Plant Bio Flashcards
1
Q
How does water enter plants?
A
- water enters root hair cells by osmosis
- there has to be a higher concentration of solutes inside the cell than in the soil water around the roots for osmosis to occur
- low concentration of water in the root hair cells
- water moves down the concentration gradient
2
Q
Describe transpiration. Where and how does it occur?
A
- Transpiration is the loss of water from leaves and stems of plants.
- Xylem vessels transport water through plant
- Water is heated in the mesophyll by sunlight and becomes vapour
- The vapour transpires out of the stroma
- Loss of water generates negative pressure and a transpirational pull on water molecules in the xylem
- Cohesion between water molecules means that the transpiration pull has a knock-on effect through the plant.
- Higher rates of transpiration lead to a faster transpiration and higher rates of water uptake.
- This theory is known as cohesion-tension theory
3
Q
Describe the structure and function of xylem tissue
A
- Xylem doesn’t have cytoplasm (they are dead): provides larger lumen making water transport more efficient (xylem is dead so water movement must be passive)
- Cell walls: thickened to make them stronger
- Spiral lignin: walls are impregnated with lignin. This strengthens walls so they can withstand low pressure without collapsing. Lignin’s may be spiral or rings. Lignin is a complex fibrous organic polymer which is strong and rigid (plant seems woody)
- Pits: holes in the walls allow for movement of water from one vessel to the next
- Xylem vessels: long continuous tubes. End cell walls are lost to allow easy flow of water
4
Q
What factors increase transpiration in plants?
A
- temperature
- light intensity
- air movement
5
Q
Describe how a potometer is used to measure the rate of transpiration.
A
- measure transpiration indirectly by looking at the water uptake.
- As transpiration occurs a bubble of air moves into the tube and towards the plant (to replace the volume of water transpired).
6
Q
Describe translocation. Where and how does it occur?
A
- Translocation is the movement of organic substances in a plant
- Active translocation happens in the phloem (requires energy)
- Sucrose is actively transported into the phloem
- H+ ions are actively transported (using ATP) out of the phloem cell.
- High H+ ion concentration gradient builds up outside the cell.
- H+ ions flow back into the cell, the energy released is used to co-transport sucrose into the phloem cell.
- High concentrations of solutes cause water uptake by osmosis
- Concentration of sucrose in phloem cells is relatively high, the water concentration is relatively low.
- Water moves down the concentration gradient from the xylem, through the membrane, into phloem cells, by osmosis
7
Q
Describe the structure and function of phloem tissue
A
- Organic molecules move in plants via phloem
- Phloem is made of living cells (unlike xylem).
- Phloem is mostly made of sieve tube members and their accompanying companion cells
- Sieve tube members are connected to one another by sieve plates to form sieve tubes (aka sieve elements).
- Companion cells are connected to their sieve tube members by plasmodesmata.
- Sieve plates have pores that allow the movement of water and dissolved organic molecules throughout the plant.
- Translocation is the movement of organic substances in plants
- Phloem cells transport contents in various directions (unlike xylem which only transport water and minerals from the roots upwards).
8
Q
What are vascular tissues? How are they arranged in a plant stem/root?
A
- Vascular tissues comprise of xylem and phloem
- Xylem in inner section of the bundle
- Phloem on the outer section of the bundle
9
Q
What is a source? Sink? Examples?
A
- A source is a site of production or storage (leaves, stems, roots)
- A sink is the destination/site of use (growing roots, fruits flowering)
10
Q
What is the pressure-flow hypothesis?
A
- Phloem transports water and solutes along hydrostatic pressure gradients
- Relatively high concentration of sucrose and water in phloem sieve tubes.
- Water is incompressible, i.e. it occupies a fixed volume.
- The walls of the sieve tubes are rigid.
- These two factors cause a build-up of hydrostatic pressure at the source.
- Water and the solutes (sucrose and amino acids) flow down the hydrostatic gradient to the sink where pressure is relatively low.
11
Q
Describe how aphid stylets and radio-actively labelled carbon dioxide are used to measuring phloem transport rates.
A
- Aphids penetrate plant tissue to reach the phloem using mouth parts called stylets. The stylet is inserted into the plant sieve element to allow the sap to be extracted.
- The high pressure inside the sieve tube pushes sap into the aphid via the stylet.
- If the aphid is anaesthetised and the stylet severed, phloem will continue to flow out the stylet and both the rate of flow and the composition of the sap can be analysed.
- The closer the stylet is to the sink, the slower the rate at which the phloem sap will come out.
- A plant is grown in the lab and one leaf is exposed for a short time to CO2 containing the radioactive isotope 14C.
- The 14-CO2 will be taken and incorporated into glucose by the process of photosynthesis. Glucose is converted into sucrose for translocation via the phloem.
- Aphids are encouraged to feed on the phloem in different locations of the stem at different times.
- The phloem is then analyzed for 14C content, and the results can be used to calculate the rate at which substances move through the phloem
12
Q
What is the function of auxin?
A
- Auxin influences cell growth rates by changing the pattern of gene expression
- Auxins are a group of hormones that have a wide range of functions in plants including: root and shoot growth, flowering, fruit development, leaf development, wound response
13
Q
How does auxin respond to sunlight? (Draw flow charts to help you understand this)
A
- Phototropism is growth of a plant towards light.
- Plant shoots exhibit positive phototropism.
- Plant roots exhibit negative phototropism.
- Auxins are produced in the coleoptile, a protective sheath around the emerging root or shoot
- Under normal conditions, auxin is distributed evenly along the shoot, causing even vertical growth
- If photoreceptors in the coleoptile detect as light stimulus from one direction, auxin is moved to the opposite sight of the growing shoot
- The uneven distribution of auxin causes increased growth on one side – and the plant grows towards the direction of the light
14
Q
Summarise the role of gene expression and auxin efflux pumps in controlling plant movement towards light. (Draw flow charts to help you understand this)
A
- Auxin efflux pumps (specialized membrane proteins) move auxin out of the cells closest to the light, using ATP as the energy source
- Cells contain an auxin receptor. When auxin binds to receptors, transcription of specific genes is promoted.
- The expression of these genes causes secretion of hydrogen ions from cytoplasm into cell walls.
- Hydrogen ions loosen connections between cellulose fibres, allowing cell expansion.
- Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
- Concentration gradients of auxin are necessary to control the direction of plant growth.
- This requires that auxin is unevenly transported amongst plant tissues
15
Q
Summarise the importance of auxin concentration gradients in controlling the direction of plant growth.
A
- Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
- Concentration gradients of auxin are necessary to control the direction of plant growth.
- This requires that auxin is unevenly transported amongst plant tissues
