9 - Transport in Plants Flashcards
What are the three main reasons that multicellular plants need transport systems?
- Metabolic Demands 2. Size 3. Surface Area to Volume ratio
Why do plants need transport systems because of metabolic demands?
Many internal or underground parts of a plant cannot make their own glucose for energy via photosynthesis, so need glucose and oxygen transported to them. Hormones and mineral ions also need to be moved.
Why do mineral ions need to be moved around a plant?
So they can be moved to all cells for protein production
Why is the surface area to volume ratio not simple in plants?
The leaves are adapted to have a large surface area to volume ratio, but when taking into account the stems, trunks and roots the overall SA:V ratio may be quite small
Where are vascular bundles in the stem of a dicot and why?
Around the edges to give strength and support
Where are the vascular bundles in the root of a dicot and why?
In the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind
Where is the main vein carrying the vascular bundle of a dicot’s leaf found and why?
The midrib of the leaf in order to help support the structure of the leaf
What other vascular tissues are found in a dicot leaf apart from the main vein?
Many small, branching veins which spread throughout the leaf and function in both transport and support
How is the main vascular bundle structured in the leaf of a dicot?
Xylem above phloem, vascular bundle roughly circular cross-section
How is the vascular bundle structured in the stem of a dicot?
Phloem on the outside, xylem on the inside, separated by a strip called the cambium)
What is the outermost layer of a plant stem called?
Epidermis
How is the vascular bundle of a dicot root arranged?
A ‘X’ shaped xylem with phloem vessels in the gaps of the X, with a ring called the endodermis enclosing all this
What is the tissue immediately surrounding a vascular bundle in a dicot root called?
Endodermis
What is the area between the vascular bundles and epidermis in a dicot root and stem called?
Cortex
What are the two main functions of xylem in a plant?
- Support 2. Transport of water and mineral ions
What is a feature of most cells found in the xylem?
Most are dead when functioning in the plant
What determines the rigidity of the xylem?
The shape of the lignin- some shapes give more support than others
What is the main part of xylem and what is its structure?
Xylem vessels, which are made up of long tubes of dead cells with no cytoplasm or end cell walls, thick, lignified walls and lots of unlignified ‘bordered pits’ in the walls for the movement of water
If lignin is the 2nd most common natural polysaccharide, then what is the first?
Cellulose
How is lignin laid down in xylem vessels’ walls?
Can be a number of ways- rings, spirals or even relatively solid tubes with lots of small unlignified areas called bordered pits
What is special about sieve tube elements?
They have no nucleus, and the tonoplast and many organelles break down, leaving them full of phloem sap with only a thin layer of cytoplasm
What are sieve plates?
Holes in the end cell walls of sieve tube elements which let the phloem’s contents through
What do companion cells do?
Add support to sieve tube elements and also carry out living functions, such as producing ATP, for them
What links sieve tube elements and companion cells?
Many plasmodesmata (microscopic holes in the cell wall)
What are some reasons that water is so important for plants?
- Its evaporation helps keep them cool 2. Raw material for photosynthesis 3. Mineral ions and products of photosynthesis are transported in aqueous solution 4. Turgor drives cell expansion 5. Turgor (a.k.a. hydrostatic) pressure in plants helps provide a hydrostatic ‘skeleton’ to keep the plant rigid
Why are mineral ions actively moved into the cytoplasm of root hair cells?
To keep the concentration of them higher than in the soil so that water moves in down a concentration gradient
What are the two pathways that water can move along the root in?
Apoplastic and symplastic
Where does water move along in the symplastic pathway?
Along the interconnected (via plasmodesmata) cytoplasm of root cells (passing around the vacuole)
How does water move along the symplastic pathway?
The cells on the very edge and end of the root have high water potentials due to the absorption of water from the soil. This water potential is higher than the water potential of the next cell along, so water moves along the root by osmosis this way.
Where does water move along in the apoplastic pathway?
Along the cell walls and intercellular spaces
How does water move along the apoplastic pathway?
It fills the open spaces between fibres in the cellulose cell wall. As water moves into the xylem, the cohesive forces between it and water further back in the cell walls creates tension that causes water molecules to be pulled along, unencumbered by the loose network of cellulose fibres in the cell walls.
