Water and Mineral Uptake in Plants (Lecture 2)

Question 1

What is diffusion?

Answer 1

The movement of a molecule down a concentration or electrochemical gradient.

Question 2

What is osmosis?

Answer 2

The movement of water across a selectively permeable membrane, usually towards a region of higher solute concentration.

We want to ‘dilute’ this high solute concentration.

Question 3

What is osmosis in energetic terms?

Answer 3

By mixing with a solute, water is moving down a free energy gradient.

Question 4

How can water flow be controlled?

Answer 4

By altering the solute concentrations in parts of the cell! This allows for the movement of water in/out of cells, and for longer distances via xylem and phloem.

Question 5

What is the role of membranes in diffusion?

Answer 5

Membranes prevent rapid diffusion of solutes.

Question 6

How do solutes (ions, organic molecules) move between cells and compartments?

Answer 6

By using transport proteins.

Question 7

What is passive transport?

Answer 7

Facilitated diffusion down a concentration gradient, using channel proteins and transporters.

Question 8

How can we move solutes against a concentration gradient?

Answer 8

Active transport, using other transport proteins. This process requires energy, in the form of ATP, or energy from an H+ gradient that was previously established using ATP.

Question 9

What are the differences in ion transport in plants, compared to animals?

Answer 9

• Plants use H+ to help with basic transport processes across membranes, NOT Na+ (which is toxic for them!)
• The cell wall is a physical barrier in plants that provides positive pressure, against osmotic pressure.

Question 10

What is the importance of turgor pressure?

Answer 10

It helps maintain the plant structure.

Question 11

For what other reason is physical pressure from the cell wall important?

Answer 11

It balances the water levels in plants.

Question 12

What are the three mechanisms that allow for the transport of solutes?

Answer 12

1. ATP-dependent proton pump (active transport): Establishes a gradient of H+ across a membrane. This creates a movement OUT, towards the outside of root hairs.
2. Co-transporters: Transport both charged and uncharged solutes AGAINST their concentration gradient (into the cell), using the energy stored in the H+ gradient across the membrane.
   H+ tags along a solute as it is led into the cell.
3. Electrochemical gradients, for ions (ion channels).

Question 13

Which transport mechanisms work together in phloem loading?

Answer 13

The ATP-dependent proton pump (H+ and membrane potential) and the co-transport mechanism of H+ and neutral solutes (H+/sucrose transporter).

Question 14

Which transport mechanisms work together in uptaking NO3- up from the soil?

Answer 14

The ATP-dependent protein pump (H+ and membrane potential) and the co-transport mechanism of H+ and ions (H+/NO3- cotransporter).

Question 15

What are ion channels important for?

Answer 15

They are critical in the control of the stomata and the regulation of water loss.

Question 16

What is water potential, and what is it composed of?

Answer 16

Water potential is a measure of free energy of water, predicting the direction of water flow in plants.

It is comprised of the PRESSURE POTENTIAL and the SOLUTE (OSMOTIC) POTENTIAL.

Question 17

What are the units of water potential?

Answer 17

Units of pressure, in megapascals.

Question 18

What is pressure potential?

Answer 18

The physical pressure on a solution, or in a cell.

Question 19

When is pressure potential 0 MPa?

Answer 19

At sea level, for an open container.

Question 20

Is the pressure potential typically positive, or negative? Why?

Answer 20

It is typically positive, due to the osmotic uptake of water.

Question 21

What is solute or osmotic potential?

Answer 21

It is proportional to the molarity of a solution.

Question 22

When is the solute potential 0 MPa, and what happens when more solute is added?

Answer 22

It is 0 MPa for pure water and goes negative as more solute is added.

Question 23

Water moves from areas of _______ water potential to ______ water potential.

Answer 23

Higher; lower

Question 24

If the solute potential is equal, water moves __________ a pressure gradient.

Answer 24

Down

Question 25

If the pressure potential is equal, water moves to the _____ solute concentration, which has ______ solute potential.

Answer 25

Greater; lower

Question 26

Plants move water into cells by pumping in ___ and __________, affecting the ________.

Answer 26

Plants can move water into cells by pumping in IONS and SOLUTES, which affect the WATER POTENTIAL (including the pressure potential).

Question 27

What function does water potential dictate?

Answer 27

Xylem and phloem

Question 28

What are the three ways in which water and minerals can move into roots?

Answer 28

• Apoplastic route: Cell wall
• Symplastic route: Plasmodesmata
• Transmembrane route: Plasma membrane

Question 29

How does the apoplastic route of water into/through the root take place?

Answer 29

Cell walls are porous and hydrophilic.
Water can diffuse through the cell wall, thanks to the abundant space between polymers.
The cell wall contains many -OH groups, with which water molecules interact easily. They are therefore attracted to the cell wall – Adhesion. The cell wall is hydrophilic!

Question 30

True or false: The apoplastic route is used for long distances?

Answer 30

False! Diffusion only works for short distances.

Question 31

What is the apoplast?

Answer 31

The non-living continuum of cell walls and extra-cellular spaces in plant tissues.

Question 32

What is the symplast?

Answer 32

The continuum of the cytosol of plant cells, connected by the plasmodesmata.

Question 33

How does the transmembrane route for the movement of water work?

Answer 33

Water moves in and out of cells via aquaporins, which are transport proteins facilitating the movement of water across membranes.

Question 34

What prevents diffusion via the apoplastic route?

Answer 34

A layer of suberin and lignin.

Question 35

What is the role of the Casparian strip?

Answer 35

It allows the root to be selective by restricting, selecting ion movement.
All water and minerals must pass into cells before getting into the xylem.

Question 36

How do most plants enhance their available surface area, for the uptake of minerals and water?

Answer 36

They associate with mycorrhizae, fungal symbionts.

Question 37

What can root systems grow into?

Answer 37

Nutrient-rich patches, by actively mining the soil.

Question 38

Can substances move between cells that are separated by cell walls, as well as plasma membranes? How?

Answer 38

Plasmodesmata (symplastic route) permits the cytosol of adjoining cells to be continuous. Thus, without crossing the membrane, substances can move from one cell to the next.

Plants can also move via the apoplastic route, but to get to the xylem, will need to cross the Casparian strip.

Question 39

If a plant cell with a solute potential of -0.7 MPa is dropped into a beaker of sucrose solution with a solute potential of -0.9 MPa, what will happen to the cell?

Answer 39

Water moves towards the LOWEST solute potential (more negative water potential), which in this case is the external environment. Water will move out of the cell, and the cell will become flaccid.

Question 40

If a plant cell with a solute potential of -0.7 MPa is dropped into a beaker of sucrose solution with a solute potential of -0.2 MPa, what will happen to the cell?

Answer 40

Water moves INTO the cell. Since the cell wall prevents expansion, the cell will become turgid.

Question 41

What will be pressure potential be in the above case?

Answer 41

WP = SP + PP
-0.2 = -0.7 + PP
PP = 0.5 MPa

Question 42

Where is the Casparian strip and what does it do?

Answer 42

It is a transverse band of suberin and lignin impregnated within the cell walls, in the ENDODERMIS (nestled between the cortex and vascular cells).

It prevents diffusion via an apoplastic pathway.