Chapter 9- Transpiration, Translocation, Plant Adaptations Flashcards
What is the cohesion-tension theory
1) water evaporates from the leaves at the top of the xylem
2) this creates a tension which pulls more water into the leaf
3) water molecules are cohesive so when some are pulled into the leaf, others follow. This means the whole column of water in the xylem from roots to leaves moves upwards.
4) water enters the stem through the root cortex cells.
What is adhesion
Adhesion is also partly responsible for the movement of water. Water molecules are attracted to the walls of the xylem vessels. This helps water to rise up through xylem vessels.
Process of transpiration
1) Water enters the roots by osmosis and is transported up the xylem until it reaches the leaves.
2) when at the leaves, water moves across membranes by osmosis and diffusion in the apoplast pathway from the xylem through the cells of the leaf, where is evaporates from the cell walls of the mesophyll cells into the air spaces.
3) the water vapour then moves into the external air through the stomata along a diffusion gradient.
Why does transpiration happen?
It is a result of gas exchange. A plant needs to open its stomata to let in carbon dioxide for photosynthesis, but this also lets out water out as there is a high concentration of water inside the leaf than in the air outside, so water moves out of the lead via the water potential gradient.
Factors affecting transpiration rates
1) light intensity- the greater the light, the faster the rate if transpiration because stomata are open when it gets light. The lighter it is, the wider they open.
2) Temperature- higher the temperature, the faster the rate of transpiration. Warmer water molecules have more kinetic energy and evaporate from the cells inside the leaf faster. This increases water potential gradient between the leaf and air, making water diffuse out of the leaf faster.
3) Humidity - the lower the humidity, the faster the rate of transpiration. The is because a lower humidity, increases the water potential gradient between the leaf and air, which increases transpiration rate.
4) wind- the more wind, the faster the rate of transpiration. Lots of air movement blows away water molecules around the stomata. This increases water potential gradient, which increases transpiration rate.
What are xerophytes + examples
They are plants that are adapted to live in dry climates. Their adaptations prevent them from losing too much water by transpiration. Examples include cacti and marram grass.
Adaptations of xerophytes
1) they have a thick waxy cuticle which reduces water loss by evaporation because the layer is waterproof.
2) they have spines instead of leaves which reduces surface area for water loss.
3) sunken stomata- they have stomata located in pits which reduce air movement and produces a humid air which reduces water potential gradient, reducing transpiration.
4) Hairy leaves to create a microclimate of still, humid air reducing water vapour potential gradient, reducing transpiration rate.
5) curled leaves trap moist air, which reduces water potential gradient, reducing transpiration rate.
What are hydrophytes + example
Hydrophytes are plants which live in aquatic habitats. An example would be water lillies
Adaptations of hydrophytes
1) air spaces in the tissues help the plants to float and can act as a store of oxygen for respiration. Floating also increases the amount of light they receive.
2) stomata are present on the upper surface of floating leaves to maximise gas exchange
3) hydrophytes often have flexible leaves and stem. The plants are supported by the water around them so do not need rigid stems for support. Flexibility helps to prevent damage by water currents.
4) thin waxy cuticle as water availability is not an issue. And wax production is a waste of energy.
What is translocation
Translocation is the movement of dissolved substances (assimilates) to where they are needed in a plant. It is an energy requiring process that happens in the phloem.
What is a source + example
The source of a substance is where it’s made (high concentration here). The source for sucrose is usually the leaves.
What is the “sink”
The sink is the area where the substance is used up (lower concentration here). Example include food storage organs, meristems in the roots, stems and leaves.
The mass flow hypothesis
1) active transport is used to load the solutes (sucrose from leaves) into the sieve tubes of the phloem at the source. This lowers water potential in sieve tubes, so water also enters tubes by osmosis from xylem and companion cells. This creates high pressure inside the sieve tubes at the source end of the phloem.
2) at the sink end, solutes are removed from the phloem to be used up. This happens by diffusion. The removal of solutes increases water potential in sieve tubes, so water also leaves the tubes by osmosis. This lowers pressure in sieve tubes.
3) this results in a pressure gradient from the source to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. The higher the concentration of sucrose at the source, the higher the rate of translocation.
What is active loading
Active loading is used at the source to move substances into the companion cells from surrounding tissues, and from companion cells into sieve tubes against a concentration gradient.
Process of active loading
1) in the companion cell, ATP is used to actively transport H+ ions out of the cell and into surrounding tissue cells. This sets up a concentration gradient as there are now more H+ ions in surrounding tissue than in the companion cell.
2) A H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down the concentration gradient). A sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ion is used to move sucrose molecules into the cell against its concentration gradient.
3) sucrose molecules are then transported out of companion cells and into the sieve tubes by the same process.
