Chapter 7.2

Question 1

Water Movement in Plants

Answer 1

• Water is transported through a plant, from its point of absorption (the root hairs of the roots) up the xylem tissue in the stem, to the leaves of the plant, where water vapour evaporates through a process called transpiration (evaporation of water vapour from the stem and leaves)
• This is a passive process, as the movement of water is being driven by the evaporation of water vapour occurring from the leaves
• Roughly 99% of the water absorbed is lost to transpiration
• There is a gradient in water potential (Ψp) between the root hair cell (high water potential) and the leaves (low water potential), that plants rely on to move water up the plant -Water also moves between cells from a high water potential to low water potential

Question 2

Movement of water in leaves

Answer 2

• In the leaves, water evaporates from the mesophyll cells resulting in water (and any dissolved solutes) being pulled from the xylem vessels (transpiration pull) into the mesophyll cells
• The water that is pulled into the mesophyll cells moves across them passively (either via the apoplastic – diffusion or symplastic – osmosis, pathways) lowering the hydrostatic pressure within the xylem vessels and creating a tension on these vessels
• Xylem vessels have lignified walls to prevent them from collapsing due to the pressure differences being created from the mass flow (all the water molecules and any dissolved solutes move together) of water upwards

Question 3

The movement of water causes

Answer 3

• Water has unique properties
• it is polar
• hydrogen bonds form between the water molecules
• Water moves from the roots to the leaves because of a difference in the water potential gradient between the top and bottom of the plant. This gradient is created because of different events occurring within the plant and due to the properties of water

Question 4

-The mass flow is helped by the what, otherwise known as the cohesion-tension theory

Answer 4

polar nature of water and the hydrogen bonds (H-bonds) that form between water molecules which results in cohesion between water molecules and adhesion between the cellulose in the cell walls and the water molecules

• So due to the evaporation of water from the mesophyll cells in the leaves a tension is created in the xylem tissue which is transmitted all the way down the plant because of the cohesiveness of water molecules.
• The cohesive force results in a continuous column of water with high tensile strength (it is unlikely to break) and the adhesive force stops the water column from pulling away from the walls of the xylem vessels so water is pulled up the xylem tissue from the roots to replace what was lost in the leaves. This mechanism is called the cohesion-tension theory

Question 5

The transpiration stream

Answer 5

-The pathway of the water from the soil through the roots up the xylem tissue to the leaves is the transpiration stream -Plants aid the movement of water upwards by raising the water pressure in the roots (root pressure) —-This is raised by actively secreting solutes (eg. mineral ions) into the xylem vessels in the root which lowers the water potential within the xylem —-This results in water from the surrounding cells being drawn into the xylem (by osmosis) thus increasing the water pressure (root pressure) -Root pressure helps move water into the xylem vessels in the roots however the volume moved does not contribute greatly to the mass flow of water to the leaves in the transpiration stream

Question 6

When answering questions about transpiration it is important to include the following keywords:

Answer 6

-Water potential gradient (between leaves and roots), Diffusion (water vapour through the stomata) -Transpiration pull (evaporation of water from the mesophyll cells pulls other water molecules from the xylem tissue) -Cohesion (between water molecules) -Adhesion (between water molecules and cellulose within the cell walls) -Cohesion-tension theory (tension present in xylem vessels causes a continuous column of water and is due to cohesive and adhesive forces) -Osmosis (water via the apoplastic or symplastic pathways in the roots and leaves)

Question 7

Water & Mineral Ion Transport: Pathways & Mechanisms

Answer 7

-Within a plant mineral ions and organic compounds (eg. sucrose) are transported by being dissolved in water. The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue -The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances -The uptake of water is a passive process and occurs by osmosis (the diffusion of water from a higher (less negative) water potential to a lower (more negative) water potential -The uptake of minerals can be passive or active and occurs by diffusion or active transport respectively -Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins -There are two pathways that water (and the dissolved solutes) can take to move across the cortex (and molecules can change between routes at any time): —Apoplastic —Symplastic

Question 8

Apoplast pathway

Answer 8

-Most water travels via the apoplastic pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem -The water moves by diffusion (as it is not crossing a partially permeable membrane) -The water can move from cell wall to cell wall directly or through the intercellular spaces -The movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway -When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway -This band is called the Casparian strip and forms an impassable barrier for the water -When the water and dissolved minerals reach the Casparian strip they must take the symplastic pathway. The presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure -As the plant ages the Casparian strip thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Question 9

Symplast pathway

Answer 9

-A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells -The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata -The movement of water in the symplastic pathway is slower than the apoplastic pathway

Question 10

Transpiration Explained

Answer 10

• The movement of water through a plants xylem is largely due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
• It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells
• Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)
• Transpiration refers to the loss of water vapour from a plant to its environment by diffusion and the transpiration stream refers to the movement of water from the roots to the leaves

