M3 Transport in Plants

Question 1

What substances are required by plant cells?

Answer 1

• O2 for respiration
• CO2 for photosynthesis
• inorganic ions
• water
• organic nutrients (produced by plant) for respiration and growth.

Question 2

What do Xylem and Phloem transport?

Answer 2

Xylem = water and mineral ions

Phloem = Sucrose and amino acids

Question 3

Water and inorganic ions are transported from the ____ to other plant parts in ____ tissue.

Organic nutrients are transported from ____ (e.g. leaves) to ____ (e.g. storage organs) in ____ tissue.

Answer 3

roots
xylem
sources
sinks
phloem

Question 4

What are the adaptations of xylem?

Answer 4

• Made from dead cells aligned end-to-end to form a continuous column.
• Lack of cell contents - increased volume
• No end walls in individual xylem elements - for rapid transport
• Narrow tubes - for cohesion of water molecules
• Lignified cell walls - strength
• Pits in cell wall - for transport of water out and around air bubbles
• Lignin deposited in spiral, annular or reticulate patterns.

Question 5

Explain cohesion-tension theory

Answer 5

Water pulled up xylem vessels as a continuous column (under tension due to transpiration) helped by:
- cohesion of water molecules to each other by continuous column.
- adhesion of water molecules to sides of xylem vessel

Question 6

Describe the process of transpiration

Answer 6

Water evaporates from cell walls of mesophyll cells into the air spaces.
Water diffuses down a water potential gradient out of the leaf through stomata.
Lost water is replaced by water taken from xylem vessel, that moves through mesophyll cells by osmosis.

Question 7

What is a ‘transpiration stream’?

Answer 7

As water molecules leave xylem in leaf (via transpiration) they must be replaced by water molecules moving up from the roots, creating a transpiration stream.

Question 8

How do water and minerals enter the xylem from the roots? How does water leave? What causes the water to flow up-stem?

Answer 8

Minerals actively transported into xylem and root hair cells. This lowers the water potential and water follows by osmosis.

This raises the hydrostatic pressure at the base of the xylem vessel and pushes water up stem. Water moves out of xylem and into leaf by osmosis creating low hydrostatic pressure and thus tension.

The hydrostatic pressure difference between the bottom and top of the xylem vessel causes water to flow upwards (by mass flow).

Question 9

What is mass flow?

Answer 9

The movement of water through the stem.

NOT diffusion or active transport as no membranes to pass through.

Question 10

What are the factors that affect the rate of transpiration?

Answer 10

• Humidity
• Wind speed
• Temperature

Question 11

How do you set up a potometer? What are the control variables?

Answer 11

• Cut the stem of plant underwater to prevent airlocks.
• Set the air bubble to zero.
• Make sure it is a closed system (no leaks)

CVs: SA of plant, time of day/light intensity

Question 12

Phloem:
Made of long thin structures called _____ ____ _______. These are arranged end to end. End walls are perforated (known as ____ ____). _______ cells are associated with sieve plates and have many _________.

Answer 12

Sieve tube elements

sieve plates

Companion

Mitochondria

Question 13

What is translocation? Where are sugars transported to and from?

Answer 13

The transport of molecules and ions through the phloem.

Sugars transported from site of production (source) to site of use/storage (sink)

Question 14

What organic and inorganic substances are transported in translocation?

Answer 14

Organic molecules: sucrose and amino acids

Inorganic ions: Potassium, chloride, magnesium, phosphate

Question 15

The rate of movement in phloem is too fast for diffusion, what theory is most likely instead?

Answer 15

Mass Flow Theory

Question 16

What are the 3 stages of Mass Flow Theory?

Answer 16

1) Transfer of sucrose from photosynthesising tissue into sieve tube elements .

2) Mass flow of sucrose through sieve tube elements.

3) Transfer of sucrose from the sieve tube elements into storage or other sink cells.

Question 17

Describe the transfer of sucrose from photosynthesising tissue to sieve tube elements.

Answer 17

Sucrose enters companion cells by facilitated diffusion, down a concentration gradient.

Hydrogen ions are actively transported (ATP) from the companion cells into the spaces within the cell wall. These hydrogen ions pass into sieve tube elements by facilitated diffusion through a co-transport protein, co-transporting sucrose with it.

Question 18

Describe the transfer of sucrose from the sieve tube elements into storage or other sink cells.

Answer 18

Sucrose moves into companion cells and then into sink cells by facilitated diffusion OR active transport.

Question 19

How does water enter the sieve tube elements?

Answer 19

Increase in sucrose lowers the water potential in sieve tube. Xylem has a much higher water potential so water moves into sieve tube by osmosis.

Question 20

How does mass flow occur in sieve tube elements.

Answer 20

Entering of water from xylem by osmosis increases hydrostatic pressure.

Water leaves sieve tube elements by osmosis, as movement of sucrose into sink cells raises water potential. Decreasing hydrostatic pressure as water moves out.

Mass flow of water occurs down the hydrostatic gradient from to source to sink.

Question 21

What supports the Mass Flow Theory?

Answer 21

+ There is pressure within sieve tubes, shown by release of sap when they are cut.
+ Increase in sucrose levels in leaf are followed by similar increases of sucrose in phloem.
+ Companion cells possess many mitochondria and readily produce ATP.
+ Metabolic poisons and/or lack of oxygen inhibits translocation of sucrose in phloem.
+ Downward flow in the phloem occurs in daylight, but ceases at night.

Question 22

What contradicts the Mass Flow Theory?

Answer 22

• Not all solutes move at the same speed
• Sucrose is delivered at approx. same rate to all regions, not faster to those with a lower sucrose concentration.
• The function of sieve plates is unclear, they would seem to hinder mass flow.

Question 23

Experiments investigating translocation: Ringing stems

Describe

Answer 23

Ring of bark and phloem removed from area of stem. The region above becomes immediately swollen.

Liquid is sampled from this region and is found to be rich in sugars and other dissolved organic substances.

Photosynthetic tissue above the removed ring grows, below, non-photosynthetic tissue withers and dies.

Question 24

Experiments investigating translocation: Tracer experiment

Describe

Answer 24

Radioactive isotope 14C to make radioactively labelled 14CO2.

The plant is then monitored using auto-radiography - cutting thin sections and placing onto an x-ray film.

Blackened areas show exposure to radiation.

Question 25

Experiments investigating translocation: Aphid tracer experiment

Describe

Answer 25

Aphid inserts stylet into phloem for sucrose.
Expose leaf to 14CO2.
Anaethetise aphids and cut off stylet - leaving it still in the phloem.
Collect drips of sap and time when radioactivity arrives at different places.