Topic 9

Xylem

Transports water and minerals from roots to leaves.

Dead cells

The composition of xylem vessels.

Passive

The primary type of movement for water in xylem, driven by transpiration pull and cohesion.

Phloem

Transports organic compounds (e.g., sugars) from sources to sinks.

Living cells

The type of cells that compose phloem, specifically sieve tube elements and companion cells.

Sieve tube elements

The main conducting cells of the phloem.

Active transport

Used to load organic compounds such as sucrose into phloem sieve tubes.

Hydrostatic pressure

The pressure created in phloem due to the uptake of water, driving the flow of phloem sap.

Companion cells

Cells that support sieve tube elements and perform metabolic functions.

Apoplast Route

A pathway for phloem loading where sucrose moves through cell walls to the companion cells.

Symplast Route

A pathway for phloem loading where sucrose travels through plasmodesmata connections between cells.

Mass flow

Movement of phloem sap from source to sink due to hydrostatic pressure gradients.

Translocation rate

The speed at which substances are transported through the phloem.

Aphid stylets

A tool used to measure translocation rate by allowing phloem sap to flow out.

Radioactively-labelled carbon dioxide

Used to trace the movement of photosynthate through the phloem.

What is transpiration?

the inevitable loss of water vapor from a plant, primarily through the stomata in the leaves.

What primarily causes transpiration in plants?

a consequence of gas exchange necessary for photosynthesis.

How do guard cells regulate transpiration?

the size of the stomatal opening, regulating water loss.

What factors affect the rate of transpiration?

Temperature and humidity

Define evaporation.

is the change of water from a liquid to a gas.

What is the cooling effect of evaporation?

Evaporation requires energy to break hydrogen bonds.

How does water act as a coolant?

Water's high latent heat of vaporization

What is the transpiration stream?

is the continuous flow of water from the roots to the leaves via the xylem.

What role does cohesion play in the transpiration stream?

allows water molecules to stick together and be pulled up the xylem.

What is adhesion in the context of the transpiration stream?

is the attraction of water molecules to the hydrophilic walls of the xylem vessels.

What generates tension that pulls water up the xylem?

Evaporation from leaves

What is cavitation in xylem vessels?

the breaking of the water column in xylem vessels due to pressure, though it is rare.

How is water absorbed by plant roots?

osmosis, due to concentration gradients established by active transport of mineral ions.

What is the role of mycorrhizae in water uptake?

increase the surface area for absorption and deliver ions to roots, benefiting both plants and fungi in a mutualistic relationship.

What is the structure of xylem vessels?

specialized tubes that transport water and minerals with lignified walls.

How do xerophytes conserve water?

strategies such as reduced leaves, thick cuticles, nocturnal stomatal openings, and water storage structures.

What adaptation do halophytes have for saline soils?

adaptations such as structures for removing salt.

What is a potometer used for?

measures water uptake in plants.

What is the purpose of controlling variables in plant experiments?

allows the isolation of the effect of the independent variable on the dependent variable.

What factors can be tested in germination experiments?

Factors like seed type, amount of water, and presence of other factors should be controlled in germination experiments.