Chapter 9 - Transport In Plants

Question 1

What is the function of cambium? (2)

Answer 1

1) separate vascular tissues

2) divide and differentiate to form new xylem and phloem tissues

Question 2

Name 2 functions of the xylem.

Answer 2

1) provides mechanical support for the plant

2) conducts water and dissolved mineral salts from the roots to the stem and leaves in one direction

Question 3

What is the function of phloem?

Answer 3

Transports sucrose and amino acids from the leaves to other parts of the plant in both directions.

Question 4

What is the structure of phloem?(3)

Answer 4

1) sieve tube cells - elongated cells that lack nuclei and have thin layers of cytoplasm
2) sieve tube elements - made of sieve tube cells that are joined end to end to form column with sieve plates in between
3) sieve plates - cross walls with many small sieve pores

Question 5

How is the phloem adapted for its function? (4)

Answer 5

1) phloem sieve tube elements have very little protoplasm and are arranged to form a continuous column. This reduces the resistance to the flow of substances within the phloem.
2) pores within sieve plates allow rapid flow of manufactured food substances
3) companion cells have numerous mitochondria to release energy for the loading of sucrose and amino acids into the sieve tubes by active transport
4) every phloem sieve tube cell has an associated companion cell to ensure its survival.

Question 6

What is a pith and what is its function?

Answer 6

It is a central region where the vascular bundles are arranged in a ring around it. It is a storage tissue.

Question 7

What is the function of the cortex?

Answer 7

It is a storage tissue.

Question 8

What is a root hair and what is its function?

Answer 8

Root hair is a a tubular outgrowth of an epidermal cell.

Function: increases surface area to volume ratio for faster absorption.

Question 9

What is translocation?

Answer 9

process by which sucrose and amino acids are transported in phloem from leaves to other parts of the plant in 2 directions.

Question 10

How can characteristics of translocation be studied? (3)

Answer 10

1) aphids
2) the “ringing” experiment
3) radioactive carbon isotopes

Question 11

Describe the procedure of using aphids to study translocation. (4)

Answer 11

1) anaesthetise the aphids with CO2 while it is feeding on a stem.
2) cut off its body such that its proboscis remains in the plant tissue.
3) analyze the liquid that exudes from the cut end of the proboscis.
4) section(isolate) the portion of the stem that contains the proboscis and examine under a microscope.

Question 12

Why is it necessary to anaesthethise the aphid while feeding?

Answer 12

To enable to body of the aphid to be cut off while the aphid is feeding. This ensures that the proboscis remains in the phloem sieve tube.

Question 13

Why do we section the stem, at the region where the proboscis is, for examination under a microscope?(aphids)

Answer 13

To determine which tissue the proboscis was inserted to.

Question 14

What tests can be used to determine to contents of the liquid that exudes from the cut end of the proboscis (aphids)?

Answer 14

1) benedict’s tests
2) biuret test
3) ethanol emulsion test

Question 15

Describe the “ringing” experiment. (4)

Answer 15

1) cut off a complete ring of bark from the main stem of one of the woody twigs. The ring is above water level.
2) set up a control with another twig.
3) ensure that the bottom end of both twigs are in contact with the water.
4) observe the twigs daily. Swelling will occur above the region with bark removed.

Question 16

Why will there be swelling above the region with barked removed?(Ringing experiment)

Answer 16

The removal of the phloem prevents the translocation of sugars to the region below the ring. The accumulation of sugars in the region just above the ring lowers the water potential of cells in that region. Net movement of water enters the region via osmosis and this results in swelling.

Question 17

Describe the procedure of using radioactive carbon isotopes to study translocation. (3)

Answer 17

1) supply radioactive carbon (14^C) to an intact leaf enclosed in a sealed chamber.
2) allow photosynthesis to take place.
3) cut a section of the stem and expose it onto X-ray photographic film. Radioactivity will be detected at the phloem.

