Gas Exchange and Mass Transport in Plants Flashcards
What do plant cells require?
- They require carbon dioxide, water and light for photosynthesis, which produces the molecules the plant needs to grow.
- They require oxygen and glucose for respiration.
What are dicotyledonous plants?
A group of flowering plants.
How does gas exchange take place in plants?
The palaside mesophyll is the site of photosynthesis where a lot of carbon dioxide is used up and a lot of oxygen is released. This creates a diffusion gradient which causes the carbon dioxide to diffuse into the stomata and through the spongy mesophyll. It also means that oxygen diffuses through the air spaces in the spongy mesophyll and out of the stomata.
How do the stomata control the movement of gases in and out of the plant?
- When the water potential outside of the guard cell is lower compared to the inside, water moves out of the guard cells via osmosis. This causes them to become flaccid and the stomata to close to prevent water loss.
- When the water potential outside of the guard cells is higher compared to the inside, water moves into the guard cells via osmosis. The guard cells become turgid which causes the stomata to open.
What is the structure of the leaves of a dicotyledonous plant and how is it adapted for gas exchange?
- The spongy mesophyll contains many small air spaces, which allow the gases to diffuse in and out of the plant leaves.
- The stomata open and close to let gases in and out.
How do dicotyledonous plants limit water loss?
- The guard cells become flaccid when the plant has a lack of water which causes the stomata to close and limits water loss.
- The stomata are at the bottom of the leaf, which causes the water vapour to accumulate. This reduces the diffusion gradient and that means less water is lost through transportation.
- The way cuticle on top of leaves reduces the amount of water loss from the surface of the leaf through evaporation.
What are xenophobic plants?
They are plants that live in dry, windy warm conditions and have adaptations that help them survive in a habit with limited water.
How are xenophobic plants adapted to survive in conditions with limited water?
- The stomata have sunken pits to trap water vapour.
- The stomata have hairs that trap water vapour.
- The plants have curved leaves to trap water.
- They have fewer stomata compared to other plants.
- They have a thick waxy cuticle, which increases the diffusion distance and decreases the diffusion of water vapour out of the leaves.
Mass Transport
How does water enter the root hairs cells.
The water potential outside of the root hair cells is higher compared to inside which means water moves into the cell via osmosis.
What is the structure and function of the xylem?
- Transport water and mineral ions from the roots to the shoots.
- The walls have lignin, which kills the cells and makes sure that the xylem forms uninterrupted tubes of dead cells with no end walls.
- The cell walls have pits which allow for the movement of substances in and out of the xylem.
Describe the cohesion-tension theory for the movement of water in the cells.
- Water evaporates off of the surface of the leaves ( transpiration).
- The water potential inside the mesophyll cells decreases, causing water to move out of the xylem cells and into the mesophyll cells via osmosis.
- This creates tension, which pulls up the continuous column of water.
- The continuous column of water forms because the water molecules are cohesive due to the hydrogen bonds forming between them and sticking together.
- The water molecules also form hydrogen bonds with the walls of the xylem and adhere to them. This makes it easier for the water columns to move against gravity.
- The whole column of water moves up so water enters the xylem through the roots to replace it, via osmosis.
Explain how the structure of the phloem is linked to its function.
- It is made up of cells called sieve tubes which have no nucleus and few organelles so that the solutes can flow through the phloem easily. The sieve tubes have sieve plates that allow solutes to pass through them.
- It has companion cells which have a lot of mitochondria which produce the ATP needed to actively transport solutes from the companion cells to the phloem l.
Explain the mass flow theory for the translocation of solutes in the phloem.
- At the sources, sucrose produced by the companion cells is actively transported to the phloem. This decreases the water potential of the sieve tubes at the source.
- This causes water to move from the xylem cells to the phloem. This increases the hydrostatic pressure inside the phloem.
- At the sources, sucrose is broken down into glucose and used for respiration. This increases the water potential inside the phloem.
- This causes water to move out of the phloem and into the xylem cells. This decreases the hydrostatic pressure inside the phloem.
- The solutes move down the pressure gradient.
What is the evidence that opposes the mass flow hypothesis?
- The solutes need to travel to multiple sinks not the ones with the highest lowest hydrostatic pressure.
- The solutes don’t just travel down the phloem they also need to travel in the other direction.
- The presence of sieve plates means the pressure must be really high to force the solutes through the phloem.
What is the evidence that supports the mass flow hypothesis?
- If the plant is punctured near the source tissues like the leaves the cell sap flows out. This indicates that at the sources there is a high pressure.
- The phloem tissue near the sources has a higher concentration of sugar compared to the sinks. This means that the sugars move into the phloem at the sources and out at the sinks.
- Removing a ring of bark from a tree prevents the translocation of sugars to the roots, causing it to die. This shows that the phloem is essential for the translocation of sugars from the sources to the sinks
What is translocation?
The movement of solutes from the sources to the sinks using energy released by ATP.
What are sources?
Where the solutes are made.
What are sinks?
The place where the solutes are used up?
