9.2 - Water Transport in Multicellular Plants

Question 1

How do plants transport water without a heart?

Answer 1

Unlike animals, plants do not have a heart. Instead, water moves through the xylem due to transpiration pull, root pressure, and capillary action.

Question 2

What are the main roles of water in plants? (5)

Answer 2

1. Provides turgor pressure for support.
2. Drives cell expansion (e.g., roots pushing through concrete).
3. Cools the plant through evaporation.
4. Transports minerals and sugars in solution.
5. Acts as a reactant in photosynthesis.

Question 3

What is turgor pressure, and why is it important?

Answer 3

Turgor pressure is the hydrostatic pressure exerted by water inside plant cells.
It supports stems and leaves, acting as a hydrostatic skeleton.

Question 4

How does turgor pressure compare to human blood pressure? What does this show?

Answer 4

Leaf cell turgor pressure ≈ 1.5 MPa (11,251 mmHg).
Human systolic blood pressure ≈ 120 mmHg.
This shows that plant cells experience much higher pressure than human arteries!

Question 5

How does water loss help plants?

Answer 5

Transpiration (water evaporation) helps to cool the plant.
Water carries mineral ions from roots to leaves.

Question 6

Why is water essential for photosynthesis?

Answer 6

Water is a raw material for photosynthesis.
It provides hydrogen ions needed to produce glucose and oxygen.

Question 7

How does water enter a plant from the soil?

Answer 7

Water moves into root hair cells by osmosis.
It then moves through the root cortex via the symplast and apoplast pathways.

Question 8

What are the symplast and apoplast pathways?

Answer 8

Symplast pathway: Water moves through the cytoplasm via plasmodesmata.
Apoplast pathway: Water moves through cell walls and intercellular spaces.

Question 9

What is the function of root hair cells?

Answer 9

Root hair cells are specialized epidermal cells that act as the exchange surface for water and mineral absorption from the soil.

Question 10

How are root hair cells adapted for water absorption? (4)

Answer 10

1. Microscopic size – allows them to fit between soil particles.
2. Large SA:V ratio – maximizes water uptake.
3. Thin cell wall and membrane – speeds up diffusion and osmosis.
4. Maintains a water potential gradient – contains solutes that attract water by osmosis.

Question 11

Why does a large surface area help in water absorption?

Answer 11

A larger surface area allows more water molecules to move into the root by osmosis, making absorption more efficient.

Question 12

Why does water move into root hair cells by osmosis?

Answer 12

Soil water has a high water potential (very few dissolved minerals).

Root hair cell cytoplasm has a lower water potential due to dissolved solutes.

Water moves down the water potential gradient into the root hair cell by osmosis.

Question 13

What substances lower the water potential in root hair cells?

Answer 13

Sugars, mineral ions, and amino acids dissolved in the cytoplasm and vacuole.

Question 14

What are the two pathways water can take across the root to the xylem?

Answer 14

Symplast Pathway – through the cytoplasm of cells via plasmodesmata.

Apoplast Pathway – through the cell walls and intercellular spaces.

Question 15

How does water move in the symplast pathway?

Answer 15

Water moves through the cytoplasm of living plant cells, connected by plasmodesmata, by osmosis.

Question 16

What maintains the water potential gradient in the symplast pathway?

Answer 16

As water leaves a root hair cell by osmosis, its water potential falls, ensuring a continuous flow of water from the soil into the root hair cell.

Question 17

How does water move in the apoplast pathway? By what forces?

Answer 17

Water moves through the cell walls and intercellular spaces, driven by cohesion and tension forces.

Question 18

Why does water continue moving in the apoplast pathway?

Answer 18

As water enters the xylem, it pulls more water behind it due to cohesion (water molecules sticking together), creating a continuous flow.

Question 19

Why do the cell walls allow easy water movement in the apoplast pathway?

Answer 19

The cellulose structure has a loose, open network that offers little resistance to water movement.

Question 20

What is the difference between a symplast pathways and a vacuolar pathway?

Answer 20

A vacuolar pathway is the same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm.

Question 21

Rank the three water movement pathways, from slowest to quickest.

Answer 21

1. Vacuolar pathway
2. Symplast pathway
3. Apoplast pathway

Question 22

What is the endodermis, and why is it important? 🌱

Answer 22

The endodermis is a layer of cells surrounding the vascular tissue (xylem and phloem) in roots. It contains the Casparian strip, which controls water movement.

Question 23

What is the Casparian strip, and what does it do?

Answer 23

The Casparian strip is a waxy band of suberin that runs around endodermal cells, creating a waterproof barrier that forces water from the apoplast pathway into the symplast pathway.

Question 24

Why is the diversion of water into the symplast pathway important?

Answer 24

It ensures that toxic solutes in soil water cannot enter the xylem, as the selectively permeable membrane filters substances.

Question 25

What role do mineral ions play in water movement into the xylem?

Answer 25

Endodermal cells actively transport mineral ions into the xylem, lowering its water potential, which causes water to move in by osmosis.

Question 26

What is root pressure, and how does it help water movement?

Answer 26

Root pressure is the force created by active transport of minerals into the xylem, which moves water upward by osmosis.

Question 27

Is root pressure the main force moving water up the xylem?

Answer 27

No, root pressure helps, but transpiration pull is the main driving force for water movement in the plant.

Question 28

What is root pressure, and where does it occur?

Answer 28

Root pressure is the pressure exerted in the xylem of plants, driving the movement of water upwards through the plant. It occurs in the roots, particularly when active transport moves water from the root endodermis into the xylem.

Question 29

How do poisons like cyanide affect root pressure?

Answer 29

Cyanide affects the mitochondria in root cells and prevents the production of ATP. Without ATP (the energy source), active transport is halted, causing root pressure to disappear.

Question 30

What is the relationship between temperature and root pressure?

Answer 30

Root pressure increases with a rise in temperature and decreases when temperature falls. This suggests that chemical reactions, which are temperature-dependent, are involved in generating root pressure.

Question 31

How do oxygen levels and respiratory substrates affect root pressure?

Answer 31

If levels of oxygen or respiratory substrates decrease, root pressure also decreases. This supports the idea that active transport requires cellular respiration to provide the necessary energy for the process.

Question 32

What is guttation, and how is it related to root pressure?

Answer 32

Guttation is the process where xylem sap is forced out of special pores at the ends of leaves under specific conditions, such as when transpiration is low (e.g., overnight). This phenomenon is evidence of root pressure, as the sap is driven up from the roots into the leaves.