy mesophyll cell evaporates off the cell, forming water vapour. Water vapour calculates in intercellular air spaces in the leaf.
4. Water vapour diffuses out of the stomata of the leaf. This is transpiration. This is due to a higher concentration of water vapour in the leaf than in air outside leaf.

Question 14

How does Humidity affect rate of transpiration?

Answer 14

As humidity increases, the rate of transpiration decreases. Humidity refers to the surrounding water vapour in the air. Thus when humidity increases, there is a higher concentration of water vapour in the surroundings than in the leaf, causing the rate of water vapour loss form the leaf via diffusion to decrease. In turn, decreases evaporation of water from the cell surface, thus decreasing the rate of transpiration.

Question 15

How does wind speed affect rate of transpiration?

Answer 15

As wind speed increases, rate of transpiration increases. When there is still air, water vapour accumulates in atmospheric air, causing the concentration of water vapour in the atmosphere to be hgier than the water vapour concentration in the leaf. As such, higher wind speeds will bring dry air to replace the humid air aroudn the leaves, increasing the rate of transpiration as water vapour now diffuses from the leaf to the surrounding atmosphere that contains a lower concentration of water vapour.

Question 16

How does temperature of the environment affect the rate of transpiration?

Answer 16

The higher the temperature of the environment, the higher the rate of transpiration. When temperature increases, the kinetic energy of water molecules on the spongy mesophyll cells also increase, causing them to evaporate at a higher rate to form water vapour. Since there is now a higher concentration of water vapour in the leaf than in the atmospheric air, water vapour will diffuse from the leaf to the atmospheric air, causing rate of transpiration to increase.

Question 17

How does light intensity affect the rate of transpiration?

Answer 17

When the light intensity increases, the rate of transpiration also increases. This is because higher light intensity causes the stomata to open more, which increases diffusion of water vapour out of the leaves into the environment as there is a higher concentration of water vapour in the leaves. This causes an increase in transpiration.

Question 18

State what wilting is and what the advantages and disadvantages of wilting are.

Answer 18

Wilting is the result of excessive transpiration. It is when rate of water loss is higher than rate of water gained.
Advantages: 1. Wilting decreases the rate of transpiration as the closing of stomata reduces excessive water loss

Question 18

State what wilting is and what the advantages and disadvantages of wilting are.

Answer 18

Wilting is the result of excessive transpiration. It is when rate of water loss is higher than rate of water gained.
Advantages:
1. Wilting decreases the rate of transpiration as the closing of stomata reduces excessive water loss
2. Wilting helps to cool down the plant due to the reduced surface area, thus it becomes less exposed to the sun

Disadvantages:

1. There is a lower rate of photosynthesis as the closed stomata prevents entry of CO2, a raw material needed for photosynthesis
2. There is a lower rate of photosynthesis as the reduced leaf surface area reduces exposure to sunlight