Mass Transport in plants Flashcards
transpiration
loss of water vapour from the stomata by evaporation
four facts that can effect the rate of this;
light intensity
temperature
humidity
wind
transpiration- light intensity
-positive correlation
-higher the light intensity, the more stomata that open
-this provides a larger surface area for evaporation
transpiration-temp
-positive correlation
-more heat means more kinnetic energy
-therefore faster moving molecules
-this increases levels of evaporation
transpiration- humidity
-negative correlation
-the more water vapor in the air, the more positive the water potential is outside the leaf
-this reduces the water potential gradient and therefore reduces evaporation
transpiration- wind
-positive correlation
-the windier it is the more humid air is blown away
-this maintains the water potential gradient
-increasing evaporation
transpiration- movement of water
-water moves up the plant from the roots against gravity
-this could several metres against gravity in large trees
this is therefore only possible due to the cohesion-tension theory
the three principles that are involved in cohesion-tension theory
- cohesion
- capillarity- adhesion
- root pressure
cohesion tension theory
cohesion
-water is a dipolar molecule (slightly negative charge on the oxygen, slightly positive charge on the hydrogen)
-this enables hydrogen bonds to form between the hydrogen and oxygen of different water molecules
this creates cohesion between water molecules
they stick together
therefore water travels up the xylem as a continuous water column
capillarity (adhesion )
-adhesion of water is when water sticks to other molecules by forming hydrogen bonds
-water adheres to the xylem walls
-the narrower the xylem the bigger the impact of the capillarity
-this is demonstrated in the straw model, the narrow the straw the less pull (sucking action) is required to draw up the liquid. this is because more water molecules are in contact with the straw walls and adhering
this adhesion helps hold the water column up against gravity
root pressure
as water moves into the roots by osmosis, it increases the volume of liquid inside the root and therefore, the pressure inside the root increases. This increase in root pressure forces water above it upwards- positive pressure-
movement of water up the xylem
- water evaporates out of stomata leaves
this loss in water volume creates a lower pressure
-when this water is lost by transpiration more water is pulled up the xylem to replace it - due to hydrogen bonds between water molecules
-they are cohesive
-this creates a column of water within the xylem
3.water molecules also adhere to the walls of the xylem
-this helps to pull the water column upwards - As this column of water is pulled up the xylem it creates tension
-pulling the xylem inward to become narrower
What does cohesive mean
Stuck together
What does adhere mean
Stick
Transport of organic substances
-phloem is the tube responsible for
-e.g sugars
-this transport is called translocation
-explained by the mass flow hypothesis
What is the phloem tissue made up of
- different cells
-one cell type is the sieve tube elements
-these cells are long and thin and arranged as a column
-the name sieve is used to describe the fact that the end walls are perforated like a sieve
-next to these cells are companion cells
Sieve tube elements
-living cells
-contain no nucleus
-very few organelles
-this is to make the cells more hollow
-therefore provides less resistance to the flow of sugars
Companion cell
-as the sieve tube elements have few organelles they depend on the companion cell for resources
-the companion cells provide atp required for the active transport of organic substances
Mass flow hypothesis
-organic substances, such as sucrose, move in a solution from the leaves where they are created in photosynthesis, to respiring cells
-the site of production is called the ‘source’ and the site of use is called the sink
Source to sink
-sucrose lowers the water potential of the source cell
-this causes water to enter by osmosis
-this increases the hydrostatic pressure in the source cell
-the respiring cell is using up sucrose, and therefore it has a more positive water potential
-as a result water leaves the sink cell by osmosis
-this decreases the hydrostatic pressure in the sink cell
This results in the source cell having a higher hydrostatic pressure than the sink cell, so the solution is forced towards the sink cell via the phloem
Translocation
-photosynthesis occurring in the chloroplasts of leaves creates organic substances e.g sucrose
-this creates a high concentration of sucrose at the site of production
-therefore sucrose diffuses down its concentration gradient into the companion cell via facilitated diffusion
-active transport of H+ occurs from the companion cell into the spaces within the cell walls using energy from ATP
-this creates a conc gradient and therefore H+ moves down their conc gradient via carrier proteins to the sieve tube element
-the increase of sucrose in the sieve tube element lowers the water potential
So water enters the sieve tube elements from the surrounding xylem vessels via osmosis
The increase in water volume in the sieve tube element increases the hydrostatic pressure causing the liquid to be forced towards the sink
Sucrose is either used in respiration at the sink or stored as insoluble starch
More sucrose is actively transported into the sink cell, which causes the water potential to decrease
This results in osmosis of water from the sieve tube element into the sink cell
Some water also returns to the xylem
The removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure decreases
Therefore the mass flow of sucrose occurs down a hydrostatic pressure gradient within the sieve tube elements
Investigating translocation
-tracer and ringing experiments
-used to prove that it is the phloem that transports the sugars and not the xylem
Tracing
-radioactively labelling carbon
-plants are provided with only radioactively labelled carbon dioxide and over time this is absorbed into the plant and used in photosynthesis to create sugars which all contain radioactive labelled carbon
-thin slices from the stems are then cut and placed on an x-ray film that turns black when exposed to radioactive material
When the stems are placed on the X-ray film, the section of the stem containing the sugars turn black
This highlights where the phloem is and shows that sugars are transported here
Ringing experiment
-a ring of bark and phloem is that the trunk swells above the removed section
-analysis of the liquid in thus swelling shows it contains sugar
-this shows that when the phloem is removed, the sugars cannot be transported and therefore proves the phloem is responsible for the transport of sugars
