3.3 TRANSPORT IN PLANTS Flashcards
Why do plants require transport systems?
Plants require transport systems for:
- metabolic demands = glucose and oxygen are produced during photosynthesis but not all areas of a plant photosynthesise. ions and hormones also require transporting
- size = some plants are small, but still grown throughout their lives. transport from root to stem tip in large plants
- large SA:V = leaves have a large SA:V for gas exchange and the absorption of light. but stems and trunks have relatively small SA:V, so diffusion in inefficient
What does the xylem do?
The xylem transports water and inorganic ions from the root to the rest of the plant
What does the phloem do?
The phloem transports assimilates from leaves to all other parts of a plant
What is the role of water in plants?
The role of water in plants:
- photosynthesis = reactant
- transport = movement of mineral ions
- stability/structure
- turgor pressure (hydrostatic) = provides plants with a hydrostatic skeleton to support stems and leaves
- cooling plants - loss of water through evaporation allows plants to keep cool
- cell expansion = it is the inward movement of water that causes cells to grow in physical size (and sometimes cause roots to penetrate pavements)
How are root hair cells adapted for water intake?
Root hair cells are adapted for water intake by having a large SA:V ratio, which gives more room for water to enter the cell, and is accompanied by a large number of cells that can be put together, and a small osmotic distance for the faster movement of water. They also have fenestrations that allow water in, and a low water potential inside the cell that allows osmosis to happen and maintains the concentration gradient. Also to maintain the concentration gradient, they allow active transport to occur.
What is the process of osmosis into the xylem?
Osmosis into the xylem:
- water moves along the cortex by osmosis
- as water moves, it increases the water potential of that cell compared to the adjacent cell
- maintains a steep concentration gradient so water can continuously travel through the pathway
What is the symplast pathway?
Symplast pathway:
- water enters cytoplasm through partially permeable cell walls
- goes to adjacent cells through plasmodesmata
- water moves by osmosis as cell water moves into the cytoplasm of adjacent cells as it increases the water potential. this maintains the water potential gradient
What is the apoplast pathway?
Apoplast pathway:
- plant cell walls are made of several layers of cellulose
- water soaks into these walls
- water seeps from cell wall to cell wall through intracellular spaces
- as water moves into the xylem, the cohesive forces pull more water along = creates tension so there is a continuous flow
Why are there two pathways (for cortex water in plants)?
Two pathways:
- importance of 2 pathways varies between plant species and different conditions
- in normal conditions = symplastic pathway is more important
- when there is high rates of transportation, the apoplastic pathway is more important
What is the Casparian strip?
Casparian strip:
- once through the cortex, water reaches the casparian strip = a band of waxy material (suberin) that runs around each of the exothermal cells, forming a waterproof layer
- water travelling along the apoplastic pathway is blocked by the strip. this water must therefore move into the cytoplasm and move via the symplast pathway until it gets through the waterproof layer
- the diversion caused by the casparian strip causes water to move into the cytoplasm via a partially permeable membrane- this excludes any potentially toxic solutes from entering living tissues
Reaching the xylem.
Reaching the xylem:
- endodermal cells move more mineral ions into the xylem by active transport
- this decreases the water potential of the xylem, relative to the endodermal cells
- this increases the rate of water moving into the xylem vessel by osmosis due to a steep water potential gradient
- the active pumping of minerals to produce this movement of water results in root pressure which contributes (independent of transpiration) to water being forced up the xylem
What is the proof of active transport in plants?
Proving active transport:
- cyanide affects mitochondria (therefore ATP production) and when cyanide is applied, root pressure disappears
- root pressure rises and falls with changes in temperature, suggesting chemical reactions (e.g aerobic respiration to produce ATP) are involved
- low oxygen levels (or other respiratory substrates) cause root pressure to fall
- xylem sap is forced out of the xylem of cut stem. this process (guttation) even occurs at night when transpiration is low, suggesting root pressure is the contributing force
What is transpiration?
Transpiration is the loss of water vapour from the stems and leaves of a plant, as a result of evaporation from cell surfaces inside the leaf and diffusion down a concentrated gradient through the stomata
How does transpiration work?
How transpiration works:
- water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and diffuse out of the stomata into the surrounding air by diffusion, down a concentration gradient
- the loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves in from the adjacent cell by osmosis (along both apoplast and symplast pathways)
- this is repeated across the leaf to the xylem. water moves out of the xylem by osmosis
Hydrostatic pressure (transpiration).
Hydrostatic pressure:
- removal of water from the top of the xylem also reduces the hydrostatic pressure
- gradient allows water to move upwards
- lignin wall prevents the vessel from collapsing under tension
