Topic 4: Water importance and xylem transport

Question 1

Key Summary: What is the {importance/roles} of water in a plant? (7 roles)

Answer 1

1. As a reactant in photosynthesis
2. To transport plant minerals/sucrose
3. For hydrolysis of bonds in chemical reactions eg. glycosidic bonds in starch to release glucose
4. As a medium for chemical reactions eg. in cytoplasm
5. Create turgor pressure to provide support
6. Temperature regulation by evaporation
7. To act as a solvent eg. for plant mineral ions

Question 2

How does water reach the parts of the plant that need it? (2 points)

Answer 2

1. transported by the xylem

2. from roots by ONE WAY MASS FLOW (upwards only).

Question 3

What are the functions of the xylem? (2 functions)

Answer 3

1. Mechanical support for the plant

2. Transport of water and {inorganic/mineral} ions

Question 4

Key Summary: Explain how xylem {tissue/vessel} is specialised for the transport of water and minerals in a plant stem. (5 points about transport + 1 point about general function)

Answer 4

1. continuous column of dead cells, with no end walls and {hollow/no cell contents/cytoplasm} so {less resistance/no obstruction} to movement of water up the plant, and allows continuous water column.
2. pits (pores) in walls to allow sideways movement of water from one vessel to another or out of the xylem.
3. lignin waterproofs the walls which keeps the water in the vessel (less water is lost).
4. cellulose and lignin in walls to provide strength/support to prevent vessels collapsing under tension.
5. wide/large cross-sectional area to transport large volumes of water.

+

(general function)
6. Lignin in {rings/spirals} to allow xylem tissue to stretch during growth

Question 5

What is the function of {lignin/secondary thickening} in xylem walls? (3 points)

Answer 5

1. Greater tensile strength
2. Extra rigidity for support
3. Waterproofing

Question 6

Key Summary: How does the arrangement of cellulose microfibrils and secondary thickening contribute to the physical properties of xylem vessels and sclerenchyma fibres? (3 points)

Answer 6

1. Cellulose microfibrils are laid down in layers, at angles to each other forming a mesh & are embedded in a matrix of hemicelluloses and pectins, which contribute to strength in different directions & allow permeability of cell wall.
2. For secondary thickening, lignin is added to the cell wall for {increased rigidity/reduced flexibility} & greater tensile strength of the cell wall & to make it waterproof.
3. {Lignin/secondary thickening} in rings or spirals to allow some flexibility & pits are present to allow water to move in and out of xylem.

Question 7

Define transpiration (2 points)

Answer 7

1. The loss of water from plant leaves (and other aerial parts),
2. by evaporation.

Question 8

(Transpiration)

Thermal energy heats water, which forms _______ ________, which _________ from leaf cells into air spaces.

_______ ________ diffuses out of leaves through _______.

Answer 8

water vapour

evaporates

Water vapour

stomata

Question 9

(Transpiration)

Water moves by _______ from the xylem into leaf cells to replace water lost by ___________, then from cell to cell.

Answer 9

osmosis

transpiration

Question 10

(Pressure gradient in transpiration)

The low hydrostatic (water) pressure in leaves ‘pulls’ the stream of water (transpiration stream) up the ______.

Answer 10

xylem

Question 11

Define adhesion (1 point)

Answer 11

1. Attraction between ‘unlike’ molecules e.g. water molecules and cellulose (in wall)

Question 12

What does adhesion help the water to do in the xylem? (1 point)

Answer 12

1. helps the water to cling to the xylem wall

Question 13

In the xylem, water moves by ______ _____ as a continuous column by __________ ________.

Answer 13

mass flow

capillary action

Question 14

Define cohesion (3 points)

Answer 14

1. Cohesion is the {attraction between like molecules / tendency of water molecules to stick together}
2. by hydrogen bonding
3. due to the {polarity/dipole/dipolar nature} of water molecules.

Question 15

________ caused by movement of water could cause xylem vessels to _________ ________.

This could break the _______ ______.

__________ of xylem wall prevents this.

Answer 15

Tension

collapse inwards (like a strongly sucked straw)

water column

Lignification

Question 16

_______ enters root hair cells by _______ from the soil, this creates a ____ hydrostatic pressure (root pressure) in roots.

There is a ____ hydrostatic pressure in leaves due to __________.

_______ moves by _______ ______ down the _________ _________ from roots to leaves via stem.

_______ moves from root hair cells, from cell to cell by ________ until it reaches the root _______.

Answer 16

Water
osmosis
high

low
transpiration

water
mass flow
pressure gradient

water
osmosis
xylem

Question 17

Key Summary: Describe water movement in the xylem

6 steps

Answer 17

1. Water evaporates from leaf cells and water vapour diffuses out of stomata, this is TRANSPIRATION, which causes water movement creating {LOW/reduced} HYDROSTATIC PERSSURE which pulls water up the xylem vessels. Water moves out of xylem through pits by OSMOSIS.
2. There is {Intermolecular force/HYDROGEN BONDING} between water molecules in xylem, so water molecules stick to each other. This is COHESION (or cohesive forces).
3. There is {Intermolecular force/HYDROGEN BONDING} between water and cellulose in xylem walls, so water molecules stick to side of xylem vessel. This is ADHESION (or adhesive forces).
4. A continuous column of water molecules is pulled, under tension, up the xylem by CAPILLARY ACTION.
5. Water moves into roots by OSMOSIS, creating a HIGH {hydrostatic pressure/root pressure}.
6. Water moves DOWN HYDROSTATIC PRESSURE GRADIENT, from high in roots to low in leaves, by MASS FLOW.

Question 18

Define cohesion-tension theory (2 points)

Answer 18

1. Water molecules are held together by HYDROGEN BONDING

2. and are pulled UP under tension.

Question 19

Define capillary action (2 points)

Answer 19

1. Minute (small) channels between CELLULOSE MICROFIBRILS of cell walls act as tiny capillaries,
2. which pulls water UP the plant