What is the Casparian Strip made of?
A waxy material called suberin
Why is water forced from the apoplastic to the symplastic pathway?
Because water travelling along the apoplastic can carry unwanted contaminants from the soil with it, and forcing it into the symplastic pathway allows the cell surface membrane to remove these contaminants
What is the Casparian strip and what is its purpose?
A waxy, impermeable layer in the cell wall of the endodermis, whose purpose is to force water from the apoplastic pathway to the symplastic pathway
What is the endodermis?
The layer of cells surrounding the vascular tissue of the roots
How is water moved from the endodermis to the xylem?
Mineral ions are actively pumped into the xylem, which creates a concentration gradient which causes water to move into the xylem via osmosis
What causes root pressure?
The active pumping of mineral ions into the xylem
What does root pressure do?
Gives water a ‘push’ up the xylem, although this is not usually the major factor in moving water up to the leaves
Where is water lost from leaf cells to?
The air spaces in the spongy mesophyll
What is transpiration?
The loss of water vapour from the stems and leaves of plants via stomata
Why do some stomata need to be open at night?
To allow oxygen in for cellular reactions such as aerobic respiration, as none is being produced by photosynthesis
What is the transpiration stream?
The movement of water from the moment it is absorbed in the roots of the leaf until it leaves as water vapour from stomata in the leaves
What is the cohesion-tension theory?
The model of water moving as a continuous stream from the roots all the way up to and across the leaves
According to the cohesion-tension theory, how does water move inside the leaf?
Water evaporates from cells in the leaves, causing their water potential to fall. This causes water to move in from an adjacent cell. This repeats all the way back to the xylem, where water moves into the first leaf cell by osmosis
According to the cohesion-tension theory, how does water move up the xylem?
Water molecules form hydrogen bonds with the carbohydrates in the walls of the xylem (known as adhesion), as well as with each other (known as cohesion), the combined effects of which cause water to move up the xylem via capillary action
What is capillary action?
Where water can be drawn up a narrow tube against gravity
What is the transpiration pull?
Where water is drawn up the xylem in a continuous stream to replace that lost in the leaves via evaporation
According to the cohesion-tension theory, how does water move along the roots?
The transpiration pull results in a tension in the xylem, helping move move water across the roots.
What are some pieces of evidence to support the cohesion-tension theory?
- When a xylem vessel is broken, in most cases air is drawn in rather than water leaking out 2. In the case above, water can no longer be drawn up the xylem vessel, as the continuous stream of water molecules is broken 3. Changes in the diameter of trees; when transpiration is at its highest in the day, the tension in the xylem is the highest, so the diameter of the tree shrinks. The opposite occurs at night when transpiration is at its lowest.
What is a common piece of equipment used to indirectly measure transpiration in plants, and what does it measure instead?
A potometer, which measures water uptake instead
How do guard cells close the stomata?
When turgor is low due to low water content, the asymmetrically thick inner cell walls of the guard cells close the stomatal aperture
How do guard cells open the stomatal aperture?
They pump in solutes by active transport, which increases their turgor. Because of cellulose hoops the cells cannot expand in width so they do so in length- because the inner wall of the cells are thicker and less flexible, the cells change shape asymmetrically and open the pore
What are 4 factors which affect the rate of transpiration?
- Temperature 2. Humidity 3. Wind speed 4. Light intensity
How does temperature affect the rate of transpiration?
- By increasing the kinetic energy of molecules, speeding up osmosis, diffusion and evaporation 2. It increases the concentration of water vapour that external air can hold before it becomes saturated
How does humidity affect the rate of transpiration?
More humid air has a higher concentration of water molecules, so there is a lower concentration gradient and the air accepts less molecules through evaporation before becoming saturated
How does wind speed affect the rate of transpiration?
Increasing wind speed blows more water vapour away from the stomata, so there is a higher concentration gradient and more water will leave the leaf
How does light intensity affect the rate of transpiration?
In higher light intensities more photosynthesis will take place, so more gaseous exchange needs to take place through the stomata and more water will be lost through them
What is most glucose transported around the plant as?
Sucrose
What is most glucose stored in a plant as?
Starch
What are assimilates?
The products of photosynthesis moving around the plant
What is translocation?
