Topic 3 - Mass Transport in Plants Flashcards
What is transpiration?
Transpiration is the movement of water out of the leaf and stem, and the flow of water through the plant.
How does water move out of the leaf in transpiration?
1) Sun shines on the leaf.
2) H2O evaporates from cells in the mesophyll.
3) This creates a high concentration of H2O vapour in air spaces in the leaf.
4) H2O diffuses out of the leaf down the water potential gradient into the air.
How does water enter the leaf in transpiration?
1) The water potential in the mesophyll cells goes down.
2) Water enters from neighbouring cells down the water potential gradient by osmosis.
3) This causes water to move out of the cells next to the xylem, giving them a low water potential.
4) H2O then moves out of the xylem by osmosis.
How is water transported in the xylem?
1) Water moves out of the top of the xylem.
2) In cohesion and adhesion, water moves up the xylem.
3) At the bottom of the xylem, there is a low water potential, so the water potential is higher in the soil.
4) Water moves by osmosis into the root from the xylem, going down the water potential gradient.
What is cohesion?
H2O molecules are attracted to other intermolecular hydrogen bonds.
What is adhesion?
H2O molecules are attracted to the sides of the xylem.
What is the mechanism that causes the movement of water up in the xylem?
The cohesion-tension mechanism.
In what process are organic substances transported in a plant?
Translocation
What is the phloem made up of?
The phloem is made of sieve tube elements stacked to form sieve tubes. Substances flow inside sieve tubes.
What are sieve cells?
Sieve cells arelong, conducting cells with narrow diameter that do not have sieve plates. They are associated with protein cells that help in the transportation of organic materials.
What are companion cells?
The function of companion cells isto support sieve elements by carrying out cell metabolism and regulation. Companion cells have a dense cytoplasm with abundant organelles, vacuoles, and large nuclei. They load and upload metabolites into the cavity of sieve elements.
What are the characteristics of a sieve cell?
1) No vacuole.
2) No nucleus.
3) Living cell.
4) Not hollow
What are the characteristics of a companion cell?
1) Contains all organelles.
2) Provides ATP required for active transport of organic substances.
Describe the process of the mass transport of organic substances in the phloem
1) At the source, solutes like sucrose are actively transported into the sieve cells from the companion cells. This decreases the water potential in the sieve tube elements.
2) Water enters the sieve tube elements from the xylem and companion cells by osmosis. This increases the hydrostatic pressure in the sieve tube elements at the source.
3) At the sink, solutes are actively transported from the sieve tube elements. This increases the water potential in sieve tube elements at the sink.
4) Water leaves the phloem by osmosis, decreasing the hydrostatic pressure at the sink. This creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink.
What are tracers and what are they used for?
1) Involve radioactively labelling carbon.
2) Thin slices from the stems are then cut and placed on x-ray film that turns black when exposed to radioactive material.
3) Sugar turns black = shows sugar is transported in the phloem.
