3.3.4.2 Mass Transport in plants

Question 1

Explain how the active transport of mineral ions into the xylem vessels in the roots results in water entering these vessels and then being moved ho the xylem tissue (hint root pressure)

Answer 1

The water potential in the xylem is reduced by the entry of ions
So a water potential gradient is established between the xylem and surrounding cells
The plasma membranes of the surrounding cells are partially permeable
So water enters the xylem via osmosis
So the volume of water in the xylem increases
And so the pressure in the xylem increases as water cannot move back down due to the concentration gradient

Question 2

The presence of an air bubble in a xylem vessel in the stem blocked the movement of water through that vessel. Use the cohesion tension theory to explain why

Answer 2

Water evaporates from the leaves
So the water in the xylem is under tension causing the water molecules to stick together
So the water is stuck together in a single column, the air bubble breaks the column which prevents cohesion

Question 3

Explain how water enters a plant root from the soil and travels through to the endodermis

Answer 3

Water enters root hair cells by osmosis (this is because the active transport of mineral ions has a created a water potential gradient causing water to move into the root hair cell)
Then water moves through the cortex by osmosis down a water potential gradient
The water diffuses through the symplastic pathway (through the cells cytoplasm) and apoplastic pathway (through the cell walls)

Question 4

Describe one piece of evidence that supports the root pressure theory and explain how it supports the theory

Answer 4

Sap is discharged from a cut stem. Only an upward force could make this happen

Question 5

The diameter of a tree is less during the day than it is at night. Explain how this supports cohesion tension theory

Answer 5

Evaporation of water through the leaves mainly during the daytime
So the tension in the xylem creates an inward pull
So the xylem vessels become narrower due to the adhesion of water molecules to the walls of the xylem vessels

Question 6

The diameter of a tree is less during the day than it is at night. Explain how this does not support root pressure theory

Answer 6

Root pressure give out an OUTWARD force pushing on the walls of the xylem
So trees would become wider not narrower as the xylem should become wider

Question 7

Using the cohesion tension theory explain the changes in the circumference of a tress throughout the day

Answer 7

Increase of light, at the start of the day, causes the stomata to open.
So water evaporates out of the stomata causing water to move across the leaf by osmosis.
Evaporation exerts force causing tension in water columns
Cohesion holds water column together
Adhesion between walls of the xylem and the water molecules results in a pulling force which causes the water columns to decrease in volume
Hence the xylem vessels and the tree trunk both become narrower

Question 8

Explain how the rate of water movement through the apoplast pathway is affected by an increase humidity of air around the leaves

Answer 8

High humidity decreases the water potential gradient so no net movement of water in or out the leaf.
This mean that the rate of water movement decreases as less water is lost by transpiration

Question 9

Which part of the cortex forms the apoplast pathway?

Answer 9

The cell walls

Question 10

Explain how the structure of the endodermis affects the passage of water by the apoplast pathway

Answer 10

The Casparian bands are waterproof so lower the water potential in the cytoplasm of the endodermis cell.
So water enters the symplastic pathway by osmosis

Question 11

Describe how water is moved through a plant according to the cohesion tension theory

Answer 11

Water evaporates from leaves 
Reducing the water potential in cells 
So water is drawn out of xylem 
This creates tension 
So cohesive forces between water molecules pull water up as a column

Question 12

Explain how water enters the xylem from the endodermis in the root and is then transported to the leaves

Answer 12

In the root the casparian strip blocks the apoplast pathway.
Active ta sport by the endodermis of ions into the xylem lowers the xylems water potential.
This results in water entering the xylem via osmosis.
Then evaporation from the leaves creates tension in the xylem.
So water molecules bind to the xylem and create a continuous water column

Question 13

What is translocation?

Answer 13

Translocation is the transport of organic substances in the phloem from source to sink

Question 14

give 2 features of xerophytic plants

Answer 14

hairs
stomata in pits
thick way cuticle
rolled leaves
spines/fewer leaves

Question 15

why do some leaves have hairs on them?

Answer 15

trap a layer of water vapour so WP gradient reduced

Question 16

why do some leaves have stomata in pits?

Answer 16

trap a layer of water vapour so WP gradient reduced

Question 17

why do some leaves have thick waxy cuticle?

Answer 17

increases diffusion distance

reduces evaporation/transpiration

Question 18

why do some leaves have rolled leaves?

Answer 18

trap a layer of water vapour so WP gradient reduced

Question 19

why do some plants have spines or fewer leaves?

Answer 19

reduce water loss by evaporation

Question 20

why might the rate of water uptake by a plant NOT be the same as the rate of transpiration?

Answer 20

1. Water used for support/turgidity;
2. Water used in photosynthesis;
3. Water used in hydrolysis;
4. Water produced during respiration;

Question 21

mass movement of sugars around the plant is known as..

Answer 21

translocation

Question 22

How are sugars transported in plant stems - describe the process.

Answer 22

1. (At source) sucrose is actively (transported) into the phloem/sieve element/tube;
2. By companion/transfer cells;
3. Lowers water potential in phloem/sieve element/tube and water enters by osmosis;
4. (Produces) high (hydrostatic) pressure;
5. Mass flow/transport towards sink/roots/storage tissue;
6. At sink/roots sugars are removed/unloaded;

Question 23

Describe cohesion-tension theory in the xylem

Answer 23

water lost due to transpiration from leaves

lowers WP in mesophyll cells

causes water to be drawn up the xylem (tension)

water molecules stick together due the the H bonds
creating a continuous column of water

water molecules also adhere to xylem wall