3.3.4.2 Mass transport in plants Flashcards
What is the cohesion-tension theory?
The cohesion-tension theory explains the movement of water in the xylem due to cohesion between water molecules and tension created by transpiration.
How does transpiration create tension in the xylem?
Transpiration causes water to evaporate from the mesophyll cells in the leaves, creating a negative pressure that pulls water up the xylem.
What is cohesion in the context of the cohesion-tension theory?
Cohesion refers to the hydrogen bonding between water molecules that helps form a continuous column of water in the xylem.
What is adhesion in the cohesion-tension theory?
Adhesion is the attraction between water molecules and the walls of the xylem vessels, aiding in the upward movement of water.
What is the function of phloem in plants?
Phloem transports organic substances, such as sucrose and amino acids, from sources to sinks in plants.
What is the mass flow hypothesis?
The mass flow hypothesis proposes that translocation occurs due to pressure differences created by the active loading and unloading of solutes in the phloem.
How does sucrose enter the phloem?
Sucrose is actively transported into the sieve tube elements by companion cells, lowering water potential and causing water to enter by osmosis.
What creates the pressure gradient in the phloem?
The active loading of sucrose at the source increases hydrostatic pressure, while unloading at the sink lowers it, creating a pressure gradient.
What is a ringing experiment and what does it show?
A ringing experiment involves removing a ring of bark (including phloem) from a stem, which interrupts translocation and causes sugars to accumulate above the ring, demonstrating the role of phloem in transport.
What is a tracer experiment in plant transport studies?
Tracer experiments use radioactive isotopes, such as 14C in CO2, to label sugars and track their movement in the phloem via autoradiography.
What evidence supports the mass flow hypothesis?
Evidence includes:
(1) Pressure differences in the phloem shown by sap release when cut,
(2) radioactive tracers showing sugar movement, and
(3) ringing experiments demonstrating phloem transport.
What evidence contradicts the mass flow hypothesis?
Evidence includes:
(1) The presence of bidirectional flow in the same sieve tube,
(2) the observation that sieve plates seem to obstruct flow, and
(3) not all solutes move at the same speed.
What factors affect the rate of transpiration?
Factors affecting transpiration rate include: (1) Light intensity - increases stomatal opening for photosynthesis, (2) Temperature - increases evaporation and diffusion rates, (3) Humidity - reduces water potential gradient when high, (4) Wind speed - removes water vapor, maintaining the gradient,
Use your understanding of the mass flow hypothesis to explain how pressure is generated inside the phloem tube. (3)
- Sucrose actively transported (into phloem);
- Lowering/reducing water potential
OR
More negative water potential; - Water moves (into phloem) by osmosis (from xylem);
Describe the cohesion-tension theory of water transport in the xylem. (5)
- Water lost from leaf because of transpiration / evaporation of water
(molecules) / diffusion from mesophyll / leaf cells;
OR
Transpiration / evaporation / diffusion of water (molecules)
through stomata / from leaves; - Lowers water potential of mesophyll / leaf cells;
- Water pulled up xylem (creating tension);
- Water molecules cohere / ‘stick’ together by hydrogen bonds;
- (forming continuous) water column;
- Adhesion of water (molecules) to walls of xylem;
The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.
Suggest one reason why. (1)
Plant has roots
OR
xylem cells very narrow;
Describe the mass flow hypothesis for the mechanism of translocation in
plants. (4)
- In source / leaf sugars actively transported into phloem;
- By companion cells;
- Lowers water potential of sieve cell / tube and water enters by
osmosis; - Increase in pressure causes mass movement (towards sink /
root); - Sugars used / converted in root for respiration for storage.
Accept starch
Give one similarity between the structure of xylem and phloem tissue.
both are made up of cells joined end to end
xylem vessels (in xylem) and sieve elements (in phloem) both lack nuclei
both are complex tissues
OR
both are made up of more than one cell type
Describe differences between the structure of xylem and phloem tissue.
xylem has lignified walls, phloem does not
xylem has a wide lumen, phloem has a small lumen
xylem has no end walls OR sieve plates, phloem does
xylem has no companion cells, phloem does
xylem has vessels, phloem does not
xylem does not have sieve tube elements, phloem does
xylem has bordered pits, phloem does not
xylem has no cytoplasm/organelles, phloem does
Suggest why plants that have a small height don’t need a xylem to survive.
have a large surface area to volume ratio
the distance that water/minerals need to travel is only short
Explain why large multicellular plants require a specialised transport system.
large plants have a small surface area to volume ratio
there is a long distance from the external surface to the cells
diffusion is not fast enough
named example of a substance that is transported in plants (e.g. sucrose, water)
Describe and explain one way in which sieve tube elements are adapted for the mass transport of sugars.
Approach 1:
no/few organelles/cytoplasm
more room for flow of sugars
OR
Approach 2:
thick walls
resist pressure
Describe and explain one way in which companion cells are adapted for the mass transport of sugars.
contain mitochondria that release energy/ATP
for active transport
Describe and explain ways in which the xylem vessels are specialised for their function.
cellulose/lignified cell walls to provide support under tension
cell walls lined with lignin for waterproofing OR to prevent water loss
cell wall made of cellulose to allow adhesion of water
no cytoplasm OR lack of cell contents OR hollow so the flow of water is easier
continuous tube/column so the flow of water is easier
pits/pores so water can be supplied to surrounding tissues/cells
wide/large diameter/lumen so a large volume of water can be transported
Explain how hydrogen bonding facilitates the movement of water through the xylem.
hydrogen bonding allows cohesion and adhesion
makes water cohesive OR stick together
allows a transpiration stream OR allows a continuous column of water
allows adhesion of water molecules to the lining of the xylem OR to cellulose molecules
What is transpiration?
the loss of water vapour from the leaves
State the environmental factors that affect the rate of transpiration.
For each factor given, explain how it affects the rate of transpiration.
temperature
increased temperature increases the rate of evaporation/diffusion
OR
at high temperatures, stomata close so transpiration stops
humidity
increased humidity decreases rate of transpiration as the water potential gradient is less steep
wind speed OR air movement
higher wind speed causes a steeper water potential gradient
OR
at high wind speed, stomata close and transpiration stops
water availability
reduced water availability causes stomata to close and transpiration stops
OR
when more water is available, there is a steeper water potential gradient between roots and leaves, so transpiration is faster
light intensity
higher light intensity increases transpiration rate as more stomata open
OR
at very high light intensity, stomata close so transpiration stops
Give two reasons why the potometer does not truly measure the rate of transpiration. (2)
- Water used for support / turgidity;
Accept: water used in (the cell’s) hydrolysis or condensation (reactions) for one mark. Allow a named example of these reactions
- Water used in photosynthesis;
- Water produced in respiration;
- Apparatus not sealed / ‘leaks’;
What is the function of xylem in plants?
“Xylem transports water and dissolved mineral ions from the roots to the stem and leaves.”
