Transport in Plants Flashcards
Define transpiration.
The evaporation of water vapour, meanly through the stomata in the surface of a leaf.
Is transpiration a passive or active process?
Passive process. It uses energy from the sun.
How does transpiration work?
It uses energy from the sun, which evaporates water from mesophyll cells.
As water vapour diffuses from the air spaces in the mesophyll and through the stomata, it’s replaced by water from the mesophyll cells. This in turn is replaced by water from the xylem in the veins of the leaf due to cohesiveness which is when some water molecules are pulled into the leaf, and others then follow.
This means that the whole column of water in the xylem moves upwards, against the force of gravity.
Water cannot be pulled up further than 10m, so how can tall trees overcome things problem?
Xylem vessels are very narrow, and water molecules tend to stick together due to the hydrogen bonds between them.
As water moves out of the xylem in a leaf, it drags other molecule behind it due to COHESION.
Because the vessels are so narrow, the column of water behind does not breaks and water is pulled up all the way from the roots. The pulling force is so great that the column of water is being stretched - its under tension. This tension in the column of water tends to pull the walls of the vessels inwards slightly.
But the lignin in the walls is strong enough to stop vessels collapsing.
Disadvantage of transpiration?
The loss of so much water by transpiration seem wasteful. However it is the unavoidable effect of the need for leaves to take in CO2 for gas exchange.
Advantages of transpiration?
- the stream of water transports mineral ions around the plant.
- the evaporation of water from the leaves has a cooling effect.
- when leaves are exposed to bright sunlight, transpiration can reduce the possibility of leaves overheating and the enzymes begin denatured.
Where is water water potential greatest?
In the root and bottom of the plant.
Where is water potential lowest?
At the top of the plant.
Define polar molecule.
A molecule with a slight positive charge in one part of the molecule, and a slight negative charge in another part.
How is water a polar molecule?
Because the hydrogen atoms have a slight positive charge.
But the oxygen atom has a slight negative charge.
Why do water molecules have a strong cohesion?
Because water is a polar molecule. (Its hydrogen ions have a slight positive charge, but oxygen atom has a slight negative charge).
Opposite charges attract one another so water has a strong cohesion between the molecules.
The cooing effect of transpiration that prevents leaves from overheating is due to water’s large…
Why is this?
Latent heat of vaporisation.
This is due to the fact that water molecules are cohesive and so relatively difficult to separate.
Define latent heat vaporisation.
The heat required to turn liquid at its boiling temperature into a gas at the same temperature.
Define solvent.
A liquid that’s able to dissolve other substances.
List 4 featured of xylem vessels that make them specialised for the transport of water.
Water is…
- polar molecules (so strong cohesion)
- latent heat vaporisation (so transpiration has cooling effect the protects leaves from overheating)
- heat capacity (so large volumes of water eg lakes don’t change temperature quickly)
- solvent (wide range of substances soluble in water, which is why solute should can be transported in blood).
What’s translocation?
The movement of solutes (eg sugars like sucrose, also amino acids) to where they’re needed in a plant.
Translocation occurs from a source of a sink.
How does translocation keep working? Ie how does it keep going from a source to a sink without the sink eventually becoming a source?
Because enzymes maintain the concentration gradient from the source to the sink by changing the solutes at the sink (eg breaking them down).
This ensures there’s always a lower concentration at the sink.
Where does translocation occur?
In phloem sieve tubes which are elongated cells joined end to end to form a chain. Where the cells meet, they have holes which us why they’re called sieve plates.
Each separate cell in a phloem sieve tube is called a…
Sieve element. It also has another cell alongside it called a companion cell.
In phloem tissues, sieve elements have another cell alongside it: companion cells. What do these do?
Companion cells provide metabolic support to the sieve elements. This is important because the cells of sieve tubes lose many of their organelles including their nucleus as they become specialised.
This allows easier flow of phloem sap through the cell, but also means they cannot repair and maintain themselves so well so rely on companion cells for many of these functions.
What are xylem tubes made of?
Tubes of dead cells linked end to end.
The ‘Mass Flow Hypothesis’ best explains phloem transport.
Describe it.
- Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source. 🏃🏽♀️🏃🏽
- This lowers the water potential inside the sieve tubes. So water enters the tubes by osmosis from the xylem and companion cells. 👬
- This creates high pressure inside the sieve tubes at the source end of phloem. ‼️
- At the sink end, solutes are removed from the phloem to be used.🚰
- This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. 👋🏼
- This lowers the pressure inside the sieve tubes. ⬇️
- The result is a pressure gradient from the source to sink end. ↔️
- The gradient pushes solutes along the sieve tubes towards the sink.
- When they reach the sink, the solutes will be used (eg respiration) or stored (eg as starch).
In translocation, what is the direction of transport?
Where the sinks are in the plant at any point in time.
Supporting evidence for Mass Flow Hypothesis.
- a radioactive tracer (eg radioactive carbon ^14C) can be used to track the movement of organic substances in a plant.
- if a metabolic inhibitor (which stops ATP production) is put into the phloem, then translocation stops - evidence that active transport is involved.
- if a phloem tube is punctured the phloem sap oozes out, suggesting it is under pressure. Phloem sap sampled from a source has a higher sucrose conc. than sap sampled from a sink - confirming that different water potentials would cause osmosis into it out of the sieve tubes into 2 locations.
Evidence against Mass Flow Hypothesis.
- sugar travels to many different sinks, not just to the one with the highest water potential, as the model suggests.
- the sieve plate would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.
What 4 things affect transpiration rate?
- light
- temperature
- humidity of the air
- wind
Explain light as a factor affecting transpiration rate.
The lighter it is, the faster the transpiration rate.
This is because the stomata open when it gets light to let in CO2 for photosynthesis. When it’s dark the stomata are usually closed, so there’s little transpiration.
Explota temperature as a factor affecting transpiration rate.
The higher the temperature, the faster the transpiration rate.
Warmer molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the conc. gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
Explain humidity as a factor affecting transpiration rate.
The lower the humidity, the higher the transpiration area.
If the air around a plant is dry, the conc. gradient between the keaf and air is increases, which increases evaporation.
Explain wind as a factor affecting transpiration rate.
The windier it is, the faster the transpiration rate. Lots of air movement blown away water molecules from around the stomata. This increases the conc. gradient, which increases the rate of transpiration.
