Mass Transport in Plants Flashcards
What substances does Cohesion-Tension Theory (transpiration) transport
Water and inorganic ions
What substances does Mass-Flow Hypothesis (translocation) transport?
Organic solutes
Where is the root hair cell located?
Epidermis
Advantage of root hair cell structure
Thinner cellulose walls so shorter diffusion distance
How does water travel from the root hair to the xylem?
Travels across the cortex cells via the apoplast pathway
What is the casparian strip?
A thick waxy waterproof band on the endodermis which blocks the apoplast pathway
What are xylem pits?
Small holes in the xylem which allow water to move laterally between vessels
Explain how water enters xylem from endodermis in the root and is then transported to the leaves (6)
- Casparian strip blocks apoplast pathway and only allows symplast pathway
- Ions and salts actively transported across cell membrane into xylem
- Down water potential gradient
- Transpiration from leaves creates cohesion between water molecules
- Which adhere to the xylem vessel walls
- Creating a continuous flow of water
Root pressure moves water through the xylem. Describe what causes root pressure (4)
- Active transport by endodermis (symplast pathway)
- Of ions/SALTS into xylem
- Lowers water potential
- Water enters by osmosis
Describe how a high pressure is produced in the leaves (3)
- Water potential becomes lower as sugar enters phloem.
- Water enters phloem by osmosis
- Increased volume of water causes increased pressure
What to use to measure rate of transpiration?
Potometer
Give 2 precautions the student should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot
- Cut shoot at a slant
- Cut shoot under water
Xerophyte adaptations
- Reduced number of stomata
- Stomata in pits
- Leaves reduced to spines
- Rolled leaves
- Thin hairs to trap water
- Thick waxy cuticle
What does phloem transport?
Sucrose and organic substances (eg: amino acids)
Sucrose structure
- Disaccharide of glucose and fructose
- Glycosidic bonds (condensation reaction, loss of water)
- Carbon, Hydrogen, Oxygen
2 cell types in phloem tissue
- Companion cells
- Sieve tube elements
What are sieve tube elements?
LIVING cells that form the tube for transporting solutes. Have no nucleus and few organelles.
What are companion cells?
Each siebe tube element has its own companion cell which carries out living functions for them.
Companion cell features:
Contain many mitocondria to synthesise ATP through AEROBIC RESPIRATION for the active transport of solutes.
How are sieve tubes connected?
Sieve plates
What is translocation?
Movement of solutes to where they are needed in a plant (e.g: shoots, roots)
What is an assimilate?
A solute that gets incorporated into the plant tissue
What is source to sink?
Translocation moves solutes from source (where assimilates are produced in higher concentrations) to sink (where assimilates are used, so lower concentration)
How is a concentration gradient maintained from source to sink?
Enzymes convert solutes at the sink to other storage substances (e.g: starch) so there is always a lower concentration at the sink than the source.
What is Mass Flow Hypothesis?
Best supported theory of how solutes are transported from source to sink by translocation.
Describe the mass flow hypothesis for the mechanism of translocation in plants (5)
- Companion cells
- Actively transport sucrose into phloem sieve tubes
- Lowering water potential in sieve cells + water enters via osmosis
- Increase in hydrostatic pressure causes mass movement towards sink end
- Sucrose used in respiration or stored as starch.
