P1 Transport in Plants Flashcards
What is translocation?
- The transport of substances from a source to sink. It is multidirectional.
- Places can be sources and sinks, eg. roots act as a sink as they are a good place for storage, but act as a source when they absorb useful substances from the soil.
- Carbohydrates transported in the phloem are called assimilates. The most common assimilate is sucrose. Plants use sucrose instead of glucose because it is less reactive.
What is the structure of the phloem?
- A long, continuous tube-like structure made of cells arranged end to end. Assimilates are transported as part of the sap in the phloem.
- There are sieve-like holes between each cell to allow the sucrose to pass through. Therefore these cells are called sieve tube elements, and the part of the cell containing the holes is called a sieve plate.
- Sieve tube elements don’t have a nucleus or many organelles, maximising the amount of space for sucrose solution to pass through.
- Companion cells sit beside sieve tube elements, and also transport sucrose, but transport sucrose from source to sieve tube element, and sieve tube element to a sink.
- Companion cells have a nucleus and other organelles, so they can provide sieve tube elements with energy and everything else they need.
Source cell –> companion cell –> sieve tube elements –> companion cell –> sink cell
What are the consequences of the removal of phloem?
- Assimilates travelling down the tree from sources (leaves) are stuck in the ring. As they build up above the ring, the tree begins to swell.
- Assimilates aren’t able to travel past the ring, meaning the roots don’t get them. As roots can’t photosynthesise to produce their own useful substances, they begin to die.
- This removal of phloem is called a ringing experiment and provides evidence that translocation occurs in the phloem.
What is hydrostatic pressure?
The pressure exerted by a fluid on the walls of a container.
Describe mass flow through sieve elements.
- Sucrose moves from the source cell to the companion cell and into the sieve tube elements, decreasing the water potential, meaning the content of the sieve tube elements is more concentrated.
- Water then moves from the xylem (very close to the phloem) into the sieve tube element by osmosis, which increases the hydrostatic pressure in the sieve tube element.
- Meanwhile sucrose leaves the sieve tube elements and enters the companion cells next to sink cells, increasing the water potential in the sieve tube elements, so water leaves due to osmosis, decreasing the hydrostatic pressure in the sieve tube element.
- Therefore sucrose solution moves down the hydrostatic pressure gradient from a higher hydrostatic pressure near the source cell, to a lower hydrostatic pressure near the sink cell, where glucose is transported into the sink cell by the companion cells.
How does sucrose move from a source cell, through companion cells to sieve tube elements?
- The movement of sucrose depends on H+ ions, where first a carrier protein moves H+ ions from the companion cell’s cytoplasm to the cell wall using active transport. This creates a concentration gradient (high concentration of H+ ions in the cell wall and a low concentration of H+ ions in the cytoplasm).
- H+ ions return to the cytoplasm via a co-transport protein, which also transports sucrose. Once in the companion cell, sucrose diffuses into sieve tube elements.
- The movement of sucrose back through companion cells to sink cells is active (required ATP).
How to conclude if evidence is for or against the mass flow hypothesis?
In the mass flow hypothesis:
1. Water moves in and out of the phloem by osmosis.
2. Pressure moves the sucrose solution.
3. It involves active processes that use ATP.
Eg. 1: when plant stem is cut, sap leaks out - this supports the MF hypothesis as sap is just dissolves substances in water, pressure moves the sucrose solution, and when something under pressure is cut, the stuff inside it leaks out - so this supports that the phloem is under pressure.
Eg. 2: when the rate of respiration increases, the rate of translocation in the phloem also increases - when the rate of respiration increases, plant cells produce more ATP, so more ATP is available for processes like active transport (which takes place in translocation), therefore this supports the mass flow hypothesis.
Eg. 3: sap near a source has a higher concentration of sucrose than sap near a sink - the concentration of sucrose affects the movement of water by osmosis, a higher concentration of sucrose at the source and a lower concentration at the sink would suggest that sieve tube elements in these locations have different water potentials, therefore water moves into the phloem by osmosis at the higher sucrose concentration, and out of the phloem by osmosis at the slower sucrose concentration - supporting mass flow hypothesis.
What is the evidence against mass flow hypothesis?
- The speed of substances - if all substances are transported under pressure, they would be expected to travel at the same speed through the phloem. However, evidence has shown that this isn’t true - sucrose dissolved in sap travels faster than amino acids.
- The function of sieve plates - sucrose solution moves down sieve tube elements, to make this as efficient as possible the phloem should provide maximum possible space for the solution, however sieve plate holes do still hinder mass flow, so they have a reduced efficiency of movement (sieve plates could exist to prevent sieve tube elements from bursting under pressure - but not confirmed)
- Sucrose delivery - sucrose should travel from areas with higher concentration of sucrose to areas with the lowest concentration of sucrose. However sucrose doesn’t always travel to areas with the lowest concentration, suggesting the movement of sucrose may not be entirely due to differences in water potentials and the resulting hydrostatic pressure.
Why do plants require a transport system?
- To meet high metabolic demands.
- Ensure nutrients reach all tissues.
- Overcome low SA:V ratio.
- Allow diffusion across large distances.
Describe gas exchange in plants.
- Gas exchange takes place in leaves via the stomata, which are opened and closed by guard cells.
- During the day, the stomata are open and both photosynthesis and respiration occur. Whereas at night the stomata close and only respiration takes place.
What is transpiration?
- The loss of water vapour through the stomata as a result of evaporation and diffusion.
- It is increases by increasing light (more stomata are open), increasing temperature (increasing kinetic energy of water molecules, therefore increasing the rate of evaporation) and increasing wind/decreasing humidity (both remove water vapour outside the leaf, increasing the water potential gradient).
Describe the transpiration stream.
- Water and minerals are actively transported into root cells, they then travel to the xylem by two different pathways:
1. Symplast pathway: water passes through the cytoplasm of cells, which are connected by plasmodesmata.
2. Apoplast pathway: water passes through cell walls until it reaches the Casparian strip (impermeable to water) and water is forced into the cell’s cytoplasm, where it travels to the xylem by the symplast pathway.
How does water travel up the xylem?
Cohesion-tension theory:
- The evaporation of water lowers the water potential in the mesophyll cells in the leaf. Water moves along a water potential gradient and pulls up a continuous column of water in the xylem.
- This puts the xylem under tension (there is negative pressure within it). Inside the column, water molecules adhere (attraction between unlike molecules) to the side of the walls, and water molecules are connected by hydrogen bonds (cohesion).
What happens when the water supply is low?
Most plants close their stomata to minimise water loss.
Adaptations of xerophytes:
- Hairs, rolled leaf and stomata in pits (all allow the plant to trap water vapour in the air, decreasing the water potential gradient).
- Closed stomata (so water cannot escape).
- Thick, waxy cuticle (reduces evaporation and increases diffusion distance).