Question 18

Why will radioactivity be detected at the phloem? (radioactive carbon isotope)

Answer 18

radioactive carbon dioxide is used to synthesize glucose during photosynthesis, which is then converted to sucrose and transported via phloem throughout the plant.

Question 19

How to study the pathway of water? (3)

Answer 19

1) allow a young plant to stand in a dilute red ink solution.
2) after a few hours, cut a transverse section of the stem and a transverse section of a portion of the root that was not immersed in ink.
3) xylem is the tissue that transports water in a plant.

Question 20

How does the water enter a plant?(5)

Answer 20

1) root hairs grow between the soil particles. They are in close contact with the surrounding soil particles.
2) each soil particle has a thin film of liquid surrounding it. The soil solution is a dilute solution of mineral salts.
3) the cell sap in the root hair has lower water potential than the soil solution due to the presence of sugars and mineral salts in cell sap. Hence, net movement of water enters the root hair by osmosis.
4) the entry of water dilutes the root hair’s cell sap. The sap of the root hair cell now has a higher water potential than that of its adjacent cell. Hence, net movement of water enters it by osmosis.
5) similarly, water passes from the cell to its adjacent cell. This process continues until the water enter the xylem vessels.

Question 21

How are mineral salts absorbed into root hair cells? (2)

Answer 21

1) diffusion - when the concentration of ions is higher in the soil solution than in the root hair cell, ions diffuse into the root hair cell.
2) active transport - when the concentration of ions is higher in the root hair cell than the soil solution, ions are taken into the cell with the use of energy.

Question 22

Why is water and mineral ions absorption decreased in water-logged soil?(too much water?

Answer 22

Water-logged soil has a very diluted soil solution. This lowers the concentration of ions and the root hair cell is unable to absorb minerals ions by diffusion. So it has to expend (use) energy to absorb mineral ions. This affects the absorption of water as the cell sap would not have as much mineral ions(as the ions are used to release energy). Thus, the water potential gradient will be less steep. This means that less water will be absorbed by osmosis.

Question 23

Define root pressure.

Answer 23

Mineral ions are pumped into the xylem vessels via active transport. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential. Water then moves from the soil into the xylem via osmosis. Root pressure is caused by this accumulation of water in the xylem pushing on rigid cells.

Question 24

What is capillary action?

Answer 24

Water move up narrow tubes due to interactions between water molecules (cohesion) and surfaces of the tube (adhesion)

Question 25

Define transpiration.

Answer 25

It is the loss of water vapor from aerial parts (parts exposed to air) of the plant, especially through the stomata of the leaves.

Question 26

Define transpiration pull.

Answer 26

It is the suction force caused by transpiration which results in water to move up the xylem.

Question 27

How does water move through a leaf?(7)

Answer 27

1) water that moves out of the mesophyll cells form a thin film of moisture around the cells.
2) water from the thin film of moisture evaporates to form water vapour in intercellular air spaces. The higher concentration of water vapour than atmosphere accumulates in the air spaces near the stomata.
3) water vapour diffuses out of the stomata into the environment due to concentration gradient between intercellular spaces of leaf and atmosphere. This is transpiration.
4) the movement of water out of the cell is to replace the thin film of moisture that has evaporated. This decreases the cell sap’s water potential.
5) the mesophyll cells absorb water via osmosis from the adjacent cells deeper in the leaf.
6) these cells, in turn, absorb water from the xylem vessels.
7) this results in the production of a suction force that pulls the column of water in the xylem vessels up.

Question 28

What are the 2 formulas to calculate rate of transpiration?

Answer 28

1) loss in mass(g)/time taken(h)

2) loss is volume(cm3)/time taken(h)

Question 29

State the importance of transpiration. (3)

Answer 29

1) transpiration pull is a major suction force for moving water and dissolved mineral salts from roots to stems and leaves for photosynthesis.
2) turgidity is maintained as water that is lost is replaced. Turgid cells keep leaves spread out widely to trap sunlight for photosynthesis.
3) evaporation of water from cells in the leaves removes latent heat of vaporisation, helping to cool the plant, preventing enzymes from denaturing due to temperature, so that glucose can still be made.

