Transport In Plants 3.3 Flashcards
What are plasmodesmata?
-gaps in the cell wall containing cytoplasm that connects two cells
What is the apoplast pathway?
-water passes through the spaces in the cell walls and between the cells
-water moves by mass flow rather than by osmosis, also dissolved mineral ions and salts can be carried with the water
-moves due to the cohesive force of water , water molecules stick together forming a continuous stream of water
-this pathway transports water faster than symplast pathway
What is symplast pathway?
-water enters the cell cytoplasm through the plasma membrane, it can then pass through the plasmodesmata from one cell to the next
-water moves by osmosis towards the xylem as each successive cell’s cytoplasm has a lower water potential
Is the water potential in a plant cell negative or positive?
-water potential of plant cell is always negative as the cytoplasm will contain mineral ions and sugars (solutes)
What happens once the cell has taken up sufficient water?
-once the cell is full of water it is described as turgid
-the water inside the cell starts to exert pressure on the cell wall called the pressure potential
-as the pressure potential builds up it reduces the influx of water
What is a potometer?
-a device that can measure the rate of water uptake as a leafy stem transpires
What is transpiration?
-the loss of water vapour from the aerial parts of a plant, mostly through the stomata in the leaves
Describe the process of transpiration
1-water enters the leaf through the xylem and moves by osmosis into the cells of the spongy mesophyll. It may also pass along the cell wall via the apoplast pathway
2-water evaporates from the cell walls of the spongy mesophyll
3-water vapour moves by osmosis out of the leaf through the open stomata. This relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf. This is known as the water vapour potential gradient. There must be a less negative (higher) water vapour potential inside the leaf than outside
What is transpiration a consequence of ?
-transpiration is a consequence of gaseous exchange
Why is transpiration important?
As water vapour is lost from the plant it draws water up the stem as a transpiration stream. This movement:
-transports mineral ions up the plant
-maintains cell turgidity
-supplies water for growth, cell elongation and photosynthesis
-supplies water that, as it evaporates , can keep the plant cool on a hot day
What are the environmental factors that affect transpiration rate?
-light intensity - in light the stomata open to allow gaseous exchange for photosynthesis. Higher light intensity increases the transpiration rate
-temperature - a higher temperature will increase the rate of transpiration e.g.it increases the rate of evaporation from the cell surfaces so that the water-vapour potential in the leaf rises
-relative humidity - higher relative humidity in the air will decrease the rate of water loss, because there will be a smaller water vapour potential gradient between air spaces in the leaf and the air outside
-air movement (wind)- air moving outside the leaf will carry away water vapour that has just diffused outside the leaf, this will maintain a high water vapour potential gradient
-water availability -if there is little water in the soil then plant cannot replace the water that is lost. If there is insufficient water then the stomata will close and the leaves wilt
What is adhesion?
-the attraction between water molecules and the walls of the xylem vessel
What is cohesion?
-the attraction between water molecules caused by hydrogen bonds
What processes help water move up a plant?
-root pressure-the action of the endodermis moving minerals into the medulla and xylem by active transport draws water into the medulla by osmosis. Pressure in the root medulla builds up and forces water into the xylem ,pushing water up the xylem, root pressure can push water a few metres up the stem
-transpiration pull- loss of water by evaporation from the leaves must be replaced from water coming up the xylem. Water molecules are attracted to each other by forces of cohesion. These cohesion forces are strong enough to hold water molecules together in a long chain or column,the whole column is pulled up as one chain. The pull from above creates tension in the column of water (why xylem is strengthened by lignin to prevent vessel collapsing under tension) This is called cohesion-tension theory
Capillary action -water molecules are also attracted to the lignin in the xylem walls. This is called adhesion. As the xylem walls are very narrow these forces of attraction can pull the water up the sides of the vessel.
What does cohesion-tension theory rely on?
-the plant maintaining an unbroken column of water all the way up the xylem
-if water column is broken in one xylem vessel, then the water column can still be maintained through another vessel via the bordered pits
What are assimilates?
-substances made by the plant, using substances absorbed by the environment
What is translocation?
-an energy- requiring process transporting assimilates ,especially sucrose, in the phloem between sources (e.g. leaves) and sinks (e.g roots, meristem)
How does active loading occur during translocation?
-companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells. It is a form of facilitated diffusion involving cotransporter proteins which allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase. As a result of that, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through plasmodesmata.
How are assimilates removed at the sink?
As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis, as a result increasing the hydrostatic pressure of the sieve tube. Therefore, water moves down the sieve tube from an area of higher pressure to an area of lower pressure. Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube. This in turn means that water leaves the sieve tube by osmosis, as a result reducing the pressure in the phloem at the sink.
-therefore the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed
What are five precautions you should take when using a potometer?
1- set it up under water to make sure there are no air bubbles inside the apparatus
2-ensure that the shoot is healthy
3-cut the stem under water to prevent air entering the xylem
4-cut the stem at an angle to provide a large surface area in contact with the water
5-dry the leaves
What is a hydrophyte ?
-a plant adapted to living in water or where the ground is very wet
What is a xerophyte?
-a plant adapted to living in dry conditions
What are the adaptions of the xerophyte Marram grass?
-leaf is rolled longitudinally so that air is trapped inside, the air becomes humid, which reduces water loss from the leaf. The leaf can be rolled more tightly in very dry conditions
-there is a thick waxy cuticle on the outer side of the rolled leaf (upper epidermis) to reduce evaporation
-the stomata are on the inner side of the rolled leaf (lower epidermis) so they are protected by enclosed air space
-the stomata are in the pits in the lower epidermis, which is also folded and covered by hairs. These adaptations help to reduce air movement and therefore loss of water vapour
-the spongy mesophyll is very dense, with few air spaces- so there is less surface area for evaporation of water
What are the adaptations of the xerophyte cacti ?
-they are succulents- they store water in their stems which becomes fleshy and swollen. The stem is often ribbed or fluted so that it can expand when water is available.
-the leaves are reduced to spines, this reduces the surface area of the leaves. When the total leaf surface area is reduced, less water is lost by transpiration
-the stem is green for photosynthesis
-the roots are very widespread to take advantage of any rain that does fall
What adaptions does the hydrophyte the water lily have?
-many large air spaces in the leaf, this keeps the leaves afloat so that they are in the air and can absorb sunlight
-the stomata are on the upper epidermis, so that they are exposed to the air to allow gaseous exchange
-the leaf stem has many large air spaces. This helps with buoyancy, but also allows oxygen to diffuse quickly to the roots for aerobic respiration
How do hydrophytes transpire?
-they have hydathodes at the tips or margins of their leaves
-these structures can release water droplets which may then evaporate from the leaf structure
Prevention-do not allow air to enter cut end/ shoot
Explanation-prevent airlock/ ensures continuous column of water
(Prevention-keep temperature/ humidity constant
Explanation-affects rate of transpiration/ evaporation of water
Prevention-keep screw clip closed
Explanation-prevents entry of water whilst measuring)
Explain how glucose produced in photosynthesis is translocated to parts of the plant where glucose is metabolised or stored (6 marks)
PHLOEM LOADING
-Glucose is converted to sucrose/ assimilate in photosynthesising cells
-sucrose loaded into phloem by apoplast route
-active transport of H+ from companion cells into surrounding cells , creates a H+ concentration gradient
-H+ moves into companion cell using a co-transport protein
-sucrose enters phloem via plasmodesmata (passive loading via simplest route)
-entry of sucrose decreases water potential of phloem/ sieve elements
-water enters from xylem / from surrounding cells this results in a higher hydrostatic pressure
MASS FLOW
-there is a bulk transport of sucrose caused by a pressure difference
-entry/ exit of water/ solutes affects hydrostatic pressure
-movement from source to sink(leaf is source)(root is sink)
-assimilate moves from phloem to sink down pressure gradient
PHLOEM UNLOADING
-sucrose diffuses from phloem to surrrounding cells, sucrose is converted back to glucose. Glucose is used for respiration or converted to starch for storage
-concentration gradient of sucrose maintained between phloem and cells
-loss of sucrose increases water potential of phloem, water leaves phloem to surrounding cells/xylem this results in a lower hydrostatic pressure pressure
-remove operculum
-use pins to hold fish (rather than hand)
-remove gills and observe under a microscope
