Xylem and Phloem Flashcards
How are xylem vessels adapted to their function? (Cell type)
- Large cells, thick cell walls
- Form a column
How are xylem vessels adapted to their function? (Production)
- Production of lignin
What is the role of lignin?
- Impregnates cellulose wall, restricting solute and water entry
- Provides Stability
What is autolysis?
Tonoplast breaks down. Organelles, cytoplasm, membrane are broken down by enzymes
What happens to end cell walls in xylem?
- They are lost or highly perforated
What two things give xylem strength?
- Cellulose microfibrils
- Lignin
What is the casparian strip?
Cell wall material deposited around endodermal cells
Why is the casparian strip different to cell walls?
- Made of suberin, and sometimes lignin
What does the casparian strip do?
- Forces solutes and water to pass through plasma membrane via symplastic route to cross endodermic level
- Regulates water and mineral intake by roots
Why is there low water potential at the bottom of the xylem?
- Loss of water by transpiration and the transpiration stream draw water up, causing low potential. Water diffuses in by osmosis
How do root hair cells gain water?
- Large surface area for diffusion
- Soil water is dilute with large water potential
- Cytoplasm of root hairs have low water potential due to dissolved ions
How does water move from root hair to xylem?
- Apoplast, diffusion between cell walls. Symplast, movement between cytoplasm
Why is cytoplasm continuous?
Plasmodesmata, narrow fluid-filled channels maintain a continuous cytoplasm
Where does water evaporate from?
Diffuses through the stomata in the leaves, then evaporated from the surface of cells lining the substomal cavity
How does water mass flow work?
- Continuous column of water drawn up through column
- Capillary action due to surface tension, cohesion, adhesion
- Pulling force on water behind
- Energy comes from the sun
What components is phloem made up of?
- Sieve elements
- Companion cells
- Parenchyma cells
- Fibres
What is the role of phloem?
Transports sucrose and amino acids
What is the source?
The cell that photosynthesyses
How is sucrose transported into companion cells?
Active transport
How does sucrose move from companion cells to sieve elements?
Diffusion
What does the diffusion of sucrose into the sieve elements cause?
Reduction of water potential
When there is low water potential, what occurs into the cells?
Osmosis
What happens to the sieve element cell following osmosis?
- Becomes turgid, hard, high pressure
What is a sink?
Region where sugars are respired or stored as starch
What do sinks include?
Roots, tubas, actively growing buds etc
What happens when there is a low concentration of sucrose in the sinks?
Sucrose passes from sieve tubes into sinks by diffusion
Why is there lower water potential in phloem by sinks?
Low sucrose means high water concentration and diffuses out
Why does sap move down the plant?
Higher pressure compared to low pressure means mass movement
What is the organelle difference between xylem and phloem?
- Xylem is dead with no organelles
- Phloem remains alive. Nucleus and most contents disintegrate, with a few organelles in a thin layer of cytoplasm. Membrane, cytoplasm, ER, mitochondira
What is most of the space in a phloem sieve tube?
Liquid filled space called a lumen
What do phloem sieve cell walls do to interact?
- Aligned holes, allowing transfer of material
What is the section between two sieve plates?
Sieve tube element
What do companion cells do?
Line the elements with more mitochondria and ribosomes, and produce and transfer materials required by sieve elements
Why are xylem vessels narrow?
More contact with the water means more adhesion
