Transport systems Flashcards
Why do plants need transport systems?
Metabolic demands
Size
Small SA: V ratio- cannot rely on diffusion alone
What are dicots?
They make seeds that contain 2 organs that contain food for the developing embryo
Herbaceous dicots- soft tissue, short life
Arborescent dicots- hard tissue, long life
Define vascular system and bundle
Vascular system- xylem and phloem
Vascular bundle- transport tissues arranged together
Xylem
Non living tissue
Transports water and mineral ions
Supports the plant
Parenchyma tissue packs around the xylem, storing food and tannin deposits (bitter chemical preventing herbivore attack)
Lignified walls so that xylem can withstand high pressures - provide mechanical strength
Water can leave the xylem through unlignified pits
Phloem
Living tissue that transports sugars in the plant
Flow can go both up and down
Transporting vessel- sieve tube element
Sieve plates allow the content pf the phloem to flow through
Phloem has no nucleus
Companion cells are linked to the sieve tube element via plasmodesmata
Companion cells provide energy to the cell
How are the roots adapted for their function?
Large SA:V ratio
Microscopic size means they can penetrate easily between soil particles
Thin surface layer- short diffusion distance
Concentration of solutes in the soil maintains a water potential gradient between the soil and roots
What are the two ways that water moves from the roots to the xylem?
Symplast pathway- water moves through the cytoplasm of cell
Apoplast pathway- water moves through the cell walls
When does water change pathways?
Water moves through the apoplast pathway until it reaches the endodermis, where there is a casparian strip. It is waterproof so water flowing in the apoplast pathway can no longer go through. It joins the symplast pathway. Water moves through the selectively permeable membrane, which excludes any potentially toxic solutes from getting to living tissue.
Describe the process of transpiration
When stomata are open to allow gas exchange, water vapour also evaporate out by diffusion down a concentration gradient
This creates a shortage of water, lowering the water potential of the cell
Water is drawn up the xylem to replace the water lost- transpiration pull
The combined effects of cohesion and adhesion lead to water exhibiting capillary action (it rises up a narrow tube against the force of gravity)
Define transpiration
The loss of water vapour from the leaves and stems of plants
What are the factors affecting transpiration?
High humidity- lowers transpiration rate
Smaller water potential gradient between the leaf and outside
Increase in temperature- Increased KE of water molecules- increases rate of evaporation of water molecules
Increasing light intensity- increasing numbers of open stomata
Less air movement (eg hairs) decreases transpiration rate as the water potential around the stomata increases
Soil- water availability
How is glucose transported from the leaves to the rest of the plant?
It is converted into sucrose for transport ( because it is not used in metabolism as readily as glucose so it less likely to be metabolised)
When it reaches the cells where it is needed it is converted back into glucose or starch for storage
Where does the transport in translocation happen?
From source- sink, transporting assimilates
Sources- green leaves/ stems, food stores in seeds
Sinks- growing roots, meristems.
Describe phloem loading - apoplast route
Hydrogen ions are actively pumped out of the companion cells using ATP
This creates a concentration gradient of hydrogen ions outside the cell
Hydrogen ions return to the cell via co transport proteins, alongside sucrose
The sucrose concentration in the companion cells increase, so they move to the sieve tube via plasmodesmata
This reduces the water potential of the sieve tube, so water moves in via osmosis
This creates turgor pressure, which drives mass transport
Describe the process of phloem unloading
Sucrose diffuses from the phloem into the surrounding cells that need it
The sucrose is converted into other forms to maintain the concentration gradient
The loss of assimilates from the phloem leads to a rise in the water potential, therefore water moves into surrounding cells or to the xylem via osmosis