The movement of assimilates dissolved in water via the phloem from sources to sinks
What is a source of assimilates?
Places where assimilates are made
What are the 3 main sources of assimilates in a plant?
- Storage organs such as tubers or tap roots when they are unloading their assimilates 2. Green leaves and stems 3. Food stores in seeds when they germinate
What is a sink of assimilates?
The parts of a plant which require assimilates, and have assimilates delivered to them
What are the 3 main sinks of assimilates in a plant?
- Actively dividing meristems 2. Roots which are growing and/or actively absorbing mineral ions 3. Any parts of the plant laying down food stores such as seeds, storage organs or fruits
What is an example of a plant organ which can act as both a source and a sink for assimilates?
Storage organs such as tubers
What is co-transport?
Where a molecule such as glucose must be transported across a carrier/channel protein with an ion, such as Na⁺, in order for the protein to allow the molecule across
What does the mass flow hypothesis state happens at sources?
Assimilates are actively loaded into the sieve tubes of the phloem. This lowers water potential, so water enter the phloem from the xylem via companion cells.
According to the mass flow hypothesis, what is the pressure at the top of the plant (the sources)?
High
What does the mass flow hypothesis state happens at the sinks?
Assimilates usually leave via diffusion to be used up here (in the sink). Removal of solutes causes water potential to rise, and water leaves by osmosis to lower it.
How does loading at the source work?
- H⁺ ions are actively pumped out of companion cells into mesophyll cells 2. A high concentration of H⁺ builds up in mesophyll cells, causing the ions to move back into companion cells, taking sucrose with it via a co-transporter protein 3. This causes a high concentration of sucrose in companion cells, so sucrose moves by diffusion into sieve tube elements, lowering water potential 4. Water moves by osmosis into the sieve tube elements from surrounding cells such as the xylem, increasing turgor pressure in the elements and causing movement of water and assimilates by mass flow to the sinks
What adaptation do companion cells have to aid active loading at the source?
Many mitochondria to provide the necessary ATP for the active transport of hydrogen
How does unloading at the sink work?
- Water and assimilates arrive in sieve tube elements in the sink 2. Sucrose diffuses into surrounding cells, which convert them into other substances such as starch 3. This lowers sucrose concentration, so more sucrose diffuses out of the sieve tube 4. The reduced sucrose in the sieve tube leads to an increase in water potential, so water moves out into surrounding cells by osmosis. 5. This reduces turgor pressure in the sieve tubes, and maintains mass flow in the phloem
What is some evidence for active loading?
- If the mitochondria in companion cells are poisoned, active loading stops 2. Advances in microscopy mean we can see it happen, and see adaptations of companion cells to help it 3. Rate of movement of sucrose is 10,000 times quicker than by diffusion
What is some evidence for translocation occurring in the phloem (as opposed to anywhere else)?
- Girdling- if you cut a ring of bark from a tree, thereby cutting off all phloem, which are found in the bark, the tree will bulge out above the cut as a new sink is formed 2. Aphids put their stylets into the bark of a tree where the phloem are and, if the fluids inside them are analysed, they can be found to contain many sugars
What are xerophytes?
Organisms adapted to survive in very dry conditions
What are hydrophytes?
Organisms adapted to survive in very wet conditions
What are some generic adaptations of xerophytes?
Thick waxy cuticle, sunken stomata, large, deep root network or many roots near the surface, less stomata, leaves reduced to spines, less leaves, hairy leaves, curled leaves, specialised water storage parenchyma tissue in succulents, loss of leaves in dry season, dormancy in dry periods
How is Marram Grass specially adapted to survive in a very dry environment?
It has a curled, ‘C’ shaped leaf, which creates a windless, humid environment for stomata to lose water into, as well as hairs near the stomata to collect water, after which it dribbles down to the roots for collection. Also has a very deep root network and an extensive system of shallow roots near the surface
What are some generic adaptations of hydrophytes?
Very thin/no waxy cuticle, many permanently open stomata on upper surfaces, reduced plant structure as plenty of water to support, wide, flat leaves to spread across surface of water, small roots, large SA of underwater stems and roots, air sacs to allow leaves and flowers to float (aerenchyma)
According to the mass flow hypothesis, what is the pressure at the sinks?
Low