Question 30

When will wilting occur?

Answer 30

When the rate of transpiration is higher than rate of absorption.

Question 31

What are 2 advantages of wilting?

Answer 31

1) excessive water loss causes stomata to close as guard cells become flaccid, reducing water loss due to transpiration
2) leaf folds up, surface area exposed to sunlight is reduced, reduce rate of photosynthesis

Question 32

Name 3 disadvantages of wilting.

Answer 32

1) stomata close, decreasing intake of CO2 and rate of photosynthesis decreases as CO2 is a limiting factor.
2) leaves reduce exposed surface area and hence decrease absorption of sunlight, decreasing the rate of photosynthesis 3) rate of photosynthesis decreases as water is a limiting factor.

Question 33

What are 4 factors that affect rate of transpiration?

Answer 33

1) humidity
2) wind
3) temperature
4) light

Question 34

How does humidity affect the rate of transpiration?

Answer 34

Humidity increases, increasing concentration of water vapour in atmosphere, reduces concentration gradient of water vapor between leaf and atmosphere, and thus decrease rate of transpiration.

Question 35

How does wind affect the rate of transpiration?

Answer 35

Increase in air movement removes water vapor, reducing concentration of water vapor around leaf. Steeper concentration gradient of water vapour between atmosphere and inside the leaf, for water vapor to diffuse quicker out of the stomata, increasing rate of transpiration.

Question 36

How does temperature affect the rate of transpiration?

Answer 36

Increase in temperature increases rate of evaporation of water from the cell surfaces into intercellular spaces. This increases the concentration gradient of water vapor between intercellular spaces and atmosphere, increasing rate of diffusion of water vapour out of stomata of leaves. Rate of transpiration increases.

Question 37

How does light affect the rate of transpiration?

Answer 37

Stomata open wider under higher light intensity, it increases the rate diffusion of water vapour out of leaf, increasing rate of transpiration.

Question 38

Explain how a potometer works. (3)

Answer 38

1) we assume that the rate of absorption is proportional to the rate of transpiration.
2) as the shoot transpires, it absorbs water from the potometer to replace what was lost during transpiration. This causes the meniscus in the capillary tube to move from near the conical flask to near the leafy shoot.
3) by measuring the distance moved by the bubble per unit time, we can measure the rate of water loss from the leafy shoot.

Question 39

List 3 precautions of conducting a potometer experiment.

Answer 39

1) allow shoot to adapt to new conditions before taking readings.
2) ensure no air bubbles are present between the point of cork and the shoot.
3) ensure point where shoot is inserted into potometer is sealed with petroleum jelly to prevent entry of air from the surroundings(which slows down the movement of air bubbles) and prevent evaporation of water from the capillary tube.

Question 40

How are vascular tissues organized in roots?(3)

Answer 40

1) the xylem and phloem are not bundled together; they alternate with each other.
2) cortex of a root is a storage tissue
3) the epidermis of the roots is the outermost layer of cells, which bears root hairs. (Piliferous layer)

Question 41

Why does most of the water vapor leave from inside of leaf instead of leaf surfaces?

Answer 41

The cuticle covering the leaf surfaces is relatively impermeable to water. However, a very small amount might evaporate directly from surfaces of the epidermal cells. This is cuticular transpiration.

Question 42

What are 2 similarities in the function of xylem and phloem tissues?

Answer 42

1) both transport substances to different parts of the plant
2) both transport substances as a solution to different parts of the plant

Question 43

What is the function of a stomata?

Answer 43

Regulate gaseous exchange. During respiration, oxygen diffuses into the cell and carbon dioxide diffuses out of the cell via stomata. During photosynthesis, carbon dioxide diffuses into the cell and oxygen diffuses out of the cell.