Question 11

The advantage of transpiration is that:

Answer 11

-It provides a means of cooling the plant via evaporative cooling -The transpiration stream is helpful in the uptake of mineral ions -The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

Question 12

Movement of water through leaves

Answer 12

-Certain environmental conditions (eg. low humidity, high temperatures) can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration) -The water vapour lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells -The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull -This transpiration pull results in water moving through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm (symplastic pathway) into the cell wall -The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water). This movement is called transpiration stream

Question 13

The role of the stomata

Answer 13

• is mainly controlled by the pairs of guard cells that surround stomata (plural, stoma is singular)
• Guard cells open the stomata when they are turgid and close the stomata when they lose water
• When the stomata are open there is a greater rate of transpiration and of gaseous exchange
• When the stomata close transpiration and gaseous exchange decrease
• As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day

Question 14

process of stroma

Answer 14

Question 15

Water movement through a leaf. Water enters the leaf as a liquid and diffuses out as water vapour through the stomata. This loss of water by evaporation and transpiration results in a water potential gradient between the leaves (low) and roots (high) causing water to move up the plant in a transpiration stream

Answer 15

Question 16

Explaining Factors that Affect Transpiration

Answer 16

• Transpiration refers to the loss of water vapour from a plant to its environment by diffusion
• The steeper the water potential (Ψp) gradient is, the greater the rate of transpiration
• Environmental factors that can cause a change in the water potential gradient are:
• Humidity (a measure of the concentration of water vapour in the air)
• Air movement / wind
• Temperature
• Light intensity
• Water availability

Question 17

How environmental factors affect transpiration table

Answer 17

Question 18

Xerophytic Plant Leaf Adaptations

Answer 18

• Xerophytes (from the Greek xero for ‘dry’) are plants that are adapted to dry and arid conditions
• Xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation

Question 19

Xeromorphic features table

Answer 19

Question 21

The source of the assimilates could be:

Answer 21

• Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)
• Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
• Food stores in seeds (which are germinating)

Question 22

The sinks (where the assimilates are required) could be:

Answer 22

• Meristems (apical or lateral) that are actively dividing
• Roots that are growing and / or actively absorbing mineral ions
• Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)

Question 23

The Sucrose Loading Mechanism

Answer 23

• Assimilates such as sucrose are transported from source to sink through the phloem sieve tubes
• Carbohydrates are generally transported in plants in the form of sucrose because:

–It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)

–It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

Question 24

Unloading of assimilates (eg. sucrose)

Answer 24

• The unloading of the assimilates (eg. sucrose) occurs at the sinks
• Scientists believe that the unloading of sucrose is similar to the loading of sucrose, with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways
• To maintain a concentration gradient in the sink tissue, sucrose is converted into other molecules. This is a metabolic reaction so requires enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)

Question 25

The apoplast and symplast pathways used when sucrose is loaded into the phloem tissue. The enlarged portion of the companion cell shows the proton and co-transporter proteins used to actively load the sucrose

Answer 25

Question 26

Loading of assimilates (eg. sucrose)

Answer 26

The molecules may move by the:

• symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion
• apoplastic pathway (through the cell walls) which is an active process

-If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an active process and therefore requires ATP as an energy source

• The large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell
• The hydrogen ions move through a cotransporter protein. While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose
• The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells
• Companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump
• This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll

Question 27

The Mass Flow Hypothesis

Answer 27

was the model initially used to explain the movement of assimilates in the phloem tissue

Question 28

An illustration of Münch’s model for mass flow in phloem tissue

Answer 28

Question 29

The translocation of phloem sap (sucrose and other organic solutes) due to a hydrostatic pressure gradient from the source to the sink

Answer 29

Question 30

Carbohydrates are generally transported in plants in the form of sucrose because:

Answer 30

• It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)
• It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

Question 31

However in phloem tissue energy is required to create

Answer 31

pressure differences for the mass flow of the organic solutes

• The pressure difference is generated by actively loading sucrose into the sieve elements at the source (usually a photosynthesising leaf or storage organ) which lowers the water potential in the sap
• This results in water moving into the sieve elements as it travels down the water potential gradient by osmosis

Question 32

The presence of water within the sieve elements increases the

Answer 32

hydrostatic or turgor pressure at the source and as solutes (eg. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs

• The pressure difference between the source and the sink results in the mass flow of water (containing the dissolved organic solutes) from the high hydrostatic pressure area to the low hydrostatic pressure area
• The mass flow of organic solutes within the phloem tissue occurs above and below the sources (which is typically photosynthesising leaves). Therefore sap flows upwards and downwards within a plant